Abstract The margins of carbonate platforms and shelves have long been of the greatest interest to stratigraphers, sedimentologists, paleontologists, and geologists of petroleum and metallic ores. Margins are the loci of reef development; they mark major changes in lithology, biotas, and depositional processes; their configuration often influence processes on the platforms or shelves behind them; and they may include major reservoirs for oil, gas, and metallic ores. Among the many riddles of platform margins, none is more challenging than the architecture and processes of progradation. What circumstances trigger episodes of progradation? What role do fluctuations in sea level have? What are the sources of sediment? What is the architecture of prograding deposits? What is the range of rates of progradation? Answers to these and associated questions have come from study of well-exposed fossil examples like those in the Permian of New Mexico and West Texas ( King, 1948 ), the Triassic Dolomites of the Southern Alps ( Bosellini, 1984 ), the Lower Cretaceous of Vercours, France ( Jacquin et al., 1991 ), numerous other Cretaceous examples ( Simo et al., 1993 ), or the Miocene of Majorca ( Pomar, 1993 ). Valuable as these fossil outcropping examples are, they often have serious limitations, e.g., difficulties in age dating and overprints of diagenesis and tectonics. The younger the example, the more likely that these limitations can be overcome. For this reason, the margins of the Bahama Banks, isolated platforms of the Late Cenozoic, have long been a focus of research.

Abstract: Two continuous core borings 8.5 km apart on the leeward margin of Great Bahama Bank provided a special opportunity to interpret the depositional history during the Late Pliocene and Pleistocene. The upper ca. 200 m of the borings presented in this paper show seaward progradation of the margin with overall shallowing. At the base of this succession are skeletal grainstones and packstones that accumulated on the proximal slope of the margin. These are succeeded by thick intervals of reefal and coral-bearing deposits that in turn are capped by nonskeletal grainstones similar to the modern sediments in the interior of the Bahama Banks. Reefal and coral-bearing limestones produce about half of the leeward progradation. In both borings, the major episodes of coral growth began on unconsolidated sediments and show indications of upward shallowing according to the interpretations of Budd and Manfrino (this volume). In Clino, the seaward boring, the major episode of reef growth was terminated by deposition of fine-grained sediment mixed with coral floatstone; in Unda, the bankward boring, reef demise was the result of subaerial exposure. In Unda, the post-reefal deposits, termed the layered cap, are largely a succession of packstones and grainstones of peloids and skeletal debris, which are interpreted as platform-top accumulations similar to those of the Holocene. Thirteen discontinuity horizons, which are distinct breaks in sedimentation and have numerous features indicative of subaerial exposure, are used to divide this interval into sea -level flooding events (rising sea level and highstands) and periods of exposure (falling sea level and lowstands) during the Pleistocene and Late Pliocene. In Clino, above the major episodes of reef development, corals and skeletal grainstones and packstones continued to be the major component of aggradation. Nonskeletal packstones and grainstones from the platform interior are present only in the uppermost 20 m. Ten discontinuity horizons, which indicate falling sea level and sea-level lowstands, are identified in the Late Pleistocene. Two magnetostratigraphic datums provide the primary dating for this upper succession (McNeill et al., this volume). Three factors show that reef development occurred first in Unda during a period of relative rise of sea level: (1) correlating the top of the Olduvai between the two borings, (2) coral assemblages, and (3) interpretations of depositional environments. Coral assemblages in Unda include Pliocene species, whereas those in Ciino are largely Pleistocene to recent (Budd and Manfrino, this volume). A drop in sea level produced a downshift of major reef growth of some tens of meters (and as much as 100 m) to Clino. Another relative rise of sea level allowed for reefal aggradation. This offlap succession of reefal deposits confirms the pre-drilling model of margin evolution developed from seismic stratigraphy ( Eberli and Ginsburg, 1989 ). The downshift in reef development during the Late Pliocene have been coincident with the onset of Northern Hemisphere ice accumulation as inferred from the deep-sea isotope record. These results confirm and expand on the role of reefs and sea-level fluctuations in the architecture of Great Bahama Bank. Reefs make a major contribution to leeward progradation, a circumstance that adds support to considering the Neogene Bank an atoll-Like structure. It is estimated that the margin shifted seaward some 10 km in approximately two million years. This episodic progradation was controlled by a hierarchy of sea-level fluctuations during the Late Pliocene and Pleistocene. Major episodes of sea-level rise allowed for aggrading reef development; higher-frequency cycles of sea-level fluctuation in the Pleistocene are recorded in the alternations of subaerial exposure and accumulation of coral-bearing limestones in Clino and the nonskeletal grainstones and packstones of Unda. The ten alternations in the Upper Pleistocene (above the Brunhes/Matnyama boundary) interval matches the number of sea-level oscillations (glacial and interglacial stages) inferred from the oxygen isotope record in the deep sea.

