Sediment Gravity Flow Deposition on a Modern Carbonate Slope Apron: Northern Little Bahama Bank: ABSTRACT

Abstract

Carbonate sediment gravity flow sedimentation north of Little Bahama Bank is initiated along a line source consisting of numerous small slope gullies on the upper slope and results in deposition of a wedge-shaped lower slope apron of coarse sediment. This broad (over 100 km; 62 mi) smooth apron (30 km, 19 mi, wide) parallels the adjacent carbonate bank margin and lies between 20 and 50 km (13 and 31 mi) seaward of the platform. The apron is marked by a relatively abrupt decrease in slope and ranges from 2° to ½° between depths of 850 and 1,400 m (2,789 and 4,593 ft). This apron is composed of a variety of medium to coarse-grained sediment gravity flow deposits of variable thicknesses, interbedded with an equal proportion of fine-grained pelagic ooze. Base on texture alone, ancient slope aprons could easily be misinterpreted as lower slope or inner fan/braided suprafan environments of a submarine fan, which operates as a point source. A detailed piston core study utilizing X-radiography, grain-size analyses, and standard petrographic techniques revealed 36 sediment gravity flow deposits displaying a spectrum of depositional characteristics. Single-layer deposits are either: (1) muddy, poorly sorted (debris flows); (2) clean, massive to inversely graded (grain flows); or (3) normally graded (turbidites). Double-layer deposits are composites of single-layer types resulting from flows that occur in two phases. They consist of either: (1) normally graded overlying muddy, poorly sorted, or (2) normally graded overlying clean, massive to inversely graded deposits. A ratio of 3:2:1 exists among debris flow, turbidity current, and grain flow deposits respectively. Debris flow deposits, up to 5.6 m (18 ft) thick, display a down-slope transition from mud to grain-supported fabrics. This transition is interpreted as a progressive downslope loss of muddy matrix due to turbulence. Grain flow deposits, up to 5.2 m (17 ft) thick, occur close to the slope break and represent deposition from flows of high concentration. Turbidites, up to 1.4 m (5 ft) thick, are ubiquitous on the apron. Typically they are simple graded basal turbidites, lacking upper Bouma-sequence laminated intervals. Some exhibit multiple-graded sequences suggesting pulsating flows. The sediment gravity flow deposits lack shallow-water sediment, but contain resedimented intraclasts derived from submarine cemented upper slope deposits (nodules) along with lithified layers and deep-water corals from the lower slope. Although textures and structures appear similar in both terrigenous submarine fan and carbonate slope apron environments, sedimentation models differ radically. Knowledge of those differences should aid in the recognition of ancient carbonate slope apron deposits. End_of_Article - Last_Page 447------------