9. Mining

Posted on 27/02/2009 by Vitor Pacheco

9.1 PRIMITIVE EXTRACTION

I include under mining all human activities concerned with the extraction of inert natural materials. As such, the extraction of stone for construction, is mining, and at the beginning, perhaps the easiest was the excavation in relatively soft sandstones, to enlarge the original caves where the people dwelt (fig. 89). Also, rock blocks shaped by natural jointing were used originally possibly only for religious purposes (fig. 163B),

Figure 163B – Stonehenge, one of the earliest human utilisation of stone.

but later they started building wall protections and houses by “dry packing” carefully selected well shaped blocks (fig. 163C).

Figure 163C – The remains of a Bronze Age village (Citânia de Briteiros, Portugal).

As technology progressed people realised  they could improve on the natural jointing by using very simple chisels for digging grooves along predetermined lines, in order to create an artificial joint through which the rock would fracture.  A good example can be seen within the Moorish castle at Sintra, Portugal (Fig. 164),  built during the 8th century.

Figure 165 - Wall of the Moorish castle where the block is located (Sintra, Portugal).

Figure 164 – Inner view of the Moorish castle wall (Sintra, Portugal).

As figure 165 shows, two lines of grooves were chiseled on the granite boulder located within the castle area, and along one of them a neat break had already been accomplished. Surely this is a block in preparation for repairing the wall.

Figure 165 - Granite boulder halfway split to shape a block for the castle wall (Sintra, Portugal).

Figure 165 – Granite boulder halfway split to shape a block for the castle wall (Sintra, Portugal).

9.2 SIMPLE EXTRACTION

With the present technology, inert materials can be extracted in many sophisticated ways. Here I just refer to some which have been extracted since time immemorial and even today are extracted in a relatively simple manner. River sand dredging (fig. 166) is one of them. This job that in the old days was done entirely manually is now done in a totally mechanised manner.

Figure 168 - Sand dredging (Douro River, Portugal).

Figure 166 – Sand dredging (Douro River, Portugal).

Note that this sand is not extracted to permit navigation, but rather for construction. A consequence of this huge and continuous suction of sand, is the rejuvenation of the river’s energy level, thus increasing enormously its erosional power, with disastrous consequences for  bridges, etc. To me this is a very short sighted approach because sand of the same quality can be obtained by minor additional processing of the fines that develop at stone crushers, which otherwise have to be discarded as waste and occupy unnecessary space requiring additional costs for the final rehabilitation.

Going now to laterite, it occurs in regions with high precipitation. India is a great example, with their well known monsoons. There, I saw laterite being mined (fig. 31D) by cutting it in blocks (fig. 167) for the construction of houses. These blocks are rather large, about 0.5 x 0.3 x 0.1 metres but they are not too heavy, so they can be handled reasonably easily. Also, I was told no cementing material is needed since, with time and the enormous amounts of rain they have, the iron in the laterite is mobilised some more and the blocks seal themselves.

Figure 167 – Laterite construction blocks (Orissa, India).

Next we have alluvial mining, which was done in the good old days by panning but now, even  though done using more mechanical methods, can still be considered a one man operation, with very few helpers.  The example I show, is for diamonds along the Vaal River, in South Africa (item 6.4.1, fig. 99), where  a rather simple but effective mechanical method of grading the clasts is being used (fig. 168)

Figure 169 - Mechanical grading of the river gravel (Ulco Area, Orange River, South Africa).

Figure 168 – Mechanical size sorting of the river gravel (Ulco Area, Vaal River, South Africa).

and figure 169 showing the owner of the enterprise doing the final sorting. It was impressive how fast his hand moved and I do not think many diamonds were missed.

Figure 170 - Final hand sorting for diamonds (Ulco Area, Orange River, South Africa).

Figure 169 – Final hand sorting for diamonds (Ulco Area, Vaal River, South Africa).

Another one man operation I came across, was the mining of a gold rich quartz vein in Zimbabwe. Actually, in this case it was a partnership of two. This time a grader was not necessary, but rather a stamping mill  (figure 170)  to crush the ore, which is also rather simple but very effective.

Figure 171 - One man gold mine operation, ore stamping mill (Bulawayo region, Zimbabwe).

Figure 170 – One man gold mine operation, ore stamping mill (Bulawayo region, Zimbabwe).

Needed too, was a vibrating table to sort the heavy material (fig. 171). Note the pale, rather thick streak of heavies at the top of the table, on the left hand side. Unfortunately for the owners, this streak was not just gold but rather predominately sulphides with minor gold specks. For the final separation they were still using mercury which makes an amalgam with the gold, and then the mercury was “boiled out”.

Figure 172 - Vibrating table (Bulawayo region, Zimbabwe)

Figure 171 – Vibrating table (Bulawayo region, Zimbabwe)

9.3 OPEN CAST

As the name indicates, open cast means mining on the surface. Of these, the simplest is the extraction of stone for construction and since there is construction everywhere, stone quarries also exist everywhere. Here I only want to show not so much the importance of the health regulations but rather, their implementation. Figure 172 shows an entirely dustless loading operation in a granite quarry in South Africa,

Figure 173 - Dustless dump loading operation in a quarry (Halfwayhouse, South Africa).

Figure 172 – Dust free dump loading operation in a quarry (Halfwayhouse, South Africa).

and figure 173 shows a limestone quarry in Portugal with so much dust, that one has difficulty in distinguishing the crusher unit on the right hand side. Probably, because Portugal belongs to the EU, its mining laws are actually more strict than those in South Africa but, obviously in Portugal there is no apparent law enforcement, since the photo was taken from a moderately important road with considerable traffic of all sorts.

figure 174 - Quarry without any dust prevention (Serra de Janeares, Portugal)

Figure 173 – Quarry without any dust prevention (Serra de Janeares, Portugal).

This is sad because quarry dust is easily prevented by simply spraying the haul roads, the blast heaps, as well as all the crushing units. That is, all the sectors where dust may develop, with the exception of course, of the dust caused by the blast, as shown in figure 174.

Figure 175 - Blasting in progress (Ulco, South Africa).

Figure 174 – Blasting in progress (Ulco, South Africa).

For obvious reasons, this regulation is extremely important and, in Portugal, where water is abundant, it is not even expensive to implement. On the other hand this is not so in many parts of South Africa like Ulco, which has a very arid climate and consequently where water is difficult to obtain. Even then however, for the workers’ health sake, the quarries are maintained dust free. Going still further, at Ulco there is not only a stone quarry, but also a cement and lime factory, that is, additional potential sectors of large quantities of dust development but, as figure 175 shows, there is none.

Figure 176 - Ulco from the air, with the township on the right, the quarry in the middle, and the factory complex on the left (South Africa).

Figure 175 – Ulco from the air, with the township on the right, the quarry in the middle, and the factory complex on the left (South Africa).

Still to do with construction and ornamental stone we look now at the extraction of marble. The interesting aspect here is that technology has already managed to do away with blasting, which used to cause a lot of wastage due to excessive and uncontrolled cracking of the rock, even under very cautious controlled methods. Nowadays a diamond wire is used, that is, a wire line impregnated with diamond chips. Figure 176 shows the control unit as well as the two sides of the wire loop. On the far side, out of the picture, there is a pulley positioned in such a fashion that the wire is in continuous contact with the marble to be cut. In other words it is the same principle as a jig saw.

Figure 177 - Wire cutter in a marble quarry (Porto Alegre region, Portugal).

Figure 176 – Wire cutter at a marble quarry (Porto Alegre region, Portugal).

I can not resist to go back to the problem of heritage misusage. The background of figure 176 is completely occupied with waste dumps, that is another example of shortsightedness, since nature is a human heritage to be preserved and not to be abused.

9.4 UNDERGROUND

Naturally mining is cheaper at the surface than underground. Hence, mining will only go underground if the desired material can no longer be extracted from the surface, or if that material does not outcrop.

9.4.1 Marble

I visited the quarry/mine shown in figure 177, in November of 1998. It is apparent that the exploitation is still within day light. In fact it is a case of cutting inwards from a central open pit. Going underground reduces the need to remove the thick overburden constituted by very weathered and broken marble, thus reducing waste removal. One can have an idea on how close the surface is, because the weathering effect is still noticeable on the upper section of the central portion, which is a structural support pillar. By the way, this pilar also shows how well the wire cutting mechanism mentioned above works.

Figure 178 - Underground mining of marble (Porto Alegre region, Portugal).

Figure 177 – Underground mining of marble (Porto Alegre region, Portugal).

Naturally, as the picture shows, this is a case of very high quality white marble, hence allowing the possibility of going underground. However, even with all these possible cost advantages, I wonder if the venture is still going. I do not have much faith in it.

9.4.2 Rock Salt

Perhaps a rather interesting and important example of early mining underground is rock salt. Obviously outcrops of such a material would not last at all since salt is so soluble. However salt diapers (item 6.5.2) are very abundant for example, in Europe, and I’m sure it has been mined since time immemorial, because it was so useful in areas away from the coast, and it is so easily mined (fig. 178).

Figure 178 – Mining a rock salt diaper (Loulé, Portugal).

Of these diapers, Portugal has a very unusual example in the vicinity of Rio Maior. Its extraction method is quite unique because that diaper is located in a rather large aquifer within a limestone succession. The water in this aquifer has diluted the rock salt to a concentration seven times greater than the one of the Atlantic Ocean, approximately 30km W of this deposit. Thus, instead of having to mine the rock salt underground, the saturated water is simply drawn from a well (fig. 178B),

Figure 178B – Rock salt extraction (Rio Maior, Portugal).

and the much cleaner salt is collected from the evaporation pans around it (fig. 178C). Supposedly this deposit has been exploited on and off since 1177.

Figure 178C – The salt pans around the well.

9.4.3 Chrome Mine at Boula, India

The chrome mine at Boula, is a good example of a mine which started at the surface, but due to the space constraints with depth, it had to opt and go underground (fig. 179).

Figure 179 – Boula chrome mine (Orissa, India).

Three chrome rich zones exist in this mine. The richest, originally mined at the pit on the left, is named Shankar. To the right within a shallower pit, is the next chrome rich zone, named Lakshmi and to the right of that, outside of the picture, there is the third chrome rich zone named Durga. The Shankar section is the deepest portion of the open cast development, at the far end of which the small little building is the engine house for the hauling of ore along an inclined shaft. On the right, within the Lakshmi pit we have the more obvious headgear of a vertical shaft. It is via these two shafts that the underground mining is done. This mine has interesting features that deserve mentioning. Notice at the centre, of picture 179, the flat portion at the higher point separating the Shankar from the Lakshmi pits. That is where the ore is sorted (fig. 180)

Figure 179 - Hand ore sorting (Chrome mine, Boula, India)

Figure 180 – Hand ore sorting (Chrome mine, Boula, India).

and piled (fig. 181). Female laborers do the sorting by hand and they are also the ones who, manually and meticulously, pack the ore on an exactly dimensioned four sided prism. This is a natural consequence of cheap labour. Note that a mechanical ore sorting machine would be far too expensive, making this venture not viable. In the same way, the packed ore does not need to be weighed, saving on the expense of such a machine. The volume of the prism is measured by tape and the tonnage is calculated using the predetermined SG of the packed ore.

Figure 180 - Manually packed ore pile (Chrome mine, Boula, India)

Figure 181 – Manually packed ore pile (Chrome mine, Boula, India).

Now, the naming of the ore zones; figure 179 is facing S and the picture was taken from a ridge formed by a fault zone with an apparent uplift to the N. On the N side of the fault, that is, behind the photographer, only one ore zone exists which was named Ganga. I find the reasoning behind the naming fascinating. If I understood it correctly, Shankar is a very important god whose wife is Lakshmi and they have a daughter called Durga. In other words, the thickest  and best developed ore zone gets the name of an important god. Next to it but not as well developed, is his wife and the weakest of the ore zones is the daughter. More, supposedly Ganga is Shankar’s lover. The affair must not be obvious so Ganga is separated by a fault, but she is important and so she is at a higher level than Lakshmi and Durga.

9.4.4 Kimberly Diamond Pipe

The Kimberly pipe (fig. 17 ) is the one which started the diamond rush in South Africa and gave the name to the rock that forms it (kimberlite). This is another example of surface mining having to go underground due to lack of space. By 1875, within the 38 acres encompassing the outcrop area of the pipe, there were hundreds of independent miners working in their separate claims as can be observed in figure 181B, showing not only the web made by the numerous cables of the active individual rock hoists, but also the depth at which they were already working.

Figure 181B – The historical Kimberley Pipe in South Africa. Photo taken in 1875, showing the existing individual rock hoists (photo obtained in the Kimberley Museum “sold in the aid of the Red Cross”).

The corresponding statistics shown in figure 182, gives a rather nice summary.

Figure 182 - Diagrammatic section and statistics of the Kimberley diamond pipe (South Africa).

Figure 182 – Diagrammatic section and statistics of the Kimberley diamond pipe (South Africa).

9.4.5 Witwatersrand Mining

I think this is the best example of the influence of mining on the surface morphology and on the important differences between surface and underground mining. Figure 183 shows Johannesburg from the air, seen from the South. Notice in the background numerous large buildings. In the mid ground, with practically no buildings, there is a reasonably sized lake and two barren portions, one close to the left extremity, which is a remaining waste dump and the other of considerably larger dimension, near to the lake is a slimes (tailings) dam. This central area is where the gold bearing sedimentary horizons of the Witwatersrand Supergroup outcrop and most of that ground still belonged to the mining houses in 1984, when this picture was taken. Rehabilitation wise, to my knowledge the majority of the material forming the waste dumps, being predominantly very hard quartzite, was reprocessed as gravel. As for the slimes dams, that was a very difficult problem. The gold ore was crushed to a very fine mesh and the gold was removed using cyanide. This means that the slimes dams are constituted by a very fined grained totally sterile material. On windy days, Johannesburg was often covered by a dust of very fine quartz particles. The solution encountered was to cover these large slimes dams with thick layers of fertile soil and vegetate them as quickly as possible.

Figure 183 - Johannesburg from the air (South Africa).

Figure 183 – Johannesburg from the air (South Africa).

These gold bearing sediments dip southwards at about 25º and the mining started going underground by about where the lake is. In other words, the houses in the foreground of the picture were built over ground that was mined pretty close to the surface. That is why, by municipal law, no houses of more than one floor were allowed on that sector. Nowadays all the Witwatersrand gold mines are underground and their head gears are characteristic of the region (fig. 184).

Figure 184 - Winklehaak Gold Mine no 1 shaft and reduction works, Evander, SA

Figure 184 – Winklehaak Gold Mine no 1 shaft and reduction works, Evander, SA.

The haulage levels are approximately 30 vertical metres apart and the staff as well as the materials are transported by very fast lifts with stations at every level (fig. 185).

Figure 185 - Underground lift station, East Driefontein Gold Mine (Carletonville South Africa)

Figure 185 – Underground lift station, East Driefontein Gold Mine (Carletonville South Africa).

All the development of the haulages used to be done by drilling and blasting (fig. 186),

Figure 186- Underground drilling team. B - Raise borer hole (East Driefontein Gold Mine, Carletonville South Africa).

Figure 186- Underground drilling team (East Driefontein Gold Mine, Carletonville South Africa).

but by the time I left, 1975, boring machines were starting to be used in main haulages and raises (fig. 187).

