and the velocity of current decreases. As this occurs the ability of the current to hold coarse material in suspension decreases, and the current drops, first, its coarsest load, and then progressively finer material as the current velocity continues to decrease. In this way the coarsest material is deposited closest to the channel or slope that the turbidity current flowed down, and the finest material is deposited further away. The same sequence of coarse to fine material is deposited upward in the turbidite bed as the current velocity decreases with time at any given location. This is how graded beds are formed, with the coarsest material at the base and finer material at the top.
Classical complete turbidite beds consist of a sequence of sedimentary structures divided into a regular A-E sequence known as the Bouma sequence, after the sedimentologist Arnold Bouma, who first described the sequence. The A horizon consists of coarse- to fine-grained graded sandstone beds, representing material deposited rapidly from suspension. The B horizon consists of parallel-laminated sandstones deposited by material that moved in traction on the bed, whereas division C contains cross-laminated sands deposited in the lower-flow regime. The D and E horizons represent the transition from mate rial deposited from the waning stages of the turbidity current and background pelagic sedimentation.
Variations in the thickness and presence or absence of individual horizons of the Bouma sequence have been related to where on the submarine fan or slope the turbidite was deposited. Turbi-dites with more of the A-B-C horizons are interpreted to have been deposited closer to the slope or channel, whereas turbidites sequences with more of the C-D-E horizons are interpreted as more distal deposits.
Many turbidite sequences are deposited in foreland basins and in deep-sea trench settings. These environments have steep slopes in the source areas, a virtually unlimited source of sedimentary material, and many tectonic triggers to initiate the turbidity current.
Molasse sequences overlie many turbidite-bear-ing flysch sequences, especially those deposited in foreland basins. Molasse consists of thick sequences of coarse-grained postorogenic sandstones, conglomerates shales, and marls that form in response to the erosion of orogenic mountain ranges. The name is derived from the classic Miocene-Oligocene-Pliocene molasse of the European foreland, deposited across much of France, Switzerland, and Germany, and overlying the Alpine flysch sequence. These sediments are up to four miles (7 km) thick on the Swiss Plateau and represent rapid erosion of the Alps. Lower parts of the molasse include shallow marine and tidally influenced sediments, overlain by alluvial fan deltas, alluvial fan complexes, and overbank deposits.
See also basin, sedimentary basin; convergent plate margin processes; orogeny; sedimentary rock, sedimentation.
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