New high-quality multibeam data depict the area located between Little Bahama Bank (Bahamas) and Blake Plateau (Leg 1) and the area located between the mouth of Exuma Sound and the San Salvador slope (leg 2).

Generalities on giant canyons.

The Carambar 2 cruise allowed the characterization of giant canyons, Great Abaco and Little Abaco, Exuma, Great Bahama (Mulder et al. 2017a; 2018; Recouvreur et al., 2020). These canyons, whose length is greater than 150 km, are of recent age for some (Middle Miocene for Great Abaco). They are all of extension parallel to the platform edge because they are aligned with deep structural accidents probably inherited from the rupture of the carbonate megabank in the Mesozoic. They are fed mostly by their flanks, either by landslides (Great Abaco), or by mud turbidites, or even by bioclasts probably evacuated by the phenomenon of tidal flushing (Exuma, Little Abaco, Great Bahama. The Exuma canyon has constructed a large lobe-shaped levee channel system, extending over about 50 km in which the coarse, bioclastic sediments deposited by gravity events during periods of high sea level are intercalated with terrigenous clays brought by the currents (see study below). Depositional systems are in place at the outlet of these canyons. The one at the outlet of Great Abaco Canyon is essentially muddy but those at the outlet of Exuma Sound appear to be more bioclastic (see detailed study below). Furthermore, the seismic data on Great Abaco Canyon suggest a recent age of the canyon (post-Middle Miocene). This recent age requires a better knowledge of the building processes of the platform and of canyons such as Grand Bahama Canyon) to confirm such an age. The analysis of seismic data intersecting the Grand Bahama Canyon should provide answers as to the beginning of the canyon's functioning and its potential structural control.

 

Leg 1

The survey during Leg 1 reveals the morphology of a giant 135-km-long canyon, the Great Abaco Canyon (GAC, Figs. 1 and 2) and its main characteristics. The canyon main axis runs parallel to the margin. The pathway shows several knickpoints and plunge pools. The most important knickpoint is underlined by an abrupt change in slope of the canyon thalweg. The last one leads to the opening towards the Blake Basin. Its head forms a vast receptacle but does not represent the main source of material at present time (Fig. 3A). The material supplied through the Little Bahama Bank (LBB) canyon systems does not reach this area which only shows lineaments related to the pathway of the Antilles current (Fig. 4B) and restricted failure scars. Most of the supply comes from the canyon flanks (Fig. 4C). In the north, tributary canyons drain the contourite deposits forming large flat plateaus above the drowned carbonate platform of the Blake Plateau. In addition, these contourite plateaus are subjected to translational slides moving towards the northern edge of the canyon forming a dissymmetric debris accumulation along the toe of the north canyon edge. Another source of sediment are two large tributaries connecting the GAC directly to the LBB upper slope. Sub bottom profiles suggest the presence of a turbiditic levee on the tributary canyon sides and inferred turbiditic activity. A channeled lobe built at its mouth (Figs 2 and 4A). Little Abaco Canyon (LAC) shows morphologic similarities with GAC but at a smaller size. However, the canyon seems more active in terms of sediment transport. Small gullies and lobes discharge into the canyon (Fig. 3B). Canyons draining the eastern part of LBB show fresh sedimentary structures (sediment waves; Fig. 4D) suggesting active sedimentary processes. These structures are made of clean sand with shallow water organisms suggesting a direct supply from the carbonate platform edge. In term of size and morphology, the GAC compares to largest canyons in siliciclastic environments. Its originality comes from the fact it is only supplied by carbonate sources. A synthesis of processes acting in LAC and GAC is presented in figure 5.

Fig. 1: Multibeam bathymetry map EM 122. Leg 1 (Mulder et al., 2017).

Fig. 2: 3D view at the mouth of the Great and Little Abaco canyons (GAC et LAC, resp.) in the abyssal plain. The escarpment represents the Blake Bahama Escarpment.

Fig. 3: Mouth of the Grand Abaco Canyon (A) and little lateral lobes (B) (Mulder et al., 2017).

 

Fig. 4: Lobes at the mouth of Little Abaco Canyon (A; Recouvreur, 2020), Furrows in the Head of Great Abaco Canyon, Slumps on the flanks of the Abaco Canyon (C; Mulder et al., 2017), Sediment waves on the lateral lobes of Great Abaco Canyon (D).