Figure 187 - Raise borer hole (East Driefontein Gold Mine, Carletonville South Africa).

Figure 187 – Raise borer hole (East Driefontein Gold Mine, Carletonville South Africa).

Stopes are the section of the mine from where the ore is extracted. Since we are dealing with a sedimentary horizon, mine-wise speaking, it has a limited thickness but an unlimited length and width. Thus wherever the grade is economical that layer of the rock sequence is entirely removed.  Figure 188 shows a stope face with the ore exposed.

Figure 188 - Stope face (East Driefontein Gold Mine, Carletonville South Africa).

Figure 188 – Stope face (East Driefontein Gold Mine, Carletonville South Africa).

To use rock pillars, is to reduce the amount of extractable ore. Thus they used wood log mats (fig. 189). The picture shows two pillars already in place and in the middle a loose pile of mats.

Figure 198 - Sotpe pillar support (East Driefontein Gold Mine, Carletonville South Africa).

Figure 189 – Sotpe pillar support (East Driefontein Gold Mine, Carletonville South Africa).

Finally, observation of my assistant, Zé (fig. 188), shows how hot it is in those mines. This picture was taken at about 1800m below surface and the rock temperature was close to 50ºC. Work is only possible because the ventilation is refrigerated.

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8. Prospecting

Posted on 20/02/2009 by Vitor Pacheco

8.1 GRASS ROOTS

Grass roots exploration is the general term for the very initial stage of prospecting that starts from a zero base, that is, neither geological maps, nor aerial photos are available, and often not even topographic maps. Of these, my first experience was in Mozambique in 1972, when communication with the outside world was a very precarious land line and some times, when we were lucky, a fax, both by means of the post office at the nearest village, which was about 150 km away. I do not think it appropriate here, to go into the prospecting work itself which consists of mapping, sampling, drilling, data interpretation  and a final data synthesising. However, under advanced prospecting I will show some photos referring to sampling which, I think, takes most of the geological time.

8.1.1 Transport

In areas of grass roots exploration, most of the times even the main roads are simple tracks across the veld. Hence a tough reliable 4 wheel drive vehicle is fundamental as this example, still in Mozambique and which was my baptism of bundu bashing, indicates. Figure 143 shows the end of my successful attempt of taking my lovely car out of a river side mud bog. I was alone, and it took me 4 hours to get it out.

Figure 143 - Bogged down in deep Africa (Porto Amélia District, Mozambique).

Figure 143 – Bogged down in deep Africa (Porto Amélia District, Mozambique).

Just for comparison purposes I also show the same kind of experience, but in Portugal in 1996 (fig. 144). This time it was easy, we only had to call the local farmer to bring his tractor and pull us out. So, not only was this in a different continent, but also 24 years later.

Figure 144 - Bogged down in paradise (Alentejo, Portugal).

Figure 144 – Bogged down in paradise (Alentejo, Portugal).

What I want to make clear is that if I had the fancy comfortable white car in Africa, even today, it would take me perhaps weeks to get it out, if at all. This because today’s sophisticated jeeps have so many complicated electronic gismos that one needs to have a highly qualified, not just mechanic, but a well equipped garage within easy reach. Unfortunately I’m now considered too old by the powers that be, to continue prospecting. One thing is sure though, if I did go, the jeep I would choose is the Indian manufactured Mahindra (fig. 145). It is incredibly robust and has a totally old fashioned simple, reliable engine that will go anywhere and the only assistance it needs is regular greasing and any simple mechanic assistant to deal with minor difficulties. Just as an interesting memory of my stay in India, notice the jeep’s front decorations with the string of flowers and the painted swastikas. This is a must to make sure the car is accepted by the gods.

Figure 145 - One of our local 4-wheel drive vehicles (Orissa, India).

Figure 145 – One of our local 4-wheel drive vehicles (Orissa, India).

8.1.2 Accommodations

Even in many remote parts of Africa it is often possible to organise a side farm building or similar locations to use as living and working quarters. When that is not possible, as in my stay in Angola, one has to organize camping facilities which must have a minimum of practicality and comfort. My full staff (fig. 146) consisted of one local geologist, one local person of the correct tribe and political affiliations, one overall organizer, two security guards (hence the guns), one cook with an assistant and two laborers. I was fortunate to find a very reliable and professional organizer, Vete Willy, who not only built our camp but also kept it going, always in impeccable conditions. He is not in the picture because, other than me, he was the only one capable of using the straight forward Zeiss Ikarex camera I had.

Figure 146 - My Angolan prospecting staff and me in the vicinity of our camp at Bentiaba.

Figure 146 – My Angolan prospecting staff and me in the vicinity of our camp at Bentiaba (Angola).

I was working for a medium sized mining company but, not so far away, there was the camp of a very large mining group, who also had to arrange a camp and whose chief geologist I became acquainted with. Since I have pictures of both camps it is interesting to put them side by side. The dimension difference is impressive. Two of my whole camps (fig. 147)

Figure 147 - The entrance to my prospecting camp (Bentiaba, Angola).

Figure 147 – The entrance to my prospecting camp (Bentiaba, Angola).

would fit within the entrance area of the other camp (fig. 148). Or putting it another way, when there are funds, much more can be done in a much shorter period, and in much more efficient working conditions.

Figure 148 - Large mining group entrance to their camping site and chief geologist’s caravan (Caama region, Angola)

Figure 148 – Large mining group entrance to their camping site and chief geologist’s caravan (Caama region, Angola)

The fleet difference is also striking. Figure 149 shows my two cars,

Figure 149 - My camp, and whole vehicle fleet, my tent and the office (Bentiaba, Angola).

Figure 149 – My camp, and whole vehicle fleet, my tent and the office (Bentiaba, Angola).

and figure 150 shows part of the, let us call opposition, fleet. Also shown in my camp is my tent in the foreground and the office  tent in the middle ground. Naturally this little office was strictly for rough work. We did have a comfortable house and office at the nearest town.

Figure 150 - Partial vehicle fleet of the opposition (Caama region, Angola).

Figure 150 – Partial vehicle fleet of the opposition (Caama region, Angola).

Going now to the eating facilities, the comparison continues to be striking. Not only is there a great difference in space, but also the accommodation and the furniture. My little dining hut (fig. 151) was built with the minimum of the essentials.

Figure 151 - The dining room of my camp (Bentiaba, Angola).

Figure 151 – The dining room of my camp (Bentiaba, Angola).

The other one even had a TV, with its dish aerial at the left edge of figure 152 . One must be fair though, I did have a satellite phone and it worked pretty well. It was not as bad as in Mozambique but, after all, I was in Angola in 1997/8, that is, 26 years later.

Figure 152 - The dining facilities of the opposition (Caama region, Angola).

Figure 152 – The dining facilities of the opposition (Caama region, Angola).

Finally, the ablution facilities. Our toilet (fig. 153) was the long drop method and to reduce unpleasant smells it was sufficiently far away, outside the camp area and on the correct side of the prevaling winds.

Figure 153 - My camp’s toilet facilities (Bentiaba, Angola).

Figure 153 – My camp’s toilet facilities (Bentiaba, Angola).

Notice that the opposition even had a water pump so that one could have a nice cleansing shower at the end of the day (fig. 154). In my case, to wash we had to go to the nearby river and use the remaining water pools during the dry season. I will never forget though, the most enjoyable showers I had. During the rainy season it practically rained every day, and often late in the afternoon, that is, at the correct time to clean all the work day dirt and sweat. I would undress in my tent, come out with the soap and use the rain as a shower. It was divinely refreshing and it lasted long enough for me to complete the job. It is definitely a lovely memory.

Figure 154 - The oppositions ablutions area (Caama region, Angola).

Figure 154 – The oppositions ablutions area (Caama region, Angola).

8.2 ADVANCED PROSPECTING

8.2.1 In the Field

After basic geological mapping, trenching is often used, especially over areas with poor or no outcrop. Additional geological mapping is done along them and, when applicable, tentative initial trench sampling will also be considered (fig. 155).

Figure 155 - Trenching along very weathered strata (Trás-os-Montes, Portugal)

Figure 155 – Trenching along very weathered strata (Trás-os-Montes, Portugal)

Nowadays, after detailed mapping as well as soil, trench and rock outcrop sampling, if the indications are positive a drilling programme will be planned. In the old days short underground adits into the hill sides would be cut or, in flatter areas they would sink small shafts from which adits would be cut, generally along strike. In present day prospecting sites it is frequent to encounter such old workings. Since geologists are eternal optimists, the assumption is that whoever was there before did not prospect well enough or, most likely, the price of the resource concerned was not high enough to make the venture viable at that stage. Obviously, these old workings are always very closely scrutinized since they will add valuable data at practically no additional cost (fig 156).

Figure 156 - Preparing to go down a prospecting shaft (Alentejo) (Portugal).

Figure 156 – Preparing to go down a prospecting shaft (Alentejo, Portugal).

Returning to the rock outcrop sampling, it is most advantageous where the outcrop is good and continuos, since it is much cheaper than drilling. In the old days the sampling was done by chipping the rock with a hammer and chisel, but now there are diamond circular saws that do not need water to cool. It makes the exercise much simpler and faster, although a bit dusty, hence the masks (fig. 157).

Figure 157 - Sampling team at work (Boula, India).

Figure 157 – Sampling team at work (Boula, India).

Figure 158 shows the sample groove and respective number.

Figure 158 - Sample groove and respective number (Boula, India).

Figure 158 – Sample groove and respective number (Boula, India).

At this stage, if all indications are positive, a drilling programme is planned and budgeted. It is now fundamental to prepare a yard (fig. 159) to store the drilling core and also a sample preparation laboratory where the samples can be cut, crushed, quartered, a portion sent to an assaying laboratory and the remainder kept for potential future use. Naturally this sample laboratory must have all the necessary equipment to prevent contamination. For the more basic prospecting facilities the core is simply split and half is sent for assaying.

Figure 159 - Initial stage of preparation of future core shed, left, and sample preparation lab, right (Boula, India).

Figure 159 – Initial stage of preparation of future core shed, left, and sample preparation lab, right (Boula, India).

Drilling especially in new areas, is done not only for sampling purposes, but primarily to assist with the identification and interpretation of the rock assemblage where the ore is located. For that, not only must each hole be meticulously geologically logged, but more important still, the core of as many of the holes as possible, must be laid side by side to facilitate in the identification and correlation of the constituents present, in order to determine the local stratigraphy, hence the need for a large yard. Figure 160 is the core yard where I was fortunate enough, at a very early period of my career, to be present during the initial stages of a diamond drilling programme in the Bushveld Igneous Complex and assist a very capable senior colleague. His  good understanding of the stratigraphic principals lead to the identification of all the individual units immediately above and below the Marensky Reef (item 2.3 Magmatic Differentiation), so necessary for a successful final synthesis.

Figure 160 - Very well planned Core shed and yard (Springs, South Africa).

Figure 160 – Very well planned Core shed and yard (Springs, South Africa).

8.2.2 In the Mine

Prospecting is not done only to find new ore resources but, just as important, it is necessary when, for example, in an already working mine, there is the possibility of exploiting an additional ore which was previously considered uneconomical. In that case, the waste dumps of the original extraction, must be reevaluated to ascertain if there is  enough of the second element to be re-qualified as ore. This is what happened at the chrome mines at Boula, India, where platinum was identified and it was hoped it might  have sufficient grade to be exploited as well. The first step to ascertain this possibility was to sample the chromite waste dumps (fig. 161). The little markers seen all over the stone pile actually form a well delineated sampling grid. It is possible that the sampling method selected, which only used chips cut from every piece of rock within the delineated square might not be adequate, but that is how it was done. The next stage was to sample the chromite ore exposed on the open cast pit (fig. 157). This would be followed by a drilling programme for which the necessary core shed and sampling lab were already being prepared (fig. 159). At that stage I left the project.

Figure 161 - Chrome mine waste dump sampled for platinum (white tags on little metal rods) (Boula, India).

Figure 161 – Chrome mine waste dump sampled for platinum (white tags on little metal rods) (Boula, India).

8.2.3 Sampling

As already mentioned, sampling is a vital item of prospecting without which a factual synthesis is not possible. Thus, its correctness and reliability is fundamental. Even though figures 162 and 163 actually represent stope sampling for grade control in a mine, they are good examples to show the basic importance of strictly adhering to a statistically predetermined grid. The yellow lines are actually the markings of each sample. When I left the gold mines the hammer and chisel chipping method was still being used, hence the shape of the area to be sampled. Careful examination of figure 162 shows very nice looking buckshot pyrite just to the left of the sampling line. This means good gold values, because there was a direct relationship between buckshot and gold. Since there is no buckshot at the sample location, its gold value will most likely be poor. However, if the sampling position is moved to include the buckshot, we are no longer dealing with a sample but rather with a bias grab specimen.

Figure 162 - Underground single sampling for gold in the Witwatersrand, South Africa.

Figure 162 – Underground single channel sampling for gold in the Witwatersrand, South Africa.

In figure 163 we are dealing with an ore horizon consisting of various conglomerate bands separated by quartzite, termed internal waste because, as it should be expected, it never carried any gold. In the present case, for a detailed study and considering the abrupt changes in thickness of the conglomerates the sampling zone consists of four adjoining sections.

Figure 163 – Underground detailed sampling for gold in the Witwatersrand, South Africa.

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7. Structure and Metamorphism

Posted on 11/02/2009 by Vitor Pacheco

7.1 General

As mentioned in the introduction, faulting, folding and metamorphism are a consequence of the movement of the plates that form the earth’s crust, and these plate movements encompass huge areas, developing topographical features with spectacular characteristics, particularly noticeable on aerial and/or satellite photographs, and giving rise to striking sceneries.

In areas of plate divergence, tension causes the ripping of the plates leading to the development of predominantly simple faults, termed normal or extensional, with the broken fragments simply falling due to gravity, and arranging themselves in an array of rifts, with the lower blocks forming “grabens” of which I show a miniature one, just to give the general idea (fig. 125), and the upper blocks form the “horsts”. Of these rifts, perhaps the best known one is the African Rift Valley.

Figure 125 – Miniature Graben (Furnas, Alentejo, Portugal).

Also, these horsts tend to have an overall flat surface because of the long erosional periods such areas were subjected to prior to the plate rupture. A good example is the highest point of Portugal, Torre, at Serra da Estrela, with an elevation of approximately 2000m and, as figure 126 shows, it has a rather dull flat top.

Figure 126 – The distinctly flat top of the highest point in Portugal (Torre, at Serra da Estrela) (view NW)

On the other hand, along converging boundaries, extreme compressional forces will cause the development of thrust faulting, intense folding, as well as extremely high temperatures giving rise to a very rugged topography with sharp peaks and associated entrenched valleys, as for example the Alps (fig. 46).

7.2 Faulting

Faulting takes place when the rocks under stress are brittle. Figure 127 shows a normal fault typical of a Diverging Boundary environment. This one shows the dragging of the right hand block, as it slid downwards relative to the left hand one. Note though, that this is not always the case, and a much more detailed study is often needed to determine the relative movement of the blocks.

Figure 127 – Normal fault with associated dragging on the downthrow side (Pedreira do Galinha, Serra de Aire, Portugal) (hight of face apr. 3m)

Relative to Converging Boundaries, I show two photos of rather small, but quite explanatory reverse faults. In figure 128, the proof that it is a reverse fault is that there is a duplication of the dyke, within the dyke/fault vertical projection. In other words, the block on the right was pushed up, being therefore, the upthrow side.

