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 been studied by means of a drilling and coring program which included drilling holes to a depth of 700 ft. Subsurface information obtained establishes the relationship between older shelf and prodeltaic river deposits and younger, progradational delta-front and river-mouth bar sediments.

Mudlumps are interpreted as the near-surface expressions of the diapiric intrusion of older shelf and prodelta clays into and through overlying bar deposits. The intrusion culminated in reverse faulting which resulted in vertical displacement of older clays by as much as 350-400 ft. In new mudlumps, found during the period of study, there are surface exposures of shelf deposits uplifted and thrust from depths of more than 350 ft. Between the diapiric clay masses are synclinal troughs filled with as much as 400 ft of rapidly accumulated, near-strandline bar sand, silt, clay, and organic material.

Rapid deposition of thick, localized masses of heavier bar sediments directly upon lighter, plastic clay leads to instability which is relieved by diapiric intrusion of the clay with the resulting formation of mudlumps.