Carbonate slope morphology revealing a giant submarine canyon (Little Bahama Bank, Bahamas)

International audienceNew high-quality multibeam data detail the morphology of the giant 135-km-long Great Abaco Canyon (GAC) located between Little Bahama Bank (LBB, Bahamas) and Blake Plateau. Knickpoints, chutes, and plunge pools mark the canyon main axis, which is parallel to the LBB margin. The canyon head covers a large area but does not represent the main source of the modern sediments. The material supplied through the LBB canyon systems originates below this head, which only shows erosive lineaments related to the pathway of currents running along the seafloor and restricted failure scars. Most of the sediment supply originates from the canyon sides. The northern canyon flank incises the Blake Plateau, which comprises contourites on top of a drowned Cretaceous carbonate platform. These deposits are susceptible to translational slides and form dissymmetric debris accumulations along the northern edge of the canyon. A large tributary drains the Blake Plateau. Two large tributaries connecting the southern flank of the GAC directly to the LBB upper slope form additional sources of sediments. Subbottom profiles suggest the presence of a sedimentary levee on the tributary canyon and of sediment gravity flow deposits. The GAC has been a permanent structure since the drowning of the Cretaceous platform, and its size and morphology are comparable to those of canyons in siliciclastic environments. The orientation of the GAC parallel to large-scale regional tectonic structures suggests a structural control. The size of the observed structures, especially plunge pools at the base of gigantic chutes, is unusual on Earth. The presence of deposits downflow of the pools suggests that the GAC results from or at least is maintained by persistent and sustained submarine gravity flows rather than by retrogressive erosion

Quaternary sedimentary processes on the Bahamas: From platform to abyss

Understanding the interaction between sediment production on and export from shallow-water areas of platforms and slopes is primordial when assessing sedimentary processes on a carbonate-platform scale. In this manuscript we explore variations in facies, sediment export, sediment deposition and reorganisation, hydroacoustic- and small-scale sedimentary structures, but also assess the variability in current systems as observed during the Quaternary for the north-facing margin of Little Bahama Bank (LBB) and compare those characteristics with features observed on other slopes surrounding LBB and Great Bahama Bank (GBB). Over the past decade, the northern margin of LBB was explored during a series of oceanographic cruises of the CARAMBAR project, which included the collection of 24.270 km2 of bathymetry data, 6.398 km of very high-resolution seismic profiles, and 42 cores covering water depths ranging from 177 m to 4873 m. This study evaluates the results obtained from the analysis of sediment cores retrieved in the Great Abaco Canyon area (GAC), located between the lower northern LBB slope, which is connected to the abyssal plain. The analysis of the shallower parts of the LBB slope relies on earlier studies and are complementary to our data, and allow for a detailed analysis of the sedimentary processes acting along the entire LBB slope. The data reveal that Quaternary sediment distribution differs when moving from the north-eastern to the north-western LBB slope. The entire LBB slope is dissected by numerous canyons. Gravity processes enriched in coarse platform components occur infrequently and are concentrated within lobes in the east. Only coarse-grained rich bank facies can concentrate coarse grains on this margin. The western LBB slope is mostly influenced by fine-grained platform export and current circulation. The deeper GAC area is dominated by pelagic sediments that are supplied from the canyon sides and through tributaries. The sediment composition confirms that pelagic sediment production and current movements determine the sediment-deposition and redistribution processes at this site. The comparison with other Bahamian slopes demonstrates that a leeward position agrees with high sedimentation rates on the slopes resulting in specific morphologic structures, such as gullies and sediment waves, related to fine-grained sediment export, whereas platform-derived coarse-grained facies are deposited downstream in larger structures, such as canyons, that are not affected by the main wind direction. Slope angle could also have an impact on grain-size export, as it appears that coarse-grained deposits are frequent on steep slopes like those bordering the Exuma Sound basin

Into the deep: A coarse-grained carbonate turbidite valley and canyon in ultra-deep carbonate setting

