Sedimentology
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Sedimentary rocks cover most 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.
Sedimentology encompasses the study of modern sediments and the processes that deposit those sediments. It also compares these observations to studies of ancient sedimentary rocks. Sedimentologists apply their understanding of modern processes to historically formed sedimentary rocks, allowing them to understand how they formed.
Uniformitarian geology, 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 finding similar features today- sand dunes in the Sahara, or Great Sand Dunes National Park near Alamosa, Colorado, U.S.A., the ancient sandstones, for example the Wingate Sandstone of Utah and Arizona of the southwest U. S. A. can be determined to have formed from eolian (wind) deposition.
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[edit] 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.
- Carbonates are composed of various carbonate minerals (most often calcium carbonate (CaCO3) precipitated by a variety of organic and inorganic processes. Typically, the majority of carbonate rocks are composed of reef material.
- Evaporites are formed through the evaporation of water at the earth's surface and are composed of one or more salt minerals, such as halite or gypsum.
- Chemical sedimentary rocks, including some carbonates, are deposited by precipitation of minerals from aqueous solution. These include jaspilite, chert, etcetera
[edit] Importance of sedimentary rocks
Sedimentary rocks provide a multitude of products which modern and ancient society has come to utilise.
- Art. Marble, although a metamorphosed limestone, is an example of the use of sedimentary rocks in the pursuit of aesthetics and art
- Architectural uses. Stone derived from sedimentary rocks is used for dimension stone and in architecture, notably slate for roofing, sandstone for load-bearing buttresses
- Ceramics and industrial materials; clay for pottery and ceramics including bricks; cement and lime derived from limestone.
- Economic geology. Sedmentary rocks host large deposits of SEDEX ore deposits of lead-zinc-silver, large deposits of copper, deposits of gold, tungsten and many other precious minerals, gemstones and industrial minerals including heavy mineral sands ore deposits
- Energy. Petroleum geology relies on the capacity of sedimentary rocks to generate deposits of petroleum oils. 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 worlds 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).
[edit] 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.
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.
Sedimentological conditions are recorded within the sediments as they are laid down; and in reference to Salvador Dalí's Persistence of memory, 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.
However, sedimentological study produces interpretations of past depositional and environmental conditions and care must be taken in analysing sedimentary rocks in a scientific manner in order to gain a picture of the events which occurred within the past.
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 fining up sequences is critical to interpretation of the sedimentary section and often the deformation and metamorphic structure of the region.
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.
[edit] Methodology of sedimentology
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 documeenting thickness, lithology, outcrop, distribution, contact relationships to other formations
- Mapping the distribution of the rock unit, or units
- Sequence stratigraphy
- Describes the progression of rock units within a basin
- Describing the lithology of the rock;
- Petrology and petrography; particularly measurement of texture, grain size, grain shape (sphericity, rounding, etc), sorting and composition of the sediment
- Analysing the geochemistry of the rock
- Isotope geochemistry, including use of radiometric dating, to determine the age of the rock, and its affinity to source regions
