Stratigraphy

Stratigraphy is the science of rock layering, with particular concern for composition, geographic distribution, and geological and chronological importance. This discipline also involves the interpretation of rock strata in terms of mode of origin and geologic history. As a main branch of sedimentology, stratigraphy generally relates the large-scale vertical and lateral similarities between units of rock layering to environment of deposition. Those relationships are defined [Page 2133]by lithologic and physical properties, geographic position, distribution, paleontological characteristics, and age relationships. Stratigraphers, using these properties and details of a sediment's composition, structure, and texture can then synthesize aspects of environmental geology and, moreover, interpret the broader aspects of Earth's geologic history.

Any stratigraphic investigation only becomes productive with a proper understanding of sedimentology, as stratigraphy is more a progeny than sibling of sedimentology. Formally, sedimentology is the study of natural sediments, both lithified (sedimentary rocks) and unlithified, and of the processes by which they form. Sedimentology includes all processes that give rise to sediment or modify it after deposition. These processes may include weathering, which breaks up or dissolves preexisting rocks; transportation; deposition; and diagenesis, which chemically and physically modifies sediment after deposition and burial, converting it into sedimentary rock. Sediments such as mud, sand, and gravel deposited by mechanical processes are known as clastic sediments, whereas those deposited predominantly by chemical or biological processes (limestones, dolomites, rock salt, chert) are known as chemical sediments.

Sedimentologists classify sedimentary rocks according to origin and size of included particles. However, the vast array of conditions by which sediments accumulate has borne a great number of sedimentary classification schemes giving rise to hundreds of sedimentary types, which invariably overlap. To complicate this system, most sedimentary rocks are, at least in part, made of several types of sedimentary rocks.

In order to make greater sense of the complexities of sedimentary relationships, classifications and hierarchal organizations were produced to give clarity and uniformity to stratigraphic analysis. Lithostratigraphic units in descending order are the supergroup, group, formation, member, and bed or stratum. The formation is the fundamental unit of stratigraphic geology, just as the cell is fundamental to biology. A formation must be lithologically distinct and large enough in scale as to be mappable. These classifications are defined strictly on the basis of lithology. Boundaries between units, including formations, are placed at the position of distinctive rock change or a distinctive erosional surface. In this manner, the divisions between units have the least possible ambiguity.

All rocks, whether metamorphic, igneous, or sedimentary, weather at the surface of the Earth. Metamorphic and igneous rocks are formed at temperatures and pressures that are not seen at the surface so when these rocks are brought to the surface they become unstable. In the absence of air, most minerals in these rocks would remain intact for millions of years; however, because of Earth's climate, water, and atmosphere, these minerals break down and convert in a predictable sequence. The new minerals that will eventually form sedimentary rocks are stable at lower temperatures and pressures and are very slow to convert.

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