Abstract: Species of reef corals are identified within lithologic units in the two Bahamas Drilling Project cores (Unda and Clino); species richness, frequencies of different colony shapes, and occurrences of indicator species are used to interpret reef environments. The results suggest that twelve Pliocene-Pleistocene coral-bearing units in the upper portions of Unda (26.2-180.8 m) were deposited under generally shallowing-upward conditions, ranging from intermediate (10-20 m) or deeper (> 20 m) forereef environments, to shallow (0-10 m) forereef environments, to shallow (0-10 m) and intermediate-depth (10-20 m) platform environments, in contrast, five Upper Miocene coral-bearing units in the lower portions of Unda (298.1-376.7 m) were deposited under generally deeper conditions, including (1) a small intermediate-depth (10-20 m) or deeper (20 to > 30 m) reefal unit and (2) intermediate-depth (10-20 m) to deeper (20 to > 30 m) forereef environments with large numbers of plate-shaped corals. Like the upper portions of Unda, eleven Pliocene-Pleistocene coral-bearing units in Clino (26.2-197.5 m) formed under shallowing-upward conditions, ranging from intermediate-depth (10-20 m) and deep (20 to > 30 m) forereef environments, to shallow (< 10 m) forereef environments, to shallow (< 10 m) or intermediate (10-20 m) platform environments. However, middle forereef deposits in the upper portions of Unda contain diverse assemblages dominated by Stylophora , whereas middle forereef deposits in Clino are dominated by Acropora . These interpretations generally correspond well with those based on lithology, and indicate that coral assemblages provide unique and important criteria for interpreting Late Cenozoic reefal deposits.

Abstract: The drilling of two core borings in the Neogene western Great Bahama Bank provided the basic descriptions of lithology for evaluations of the source and mode of deposition, diagenetic patterns, and the role of relative sea-level fluctuations. Comparison of these parameters with those of the Holocene revealed a consistent and general relationship between the position of sea level and the source and mode of deposition. The bulk of the slope sediment, more than 80%, was sourced by extensive offbank transport of fine-grained suspended sediment and minor pelagic sediment when the platform was submerged During sea-level lowstands, only minor amounts of reworked, coralgal sediment were deposited and rapidly cemented on the slope, whereas subaerial exposure surfaces developed on the platform and the margin. During the subsequent initial rise of sea level, marine hardgrounds developed on the slope and, in some cases, at the margin The marginal coralgal and skeletal shallow-water depositional environments were either reworked or retreated until sediment production was in equilibrium with sea level and offbank sediment transport of fines resumed. At least three different frequencies of sea-level fluctuations are recorded on the slope as sedimentary sequences, with a thickness ranging from meter scale up to approximately 170 m. At least five, and possibly seven, larger-scale sedimentary sequences ( A-G ), varying in age from late Miocene to Pleistocene, are recognized and correlated in both cores. Each sequence is characterized by distinct variations in composition, texture, and degree of cementation. At the margin, the depositional signature is less distinct and only the largest-scale sea-level fluctuations are recognized, local accumulation and removal by erosion of sediment at the shallow-water margin are suggested to obscure the recognition of sea-level-genera ted sedimentary packaging Two sequence boundaries are associated with global climatic events: (1) the expansion of the Antarctic ice Sheet and postulated fall of sea level of approximately 50 m at the Miocene-Pliocene boundary (sequence D ), and (2) the onset of the first post-Miocene cooling in the late Pliocene at approximately 3.2-2.8 Ma (sequence C ). The onset of first post-Miocene cooling (sequence C ) is associated with dramatic changes in depositional style: (1) from mixed skeletal and nonskeletal to predominantly peloidal, (2) increased rates of accumulation on the slope, or from predominantly aggrading to prograding, and (3) an increase in the frequency of sea-level fluctuations. it is suggested that these changes are associated with the change from a low-angle ramp to a flat-topped platform. Slope systems of well-documented ancient examples of carbonate platforms have usually steep flanks, are dominated by reef-margin-derived sand and rubble, and have a low potential of recording sea-level fluctuations. In contrast, as a result of intimate relationships between sediment sources, modes of accumulation, and relative position of sea level, the Neogene low-angle slope of western Great Bahama Bank contains a high-resolution archive of sea-level fluctuations.

