TIMING OF DIAPIR GROWTH AND CAP ROCK FORMATION, DAVIS HILL SALT DOME, COASTAL TEXAS

The Davis Hill dome is a relatively large salt stock located in the northeastern part of the Houston salt dome province. The minimum depth is about 800 ft to the cap rock and about 1,200 ft to salt. The source layer (Jurassic Louann Salt) lies at a depth of at least 30,000 ft. Seismic and well logs interpretation suggests that diapir growth took place from Late Cretaceous to Plio-Pleistocene. Passive growth (downbuilding) of the dome ceased during the Early Oligocene (Vicksburg) and was followed by active growth (upbuilding) of the dome. The rate of dome growth during lower Wilcox deposition was about 1,700 ft / M.y. and increased to 2,500 ft / M.y. during middle and upper Wilcox time. After deposition of the Wilcox Group, the growth rate substantially decreased down to 390 ft / M.y. during Claiborne time, and to 245 ft / M.y. during Jackson and Vicksburg times. Present-day topographic relief at Davis Hill is 175 ft above the surrounding coastal plain. A radial pattern of oxbow lakes above the dome and older age of the Davis Hill from surrounding rocks suggest that the dome is still rising.

Studies of cores indicate that the cap rock is about 440 ft thick and comprises, from bottom to top, anhydrite, gypsum and calcite zones. The anhydrite zone represents the less soluble components of the salt that accumulated as the halite was dissolved by pore fluids. The anhydrite zone locally contains abundant celestite that probably formed as the result of interaction with an external Sr-bearing brine. The calcite zone formed by bacterial alteration of sulfate accompanying hydrocarbon destruction. The calcite zone contains significant amounts of sulfide minerals and oil. The sulfide-rich calcite zone is approximately 40 ft thick and is dominated by locally massive iron sulfides. These sulfide concentrations resulted from the interaction of deep heated metal-bearing formational brines with bacterially derived reduced sulfur in the ambient cool ground waters.

The systematic accumulation of anhydrite cap rock layers by underplating as the salt diapir is dissolved allows a cap rock stratigraphic succession to be defined on the basis of texture and accessory minerals. These relationships can be used to constrain the timing of dome growth and cap rock formation.