With very few exceptions, materials that have been weathered and/or eroded, are transported elsewhere and eventually settle. Sedimentation is the processes to which these materials are subjected to until they are deposited, and transport is the main mechanisms involved. This transportation can be done either in solution or in the solid state. In the case of solutions, only water is relevant, but these sediments are not very common and will be discussed under Non-Clastic Sediments (item 6.5). The other materials occur as fragments, termed clasts and, depending on their size, are transported either in suspension, or, if they are considerably larger and/or heavier, they move by saltation. That is, in a series of short intermittent hops close to the ground. Depending on the transporting mechanism, the clastic sediments are subdivided into:
Terrestrial – Aeolian; Glacial; Aluvial; Delta/Estuary
Marine – Wave Dominated; Tide Dominated; Bathyal/Abyssal.
6.1 Aeolian Environment
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).
Aeolian deposits consist of dunes formed by rather fine grained sand and characteristically have large scale cross-bedding, which refers to the internal arrangement of the sedimentary layers 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.
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.
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 are:
Dome-shaped dunes, consisting of low, circular, isolated mounds;
Transverse dunes, consisting of almost straight sand ridges at about right angles to the predominant winds;
Longitudinal dunes, also consisting of long sand ridges, but this time oriented along the vector resulting from two converging wind directions;
Barchan dunes, have a crescent shape with the horns extending downwind.
6.2 Glacial Environment
There is very little to say about sedimentation related to glaciation, since most of the associated deposits can be better fitted under the alluvial 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),
and a rather small one in Portugal (fig. 93).
6.3 Alluvial Environment
This is the general term for the depositional environments related to the flow of water on land.
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 represents 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, with figure 94 showing 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).
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),
as well as clast roundness, and they tend to occur within relatively long confined channels (fig. 98).
Fluvial depositional environments refer to sediments related to rivers that are past the transient stage.
220.127.116.11 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).
Detailed observation of this type of environment shows quite nicely the relationship between adjoining bars of coarser and finer clasts (fig. 99B).
Naturally this alluvial high energy regime is also a sorting mechanism, where the most important factor is mass rather than volume and an obvious example is shown in figure 100 where we have large well rounded quartz pebbles with an SG of 2.6, associated with much smaller, also well rounded, pebbles of pyrite (buck shot), with a significantly higher SG of 5.0. Gold, with an even higher SG of 18, consequently needing much smaller dimensions is also present. In fact, it is seldom visible by the naked eye. In this case though, if my memory does not fail me, at the bottom right hand corner of the picture, those small shiny specks are gold. Returning to figure 99, it is 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).
The meandering stage of the river is when high levels of deposition really begin. Meandering starts developing with the reduction of the water velocity, where the heavier gravel and sand start to deposit, preferentially at the inner side of the bends, where the speed is least, thus forming point bars which, with time enhances those bends. The sand concentration is quite distinct in figure 102.
During flood periods, the river overflows its banks, dramatically reducing the flow speed and depositing the very fine silts and muds in suspension, giving rise to rich agricultural soils over the flood plains, also quite distinctly seen.
Hence, these marshy areas contain predominantly silts and muds which, during the dry season desiccate, forming very characteristic mud cracks. Figure 102B, shows a present day example, where mud cracks had started to develop during a previous dry period, but the area was again flooded in the rainy season
and figure 102C, shows consolidated mud cracks as well as casts.
A magnificent text book example though, is the one seen in figure 102D,
where even the actual curling at the edges of the mud crack slabs was preserved (fig. 102E). As for the slight ripples along the edges and the indentations at the centre of some of the mud plates, I assume they were caused by rain drops, just before there was a new inflow of sediments which covered and thus preserved them.
Finally, under this kind of fertile flood plane 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 type of environment can be seen where there is a significant tidal variation, because the mud-flats get exposed (fig. 102F).
When these sedimentary layers are preserved, the signs of the various types of animal burrowing gives very distinct characteristics to those horizons that are termed bioturbated beds (fig. 102G).