Abstract: Independent analyses to date prograding carbonate platform-margin sediments have enabled development of an integrated chronostratigraphy of the west margin of the Great Bahama Bank (GBB). The chronostratigraphy permits interpretation of periods of deposition, nondeposition, and erosion on the platform and margin. The integrated chronostratigraphy is based on planktic foraminiferal biostratigraphy, calcareous nannofossil biostratigraphy, strontium-isotopestratigraphy, and magnetostratigraphy. Critical to interpreting each type of stratigraphy is a clear understanding of slope dynamics and the depositional system. The biostratigraphic data, in particular, are affected by the extreme dilution of key microfossils by platform-derived sediments during margin progradation. Because of this factor, the highest abundance of microfossils is restricted to thin units of pelagic sediment, deposited during temporary intervals when platform sediment supply was greatly reduced. However, these selective microfossil concentrations are also more likely to yield premature last-appearance datums and delayed first-appearance datums than the diluted intervals, because they represent short periods of time. Despite these problems, the microfossils provide critical age control. The detailed chronostratigraphy allows interpretation of a well-constrained platform-margin evolution. Core Unda, the more landward location, penetrated the oldest sediments (earliest late Miocene). The more seaward core Clino, although deeper, bottomed in younger sediments (latest Miocene). Three major progradational episodes were delineated using seismic stratigraphy, lithostratigraphy, and information on depositional age. Progradation occurred during the late Miocene, late early Pliocene, and latest Pliocene. In the Pliocene shelf and ramp setting margin progradation began during a highstand, but also occurred in a forced-regression-type situation during a fall in sea level. Rapid reef progradation occurred near the end of the Pliocene and early Pleistocene, when the platform had aggraded to a nearly horizontal surface. The transformation from a ramp-type platform topography to a flat-topped platform culminated in the middle Pleistocene. Age constraints across the west margin of GBB indicate that the seismic reflectors that constitute sequence boundaries are synchronous, within our age resolution. As expected in a slope setting, the sequence boundaries represent condensed time periods of both erosion and nondeposition. Downslope plankton-rich onlapping units are correlated to upslope marine hardgrounds and are thought to represent periods of falling sea level. Subsequent high-resolution dating of additional basinal and shallow-platform borings will provide a rich archive of paleoceanographic changes and will serve as a direct link between the deep-ocean and shallow-platform accumulations.

