In
geology, clastic wedge usually refers to a thick assemblage of
sediments--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 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 are the
Catskill Delta in Appalachia and the sequence of
Jurassic and
Cretaceous 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 are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...
al margins such as the
Gulf Coast; 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 crea ...
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 co ...
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 ...
, 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 flys ...
, 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. ]
See also
*
Foreland basin
A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...
References
Sedimentology
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