--> Abstract: Deep-Water Coarse- and Fine-Grained Sediments and What Seismic Analyses May Overlook, by Arnold H. Bouma; #90078 (2008)
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Deep-Water Coarse- and Fine-Grained Sediments and What Seismic Analyses May Overlook

Arnold H. Previous HitBoumaTop
Geology and Geophysics, Texas A&M University, College Station, TX

No two deep-water deposits are completely alike, but as a whole, they can be divided into coarse-grained (active margin) and fine-grained (passive margin) deposits. Coarse-grained fans normally carry gravel and sand with some fine-grained sediments that are canyon-fed, cross a narrow shelf, and can also move sediment during times of high sea level. The outer fan receives very little sand.

Fine-grained fans commonly originate from mountains located more than a thousand miles from the ocean. Sand, silt, and clay form the grain sizes. The sediments are of deltaic origin, are normally transported to deep water during regressions, and are deposited at the middle and outer fan.

The late Permian Tanqua Karoo deposits in SW South Africa are presently horizontal, which makes it possible to measure about 80% of all sediments that were deposited in the outer fan. Similar distributions exist at the Mississippi Fan (Gulf of Mexico) and the Pennsylvanian turbidites in Arkansas (USA).

Sediment deposition gradually thins, slows down, and breaks into fingers at the lower fan. At about 5 m thickness, the movement stops. Somewhere in the middle fan, a second finer-grained flow starts, gradually thickens, and stops somewhere past the outer fan. The second flow starts to thin somewhere beyond the first flow. Both flows carry a sufficient amount of silica as to make it basically impossible to recognize seismically how far the first deposition went, and where the second stopped. Studies are needed to reduce bypassing.

 

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas