Abstract

Thirty closely spaced piston cores from Columbus Basin, a 2700-sq mi re-entrant in the southern Great Bahama Bank, were studied in an effort to describe, in detail, the morphologies and lateral changes in faunal and floral composition, texture, internal structures, and mineralogy of individual bioclastic turbidites.

Attention was concentrated on the turbidites of a small northern sub-basin, and resulted in the reliable correlation and description of six such sedimentary units. Correlation was based on the color of the pelagic units, the relative and absolute positions of the turbidites in the cores, the absolute ages of the pelagic sediments, the coiling ratios of Globorotalia truncatulinoides, the Globorotalia menardii complex, and the thickness, composition, and texture of the turbidites.

Turbidity currents, originating on the upper parts of the steep slopes of the shallow carbonate banks, have flowed a maximum of 15 mi into the basin from two dominant directions, the northeast and the northwest. These flows have occurred with a frequency of about one every 3000 to 6000 yrs and have resulted in turbidites with volumes of the older of 108 m3. The individual flows are tongue shaped, and there is some indication that the thickest portions are near the head of the flow. Relatively little fine material is deposited in the basin by turbidity currents.

The following marked lateral changes were observed with increasing distance of transport from the source as well as distance from the flow axis: (1) decrease in turbidite thickness; (2) decrease in abundance of Halimeda, oöids, and pellets; (3) increase in relative abundance of planktonic forams; (4) decrease in the abundance of high-magnesium calcite and an increase in the abundance of low-magnesium calcite; (5) finer grain size; and (6) more turbidites beginning with the Bouma B unit of horizontal stratification than the Bouma A unit.

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