Sedimentary Rocks and Sedimentary Structures:
How to 'Read' Sedimentary Rocks
Sedimentary rocks fall into three general categories base on origin and composition:
These rocks are composed of fragments of pre-existing rocks. The fragments are classified by their size (see below). The fragments may be composed of single minerals, such as clay grains, mica grains, quartz grains, or feldspar grains. However, they may also be fragments of rocks, such as shale clasts, granite pebbles, etc.
Bioclastic rocks are composed of organic debris such as plants (lignite, coal) or shells (coquina, fossiliferous limestone).
Chemical rocks precipitate from water. These include carbonates (limestone and dolomite), evaportites (halite, gypsum, anhydrite), and chert (SiO2).
Textures of Sedimentary Rocks
Most sedimentary rocks are derived by processes of weathering, transportation, deposition, and diagenesis. The final texture (grain size, shape, sorting, mineralolgy, etc.) in a sediment or sedimentary rocks is dependent on process that occur during each stage. The points below summarize the basic factors affecting rock texture.
Because of their detrital nature, any mineral can occur in a sedimentary rock. Clay minerals, the dominant mineral produced by chemical weathering of rocks, is the most abundant mineral in mudrocks. Quartz, because it is stable under conditions present at the surface of the Earth, and because it is also a product of chemical weathering, is the most abundant mineral in sandstones and the second most abundant mineral in mudrocks. Feldspar is the most common mineral in igneous and metamorphic rocks. Although feldspar eventually breaks down to clay minerals and quartz, it is still the third most abundant mineral in sedimentary rocks. Carbonate minerals, either precipitated directly or by organisms, make up most biochemical and chemical sedimentary rocks, but carbonates are also common in mudrocks and sandstones.
The longer a mineral is in the weathering and transportation cycles of sedimentary rock forming processes, the more likely it is to break down to a more stable mineral or disappear altogether. Thus, we can classify sediments on the basis to which they have achieved mineralogical maturity.
Sorting refers to the uniformity of grain size in a sediment or sedimentary rock. Particles become sorted on the basis of density because of the energy of the transporting medium. High energy (high velocity) currents can carry larger fragments. As the energy or velocity decreases, heavier particles are deposited and lighter fragments continue to be transported. This results in sorting due to density. If the particles have the same density, such as all grains of quartz, then the heavier particles will also be larger, so the sorting will take place on the basis of size. We can classify this size sorting on a relative basis: well-sorted to poorly-sorted.
Beach sands and dune sands tend to be well-sorted because the energy of the waves or wind is usually rather constant. The coarser grained sediment is not carried in because the wave or wind velocity is too low to carry such large fragments, and the finer grained sediment is kept in suspension by the waves or wind.
Mountain streams, because they have many turbulent eddies where the velocity of the stream changes suddenly usually show poorly-sorted sediment on the bottom of the stream channel. Similarly, glacial till, because it is deposited in place as glacial ice melts, and is not transported by water, tends to show poor sorting.
During the transportation process, grains may be reduced in size due to abrasion. Random abrasion results in the eventual rounding off of the sharp corners and edges of grains. Thus, the degree of rounding of grains gives us clues to the amount of time a sediment has been in the transportation cycle. Rounding is classified on relative terms as well. Note that rounding is not the same a sphericity. Sphericity is controlled by the original shape of the grain. The longer the sediment is transported, the more time is available for grains to lose their rough edges and corners by abrasion.
Structures of Sedimentary Rocks
The process of deposition usually imparts variations in layering, bedforms, or other structures that give clues to environment in which deposition occurs. Such things as water depth, current velocity, and current direction can be sometimes be determined from sedimentary structures. Thus, it is important to recognize various sedimentary structures so that we can interpret the clues that they offer to these conditions. Also, because sedimentary rocks can be deformed by folding and faulting long after the deposition process has ended, it is important to be able to determine which was up in the rock when it was originally deposited, especially if one is going to use the rocks to determine the sequence of events that occurred or interpret the geologic history of an area. Features that tell us which way was up are often referred to as top and bottom indicators.
Stratification and Bedding
1) Layering (bedding)
Layering is usually described on the basis of layer thickness as per the table below. Other distinctive types of layering are described below.
2) Cross Bedding
Consists of sets of beds that are inclined relative to one another. The beds are inclined in the direction that the wind or water was moving at the time of deposition. Boundaries between sets of cross beds usually represent an erosional surface. Cross bedding is very common in beach deposits, sand dunes, and river deposited sediment. Individual beds within cross-bedded strata are useful indicators of current direction and tops and bottoms. Note how the beds become asymptotic to the lower boundary on which they were deposited.
3) Graded Bedding
As current velocity decreases, the larger or more dense particles are deposited first, followed by smaller particles. This results in bedding showing a decrease in grain size from the bottom of the bed to the top of the bed. This gives us a method for determining tops and bottoms of beds, since reverse grading will not be expected unless deposition occurs under unusual circumstances. Note that reverse graded bedding cannot occur as current velocity increases, because each layer will simply be removed as the current achieves a velocity high enough to carry sediment of a particular size.
These Sedimentary structures tell us about water currents, wind direction, and climate conditions.
1) Ripple Marks
Ripple marks are characteristic of shallow water deposition. They are caused by waves or winds piling up the sediment into long ridges.
Asymmetrical ripple marks can give an indication of current direction when formed in water, and when formed by wind, give wind direction.
Symmetrical ripples form when the water moves back and forth. Symmetrical ripple marks occur in environments where there is a steady back and forth movement of the water, such as tidal action.
These structures result from the drying out of wet sediment at the surface of the Earth. The cracks form due to shrinkage of the sediment as it dries. In cross section the mudcracks tend to curl up, thus becoming a good top/bottom indicator. The presence of mudcracks indicates that the sediment was exposed at the surface shortly after deposition, since drying of the sediment would not occur beneath a body of water.
Casts and Molds
Any depression formed on the bottom
of a body of water may become a mold for any
sediment that later gets deposited into the depression. The body of
sediment that takes on the shape of the mold is referred to as a
Tracks and Trails
These features result from organisms moving across the sediment as they walk, crawl, or drag their body parts through the sediment.
Any organism that burrows into soft sediment can disturb the sediment and destroy many of the structures discussed above. If burrowing is not extensive, the holes made by such organisms can later become filled with water that deposits new sediment in the holes. Such burrow marks can be excellent top and bottom indicators.