Abstract
The initial sediment lithification reactions start with complex interactions involving all components of the sedimentary material (minerals, surface water, decomposing organic matter and living organisms). This is the eogenesis domain (0 to 2000 m below seafloor), covering a burial interval ranging from the interface with the biosphere down to depths where physical compaction processes become predominant. Compared to studies performed on sedimentation (and sedimentary dynamics) and on deep diagenesis (mesogenesis), there is a true lack of data concerning diagenetic processes occurring during eogenesis, in particular concerning siliciclastic diagenesis. However, shallow sediments within the eogenesis domain undergo intense deformation and fracturing. In clay-rich sediments the created faults are organized in polygons due to the volumetric contraction leading to a volume loss during burial. The polygonal fault systems (PFS) have been identified in many basins worldwide, such as in the China Sea, in the Australian Eromanga Basin, in the Lower Congo Basin, in the Danish Central Trough, in the Canadian Atlantic margin and in the Irish Sea. These area are all located in petroleum provinces, either onshore or at water depths ranging from 200 to 1500 m. During the Garanti Cruise in May-June 2017, giant polygons have been identified on the slope of the Caribbean sea, west of Grenada Basin, between 1800 and 2500 m water depth. On seismic profiles the polygonal faults are characterized by an intense dimming of reflections on both edges of the fault planes suggesting that fluids are currently migrating upward. They affect a 700 to 900 m thick interval and they can locally reach the modern seafloor where they form polygons visible on multibeam data. On chirp profiles, the polygons have very steep flanks, defining rectilinear depressions (or furrows) that are 40 m deep compared to the regional slope. Various mechanisms have been referenced in the literature as responsible for polygonal fault initiation and propagation, such as diagenetic transformations or reactivation by sediment loading for instance. Four hypotheses are actually proposed to explain the formation of these polygonal faults: i) syneresis related to colloidal properties of such fine-grained sediments, ii) density inversions and associated hydrofracturing, iii) smectite-rich clays causing residual friction at low burial depth and iv) grain dissolution in incemented media inducing a decrease in horizontal stress that leads to shear failure and shear strain localization. In the Grenada basin it seems that the volumetrical contraction starts very early after deposition suggesting that the smectite-rich clays play a key role in the formation of polygons. This is compatible with the volcano-clastic context in the area where clays may come from the in-situ alteration of volcanic material.