6.4 Marine Environment
Under sedimentation, we are interested in prograding coast lines, that is, where sedimentation is taking place, with a consequent shore accretion, Figure 73B shows very nicely a prograding situation with a well defined on-lap phase. As for our example of the Catumbela River in Angola, it is impressive that even though the river is quite large and carries vast quantities of water, its mouth is actually parallel to the cost, pointing northwards (fig. 103).
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).
Due to an initial coastal reentry, the pro-graded area became quite wide and that is where Lobito developed (fig. 105),
with the harbor having been formed by the natural extension of the off shore bar created by the Catumbela river sediments (fig. 106).
A very conspicuous feature of these arenaceous coastal environments is the development of ripple marks. The example shown in figure 106A is rather striking because the lower bedding plane has current 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 geological time. Significant too is the orientation difference between the two sets of ripples.
Also, not noticeable in figure 106B, is the fact that a section across a sandstone formed by the progressive development of ripple marks shows crossbedding with the same visual characteristics as the ones developing in aeolian dunes (fig. 91), but with a much smaller scale. The example I present has the crossbedding unusually enhanced by pyrite clasts (fig. 107). It is important to note that the scale difference between these two crossbedding structures represents hydrodynamic settings that are not comparable.
Finally, as already mentioned, these environments are very rich in animal life and the accumulation of mollusk shells often give rise to bioclastic limestones (fig. 108).
6.4.2 Shallow Marine
Going away from the coast but still within the continental shelf, in areas with a tropical climate, there is the formation of fossiliferous limestones consisting primarily of coral skeletons originating from the barrier reefs (fig. 109)
Further away from the coast, outside the continental shelf, very little deposition takes place on the sea floor, other than the very slow accumulation of dead plankton and the skeletons of deep water dwellers. Some of the plankton inhabitants have silicious shells and others as well as some algae, produce minute amounts of calcium carbonate. The accumulation of these products on the sea floor often consolidate forming a succession of alternating layers of biogenic limestone and chert. Such a succession is the absolute proof of deposition in a pelagic environment.
A very good example of these limestones is the thick succession of the Transvaal Dolomite in South Africa, that contains very abundant inter-layered chert bands (fig. 110). For diamond drilling purposes this is absolute agony, because the very sharp alternation between the relatively soft limestone and the thin but very hard chert bands, destroys the drilling crowns in no time at all.
6.5 Non-Clastic Sediments
These are sedimentary rocks formed by the precipitation of chemical compounds carried in solution by the water. Naturally the components of these rocks will be those that are most soluble, like salt, gypsum, chert and calcite.
6.5.1 Chemical Precipitates
This is the general term for rocks originating from the precipitation of the substances under saturated solutions. Of these, the most abundant by far, are limestones which are constituted almost entirely by calcite. They are generally fine grained and range in colour from almost white, when very pure, to varying shades of gray depending on the quantity of organic matter present.
Often these limestones contain levels rich in chert nodules that originate from a diagenetic silicification within the rock in which they occur (fig. 111). In other words, these chert nodules represent a post depositional modification with no significance relative to the identification of the depositional environment, as opposed to the biogenetic assemblages mentioned above.
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 (fig. 112 ). It has a fan shape and is being built by a stream falling over a Northern Cape dolomite ridge. The water flowing over the ridge is saturated with the calcite it dissolved, and than precipitates covering every surface along which the water runs, thus causing the growth of the fan.
A closer view of the continuously growing ridge caused by the accumulation of calcite along the stream is shown in figure 113.
In fact, one can see growing grass stems being coated by calcite (fig. 114).
Diamond drill core cutting through the fan sequence proves that its development was caused by the continuous coating of whatever materials over which the saturated waters flowed (fig. 116).
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. 117).
Calcite and gypsum may also form in desert pools, 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. 118).
It is under these conditions that the gypsum crystals known as desert roses form (fig. 119).
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. 120),
since it forms a rather solid rock with a very high content of iron and very little else (fig. 121). When aluminium predominates we have bauxite. 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.
Also, 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. 122),
with predominantly a pisolitic texture (fig. 123)
When, due to erosion these iron rich layers are exposed, they form “crusts” because they are 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.
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. 124). 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.