Une analyse de données de sondeur multifaisceaux, de sondeur de sédiments (Chirp) et de sismique multitraces, présente la morphologie et la dynamique sédimentaire actuelle ainsi que l’évolution architecturale et stratigraphique du Néogène-Quaternaire opérant le long d’une marge leeward, au nord-ouest du Great Bahama Bank. L’analyse morpho-bathymétrique révèle un talus dominé par de la boue aragonitique, et une grande variété de structures sédimentaires, liées à des processus hydrodynamiques diversifiés. Les courants de density cascading associés aux courants de marée et aux vents d’est représentent les mécanismes de transports édimentaire dominant le long de la marge. La zone de Bimini présente localement un talus court et abrupt,stabilisé en bordure de plate-forme par une barrière récifale, ce qui limite l’export off-bank de sédiments.L’architecture stratigraphique montre une évolution complexe du talus au cours du Néogène. La fin du Paléogène est marquée par un talus continu aggradant détaché de la plate-forme par un escarpement sur faille. Ilpasse à un système très peu incliné de type slope-apron, attaché à la plate-forme au Miocène et se termine par un système en accrétion fortement incliné au Pléistocène. Bien que les dépôts de talus soient dominés par de la boue, ils présentent des variations latérales rapides du Sud au Nord tout au long du Néogène (< 30 km) avec la mise en place de tabliers turbiditiques, de nappes de débrites, et de larges Mass Transport Complexes. Ces faciès interagissent depuis le Langhien avec le Santaren Drift qui s’étend et migre progressivement le long du talus jusqu’à aujourd’hui. Le maximum d’empiétement des contourites se produit au Pliocène supérieur et coïncide avec une réorganisation océanographique globale ainsi que des changements climatiques majeurs dans l’hémisphère nord liés à la fermeture de la Central American Seaway. / An analysis of multi-beam echo sounder, sub-bottom profiler (Chirp) and multichannel seismic,highligths the present-day sedimentary dynamics and the Neogene-Quaternary architectural and stratigraphicevolution along the northwestern leeward margin of the Great Bahama Bank.The morpho-bathymetric analysis reveals an aragonite mud-dominated slope, and a broad spectrum ofsedimentary structures, related to various hydrodynamic processes. The density cascading currents associatedwith tidal currents and prevailing easterly winds correspond to the dominant transport mechanisms operatingalong the margin. The Bimini area displays a short and steep slope, stabilized at shelf edge by rimmed reefalbarrier, which constrains the off-bank export of materials.The stratigraphic architecture shows a complex evolution of the slope during the Neogene. The end ofthe Paleogene is marked by a continuous aggrading slope detached from the shelf by a fault escarpment. It passesinto a low angle slope-apron attached to the platform in the Miocene, and ends with an accretionary system witha steepened slope in the Pleistocene. Although the slope deposits are mud-dominated, they show rapid lateralvariations (< 30 km) from South to North throughout the Neogene with the establishment of turbidite aprons,debrite layers and large Mass Transport Complexes. These facies interact since the Langhian with the SantarenDrift which gradually extends and migrates along the slope until today. The maximum extent of the drift occursduring the upper Pliocene and coincides with a global oceanographic reorganization and major climate changesin the northern hemisphere, related to the closure of the Central American Seaway.

Cette étude présente une reconstruction du fonctionnement sédimentaire au Quaternaire de deux pentes carbonatées des Bahamas (leeward et windward). Elle est basée sur les analyses stratigraphique et sédimentaire de 34 carottes principalement collectées lors de la mission CARAMBAR (2010). Une étude stratigraphique à haute résolution a été développée à partir de plusieurs outils stratigraphiques, dont l’écostratigraphie basée sur les assemblages de foraminifères planctoniques qui a permis de dater l’ensemble des séquences sédimentaires en s’affranchissant des problèmes liés à la diagenèse précoce. L’analyse sédimentaire des carottages a permis de proposer une nouvelle classification des sédiments carbonatés meubles qui caractérise les fractions dominantes du sédiment au moment du dépôt. Elle permet notamment de discriminer les dépôts fins et d’identifier les processus de transport off-bank, les dépôts contouritiques, gravitaires ou légèrement diagenétisés. Les processus de transport off-bank permettent l’accumulation de boue carbonatée de plate-forme (aiguilles d’aragonite et silts) au niveau des pentes des Bahamas lorsque la plate-forme est ennoyée. Les derniers 4ka cal BP, le MIS 5e et le début du MIS 11 sont les principales périodes d’accumulation sédimentaire au cours des derniers cycles climatiques. L’érosion intra-pente, l’action des courants de fond et les processus de diagenèse précoce ont également été mis en évidence. La pente leeward se caractérise par une sédimentation très importante (wackestone) alors que la pente windward a conservés a morphologie pliocène avec localement des accumulations sédimentaires quaternaires de faible épaisseur (wackestone à packstone). / This study proposes a reconstruction of the quaternary sedimentary history of two Bahamian slopes (leeward and windward). This work is based on the stratigraphicand sedimentary analysis of 34 marine cores recovered mainly from the CARAMBAR oceanographic cruse (2010). A high-resolution stratigraphic study was conducted by coupling several tools including ecostratigraphy based on planktonic foraminifera assemblages, which allowed dating all sedimentary sequences despite early diagenesis processes. The sedimentary analysis of the marine cores proposes a new classification for carbonate sediment. It characterizes the dominant particle distributions at the time of deposition and allows for discriminating the fine sediments and for determining the depositional processes such as off-bank transport, contouritic or gravity deposits, and slightly altered sediment. Off-bank transport is the mainprocess responsible for high-accumulation of platform carbonate ooze (aragonite needles and silt) on the Bahamian slopes. It predominates during major flooding of the platform, so during the last 4 ka cal BP, the MIS 5e and at the on set of the MIS11. Intra-slope erosion, bottom currents, and early marine diagenesis processes have also been highlighted. The leeward slope is characterized by a high accumulation of wackestone sediment where as the windward slope preserved its Pliocene morphology with local quaternary sediment accumulation (wackestone to packstone).

