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Clast-Contact Conglomerates in Submarine Canyons: Possible Subaqueous Sieve Deposits

M. Shannon Fitzgerald

Thick, coarse, clast-contact conglomerates in submarine canyon fill have previously been attributed to rock-fall, grain-flow, or winnowing processes. However, these processes do not adequately explain some thick conglomeratic sequences. The proposed process of subaqueous sieve deposition could account for these clast-contact conglomerates.

Subaerial sieve deposition has been documented on small-scale fan models and on alluvial fans. A subaerial sieve deposit begins as a debris flow which at some point "freezes up." The matrix is then lost by subsequent filtration or outflow, and the emplacement of a clast-contact gravel ensues. A subaqueous sieve deposit would be slightly modified in that the matrix would not be lost by filtration into the submarine canyon floor, but rather by outflow at the terminus of the lobe immediately after deposition, or possibly from the top and/or sides of the "freezing" flow mass during transport. Besides forming in submarine canyons, subaqueous sieve deposits might also occur in paralic, submarine fan channel, and base-of-the-slope settings.

In substantiating the existence of subaqueous sieve deposits, the sedimentary structures and grain-size data from recent sieve deposits on alluvial fans are compared to those of ancient submarine canyon deposits. Numerous similarities are found supporting this new method of deposition. Some discrepancies are encountered, but these are expected due to modifications caused by an aqueous medium.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.