9. Mining

9.1 PRIMITIVE EXTRACTION

I include under mining all human activities concerned with the extraction of inert natural materials. As such, the use of stone for construction, is mining. 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), but later they started building wall protections and houses by “dry packing” carefully selected well shaped blocks.

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

As technology progressed people realized that 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, as shown in figure 164.

Figure 164 - Granite boulder halfway split to shape a block for the castle wall (Sintra, Portugal).

This granite block is located within the Moorish castle at Sintra (Fig. 165), which was built not very much earlier than 1100 AD.

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

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

Another, interesting example, is the extraction of rock salt in a deposit near Rio Maior, in Portugal. Its extraction method is quite unique. The deposit is located in a rather large but enclosed aquifer within a limestone succession containing a salt diapir. The water of this  aquifer dilutes the rock salt, reaching a concentration seven times larger than the one of the Atlantic at the coast 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. 166)

SaltWell

Figure 166 – Well into waterlogged rock salt deposit (Rio Maior, Portugal)

and the much cleaner salt is collected from the salt pans around it (fig. 166A). Supposedly, the extraction of this salt was initiated in 1177.

Figure 166A - Salt pans for recovering the dissolved rock salt (Rio Maior, Portugal)

Figure 166A – Salt pans for recovering the dissolved rock salt (Rio Maior, Portugal).

Finally, laterite occurs in regions with high precipitation. India is a great example, with their well known monsoons. There, I saw laterite being mined (item 6.6.3, fig. 124) by cutting it in blocks (fig. 167) for the construction of houses. These blocks are rather large, about 0.5 x 0.2 x 0.1 metres but apparently they are not too heavy, so they can be handled reasonably easily. Also, I was told that no cementing material is needed since, with time and the local enormous rain fall, the iron in the laterite is mobilized and the blocks seal themselves.

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

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

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. 168) is one of them. This job that in the old days was done entirely manually is now done in a totally mechanized procedure.

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

Figure 168 – 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, thus increasing enormously its erosional power, with disastrous consequences on the river bed, bridges across, 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 for alluvial mining, this was done in the good old days by panning but now, even  though done more mechanically, it can still be considered a one man operation, with very few helpers. The example I show, is for diamonds along the Orange River, in South Africa (item 6.4.1, fig. 99). Figure 169, shows a rather simple but effective mechanical method of grading the clasts,

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

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

and figure 170 shows 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 170 – Final hand sorting for diamonds (Ulco Area, Orange River, South Africa).

Now for another one man operation, we go to a gold rich quartz vein operation in Zimbabwe. In fact, in this case it was a partnership of two persons. Here we do not need a grader but rather a crushing unit, stamping mill (figure 171),

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

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

which is also rather simple and very effective. Figure 172, shows a vibrating table which sorts the heavy material. Note the pale, rather thick streak of material at the top of the table. Unfortunately this streak is not just gold but rather predominately pyrite with minor gold specks. For the final separation they were still using mercury which makes an amalgam with the gold, and then the mercury is “boiled out”.

Figure 172 - Vibrating table (Bulawayo region, Zimbabwe)

Figure 172 – Vibrating table (Bulawayo region, Zimbabwe)

9.3 OPEN CAST

As the name indicates, open cast means mining on the surface. Of these, the simplest are 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 173 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 173 – Dustless dump loading operation in a quarry (Halfwayhouse, South Africa).

and figure 174 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 implementation, because 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 174 – Quarry without any dust prevention (Serra de Janeares, Portugal)

Quarry dust is prevented by continuously spraying the haul roads, the blast heaps, as well as all the crushing units. That is, all the sectors where dust may develop. The only dust observable in a South African quarry is that caused by the blast (fig. 175).

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

Figure 175 – 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 apply. 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 than however, for sake of the health of the employees, the quarries are maintained dust free.

Going still further, Ulco is not a stone quarry, but rather a cement and lime factory which means, another potential sector of large quantities of dust development. Figure 176 shows the Ulco factory and quarry from the air and practically no dust is noticeable, even though as the surrounding vegetation indicates, the local climate is rather arid.

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 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).

Still to do with construction and ornamental stone we go now to the extraction of marble. The interesting aspect here is that technology has already managed do away with blasting which used to cause a lot of wastage due to cracking of the rock, even under very cautious controlled blasting. The method now use is a wire line impregnated with diamond chips. Figure 177 shows the control unit and careful observation shows two wires, which are the two sides of a closed wire loop. On the other side there is a pulley located 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.

I can not resist to go back to the problem of heritage misusage. The background of figure 177 is completely filled with waste dumps, that is another example of shortsightedness, since nature is a human heritage to be preserved and not to be abused.

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

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

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 Mining Underground

I visited this quarry or mine, I’m not sure what to call it, 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 processing. One can have an idea on how close the surface is because the surface weathering is still noticeable on the upper section of the central portion, which is a structural support pillar. Also, it is apparent that the marble is of a very high quality. However, even with all these possible cost advantages, I wonder if the venture is still going. I do not have much faith in it.

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

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

This picture also shows how well the wire cutting (fig 177) system mentioned above, works.

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. 178).

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 Laxmi 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 Laxmi 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 Laxmi 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 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 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 Laxmi, 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 smallest 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 Laxmi and Durga.

9.4.3 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: A – 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, give 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).

9.4.4 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 all the large buildings, followed in the mid ground by an area with practically no buildings showing two rather barren ares, one close to the left extremity, which is a remaining waste dump. The one, nearer to the middle and with a considerably larger area is a slimes dam. In front of the slimes dam there is a reasonably sized lake. This central area is where the gold bearing sedimentary horizons of the Witwatersrand Supergroup outcrop. When this picture was taken in 1984, most of that ground still belonged to the mining houses.

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 staring 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 refrigeration is used in the ventilation.

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8. Prospecting

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 synthesizing and interpretation. 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 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 considered too old by the powers that be, to continue prospecting. One thing is for 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 the 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 person, 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 camera.

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.

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 organize 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

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. Since geologists are eternal optimists, it is very frequent to encounter such old mine workings in many present day prospecting sites. The assumption is always that whoever was there before did not look well enough, or most likely, the price of the metal 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 very continuos, since it is much cheaper than drilling. In the old days the sampling was done by chipping the rock with a chisel and hammer but now there are diamond circular saws that do not need water to cool and make the exercise much simpler and faster, although a bit dusty and hence the masks in figure 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).

So, all is well and a drilling programme is planned and budgeted. It is now fundamental to prepare a yard 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 (fig. 159). 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 away.

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).

Returning to the core yard, for an effective and thorough study of the diamond drill core, especially in new areas, not only must each hole be meticulously geologically logged, but perhaps even more important, the core of as many of the holes available as possible must be laid side by side, to assist in the correlation of all the existing stratigraphic features, in order to develop a local and/or regional succession. Hence, the larger the yard, the easier it is. Figure 160 is the core yard where I was fortunate enough, at a very early period of my career, to be present during the early stages of the drilling programme in the Bush Veld Igneous Complex and assist a senior colleague. His very good stratigraphic experience permitted the identification of all the individual stratigraphic 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).

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 metal which was previously considered waste. In that case, the waste dumps of the original extraction, must be reevaluated to ascertain if it contains enough of the second metal to be re-qualified as ore. This is what happened at the chrome mines at Boula in India, where platinum was though to have sufficient grade to be exploited as well (fig. 161).

The little markers seen all over the chromite waste dump, 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 be misleading, but that is how it was done. Also, prospecting within a working mine must continue throughout its life time to maintain a detailed advanced knowledge of the location and grade of the ore ahead of the working faces.

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).

Finally an important point about the reliability of sampling. Even though the two following figures are actually mine stope sampling for grade control, it is important to make it absolutely clear that sampling must strictly adhere to a specified 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. Careful examination of figure 162 shows very nice looking buckshot pyrite just to the left of the sampling line, which means good gold values, but no buckshot at the actual sampling location. 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 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 Mines, South Africa

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7. Structure and Metamorphism

7.1 Faulting

I have very few structural photos, mainly because only the little ones fit within a picture frame, and of these very few are worth photographing. Faulting occurs when the rocks under stress are brittle and therefore break instead of folding. In very broad terms I suppose we can divide the faults into two groups. The simple faults associated with Diverging Plate Boundaries, where normal (extensional) faults predominate, and Converging Boundaries where thrust faults are the norm. It is then reasonable to start with a photo of a miniature graben (fig. 127), which, when it has its normal dimensions is the main form of the rift valleys.

Figure 127 -  Miniature Graben (Furnas, Alentejo, Portugal).

Figure 127 – Miniature Graben (Furnas, Alentejo, Portugal) (escarpment approximately 2.5 m high).

Of the simple faults, I only have rather small but quite explanatory reverse examples. In the case of 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 upthrow side is the one on the right.

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

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

In figure 129 the dragging of the sediments on the upthrow side make quite clear which way the movement went.

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).

Both the above photos are from road side cuttings, that is, natural cross sections, which is the easiest way to visualize 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 mapped geology is plotted on a surface map, where 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 it is only observable because the fault is tiny. As it 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 leveled 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).

On the Converging Plates environment, I have a, not so clear, almost horizontal, thrust fault possibly belonging to the European Variscan Orogeny and cutting across the Lower Alentejo Flysh Group in Portugal (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 that results from the friction along a fault plane.  Figure 131, is an example of a thrust fault, because  the striations are almost horizontal. Also, the movement of the block nearest to the observer, was from the right to the 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, defined as a compact rock with a streaky or banded structure produced by the extreme granulation and shearing of the rocks which have been pulverized during the thrusting, in other words, a micro-breccia (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 evident 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 striking 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 that 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)

7.2 Folding

Folding takes place when the rocks are sufficiently plastic. That is, it occurs at considerable depths under compressional conditions, which means, Converging Plates environments. The simplest   folds are the syncline where the fold forms the bottom of a trough, and the anticline when 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 – 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.

7.3 Metamorphism / Metasomatism

This is the part of geology I like least and as such the one of which I know even less. However, I do have some photos that look interesting. As mentioned before, within the subduction zone area of converging plates, solid rocks may be remobilized to the extreme of becoming magma again. Figure 140 is an example of high level remobilization forming a gneiss, with the white streaks representing the remaining evidence of some thin quartz rich sedimentary beds and it is amazing how distorted they are.

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 existence of a resistant sedimentary remnant, it would not be possible, by the naked eye, to determine if it was a granite or a gneiss.

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

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

Finally greisen, which is defined by the Glossary of the Geological Society of America, as a light coloured rock containing quartz, mica and fluorine rich minerals resulting from the hydrothermal alteration of granite. A rock identical to the one shown in figure 142, was quite common in the Castromil area in 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 these 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.

7.4 Stylolites

Stylolites, as the Wikipedia states, “are serrated surfaces at which mineral material has been removed by pressure dissolution, in a process that decreases the total volume of rock. Insoluble minerals remain within the stylolites making them visible”. This volume reduction may be caused by overburden or tectonic pressures. They are very common in limestones but are generally quite difficult to recognize, as for example the one shown in figure 114, 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 and that is why I am showing it here.

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

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6. Sedimentation

With very few exceptions, materials that have been weathered and/or eroded, are transported elsewhere and eventually settle. Sedimentation is the study of the processes to which these materials are subjected to until they are deposited, and transport is one of 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 discussed later under Non-clastic Sediments (item 6.6). The other materials are termed clasts and, depending on their size, are transported either in suspension, or by saltation, which refers to the much larger particles that move in a series of short intermittent hops close to the ground.

Relative to the transporting method, clastic sediments can be subdivided into: Terrestrial – Aeolian; Glacial; Piedmont; Fluvial. Marine – Delta/Estuary; Wave Dominated; Tide Dominated; Bathial/Abyssal.

6.1 Aeolian

As 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, that is, with 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. Cross-bedding refers to the internal arrangement of the sedimentary layers consisting of minor beds or laminae inclined to the principal bedding planes. In the case of dunes, most sets of cross-strata have dips greater than 10º and deposition is done on the downwind side. Occasionally different sets of cross-strata are separated by low angle strata sets which represent deposition near the base of the dune. 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.

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

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

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 stratification in a coastal dune (Magoito, Portugal)(fig. 57B).

Figure 91 – Cross bedding in a coastal dune (view approximately 7 m high) (Magoito, Portugal)

Large sand deserts are common in areas of low relief and the various types of dunes that exist are a consequence of the local prevailing winds. The best known of these 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

There is very little to say about sedimentation related to glaciation, since most of the associated deposits can be better fitted under the fluvial heading. Perhaps the only sediment type that can be directly related to glaciers is the tillite, which is an unstratified, very poorly sorted sediment, composed mainly of fine clasts, but containing disseminated large to very large boulders generally striated and faceted. Predominantly these tillites cover a relatively small area, occur across the glacial valley and have a mound shape. In that case they are termed moraines and represent the site where the glacier melted away during a moderately long period. If large enough, these moraines act as dam walls, forming 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 Piedmont

The term Piedmont means, 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 alternating rather laminar beds of 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. I also include here sediments formed on the hill slopes as well as mountain river valleys. That is, those characterised by high energy environments, but with short periods of action and thus with very limited transport and consequent poor sorting. If the  rock scree shown in figure 46 had consolidated, it would be classified as a piedmont type deposit, more specifically, a debris flow, which is when large clasts predominate. One of the depositional facies of the gold bearing Ventersdorp Contact Reef (VCR), which stratigrafically lies at the very top of the Witwatersrand Supper Group, is a classic example of a debris flow (fig. 94).

Figure 94 - Debris flow type Ventersdorp Contact Reef (East Driefontein Gold Mine S. Africa).

Figure 94 – Debris flow type Ventersdorp Contact Reef (East Driefontein Gold Mine S. Africa).

Here we can see two separate flow periods of large clasts with an intermediate, shorter flow period of coarse grained sand. Notice also how poorly rounded, packed and sorted the clasts are. Often too, the composition of these clasts is very varied (figs. 95 and 96).

Figure 95 - example of clast composition variation within the VCR (East Driefontein Gold Mine S. Africa).

Figure 95 – Example of clast composition variation within the VCR (East Driefontein Gold Mine S. Africa).

Figure 96 - another example of clast composition variation within the VCR (East Driefontein Gold Mine S. Africa).

Figure 96 – Another example of clast composition variation within the VCR (East Driefontein Gold Mine S. Africa).

Following now to steeply inclined valleys with their intermittent streams still in their primary stages, the resulting sediments are a consequence of flash floods. Naturally they show very poor sorting (fig. 97),

Figure 97 - Stream flood sediment showing irregular cross bedding and very poor sorting (Barberton Mountain Land, South Africa).

Figure 97 – Stream flood sediment showing irregular cross bedding and very poor sorting (view approximately 2 m high) (Barberton Mountain Land, South Africa).

as well as clast roundness, and they tend to occur within relatively long confined channels (fig. 98).

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

Figure 98 – Stream channel VCR showing the steep lateral contact (stope hight approximately 1.5 m) (East Driefontein Gold Mine S. Africa).

6.4 Fluvial

6.4.1 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 Orange 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 bars, mined for alluvial diamonds (view approximately 2 m high) (Orange 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.

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).

Returning to figure 99, It is 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. 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).

6.4.2 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 enhance those bends. The sand concentration is quite distinct in figure 102. During flood periods, the river overflows its banks, dramatically reducing the flow speed, thus 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).

6.5 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. Returning to the Angolan Catumbela River, but this time to its mouth, we notice that even though the river is quite large and carries vast quantities of water, its mouth is actually parallel to the cost, 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 the town of Lobito  developed there (fig. 105),

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

Figure 105 – Wide pro-grade 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).

The example above, shows a sandy river mouth bar depositional environment prograding mainly northwards but also slightly westwards and the movement of the sands will be similar to that of the dunes, that is, forming distinct cross-bedding which in this coastal environment has a considerably smaller scale. I only show one example, where the cross bedding is unusually outlined by pyrite clasts (fig. 107).

Figure 108 - Cross-bedding enhanced by pyrite in Main Reef internal waste (East Driefontein Mine, Carletonville, S. Africa).

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

Another feature of these arenaceous coastal deposits are the ripple marks. The example in figure 108 is rather striking because the lower bedding plane has climbing ripples, that is, asymmetric, indicating water flow, suggestive of an environment still under the influence of the river. On the other hand, the next bedding plane only 5 cm above, has wave ripples, that is, symmetric, caused by wave action, most likely shore line. Hence, quite a marked facies change, possibly within a relatively short time. Significant also is the orientation difference between the two sets of ripples.

Figure  108 - Ripple marks in quartzite (Ferro Quarry, Pretoria, S. Africa).

Figure 108 – Ripple marks in quartzite (Ferro Quarry, Pretoria, S. Africa).

Going now to the marshy areas where there is a predominance of finer detrital material, we have the development of silty and muddy sectors which, during the dry season desiccate, forming very characteristic mud cracks. Figure 109, shows a present day example where the water has not yet dried altogether, but the first signs of the mud cracking are already evident.

Figure 109 - Water puddle on clay rich ground showing the beginning  of mud crack formation (Salvadorinho clay pit, Rocio ao Sul do Tejo, Portugal).

Figure 109 – Water puddle on clay rich ground showing the beginning of mud crack formation (Salvadorinho clay pit, Rocio ao Sul do Tejo, Portugal).

In figure 110, we have consolidated mud cracks and casts.

figure 110 - Cast of consolidated mud cracks in mudstone (Transvaal, S. Africa).

figure 110 – Cast of consolidated mud cracks in mudstone (Transvaal, S. Africa).

A magnificent text book example tough, is the one in figure 111,

Figure 111 – Consolidated mud cracks in limestone (Ulco. S. Africa).

where even the actual curling at the edge of the mud crack slabs was preserved (fig. 111B). As for the slight ripples along the edges and the indentations at the centre 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.

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

Finally, under this kind of natural fertile environment, we should expect abundant fauna and flora. The richest portion must surely be the estuary where the  river water rich in all sorts of dissolved organic matter, contacts the ocean’s salt water. Evidence of the fertility of this environment is well preserved where there is a significant tidal variation, because the mud-flats get exposed (fig. 112).

Figure 112 - Present day mud flats rich in burrowing animals (Sado River Estuary, Portugal).

Figure 112 – Present day mud flats rich in burrowing animals (Sado River Estuary, Portugal).

and allow the signs of the various types of animal burrowing to be preserved giving rise to what are named, bioturbated beds (fig. 113).

Figure 113 – Preserved burrowing casts in a clayey limestone (bioturbated) (Carcavelos, Portugal)

It is also in this environment that we have the bioclastic limestones, that is, those formed by the accumulation of mollusk shells (fig 113B).

Figure 113B – Bioclastic sedimentary layer (Azenhas do Mar, Portugal)

Likewise, already within the marine environment, but still quite close to the coast there are the fossiliferous limestones formed primarily by coral skeletons originating from the barrier reefs (fig. 113C).

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

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

6.6 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 in water, like salt, gypsum, chert and calcite.

6.6.1 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. Occasionally these limestones contain interlayered  horizons with irregularly shaped chert nodules formed by the diagenetic silicification within the rock in which they occur (fig. 114).

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

Figure 114 – Close up of silica nodules within a chemically precipitated limestone (Serra do Sicó, Portugal).

In the thick successions of the Transvaal Dolomites in South Africa we do not have nodules but rather very abundant inter-layered chert bands (fig. 115). For diamond drilling purposes this is absolute agony because the very sharp alternation between the relativily soft limestone and the thin but very hard chert bands, destroys the drilling crowns in no time at all. Even though these chert layers are not a chemical precipitate but rather formed by a normal sedimentation process, I mention them here, to put the two types of chert common in limestone together. These latter cherts are formed by the accumulation on the sea floor of the siliceous shells of radiolaria, which are one of the many animal species forming the sea plankton.

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

Figure 115 – Column of well marked alternating dolomite and chert (positive relief) (view approximately 10 m high) (Sabie River. S. Africa)

Perhaps one of the most striking examples of a chemically precipitated limestone is  travertine, 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, extending from a section of an escarpment formed by the Northern Cape dolomites. It has a fan shape and is being built by the continuous precipitation of calcite from a stream falling over the dolomite ridge and saturated with the limestone it dissolved before reaching the ridge (fig. 116 ), and

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

Figure 116 – 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 117.

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

Figure 117 – 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. 118).

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

Figure 118 – Calcite coating around a growing grass stem (Ulco, South Africa).

Diamond drill core cutting through the fan sequence shows that its development was caused by the continuous coating of whatever materials over which the saturated waters flowed (fig. 119).

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

Figure 119 – Diamond drill core cutting through the Gorokop travertine fan (Ulco, South Africa).

6.6.2 Evaporites

Also defined by the he American Geological Institute glossary, an evaporite is a rock composed primarily by a solution that became concentrated due to the evaporation of its solvent, for example, the present day coastal salt pans. Evaporites of gypsum are also quite frequent in the geological record and can cover quite large areas like the one I saw in Angola. Unfortunately I did not have a camera, but I collected a specimen (fig. 120).

Figure 120 – Lovely specimen of fibrous gypsum (Angola)

Calcite and gypsum may also form in desert pool, as well as from underground water brought to the surface by capillary action which, on evaporating leaves behind a residue of calcite and gypsum. This latter process may form a surface crust of soluble salts and the process is known as calcification or more generally efflorescence (fig. 121).

Figure 122 - distinct hard calcrete layer half way down the succession (Namibe desert, Angola).

Figure 121 – Distinct hard calcrete layer half way down the succession (Namibe desert, Angola).

It is under these conditions that the gypsum crystals known as desert roses form (fig. 122).

Figure 123 - desert rose crystals (Namibia).

Figure 122 – Desert rose crystals (Namibia).

6.6.3 Laterization

I’m not too sure wether to classify laterization as weathering, or as the only example of sedimentation without transport. Dana defines it as a prolonged weathering of Fe and, or Al rich rocks under tropical conditions. A Fe/Al colloidal is formed and most of the other substances are dissolved and washed away. That is, the very insoluble materials concentrate in situ making the soils very infertile (fig. 123),

Figure 124 - Laterite field, rather barren compared to the lush vegetation in the background (Orissa, India).

Figure 123 – Laterite field, rather barren compared to the lush vegetation in the background (Orissa, India).

since it forms a rather solid rock with a very high content of iron and very little else (fig. 124). When aluminium predominates we have bauxite.

Figure 125 - close up of the laterite (Orissa, India).

Figure 124 – Close up of the laterite (Orissa, India).

Thus, laterization is practically the opposite to what happens to limestone which, because it is so soluble in acidic waters, is almost totally washed away leaving caves behind. On the other hand, depending on the acidity of the water, the iron originating from rock weathering may be dissolved by the infiltrating waters, and transported to the water table. With a change on the ground water acidity level, the iron oxide may precipitate at various depths within the soil profile, generally as irregular, thin limonite rich layers fig. 125),

Figure 125 – Thin crust of limonite capping over a very clean sandstone (Lizandro River mouth, Portugal).

with predominantly a pisolitic texture (fig. 125B)

Figure 125B – Limonite crust showing pisolitic texture (Lizandro River mouth, Portugal).

When, due to erosion these irregular thin layers are exposed, they form “crusts” because they tend to be more weather and erosion resistant. These horizons are termed duricrusts or ferricrete and fall under the general group of laterites, even tough their formation is practically the opposite of the much more voluminous bodies formed by the “in situ” concentration of iron mentioned above.

6.6.4 Geodes

As already referred to in item 2.2.1 (volcanic rocks), geodes are a consequence of the precipitation of saturated chemical solutions within voids. I come back to them here simply because of the exceptional case of the enormous selenite crystals at the Naica caves in Chihuahua, Mexico (fig. 126). Even though this is another picture that I grabbed, this time from the Guardian Newspaper, I think these geodes at Naica must surely be the largest in the world.

Figure 126 - Giant sized geode of selenite crystals (Naica caves, Chihuahua, Mexico).

Figure 126 – Giant sized geode of selenite crystals (Naica caves, Chihuahua, Mexico).

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5. Unconformity

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 and, or depositional 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 plane of that break represents an interruption of the cycle of sedimentation caused by a cycle 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 at 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 plane 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). In figure 81 there is a layer of sandstone overlain by a thin band of, not clearly observable mixed materials, transgressing upwards into a chemical precipitated limestone. The mixed materials band is the plane of disconformity because it represents a time period when there was no sedimentation, but rather erosion as well as mixing of clasts.

Figure 81 - Contact between a lower clastic succession and an evaporite (limestone), with a relatively narrow band representing a period of disturbance (disconformity/diastem).

Figure 81 – Contact between a lower clastic succession and a chemically precipitated limestone, with a relatively narrow band representing a period of disturbance (disconformity/diastem).

The other example is shown in figure 31B, where the lowermost lava outflow has weathered to a paleosol. That is, that 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 plane 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 shales below belong to the Lower Alentejo Flysch Group, of Devonian age. Thus, although irregular, this plane demarcates a very long period of erosion.

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 (Vale do Gaio, Portugal).

More common, is a very well defined, smooth plane surface, cutting at a distinct angle across a succession of older sediments (fig. 83). This is a very good example of an inter-regional unconformity plane because it separates the Witwatersrand sedimentary column from the thick pile of Ventersdorp 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, the angular difference between the sediments bellow and above is minimal, 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-sedimentational period for algae to grow and to capture suspended gold.

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 units, 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). 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 at 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 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 cast dimensions are identical, but the Angolan conglomerates are much more robust.

Figure 88 – Pebble beach (Brighton, England).

5.3 Non Conformity

A plane separating a strongly eroded plutonic, or a massive metamorphic body below, from a sedimentary body above, is termed a non conformity. I have seen very few of these and none could really be photographed. Even this one in Angola is not that clear, but for completeness sake and the fact that it represents a huge time gap, I am including it (fig. 88B).

The very thick column of Karroo age sandstone, Triassic, is overlying basement granite of Precambrian age and the wavy contact is reasonably well noticeable. Thus, this is indeed a real inter-regional unconformity.

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4. Erosion

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, in the examples shown here, gravity is by far, the main agent. The simplest is the tumbling of rock debris down a steep mountain slope.  Clearly this rock debris had to be disintegrated 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 that as they thicken, widen the joints along which they grow. These dislodged fragments tend to accumulate on minor ledges (fig. 46) and may suddenly be displaced by, for  example a rain storm, and violently fall down the slope forming a rock fall deposit.

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

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

Much more serious, for safety reasons, are rock falls down coastal escarpments along popular tourist beaches, as the fairly recent example shown in figure 46B demonstrates. Examination of the picture shows at the crest of the ridge a ledge of erosion resistant limestone underlain by a clay rich, poorly consolidate sandstone, very easily erodible.

Figure 46B - Evidence of a recent rock fall (Arrábida, Portugal)

Figure 46B – 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. 46C) and eventually the collapse of large blocks, with potential, rather serious consequences.

Figure 46C – Ground cracks development cause by undercutting due to erosion (Praia das Avencas, Parede, Portugal)

Also very important in regions with good rain is the down hill flow of the upper fraction of the weathered rock/soil, due to gravity and known as soil creep. This can be seen in figure 46D, where the upper sector of the moderately well bedded weathered shales have bent slightly downhill because of the steep surface slope as well as the lubricating effect of the water.

Figure 46D – Soil creep in graywakes causing the drooping of the section nesrest to the surface (Vide, Portugal)

Again, serious repercussions may occur when, after torrential rains soils become supper saturated and suddenly a large segment is released causing a land slide (fig 47).  This is naturally aggravated when the ground is partially or totally denuded due to land misuse.

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

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

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 climates. 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 sand is entirely covering the Canarias Archipelago, Madeira Island as well as a large sector of the Atlantic Ocean. Often, on the Eastern side of Las Palmas, there is a distinct haze in the air and I wonder if it is not due to the desert dust ( fig. 2).

Figure 48 - Dust storm from the Sahara

Figure 48 – Dust storm from the Sahara

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. That is, a surface covered by a carapace of clasts too large to be transported by the wind. Further, this carapace of larger clasts, due to the continuous blasting of the sand in movement over it, becomes highly polished. My example (fig. 49) is not a particularly good one, but it gives the general idea.

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

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

The effect of the sand blasting is noticeable not only on desert floors, but also on outcrops of all types. For example, in the Golden Gate region of South Africa, there is a sandstone formation termed 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 - Closer view of one of the caves (Brandwag, South Africa).

Figure 51 – Closer view of one of the 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).

It is not surprising than, that the blasting sand on a windy day at the beach hurts the legs so much. See how it pits a granite outcrop at one of the Cape Town beaches (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 it goes, even with a tough fresh granite (fig. 54). Notice also a suggestion of striations at the top section of figure 53.

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).

This effect is quite different from that on a limestone outcrop at the Abano beach near near Cascais, Portugal (fig. 54B).

Figure 54B - Effect of sand blasting on a limestone outcrop (Praia do Abano, Sintra coast, Portugal

Figure 54B – Effect of sand blasting on a limestone outcrop (Praia do Abano, Cascais, Portugal)

In the latter case pitting is not visible but there is a very distinct although fine, striation. I think the difference is because in Cape Town the sand is coarse grained and most likely the prevailing winds are almost perpendicular to the granite outcrop. On the other hand  at the Abano beach, the quartz sand is very fine grained and the prevailing wind is quite oblique to the outcrop.

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. 54C) as well as causing very characteristic striations and faceting on the rock outcrops as well as the clast carried within the ice.

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

Figure 54C – 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. Further, 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. A good example is 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 destroied  in Angola as a consequence of poor farming procedures (fig. 57).

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

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

And how deep the destruction is (fig. 58).

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

Figure 58 – Bad lands locally termed “moonscape” – close view (Vicinity of Lobito, 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, a river tends to be rather well behaved and its 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)

At this stage, I will only mention 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 with 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 of about 800m (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 coming from the plateau form spectacular falls over such escarpments. The Victoria Falls on the Zambezi River, is the greatest example (fig. 63), and it 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 (fig. 64).

Figure 64 -  Sight of previous location of the Victoria falls (Zambezi River, Zimbabwe).

Figure 64 – Sight of previous location of the Victoria falls (Zambezi River, Zimbabwe).

As the saying goes, “soft water will eventually cut through hard rock”. However, the erosional work is actually due to the saltating, whirling pebbles carried in suspension by the rivers and 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 where 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).

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).

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 under the effect of erosion, as is confirmed by the government geographical survey study showing that the sea elevation has risen 15 cm in the last 100 years. The data for this study is collected by an instrument installed at the Cascais sea side in 1882 and which continues in full operation (fig. 72B).

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

It has produced since its establishment, a continuous graph of the tidal variations as well as mean sea level elevations (fig. 72C).

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

Another very impressive example of ocean transgressions and regressions through time, is the Roman galleries found under Rua da Prata in the section of Lisbon close to the Tagus River margin (fig. 72D).

Figure 72D - Entrance to the Roman galleries below the street (Lisbon, Portugal)

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

These galleries were build at about the first century AC and are thought to be the foundations of some important building. The arches are just over 2 meters high and about another 2 meters below the street level (fig. 72E). They are now completely under water which was surely not the case when they were constructed. 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 72E - View from one of the galleries to the entrance steps (Lisbon, Portugal

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

The present water table in this part of Lisbon is less than 2 metres below the street and these galleries are permanently left under water to prevent ground subsidence and the consequent damage to the buildings above. However, once a year the water is pumped out for a general structural inspection and it is then open for a public visit.

