Salt pillows and localization of early structures: case study in the Ucayali basin (Peru).In Nemcok

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Sedimentology of Modern Bahamian Carbonate Slopes: Summary and Update

International audienceSlopes adjacent to the Bahamian carbonate platform revealed a large variety of depositional processes. In this study, we present a synthesis summarizing 109,000 km2 of bathymetric and reflectivity data with ∼7,900 km of seismic lines and 311 m of sediment cores that were obtained over the last 50 years. These data are used to develop a conceptual model of sedimentation patterns on Quaternary carbonate slope systems and their interaction with the adjacent shallow-water carbonate platforms. Our data highlight that during the Quaternary, factors controlling large-scale sedimentation on Bahamian slopes have numerous similarities as they have higher sedimentation rates during interglacials. At a small scale, every slope has its own characteristics that are contemporary controlled by two main characteristics: (a) facies on the adjacent shallow-water platform, and (b) the impact of shallow- and deep-water currents. Large-scale tectonics influence sediment deposition as it determines the position of the islands and impacts platform facies distribution

Carbonate slope morphology revealing sediment transfer from bank-to-slope (Little Bahama Bank, Bahamas)

International audienceNew high-quality multibeam and high-resolution seismic data reveal new observations on sedimenttransfer and distribution and margin morphometrics in the uppermost slope of Northeastern LittleBahama Bank between 20 and 300 m water depth. The echofacies/backscatter facies show an alongslopesediment distribution forming successive strips. The upper part of the uppermost slope corresponds tothe alternation of several submerged coral terraces and escarpments that could be related to LateQuaternary sea-level variations. The terraces could either be related to periods of stagnating sea-level orslow-down in sea-level change and therefore increased erosion by waves, or periods of accelerated sealevelrise since the Last Glacial Maximum. Terraces could therefore be related to coral construction anddrowing. The medium part corresponds to the marginal escarpment, a steep cemented area. The lowerpart of the uppermost slope shows a discontinuous Holocene sediment wedge with varying thicknessbetween 0 and 35 m. It is separated from the upper part by a zone of well-cemented seafloor associatedwith the marginal escarpment. Passing cold fronts result in sediment export caused by density cascading.The associated sediment fall-out and convective sedimentation can generate density currents that formthis wedge and eventually flow through linear structures on the upper slope. The survey reveals thepresence of recently active channels that extend over the entire uppermost slope and interrupt thewedge. The channels connect shallow tidal channels to submarine valleys connected to the proximal partof canyons. They directly feed the canyons with platform-derived sediment forming low-densityturbidity currents and could supply the deepest part of the system with coarse-grained sedimentdirectly exported from the carbonate platform

New insights into the morphology and sedimentary processes along the western slope of Great Bahama Bank.

New high-quality multibeam and seismic data image the western slope of the Great Bahama Bank and the adjacent floor of the Straits of Florida. The extensive survey reveals several unexpected large- and small-scale morphologies. These include bypass areas, channel-leveelobe systems, gullied slopes, and products of slope instabilities at various scales, including long slump scars at the lower slope and mass transport complexes that extend ~30 km into the adjacent basin floor. The toe of the slope is irregularly covered with deep-water carbonate mounds. The abundance of the individual morphological features varies from north to south. From 26°00'N to 25°20'N, the slope is dissected by numerous deep canyons that abruptly end southward, where the slope is characterized by a smooth lower portion and small regularly spaced furrows in its upper part. Further south, two long (25-50 km) scars document instability at the lower slope. One of these scars is the source area of a large mass transport complex. In addition to this large-scale feature, several types of gravity-induced sedimentary processes are revealed. Most of the morphologies and inferred processes of this carbonate system are similar to those observed in siliciclastic systems, including mass transport complexes, gravity currents initiated by density cascading, and overspilling channeled turbidity currents. For the first time, a clear asymmetric channel-levee system has been identified along the slope, suggesting similitude in sorting processes between carbonate and siliciclastic systems and enhancing the reservoir-bearing potential of carbonate slopes. Notable differences with siliciclastic systems include: the lack of connection with the shallow and emerged part of the system (i.e., bank top), and the small size of the sedimentary system. © 2012 Geological Society of America

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

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

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