Gulf of Mexico Paleogene "Whopper Sand" Sedimentology: Hypersaline Drawdown Versus Low-Salinity Hyperpycnite Models

Roger Higgs
Geoclastica Ltd, Marlborough, United Kingdom

Billions of barrels of oil have been found since 2001 in the Paleocene-Eocene, Wilcox-equivalent "Whopper Sand" turbidites, deposited on the Gulf of Mexico abyssal plain, anomalously far (100s km) from the coeval shelf margin. Major evaporative drawdown (1-2 km), after tectonic damming of the Gulf (Cuban orogeny), has been proposed to explain (1) turbidite deposition so far basinward (shore advance), and (2) deep paleocanyons incising the shelf margin and slope. This model suffers from a lack of associated evaporites, and from the unlikelihood of southern Gulf aridity (evaporation) outweighing inflow from humid (peaty) northern deltas. An alternative, "opposite", low-salinity model is as follows. During three Paleocene-Eocene eustatic superlows, each involving a fall of about 100 m (Haq chart), world sea level fell toward or below the level of the Gulf's lowest inlet/outlet (sill), such that inflow from the ocean was reduced or cut (cf. Quaternary Black Sea). River inflow exceeded evaporation, desalinating the Gulf, turning it brackish or even, at times, fresh ("Lake Mexico" proposed here). Reduced salinity meant that river-fed (hyperpycnal) turbidity currents of long duration (weeks), already known to transport silt far out (100s km) onto modern marine abyssal plains, would have become more frequent and more sustained, carving the canyons and supplying the Whopper. Coriolis turning of unchanneled basin-floor flows impedes prediction of proximality trends, vital for exploration and development. Proper outcrop analogs of the Whopper low-salinity abyssal hyperpycnites may exist only in collisional accretionary complexes, because abyssal plains are ultimately subducted. Partial analogs are Carboniferous and Permian formations interpreted by the author as lacustrine hyperpycnites, but deposited above storm wave base (Brushy Canyon, Bude, Ross, Laingsburg, Skoorsteenberg).

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