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Here we develop further models to explain how some of the diverse sedimentary rocks are related to each other. Furthermore, these sediments can be related to tectonics, depositional environments, sedimentary sequence position, sea level changes and the whole collection of processes by which sediments and environments systematically evolve downstream within ideal long and short depositional systems.
A diagram for the evolution of sedimentary rocks in both long and short systems is illustrated at this evolutionary diagram. The discussion begins with the long system and then examines a short system. The models are ideal, and makes some assumptions you should be aware of.
First Assumption: Sediments, sedimentary structures and sequences, and depositional environments all evolve in unison downstream. In the real world these may not evolve synchronously.
Second Assumption: The sourcelands are simple; they are uniform in composition, even if of mixed parent rocks, and do not have complex tectonic histories.
Third Assumption: Environments are found in their ideal state without significant transitions from environment to environment.
You may want to print out " The Evolution of Depositional Environments, Sedimentary Rocks, and Rock Sequences". Refer to it frequently while reading about the model; the model is as much visual as verbal. (Note that the model is in landscape mode and you will have to set your printer accordingly.)
The Long-System Model Upper Path on evlutionary diagram:
We begin the long-system model in the upper left with a tectonically active fault-block mountain built on a continental sourceland. On a continent the mountains are composed of felsic igneous rocks (e.g. granites, granodiorites, syenites). They undergo primarily mechanical weathering to produce coarse, angular weathering products. These angular fragments are deposited rapidly at the base of the mountain in alluvial fan environments during torrential rains, producing deposits of arkose (feldspathic) breccia. Because the sediments are dumped rapidly they form coarse grained, disorganized, matrix supported deposits called debris flows.
Some of the debris flow deposits get buried in the alluvial fan and become part of the geologic record. Most of the sediment continues to move downstream. During transportation the angular breccia fragments are abraded and rounded and the rock evolves into an arkose conglomerate. Arkose conglomerates are common in alluvial fan and braided river environments.
Transportation in a braided river is very different from a debris flow. In a braided river gravel and sand particles roll and bounce along the bottom in such a way that two characteristic types of bedding called L-Bars (gravel beds) and T-Bars (large planar cross beds) form. The gravel in the L-Bars is grain supported, meaning the cobbles, pebbles, boulders are touching and support each other. This is in contrast to the debris flow which are typically matrix supported with the gravel not touching and supported by the smaller particles in between.
Near the sourceland, in alluvial fan and braided river environments, energy is high and sediment moves by a combination of water pushing the grains and gravity helping the particles slide and role down steep gradients. The farther downstream however, the less the gradient, and the less gravity is able to help move the particles. More and more the primary energy moving the particles is the force of running water. Although this force is great during a flood and easily moves cobbles and boulders, during more normal flow the gravel is sorted out and left behind upstream. By the time the distal braided river is reached most of the gravel is gone (meaning L-Bars no longer form) and T-Bars made of coarse arkose sandstone dominate as the environmental energy falls.
By the distal braided river chemical weathering is replacing mechanical weathering as the major process. The feldspars so common in the arkose sediments upstream are decomposing to clays, and minerals in solution. The result is the sediments are evolving from feldspar rich arkose sandstones to more clay rich subarkose wackes. That is, as the feldspars weather they turn into clay, and as the amount of feldspar sand declines the relative amount of quartz sand increases.
Subarkose wackes in particular are common in the meandering river environment because meandering rivers form best from sediments with abundant silt and clay. Look at the point bar sequence typical of meandering rivers. Notice all the silts and clays at the top of the sequence which are deposited in the flood plain. Chemical weathering continues in the meandering river so that eventually all the felspar is gone and the sediments consists of only quartz and clay, a quartz wacke.
Thus, by the time the meandering river reaches the shoreline transitional environments we are dealing with just two undissolved products, quartz and clay. At the beach, or wave-washed mouth of a delta then the high energy of the waves continuously and efficiently sorts the sediment leaving the sand behind on or near the shore while the clay drifts offshore. (Note that silt is also a common component here, it consisting largely of QFL grains smaller than sand.) And since by this time weathering is complete the beach sands are quartz arenites, although some may still contain minor percentages of feldspar.
In this long system model, once we reach the beach we have also reached an epicontinental sea on a tectonically stable craton. Under these conditions the simple, ideal model for the evolution of sedimentary rocks is in force and the result is a quartz sandstone >> shale >> limestone sequence deposited on the beach, shelf, and far shelf environments. If the shelf is frequented by storms then the sediments are deposited in hummocky sequences.
The limestone deposited in the far shelf comes from CaCO3 dissolved in the river waters during the weathering of feldspars. Limestones (carbonates) typically do not form in the presence of clastics, such as sandstone and shale, and so are not deposited until the far shelf environment, beyond the reaches of shale deposition. Much or most of the carbonate is generated when organisms extract it from sea water to form their skeletons, although some, mainly micrites, may be biochemically precipitated.
We now switch to the lower left of the evolutionary model and begin the sediment evolution process with a complex sourceland of igneous, sedimentary and metamorphic rocks, such as are produced by a mountain building process. This sourceland would most likely contain feldspars, but the weathering of all the complexity of rocks here would produce mostly abundant lithic fragments so the model begins with a mechanically weathered, coarse grained lithic breccia.
But just as with the arkose breccia above, this sediment begins its transportation down a long system, undergoing all the transformations of sorting and chemical weathering that happen along the way. So the lithic breccia will evolve by rounding into a lithic conglomerate, and by sorting into a lithic sandstone, and by chemical weathering into a sublithic wacke, and finally to a quartz wacke deposited in a meandering river.
