Mudlumps: Diapiric Structures in Mississippi Delta Sediments1
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.
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“Diapir” and “diapirism” come from the Greek diapeirein, which means “to pierce.” Diapirism sensu lato is a process by which earth materials from deeper levels have pierced, or appear to have pierced, shallower materials; it is divided into magmatic intrusion and diapirism sensu stricto on the basis of the temperature at which piercement occurs. Diapirs s.s. are composed of evaporites, argillaceous sediments, coal, peat, ice, serpentine, or other earth materials which have the critical characteristics of low equivalent viscosity and low density. These materials range in age from Precambrian to Recent. Diapirs are found in all parts of the world except the shield areas. They have many forms, ranging from smoothly rounded pillows to complexly injected laminae, are either connected with or disconnected from the “mother” bed, and are present either at the surface, where they form distinctive features, or at considerable depth. Diapirs have well-developed internal structures indicative of an origin by flow. Strata around a diapir may be strongly affected structurally and/or stratigraphically by the diapir, or they may be unaffected. Field and model studies indicate that diapirs have developed as a result of horizontal compression, gravitational instability, or both. Diapiric structures of various types contain large quantities of oil and gas, sulfur, salt, and potash and are important for underground storage and nuclear testing.