Formation of the Gulf of Mexico Salt Basin

Friday, 19 December 2014
Ian O Norton, Harm J Van Avendonk, Gail Lynn Christeson and Drew R Eddy, University of Texas at Austin, Austin, TX, United States
Recently acquired seismic refraction data in the northern Gulf of Mexico (GOM) have provided new insights into the basin’s crustal structure. We use the four refraction profiles to build regional-scale crustal sections across the GOM, and then use these profiles as the basis for basin modeling of crustal subsidence through time. Basin modeling includes flexural backstripping of the sediment load and correction for thermal subsidence, with the aim of calculating the shape of the basin at the time (Callovian) of salt deposition. The age of salt deposition relative to rifting events is debated, with opinions ranging from salt being synrift to entirely postrift. We suggest that salt was deposited near the end of rifting, close to the time of initiation of sea floor spreading. This interpretation is based partly on reconstructing possible water depths at the start of salt deposition, using the backstripping method. If water depths were too deep, i.e. sea floor spreading already established, salt thickness based on isostatic balance would be far too large. If water depths were too shallow, i.e. little crustal thinning, salt thickness would be too thin.  We can compare the outcome of this analysis with the distribution of evaporites in the Gulf of Mexico basin, which may have formed a 4 km thick layer in some areas, though these salt deposits have subsequently been remobilized. Crustal structure from the refraction data shows crustal thicknesses of 8-15 km under the salt basin. The seismic velocity structure of the thinned crust suggests that at least some of the basement was formed by magmatic intrusions. If we correct for the inferred stretching of the continental margin and thermal subsidence, we obtain a plausible depth for the margin at Callovian time.  Velocity structure from the refraction data plus observations of SDRs in the eastern GOM are consistent with the margins of the GOM having a significant synrift volcanic component. We suggest that this volcanic component is a prerequisite for formation of marginal salt basins like the GOM, Aptian South Atlantic and present-day Red Sea.