The abyssal giant pockmarks of the Black Bahama Escarpment: Relations between structures, fluids and carbonate physiography

Abstract. This study reports the discovery of spectacular abyssal giant pockmarks located at the toe of the Bahamian carbonate platform, along the Black Bahama structurally-controlled Escarpment (BBE) that exhibits up to 4 km of submarine elevation above the San Salvador Abyssal Plain (SSAP). Analysis of seismic reflection and bathymetric data collected during the CARAMBAR 2 cruise revealed the presence of 29 pockmarks; their water depths range from −4584 m to −4967 m whereas their bathymetric depressions are elliptical in shape, range in diameter from 255 m to 1819 m, and in depth from 30 m to 185 m. The pockmarks alignment trends parallel to the BBE as well as the structural lineaments of the area, exclusively between 2200 and 5000 m from its toe, and overlies a buried carbonate bench in which a high-amplitude seismic anomaly has been detected. The pockmark density interestingly increases where the recognized structural lineaments intersect the BBE.

The aforementioned observations suggest an atypical relationship between the spatial occurrence of the abyssal fluid releases, the carbonate platform tectonic structures, the buried carbonate bench that underlies the hemipelagites in the San Salvador abyssal plain and the physiography of the area. Indeed, the ground water entrance during low-level stands, the dissolution of evaporites by meteoric water, the platform-scale thermal convection and the seawater entrance at the platform edge most probably act in concert to favor the circulation of brines and therefore the corrosion within the Bahamian carbonate platform. These mechanisms are particularly efficient along the structural heterogeneities (i.e. faults and fractures) which act as fluid conduits and control the physiography of the area by maintaining the location of the sedimentary pathways. The dense fluids migrate along the faults towards the BBE free edge and are subsequently trapped into the buried carbonate bench that laterally disappears below the low-permeability deep-sea hemipelagics of the SSAP. In consequence, the trapped corrosive fluids dissolve the carbonates preferentially along the tectonic structures such as the Samana Fracture Zone, at the origin of the BBE curvature and triggers collapse-structures in the overlying fine-grained deposits generating giant pockmarks. This structurally-directed process of dissolution is believed to have played a major role in the BBE 5–6 km erosional retreat and also probably explains the occurrence of plunge pools in the area.