No abstract

Summary The contribution of fine-grained, bank-derived versus pelagic-derived carbonate sediment in periplatform oozes has been evaluated by means of a detailed, quantitative carbonate mineralogy study of 35 surface sediment samples from north of Little Bahama Bank. Our data indicate that large volumes of aragonite and magnesian calcite are being exported along the open-ocean margin of Little Bahama Bank during the present high-stand of sea-level. Percentages of mud-size aragonite + magnesian calcite display a strong negative correlation (r = −0.912) with distance from the edge of the bank, whereas percentages of calcite show a strong positive correlation (+0.914). Immediately adjacent to the northern, windward margin of Little Bahama Bank, 86% of the fine-grained carbonates are bank-derived. This value decreases in a linear fashion to 50% bank-derived muds at a distance of 50 km from Little Bahama Bank. Direct seaward projection of our data suggests that fine-grained, metastable, shallow-water carbonate sediments are transported laterally at least 120 km from open-ocean bank margins. This has important implications for the global budget of CO 2 , the diagenetic potential of periplatform oozes, and the growth potential of carbonate slopes.

The open‐ocean carbonate slope north of Little Bahama Bank consists of a relatively steep (4°) upper slope between water depths of 200 and 900 m, and a more gentle (1–2°) lower slope between depths of 900 and 1300+ m. The upper slope is dissected by numerous, small, submarine canyons (50–150 m in relief) that act as a line source for the downslope transport of coarse‐grained carbonate debris. The lower slope is devoid of any well‐defined canyons but does contain numerous, small (1–5 m) hummocks of uncertain origin and numerous, larger (5–40 m), patchily distributed, ahermatypic coral mounds. Sediments along the upper slope have prograded seaward during the Cenozoic as a slope‐front‐fill seismic facies of fine‐grained peri‐platform ooze. Surface sediments show lateral gradation of both grain size and carbonate mineralogy, with the fine fraction derived largely from the adjacent shallow‐water platform. Near‐surface sedimentary facies along the upper slope display a gradual downslope decrease in the degree of submarine cementation from well‐lithified hardgrounds to patchily cemented nodular ooze to unlithified peri‐platform ooze, controlled by lateral variations in diagenetic potential and/or winnowing by bottom currents. Submarine cementation stabilizes the upper part of the slope, allowing upbuilding of the platform margin, and controls the distribution of submarine slides, as well as the headward extent of submarine canyons. Where unlithified, sediments are heavily bioturbated and are locally undergoing dolomitization. Upper slope sediments are also ‘conditioned’eustatically, resulting in vertical, cyclic sequences of diagenetically unstable (aragonite and magnesian calcite‐rich) and stable (calcite‐rich) carbonates that may explain the well‐bedded nature of ancient peri‐platform ooze sequences. Lower slope sediments have prograded seaward during the Cenozoic as a chaotic‐fill seismic facies of coarse‐grained carbonate turbidites and debris flow deposits with subordinate amounts of peri‐platform ooze. Coarse clasts are ‘internally’derived from fine‐grained upper slope sediments via incipient cementation, submarine sliding and the generation of sediment gravity flows. Gravity flows bypass the upper slope via a multitude of canyons and are deposited along the lower slope as a wedge‐shaped apron of debris, parallel to the adjacent shelf edge, consisting of a complex spatial arrangement of localized turbidites and debris flow deposits. A proximal apron facies of thick, mud‐supported debris flow deposits plus thick, coarse‐grained, Ta turbidites, grades seaward into a distal apron facies of thinner, grain‐supported debris flow deposits and thinner, finer grained Ta‐b turbidites with increasing proportions of peri‐platform ooze. Both the geomorphology and sedimentary facies relationships of the carbonate apron north of Little Bahama Bank differ significantly from the classic submarine fan model. As such, a carbonate apron model offers an alternative to the fan model for palaeoenvironmental analysis of ancient,...