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Abstract: Quaternary Geology of Weeks and Cote Blanche Islands Salt Domes

Whitney J. Autin, Richard P. Mc Culloh

Weeks and Cote Blanche Islands, part of south-central Louisiana's Five Islands, are piercement-type salt diapirs uplifted from about 17 km deep. Both domes are nearly circular in plan, with maximum elevations of approximately 52 m and 30 m MSL, respectively. Surficial sediment from both islands are loess and silty colluvium overlying a paleosol developed in the Pleasant Bluff Alloformation, part of the late Pleistocene Prairie Complex. The loess represents a single genetic unit, whose thickness variations are chiefly related to slope erosion and sediment reworking. A perimeter flanking silty colluvium has accumulated at the toe-slope position of the islands. Geomorphic evidence of salt-induced uplift includes surface lineations, linear gullies, and excessively steep land urface topography coincident with steep topography on the top of the underlying salt. Active dissolution of salt is a likely factor in the evolution of both natural and human-affected collapse sinkholes.

Shear fractures with high-angle average dips occur in the Mt. Pleasant Bluff Alloformation. Fractures are best developed in clayey sediment but also occur in sandy and loamy sediments. Apparent conjugate shear pairs observed in cores are believed to reflect extensional stress associated with domal uplift.

The Quaternary geologic history of Weeks and Cote Blanche Islands is complex, beginning with middle-to-late Wisconsinan deposition of the Mt. Pleasant Bluff Alloformation in a low-relief alluvial plain concurrent with soil development. Predominantly sandy lithofacies at Weeks Island reflect uplift of a fluvial channel belt, whereas the clayey sequences at Cote Blanche Island indicate uplift of a backswamp. The stratigraphy of loess and colluvial silt indicates the islands were emergent during late Wisconsinan loess deposition. Landscape dissection, distribution of colluvium, and shearing of Quaternary sediment reflect continued uplift after loess deposition. Surficial geological data suggest that Quaternary sediments and the present landscape are responding to active uplift and salt d ssolution.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana