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Inner--Shelf Hardbottom Areas, Northeastern Gulf of Mexico
William W. Schroeder (1), Albert W. Schultz (2), John J. Dindo (3)
Indurated sediments are common on the seafloor of offshore Alabama and NW Florida in water depths of 60 to 130 ft (18 to 40 m). Hardbottom areas include reef-like outcrops with up to 6.5 ft (2 m) of relief above the surrounding seafloor, moderately sloping ridges of rock rubble and shell hash, and also 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 prior to growth of the epifauna.
Four types of hardbottom rock are identified: (1) Massive to nodular sideritic sandstones and mudstones are widely distributed in the central and western portions of the area. (2) Slabby aragonite-cemented coquina and sandstone occur in the central part of the area, mostly as rubble associated with storm-related ridges of shell and sand. (3) Dolomitic sandstone occurs in small irregular outcrops, and is compositionally related to (2). (4) Calcite-cemented algal calcirudite occurs in reef-like 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 depositional and cementational patterns, but also a response to modern shelf processes such as energetic storm events.
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