Sedimentology encompasses the study of modern sediment
s such as sand
, and clay
, and the processes that result in their formation (erosion
. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks
and sedimentary structures
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
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 dune
s to dunes preserved in ancient aeolian
sandstones—geologists reconstruct past environments.
Sedimentary rock types
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
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
** 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
are composed of various carbonate minerals
(most often calcium carbonate
)) precipitated by a variety of organic and inorganic processes. Typically, the majority of carbonate rocks are composed of reef
s are formed through the evaporation of water at the Earth's surface and most commonly include halite
* Chemical sedimentary rocks, including some carbonates, are deposited by precipitation of minerals from aqueous solution. These include jaspilite
Importance of sedimentary rocks
Sedimentary rocks provide a multitude of products which modern and ancient society has come to utilise.
, 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
, a meta-shale
, for roofing
for load-bearing buttress
s and industrial materials: clay
s including brick
derived from limestone
* Economic geology
: sedimentary rocks host large deposits of SEDEX
ore deposits of lead
, large deposits of copper
, deposits of gold
, 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 oil
and oil shale
are found in sedimentary rocks. A large proportion of the world's uranium
energy resources are hosted within sedimentary successions.
: sedimentary rocks contain a large proportion of the Earth's groundwater aquifer
s. 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).
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; 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.
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.
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.
The principle of cross-cutting relationships
states that whatever cuts across or intrudes into the layers of strata is younger than the layers of strata.
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
* Descriptions of rock core (drilled and extracted from wells during hydrocarbon exploration)
* Sequence stratigraphy
** Describes the progression of rock units within a basin
* Describing the lithology
of the rock;
; 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
The longstanding understanding of how some mudstone
s form has been challenged by geologists at Indiana University (Bloomington)
and the Massachusetts Institute of Technology
. The research, which appears in the December 14th, 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."
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."
Considerable recent research into mudstone
s has been driven by the recent effort to commercially produce hydrocarbons from them, in both the Shale gas
and Tight Oil
(or Light Tight Oil) plays.
[Robert G. Loucks, Robert M. Reed, Stephen C. Ruppel, and Daniel M. Jarvi]
"Morphology, Genesis, and Distribution of Nanometer-Scale Pores in Siliceous Mudstones of the Mississippian Barnett Shale"
Journal of Sedimentary Research, 2009, v. 79, 848-861.
* Clastic rocks
* Important publications in sedimentology
* Oil shale
* Ore genesis
* Rock formation
* Sequence stratigraphy
* Vegetation-induced sedimentary structures