Abstract: Two cores drilled on the western margin of Great Bahama Banks penetrated a total of 1131 m of Quaternary to Late Neogene platform and periplatform carbonate sediments. These carbonates show a wide range of diagenetic textures ranging from largely unaltered primary carbonates to texturally mature limestones and dolostones. Although some of these carbonates have been diagenetically altered under freshwater conditions and show classic geochemical and petrographic indicators of such, other rocks exhibit similar petrographic features yet have never experienced meteoric influences. These features include micrite envelopes, moldic porosity, blocky spar cementation, and aragonite neomorphism The core Unda (453.8 m) includes three successions of shallow-water platform to reef sediments that alternate with deeper shelf deposits of silt to coarse sand. Clino, the more distal core, penetrated 677.3 m. It contains an upper reef to platform (21.6-140 m) overlying a thick package of deep forereef to upper and lower slope sediments (140-677.3 m). Three diagenetic zones were identified on the basis of similar diagenetic fabrics. Diagenetic zone 1 (Unda 0-108.08 m; Clino 0-152.71 m) is characterized by minor early marine diagenesis overprinted by numerous episodes of subaerial exposure with caliches, large-scale dissolution, and blocky spar cementation. Diagenetic zone II includes the lower reef and platform in Unda (292.82360.28 m) and has the most complex alteration in these cores. Minor early marine diagenesis was followed by several episodes of meteoric diagenesis, Most of the early fabrics, however, were destroyed during pervasive dolomitization in the burial environment. Diagenetic zone III includes deeper shelf to slope facies in both cores (Unda 108.08-292.82 m; 360.28-452.94 m; Clino 152.71-677.27 m) and shows only marine to marine-burial diagenesis. The exact fabrics within these intervals varies by lithology. Peloidal packstones to grainstones show minimal alteration (primarily compaction, minor dolomitization, and some recrystallization). Skeletal grainstones show two different styles of modification. Most commonly the grainstones are characterized by nearly complete dissolution of aragonitic components, minor cementation, and 10% to 20% early burial dolomite. Grainstones interbedded with the tight peloidal sediments, on the other hand, show nearly complete blocky spar cementation along with neomorphism of aragonitic skeletal grains (peloids were dissolved). Several marine hardgrounds with penecontemporaneous dolomite and/or phosphate and blackening also occur within the deeper-water facies. Each diagenetic zone in Clino and Unda is characterized by fabrics developed in one or more diagenetic environment. The larger-scale controls, therefore, are those that govern the diagenetic environment. For these young sediments, the most important are the depositional system (i.e., reef and platform versus deeper margin to slope) and the sea-level history. As a result, the diagenetic zones largely coincide with the depositional successions and seismic sequences identified in these cores. This study shows that the end product of marine to marine-burial diagenesis appears very similar to the end product of alteration in the phreatic meteoric environment i.e., a limestone composed of LMC and minor dolomite with micrite envelopes, moldic porosity, blocky spar cementation, and aragonite neornorphism. On this basis, a reevaluation is needed of the criteria for recognizing meteoric diagenesis in ancient carbonate sequences. Without clear physical evidence of subaerial exposure (e.g., caliche horizons or vadosecements) or chemical evidence of meteoric fluids (e.g., negative δ 18 O) great care is needed to identify meteoric diagenesis, In addition, the large-scale (> 100 m) sea-level lowstands of the latest Pleistocene did not result in significant phreatic meteoric diagenesis, perhaps because the fresh-water lens is too far below the recharge zone.

Abstract: Geochemical measurements (Ca 2+ , Sr 2+ , Mg 2+ , Cl − , SO 4 2- , and 87 Sr/ 86 Sr) have been made on fluids obtained from two deep boreholes (Unda and Clino) drilled on the Great Bahama Bank. As a result of the fact that these holes were drilled through cemented Limestones, it was not possible to squeeze sediments to obtain interstitial pore waters, and instead the fluids were retrieved using a combination of pumping from the surface and passive water samplers deployed during logging. Geochemical analyses show that it is possible using these techniques, combined with tritium, to define the extent of contamination of these pore fluids by surface waters. Consequently, with appropriate corrections for surface contamination, estimates of the interstitial concentrations of Ca 2+ , Sr 2+ , Mg 2+ , Cl − , and SO 4 2- and 87 Sr/ 86 Sr ratios can be obtained. In both holes, dissolution and precipitation of carbonate minerals such as aragonjte, calcite, and dolomite are taking place in slightly modified sea water, leading to significant Increases in the concentration of Ca 2+ and Sr 2+ and decreases in Mg 2+ . The absence of significant geochemical gradients in the upper zone of Unda suggests that there is substantial influx of modern seawater. Deeper in Unda and throughout Clino there are large increases in Sr 2+ and Ca 2+ suggesting a more closed diagenetic system. In both Clino and Unda the 87 Sr/ 86 Sr ratio of the pore fluids, when corrected for surface contamination, was either similar to or more radiogenic than the co-occurring sediment. This not only suggests that there is not a significant upward vertical flow through the platform, but also supports the notion that the 87 Sr/ 86 Sr ratios of carbonates in such settings can be used to give the oldest possible age of deposition and the youngest possible age of diagenesis.

