ALBERT

Notes: Previous studies on early submarine diagenesis of periplatform carbonates have implied that these originally polymineralic (aragonite, magnesian calcite, calcite) sediments are susceptible to early diagenesis only in current-swept open seaways or where surficially exposed by erosion on the seafloor. It has also been proposed that while in the shallow subsurface, periplatform oozes retain their original mineralogy for at least 200,000–400,000 yr and remain unlithified for tens of millions of years.Evidence is reported here for extensive calcitization and selective lithification of periplatform oozes of late Pleistocene age in two piston cores collected from water depths of ∼ 1,000 m north of Little Bahama Bank. It is shown that shallow (〈30 m) subsurface diagenesis can significantly alter the original mineralogy of periplatform oozes to predominantly calcite in less than 440,000 yr, and that cementation by calcite can produce chalk-ooze sequences within the same time-frame. Periplatform oozes that originally contain a high percentage of bank-derived magnesian calcite appear to have a higher diagenetic potential than those originally low in magnesian calcite. Shallow subsurface calcitization and fithification greatly reduce the diagenetic potential of periplatform carbonates, and chalk-ooze sequences apparently can persist for tens of millions of years and to burial depths of at least 300 m.Shallow subsurface diagenesis, at water depths 〉 1,000 m, proceeds via dissolution of magnesian calcite and aragonite and reprecipitation of calcite as allochem fillings, exterior overgrowths and cement. It is speculated that density-driven ‘Kohout convection‘, where seawaters under-saturated with respect to magnesian calcite and aragonite and saturated/supersaturated with respect to calcite flow through the margins of carbonate platforms, is the primary driving mechanism for shallow subsurface diagenesis. Removal of Mg during early stages of deep seafloor and shallow subsurface diagenesis should increase the Mg content of interstitial waters which is likely to increase the ‘dolomitizing potential’ of Kohout convection fluid flow.

Notes: 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, open-ocean carbonate slope sequences.

Notes: Carbonate ramps are gently sloping depositional surfaces where shallow-water, coarse-grained facies pass basinward into fine-grained, deep-water sediments, with no abrupt change in slope. The objectives of this study are: (i) to integrate the depositional processes recorded in the Pleistocene stratigraphy of the west Florida outer ramp into a palaeoclimatic and palaeoceanographic framework for the eastern Gulf of Mexico; and (ii) to examine the origin of mineralogical and sedimentary cycles in the light of pteropod and planktonic foraminiferal populations corresponding to climatic oscillations.Aragonitic, pteropod-rich sediments with large amounts of insoluble residue occur in sediments deposited during glacial intervals; sandy calcitic sediments with abundant planktonic foraminifera accumulate during interglacials. These cycles reflect variations in biological productivity of pelagic pteropods and planktonic foraminifera, rather than preferential dissolution of either aragonitic or calcitic fractions. Species assemblages suggest that the productivity cycles are linked to changes in upwelling intensity at the margins of the Loop Current and variations in water mass salinities, as well as terrigenous dilution from the Mississippi Delta. These cycles are the response to Pleistocene glacial-interglacial oscillations, controlled by Milankovitch orbital parameters.Although the organisms contributing to deep-water carbonate environments have changed through geological time, facies patterns, as well as sedimentary textures and structures, identified in the west Florida sediments provide criteria for recognition of ancient ramps. An understanding of the processes on a modern ramp slope, such as west Florida, may prove valuable in palaeoclimatic and palaeoenvironmental analysis of epicontinental carbonate sequences and ramps in the rock record.

Notes: Analyses of high resolution, seismic reflection profiles and surface sediment samples indicate that the Cat Island shelf is presently in an incipiently drowned state. This small carbonate bank is characterized by a thin (〈4 m), coarse-grained, relict sediment cover, along with limited reef development, and a relatively deep (20–30 m) margin indicating that it has been unable to ‘keep-up’ with Holocene sea-level rise.Early flooding at relatively high rates of sea-level rise (4 m kyr-1, 5–8 × 103 yr BP) in conjunction with small bank size and relatively low elevation, led to a reduced rate of carbonate accumulation and incipient drowning. The shelf edge currently lies beneath the zone of maximum carbonate production and exposes the interior shelf to open marine conditions which may result in permanent drowning if it is unable to ‘catch-up’ with continued sea-level rise. Sediment facies patterns are largely oriented perpendicular or oblique to the shelf edge and appear to be controlled by shelf circulation patterns focused by bank-margin reentrants.In comparison with most of the northern Bahamas, the Cat Island shelf was flooded earlier and at relatively higher rates of Holocene sea-level rise which led to selective drowning, implying that carbonate platforms need not drown synchronously over widespread areas as commonly thought. The potential rock record of this incipient drowning event would be a thin, open-marine sand sheet of highly degraded cryptocrystalline and aggregate grains associated with poorly developed reefs.

Notes: Abstract Serpentinites and spilitic basalts recovered at depths of 1000 m from Ascension Submarine Canyon northwest of Monterey Bay, California indicate that Franciscan basement is present immediately to the west of the San Gregorio Fault. This new information, together with published geological/geophysical data, support previous suggestions that the offshore western boundary of the Salinian block (Sur-Nacimiento Fault) has been tectonically truncated by the San Gregorio Fault and has been displaced by as much as 90 km to the northwest since the mid-late Miocene.

Notes: Abstract Interpretation of seismic reflection data reveal evidence of a Cenozoic fault (Walkers Cay Fault) north of Little Bahama Bank. This fault strikes N15–30°E, perpendicular to the adjacent bank margin and offsets a late Oligocene reflector by as much as 100 m. The origin of this near-surface fault is uncertain, but its location and strike are nearly coincident with an independently mapped basement fault. Walkers Cay Fault may be the result of recurrent faulting, implying intermittent basement fault movement during the post-rift history of the northern Bahamian continental margin.