With all these very strong signs of the present day sea transgression, it is  difficult to understand constructions, often extensive, very 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 line  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)

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 this is quite impressive.

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 accompanied by currents, or if the coast has structural weaknesses. Figure 75 shows how an almost vertical fault gouge of very weathered 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.

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 most of the 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 within the estuary proper on the northern side, 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 near to the sea. That sector was exposed during the particularly stormy 2008 winter and no extra sand was dumped there. The rock is green because the algae have already had time to colonize the outcrop. As for the limestones in the mid and foregrounds, they 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. Observing the relative hight of the outcrops one can have an idea on the considerable amounts of sand previously dumped.

DSC04338

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

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 31 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. 16). 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).

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, Northumberland National Park, Great Britain

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

3.1 General

3.1.1 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).

This differential weathering characteristics can also be noticed in the variability between two different members of a rock outcrop such as in figure 31, where a dolorite dike is totally weathered away and the limestone through which it intrudes, being more weather resistant remains unaltered, thus causing a very sharp topographical contrast.

Figure 31 - Totally weathered dolorite dyke cutting through a limestone succession (Boca do Inferno, Cascais, Portugal).

Figure 31 – Totally weathered and washed away dolorite dyke cutting through a limestone succession (Boca do Inferno, Cascais, Portugal).

3.1.2 Soil Formation

The above example is a rather exceptional case because more often we have reasonably flat areas with uniform rock outcrops where the weathering is widespread, giving rise to the development of soils, as shown in figure 31B. On the right side of the photo, the lowermost member of the rock sequence which has a blotchy wine colour, is covered by a layer of very angular rock fragments and above that we have a layer of fairly unaltered lava. The wine coloured horizon is also a basaltic lava and its colour is caused by the complete weathering of its minerals, which are rich in iron and the blotching indicates mineral assemblages with different chemical composition. If this altered material that is now a soil, had been transported, the blotches would have disappeared because of the mixing during transport. Thus, in this case we have an “in situ” paleosol.

Figure 31B – Basic lava weathered to soil overlain by a layer of hill slope debris, covered by moderately fresh lava (Tagus estuary, Oeiras, Portugal)

3.2 Special Aspects

3.2.1 Weathering Cups

The difference in reactability of the various minerals constituting a rock give rise to interesting surface features. One such case is the formation of cup shaped holes on granite outcrops on mountain tops (fig. 31C). I presume these remarkably circular depressions are caused by the weathering of the feldspars and other chemically unstable minerals into clays, initially in single rain drops size cups. At the next rain stage, the very light clay washes away, enlarging the cup and exposing newer levels of rock to weathering. In the mean time, the much more weather resistant quartz grains present get loosened and acumulate at the bottom of the cup as shown in the picture.

Figure 30B - Granite weathering in the form of very circular cups (see keys for scale) (Serra de Freita, Arouca, Portugal)

Figure 31C – Granite weathering in the form of very circular cups (see keys for scale) (Serra de Freita, Arouca, Portugal)

When the rain is stronger, these quartz grains will also be washed away, thus allowing the continuation of the enlargement of the cup, which can eventually reach significant dimensions like the ones at Marinieche in France, as was shown to me by Jean Jacques Espirat, another geologist. In fact, after his communication, I changed my mind about a depression in a granite boulder, with a mouth diameter of just over 1 meter, which I photographed in the vicinity of the Orange River (Agrabis Falls), in South Africa (fig. 31D). I had originally interpreted it as being a river pothole, at a site which, although with an elevation somewhat higher than the present river bed, I presumed it was where the river previously flowed. Perhaps another point in favour of this new interpretation is the fact that river potholes tend to be deep and have a proportionally narrow mouth, as against these weathering cups which appear to be shallow and have a relatively large mouth.

Figure 31D – Very large weathering cup (diameter over 1 m) (Agrabis Falls, Orange River. South Africa).

3.2.2 Boxwork Weathering

The larger the surface area being affected by the weathering, the more effective it will be. That is, the more cracked the original rock, the sooner it will become weathered, since the cracks greatly increase the area exposed. Cracks develop, for example, due to the influence of the temperature variations from night to day, causing consecutive expansions and contractions. Along these cracks the weathering will have a larger surface of impact and it will often form troughs. This is well demonstrated in figure 31E where the weathering effect on a gray limestone has such a similarity to the skin of an elephant, even the colour, that in South Africa this type of weathering is known as “elephant’s hide weathering”.

Figure 31E - "Elephant hide" type weathering on a grey limestone (Praia do Abano, Sintra, Portugal

Figure 31E – “Elephant hide” type weathering on a grey limestone (Praia do Abano, Sintra, Portugal

On the other hand the joints could have been previously filled with a more resistant material in which case they will tend to form ridges (fig 32),

Figure 32 - Boxwork weathering on flisch sandstones (Cabo Sardão, Portugal).

Figure 32 – Boxwork weathering on flisch sandstones (Cabo Sardão, Portugal).

giving rise to a surface texture with the obvious name of box-work, which comes in all sizes and is quite noticeable in limonite rich sediments (fig. 33).

Large scale box-work weathering from a Fe rich horizon (Castro Verde, Portugal).

Figure 33 – Large scale box-work weathering from a Fe rich horizon (Castro Verde, Portugal).

Another example is the spectacular weathering sequence initiated with the alteration of olivine to serpentine (fig. 34).

Serpentine after olivine, notice the initial formation of a box-work texture (Boula, india).

Figure 34 – Serpentine after olivine, notice the initial formation of a box-work texture (view approximately 100 x70 cm) (Boula, india).

This is followed by a marked increase in the box-work texture, and the alteration of serpentine to breunerite (fig. 35).

Breunerite after serpentine with a distinct box-work texture (Orissa, India).

Figure 35 – Breunerite after serpentine with a distinct box-work texture (view approximately 100 x70 cm) (Orissa, India).

3.2.3 Jointing in Weathering

Much better defined cracks (joints) develop when buried rocks are released from their surrounding pressure. Mining for example, when the ore and associated waste is extracted, relieves the surrounding rock body from its original tension. Figure 36 shows the cracks developed by the pressure release caused by the mining of this dunite pipe. Humidity concentrated along these cracks thus accelerating the weathering which altered the dunite into magnesite, now framing the joints so distinctly.

