ABSTRACT

River-mouth depositional patterns are modified by sediment deformational processes of sufficient magnitude to severely endanger bottom-supported structures. Several types of deformations are present, including a) peripheral slumping, b) differential weighting and diapirism, c) radial tensional faulting, d) mass wasting and flowage induced by wave motion and degassing, and e) deep-seated clay flowage. High depositional rates occur near the river mouth and decrease seaward, and with time the bar oversteepens; rotational slump planes form peripheral to the bar front, moving sediment into deeper water. These blocks have longitudinal dimensions of 60 to 600 meters and lateral dimensions of 200 to >600 meters. Differential loading by denser bar sands overlying low-density clays results in vertical and seaward flowage of the clays contemporaneously with seaward bar progradation. Diapiric folds and spines (mudlumps) intrude into delta-front sediments on the seaward side of the deforming load, vertical movement affecting sediments to depths in excess of 200 meters. The seaward extrusion and continued movement of clays arch the overlying delta-front sediments, and this stress is relieved by small graben faults oriented radial to the deforming load or delta lobe. The grabens have widths from 50 to 500 meters and lengths of several kilometers. The finer grained river-mouth sediments contain high percentages of methane and CO₂ gases, which are formed by bacterial decomposition of organics. Passage of hurricane waves produces bottom pressure perturbations, forcing the entrapped gas upward, causing loss of sediment strength, and allowing subaqueous mass movement to occur. The weight of the modern delta has depressed underlying Pleistocene sands some 120 meters, causing squeezing and flowage of clays onto the continental shelf at water depths greater than 150 meters. Large-scale slumping and faulting near the continental shelf result from this clay flowage.