Although carbonates hold more than 60 percent of the world's oil reserves, they, nevertheless, exhibit much lower average recovery factor values than terrigenous sandstone reservoirs. Thus, utilization of advanced enhanced oil recovery (EOR) techniques such as high pressure CO2 injection may normally be required to recover oil in place in carbonate reservoirs. This study addresses how different rock types can influence the seismic monitoring of CO2 sequestration in carbonates. This research utilizes an elastic parameter, defined in a rock physics model of poroelasticity and so-­called as the frame flexibility factor, to successfully quantify the carbonate pore types in core samples available from the Great Bahama Bank (GBB). This study shows that for carbonate samples of a given porosity the lower the frame flexibility factors the higher is the sonic wave velocity. Generally, samples with frame flexibility values of <4 are either rocks with visible moldic pores or intraframe porosity; whereas, samples with frame flexibility values of >4 are rocks with intercrystalline and microporosity. Hence, different carbonate pore geometries can be quantitatively predicted using the elastic parameters capable of characterizing the porous media with a representation of their internal structure on the basis of the flexibility of the frame and pore connectivity. In this research, different fluid substitution scenarios of liquid and gaseous CO2 saturations are demonstrated to characterize the variations in velocity for carbonate-specific pore types. The results suggest that the elastic response of CO2 flooded rocks is mostly governed by pore pressure conditions and carbonate rock types. Ultrasonic P-­wave velocities in the liquid-­phase CO2 flooded samples show a marked decrease in the order of 0.6 to 16 percent. On the contrary, samples flooded with gaseous-­phase CO2 constitute an increase in P-­wave velocities for moldic and intraframe porosities, while establishing a significant decrease for samples with intercrystalline and micro-­porosities. Such velocity variations are explained by the stronger effect of density versus compressibility, accounting for the profound effect of pore geometries on the acoustic properties in carbonates. The theoretical results from this research could be a useful guide for interpreting the response of time-­lapse seismic monitoring of carbonate formations following CO2 injection at depth. In particular, an effective rock-­physics model can aid in better discrimination of the profound effects of different pore geometries on seismic monitoring of CO2 sequestration in carbonates.

Observations of modern carbonate depositional environments and their accompanying depositional models have been used for decades in the reconstruction and interpretation of ancient carbonate depositional environments and stratigraphic successions. While these Holocene models are necessary for interpreting their more ancient counterparts, they inherently exclude important factors related to the erosion, diagenesis, and ultimate preservation of sediments and sedimentary structures that are ubiquitous in shallow marine carbonate environments. Andros Island, Bahamas is an ideal location to examine the validity of Holocene conceptual models, where geologically young (Late Pleistocene) limestones can be studied immediately adjacent to their well-documented modern equivalents. For this study, two 3D ground-penetrating radar (GPR) datasets (200 MHz and 400 MHz) were collected at a schoolyard in northwest Andros. These surveys reveal the geometries and internal characteristics of a peloidal-oolitic sand wave and tidal channel in unprecedented detail. These two prominent features are underlain by low-energy lagoonal wackestones and packstones, and are bordered laterally to the northwest by wackestones-packstones intermixed with thin sheets of peloidaloolitic grainstone. A deeper radar surface is observed at approximately 6 m depth dipping gently to the west, and is interpreted to be a karstified exposure surface delineating the base of a complete depositional sequence. Interpretation of the 3D radar volumes is enhanced and constrained by data from three cores drilled through the crest and toe of the sand wave, and through the tidal channel. This study is the first of its kind to capture the complex heterogeneity of a carbonate depositional package in three dimensions, where various depositional environments, sedimentary structures, and textures (mudstone to grainstone) have been preserved within a small volume.The results from this study suggest that the degree of vertical and lateral heterogeneity in preserved carbonate successions is often more complex than what can be observed in modern depositional environments, where sediments can generally only be observed in two dimensions, at an instant in time. Data from this study demonstrate the value of using two overlapping GPR datasets at differing resolutions to image the internal characteristics of a complete carbonate depositional package in three dimensions. From these datasets, a depositional model similar to other Holocene and Pleistocene carbonate depositional models is derived.