 

The synthesis between the Carambar 1 (Leg 2) and Carambar 2 cruises makes it possible to carry out a synthesis of the gravity processes of sediment transfer from the platform to the deep basin (Fig. 5).

 

Fig. 5: 3D view of the processes acting on the north Great Bahama Bank and main sedimentary transfers.

 

Leg 2

The survey during leg 2 details the southern part of Exuma Sound, and its unchartered transition area to the deep abyssal plain of the Western North Atlantic bounded by the Blake Bahama Escarpment (BBE) between San Salvador Island and Samana Cay, referred here to the San Salvador abyssal plain (SSAP) (Fig. 6). The transition area is locally referred to as Crooked Island Passage (CIP), loosely delineated by Crooked, Long, and Conception Islands, Rum and Samana Cays. Surprisingly in such a pure carbonate landscape, the newly established map reveals the detailed and complex morphology of a giant valley formed by gravity flow processes originated in Exuma Sound, in addition to many secondary slope gullies and smaller tributaries draining the CIP upper slopes. The valley referred here as the Exuma Canyon system starts with a perched system with low sinuosity, characterized by several flow restrictions and knickpoints initiated by the presence of drowned isolated platforms and merging tributaries. The valley abruptly transforms into a deep incised canyon, rivaling the depth of the Colorado Grand Canyon, through two major knickpoints with outsized chutes exceeding several hundreds of meters in height, a total of 1600-1800 m. The sudden transformation of the wide valley into a deep narrow canyon is characterized by outsized submarine depressions formed after carbonate dissolution by upward-moving sulfate-rich fluids and preserved by the erosive action of gravity flows (see below). The high kinetic flow energy, constrained by this narrow and deeply incised canyon, formed, when it is released at its mouth in the abyssal plain, a wide deep-sea channel with well-developed levees and fan, made of coarse-grained carbonate defined layers separated by fine carbonate sediments mixed with fine siliciclastics transported along the BBE by the energetic Western Boundary UnderCurrent.

Fig. 6: EM 122 Multibeam bathymetry map of the area at the outlet of Exuma, mapped during the Leg 2 (Mulder et al., 2018).

Morphology and core facies of a modern turbidite system

High-quality multibeam echosounder swatch bathymetry, backscatter, cores data and very high resolution (VHR) 2D seismic profiles collected during the Carambar 2 cruise along the Exuma Sound-San Salvador system of Great Bahama Bank (GBB) constitute an exceptional dataset that can be used to investigate morphologies, sedimentary facies, transfer processes geometry and evolution from confined to unconfined system at 2000 to 5000 m water depth in modern carbonate settings. Interpretation of surficial facies and the distribution of acoustic facies and echofacies display an important variety of facies and processes as well as for penetrative data.

Moreover, unique workflow was created mixing bathymetric, acoustic facies, core facies and echofacies in order to predict facies distribution and properties encountered in a deep carbonate system. We present here only the lithologic facies and their correlation with acoustic data (Fig. 7).

Fig. 7: Lithologic facies in cores (Polard-Taine et al., subm.; Polard-Taine, 2023)

 

Sedimentary facies mapping

The sedimentary facies mapping on the reflectivity map was realized using the facies classification. In the Exuma Channel axis, all cores show Net to gross > 0.5 with sandy beds from 15 cm to 1 m facies F.2 to F.7 corresponding to high density gravity flow (sensu Mulder and Alexander, 2001) corresponding to the term Ta/b of Bouma, F3 to F8 of Mutti (1992) and B1/E1 of Busson (2018) classification. In the proximal area of the Exuma Channel muddy and sandy facies without grading are present incorporating platform or slope clasts which have been interpreted by (Busson, 2018) as resulting from large scale collapse.

 

Fig. 8: Reflectivity map with superposed lithologic facies (Mulder et al., 2018).