Abstract: The Sr-isotope composition of pore fluids and co-occurring sediments (carbonates, phosphorites, and dolomites) have been measured from two cores retrieved from western margin of the Great Bahama Bank. The Sr-isotope compositions of the pore fluids, corrected for possible contamination by surface waters using tritium, indicate that the formation fluids have 87 Sr/ 86 Sr ratios less radiogenic than modern sea water, but in all instances considerably more radiogenic than the co-occurring sediments and rocks. Assuming that similar conditions of fluid movement have been prevalent since deposition, the Sr-isotopic composition of the sediments has been used to constrain the age of deposition and diagenesis. The 87 Sr/ 86 Sr ratios of the solid components can be treated in two ways. First, if it is assumed that the chronostratigraphy determined from biostratigraphic and magnetostratigraphic methods is correct, then the Sr-isotopic composition can be used to place constraints on the timing of diagenesis. Second, if the diagenetic alteration occurred relatively soon after deposition, then the Sr-isotope composition can constrain the age of deposition. The data from the sediments show that the 87 Sr/ 86 Sr ratios are in most instances close to the 87 Sr/ 86 Sr ratio calculated from the chronostratigraphic age. The only substantial disagreement between the Sr-isotope age and the biostratigraphic/magnetostratigraphic age occurs between 300 and 500 mbsf in Clino. This could indicate either recrystalli-zation after deposition by more radiogenic solutions or slight uncertainties in the chronostratigraphy in this interval.

Abstract: The downhole logs included in the pocket of this volume include the geophysical, geochemical, lithologic and petrophysical logs of borings Clino and Unda. Data displayed include mineralogy, stable isotopes, recovery, lithology, depositional fades, trace fossils, seismic sequences, porosity, and permeability. Geophysical logs include gamma ray, caliper, density, neutron, sonic velocity, and neutron porosity. These logs are plotted to the same scale to allow direct comparison of the data from the two borings. The upper Pliocene and Pleistocene reefs and platform-top deposits are readily recognizable on geophysical logs where they show higher amplitudes of density and velocity than the forereef and slope deposits. Some of the peaks coincide with subaerial exposure surfaces and their associated cemented intervals. It is not however possible to distinguish reef from platform top using logs alone. The Miocene reef facies in Unda stands out from surrounding slope facies with higher density and sonic velocity and much greater variability. The fine-grained slope facies show consistent low density and sonic velocity broken by interruptions or hardgrounds. The background values gradually increase with depth. The interruptions of platform-derived sediment are characterized by sonic velocity and density peaks of less than a meter to a few meters in thickness alternating with low velocity/density beds of similar thickness. Hardgrounds are identified by sharp gamma peaks and usually also have density and sonic velocity peaks either at or just below the hardground.

