--> Abstract: Direct Transfer of Sand from Shelf-Edge Deltas to the Continental Slope, by David Mohrig, Michael P. Lamb, and Jeffrey Nittrouer; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Direct Transfer of Sand from Shelf-Edge Deltas to the Continental Slope

David Mohrig1; Michael P. Lamb2; Jeffrey Nittrouer1

(1) Jackson School of Geosciences, Univ of Texas at Austin, Austin, TX.

(2) Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA.

The conditions for direct river input of sand into the deep-marine environment are evaluated using results from recent studies that (1) define the properties of sand transport in zones of backwater flow for coastal rivers, (2) define the conditions for producing hyperpycnal flows, and (3) define the extra sediment entrained into coastal rivers by shelf erosion during early fall, late fall and early rise in a sea-level cycle. Recent work has documented that a densimetric Froude number can be used to predict the local conditions associated with plunging of a river plume and generation of hyperpycnal flow. Plunging occurs when the local densimetric Froude number equals roughly 0.5. Generation of these river-fed turbidity currents is correlated with the marine bed slope because a higher slope tends to shorten the distance to the plunge point, minimizing settling and preserving high suspended-sediment concentrations in the river plume. Therefore, shelf-edge deltas and the continental slope promote generation of hyperpycnal flows, particularly when compared to topography commonly observed in front of highstand deltas. Generating hyperpycnal flows is also directly tied to suspended-sediment concentrations in the river water at the river mouth. Recent laboratory studies are helping to define the additional sediment delivered to a shelf edge during early fall, late fall and early rise in sea level. In addition, recent field studies on the lowermost Mississippi River document how backwater mechanics can raise suspended-sand flux more than 100-fold during large floods. Insight from these sediment-charged floods and the enhanced suspended sediment associated with rivers eroding shelf sediments will be used to constrain environmental conditions associated with direct transfer of sand from shelf-edge deltas to the lower continental slope.