Introducion:
The geologic processes operating on Earth’s surface produce only subtle changes in the landscape during a human lifetime, but over a period of tens of thousands or millions of years, the effect of these processes is considerable. Given enough time, the erosive power of the hydrologic system can reduce an entire mountain range to a featureless lowland. In the process, the eroded debris is transported by rivers and deposited as new layers of sedimentary rock.
A series of sedimentary rock layers may be thousands of meters thick. When exposed at the surface, each rock layer provides information about past events in Earth’s history. Such is the case in the Moenkopi Formation of southern Utah shown in the panorama above.
The various shades of red and white occur in the thin beds of siltstone and mudstone deposited on an ancient tidal flat about 220 million years ago. Thin layers of siltstone and shale each contining ripple marks, mudcracks, and rain imprints combine to tell the history recorded in the rock now exposed in this colorful cliff. The record of Earth’s history preserved in sedimentary rocks is truly remarkable. Each bedding plane is a remnant of what was once the surface of Earth. Each rock layer is the product of a previous period of erosion and deposition. In addition, details of texture, composition, and fossils are important records of global change, showing how Earth evolved in the past and how it may change in the future. To interpret the sedimentary record correctly, we must first understand something about modern sedimentary systems, the sources of sediment, transportation pathways, and places where sediment is accumulating today, such as deltas, beaches, and rivers. The study of how modern sediment originates and is deposited provides insight into how ancient sedimentary rocks formed. Fossils preserved in sedimentary rocks not only reveal the environment of deposition but also the pace and course of evolution through Earth’s long life.
Apart from their scientific significance, the sedimentary rocks have been a controlling factor in the development of industry, society, and culture. Humans have used materials from sedimentary rocks since the Neolithic Age; flint and chert played an important role in the development of tools, arrowheads, and axes. The great cathedrals of Europe are made from sedimentary rock, and the statues made by the artists of ancient Greece and Rome and during the Renaissance would have been impossible without limestone. Fully 85% to 90% of mineral products used by our society come from sedimentary rocks. Virtually our entire store of petroleum, natural gas, coal, and fertilizer come from sedimentary rocks. Sand, gravel, and limestone are the raw materials for cement. Sedimentary rocks are also important reservoirs for groundwater, and host important deposits of copper, uranium, lead, zinc, as well as gold and diamonds.
Major Concepts:
1.Sedimentary rocks form at Earth’s surface by the hydrologic system. Their origin involves the weathering of preexisting rock, transportation of the material away from the original site, deposition of the eroded material in the sea or in some other sedimentary environment, followed by compaction and cementation.
2. Two main types of sedimentary rocks are recognized: (a) clastic rocks and (b) chemically precipitated rocks, including biochemical rocks.
3. Stratification is the most significant sedimentary structure. Other important structures include cross-bedding, graded bedding, ripple marks, and mud cracks.
4. The major sedimentary systems are (a) fluvial, (b) alluvial-fan, (c) eolian, (d) glacier, (e) delta, (f) shoreline, (g) organic-reef, (h) shallow-marine,
(i) submarine fan, and (j) deep-marine.
5. Sedimentary rock layers can be grouped into formations, and formations can be grouped into sequences that are bound by erosion surfaces. These
formations and sequences form an important interpretive element in the rock record.
6. Plate tectonics controls sedimentary systems by creating uplifted source areas, shaping depositional basins, and moving continents into different climate zones.
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