Abstract: Results from analyses of physical properties on discrete samples from Miocene to Pleistocene carbonates of two core borings in the Great Bahama Bank indicate that sonic velocity in carbonates is, unlike in siliciclastic sediments, not a function of burial depth. Velocity is much more controlled by the combined effect of (1) the depositional lithology of the carbonate sediment and (2) the diagenetic alterations that affect porosity and porosity type. In order to simulate in situ conditions in the laboratory, ultrasonic compressional-wave and shear-wave velocities of 89 carbonate samples were measured under varying confining pressures and stable pore-fluid pressures. The results are correlated with other petrophysical data such as porosity, density, or permeability as well as with lithological parameters taken from thin-section observations and X-ray analyses. Compressional-wave velocity (Vp) ranges between 1900 and 6400 m/s, and shear-wave velocity (Vs) between 1000 and 3500 m/s, which is a remarkable range for rocks with limited mineralogical variation like these carbonates. Because of their coarser grain size and their higher diagenetic potential, shallow-water carbonates have a wider range and reach higher velocities than deeper-water carbonates. The main factor that controls velocity is porosity, which shows a clear inverse correlation with velocity. However, we observe a large scatter in the velocity-porosity diagram, which is an effect of carbonate-specific pore types. The characteristic influence of these different pore types on the elastic properties explains why rocks with the same porosity can have very different velocities. For example, rocks with mainly moldic porosity have velocities that are more than 2500 m/s faster than same porous rocks with mainly interparticle porosity. The effect of carbonate mineralogy on the velocities is overshadowed by the effects of porosity and pore types. Completely dolomitized rocks are the fastest but also the slowest samples of the dataset. Rather than being controlled by the dolomite content, velocity is more a result of the type of dolomite and the associated porosity and pore type: fabric-destructive sucrosic dolomitization decreases velocity dramatically, whereas fabric-preserving, crystalline mimetic dolomitization results in very high velocities. Velocities measured in the laboratory are in general comparable with those obtained by the sonic wire-line log, though some differences occur in the velocity range, which are mainly caused by the difference in measurement scales of the two techniques. The continuous recording of the sonic log completes and improves the correlation of the downhole velocity variations with the changes in lithology. Many of the major lithological boundaries that are observed in the cores and that have sequence stratigraphic significance also have a distinct signature on the sonic log.

Abstract: Permeability was measured on cores from two wells (Clino and Unda) from the western margin of Great Bahama Bank. Permeability varies from < 1 md to > 8000 md and shows no simple correlation with porosity. Pore types, however, are important in determining permeability. Macropore types include intergranular, intragranular, intercrystalline, moldic, and vuggy to cavernous. Micropore types include both primary and secondary intercrystalline pores. Moldic pores occur as either isolated molds or connected into an open pore network. On the basis of the most abundant pore type, samples with intercrystalline porosity show the highest permeability (median = 1041 md), followed by those with connected-moldic pores (median = 529 md), and then by those with intergranular pores (median = 123 md). Moldic pores are generally considered not to contribute to permeability because they are often isolated from other pores. Many of these samples, however, have moldic pores that are well connected in a continuous pore network, As a result, these molds contribute to, and even control, the permeability. The permeability distribution in the cores can be related to the original lithology and to diagenesis, because both control the pore types present. Meteoric diagenesis of shallow-water facies leads to generally low matrix permeability (median = 9 md), but the permeability is controlled by vuggy to cavernous porosity that we cannot measure. Deeper-water facies were altered in the marine burial environment, and are characterized by either moldic porosity with minimal cementation, or neomorphism with nearly complete cementation. Peloidal sediments compacted early and show little diagenetic alteration. The poorly cemented skeletal intervals have moderate to high permeability (range 0.1-8445 md) generally controlled by the degree of connectivity between moldic pores. The peloidal and well-cemented skeletal intervals have low permeability (range 0.1-198 md), reflecting the controlling role of microporosity. Permeability in the dolomitized portion of Unda, the more proximal core, is high (median = 1274 md) due to the predominance of intercrystalline porosity. On the basis of this study, the most important factor controlling permeability in these young carbonates is the specific pore type rather than the amount of porosity. In addition, the principal pore type can be related to the diagenesis and, to a lesser degree, the depositional facies. Although intercrystalline porosity from dolomitization and vuggy porosity from meteoric diagenesis have long been known to control permeability where they occur, this study also documents the importance of the connected-moldic porosity that forms during marine burial diagenesis.