Figure 128 – Reverse fault/dike (Boula, India).

In figure 129 the dragging of the sediments on the upthrow side makes quite clear which way the movement was. In other words, the block on the right was pushed up, being therefore, the upthrow side.

Figure 129 - Reverse fault with drag of sediments on the up-throw side (Cape Peninsula, South Africa)

Figure 129 – Reverse fault with drag of sediments on the up-throw side (Cape Peninsula, South Africa).

All the fault photos shown so far, are from artificial land cuttings that is, they show cross sections, which is the easiest way to visualise a fault. However, this is not the most common view of a fault when doing field work. In fact, with the large ones we only become aware of them when the  geology is plotted on a map, and the fault appears as a linear structure offsetting the stratigraphic succession across which it cuts. Figure 129B is an exceptional example because it shows the fault and its displacement on a horizontal land surface and this is only observable because the fault is tiny. As can be seen, the sedimentary beds dip towards the photographer and, to the right of the fault, the bedding plane strike lines are further away. Thus, the right hand side fault block is the downthrow side since it dropped relative to the other one and, as the erosion levelled the ground, the strike lines were displaced towards a higher bedding plane position. As we used to say, the downthrow side moves up-dip.

Figure 129B – Plan view of a fault with the downthrow side on the right hand block (actual fault throw possibly less than 0.5 m) (Praia das Avencas, Parede, Portugal).

Of the thrust faults, which tend to be quite flat, because the movement within converging plate boundaries is mainly parallel to the earth’s surface, the only one I have is not so clear, but it gives the general idea  (fig. 130).

Figure 130 – Almost horizontal thrust fault (Odeceixe, Alentejo, Portugal) (escarpment hight, over 15 m).

Other items related to faulting are:

– Slickensided fault plane, defined as the polished and smoothly striated surface resulting from the friction along a fault plane.  Figure 131, is an example of a strike-slip fault, because  the striations are almost horizontal. Also, the movement of the block on the observer’s side was from right to left, since the sharper angle of the humps tend to be predominantly on the left side.

Figure 131 - Slickensided fault plane (Boula, India).

Figure 131 – Slickensided fault plane (Boula, India) (view hight approximately 3m).

– Fault breccia, defined as the assemblage of angular fragments resulting from the crushing and shattering of rocks during the movement on the fault, is shown in figure 132.

Figure 132 - Fault breccia (Tete Region, Mozambique).

Figure 132 – Fault breccia (Tete Region, Mozambique).

– Mylonite is defined as a compact rock with a streaky or banded structure produced by the extreme granulation and shearing of the rock pulverised during the thrusting. The development of very high pressure and temperature causes partial melting, giving the rock the appearance of a micro-brecciated toffee (fig. 133).

Figure 133 - fault plane filled with mylonite (Buffelsfontein Mine, Stillfontein, South Africa).

Figure 133 – Fault plane filled with mylonite (Buffelsfontein Mine, Stillfontein, South Africa).

Finally, as mentioned above, faulting takes place under brittle environments. The stress release in such fields tend to develop fault sets that are close to perpendicular to each other and they are termed Conjugate Faults. Again, evidence of such occurrences are far more easy to visualise in a global scale and become very distinct when they are sufficiently strong to control the geomorphology . Once more I make use of Google maps to show a rather impressive example, where the Douro River, in the North of Portugal, at one stage flows on an almost straight line in a SSW direction, to sharply veer to another very straight line, but now in a WNW direction (centre of figure 133B). Those lines are so straight that there can be no doubt they represent a strong set of conjugate faults.

Figure 133B - Example of geomorphological control by conjugate faults (Douro River, Portugal)

Figure 133B – Example of geomorphological control by conjugate faults (Douro River, Portugal) (after Google)

Just to finalise faulting, since I have two rather nice photos, I must mention, but only briefly, the most important ones which are the transform and their associated transcurrent faults which are intrinsically related to the tectonic plate movement. Wikipedia defines them as:

  • Transform Fault is a plate boundary strike-slip fault where the motion is predominantly horizontal. It ends against another one, or at the end of the plate.
  • Transcurrent Fault is closely related to the transform fault, but is far less persistent, terminating without a junction.

I show the transcurrent fault first (fig. 133C) because of the observable impressive corresponding detail. As it can be seen, at the near side of the photo, the sediments have an approximately N-S strike with a gentle eastwards dip. In the mid ground though, over an extent of about 200 m, the strike changes abruptly to almost E-W, with a moderate northerly dip. At the promontory on the background, the strike reverts to N-S. The area where the sediments strike E-W, is surely the fault drag zone and, in my interpretation, the northerly dip indicates a westwards movement of the northern block relative to the southern one, and this also explains the promontory. That is, there was a sinistral movement. I consider this a transcurrent fault, because its extent inland is rather limited.

Figure 133C – Costal escarpment, showing a transcurrent fault and the associated dragging of the nearby sediments (view N) (Magoito beach, Portugal)

Going now to the transform fault. Figure 133D shows the large promontory such a fault caused at Nazaré. Again we are dealing with a sinistral movement, since the promontory is on the North side. This time, the fault extends kilometres inland and, westwards, continues towards where the Central Atlantic Ridge was at the time when the ripping occurred. The other consequence of this ripping was the formation of a deep underwater canyon that permits the development of the local huge and famous surfing waves.

Figure 133D – The large promontory formed by the Nazaré transform fault (view N) (Nazaré, Portugal)

7.3 Folding

Folding takes place when the rocks are sufficiently plastic. That is, it occurs at considerable depths under compressional conditions, which means, a Converging Plates environment. The simplest folds are the syncline, where the fold forms the bottom of a trough, and the anticline, where the fold forms the crest. Figure 134 shows a simple synclinorium with a broad anticline on the left, followed by a broad syncline on the right.

Figure 134 - Simple synclinorium (Northern Cape, South Africa).

Figure 134 – Simple synclinorium (Northern Cape, South Africa).

In figure 135 we have a small anticline, well delineated because of the thinly bedded sediments on which it occurs.

Figure 135 - Small syncline (Cape Folded Belt, South Africa).

Figure 135 – Small anticline (about 3 m high) (Cape Folded Belt, South Africa).

In the case of figure 136 the anticline is quite tilted, that is, overturned.

Figure 136 - Overturned anticline (Odeceixe, Alentejo, Portugal).

Figure 136 – Overturned anticline (Odeceixe, Alentejo, Portugal).

Both these anticlines are small scale examples of the gigantic folding common in orogenic regions. The first within the Cape Folded Belt in South Africa and the second within the Lower Alentejo Flysh Group in Portugal. Much more complete, but very seldom observable, is the grand view of a regional overturned synclinorium with a dimension of kilometers, also within the Cape Folded Belt (fig. 137).

Figure 137 - Very large overturned synclinorium Cape Folded Belt, South Africa).

Figure 137 – Very large overturned synclinorium, Cape Folded Belt, South Africa).

Just to finalize folding, figures 138 and 139 show two examples of chevron folds, which are most aptly named.

Figure 138 - Chevron folding, Cabo Sardão, Alentejo, Portugal.

Figure 138 – Tight chevron folding, Cabo Sardão, Alentejo, Portugal (view about 3 m).

 

Figure 139 - Chevron folding, Barberton Mountain Land, South Africa.

Figure 139 – Chevron folding, Barberton Mountain Land, South Africa.

7.4 Metamorphism / Metasomatism

Wikipedia defines metamorphism as the change of the minerals and structure of a preexisting rock by extreme heat and pressure, without causing its complete melting into a magma. This is the part of geology I like least and as such the one of which I know even less. Within the subduction zone, solid rocks may be remobilized to the extreme of becoming magma again. In the case of only partial remobilisation, when the initial rocks originate from a continental mass, they will have an acid composition and will be metamorphosed to gneisses. Figure 140 is an example of high level remobilization giving rise to a gneiss, where the white streaks are the remaining evidence of some thin quartz rich sedimentary beds which now give an appearance of flowing.

Figure 140 - Flow folding in gneiss (Okiep, South Africa).

Figure 140 – Flow folding in gneiss (Okiep, South Africa).

In the case of figure 141, if it was not for the presence of a resistant sedimentary remnant (xenolith), it would not be possible, by the naked eye, to determine if that rock was a granite or a gneiss.

Figure 141 - Resistant remnant in gneiss (Okiep, South Africa).

Figure 141 – Resistant sedimentary remnant in gneiss (Okiep, South Africa).

Finally, the rock shown in figure 142, on first impression can very easily be considered a granite conglomerate, but in fact it is a greisen, defined by Wikipedia, as a light coloured rock containing quartz, mica and fluorine rich minerals. It results from the hydrothermal alteration of a granite, during the high gas and water rich cooling stages of emplacement. The fluids are forced into the interstitial spaces, thus giving the rock that conglomeritic appearance. Such a rock is quite common in the Castromil area of Portugal where I prospected for gold. Unfortunately the photographs from there were mislaid so I opted to use this one from Okiep in South Africa.

Figure 142 - Possible greisenised granite (Okiep, South Africa).

Figure 142 – Possible greisenised granite (Okiep, South Africa).

At Castomil I was told by a Chilean geologist that those rocks were definitely greisen and in Chile it was always associated with thrust faults in the vicinity of magmatic rocks. That is, identical to the Castromil circumstances.

7.5 Stylolites

Wikipedia defines diagenesis as “the processes that cause changes in a sediment after its deposition, but prior to its final lithification”. One of such processes gives rise to stylolites which again Wikipedia defines as “serrated surfaces from which mineral materials have been removed by pressure dissolution, in a process that decreases the total volume of rock, and the insoluble minerals remain within the stylolites making them visible”. Such  structures are very common in limestones but are generally quite difficult to recognise, as for example the one shown in figure 111, just to the left of the pen. However, the one shown in figure 142B is the largest I have ever seen and it is very well highlighted by weathering. That is why I had to show it somewhere.

Figure 142B – Very well developed stylolite structure (Tagus River front, Oeiras, Portugal).

Since diagenesis to a certain extent may be considered the first step towards metamorphism, I opted to show this picture here, rather than under sedimentation, as a post depositional process.

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6. Sedimentation

Posted on 08/02/2009 by Vitor Pacheco

In by far the majority of cases, materials that have been weathered and/or eroded, are transported elsewhere and eventually settle. Sedimentation is the processes to which these materials are subjected to until they are deposited, and transport is the main mechanisms involved. This transportation can be done either in solution or in the solid state. In the case of solutions, only water is relevant, but these sediments are not very common and will be presented under Non-Clastic Sediments (item 6.5). The other materials occur as fragments, termed clasts and, depending on their size, are transported either in suspension, or, if they are considerably larger and/or heavier, they move by saltation. That is, in a series of short intermittent hops close to the ground. Or, if they are really heavy they simply roll along the surface.

As for the transporting mechanism, the clastic sediments are subdivided into:

Terrestrial – Aeolian; Glacial; Aluvial; Delta/Estuary

Marine – Wave Dominated; Tide Dominated; Bathyal/Abyssal.

6.1 Aeolian Environment

Like with erosion, we start with the least energetic agent, air. Aeolian sediments range in the very lower volumetric size in suspension, and a marginally larger dimension in saltation. For wind to be an effective erosion/sedimentation agent, the ground must be unprotected which, under general conditions, means little or no vegetation. Hence the aeolian sedimentary environment is restricted to deserts (fig. 89) and unprotected coast lines (fig. 90).

Figure 89 - consolidated desert dunes (Namibe, Angola).

Figure 89 – Consolidated desert dunes (Namibe, Angola).

Aeolian deposits consist of dunes formed by rather fine grained sand and characteristically have large scale cross-bedding, which refers to the internal arrangement of the sedimentary layers being formed by minor beds or laminae inclined to the principal bedding planes. Deposition takes place on the downwind side and, at the base, the cross strata sets have a low angle, but upwards the dips increase to over 10º. 

Figure 90 - Consolidated coastal dune (Magoito, Portugal).

Figure 90 – Consolidated coastal dune (Magoito, Portugal).

It is easier to identify cross-bedding in consolidated rocks.  I have only seen consolidated desert dunes in the Namibe desert in Angola, but no cross stratification was apparent and, as figure 89 shows, if anything, planar stratification seems to be more likely.

On the other hand, the cross bedding is absolutely distinct at the Magoito consolidated coastal dune. In figure 91 we see a set of moderately steep cross beds at the top, with an almost flat set below, but still quite high up the present dune’s cross section. Since these flat sets tend to form at the base, we must assume that the upper set represents the base of the dune during a later migration stage.

Figure 91 – Cross bedding in a coastal dune (view approximately 4 m high) (Magoito, Portugal)

Large sand deserts are common in areas of low relief and the various types of dunes are a consequence of the local prevailing winds. I have no examples of these, but the best known ones are:

Dome-shaped dunes, consisting of low, circular, isolated mounds;

Transverse dunes, consisting of almost straight sand ridges at about right angles to the predominant winds;

Longitudinal dunes, also consisting of long sand ridges, but this time oriented along the vector resulting from two converging wind directions;

Barchan dunes, have a crescent shape with the horns extending downwind.

6.2 Glacial Environment

Sedimentation related to glaciation is limited. Its only true representative is the tillite, which is an unstratified, very poorly sorted sediment, composed mainly of fine clasts, but containing disseminated large to very large boulders often striated and faceted. Tillites predominantly cover relatively small areas, other than when they are related to extensive ice ages. I  have no representative photos, but I feel I must mention it here as a matter of completeness. The most common present day examples occur across glacial valleys, have a mound shape, and are then termed moraines, forming at the site where the glacier melted during a moderately long period.  If large enough, these moraines act as dam walls, giving rise to the well known elongated glacial lakes, of which I show a nice sized one in Slovenia (fig 92),

Figure 92 - Glacier lake (Bled, Slovenia).

Figure 92 – Glacier lake (Bled, Slovenia).

and a rather small one in Portugal (fig. 93).

Figure 93 - Lagoa Comprida, Portugal.

Figure 93 – Lagoa Comprida, Serra da Estrela, Portugal.

6.3 Alluvial Environment

6.3.1 Ephemeral Stage

6.3.1.1 Ravine Fill

The most basic of these sediments are those deposited on steeply inclined valleys caused by ravinement, (fig. 58), where the resulting sediments are a consequence of flash floods. That is, they are characterised by high energy environments, but with short periods of action. Naturally they show very poor sorting as well as roundness, and they tend to occur within relatively long confined channels with very steep sides, for which I present the following examples:

  • Figure 94 shows the present day infilling of a ravine, with a very unsorted assemblage of predominately small clasts.

Figure 94 – Present day infill of a ravine (view approximately 2 m high) (Barberton Mountain Land, South Africa).

  • Figure 95 shows a close up of the infill at the edge of a ravine cutting across a Jurassic age limestone where again, the clasts are very unsorted and very poorly rounded.

Figure 95 – Infill of a ravine cutting a limestone succession (Arrábida Mountain range, Portugal) – Close up of the edge (quarry face, cutting done by diamond wire) (hight of picture, approximately 2 m)

However, at the bottom of the ravine there is a considerable concentration of the larger and somewhat better rounded clasts (fig. 96).

