Sedimentary rocks cover up to 75% of the Earth's surface, record much of the Earth's history, and harbor the fossil record. Sedimentology is closely linked to stratigraphy, the study of the physical and temporal relationships between rock layers or strata.
The premise that the processes affecting the earth today are the same as in the past is the basis for determining how sedimentary features in the rock record were formed. By comparing similar features today to features in the rock record—for example, by comparing modern sand dunes to dunes preserved in ancient aeolian sandstones—geologists reconstruct past environments.
Sedimentary rock types
Middle Triassic marginal marine sequence of siltstones and sandstones, southwestern Utah.
There are four primary types of sedimentary rocks: clastics, carbonates, evaporites, and chemical.
Clastic rocks are composed of particles derived from the weathering and erosion of precursor rocks and consist primarily of fragmental material. Clastic rocks are classified according to their predominant grain size and their composition. In the past, the term "Clastic Sedimentary Rocks" were used to describe silica-rich clastic sedimentary rocks, however there have been cases of clastic carbonate rocks. The more appropriate term is siliciclastic sedimentary rocks.
Organic sedimentary rocks are important deposits formed from the accumulation of biological detritus, and form coal and oil shale deposits, and are typically found within basins of clastic sedimentary rocks
Evaporites are formed through the evaporation of water at the Earth's surface and most commonly include halite or gypsum.[5]
Chemical sedimentary rocks, including some carbonates, are deposited by precipitation of minerals from aqueous solution. These include jaspilite and chert.
Energy: petroleum geology relies on the capacity of sedimentary rocks to generate deposits of petroleumoils. Coal and oil shale are found in sedimentary rocks. A large proportion of the world's uranium energy resources are hosted within sedimentary successions.
Groundwater: sedimentary rocks contain a large proportion of the Earth's groundwater aquifers. Our understanding of the extent of these aquifers and how much water can be withdrawn from them depends critically on our knowledge of the rocks that hold them (the reservoir).
Basic principles
Heavy minerals (dark) deposited in a quartz beach sand (Chennai, India).
The aim of sedimentology, studying sediments, is to derive information on the depositional conditions which acted to deposit the rock unit, and the relation of the individual rock units in a basin into a coherent understanding of the evolution of the sedimentary sequences and basins, and thus, the Earth's geological history as a whole.[citation needed]
The scientific basis of this is the principle of uniformitarianism, which states that the sediments within ancient sedimentary rocks were deposited in the same way as sediments which are being deposited at the Earth's surface today.[citation needed]
Sedimentological conditions are recorded within the sediments as they are laid down; the form of the sediments at present reflects the events of the past and all events which affect the sediments, from the source of the sedimentary material to the stresses enacted upon them after diagenesis are available for study.[citation needed]
The principle of superposition is critical to the interpretation of sedimentary sequences, and in older metamorphic terrains or fold and thrust belts where sediments are often intensely folded or deformed, recognising younging indicators or graded bedding is critical to interpretation of the sedimentary section and often the deformation and metamorphic structure of the region.[citation needed]
Folding in sediments is analysed with the principle of original horizontality, which states that sediments are deposited at their angle of repose which, for most types of sediment, is essentially horizontal. Thus, when the younging direction is known, the rocks can be "unfolded" and interpreted according to the contained sedimentary information.
The principle of lateral continuity states that layers of sediment initially extend laterally in all directions unless obstructed by a physical object or topography.
Centripetal desiccation cracks (with a dinosaur footprint in the center) in the Lower Jurassic Moenave Formation at the St. George Dinosaur Discovery Site at Johnson Farm, southwestern Utah.
The methods employed by sedimentologists to gather data and evidence on the nature and depositional conditions of sedimentary rocks include;
Measuring and describing the outcrop and distribution of the rock unit;
Describing the rock formation, a formal process of documenting thickness, lithology, outcrop, distribution, contact relationships to other formations
Mapping the distribution of the rock unit, or units
Petrology and petrography; particularly measurement of texture, grain size, grain shape (sphericity, rounding, etc.), sorting and composition of the sediment
The longstanding understanding of how some mudstones form has been challenged by geologists at Indiana University (Bloomington) and the Massachusetts Institute of Technology. The research, which appears in the December 14, 2007, edition of Science, counters the prevailing view of geologists that mud only settles when water is slow-moving or still, instead showing that "muds will accumulate even when currents move swiftly." The research shows that some mudstones may have formed in fast-moving waters: "Mudstones can be deposited under more energetic conditions than widely assumed, requiring a reappraisal of many geologic records."[6]
Macquaker and Bohacs, in reviewing the research of Schieber et al., state that "these results call for critical reappraisal of all mudstones previously interpreted as having been continuously deposited under still waters. Such rocks are widely used to infer past climates, ocean conditions, and orbital variations."[7]
Considerable recent research into mudstones has been driven by the recent effort to commercially produce hydrocarbons from them as unconventional reservoirs, in both the shale gas and tight oil (or Light Tight Oil) plays.[8]
Recent research by an Australian sedimentologist, Dutkiewicz, has described how geocirculation is related to global temperatures and climate change. The research described carbon and water circulation, and impacts of heat on current and future capacity of carbon capture by the ocean. [9]
^Raymond Siever, Sand, Scientific American Library, New York (1988), ISBN0-7167-5021-X.
^Georges Millot, translated [from the French] by W.R. Farrand, Helene Paquet, Geology Of Clays - Weathering, Sedimentology, Geochemistry Springer Verlag, Berlin (1970), ISBN0-412-10050-9.
^Gary Nichols, Sedimentology & Stratigraphy, Wiley-Blackwell, Malden, MA (1999), ISBN0-632-03578-1.
^Donald R. Prothero and Fred Schwab, Sedimentary Geology: An Introduction to Sedimentary Rocks and Stratigraphy, W. H. Freeman (1996), ISBN0-7167-2726-9.
^Edward J. Tarbuck, Frederick K. Lutgens, Cameron J. Tsujita, Earth, An Introduction to Physical Geology, National Library of Canada Cataloguing in Publication, 2005, ISBN0-13-121724-0