Abstract: During the Neogene, western Great Bahama Bank prograded more than 25 km into the Straits of Florida. This progradation occurred in pulses seen on seismic lines as distinct seismic sequences. Two core holes (Unda and Clino) were drilled through the proximal parts of these prograding carbonate sequences to test several fundamental questions in carbonate sedimentology, diagenesis, and sequence stratigraphy. The sequence strarigraphic objectives of the drilling were to (1) provide information on facies within the seismic sequences, (2) test if predicted facies changes coincide with seismic sequence boundaries and inferred systems tracts, (3) determine the ages of sequence boundaries, and (4) test whether the prograding sequences were sea-level controlled, and if so, provide information about the timing and amplitude of these relative changes in the later part of the Neogene. The cores at sites Unda and Clino penetrated eight seismic sequences that have a Mid/Late Miocene to Recent age. The sedimentary successions in the cores display a repetitive pattern of changing facies that can be related to changes in sea level. Sea-level falls are indicated on the slope by either hardgrounds, firmgrounds, or coarser-grained beds, whereas caliche horizons or karst surfaces document exposure on the platform. Aggrading shallow-water carbonates and thick fine-grained slope deposits are interpreted to be deposits of sea-level highstands. These lithologic indications of changing sea level coincide with the interpretation of the seismic sequence architecture. Seismic sequence boundaries correlate to horizons and/or sedimentary packages indicative of sea-level falls, whereas prograding pulses coincide with the thick slope sections deposited during sea-level highstands. In addition to the facies there is a strong correlation between diagenesis and the seismic sequences. This relationship can be seen in both the paragenetic sequences and the variations in mineralogy. In core Unda, all paragenetic boundaries coincide with sequence boundaries. In the slope section of Clino, mineralogical changes occur across diagenetic horizons, e.g., hardgrounds, which also coincide with seismic sequence boundaries. These surface-bounded changes in diagenesis give strong indications that changes in sea level alter the diagenetic potential of the carbonates within the sequence stratigraphic pattern. Petrophysical analyses document that the combination of changing facies and diagenesis across sequence and systems-tract boundaries produces the necessary impedance contrasts for seismic reflections along stratigraphic surfaces. The ages of the eight seismic sequence boundaries were determined using a combination of biostratigraphy, Sr-isolope stratigraphy, and magnetostratigraphy. The ages of the sequence boundaries are the same in both drill sites and do not cross seismic reflections, documenting that seismic reflections separate sedimentary packages of different ages. This age consistency confirms one of the basic assumptions of sequence stratigraphy, i.e., that seismic reflections are time lines and have chronostratigraphic significance. The duration for the deposition of the individual sequences is between 0.5 and approximately 1 Myr, indicating that the sequences are produced by third-order sea-level fluctuations. The sequence architecture reflects a long-term pattern of sea-level changes that is in good agreement with the known global sea-level history. A general lowering of sea level at the end of the Middle Miocene resulted in progradation of the platform and the development of a margin at drill site Unda. At about 8.9 Ma progradation was interrupted and the margin stepped back because of a sea-level rise. The lowering of sea level in the latest Miocene and the subsequent sea-level lowstand that lasted during most of the Messinian is recorded in Unda by the development of a reef on the former slope deposits and recognized on the seismic line as a major erosional unconformity at the margin. Backstepping of the platform margin in the early Pliocene indicates a rapid flooding, which is also a major transgression on the global chart. The high-amplitude sea-level changes in the Pleistocene are recorded as laterally stacked sequences with increasingly higher angles in the foresets.

Abstract Subsurface Geology of a Prograding Carbonate Platform Margin, Great Bahama Bank: Results of the Bahamas Drilling Project - This volume will be of special interest to carbonate sedimentologists, geochemists, petroleum geologists, engineers, and seismologists. It addresses fundamental aspects of prograding carbonate platforms in a Neogene example from Great Bahama Bank. A remarkable seismic profile, which imaged the prograding margin, provided the seismic stratigraphic framework. Two continuouslycored and logged borings on the profile produced the ground truth for testing and characterizing processes: lithologies and ages of sequence boundaries; influence of sea level fluctuations on progradation, controls on impedance contrasts in carbonates; fluid flow through the submerged margin; log responses of different lithologies; and the origin, ages and depositional environments of the platform top and prograding clinothems. The new findings on diagenesis are of special interest, including complete mineral stabilization in seawater, early burial dolomitization related to sequence boundaries and how diagenesis controls sonic velocity and permeability.

