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
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.
Importance of sedimentary rocks
Sedimentary rocks provide a multitude of products which modern and ancient society has come to utilise.
Art:
marble, although a
metamorphosedlimestone, is an example of the use of sedimentary rocks in the pursuit of aesthetics and art
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 Our understanding of the extent of these aquifers and how much water can be withdrawn from them depends critically on our knowledge of the rocks that hold them (the reservoir).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
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.
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