Sedimentary Rock Evolution


 

Sedimentary Rock Evolution 
 
The "Introduction To Sedimentary Rocks" presents an ideal model illustrating the origin of the three most abundant kinds of sedimentary rock: Quartz Sand, Shale, And Limestone. Sedimentary particles and rocks, however, are much more diverse than just these three main types- see the Alphabetical List Of Sedimentary Rocks. Environmental models explain how some of these sedimentary rocks are related to each other. 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 models are ideal, and make some assumptions:
  • 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.

~The Long-System Model - Upper Path on Evolutionary Diagram:~
 
     
Alluvial Fan & Debris Flow
The pathway for the long-system model begins (Upper Left) with a tectonically active fault-block mountain on a continental sourceland. The mountains are composed of Felsic Igneous Rocks (e.g. Granites, granodiorites, Syenites). They undergo primarily mechanical weathering to produce coarse, angular fragments and these 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 flow deposits. 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. 
 
Near the sourceland of alluvial fan environment, energy is high and sediment moves through a combination of water and gravity which moves particles down steep gradients by rolling and sliding. Further downstream, the gradient decreases, and particles cannot move as readily and so the primary energy moving the particles becomes the force of running water. Although this force is great during a flood and easily moves cobbles and boulders, during normal to base flows the larger gravel is sorted out and deposited in the upstream reaches. 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. 
 
Braided Stream 
By the time the stream transforms into a Distal Braided River, most of the gravel is gone (meaning L-Bars no longer form) and T-Bars made of coarse Arkose Sandstone dominate corresponding with lower energy.Sediment transport in a braided river differs from transport in 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 where gravel is supported by the smaller particles in between. Chemical weathering replaces mechanical weathering as the major weathering agent in a braided river environment. The feldspars common in the arkose sediments upstream decompose to Clays and minerals in solution such that sediments evolve 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. 
     
Meandering Stream 
As energy and gradient decrease further, the stream transitions into a meandering river environment. Subarkose wackes are common in the meandering river environment because meandering rivers. Chemical weathering dominates in the meandering river so that eventually all the felspar is gone and the sediments consist of Quartz and Clay- a Quartz wacke.
   
Coastal 
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, the high energy of the waves continuously and efficiently sort the sediment leaving the sand behind on or near the shore while the Clay drifts offshore. Weathering is complete and the beach sands are Quartz Arenites, although some may still contain minor percentages of feldspar.
     
Under these conditions the Simple, Ideal Model for the evolution of sedimentary rocks is in effect and the result is a Quartz Sandstone >> Shale >> Limestone sequence deposited on the beach, shelf, and far shelf environments, respectively. 
     
Shelf 
The Limestone deposited in the far shelf is derived 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. 

~Lower Path~   
 
In the lower path of the Evolutionary Model, sediment evolution process begins with a complex sourceland of igneous, sedimentary and metamorphic rocks. This sourceland would most likely contain Feldspars, but the weathering of the diverse rocks would produce mostly abundant Lithic Fragments. Therefore, the model begins with a mechanically weathered, coarse grained Lithic Breccia
     
Similar to the long pathway, sediment begins its transport down a steep gradient, undergoing sorting and chemical weathering in transit. The lithic Breccia will evolve by rounding into a Lithic Conglomerate, followed by sorting into a Lithic Sandstone, followed by chemical weathering into a sublithic wacke, and finally to a Quartz wacke deposited in a meandering river. The final sorting into the epicontintental sea results in the Quartz Sandstone >> Shale >> Limestone sequence of the simple ideal model.

It does not matter the type of sourceland the sediment comes from; if all processes of weathering and sorting are allowed to go to completion the end products are always the same. The Quartz Sandstone, Shale, and Limestone end products 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 the sediment passes through are short-lived.  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 lesser 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.
     
There are typically no meandering river or coastal environments so the lithic (arkosic) Conglomerates and lithic (arkosic) sandstones of the braided river are dumped directly onto the beach, constructing a beach made of gravel and coarse sand of dark lithic (feldspathic) particles. Because the sourceland is close, large volumes of lithic (arkosic) sediment continue to pour onto the narrow beach, but much of the sediment bypasses the beach to travel down the continental 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.

Wacke Quartz Conglomerates and Quartz Conglomerate are not that uncommon in the geologic record. Quartz Conglomerates are formed when all the terrestrial environments have been eroded and transported out of the environment and large quartz bodies (pegmatites) remain. The Quartz may be eroded down to gravel-size, but it remains as a lag deposit. Initially it might be Quartz mixed with Clay, but as time goes by the Clay is sorted out leaving behind pure Quartz (sandy) Conglomerates.

~Conclusions~
 
First Deductive Argument
 
FIRST 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 environment in which it was deposited.

SECOND PREMISE: Environments evolve in systematic and predictable ways from sourceland to Basin floor in each tectonic regime. 

CONCLUSION: The compositional and textural characteristics of a sediment change in corresponding systematic and predictable ways from the sourceland to the basin.
 
Second Deductive Argument 
 
FIRST 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: Sedimentary Structures and rock sequences of structures change in corresponding systematic and predictable ways from the sourceland to the basin.
 
Contributed by Lynn Fichter 
Thursday, October 16, 2014
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