River-mouth depositional patterns are modified by sediment deformational processes of sufficient magnitude to endanger severely bottom-supported structures. Several types of deformations are present and include (a) peripheral slumping, (b) differential weighting and diapirism, (c) graben faulting, (d) mass wasting by sediment degassing, and (e) deep-seated flowage. High depositional rates are present near the river mouth and decrease seaward; with time, the bar front oversteepens and rotational slump planes form peripheral to the bar front, moving sediment into deeper water. These blocks have longitudinal dimensions of approximately 200 to 2,000 ft and lateral dimensions of 600 to greater than 2,000 ft. Differential loading by denser bar sands overlying low-density clays esults 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 500 ft. The seaward extrusion and continued movement of clays arch the overlying delta-front sediments, and this stress is relieved by small graben faults oriented radially to the deforming load or delta lobe. The grabens have widths from 150 to 1,500 ft and lengths of several miles. The finer grained river-mouth sediments contain high percentages of methane and CO₂ gases, formed by bacterial decomposition of organics. Passage of hurricane waves produces bottom-pressure perturbations, forcing the entrappe gas upward, causing loss of sediment strength and allowing mass movement. The weight of the modern delta has depressed underlying Pleistocene sands about 400 ft, causing squeezing and flowage of clays onto the continental shelf at water depths greater than 300 ft. Large-scale slumping and faulting near the continental shelf result from this clay flowage. These processes are contemporaneous with deposition and play an important role in initiating a depocenter.

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