But again, as in the arkosic long system above, these final weathering products get dumped on the beach at the edge of an epicontinental sea on a tectonically stable craton. The final sorting and precipitating processes result in the quartz sandstone >> shale >> limestone sequence of the simple ideal model.
The point here is that it does not matter what kind of sourceland you start with, if all processes of weathering and sorting are allowed to go to completion the end products are always the same. In this sense the quartz sandstone, shale, and limestone of the simple ideal model are like attractors, and all sediments regardless of their starting composition are irresistibly " attracted" to these end products because they are the most stable outcomes of the sedimentary processes.
The Short-system Model
Short-systems form in tectonically active regions where high mountains are built very near the depositional basin (evolutionary model - lower left). Under these conditions, both the distance and the time to final deposition are short and systematic changes in texture, sorting, and particle maturity to end member compositions are not possible.
Short systems form under many circumstances, both continental and oceanic. A continental fault block mountain, like the one in the long-system model above, could be a short system if a sea existed at the base of the mountain. In the Cordilleran Orogeny both a short system and a long system are present.
In a short system the number of environments and distance from sourceland to basin is reduced And because these systems rapidly go from high mountains to deep basins the environments that do remain are often shorter than normal. As a result the mechanical energy in each environment is dissipated over a shorter distance making it more difficult for sorting processes to work efficiently. The result is coarser, more poorly sorted sediments. Also, chemical weathering does not have as much opportunity to work and so less clay is generated and quartz does not have an opportunity to increase much at the expense of feldspar and lithics. The end result is less mature sediments at each stage in the sequence.
Still, as in the long-system, the first depositional environment is the alluvial fan at the base of the mountain. The sediment will be coarse-grained, unsorted, unstratified, matrix supported debris flows of lithic (or arkosic) breccias. As the lithic breccia moves downstream the angular particles become rounded to form lithic (or arkosic) conglomerates. They are deposited mostly in grain supported gravel L-Bars of a braided river.
So far the short-system is similar to the long-system. Below the braided river, however, the short-system differs dramatically. There is no long, meandering river, or delta, or shelf environment, and all of the processes of sorting and weathering which takes place in these environments do not happen. Instead, the lithic (arkosic) conglomerates and lithic (arkosic) sandstones of the braided river are dumped directly onto the beach. This is no beach of white quartz sand; it is a gravel and coarse sand beach of dark lithic (feldspathic) particles.
Imagine being able to study samples of the braided river sands/conglomerates and beach sands/ conglomerates; how could you tell them apart? They may be deposited only a few dozen yards apart and will look virtually identical.
Offshore, perhaps only a few hundred yards or so from the beach, the floor of the sea drops away suddenly and steeply to a long underwater slope which descends perhaps thousands of feet to the basin floor below. Because the sourceland is close, large volumes of lithic (arkosic) sediment continue to pour onto the narrow beach, which cannot hold it all. Much of the sediment bypasses the beach to pour down the slope as turbidity currents forming a submarine fan environment.
The sediments in the submarine fan may be lithic conglomerates or sandy lithic conglomerates in the proximal fan, but distally they become (sub)lithic wackes. Finally on the ocean floor, beyond the outer edges of the fan, silts and shales, black in color from the low oxygen conditions, are deposited in the basin floor environment.
The most important message of the short-system is that all the processes of sediment evolution are cut short. As a result immature sediments are deposited in depositional environments which may be atypical in their characteristics (larger sizes, less maturity, etc.). By extension the longer the system the more it will begin to resemble the ideal long-system. Eventually even a sourceland supplying dominantly lithic sediments to a long system will, by the end of the long system, produce the classic quartz sandstone >> shale >> limestone sequence of the simple ideal model.
There is only two rocks remaining on the evolutionary chart we have not discussed. These are the wacke quartz conglomerates and quartz conglomerate near the top center of the chart. Quartz for all intents and purposes does not weather. Grains of quartz which are sand size eventually become sandstones, but what about large chunks of quartz, such as might be formed in a pegmatite (very coarse felsic igneous rock) or vein quartz precipitated by hydrothermal fluids. These will form quartz gravel. This quartz gravel is not going to go away, and in a complete long-model it is not likely to make it all the way to the beach. Yet quartz conglomerates are not that uncommon in the geologic record. What happens is, the quartz conglomerates just sits around, forever, waiting until the mountain is completely eroded to sea level (a peneplain). When all the terrestrial environments have disappeared the quartz gravel remains as a lag deposit (because it lags behind until everything else is gone). Initially it might be quartz mixed with clay, but as time goes by the clay is sorted out leaving behind pure quartz (sandy) conglomerates.
The information in this model is summarized with two deductive arguments:
First Deductive ArgumentFIRST PREMISE: The composition (the size, shape, sorting, and content) of a sedimentary rock is largely dependent on the tectonic regimes in which the sediment forms and the depositional environments in which it was deposited.
SECOND PREMISE: Environments evolve in systematic and predictable ways from sourceland to basin floor in each tectonic regime.
CONCLUSION: Therefore, the compositional and textural characteristics of a sediment change in corresponding systematic and predictable ways from the sourceland.
Sediment texture and composition are rock properties which help determination the geologic history of a basin. Sediment textures, sedimentary structures, and structural sequences shed light on the nature of the transporting and sorting agents, and provide clues to the final environment of deposition.
Second Deductive ArgumentFIRST PREMISE: Sedimentary structures and sequences of sedimentary structures found in a sedimentary rock are determined by the processes characteristic of each particular depositional environment.
SECOND PREMISE: Depositional environments evolve in systematic and predictable ways downstream.
CONCLUSION: Therefore, sedimentary structures and rock sequences of structures change in corresponding systematic and predictable ways from the sourceland to the basin.
|Last Update: 10/26/00||