Subaqueous Delta

The subaqueous component of deltas lies from the low tide line to the farthest portion of fluvial sediment deposits in the prodelta. In general, the sediments fine seaward. The coarser sands are deposited near the river, and finer sediments are deposited offshore out of suspension or from mass movement. The water depth of the subaqueous delta can range from 50-300 meters depending upon coastal conditions. The width of the subaqueous delta can also range from a few kilometers to tens of kilometers. One factor controlling the lateral extent of this component is the dominance affecting the delta (fluvially, wave, or tide; back to the main page). Deposits associated with the subaqueous delta include distributary-mouth-bar deposits, subaqueous slump deposits, and prodelta deposits.

Distributary-mouth-bar deposits include the distributary-mouth bar and river-mouth tidal-range deposits. Distributary-mouth bar and river-mouth tidal-range deposits consist of gravels and sands. These deposits settle out early in the travels of the sediment load. The delta front, located landward of the prodelta slope, may be influenced by high energy waves and long shore currents producing well sorted, cross bedded sheet sands from reworking. Biologically, these deposits contain local packages of transported organics. Faulting may also occur as the deposit accumulates sediment and grows larger. Subaqueous slump deposits also occur in the subaqueous component of a delta. These deposits are caused by the force of gravity acting on the increasing weight of the expanding deposit. As sediments accumulate along a slope or gradient, the force of gravity acts on the deposit and causes it to slump, slide, or flow. Small faults (listric) and fractures are associated with subaqueous slump deposits. Prodelta deposits consist of the finest silts and clays that have traveled the farthest of the sediment load. These sediments fall out of suspension producing the prodelta distal bar and seaward prodelta deposits. The prodelta distal bar is comprised of alternating, laminated sands, silts, and shales. Sedimentary structures present in the distal bar include climbing ripples, wave ripple marks, and current ripple marks. There is also a small amount of fauna in these deposits. The distant seaward prodelta deposits consist of colorful silts and clays. These sediments biologically include scattered shells and microfauna. These packages of laminated silts and clays generally fine upward.