Figure 96 – Close up of conglomerate infill at the base of the ravine shown in figure 95

  • Figure 97 shows another example of the infilling of the base of a ravine. Notice the similarity with the infill at the base of the ravine in the limestone of the previous figure, but for this example I use the gold bearing Ventersdorp Contact Reef (VCR), which lies stratigraphically at the top of the Witwatersrand Supper Group, in South Africa, and where siliciclasts predominate.

Figure 98 - Stream channel VCR showing the steep lateral contact (East Driefontein Gold Mine S. Africa).

Figure 97 – VCR infill at the base of a Ventersdorp age ravine (stope hight, approximately 1.5 m) (East Driefontein Gold Mine S. Africa).

6.3.1.2 Piedmont

Next there is the Piedmont, meaning a deposit at the base of a hill, and its most important sediment type is the alluvial fan which is a low, outspread, relatively conical succession of rather laminar beds comprised by poorly sorted clasts, deposited by water flash flows from narrow mountain valleys upon a flat surface. Each of these stacked sedimentary layers represent individual flush discharge periods caused by heavy rain downpours. When large clasts predominate, the assemblage is termed a debris flow and this was the principal environment during the deposition of the VCR, with figure 98 showing a classical example of two separate flow periods of large clasts, separated by a shorter flow period of coarse grained sand. Notice also how poorly rounded, packed and sorted the clasts are.

Figure 98 – Debris flow type Ventersdorp Contact Reef (East Driefontein Gold Mine S. Africa).

Often too, the composition of these clasts is very varied (fig. 98B).

Figure 98B – Example of clast composition variation within the VCR (East Driefontein Gold Mine S. Africa).

6.3.2 Mature Stage

6.3.2.1 Braided Streams

Of the different fluvial facies, the first important stage of deposition is perhaps the braided flow regime, where the master stream separates into numerous channels. The energy level is still very high and the deposition of heavy clasts predominates, forming inter-tonguing lenses or small sheets of sediments characterized by cut and fill structures and abrupt changes in particle sizes. A reasonably contemporaneous example can be seen along the Vaal River near Ulco in South Africa, where inter-tonging sand and gravel bars occur and the gravel sections are locally mined for alluvial diamonds (fig. 99).

Figure 99 - Braided river gravel beds, mined for alluvial diamonds (Orange River, Northern Cape, S. Africa).

Figure 99 – Braided river gravel bar, mined for alluvial diamonds (view approximately 4 m high) (Vaal River, Northern Cape, S. Africa).

Detailed observation of this type of environment shows quite nicely the relationship between adjoining bars of coarser and finer clasts (fig. 99B).

braided seds

Figure 99B – Close up of inter-fingering grit and sand bars within a braided river sedimentary environment (Lizandro River mouth area, Portugal)

Naturally this alluvial high energy regime is also a sorting mechanism, where the most important factor is mass rather than volume and an obvious example is shown in figure 100 where we have large well rounded quartz pebbles with an SG of 2.6, associated with much smaller, also well rounded, pebbles of pyrite (buck shot), with a significantly higher SG of 5.0. Gold, with an even higher SG of 18, consequently needing much smaller dimensions is also present. In fact, it is seldom visible by the naked eye. In this case though, if my memory does not fail me, at the bottom right hand corner of the picture, those small shiny specks are gold. Returning to figure 99, it is now more easy to understand why the search for diamonds concentrates on the gravel lenses, even though the diamond’s SG is not that high, 3.5.

Figure 100 - Channel type VCR rich in “buckshot” pyrite (East Driefontein Gold Mine, Carletonville, S. Africa).

Figure 100 – Channel type VCR rich in “buckshot” pyrite (sample approximately 20 x 10 cm) (East Driefontein Gold Mine, Carletonville, S. Africa).

Grading, defined as the gradual upward reduction of clast size within a stratigraphic succession, is caused by the diminishing flow strength of the transporting medium. Thus, it must also be included here because it happens as the river reduces speed when reaching a section of its course with a lower gradient. Once again we have a very good text book example from the VCR (fig. 101).

Figure 101 - Graded bedding in VCR (East Driefontein Gold Mine, Carletonville, S. Africa).

Figure 101 – Graded bedding in VCR (East Driefontein Gold Mine, Carletonville, S. Africa).

6.3.2.2 Meandering

The meandering stage of the river is when high levels of deposition really begin. Meandering starts developing with the reduction of the water velocity, where the heavier gravel and sand start to deposit, preferentially at the inner side of the bends, where the speed is least, thus forming point bars which, with time enhances those bends. The sand concentration at the inner side of the bends is quite distinct in figure 102.

During flood periods, the river overflows its banks, dramatically reducing the flow speed and depositing the very fine silts and muds in suspension, giving rise to rich agricultural soils over the flood plains, also quite distinctly seen.

Figure 102 - The Catumbela River meanders and its mouth into the Atlantic Ocean (Angola).

Figure 102 – The Catumbela River meanders and its mouth into the Atlantic Ocean (Angola).

Hence, these marshy areas contain predominantly silts and muds which, during the dry season will desiccate, forming very characteristic mud cracks. Figure 102B, shows a present day example, where mud cracks have formed very recently,

Figure 102B – Recent mud crack formation (Praia do Telheiro, Algarve, Portugal).

and figure 102C, shows consolidated mud cracks as well as casts.

figure 102C - Cast of consolidated mud cracks in mudstone (Transvaal, S. Africa).

figure 102C – Cast of consolidated mud cracks in mudstone (Transvaal, S. Africa).

A magnificent text book example though, is the one seen in  figure 102D,

Figure 1102D - Preserved mud cracks in limestone (Ulco. S. Africa).

Figure 102D – Preserved mud cracks in limestone (Ulco. S. Africa).

where even the actual curling at the edges of the mud crack slabs was preserved (fig. 102E). As for the slight ripples along the edges and the indentations at the centre of some of the mud plates, I assume they were caused by rain drops, just before there was a new inflow of sediments which covered and thus preserved them.

From the above, it is apparent therefore that mud cracks, seen forming in a present day soil in figure 102B, when covered by new sediments and preserved, are another very useful indicator of a palaeosol, to add to those mentioned under weathering (item 3.2).

figure 111B - Close view of one of the mud crack plates (Ulco. S. Africa)

figure 102E – Close view of one of the mud crack plates (Ulco. S. Africa)

6.3.2.3 Estuary/Delta

Within this kind of fertile flood plane environment, we should expect abundant fauna and flora, and the richest portion must surely be the estuary or delta, where the river water rich in all sorts of organic matter, contacts the ocean’s salt water. Evidence of the fertility of this type of environment can be seen where there is a significant tidal variation, because the mud-flats get exposed (fig. 102F).

Figure 102F - Present day mud flats rich in burrowing animals (Sado River Estuary, Portugal).

Figure 102F – Present day mud flats rich in burrowing animals (Sado River Estuary, Portugal).

When these present day soils are preserved, forming future palaeosols, the signs of the various types of burrowing animals gives very distinct characteristics to those horizons, termed bioturbated (fig. 102G).

Figure Preserved burrowing casts in a clayey limestone (bioturbated) (Carcavelos, Portugal).

Figure 102G – Preserved burrowing casts in a clayey limestone (bioturbated) (Praia do Guincho, Cascais, Portugal).

What is more, this type of depositional environment has been present for a very long time indeed, as demonstrated by the Ordovician bioturbated horizon shown in figure 102H, which also shows the casts of two large tree trunks.

Figure 102H – A magnificent example of bioturbation in an Ordovician mudstone (Penha Garcia, Portugal) (hight of escarpment approx. 2 m)

6.4 Marine Environment

6.4.1 Coastal

Under sedimentation, we are interested in prograding coast lines, that is, where sedimentation is taking place, with a consequent shore accretion. Figure 73B shows very nicely a prograding situation with a well defined on-lap phase. As for our example of the Catumbela River in Angola, it is impressive  that even though the river is quite large and carries vast quantities of water, its mouth is actually parallel to the coast, pointing northwards (fig. 103).

Figure 104 - The mouth of the Catumbela River (Angola).

Figure 103 – The mouth of the Catumbela River (Angola).

This is caused by the northwards flowing Benguela sea current which is even stronger. Hence, as the river water impacts the ocean current, its speed is dramatically reduced and the bed load particles are deposited,  forming a sand barrier termed off shore bar, which progressively grows northwards. With time, this bar widened and the shore line pro-graded giving rise to sand banks, lagoons and marshes (fig. 104).

Figure 105 - The coast line just N of the river mouth with a pro-grading marshy shore line.

Figure 104 – The coast line just N of the river mouth with a pro-grading marshy shore line (Catumbela River, Angola).

Due to an initial coastal reentry, the pro-graded area became quite wide and that is where the town of Lobito  developed (fig. 105),

figure 106 - Wide pro-grade coast line with the Lobito harbor.

Figure 105 – Wide prograding coast line with the Lobito harbor (Angola).

with the harbor having been formed by the natural extension of the off shore bar created by the Catumbela river sediments (fig. 106).

figure 106 - The sand barrier protecting the Lobito harbour (Angola).

figure 106 – The sand barrier protecting the Lobito harbour (Angola).

A very conspicuous feature of these arenaceous coastal environments is the development of ripple marks. The example shown in figure 106B is quite striking because the upper bedding plane has current ripples, that is, asymmetric, indicating water flow, suggestive of an environment still under the influence of a river or, with sea currents. On the other hand, the lower one only 5 cm above, has symmetric ripples, indicative of still waters where the ripples are developed by waves caused by the wind. Hence, there is quite a marked facies change, possibly within a relatively short geological time. Significant too is the orientation difference between the two sets of ripples.

Figure 109 - Ripple marks in quartzite (Ferro Quarry, Pretoria, S. Africa).

Figure 106B – Ripple marks in quartzite (photo width, approximately 1.5 m) (Ferro Quarry, Pretoria, S. Africa).

It is worth noting too that a section across a sandstone formed by the progressive development of ripple marks shows crossbedding. See figure 107 for an ordinary example,

Figure 107 – Crossbedding in sandstone (photo width approximately 15cm) (Azenhas do Mar, Portugal)

and figure 107B for a rather unique case where the cross-bedding is enhanced by pyrite.

Figure 107B – Cross-bedding enhanced by pyrite in Main Reef internal waste (East Driefontein Mine, Carletonville, S. Africa).

Also, the visual characteristics of this cross-bedding looks rather similar to those developing in (fig. 91),as well as alluvial dunes (fig. 73B), but at a much smaller scale. This difference is important because the physical forces involved have completely different dynamic characteristics and thus are not comparable, even though they have similar appearances.

Finally, as already mentioned, these beach environments can be very rich in animal life and when there is a limited supply of inorganic clasts, assemblages composed entirely by the accumulation of mollusk shells may occur, giving rise to bioclastic limestone beds (fig. 108).

Figure 108 Bioclastic sediment layer (Carcavelos Beach, Portugal).

6.4.2 Shallow Marine

Going away from the coast but still within the continental shelf, in areas with a tropical climate, there is the formation of fossiliferous limestones consisting primarily of coral skeletons originating from the barrier reefs (fig. 109)

Figure 113C — Coral reef limestone (Cabo Raso area, Cascais, Portugal)

Figure 109 — Coral reef limestone (Cabo Raso area, Cascais, Portugal)

6.4.3 Continental Slope

Turbidites are defined by Wikipedia as being formed by turbidity currents that develop at the edge of the continental slope, and consist of a gravity driven turbid mixture of sediments temporarily suspended in water. I know practically nothing about them and have no related photographs, but they must be mentioned, since they are the only true clastic sediments occurring in this environment and form a very significant portion of the sedimentary group of rocks.

6.4.4 Pelagic

Further away from the coast, outside the continental shelf, very little deposition takes place on the sea floor, other than the very slow accumulation of dead plankton and the skeletons of deep water dwellers. Some of the plankton inhabitants have silicious shells and others as well as some algae, produce minute amounts of calcium carbonate. The accumulation of these products on the sea floor often consolidate forming a succession of alternating layers of biogenic limestone and chert. Such a succession is the absolute proof of deposition in a pelagic environment. A very good example of these limestones is the thick succession of the Transvaal Dolomite in South Africa, that contains very abundant inter-layered chert bands (fig. 110). For diamond drilling purposes this is absolute agony, because the very sharp alternation between the relatively soft limestone and the thin but very hard chert bands, destroys the drilling crowns in no time at all.

Figure 115 - Column of well marked alternating dolomite and chert (positive relief) (Sabie River. S. Africa)

Figure 110 – Column of well marked alternating dolomite and chert (positive relief) (view approximately 10 m high)  (Sabie River. S. Africa)

6.5 Non-Clastic Sediments

These are sedimentary rocks formed by the precipitation of chemical compounds carried in solution by the water. Naturally the components of these rocks will be those that are most soluble, like salt, gypsum, chert and calcite.

6.5.1 Chemical Precipitates

This is the general term for rocks originating from the precipitation of the substances under saturated solutions. Of these, the most abundant by far, are limestones which are constituted almost entirely by calcite. They are generally fine grained and range in colour from almost white, when very pure, to varying shades of gray depending on the quantity of organic matter present.

Often these limestones contain levels rich in chert nodules that originate from a diagenetic silicification within the rock in which they occur (fig. 111). In other words, these chert nodules represent a post depositional modification with no significance relative to the identification of the depositional environment, as opposed to the biogenetic assemblages mentioned above.

Figure 114 - Close up of silica nodules within a limestone evaporite (Serra do Sicó, Portugal).

Figure 111 – Close up of silica nodules within a chemically precipitated limestone (Serra do Sicó, Portugal).

Travertine is another striking chemical precipitate, defined by the glossary of the American Geological Institute as a dense, finely crystalline concretionary  limestone formed by the rapid chemical precipitation of calcium carbonate from saturated solutions  on to the surfaces over which it flows. A contemporaneous example occurs at Ulco, South Africa (fig. 112). It has a fan shape and is being built by a stream falling over a Northern Cape dolomite ridge. The water flowing over the ridge is saturated with the calcite it dissolved, and than precipitates covering every surface along which the water runs, thus causing the growth of the fan.

Figure 118 - General view of the fan of precipitated calcite from a saturated stream.

Figure 112 – General view of the fan of precipitated calcite from a saturated stream.

A closer view of the continuously growing ridge caused by the accumulation of calcite along the stream is shown in figure 113.

figure 119 - Close up of the mound created by the stream (Ulco, South Africa)

Figure 113 – Close up of the mound created by the stream (Ulco, South Africa)

In fact, one can see growing grass stems being coated by calcite (fig. 114).

Figure 120 - Calcite coating around a growing grass stem (Ulco, South Africa).

Figure 114 – Calcite coating around a growing grass stem (Ulco, South Africa).

Diamond drill core cutting through the fan sequence proves that its development was caused by the continuous coating of whatever materials over which the saturated waters flowed (fig. 115).

Figure 121 - Diamond drill core cutting through the Gorokop calcite fan (Ulco, South Africa).

Figure 115 – Diamond drill core cutting through the Gorokop travertine fan (Ulco, South Africa).

6.5.2 Evaporites

Also defined by the American Geological Institute Glossary, an evaporite is composed primarily by a solution that became concentrated due to the evaporation of its solvent, for example the present day coastal salt pans, of which I show here a  rather unique example because it occurs in the volcanic Sal Island of the Cabo Verde Archipelago, within the crater of an extinct volcano. The base of the crater is below sea level and, due to the lavas being very fractured, the sea water seeps into the crater, where the water evaporates because of the intense heat caused by the local desert climate, within a totally enclosed area, just like in a cooking pot (fig. 115B).

