ABSTRACT

The lower part of the Buckner Anhydrite Member of the Haynesville Formation (Upper Jurassic) was deposited as shallowing upward cycles of subaqueous to subaerial deposits on the north flank of the Wiggins Arch in northeastern Mobile Country, Alabama. The unit studied conformably overlies the Smackover Formation and is generally evaporite dominated. The Buckner Anhydrite averages about 35 m (115 ft) in thickness and has been buried to depths of 5.5 km (> 18,000 ft). In spite of this deep burial, it has suffered little deformation since alteration of gypsum to anhydrite.

Subaqueous evaporites in the Buckner are dominated by bedded massive, bedded mosaic, bedded nodular, bedded nodular mosaic, and mosaic structural types. Buckner cycles begin with the selenite facies in which bedded nodular mosaic and bedded mosaic structural styles often display a preferential vertical alignment of anhydrite nodular pseudomorphs after gypsum. Selenite crystals were precipitated on the sediment surface possibly below wave base but within the photic zone. Gypsarenite, which vertically succeeds selenite, is a laminated deposit in which ripples and low-angle cross-beds indicate clastic deposition above wave base. The upper part of this facies may have been subaerially exposed. Stromatolite domes formed in relatively quiet water areas.

Subaqueous evaporites of this unit commonly grade upward into a generally poorly laminated anhydrite often containing burrow structures, detrital silt, and possible root structures. This may have been a gypsite facies which formed in the zone of soil moisture. Often this facies is located stratigraphically just below probable storm deposits and tidal flat-sabkha deposits which accumulated under subaerial conditions. These deposits are located at the top of the cycle. Subaerial conditions are indicated by desiccation cracks, an influx of oxidized red silt and clay, and erosional scours.

There probably was not a significant change in water depth at the transition from the Smackover Formation to the Buckner Anhydrite. Subaqueous Buckner evaporites were deposited conformably over subtidal deposits at the top of the Smackover. The formation of microcrystalline dolomite at the top of the Smackover probably occurred below the sediment/water interface.

Relative thickness of cycles, facies correlations, and the frequency and duration of subaerial exposure indicate the relative paleotopograpic position of various locates in the area. Buckner cycles were controlled by the rate of evaporite aggradation versus sea level rise and/or rate of subsidence.