Subsidence and Collapse at Texas Salt Domes

Subsidence at salt domes results from man-induced and natural removal of salt, cap rock, minerals within the cap rock, and supradomal fluids. In the Houston diaper province, Frasch sulfur mining as caused subsidence bowls and collapse sinkholes at 12 of the 14 sulfur productive domes. Vertical subsidence owing to sulfur mining exceeds 35 ft (11 m) at Boling and Orchard Domes. Aerial photography from 1941 to 1979 documents continuous enlargement of one subsidence feature at Orchard Dome at the lateral rate of approximately 3 acres/year (12,000 m2). Subsidence mechanisms at salt domes vary between two end members-trough subsrdence and collapse. Trough subsidence is characterized by ductile deformation (gentle downwarping) centered over the zone of extraction and is expressed at the surface as a broad subsidence bowl. This process occurs when sulfur is evenly disseminated throughout relatively flat-lying cap-rock matrix. Collapse occurs when roof slabbing falls into a subsurface void that results from processes such as the extraction of thick-bedded, relatively pure sulfur. It is a form of brittle failure, and collapse sinkholes form when the zone of broken rock migrates to the surface. Typically, both subsidence bowls and collapse sinkholes develop at Frasch sulfur mines; the dominant landform depends on the structural and stratigraphic position of sulfur in the cap rock. Subsidence bowls predominate at Moss Bluff, Fannett, and Spindletop Domes, whereas collapse sinkholes are the typical form at Orchard Dome. The surface of Boling Dome, a site of extensive sulfur production, exhibits excellent examples of both types.