Figure 115B – Natural salt evaporating pan at the base of a volcanic crater (Sal Island, Cabo Verde)

In nature it may happen that a sedimentary layer of salt which has a relatively low SG, is covered by another layer of material with a considerably higher SG. If later, during the post depositional compaction stage that mass of salt is subjected to unstable adjustments, that may cause an upwards high velocity movement of the salt through the higher density layer covering it and that, when in a large scale, forms a diaper.

Gypsum evaporites are also quite frequent, but with considerably smaller dimensions, and I saw a relatively large deposit in Angola. Unfortunately I did not have a camera, but I collected a specimen (fig. 116).

Figure 116 – Lovely specimen of fibrous gypsum (Angola)

Also, gypsum as well as calcite may form in desert pools, or from underground water brought to the surface by capillary action. It is under these conditions that the gypsum crystals known as desert roses form (fig. 117).

Figure 123 - desert rose crystals (Namibia).

Figure 117 – Desert rose crystals (Namibia).

Posted in Geology, Sedimentary Rocks | 2 Comments

5. Unconformity

Posted on 27/01/2009 by Vitor Pacheco

A sediment is a mass of material that originates from the weathering and or erosion of the earth’s surface substances which, in the majority of cases, have been transported and deposited elsewhere. Naturally, the transporting agents are the same as those that cause the erosion, that is, wind, ice and water. These different transporting agents, acting within various environments, will give the sediments distinctive characteristics, termed facies. As such, the interpretation of a sedimentary succession consists on the identification of their respective depositional environments and the history of the various sedimentation stages. It is therefore useful to group the different parts of a stratigraphic column according to the different transporting agents as well as the environments under which they were deposited. The larger the differences between the depositional conditions across a sedimentary pile, the more likely it will be that a non-depositional cycle separates them. Hence these breaks are ideal markers for stratigraphic groupings.

These surfaces are termed unconformities and the American Geological Institute Glossary defines them as the break in the geological record where a rock unit is overlain by another that is not next in the stratigraphic sequence. That is, the surface representing that break denotes an interruption of the cycle of sedimentation caused by a period of no sedimentation and, or weathering as well as erosion.

For in depth stratigraphic studies, unconformities are rated according to the time span and areal extent which they cover. An inter-regional unconformity tends to cover wide areas, separating distinctly different depositional episodes, occurring during significantly different time periods. The other unconformities which only cover limited areas mainly at the edge of sedimentary basins are termed intra-formational unconformities. For the purpose of this presentation, unconformities will simply be subdivide into:

5.1 Disconformity (Diastem)

When the surface of the disturbance  separates essentially parallel strata, we are dealing with a disconformity. That is, we are dealing with a short geological time break, since there was not sufficient time for the older rocks to be tectonically disturbed. I think the terms disconformity and diastem are equivalent because there is little difference between having had a short period of erosion (disconformity) or a non depositional period (diastem). This type of sedimentation break is well exemplified in figure 81 where not only is the surface separating the two horizons sharp, but also the characteristics of the sediments on either side are considerably different, with the lower horizon showing a high level of sorting and the one above, the opposite. There is therefore a definite change in sedimentation characteristics across that plane, indicative of a stage of erosion, but the time break was not long enough to allow additional alterations.

Figure 81 – Very distinct contact, disconformity, between a fine grained well sorted white sandstone below, and a poorly sorted reddish arenaceous assemblage above (hight of vertical section about 3 m) Foz do Falcão, Portugal)

The other example shown in figure 31B, although referring to a volcanic sequence, is just as good since the principals are exactly the same. In this case the lowermost lava outflow has weathered to a paleosol. That is, the lowermost lava band was exposed at the surface during a long enough period to be weathered completely into a soil. The layer above consists of an accumulation of partially weathered lose lava fragments, either transported from nearby, or eroded “in situ”. Covering it all there is a younger fresh lava outpour. Hence, the boulder horizon is the disconformity plane distinctly indicating a period of weathering and erosion.

5.2 Angular Unconformity

When the unconformity surface separates an older tilted or folded strata from a younger undisturbed, or less disturbed strata, we have an angular unconformity (fig. 82). Note how uneven the erosion surface (unconformity) plane is, in this example. The arkose above the unconformity belongs to the recent Tagus and Sado estuary sedimentary column and the metamorphosed contorted shales below belong to the Lower Alentejo Flysch Group, of Devonian age. Thus, although irregular, this plane demarcates a very long cycle of erosion and, these irregularities can be much more deeply incised, when there is intense ravinement.

Figure 82 – Irregular unconformity plane separating highly contorted flysch sediments of Devonian age below, from a horizontally laying arkosic sandstone of the Tagus-Sado estuary (view approximately 3 m high) (Vale do Gaio, Portugal).

Now, going away from the mountainous terrane towards areas with a more even topography, better defined, smoother plane surfaces, start appearing, cutting at a distinct angle across the older sedimentary succession. Figure 83 is a very good example of an inter-regional unconformity plane. It separates the Witwatersrand sediments from the thick pile of Ventresdorp Lavas. Notice also, at the right hand side of the photo, how the conglomerate becomes thicker as it steps from a hard quartzite onto a soft shale surface. In other words, it is the same principle as in Peneplanation (item 4.4.2.2), but at a much smaller scale.

Figure 83 - Typical case of an Angular unconformity (Ventersdorp Contact Reef, East Driefontein Gold Mine, Carletonville, S. Africa).

Figure 83 – Typical case of an angular unconformity (Ventersdorp Contact Reef, East Driefontein Gold Mine, Carletonville, S. Africa).

Naturally there are all sorts of variations on the theme. In the example shown in figure 84, due to the limited range of the photograph, there is no apparent angular difference  between the sediments bellow and above the conglomerate, locally known as the Carbon Leader, but there is no doubt that an unconformity is present, because additionally to the angular difference between the strikes and dips of the sediments on either side of the unconformity, we have a marked difference in the characteristics of the quartzites. Also, there are occasional scattered, well rounded pebbles and, most important, the Carbon Leader, although very seldom thick, is one of the richest gold bearing horizons within the Witwatersrand Supper Group. That is, there was sufficient erosional/satatic time for the gold to concentrate.

Figure 84 - very even unconformity plane with disseminated scattered pebbles - notice distinct difference between the quarzites on either side (Carbon Leader. East Driefontein Mine, Carletonville S. Africa).

Figure 84 – Very even unconformity plane with disseminated scattered pebbles – notice distinct difference between the quartzites on either side (the conglomerate thickness is 3 cm) (Carbon Leader. East Driefontein Mine, Carletonville S. Africa).

Finally, to my knowledge unique in the Carbon Leader, locally there are areas where the unconformity is defined by a single algal mat, seldom exceeding 3 mm in thickness which, in the majority of cases, contains abundant interstitial gold (fig. 85). Once more, there must have been a sufficiently long erosional and or non-sedimentation period for algae to grow and capture gold suspended in the sea water.

Figure 85 - Algal mat with abundant interstitial gold (Carbon Leader. East Driefontein Mine, Carletonville S. Africa).

Figure 85 – Algal mat with abundant interstitial gold (Carbon Leader. East Driefontein Mine, Carletonville S. Africa).

Returning to figure 82, notice the layer of weathered flysch defining the bottom contact of the unconformity. In a present day surface occurrence, it is very frequent to encounter in situ weathered rock, that is soil, overlain by another layer of soil that was transported. Since these two adjoining soils may have very different textures and structural strengths, these must be clearly determined by the civil engineers, when evaluating the strength of the foundations needed for large constructions. Fortunately, to assist in the identification of these distinctly different elements, there is often an erosional period sufficiently long to allow the accumulation of a layer of larger clasts while the lighter material was washed away (fig. 86). In other words, we have the same principle as the desert pebble screen (fig. 49), but this time with water. This coarse clast layer separating in situ, from transported soil is termed  the pebble marker.

Figure 86 - Poorly sorted beach horizon conglomerate separating weathered rock (in situ soil), below, from transported soil above - pebble marker (Simonstown, South Africa).

Figure 86 – Poorly sorted beach horizon conglomerate separating weathered rock (in situ soil), below, from transported soil above – pebble marker (Simonstown, South Africa).

Another unique example of the weathering of outcropping rocks during the weathering/erosion cycle, is shown in figure 87, where an erosional period sufficiently long to allow the formation of a conglomerate, was “sealed” by an outpour of submarine lava with its pillows, which fell on top of an unconsolidated conglomerate, squeezing it between the pillows, not only the conglomerate pebbles, but also the underlying soil, soft due to the weathering.

Figure 87 - Lava pillows squashing Ventersdorp Contact Reef (VCR) and underlying soil (East Driefontein Gold Mine S. Africa).

Figure 87 – Lava pillows squashing Ventersdorp Contact Reef (VCR) and the underlying paleosol (East Driefontein Gold Mine S. Africa).

Present day examples of angular unconformities are the occurrence of shore line gravel beds (fig. 88), indicating a marine transgressive lag. The larger the boulders, the higher the energy of the erosional agent. In Southern Angola I saw some coastal conglomerates with no noticeable matrix and with boulders much larger than the ones shown in figure 88, but I did not have the opportunity of photographing them. Perhaps these southern Angolan coastal conglomerates are equivalent to those of the Cape Peninsula (fig. 86) and I think they are related to the isostatic uplift of the Southern African continental plate. The clast dimensions are identical, but the Angolan conglomerates are much more robust.

Figure 88 – Pebble beach (Brighton, England).

5.3 Non Conformity

A surface separating a strongly eroded plutonic body, or a massive metamorphic assemblage below, from a sedimentary body above, is termed a non conformity. I have seen very few of these and rarely is there one which will fit in a photograph. Related to a contact with a plutonic body, all I have is this one from Angola (fig. 88B). It is not that clear but, the time gap it represents is very significant, since it separates a very thick column of Karroo age sandstone, Triassic, covering a basement granite of Precambrian age and the wavy contact is reasonably noticeable.

Figure 88B - Non-conformity between Karroo age sandstone and basement granite (Leba Escarpment, Angola)

Figure 88B – Non-conformity between Karroo age sandstone and basement granite (escarpment hight, over 100 m) (Leba Escarpment, Angola)

As for the cover of a massive metamorphic body, I have a beautiful example indeed. The Praia do Telheiro non conformity, which is really magnificent because, according to https://geossitios.progeo.pt, the thin horizontal parting seen in approximately the middle of figure 88C, has highly metamorphosed Upper Carboniferous turbidites below, and above, it has an Upper Triassic limestone band covered by an iron rich clayey sandstone.

Figure 88C – The Praia do Telheiro non conformity separating Upper Triassic sandstones above, from highly metamorphosed Upper Carboniferous turbidites below (Algarve, Portugal).

 

 

Posted in Geology, Unconformity | 2 Comments

4. Erosion

Posted on 22/01/2009 by Vitor Pacheco

According to the American Geological Institute Glossary, erosion is the natural process whereby the materials of the earth’s crust are loosened or worn away and simultaneously removed to another place. These natural processes, in an increasing order of power, are gravity, wind, ice and water.

4.1 Gravity

Gravity contributes to any natural removal mechanism. However, there are cases where only gravity is relevant but, as shown in the following examples, the effect of water as a lubricant is very significant:

  • The simplest case is the tumbling of rock debris down a steep mountain slope. Clearly this rock debris had to be fragmented and dislodged first, and that may be done by water seeping through the rock cracks, turning into ice and expanding, thus causing the widening of the cracks. Another important agent is plant roots. As they thicken, they widen the joints along which they grow. Eventually these dislodged fragments may suddenly be displaced by, for example a rain storm, and violently fall down the slope forming a rock fall deposit (fig. 46).

Figure 46 - Rock debris accumulation down slope (Bled, Slovenia).

Figure 46 – Mountain side rock scree accumulation (Bled, Slovenia).

  • Or, where there is a very limited vegetation cover, the dislodgment of large solid rock masses (fig. 46B) may occur due to water seeping through existing cracks thus lubricating bedding planes as well as fractures parallel to the topography.

Figure 46B – Distinct scars caused by two large rock falls (Ponta to Calhau, S. Vincent Island, Cabo Verde).

  • Further, in regions with good rain, the lubricating effect of the water may cause the down hill flow of the upper fraction of weathered rock/soil, know as soil creep. This can be seen in figure 46C, where the upper sector of the moderately well bedded weathered shales have drooped slightly down the steep surface slope.

Figure 46C – Soil creep in graywakes causing the drooping of the shale section nearest to the surface (Vide, Portugal)

  • Further still, after torrential rains, soils become super saturated with water, and a large segment may suddenly be released, causing a land slide (fig 46D). This is naturally aggravated when the ground is partially or totally denuded due to land misuse, in the present case poor farming methods.

Figure 47 - Land slide on farm land caused by the monsoons (Orissa, India).

Figure 46D – Land slide on farm land caused by the monsoons (Orissa, India).

Another important factor is under cutting. Take for example figure 47, showing at the crest of the ridge, a ledge of erosion resistant limestone underlain by a clay rich, very easily erodible, poorly consolidated sandstone.

Figure 47 – Evidence of a recent rock fall (Arrábida, Portugal)

As the the ledge becomes progressively more undercut, gravity will initially cause the development of weakness cracks (fig. 47B) and eventually the collapse of large blocks, with potentially rather serious consequences.

Figure 47B – Ground crack development caused by wave undercutting (Praia das Avencas, Parede, Portugal).

4.2 Wind

The places where the erosional effect of the wind is most noticeable is where the ground has poor or no protective vegetation cover, that is, in arid regions. Figure 48 is not mine. I got it from the internet, but it is so spectacular that I felt it must be included. It is impressive to see how the Sahara dust is entirely covering the Canaries Archipelago, Madeira Island, as well as a large sector of the Atlantic Ocean.

Figure 48 – Dust storm from the Sahara

Often, on the Eastern side of Tenerife, there is a distinct haze in the air and I wonder if it is not due to the desert dust ( fig. 48B).

Figure 48B – Possible Sahara dust over the Canary Islands).

Interestingly, I had the opportunity of observing the result of such a dust cloud in Carcavelos, Portugal, on the 16th of March 2022 (fig. 48C). As can be seen, it is quite spectacular, giving rise to a pinkish dull light which lasted for 2 days, after which we had a short drizzle and, to our surprise, our car wind screen was covered with tiny mud blobs, which obviously were the consequence of the dust coagulating in small droplets of the saturated water vapour in the atmosphere.

Figure 48C – The effect of a Sahara dust cloud over Carcavelos, Portugal.

Not as spectacular but still most impressive is how the wind causes the dunes to move, and the speed at which it happens. Figure 49 shows a section of a boardwalk completely covered by a dune. This is occurring in an area where recovery is being desperately attempted. I do not know how long it took for the sand to move over the boardwalk, but I imagine it could have happened in 2 years or perhaps less.

Figure 49 – Dune movement causing the sand to cover  a hiker’s path (Guincho coast, Cascais, Portugal).

Naturally wind is also a magnificent sorting mechanism since the stronger it is, the larger the grains it will be able to carry. As a consequence, one of its effects is the formation of what is known as a desert pavement (fig. 49B). That is, a surface covered by a shield of clasts too large to be transported by the wind.

Figure 49 - Partial desert pebble screen (Namibe Desert, Angola).

Figure 49B – Partial desert pebble screen (Namibe Desert, Angola).

Due to the blasting of sand in movement, the shield of larger clasts, or the exposed rock outcrop, becomes highly striated, as also shown in figure 49C.

Figure 49C – Striations caused by sand blasting (Guincho, Cascais coast, Portugal).

This sand blasting effect is noticeable not only on desert floors, but also on outcrops of all types. For example, in the Golden Gate region of South Africa, where there is a sandstone formation termed, for obvious reasons, the Cave Sandstone (fig. 50).

Figure 50 - The Cave Sandstone, with abundant aeolian caves (Brandwag, South Africa).

Figure 50 – The Cave Sandstone, with abundant aeolian caves (Brandwag, South Africa).

Not entirely coincidental, this sandstone has an aeolian origin, meaning that it is rather fine grained and poorly cemented, thus easily erodible. At the mountains where these photos were taken, the vegetation cover is poor and the wind may be very strong, giving rise to numerous caves caused by the blasting of the sand carried by the wind. A closer view of these caves shows how different they are from those formed by weathering. These wind caves have an elongated half pipe shape created by the tunneling effect of the wind flowing through the valleys (fig. 51).

Figure 51 – Close view of one of the wind caves (Brandwag, South Africa).

The actual detail of the wind blasting has an interesting honey comb appearance (fig. 52).

Figure 52 - Detail of the holes caused by the sand blasting (Brandwag, South Africa).

Figure 52 – Detail of the holes caused by the sand blasting (Brandwag, South Africa).

This sand blasting may occur in any exposed area and the next example, from one of the Cape Town beaches, is very impressive too (fig. 53).

Figure 53 - Sand blasted granite outcrop(Cape Peninsula, South Africa).

Figure 53 – Sand blasted granite outcrop (Cape Peninsula, South Africa).

And how deep the pitting is, even with a tough fresh granite (fig. 54).

Figure 54 - close up of sand blasted granite outcrop (Cape Peninsula, South Africa).

Figure 54 – Close up of sand blasted granite outcrop (Cape Peninsula, South Africa).

Interesting to observe too, is how different the pitting of the granite is from the striations of the limestone, but that is simply because at the Cape Town beach the blasting was done perpendicular to the granite, and at Guincho, the basting angle is quite oblique.

4. 3 Ice

The first action of ice erosion, as mentioned above (item 4.1), is the enlargement of rock fractures caused by the volume increase as the water contained within freezes (frost shattering). Although very effective, this type of rock breaking is rather localized. Far more spectacular are the glassiers. As they flow, the clasts being transported scrape against the valley wall, shaping a typical “U” section (fig. 54B) as well as causing very characteristic striations and faceting on the rock outcrops as well as the clasts carried within the ice.

Figure 54A - Typical U shaped glacial valley (Manteigas, Portugal)

Figure 54B – Typical U shaped glacial valley (Manteigas, Portugal)

4.4 Water

4.4.1 Initial Stage

By initial stage I mean the effect of falling rain. Before the impact of climate change started being felt, in Europe as well as other temperate climate areas, rain tended to be very gentle and hence ineffective as an erosional agent. In fact, it used to impress me that in a country with long and very dry summers like Portugal, the farmers do not bother to do contour plowing, even when their fields are on steeply inclined ground. Worst still, they all plow up and down the hill, because it is more convenient I suppose. In Africa on the other hand, where rain is predominantly torrential, contour plowing was the absolute norm to prevent “bad lands”. Figure 55 is an example, but in this case the vegetation denudation was due to overgrazing. As it is apparent, the area being eroded has practically no vegetation cover. The serious consequence, is that it is almost impossible to rehabilitate bad lands.

Figure 55 - Initial development of bad lands (Barberton Mountain Land, South Africa).

Figure 55 – Initial development of bad lands (Barberton Mountain Land, South Africa).

So, for bad lands to develop, the ground must be relatively barren and the rain must be of the torrential type. It is not surprising therefore that the best examples are  in arid regions, where although seldom, when it rains, it pours and the surface is practically barren of protective vegetation. Good examples can be seen on the Eastern side of Tenerife, that is, the arid side. Figure 56 shows how the bad lands effect spreads once the protective upper hard lava flow cover has been cut through and the poorly consolidated underlying pyroclastic horizon is exposed.

Figure 56 - Bad land formation in arid country (Tenerife Island).

Figure 56 – Bad land formation in arid country (Tenerife Island).

Finally, to show how destructive bad lands can become, see the huge area ruined  in Angola as a consequence of poor farming procedures and how deep the destruction is (fig. 57).

Figure 57 - Bad lands locally termed “moonscape” Overall view (Vicinity of Lobito, Angola).

Figure 57 – Bad lands locally termed “moonscape” – overall view (Just N of the Cuanza River mouth, Angola).

The situation there is by far the worst I have seen, and it will become even worst, because that area has a tropical climate where torrential rain occurs all the time. That is, there will be a very rapid increase of the erosion rate, forming deep gullies in a process known by the very explicit term of ravinement, very clearly shown in figure 58.

Figure 58 - Bad lands locally termed “moonscape” - close view (Vicinity of Lobito, Angola).

Figure 58 – Bad lands locally termed “moonscape” – close view showing deep ravinement (Just N of the Cuanza River mouth, Angola).

4.4.2 Rivers

4.4.2.1 Erosional Action

Continuing with arid regions, torrential rains cause flash floods and river beds that for most of the year are dry (fig. 59), suddenly carry enormous volumes of water with a huge content of suspended materials of all sizes, thus capable of destroying whatever is in its path.  I did see this type of violent flash flood only once and I was so amazed that by the time I reacted, there was nothing to photograph.

Figure 59 - Typical temporary river bed in a semiarid region (Bentiaba, Angola).

Figure 59 – Typical ephemeral river bed in a semiarid region (Bentiaba, Angola).

However, the effects over an extended period can easily be photographed, since they tend to form quite impressive canyons, especially in areas where the sediments are poorly consolidated (fig. 60).

Figure 60 -Desert canyon (Namibe Desert, Angola).

Figure 60 – Desert canyon (Namibe Desert, Angola).

4.4.2.2 Peneplanation

Overall, rivers are rather well behaved and their erosional effect tends to form a very distinct V Section (fig. 60B), quite different from that of the glacier (fig. 54C).

Figure 60B – V shaped valley section typical of erosion by a river (Vide, Serra da Estrela, Portugal)

For this presentation I will only refer to the peneplanation effect caused by river erosion, and the example I’ll use is Southern Africa.  Due to isostatic adjustment, this land mass has become a plateau with an altitude generally above 1000 m and has a very sharp drop from the old Southern Africa plateau on to what was originally the continental shelf and now forms the coastal planes. A magnificent portion of this plateu is the Leba area in Southern Angola, where we can look down an almost vertical escarpment about 800m deep (fig. 61),

Tundavala escarpment, Angola

Figure 61 – View from the highveld to the lowveld through the Tundavala gorge (Leba, Angola).

and where impressive civil engineering was required to circumvent the problems of constructing a road down such a steep escarpment (fig. 62).

Figure 62 The road down the escarpment (Serra da Leba, Angola).

Figure 62 The road down the escarpment (Serra da Leba, Angola).

Rivers running along the plateau form spectacular falls over such escarpments. The Victoria Falls on the Zambezi River, is the greatest example (fig. 63), and shows the immense amount of energy contained in a river of those dimensions.

Figure 63 - Victoria Falls (Zambezi River, Zimbabwe)

Figure 63 – Victoria Falls (Zambezi River, Zimbabwe)

It is not surprising therefore that they are capable of cutting back into the escarpment and entrench themselves. Also, the present plateau was originally a flat coastal plane with typical meandering rivers. Thus the entrenching will maintain that meandering, making it quite spectacular (fig. 64).

Figure 64 – Incised river meandering (Umzimkulo River, South Africa).

The actual act of erosion is done by the saltating, whirling pebbles carried by the rivers which, on impact against the river bed rocks will grind them away. The most spectacular effect of this erosion, is the formation of potholes created at every ledge over which they fall (fig. 65),

Figure 41 Formation of potholes in Transvaal Dolomites - view of the upper section (Bleyder River, S. Africa).

Figure 65 Formation of potholes in Transvaal Dolomites – The upper section (view approximately 3 m high) (Bleyder River, S. Africa).

and figure 66 shows the actual grinding pebbles at the bottom of one of the potholes. Notice how well rounded these pebbles have become.

Figure 66 - See abundant well tumbled clasts at the base of one of the potholes (Bleyder River, S. Africa).

Figure 66 – Abundant well tumbled clasts at the base of one of the potholes (view approximately 3 m high) (Bleyder River, S. Africa).

Naturally this pothole effect does not occur only in soft rocks. It is just as common in granites. Figure 67 shows potholes in granite at a gorge cut by the Zambezi River, downstream from the Victoria Falls.

Figure 67 - Potholes in granite in the Zambezi River (Cahora Bassa Gorge Mozambique).

Figure 67 – Potholes in granite in the Zambezi River (rock face approximately 2 m high) (Cahora Bassa Gorge Mozambique).

The consequence of these entrenchments are spectacular canyons, with figure 68 showing a panoramic view of the Bleyder river canyon seen from the high veld,

Figure 69 - The Blyder River Canyon seen from the highveld (South Africa).

Figure 68 – The Blyder River Canyon seen from the highveld (South Africa).

and figure 69 giving an idea of its depth.

Figure 70 - Blyder River, view down the canyon (South Africa).

Figure 69 – Blyder River, view down the canyon (South Africa).

With time, the rivers will not only cut back into the plateau but also sideways, with the assistance of their tributaries as well as the continuous collapse of their steep margins. From canyons, erosion will progress to an abundance of semi isolated peaks (fig. 70),

Figure 71 - The Three Rondavels (Blyder River area, South Africa)

Figure 70 – The Three Rondavels (Blyder River area, South Africa).

followed by a period of very isolated hillocks (fig. 71), and on to the final peneplane stage.

Figure 72 - Isolated hillocks (Golden Gate, South Africa).

Figure 71 – Isolated hillocks (Golden Gate, South Africa).

What is also apparent in figure 70 is that the preservation of the hillocks is due to the protection of its upper portion against weather/erosion by a layer of a more resistant bed. In the present case, the protection agent is hardened quartzites/sandstones.

4.4.3 Oceans

The wave action against the coast is another very powerful erosional agent and the undercutting mechanism, already mentioned under gravity, is one of the most important processes (fig, 72). In fact, the impressive crack shown in figure Figure 47B is a magnificent illustration, since that crack is located just above the present picture, where the consequence of the waves incessant undercutting is obvious.

Figure 72 – Example of wave erosion undercutting an  easily erodible impure limestone bed (view approximately 4 m high) (Praia das Avencas, Parede, Portugal)

The Portuguese coast is in a transgressive stage, that is, the sea is rising. A very impressive example is the Roman galleries located under Rua da Prata in the section of Lisbon close to the Tagus river margin (fig. 72B).

Figure 72B – Entrance to the Roman galleries below the street (Lisbon, Portugal)

Presently the water table at that site is less than 2 metres below the street level and these galleries are completely flooded. To prevent ground subsidence and the consequent damage to the buildings above, only once a year is the water pumped out for a general structural inspection, and it is then open for a public visit.

These galleries, built during about the first century AC, are thought to be the foundations of some important building. Its, just over 2 meters high arches, are now about 2 meters below the street (fig. 72C), but were above ground at the time of construction. In other words, the sea level is today some 8 metres higher.

Naturally this sea level change was not at all regular and continuous since then. There must have been quite a few ups and downs as exemplified by at least one minor ice age during the last millennium.

Figure 72C – View from one of the galleries to the entrance steps (Lisbon, Portugal)

A much more recent example is provided by the data collected by the government’s geographical survey by means of an instrument installed at the Cascais sea side in 1882, and still in full operation (fig. 72D).

Figure 72D – The tide gauge building (Cascais, Portugal)

It has produced a continuous graph of the tidal variations as well as mean sea level elevations (fig. 72E), indicating that the sea elevation has risen 15 cm in the last 100 years. Again, I have no doubt that a detailed observation of that data will show marked irregularities with, I’m sure, a significant increase in the near past.

Figure 72E – The mechanism that produces the continuous graph of the tidal and sea level variation (Cascais, Portugal)

Concluding, sea transgression means coastal erosional. It is therefore difficult to understand that even with all these very strong signs, constructions often extensive, continue being done so close to the edge of the ocean, with no apparent regional structural study. Dramatic examples can be seen along the Estoril coast where some expensive apartments were built right against the receding coast  as can be seen in figure 73, where it appears that undercutting is already taking place at the base of the white building.

Figure 77 - Building foundation being undercut by sea erosion (Estoril Coast, Portugal)

Figure 73 – Building foundation being undercut by sea erosion (Estoril Coast, Portugal)

Confirming this, I cannot resist showing how fast this coast line receding can be. Figure 73B, taken in June 2007, shows an apparently resistant looking bed of sandstone.

Figure 73B – Cross bedded sandstone at a seaside cliff (photo taken in June 2007) (Azenhas do Mar, Portugal).

Now see figure 74 of exactly the same place, but photographed in July 2011. Note that the large block with a topographical marker at the top has disappeared. This collapse must have happened practically instantaneously. Unfortunately I only returned to the sight 4 years later. Still, I think it is quite impressive, but also rather worrying, because at the crest of the escarpment there was a FOR SALE sign, and somebody might just build a house there.

Figure 74 – Same sandstone outcrop as the one shown in the previous figure (photo taken in July 2011) (Azenhas do Mar, Portugal)

The coastal erosion action is even more enhanced if there are structural weaknesses present. Figure 75 shows how an almost vertical fault crevice filled with a very easily weathering material acted as an erosional focus and a “blow hole” developed.

Figure 78 - Blow hole in a limestone sequence (Praia da Adraga, Portugal).

Figure 75 – Blow hole in a limestone sequence (depth approximately 5 m) (Praia da Adraga, Portugal).

Far more spectacular though, is figure 75B showing how careful one must be when contemplating living on a cliff by the sea side. Not only is the destroying action of the waves perfectly clear with the gaps on the horizontal limestone succession but, compounding the weakness of the cliff, there is the vertical highly weathered dolorite dyke, almost completely converted into a brown soil, as well as a very open joint, most likely associated with the dyke. The fact is, this photo was taken in 2013 and, most likely for safety reasons, by 2017 the house had already been demolished.

Figure 75B Precarious location for a construction of any sort (Azenhas do Mar, Portugal)

Figure 75B Precarious location for a construction of any sort (Azenhas do Mar, Portugal)

Still related to the wave erosion and returning to the Tagus river estuary, its margins have long rocky sections and overall the river flows quite strongly. Sand needs to be dredged from the centre of the river to maintain it navigable for large tonnage shipping. In an attempt to develop beach tourism, the local municipalities dump that sand at the beach fronts. In places, this is a loosing battle because in winter, due to violent storms, that sand is washed away. For example, according to the news media, for the 2014 holiday period, one million cubic metres of sand had to be dumped at the Caparica beach, which is at the open sea just to the South of the Tagus river estuary. In another case, this time on the northern side of the estuary proper, we have the same kind of evidence at the Carcavelos beach. Notice the difference between figures 76, where the beach is wide nice and sandy before the stormy weather,

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Figure 76 – Broad and sandy Carcavelos beach before the winter storms (Portugal)

and figure 77, with practically no sand. More, in the latter figure, notice the green coloured limestone in the background. That sector was exposed during the particularly stormy 2008 winter and no extra sand was dumped to cover it. The rock is green because the algae have already had time to colonize the outcrop, which is not the case for the limestones in the mid and foregrounds, that were uncovered during the very strong storms of January 2014. The photo was taken in March and that is why the rocks still have a very clean appearance.

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Figure 77 – The same beach stripped of sand by consecutive storms

By April 2015, for the start of the Summer season, the municipality covered the whole of the previously denuded area (fig. 77B), with the exception of the tip of the green sector of the outcrop seen in figure 77.

Figure 77B – The Carcavelos beach once again covered with imported sand, for the 2015 holiday season (Portugal)

Then, confirming that it is a loosing battle, came the rather stormy winter of 2016 which promptly washed away a large quantity of the sand recently dumped, with marked consequences, as shown in figure 77C, this time taken from the sea side.

Exactly the same area, after the January 2016 storms, view from the sea side. The person with the red jacket is standing next to the algae covered limestone outcrop just seen in figure 4.29A (erosSeaCarcPost2.jpg).

Figure 77C – Exactly the same area, as figure 77B, after the January 2016 storms, view from the sea side. The person with the red jacket is standing next to the algae covered limestone outcrop just visible in the previous figure.

At that stage, the local municipality appears to have changed policy, as interpreted by the vast amounts of sand dumped there at the end of winter. My rough estimate is that something like 250000 cubic metres of sand were dumped then. Now, they replenish the beach sand continuously, in a procedure known as “beach nourishment”.

4.5 Geomorphological Control

Wikipedia defines geomorphology as the study of landforms and the processes that change them. From what has been shown so far it is obvious that erosion is a very important factor. Also very important are the characteristics of the outcropping rock assemblages, especially when they present very distinct structural features, of sufficiently large dimensions. In such cases we have a well marked geological control of the earth’s morphology. Representative photos have already been presented. In the case of figure 14 we have the remarkable, kilometres long sharp ridge  caused by a dike which is obviously more resistant to weathering than the surrounding rocks. However, in figure 30B we have exactly the opposite, that is, a very linear sharp trough also caused by a dike, which in this case is far less resistant to weathering. Another very sharp topographical contrast, this time quite circular, is caused by a carbonatite plug (fig. 17). Additionally, observe the impressive outcrop of a granite pluton (item 2.2.3) surrounded by an otherwise well peneplaned area (fig. 78).

Figure 78 - Huge outcrop of a granite pluton on an area reasonably peneplaned (Niassa Province, Mozambique).

Figure 78 – Huge outcrop of a granite pluton surrounded by a  reasonably well peneplaned area (Niassa Province, Mozambique).

Anyway, perhaps the simplest example is a river running along an almost straight line valley, because of the fault which controls its flow direction (fig. 78B).

Figure 78B – The strike of a fault controlling the river valley direction (Malcata mountain, Portugal)

Also, figure 79 shows how sloping strata influences the land morphology, with on one side  the slopes of the hill conforming to the bedding planes and on the other, cutting through the strata, causing a much steeper slope. In fact, this bedding plane is so smooth that it looks almost like a tilted billiards table. However on high magnification, it can be noticed that the sandstone succession actually consists of a column of relatively thin layers, showing a stepping down succession towards the valley at the foreground.

Figure 79 - Stratigraphic geomorphological control (Northumberland National Park, Great britain

Figure 79 – Stratigraphic geomorphological control caused by sloping sediments (Northumberland National Park, Great Britain)

The same can be seen in a much grander scale in figure 80.

Figure 80 - Stratigraphic geomorphological control, Magalisberg Mountains, Transvaal (South Africa).

Figure 80 – Stratigraphic geomorphological control caused by sloping sediments (Magalisberg Mountains, Transvaal South Africa).

This geomorphological control has been used advantageously since time immemorial. Figure 80B shows how the Romans selected the top of the ridge to build a defensive wall to prevent attacks from those “savages”, the Scots. Obviously the Scots lived on the northern side, that is the steep one and the Romans, on the southern side, had an easy slope to approach their fortifications.

Figure 80B – Hadrian’s Wall at the crest of a sloping sandstone sequence (Northumberland National Park, Great Britain)

Another much more modern example is the utilisation of a well developed quartz vein (fig. 80C), for the location of a hydroelectric dam at Chicamba Real in Mozambique.

Figure 80C – Hydroelectric dam positioned against a well developed quartz vein (Chicamba Real, Mozambique).

Posted in Erosion, Geology | 4 Comments

3. Weathering

Posted on 18/01/2009 by Vitor Pacheco

3.1 Initial Stage

Weathering is the chemical alteration caused by the atmospheric water in vapor and/or liquid form, due to the reacting agents carried in solution. Thus, this was the first step in the alteration of the original consolidated rocks forming the initial land masses. Figure 29 shows how the most reactive minerals are preferentially affected.

Crumbling of weathered material within Flysch shale (Furnas, Alentejo, Portugal).

Figure 29 – Crumbling of weathered material within Flysch shale (Furnas, Alentejo, Portugal).

Also, the less soluble materials of the weathered rock will precipitate in the immediate vicinity, along fissures and/or bedding planes through which the water percolates.  For example, the manganese dendritic growths frequently found on bedding planes and joints (fig. 30).

Figure 30 - Manganese dendritic growths on a quartzite bedding plane (Peninsula Quarry, Cape Peninsula, South Africa).

Figure 30 – Manganese dendritic growths on a quartzite bedding plane (Peninsula Quarry, Cape Peninsula, South Africa).

These differential weathering characteristic can also be noticed in the variability between two different members of a rock outcrop, such as in figure 30B. In fact in this example one can see a progression of the weather-erosion (item 4) sequence. Behind the photographer there is fresh dolerite indistinguishable from the limestone across which the dyke cuts. This is followed by the pale brown section of very weathered dolerite already forming a trough along which rain water will converge accelerating the weathering as well as transporting that weathered materials in to the sea, by which section the dyke has been completely removed, thus forming a very sharp topographical contrast.

Figure 30B – Totally weathered and partially washed away dolerite dyke cutting through a limestone succession (Cabo Raso, Cascais, Portugal)

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Posted in Geology, Sedimentary Rocks, Weathering | 5 Comments

2. Igneous Rocks

Posted on 31/12/2008 by Vitor Pacheco

2.1 Origin and Composition

Convection cells must have developed in the Earth’s Mantle at a very early stage, consequently initiating the differentiation of the elements composing the original magma. Just like foam in a boiling pot, the less dense elements accumulated at the top of the up flow side of the convection cells, concentrating at the surface and consolidating into a “crust”, thus creating the continents.

These lighter rocks, silicon rich, are classified as oversaturated (acid), and contain an abundance of quartz. They encompass the granite family, and its volcanic equivalent is termed rhyolite. Of the remaining magma, the most common member and the one which forms the ocean floors, does not have enough silicon for quartz to form, is classified as saturated, and its most common rock family is the gabbro, with its lavas termed basalt. The rocks with least silicon content are classified as undersaturated (alkaline), and one of its rock types is peridotite.

It is easy to understand that along crustal plate diverging boundaries, numerous cracks will form through which the fluid magma from the mantle will be injected. Hence, igneous rocks associated with diverging boundaries, if within an ocean, will have a basaltic composition and form a ridge along the fissures separating the plates. Naturally, all the portions of the fissure’s ridge emerging above the ocean surface form islands, and the best known of these oceanic ridges is the one at the centre of the Atlantic.  Also, this is the reason why, with very few exceptions, most of the existing islands are constituted by basaltic rocks like the Azores archipelago, and they are termed Oceanic Islands (fig. 1).

Figure 1 – Typical oceanic island, formed by a volcano (Pico Island, Azores).

There are some exceptions like the Seychelles islands, which have a granitic composition because they actually represent a remanent that stayed behind when the Indian, Australian and African plates parted ways. In fact Madagascar must also be included, either as a micro continent or an oversized island. Such islands are termed Continental Islands.

If, on the other hand, the plate divergence is within a breaking continent like the Rift Valley in Africa, the igneous rocks will be basaltic, but only if the magma being tapped is from the mantle.

At converging boundaries, where the rock masses are under compression, it is not so straight forward because:

  • When converging plates have identical densities (continents), the collision, obduction, will cause the formation of mountains, like the Himalayas, formed by the impact of the Indian and the Asia plates.
  • Or, if one of the plates is heavier, it will be subducted under the other one, forming a trench like the one along the Asian western margin with its associated volcanic ring. 

So, for converging plates, I think that in the majority of cases, the igneous rocks originate from the melting of the local rocks due to the incredibly high temperatures and pressures caused by the friction developed during compression. Thus, their composition will differ according to their relative location, with basic rocks for the sector close to the subduction trench, because they may be fed by oceanic floor rocks. Within the continental masses, acidic rocks will predominate.

2.2 Type of Occurrence

2.2.1 Volcanic Rocks

Molten magma is continuously being spewed from the mantle through all sorts of existing fractures. If ejected into the atmosphere, it is known as lava,  with the resulting rocks termed volcanic. The ducts through which the lava pours have predominantly a pipe like form and, as the magma cools it creates the well known volcano conic shapes (fig. 1B). Further, because the surrounding atmospheric temperature is markedly lower, the lava will cool very rapidly and the resulting rock will tend to be fine grained

Figure 1 - The top of the Teide volcanic cone.

Figure 1B – The  Teide volcano (Tenerife, Canarias Archipelago).

Also, volcanic activity tends to be periodic and, if the time between separate phases is long enough, the original magma will have solidified, forcing new craters to develop during successive new stages, thus frequently causing de development of a maze of craters of different ages (fig. 2).

Figure 2 – The maze of volcanic craters forming the surface of S. Miguel Island, Azores

The volcanic exhalations may be gentle and fairly continuous, taking the form of a very fluid mass termed lava flows, which will  spread down the volcanic cone and form a fan at the base. In the example shown on figure 3 from Tenerife, the fan actually entered into the sea, and that is where the town of Garuchio was built.

Figure 3 - Town built on a lava flow fan into the sea (Puerto de la Cruz, Tenerife).

Figure 3 – Town built on a sea level lava flow fan (Garuchio, Tenerife).

If, on the other hand, the magma has a high gas content, more violent outpours will occur in the form of ash, termed pyroclastic, with small fragments predominating. In the example shown in figure 4 there is a gentle lava flow capping such a pyroclastic explosive outburst, with the larger clasts being easily identified because of their much darker colour, and this is possibly because they represent fragments of already consolidated lava only spewed out of the pipe at this stage.

Figure 3 - Layer of volcanic ash (pyroclasts) overlain by basalt (Tenerife, Canarias Archipelago).

Figure 4 – Layer of volcanic ash (pyroclasts) overlain by basalt (view approximately 6 m high) (Tenerife, Canarias Archipelago).

These pyroclastic explosive bursts are due to the high gas content of the magma, as well as the stage of consolidation of the lava being spewed out. In extreme cases we will have volcanic breccias (fig. 4B).

Figure 4B – Volcanic breccia (Barberton Mountain Land, S. Africa)

The appearance of the consolidated lava will also be affected by:

• its degree of plasticity, which, when very high gives a very contorted appearance (fig. 5);

Figure 5 – Contorted appearance of a plastic lava flow (Pico Island, Azores)

•  the rate of cooling which, when very rapid, yields volcanic glass, termed obsidian (fig. 6);

obsidian

Figure 6 – Lava field with abundant obsidian (black), (Tenerife, Canarias Archipelago).

•  high fluidity as well as gaseous content, will cause the lava to be very porous, pumice stone, and the porosity will make these rocks very light (fig. 7).

Figure 7 — Demonstration on how light the pumice stone is (Tenerife, Canarias Archipelago).

Figure 7 — Demonstration on how light the pumice stone is (Tenerife, Canarias Archipelago).

  • Or, if the lava is very viscous, the gaseous content will not be so evenly distributed but rater form lager bubbles within the lava (fig. 7B).

Figure 7B – Hollows caused by air bubbles in a viscous lava (Pico Island, Azores).

  • The porosity will allow water to flow through the hollows and, with time, the substances under solution will precipitate filling them, and giving rise to what is known, as amygdaloidal lava (fig. 8).
Figure 8 - Amygdaloidal lava( Ventersdorp lavas, Carletonville, S. Africa).

Figure 8 – Amygdaloidal lava (Ventersdorp lavas, Carletonville, S. Africa).

  • When the size of those hollows is sufficiently large we have the formation of the famous agates and geodes (fig. 9), which will tend to broadly have a spherical form but may reach quite a considerable size and present a huge variety of internal shapes. The term agate is used when the precipitate is not crystalline, and geode when it is.
Figure 9 — Geodes from the Karroo lavas (Lebombo Mountains, Mozambique).

Figure 9 — Agates/geodes from the Karroo lavas (Lebombo Mountains, Mozambique).

• Lava that flows into the sea freezes as it tumbles in and forms very characteristic spherical shapes, termed pillows. As these pillows fall on top of those already settled, if the lava is still sufficiently plastic, its lower portion will become sort of squeezed between the ones more solid below (fig. 10).

Figure 10 - Outcrop of pillow lavas (Barberton, S. Africa).

Figure 10 – Outcrop of pillow lavas (Barberton, S. Africa).

If, on the other hand the pillows fall on soft ground, their spherical shapes are preserved as they squeeze the soil below (fig. 11).

Figure 11 - Pillow lavas overlying VCR (East Driefontein Mine, Carletonville, S. Africa).

Figure 11 – Pillow lavas overlying VCR (East Driefontein Mine, Carletonville, S. Africa).

• Lava cooling on land often develops a very characteristic hexagonal jointing, columnar. This occurs both with basalt (fig. 12).

Figure 12 – Volcanic plug basalt showing columnar jointing (view approximately 6 m high) (Mafra region, Portugal).

as well as rhyolite (fig. 13).

Figure 13 - Close up of columnar rhyolite (Castro Verde, Portugal).

Figure 13 – Columnar rhyolite (view approximately 2 m high) (Castro Verde, Portugal).

Because volcanos produce a variety of materials, from lavas to pyroclasts, their settling characteristics give rise to rock assemblages considerably similar to those of sedimentary rocks (Item 6), as nicely exemplified by the rock assemblage at Pico do Arieiro in Madeira. As can be seen (fig. 13B), there are horizontal lava layers with distinct columnar jointing and above those we have a thick succession of “cross bedded” pyroclastic horizons.

Figure 2.10 - Spectacular cross section of a volcanic rock rock column (Pico do Arieiro, Madeira).

Figure 13B – Spectacular cross section of a volcanic rock assemblage (Pico do Arieiro, Madeira).

Within this upper assortment there are beds that range from poorly sorted (fig. 13C),

Figure 13C - Column of poorly sorted pyroclasts (hight approximately 2.5m).

Figure 13C – Column of poorly sorted pyroclasts (hight approximately 2.5m) (Pico do Arieiro, Madeira).

to moderately well sorted but rather coarse grained (fig. 13D).

Figure 13C - Assemblage of rather coarse pyroclasts (width of picture, approximately 30 cm) (Pico do Arieiro, Madeira).

Figure 13D – Assemblage of rather coarse pyroclasts (width of picture, approximately 30 cm) (Pico do Arieiro, Madeira).

