Inner-Shelf Hardbottom Areas, Northeastern Gulf of Mexico
William
W. Schroeder, Albert W. Shultz, John J. Dindo
Indurated sediments are common on the sea floor of offshore Alabama and northwestern Florida in water depths of 18 to 40 m. Hardbottom areas include reeflike outcrops with up to 2 m of relief above the surrounding sea floor, moderately sloping ridges of rock rubble and shell hash, and areas of surficial rock and shell rubble with little or no relief. Although most of these hardbottoms are classified as "live" (i.e., they support a diverse encrusting epifauna), their composition and texture indicate formation under different prior conditions.
Four types of hardbottom rock are identified: (1) massive to nodular sideritic sandstones and mudstones, widely distributed in the central and western portions of the area; (2) slabby aragonite-cemented coquina and sandstone in the central part of the area, mostly rubble associated with storm-related ridges of shell and sand; (3) dolomitic sandstone in small irregular outcrops, compositionally related to (2); and (4) calcite-cemented algal calcirudite in reeflike knobs in the southeastern portion of the area. In all but (4), mineralogy and isotope ratios suggest that cementation took place via marine rather than fresh waters, and that methane and plant detritus served as carbon sources.
Most mappable hardbottom areas are rubble zones on northeast-facing slopes of gentle low-relief shore-oblique ridges. Thus, present-day extent of hardbottoms is a function not only of original deposition and cementation patterns but also represents a response to modern shelf processes such as energetic storm events.
AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.