Magnesite after dunite, developed along the pressure release joints around the pipe mined for platinum (Bushveld Igneous Complex, South Africa).

Figure 36 – Magnesite after dunite, developed along the pressure release joints around the dunite pipe mined for platinum (view approximately 2.5 m high) (Bushveld Igneous Complex, South Africa).

On a much larger scale, are the joints formed during the uplifting of rock masses caused by isostatic adjustment. A good example is seen in granite outcrops, since they are formed at great depths and thus at very high pressures. Erosion will eventually bring these large igneous rock masses to the surface reducing dramatically the surrounding pressure and causing them to break into large blocks. Since the weathering will be most intense at the corners of these blocks, these tend to become rounded, as nicely demonstrated in figure 36B.

Figure 36B – Small scale exfoliation in a dolorite dike (Cascais coast, Portugal)

Hence the decomposition becomes more concentric causing a pealing effect, like an onion (exfoliation). In a grand scale like in a granitic land surface, this can give rise to the very characteristic geomorphology of upstanding huge egg shaped solid granite boulders, known as inselbergs (fig. 37).

sugarloaf

Figure 37 – The most famous inselberg in the World (Sugar Loaf – Rio de Janeiro).

A very unusual effect of the decompression on a granite body and consequent enhancement of the exfoliation by weathering is shown in figure 38 where the granite boulders resemble piles of pancakes. It is possible that this is a consequence of the lamination effect, caused by prior strong shear pressures on the granite mass.

exfoliation

Figure 38 – Unusual exfoliation in granite (granite pile approximately 1 m high) (Serra de Montesinho -Trás-os-Montes. Portugal).

3.2.4 Sink Holes

Perhaps the most devastating weathering effect, is the one giving rise to sink holes. In acidic waters, limestones and dolomites are incredibly soluble rocks and thus very easily weathered. The effect on outcrops, termed “karst”, gives rise to a very irregular surface with rather deep hollows (fig. 39).

Typical example of karst topography (Cabo Carvoeiro, Portugal).

Figure 39 – Typical example of karst topography (Cabo Carvoeiro, Portugal).

This weathering action continues under the surface anywhere above the ground water table. Thus, where the water table is sufficiently deep, the limestone will continue weathering above that and caves will form (fig. 40).

Inside the Sterkfontein Caves.

Figure 40 – Inside the Sterkfontein Caves (Transvaal, South Africa).

Figure 41 shows the difference between solubility levels. Notice the marked contrast between the very weathered, soluble, limestone bed, above, and an insoluble chert horizon below.

 Highly weathered limestone horizon (above), and unweathered chert band below (Sterkfontein Caves,Transvaal, South Africa).

Figure 41 – Highly weathered limestone horizon (above), and unweathered chert band below (view approximately 60 x40 cm) (Sterkfontein Caves,Transvaal, South Africa).

If the conditions remain constant for long periods, the weathering of the limestone will continue, the caves will increase in dimensions, reaching a stage when the ceilings, generally in the form of a vault, will no longer support the mass of ground above, and will collapse, forming sink holes. Even small sink holes can cause significant damage like the one shown under the railway line (fig. 42).

Small sink hole under a railway line (Bufulsfontein Mine - Stilfontein - S. Africa).

Figure 42 – Small sink hole under a railway line (Bufulsfontein Mine – Stilfontein – S. Africa).

On the other hand, particularly interesting is the double sink hole shown in figure 43, where initially the dome collapsed, but somehow its central sector maintained its shape, to eventually disintegrate at a later stage. For a matter of scaling, the markings on the right side of the photo are car tracks.

 Reactivated large sink hole (Carletonville, S. Africa).

Figure 43 – Reactivated large sink hole (Carletonville, S. Africa).

In the mining town of Carletonville, South Africa, there are/were 4 deep gold mines which pumped to the surface in excess of one million cubic meters of water per day, in order to maintain the underground workings sufficiently dry to enable the mining operations. As a consequence, the ground water table dropped tremendously in some places to more than 500m below surface, causing the development of large areas with perfect conditions for the formation of caves which naturally progressed into numerous sink holes. This eventually led to the evacuation  of a few of the mining villages, because of the collapse of some of the constructions and the unfortunate death of some of the inhabitants.

Much more frightening though, is what occurs in Slovenia where the village of Skocjanske (fig. 44).

the village of Skocjanske suspended amongst limestone arches separating different sink holes (Slovenia).

Figure 44 – the village of Skocjanske suspended amongst limestone arches separating different sink holes (Slovenia).

is located on top of arch remnants separating old sink holes (fig. 45). Having lived in Carletonville, I do not understand how a village can continue being inhabited under such conditions.

View from below of a sink hole face (Slovenia).

Figure 45 – View from below, of a sink hole face (escarpment approximately 10 m high) (Slovenia).

Posted in Geology, Sedimentary Rocks, Weathering | 5 Comments

2. Igneous Rocks

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. The less dense elements, silicon rich, accumulated at the top of the up flow side of the convection cells, just as foam in a boiling pot. Thus, this lighter material concentrated at the surface and consolidated creating the continents which are therefore formed by silicon rich rocks containing an abundance of quartz and are classified as oversaturated (acid). They encompass the granite family, of which the volcanic equivalent is rhyolite. Of the remaining magma, the most common member and the one which forms the oceanic floors, does not have enough silicon for quartz to form, is classified as saturated, and its most common rock family is the gabbro, with basalt as its volcanic equivalent. 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 can flow. Thus, igneous rocks associated with diverging boundaries, if within an ocean and forming its ridge, like the one along the centre of the Atlantic, will have a basaltic composition since its source is also basaltic. If the divergence is within a continent breaking up like the Rift Valley in Africa, the igneous rocks will be basaltic, but only if the magma being tapped is from the mantle.

Along converging boundaries, where the rock masses are under compression, it is not so straight forward, especially since either the two plates are compressing against each other, or the heavier density plate is being subducted under the other one. So, 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 in accordance to their relative location, with basic rocks for the sector close to the subduction trench because they will be fed by oceanic floor rocks. Within 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, and the ducts through which the lava pours are the volcanos. 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. Nowadays volcanos typically have pipe like structures through which the magma flows and as it cools, it creates the well known conic shapes (fig. 1).

Figure 1 - The top of the Teide volcanic cone.

Figure 1 – The top of the Teide volcanic cone (Tenerife, Canarias Archipelago).

Also, they frequently develop lateral vents (fig. 2). However, magma may also outpour along fissures as presently in Iceland and in the past, for example during the Karroo volcanicity (Jurassic), in South Africa.

volcvent

Figure 2 – lateral volcanic vent of the Teide (Tenerife, Canarias Archipelago).

Lava flows will enlarge the volcanic cone and spread in a fan shape at the base. In the example shown on figure 3 in 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 lava flow fan into the sea (Garuchio, Tenerife).

Volcanic exhalations may be gentle and fairly continuous, in which case it takes the form of a very plastic fluid termed lava flow, as for example the upper dark layer of figure 4. Or, like the lower layer of the same figure, the out pour may take the form of ash, termed pyroclastic, with small fragments predominating, but  larger clasts may also be common and in the present case they are easily identified because of their much darker colour.

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 magma high gas content, 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);

Contorted lava

Figure 5 – Contorted appearance of a very plastic lava flow (view approximately 1 m high) (Tenerife, Canarias Archipelago).

•  the rate of cooling which, when very rapid, yields volcanic glass, 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).

Further, this porosity will allow water to flow through the hollows and, with time, the diluted substances will precipitate and fill the holes, 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 shape 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 units, termed pillows. As these pillows fall on top of of those already settled and 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 and they squeeze the paleosol 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 develop a very characteristic hexagonal jointing, columnar. This occurs both with basalt (fig. 12).

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

as well as rhyolite (figs. 13).

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

Figure 13 – Close up of columnar rhyolite (view approximately 2 m high) (Castro Verde, Portugal).

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 narrow and very long (fig. 14). As such, the magmas filling these fissures will cool quite fast and the resulting rocks will predominantly be fine to medium grained.  If these intrusives are parallel to the surrounding strata they are termed sills and when cutting across, they are called dykes.

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

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

Also, these fractures 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).

Hypabysal rocks occasionally also have pipe like forms which may have considerably large diameters, hence taking longer to cool and becoming therefore more coarse grained. They are predominantly associated with rifting and if I’m not mistaken, their magma source is very deep, as with carbonatites (fig. 16),

Figure 16 - Aerial view of a large carbonatite plug outcrop on a peneplane (Central Angolan Plateau).

Figure 16 – Aerial view of a large carbonatite plug outcrop on a peneplane (Central Angolan Plateau).

kimberlites (fig. 17), and some others.

Figure 17 — Kimberly diamond mine (South Africa).

Figure 17 — Kimberly diamond mine (South Africa).

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 (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 clasts 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. However the brecciated wall-rock shows very little movement. For example, the position of the very large chromite clast 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 clasts 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 clast 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).

Also, with the exception of the marginal areas of contact and the fact that they generally have very large volumes, this magma has a very long time to cool, allowing the development of coarse grained rocks. 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)

2.3 Magmatic Differentiation

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

2.3.1 Differential Crystal Settling

While cooling within the intruded chamber, further differentiation took place 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. The reason why these two cases are so spectacular is because both assemblages consist of a light coloured member, peridotite in India and anorthosite in South Africa, inter-layered with a black member, chromite. Also, the SG 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).

The similarity between a normal sedimentation process and the crystal settling in these two cases is remarkable. So much so, that initially a school of geology in South Africa believed the B. I. C. to be an assemblage of  metamorphosed sediments. Take also the example shown in figure 25. I have never seen such perfect graded bedding in real 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 where the heavier clasts are the ones that reach the bottom first.

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).

Another example, still with close similarities with sedimentation, but now with igneous crystal settling characteristics more apparent, is the occurrence observed at the sector of this rock sequence where the locally termed pyroxenite boulder horizon occurs. This member of the succession is approximately 50cm above a very well defined and continuous pyroxenite band and consists of a layer of spotted anorthosite, containing scattered coarse grained pyroxenite nodules with an average diameter of 15 cm (fig. 25B).

Figure 25B – 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 the “boulders”, considerably larger than normal, appears to have fallen through the semi fluid mush of the already settled pyroxenite band. Note that the “boulder” was not entirely solid, since it looks as if it is rather frayed at the edges. Both these photos 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 beds (Bafokeng Mine, Rustemberg, South Africa).

2.3.2 “Pot Holes” Within the Marensky Reef

The Marensky Reef (MR) is a platinum bearing, generaly conformable horizon of the B. I. C.. It is accepted that this band is the first layer after a new magma influx was injected into the settling chamber, bringing the platinum and also raising the environmental temperature. That is the reason why the MR has a pegmatitic texture with a much coarser grain size than that of the lower layers. This temperature rise also caused the development of convection currents within the settling chamber causing what are locally called “potholes” and for which a tentative explanation follows:

Figure 27 was taken underground at the face of a MR  stope. The right hand portion of the picture is a pegmatitic pyroxenite, with practically a vertical contact, representing the  edge of a MR “pothole”. On the left side of the ruler, we have a mottled anorthosite, filling in the centre of the “pothole”, with vague suggestions of normal horizontal layering, due to a latter period of crystal settling.

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

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

Figure 28 is an interpretative cross section along a diamond drill hole which intersected a different “pothole”, but I think helps to understand the situation. M3 and M2 are anorthosites that cover a normal MR, shown in pink at the upper section of the diagramme. Below that, the bore hole intersected another mottled anorthosite interpreted as the inner fill of the pothole. Next comes the MR horizon again, this time consisting of a very thin chromite seam. Following is a norite footwall below which we have the final segment of MR at the base of the “pothole”, and consisting of a rather thick chromite horizon very rich in platinum. Thus we have a situation indeed similar to an ordinary river pot hole with irregularities close to the bottom, where the heavier materials concentrate.

Very important as well is that, as logically expected, the footwall below the base of the “pothole” is not the same as the horizon under an ordinary MR, but rather a unit which is stratigraphically considerably lower. Some of these “potholes” actually cut down more than 5m through the presumably semi solid mush within the magma chamber.

Figure 28 - Diagrammatic interpretation of a “pothole” edge intersected by a surface diamond drill prospecting hole (Maricana, South Africa).

Figure 28 – Diagrammatic interpretation of a “pothole” edge intersected by a surface diamond drill prospecting hole (Maricana, South Africa).

Posted in Geology, Igneous Rocks | 2 Comments

1. Introduction

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 between 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 involved, thus Metamorphosing them. Further, the mechanisms involved and the distortion caused in 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 surface of the Earth.

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 plane.

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

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.

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