Mudlump islands are surface manifestations of intrusive clay masses that result from depositional processes at the mouths of major Mississippi River distributaries. The stratigraphy and structure of mudlumps at the South Pass mouth have recently been studied through a drilling and coring program which included holes to depths of 700 feet. Subsurface information obtained establishes the interrelationship between older shelf and prodeltaic river deposits and younger, progradational delta front and river mouth bar sediments.

Mudlumps are interpreted as being near-surface expressions of older shelf and prodelta clays diapirically intruded into and through overlying bar deposits. The intrusion culminates in shallow-angle thrust faulting which has resulted in vertical displacement of older clays as much as 350 to 400 feet. New mudlumps, revealed during the period of study, display surface exposures of shelf deposits uplifted and thrust from depths in excess of 350 feet. Between diapiric clay masses are synclinal troughs filled with as much as 400 feet of rapidly accumulated, near-strandline bar sands, silts, clays, and organic material.

Rapid depositional of thick, localized masses of heavier bar sediments directly upon lighter, plastic clays leads to an unstable situation which is relieved by diapiric intrusion of the clays with the resulting formation of mudlumps.

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