Abstract Sequence stratigraphic interpretations of carbonate platform margins are based to a large degree on concepts of variable timing and nature of deposition relative to fluctuations in sea level. Quaternary platform margins, such as those found in the Bahamas, provide a unique opportunity to calibrate the sedimentary record because of the well-constrained nature of sea-level history during this period. Detailed observations and sampling from a research submersible combined with high-resolution radiocarbon dating in the Tongue of the Ocean, Bahamas, have enabled us to document variations in deposition along the upper parts of the marginal slope during the most recent rise in sea level. We have found that the steep marginal slopes around the Tongue of the Ocean record deposition during the early rise of sea level following the last lowstand some 18–21 Ka. Coarse-grained skeletal sands, gravel, and boulders derived from reefs growing along the overlying escarpment were deposited on slopes of 35–45º and cemented in place within a few hundred years. Deposition by rockfall and grainflow resulted in a series of elongate lenses oriented parallel to the slope. These lenses are generally less than 0.5 m thick and pinch out downslope within tens of meters. Repeated deposition and cementation produced slope deposits that are both laterally discontinuous and internally heterogeneous. Radiocarbon dating of skeletal components and cements indicate that active deposition on the slopes ceased approximately 10,000 years ago as sea level rose above the escarpment and began to flood the top of the Great Bahama Bank. Fine-grained, nonskeletal sands and muds derived from the platform are presently bypassing these slopes and are deposited downslope as a wedge of sediment with slope declivities of 25–28º. Cracks and slide scars are a common feature of the steep-cemented slopes. The cracks are a few centimeters wide and may extend for tens of meters across the slope with an arcuate, convex-up expression. The slide scars are generally a few meters wide by several meters long and cut back into the slope a few meters to less than 1 m, although one large example is 30 m wide, extends downslope for 75 m, and has exposed 10 m of the interior of the slope. Transects downslope from slide scars show that large blocks of the slope, some in excess of 10 m across, have been transported for tens or hundreds of meters downslope. The release and transport of such blocks may be one mechanism by which turbidity currents are initiated in deeper slope environments.

Abstract Seismic profiles across the top of northwestern Great Bahama Bank reveal that the modern bank is formed by the coalescence of three smaller platforms. This coalescence resulted from progradation of the bank margin during the Cenozoic. Since the Late Cretaceous, vertical aggradation on the banks has been approximately 1,500 m, whereas the leeward bank margin has migrated as much as 25 km, indicating that lateral growth can dominate the growth direction of a healthy platform with the capability of transporting offbank excess sediment. In addition, asymmetric progradation of the leeward margin indicates the important role of physical energy on the direction of platform expansion. A phase of filling preceded the progradation and led to shallowing of the intraplatform seaways and reduction of their slope height and declivity. Progradation began when slopes were approximately 500 m high and/or had angles less than 5°. The pattern and direction of progradation is governed by prevailing currents and changes in sea level. Progradation occurred in pulses that are recognized in the seismic lines as discrete sequences of sigmoidal clinoforms. Each prograding sequence is interpreted as the record of a relative rise and highstand of sea level, when sediment production on the bank top was high and cross-bank currents moved excess sediment offbank. Repeated fluctuation of sea level resulted in a series of horizontally stacked sigmoidal sequences that form two distinct bundles of sequences. Correlation with the Great Isaac exploration well suggests that progradation began in the late Oligocene in the Straits of Andros, a buried intraplatform seaway, and in the late Miocene in the Straits of Florida. If these age assignments are correct, the two bundles of sequences coincide with the youngest second-order cycles TB2 and TB3 of the sequence stratigraphy curve of Haq and others (1987). The number of sequences within the bundles nearly matches the number of third-order cycles in the curve of Haq and others (1987). Furthermore, the shape of the onlap curve in the Bahamas is similar to that of the curve determined in siliciclastic terrains. Assuming that the correlation between the Bahamas curve and the global curve is correct, we propose specific ages for the sequence boundaries and a demonstration of the potential of platform carbonates to provide a legible record of the relative sea-level fluctuations of a second- and third-order magnitude.