International audienceNew high-resolution multibeam mapping images detail the southern part of Exuma Sound (Southeastern Bahamas), and its unchartered transition area to the deep abyssal plain of the Western North Atlantic, bounded by the Bahama Escarpment extending between San Salvador Island and Samana Cay. The transition area is locally referred to as Exuma Plateau. The newly established map reveals the detailed and complex morphology of a giant valley draining a long-lived carbonate platform from its upper slope down to the abyssal plain. This giant valley extends parallel to the slope of Long Island, Conception Island, and Rum Cay. It starts with a perched system flowing on top of a lower Cretaceous drowned main carbonate platform. The valley shows low sinuosity and is characterized by several bends and flow constrictions related to the presence of the small relict isolated platforms that kept alive longer than the main platform before drowning and merging tributaries. Turbidite levees on either side of the valley witness the pathway of multiple gravity flows, generated by upper slope over steepening around Exuma Sound through carbonate offbank transport, some of them locally >15°, and resulting slumping. In addition, additional periplatform sediments are transported to the main valley through numerous secondary slope gullies and several kilometre-long tributaries, draining the upper slopes of cays and islands surrounding Exuma Plateau. Some of them form knickpoints indicating surincision of the main Exuma Valley which is consistent with an important lateral supply of the main Exuma Valley. Prior to reaching the abyssal plain, the main valley abruptly evolves into a deep canyon, 5 km in width at its origin and as much as 10 km wide when it meets the abyssal plain, through two major knickpoints named “chutes” with outsized height exceeding several hundred of meters in height. Both chutes are associated with plunge pools, as deep as 200-m. In the deepest pools, the flows generate a hydraulic jump and resulting sediment accumulation. When the canyon opens to the San Salvador abyssal plain, the narrow, deep, and strong flows release significant volume of coarse-grained calcareous sediments in numerous turbidite layers interbedded with fine mixed siliciclastic and carbonate sediments transported by the Western Boundary Undercurrent (WBUC) along the Bahama Escarpment. Carbonate gravity flows exiting the canyon decelerate at the abyssal plain level and construct a several-kilometre-wide coarse-grained deep-sea turbidite system with well-developed lobe-shape levees, partially modified by the flow of strong contour-currents along the Bahama Escarpment

Lower Jurassic Bahamian-type facies in the Choč Nappe (Tatra Mts, West Carpathians, Poland) influenced by paleocirculation in the Western Tethys

The Lower Jurassic (upper Sinemurian) of the Hronicum domain (Tatra Mts., Western Carpathians, Poland) represents typical tropical shallow-water carbonates of the Bahamian-type. Eight microfacies recognized include oolitic-peloidal grainstone/packstone, peloidal-bioclastic grainstone, peloidal-lithoclastic-bioclastic-cortoidal grainstone/packstone, peloidal-bioclastic packstone/grainstone, peloidal-bioclastic wackestone, spiculitic wackestone, recrystallized peloidal-oolitic grainstone and subordinate dolosparites. The studied sediments were deposited on a shallow-water carbonate platform characterized by normal salinity, in high-energy oolite shoals, bars, back-margin, protected shallow lagoon and subordinately on restricted tidal flat. Some of them contain the microcoprolite Parafavreina, green alga Palaeodasycladus cf. mediterraneous (Pia) and Cayeuxia, typical of the Early Jurassic carbonate platforms of the Western Tethys. The spiculite wackestone from the upper part of the studied succession was deposited in a transitional to deeper-water setting. The studied upper Sinemurian carbonates of the Hronicum domain reveal microfacies similar to the other Bahamian-type platform carbonates of the Mediterranean region. Thereby, they record the northern range of the Lower Jurassic tropical shallow-water carbonates in the western part of the Tethys, albeit the thickness of the Bahamian-type carbonate successions generally decrease in a northerly direction. The sedimentation of the Bahamian-type deposits in the Hronicum domain, located during the Early Jurassic at about 28^{\circ}N, besides other specific factors (i.e., light, salinity, and nutrients) was strongly controlled by the paleocirculation of warm ocean currents in the Western Tethys