 

Geometry and evolution of a carbonate deep-sea depositional system

The high resolution 2D seismic data collected during Carambar 2 cruise and wells collected during Ocean Drilling Program and Bahamas Drilling Project have been used to describe and map extensive Neogene - Quaternary carbonate turbiditic system on the windward margin across Exuma - San Salvador depositional system (Fig. 8). The data reveal a giant 400 km long turbiditic complex which has accumulated more than 600 m thick sediments in ± 12 Ma, at water depth ranging from 1000 and 5000 m. Stacked channels/levees and sedimentary lobes represent 150 m thick sediments on average. The seismo-facies analysis has permitted to highlights a large catalogue of channel levee geometries and sedimentary processes from debris flow to low density gravity flow deposits. The high-resolution seismic stratigraphy allowed to identify three seismic units and eight sub-units from lower Tortonian to present. A robust stratigraphic link could be established between the platform and the windward margin (Exuma Sound) which presents predominantly a base of slope apron system.

In addition, a common stratigraphy is established between the leeward and windward margins of the Great Bahama Bank and the platform allowing the definition of seven remarkable intervals in terms of geometry, accumulation and facies (Fig. 9). The description and the comparison of the morphology, the stratigraphy and the facies evolution of these intervals show an important dissymmetry between the two margins. This study shows that: 1. a leeward or windward slope can present important variations along time in term of morphology, facies and accumulation; 2. Leeward and windward systems are globally dissymmetrical; 3. Their stratigraphic response differs largely according to the same controlling factor; 4. The study and the understanding of a carbonate system is possible thanks to the integration of multiple parameters, some of which being very rarely considered. They have a major influence such as controlling the spatial relationship between the source of the carbonate sediments and the carbonate slope, the nature of gravity deposits, and finally the nature of the platform escarpment, whether it is inherited, erosive or tectonic and the presence of contour currents.

 

Geometry and evolution of a deep-sea carbonate system

Fig. 8. Conceptual model illustrating the evolution of the platform GBB to basin Exuma Sound system from Tortonian to Quaternary. A. Mid - late Pleistocene: Constructive upper slope attached system. B. Late Pliocene early Pleistocene: Constructive upper slope detached system. C. Early Pliocene: Flooding and detached system. D. Messinian: Bypass and erosional slope detached system. E. Tortonian: Attached system - Toe of slope accumulation. F. Wheeler diagram from Tortonian to Quaternary (Polard-Taine et al., subm.; Polard-Taine, 2023).

Fig. 9 : Sequence stratigraphic correlation leeward ¿ windward margins and platform through the Great baham Bank calibrated on sea-level changes (Miller et al., 2011) (Polard-Taine et al., subm.; Polard-Taine, 2023).

 

Submarine depressions at canyon mouth and at BBE toe

Submarine depressions (Fig. 10) are common features on the eastern Bahamian seafloor but the genesis of the deepest ones (>  4000 m in water depth) is still not well understood. We conducted a morphometric analysis and compared them to a worldwide database of rounded depressions, whatever their genesis is. The deep Bahamian depressions are large elongated structures, among the largest on Earth, with a width greater than 1000 m and a depth sometimes greater than 200 m. They extend at the toe of the Blake Bahama Escarpment (BBE), one of the tallest escarpments on Earth (Fig. 11). Some of them align parallel to the BBE. Other depressions align along large submarine canyon axes. When aligned along canyon axes, the depressions closest to the canyon head are flanked by a topographic high interpreted as a slope-break deposit, i.e. sediment deposited after flow expansion following a hydraulic jump. Turbidity currents in the carbonate canyon system are not permanent processes, but are rather triggered during sea-level high stands when the carbonate platform is flooded. In addition, some depressions are not located in canyon axes. Consequently, the size and location of the depressions are not likely explained by a simple plunge pool mechanical erosion. Rather, our data suggest that all depressions could be initiated by giant karstic dissolution structures (dolines or sinkholes). Under interpretation, those located in canyon axes are sporadically refreshed by carbonate-laden turbidity currents. The height of the outsized chutes marking the crossing of the BBE by the canyon mouth generates a hydraulic jump allowing sediment deposition towards the bottom of the depression. Large depressions observed at this location in the Bahamas were the result of an initial dissolution phase related to retreat of the BBE and the more recent sediment-laden flow activity. The depressions orientated along canyon axes facilitated the regressive erosion that formed the canyons. At present, the depressions located at canyon mouth act as regular plunge pools.

 

Fig. 10 : Mapping of giant depressions at the mouth of Little Abaco Canyon (Mulder at al., 2022).

Fig 11 : Submarine depressions at the mouth of canyons and at the toe of BBE (Mulder at al., 2022).