Anatomy of a modern open‐ocean carbonate slope: northern Little Bahama Bank

Abstract

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,...

Lithification and porosity evolution in Permian periplatform limestones, Midland basin, Texas

“…Relativelylittlewasknown aboutstabilization and lithification of such deposits (e.g., Hollister et al 1972;Mullins 1983Mullins , 1986and earlierpapersbyFriedman 1964, 1965andGevirtz and Friedman 1966 prior to the availability about25 years ago of coresof deep-sea deposits provided by the DSDPand ODP. Since that time, however, the studies of Gomberg and Bonatti(1970), Milliman and Muller(1977), and especially, Schlager and James (1978) and Mullins and co-workers (Mullins 1983;Mullins et al 1980Mullins et al , 1984Mullins et al , 1985Mullins 1988aand b, 1992) haveprovided a wealth of information on the diagenesis attending the progressive burial of periplatfonn carbonates. Thesepapersspecifically dealt with mineralogic stabilization and theresulting formation of lowMg calcite-cemented limestones, mostly in upper Cenozoic periplatfonn deposits.…”

Sedimentology of a Middle Jurassic Base-of-Slope environment, Cutri Formation, Mallorca

“…Similar U-shaped back-filled channels cutting across similar lithofacies have been described by Mullins et al (1984), Phelps and Kerans, (2007), Noda andToshimitsu (2009), andSami et al (2010). Gully fills that cut into slope mudstone and are similarly plugged with coarser sediment are described from the Oligo-Miocene Numidian Flysch, Tunisia (Sami et al, 2010), the Devonian Prongs Creek Formation, Yukon (Mullins and Cook, 1986), This cliff is underlain and overlain by pack-grainstone packages and these packages are interbedded with wackepackstones.…”

“…Mullins and Cook, 1986;Hüneke and Krienke, 2004), and feed sediment into a distinctively linear apron complex that forms parallel to the platform (e.g. the modern Bahamas slope, Mullins et al, 1984 andthe Tuxpan Platform, Janson et al, 2011). This apron system is characterised by multiple sources along a margin rather than a single point source.…”

Toe-of-slope of a Cretaceous carbonate platform in outcrop, seismic model and offshore seismic data (Apulia, Italy)

“…6), where we noticed a gradual change in the composition on the slope from layers rich in debris to almost pure pelagic layers. This is most likely to happen on the "rise" at the toe-ofslope that typically dips at 1-2° (Schlager and Chermak 1979;Mullins et al 1984). On the rise, debris layers gradually give way to fine basin sediment.…”