SEDIMENTARY ROCKS
Sedimentary Rocks are formed by the accumulation and subsequent consolidation of sediments into various types of rock. The key is the sediments. Sediments are unconsolidated material and have different origins. Ultimately, the origin of these sediments is the weathering, erosion and/or the chemical breakdown of other rocks. These "other" rocks could be igneous, metamorphic or even other sedimentary rocks. The type and size of the sediments and how they are formed will lead to the classification of the different sedimentary rocks listed below. There are three main subclasses of sedimentary rocks. They include biochemical, clastic and evaporative sedimentary rocks. Biochemical sedimentary rocks are formed from organic processes that involve living organisms producing the sediments. These living organisms can be snails and clams whose discarded calcium carbonate shells can form limestone. But it also includes swamp plants whose organic debris can produce coal if conditions are right. Although the origin of the sediments are organic, most of the chemicals that the living organisms used to produce their shells or their body parts have origins from previous rocks. Thus these are sedimentary rocks, but with a biogenic intermediate so to speak. Clastic sedimentary rocks are sometimes considered true sedimentary rocks as they are composed directly of the sediments or fragments from other rocks. These fragments are called clasts, hence the term clastic. Generally they are further defined by the size of their clasts which range from nearly boulder size in the coarsest conglomerates and breccias to very fine grain size (<0.004 mm) in shale. Often clastic rocks can have their clasts analyzed and the original source rocks can be determined if the clasts have not been moved too far from the source or have not been worked and reworked into sedimentary rock after sedimentary rock. Glacial till from a large continental glacier can be analyzed and the origin of the glacier's debris could be determined and therefore the possible path of the glacier could be known. Evaporative or chemical sedimentary rocks are formed from the generally inorganic deposition of chemicals, usually through evaporation of a chemical rich solution. These chemicals generally have their origin from the chemical weathering of other rocks or other sediments. The sodium and chlorine in halite (salt) comes from the sodium and chlorine in other rocks that have dissolved over the years. Unlike clastic sedimentary rocks the direct origin of the chemicals is rarely easy to identify. The chemicals could come from magma deep in the crust of the Earth, rocks that dissolved in the ocean billions of years ago or from an outcrop in the hillside next to a playa lake. Sometimes the origin can be figured out and sometimes there is no way to know the originating source. No matter what type of sedimentary rock, water is almost always a key component. The only real exception to this is desert wind blown sediments. All other rock types involve water is some way and generally in an important way. The biochemical rocks come from water born organisms. Clastic rocks are usually water transported, sorted and deposited. Evaporative rocks are of course derived from chemicals dissolved in water. Where there is water, there are sedimentary rocks being formed. Sedimentary geologists must have an understanding of hydrology in order to understand their subjects. Another common factor to sedimentary rocks is that they originate on the surface of the Earth unlike most igneous and metamorphic rocks which originate in the interior of the Earth's crust. Geologists can actually see many sedimentary rocks form or at least see the sediments that will become sedimentary rocks prior to their lithification. The lithification of the sediments is usually accomplished by a
cementing agent. Once the sediments are no longer loose sediments, but cemented
together grains or crystals, they become a rock. What happens to sediments
from this stage on is called diagenesis. Diagenesis is important to study
in sedimentary rocks. It includes the study of the compaction of the rock,
physical conditions, chemical alterations and biological interactions. Cementation
is usually the first aspect of diagenesis, but cementation can be episodic,
reversed and recementation can occur. Compression of the rocks can change
the banding and increase chemical alterations. Chemicals can leave or enter
a rock through pore space waters and minerals can crystallize or dissolve.
Temperature increases can alter the characteristics of the rock as well.
If too much heat and pressure occur during diagenesis then the rock may wander
into the regime of metamorphism. At times geologist will argue over the
boundary between a highly diagenetically altered sedimentary rock and a low
grade metamorphic rock. Diagenesis is extremely important in understanding
the history and the resulting effects on the characteristics of sedimentary
rocks.
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