geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...

, clastic wedge usually refers to a thick assemblage of

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...

s--often

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

-shaped in profile--eroded and deposited landward of a

mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually highe ...

chain; they begin at the mountain front, thicken considerably landwards of it to a peak depth, and progressively thin with increasing distance inland. Perhaps the best examples of clastic wedges in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

are the Catskill Delta in Appalachia and the sequence of

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

and

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

sediments deposited in the Cordilleran foreland basin in the Rocky Mountain region. Not all clastic wedges are associated with mountains. They are also characteristic of passive

continent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions In geography, regions, otherwise referred to as zones, lands or territories, are areas t ...

al margins such as the

Gulf Coast The Gulf Coast of the United States, also known as the Gulf South, is the coast, coastline along the Southern United States where they meet the Gulf of Mexico. The list of U.S. states and territories by coastline, coastal states that have a shor ...

; these are quiescent environments, where sediments have accumulated to great thickness over a long period of time. These

passive margin A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting cre ...

continental shelf sediment sequences are termed

miogeocline A miogeocline is an area of sedimentation which occurs along the passive margin of a continent. The deposits occur as typically shallow water clastic sediments which thicken seaward to form a clastic wedge parallel to a tectonically quiescent coa ...

s. Clastic wedges are often separated into one of two distinct types:

flysch Flysch () is a sequence of sedimentary rock layers that progress from deep-water and turbidity flow deposits to shallow-water shales and sandstones. It is deposited when a deep basin forms rapidly on the continental side of a mountain building epi ...

, mostly dark shales that originate from moderate to deep marine water; and

molasse __NOTOC__ The term "molasse" () refers to sandstones, shales and conglomerates that form as terrestrial or shallow marine deposits in front of rising mountain chains. The molasse deposits accumulate in a foreland basin, especially on top of flysc ...

, which is composed mainly of red sandstones, conglomerates and shales that were deposited in terrestrial or shallow marine environments.Stanley, Steven M. ''Earth System History.'' New York: W.H. Freeman and Company, 1999.

References

Sedimentology {{geology-stub

See also

References