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Modern and Ancient Sediment Waves in the Deepwater Campeche Basin, Offshore Southern Mexico: Contourites or Turbidites?


A series of sedimentary wave bedforms exist on the modern seafloor and throughout the Cenozoic in the deepwater Campeche Basin, offshore southern Mexico. The modern features are interpreted as deposited by either along slope, bottom-current (contourite) processes or turbidite-driven, regionally unconfined sediment gravity flows, while the ancient features occur in close association with large 3rd order submarine channels and are interpreted as turbidite deposits generated by channel overspill (cyclic steps). Modern features are characterized by a series of linear waves up to 20km long, 50m in height with wavelengths of 2-4km that are parallel to bathymetric contours. In seismic cross section, these features are strongly aggradational, up to 500m thick, wavy, continuous reflections that exhibit both symmetric and asymmetric stacking geometries. Symmetric reflection features exhibit both vertical and subtle upslope migration tends and are interpreted as antidunes. Asymmetric bedforms generally exhibit upslope migration directions and are interpreted as either contourites or turbidite-driven cyclic step deposits. The interpreted antidunes have been tested in a nearby DSDP well and are comprised of predominantly carbonaceous ooze lithologies, while the contourites/cyclic-steps have not been tested. However recent seafloor images suggest a similar very fine-grained sedimentary cover for much of the region. The older and deeper sediment waves are similar to the modern/recent features in map view, however their orientation is orthogonal to bathymetric contours and parallel to large channels. In cross section they occur as shingled seismic reflections that are subparallel to bounding surfaces. They do not exceed 150m total thickness and are typically confined to within 30km of channel margins. These features are interpreted as overbank cyclic-step deposits, similar to features observed offshore southern California. Unlike the overlying, modern, fine-grained sediment waves, these features show evidence of variable sediment grain size. Well data in proximal channel overbank regions suggest sand-grade sediment while polygonal faults are noted in seismic sections far away from channels. These observations suggest that the grainsize of sedimentary waves in the distal Campeche basin are typically fine-grained, but coarser sediment can occur on the flanks of large channels, offering a possible new exploration concept in the basin.