ABSTRACT

Jefferson Island, the northernmost of the Five Islands salt domes of coastal Louisiana, experienced major ground failure and mine flooding on November 20, 1980. The development of a collapse sinkhole in Lake Peigneur, which overlies the Jefferson Island dome, resulted in the loss of 68.2 ac (27.6 ha) of land along the southeastern lake margin. Although this failure was man-induced, the observed processes provide insight into collapse structure development on salt domes by salt dissolution and caprock alteration.

The sinkhole was triggered by a deep-seated instability that initiated a series of mass movements. The morphology and spatial distribution of these movements depended on variations in surficial lithology, water-holding capacity, and yield strength of affected sediments. Multiple retrogressive rotational slides developed in Pleistocene clays along the southern shoreline because the clay's cohesive strength provided little resistance to tensional release. Compound slides developed in loessial sediments along the southeastern shoreline because frictional resistance within the loess retarded tensional release. The rate of sediment dewatering and variations in sediment texture and thickness produced variations in mudflow morphology in the Holocene muds on the lake bottom.

Available evidence indicates that Jefferson Island has previously experienced surficial subsidence during its geologic development. At least two distinct rates of subsidence have occurred during modern times -- incremental subsidence measured near the salt mine and episodic subsidence, such as occurred on November 20, 1980. The topographic low overlying Jefferson Island resulted from the dissolution of salt and caprock and formation of a collapse structure. Geomorphic and stratigraphic evidence of episodic subsidence includes late Pleistocene meander scars cut by the lake's shoreline, V-shaped Holocene mud fills beneath portions of the lake bottom, and brecciation and cavity development within the caprock associated with a complex history of secondary alteration. Natural incremental subsidence was probably occurring as well; however, its relative contribution cannot be confidently estimated.