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New Ideas on the Timing and Paleogeography of Salt Deposition in the Gulf of Mexico: Mark Rowan

calendar April 10, 2020

Mark Rowan discusses evaporite deposition in the Gulf of Mexico and how new ideas on its timing have important implications for both pre- and suprasalt exploration.

The Gulf of Mexico (GoM), despite being one of the most studied salt basins in the world, remains an enigma in terms of the timing and paleogeography of evaporite deposition. But new data and ideas are changing how we think about the deep framework of this prolific basin.

The salt has traditionally been considered to be Callovian (upper Middle Jurassic), but with effectively no supporting data due to suprasalt strata with no age control and a lack of presalt penetrations. Recently, though, Sr isotopes have yielded ages ranging over roughly 5 my from the Bajocian to the Callovian. Well data from the southern GoM onshore and shelf show that the cessation of evaporite deposition was gradational, with interbedded carbonates and anhydrite that continued into the Oxfordian and Kimmeridgian in a hypersaline sabkha environment with up to 3X normal ocean salinity. In coeval salt basins from onshore Mexico, Sr and biostratigraphic data indicate ongoing evaporite and minor carbonate deposition from the Bajocian through the Kimmeridgian.

Other traditional views are that the salt was deposited near sea level and that the salt was almost pure halite. But these are being challenged by new ideas triggered in large part by the much improved imaging provided by modern seismic data. More researchers are coming around to a model in which the salt basin had considerable relief, ranging from close to sea level in proximal areas to 2 km or more in the basin center. However, whether the basin was filled mostly with brine or mostly with air is still a matter of debate. Moreover, the salt appears to be a typical layered evaporite sequence with at least locally significant proportions of non-halite lithologies. This can be seen in folded intrasalt layers within the cores of deep anticlines in the NW and SW GoM and in the “Sakarn” series in the NE GoM (with an equivalent offshore Yucatán), a deformed layered sequence coeval with at least part of the Louann/Campeche salt.

These new ideas have critical implications for subjects ranging from both pre- and suprasalt exploration, to plate tectonics and Jurassic paleogeography. They, along with the fundamentals and styles/processes of salt tectonics, will be addressed in Salt Tectonics of the Gulf of Mexico, the GeoLogica course running in Houston from 13–14 August, 2024.