Finally, at the ending stage of the volcanic activity when the lava deep within the crater is still very hot the phreatic water in contact with that material reaches high temperatures and is expelled into the atmosphere as geysers or fumaroles, often with the water dissolving some of the lava substances, of which the most noticeable is sulphur (Fig. 13E).

Figure 13E – A fumarole with boiling yellowish water, and showing a thick coating of sulphur covering the rock on the left hand side (S. Miguel Island, Azores).

2.2.2. Hypabyssal Rocks

A significant proportion of the magma flowing through the tension cracks will actually consolidate along them. The resulting rocks are termed hypabyssal, that is, intermediate between plutonic and volcanic. The majority of the ducts through which magma flows are relatively narrow. As such, the magmas filling these fissures will cool quite fast and the resulting rocks will be predominantly fine to medium grained. If these intrusives are parallel to the surrounding strata they are termed sills (fig. 13E).

Figure 13E – A sill with limestone beds capping it (Samarra River mouth, Portugal).

When cutting across, they are named dykes which, as shown in figure 14, can be very long. 

Figure 14 - Aerial photo of a dyke outcrop on a peneplane (Central Angolan Plateau).

Figure 14 – Aerial photo of a very long dyke outcrop on a peneplane (Central Angolan Plateau).

Also, these fissures are a consequence of the breaking away of continental plates, and fracturing of non homogenous brittle materials usually have associated splitting, termed conjugate faulting. Thus dykes tend to occur in conjugate sets (fig. 15).

Figure 15 - Set of conjugate dykes (Estoril beach, Potugal).

Figure 15 – Set of conjugate dykes (Estoril beach, Potugal).

Hypabyssal rocks occasionally have pipe like forms, many of them corresponding to the volcanos, and they may have considerably large diameters. For those that did not actually reach the surface, their magma will take longer to cool, thus becoming more coarse grained. When they occur along rifting lines and their magma source is very deep, that is from the mantle, it may have an undersaturated composition like the kimberlites (fig. 16),

Figure 16 — Kimberly diamond mine (South Africa).

or they may already show a considerable level of differentiation like the carbonatites (fig. 17).

Figure 17 – Aerial view of a large carbonatite plug outcrop on a peneplane (Central Angolan Plateau).

Volcanic breccias are moderately frequent (fig. 4B), but I think the Boula Igneous Complex in India is a rather unique example (fig. 18)

Figure 18 - Ultramafic Igneous breccia.

Figure 18 – Ultramafic Igneous breccia (block approximately 2.5 m high) (Boula, Orissa, India).

In fact I put it here rather than with the volcanic rocks, because, according to Augé and Thierry, this breccia was caused by a violent explosion within the magma ducts with the fragments belonging to the intruded, rather than the intruding rock and it must have happened at a considerable depth since the intruding basalt is very coarse grained, often pegmatitic (fig. 18A).

Figure 18A – Brecciated chromite fragments within a pegmatitic basalt (Boula, Orissa, India).

However the brecciated wall-rock shows very little movement. For example, the position of the very large chromite fragment shown in figure 19, is very close to its initial position relative to sector of the chromite lens unaffected by the explosive burst.

Figure 19 - Igneous breccia containing chromite clasts (Boula, Orissa, India).

Figure 19 – Igneous breccia containing chromite clasts (view approximately 16 m high) (Boula, Orissa, India).

Other than the in situ shattering, what we had was the rotation of the fragments within a very hot chamber which partially melted the wall-rock (fig. 20).

Figure 20 - Metasomatised igneous breccia clast showing roundness and concentric reaction rim due to partial melting (Boula, Orissa, India).

Figure 20 – Metasomatised igneous breccia fragment showing roundness and concentric reaction rim due to partial melting (Boula, Orissa, India).

2.2.3 Plutonic Rocks

Plutonic rocks are formed by magmatic intrusions at great depths. Since we are dealing with a fluid intrusion, the contacts with the surrounding rocks tend to be irregular (fig. 21).

Figure Figure 21 - Granite/limestone intrusive contact (Sintra Mountain, Portugal).

Figure Figure 21 – Granite/limestone intrusive contact (Sintra Mountain, Portugal).

Further, even though these intrusions occur at great depths, the host rocks are still rather brittle and the magma intrudes through bedding planes and joints, forming a maze of dikes and sills across the host rock  in the immediate vicinity of the pluton (fig. 21B).

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Figure 21B – Maze of granitic dykes and sills cutting the limestones surrounding the Sintra Granite (escarpment hight, about 20 m) (Sintra Mountain, Portugal)

The other consequence of these intrusions taking place at great depths and the fact that they generally have very large volumes is that, with the exception of the marginal areas of contact, this magma has a very long time to cool, allowing the development of coarse grained rocks. Often, since some of the substances crystallise more easily than others, they grow proportionally lager, like feldspar crystals in granite. In such cases they have a porphyritic texture (fig. 21C).

Figure 21C – A porphyritic granite where the much larger feldspar crystal resemble horse teeth (Belmonte, Portugal).

Or, when the magma is rich in volatiles it often has associated hydrothermal pegmatitic (ultra coarse grained) veins, giving rise to magnificently well developed crystals (fig. 22).

Figure 22 - Pegmatitic minerals: book of muscovite (back) (Perth, Canada); black tourmaline, red and green tourmaline and blue beryl (front) (Ligonha, Mozambique); Wolframite (Panasqueira, Portugal)

Figure 22 – Pegmatitic minerals: book of muscovite (back) (Perth, Canada); black tourmaline, red and green tourmaline and blue beryl (front) (Ligonha, Mozambique); Wolframite (Panasqueira, Portugal)

Of these pegmatitic crystals, perhaps the most impressive example I have ever come across, are those in the Naica caves in Chihuahua, Mexico. There, as stated by Wikipedia, the enormous selenite crystals originate from the hydrothermal fluids injected from the magma chamber below, into the surrounding limestone sediments. The figure I show, 22B, is not mine, I grabbed it from the Guardian Newspaper but, it is so spectacular that I felt I should add it here.

Figure 22B – Giant sized selenite crystals in a pegmatite pocket (Naica caves, Chihuahua, Mexico).

2.3 Magmatic Differentiation

Magmatic differentiation was already mentioned (item 2.1), but here I want to refer to two rather unique examples, the Boula Igneous Complex in India and the Bushveld Igneous Complex (BIC) in South Africa. Both these igneous lopoliths have a basic to ultrabasic composition, meaning that the intruding magmas had already had a significant amount of chemical differentiation from the initial mantle magma.

2.3.1 Differential Crystal Settling

Further, within a portion of the intruded chambers of the above mentioned igneous complexes, additional differentiation took place during the cooling process, due to the rate of settling of the various minerals as they crystallised at the top, the coolest area, and slowly dropped to the bottom. What makes these two cases unique though, is that both assemblages consist of a light coloured member, peridotite in India and anorthosite in South Africa, inter-layered with a black member, chromite. Very important too, is that the specific gravity of the latter is far higher than either of the other two, thus allowing for a much more clear separation of the respective minerals  (figs. 23 and 24).

Figure 23 - Magmatic differentiation by crystal settling - A -Boula, Orissa, India.

Figure 23 – Magmatic differentiation by crystal settling (view approximately 30×20 cm) (Boula, Orissa, India).

 

Figure 24 - Dwars River, South Africa.

Figure 24 – Magmatic differentiation by crystal settling (Dwars River, South Africa).

Concentrating now on the BIC, the portion where differentiation took place is termed the Critical Zone, and there, other types of crystal differentiation also occur, like the graded bedding seen in figure 25. I have never seen such perfection in sediments. In the present case, we have granular magnetite forming the base of the sequence, with feldspar crystals progressively increasing in quantity upwards, just like in sediments, with the heavier clasts reaching the bottom first. This impressive similarity between normal sedimentation and crystal settling, initially lead a school of geology in South Africa to believe that the BIC was a metamorphosed sedimentary sequence.

Figure 25 - Graded bedding by crystal settling (Dwars River, South Africa).

Figure 25 – Graded bedding by crystal settling (view approximately 1 m high) (Dwars River, South Africa).

This remarkable similarity is impressive, but it is fundamental to draw a distinction between the two. For that I use the sequence in  the immediate vicinity of the platinum rich Merensky Reef, and I start by presenting the stratigraphic column of that sector, with its magnificently well defined and easily correlatable continuous stratigraphic sequence, including the consistent thicknesses of the various constituents, summarised in figure  25B.

From the top, and using the Impala Platinum Mines terminology, we have the hangingwall 1 (HW1) which is a norite, followed downwards by the Bastard Reef, constituted by a medium grained pyroxenite termed “bastard” because it contains no platinum. Below we have the middling 3 (M3) horizon of mottled anorthosite, followed by the M2 and M1 of spotted anorthosite and norite respectively. Below that we have the Merensky Reef which is presented in more detail further down.

Figure 25B – The Bushveld Igneous Complex column of sediments in the vicinity of the undisturbed Merensky Reef (not to scale).

What is of interest now is the occurrence observed at the sector of the sequence where the locally termed “pyroxenite boulder horizon” occurs (fig. 25B) at the base of footwall 6 (FW6). The bottom contact of this member of the succession is a very well defined and continuous pyroxenite band above which there are scattered coarse grained pyroxenite nodules with an average diameter of 15 cm (fig. 25C).

Figure 25C – Normal pyroxenite “boulder” horizon, about 50 cm above the distinct pyroxenite band (Bafokeng Mine, Rustemberg, South Africa).

However, as shown in figure 26, one of these “boulders”, considerably larger than normal, appears to have fallen through the semi fluid mush of the already settled pyroxenite band. Note that the “boulder” could not possibly be entirely solid, since it looks as if it is rather frayed at the edges. Photos 25C and 26 were taken along one of the mine adits, within 2 m of each other, and I think this example is rather useful in helping to understand the notion of a crystal settling environment.

Figure 26 - Pyroxenite “boulder” falling through pyroxenite beds (Bafokeng Mine, Rustemberg, South Africa).

Figure 26 – Pyroxenite “boulder” falling through pyroxenite band (Bafokeng Mine, Rustemberg, South Africa).

2.3.2 “Pot Holes” Within the Merensky Reef

The platinum bearing Merensky Reef (MR) is generally a conformable horizon of the BIC and it is accepted that this band is the first layer after a new magma influx was injected into the settling chamber, raising its temperature and introducing platinum. That is why the MR has a pegmatitic texture, with a much coarser grain size than the layer immediately below, the approximately 3 m thick norite forming the footwall 1 (FW1). Further, the temperature rise also lead to the development of convection currents within the settling chamber, causing irregular whirls that in places disturbed the already settled crystals, developing what are locally termed “potholes”.

My first example of these potholes was chosen because it fits into the frame of a photograph. Even though it is in black and white, the added markings make it quite clear why these irregularities are known as potholes (fig. 26B). It also shows that the MR is formed by a pegmatitic pyroxenite with discontinuous thin chromite seams at the base as well as at the top, and this is covered by a medium grained pyroxenite. Further, at the centre of the photo, the MR pegmatite has “cut” through the FW2, a 50 cm thick anorthosite band, as well as about 30 cm into the FW3. Note though that, observation of the undisturbed sequence (fig. 25B), indicates that in fact, what figure 26B shows is only the bottom portion of a considerably larger pothole, since the FW1 is not at all present. That is, this pothole actually reaches a total depth of about 4 m, with only its central lowermost portion being visible in the photo.

Figure 26B – Example of a pothole in the Merensky Reef (Bafokeng Mine, South Africa).

This example is exceptional though, because predominantly the potholes are much larger, like the one shown in figure 27 where we see only a fraction of the pothole with, at the right hand side, a MR pegmatitic pyroxenite contacting practically vertically with a mottled anorthosite,  interpreted as the filling of the centre of the pothole and shows a vague suggestion of horizontal layering, corresponding to a latter more quiet period of crystal settling.

Figure 27 - Marensky reef “pothole” edge (Bafokeng Mine, Rustemberg, South Africa).

Figure 27 – Merensky reef “pothole” edge (Bafokeng Mine, Rustenberg, South Africa).

Finalising, figure 28 shows a diamond drill hole that intersected a large pothole and I think it adds to understanding the situation. The core is cut in half and laid on a corrugated iron tray.

Figure 28 – Surface diamond drill hole intersection of a “pot hole” edge (Maricana, South Africa).

Figure 28A is the interpretation section and it is more complete because of the camera’s dimensions limitation. The M3 and M2 horizons are present but, even though the M1 is not, we can accept that the MR (pink band), on the upper portion of the diagramme is reasonably close to its undisturbed position. Further down, the borehole intersected another mottled anorthosite, interpreted as the inner fill of the pothole. Next comes another MR horizon, this time consisting of a very thin chromite seam. Following, is a anorthositic norite (FW3) below which we have the final segment of MR at the base of the pothole, consisting of a rather thick chromite horizon very rich in platinum, underlain by a mottled anorthosite interpreted as representing FW4. In other words, this “pothole” has an approximate depth of just over 12 m.

Figure 28A – Diagrammatic interpretation of the “pothole” intersection (not to scale).

Posted in Geology, Igneous Rocks | 2 Comments

1. Introduction

Posted on 31/12/2008 by Vitor Pacheco

Geology is the study of the Earth’s crust and its continuous process of modification. At the beginning of solidification, approximately 3,9 Ga (giga annum = 109 years) ago, there were only Igneous Rocks, originating from the consolidation of the initial magma. With time these disintegrated due to the atmospheric chemical and physical actions, termed Weathering and Erosion respectively and the altered products were transported by gravity, air or water, and eventually deposited elsewhere, forming the Sedimentary Rocks.

The heat emanating from the Earth’s core is transferred to the mantle, therefore keeping it fluid and giving rise to convection currents which causes the Earth’s crust to be on a continuous reshaping mode. This is magnificently well explained by the Plate Tectonic principle, with plates breaking away from each other at the Diverging Boundaries, and pushing against each other at the Converging Boundaries. Very briefly, we can have diverging boundaries where a continental mass is being split into two, or the split has a continent on one side and an ocean on the other, or there is ocean on both sides of the split, being the Mid Atlantic Oceanic Ridge the classic example of the latter. With converging boundaries we have for example the Himalayas being a consequence of the convergence between two continents, and the American western coast exemplifies a continental plate overriding an oceanic margin, the Pacific Ocean.

At the converging boundaries, with the impact of two plates, especially if both contain continental masses, we have the development of huge amounts of kinetic energy, a large proportion of which will be converted into heat. This, as well as the contact with the igneous rocks which continue being spewed from the mantle, causes a very marked change in the general characteristics, texture and composition of the rocks affected, that is, Metamorphosing them. Finally, the mechanisms involved and the distortion caused to the rock masses when they are broken apart, or pushed together and its consequences, is studied under Structural Geology.

Following, is an introductory presentation of these geological principles and mechanisms accompanied by examples photographed by me during my career. The sequence I will use is:

Igneous Rocks – after all these were the first solid substances on the Earth’s surface.

Weathering – which, it is fair to accept, was the initial action on the rocks by the atmosphere.

Erosion – occurring at the same time as weathering and adding to it.

Unconformity – which is a very important stratigraphic reference surface.

Sedimentation – since that is a consequence of the preceding actions.

Structure and metamorphism – topics I know even less about.

Prospecting – because, after all, in my days, that was what geologists were for.

Mining – again, that was why prospecting was done.

Posted in Geology, Introduction | 1 Comment