--> Clast-Contact Conglomerates in Submarine Canyon Fill: Possible Subaqueous Sieve Deposits, by M. Shannon Fitzgerald and Donn S. Gorsline; #91024 (1989)

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Clast-Contact Conglomerates in Submarine Canyon Fill: Possible Subaqueous Sieve Deposits

M. Shannon Fitzgerald, Donn S. Gorsline

A new mechanism is proposed for the deposition of gravel from high-concentration sediment gravity flows in submarine canyons. Modeled after Roger LeB. Hooke's model of sieve deposition on alluvial fans, this new mechanism is called "subaqueous sieve deposition." Alluvial-fan sieve deposition occurs when a fluidal flow can no longer transport its coarsest material due to a break in the slope or an increase in fan surface permeability. The deposited boulders act as a sieve that allows water and finer material to pass through the gravel lobe while trapping coarser material. In the case of subaqueous sieve deposition, the freezing of a high-density turbidity current's basal traction carpet or of a flow intermediate between a noncohesive sandy debris flow and a modified grain low causes emplacement of the gravel on the submarine canyon floor. Immediately upon deposition, the sand matrix continues to flow out of the front and sides of the gravel lobe due to (1) the residual momentum of the low, and (2) the lighter weight of the sand as compared to that of the gravel. Cobbles on the edge of the gravel lobe may be pushed or rafted a short distance beyond the lobe's margin by the outflowing sand matrix. An important difference between subaerial and subaqueous sieve deposits is that the subaerial sieve deposits are open framework, whereas the subaqueous sieve deposits contain little, if any, open framework.

The necessary conditions for subaqueous sieve deposition include (1) a coarse sediment load (cobble to pebble-sized clasts) with a minimum of finer grained material, and (2) a decrease in the competency of the transporting flow due to a decrease in channel-floor slope gradient, the incorporation of water into the flow, or a barrier formed by a topographic high. Theoretically, subaqueous sieve deposits could occur in other submarine environments that meet these requirements.

Grain-size measurements from photographs and sedimentary structure data were taken on different types of recent alluvial fan deposits in Deep Springs Valley, California. These measurements were compared to similar data collected from the submarine canyon conglomerates of the Carmelo Formation at Point Lobos, California. Bivariate diagrams of grain-size distribution variables show that subaerial sieve deposits and subaqueous sieve deposits more closely resemble each other than they do any other alluvial fan and submarine canyon deposits.

Certain sedimentary structures of the subaqueous sieve deposits, such as having the coarsest clasts at or beyond the terminus of the lobe, could be formed by another process that has been proposed in the literature to explain these features. If the gravel beds were deposited with an accompanying sand layer, the movement of the sand layer over the stationary gravel also could pluck or push clasts from the margins of the gravel lobe. The two processes are not mutually exclusive and could operate simultaneously. Other features in the submarine canyon deposits, such as sandstone halos around conglomeratic lobes encased in conglomerate, indicate that subaqueous sieve deposition is a distinct process.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.