SEPTEMBER 2015 LUNCHEON MEETING

SEPTEMBER 2015 LUNCHEON MEETING

Ian Norton
of the Bureau of Economic Geology
presents
“Formation of the Gulf of Mexico Salt Basin”

11:30 AM Wednesday, September 16, 2015
at the Cascades
4511 Briarwood Road
Tyler, TX 75709

BIOGRAPHY

Ian Norton grew up in S. Africa and attended college there. He attended MIT while completing his PhD, which he finished in S. Africa. Ian then moved to the oil patch, first in S. Africa, then at ExxonMobil for 26 years. He now has a part-time position at the Institute for Geophysics at UT Austin.

ABSTRACT

“Formation of the Gulf of Mexico Salt Basin”
by
Ian Norton, Harm Avendonk, Gail Christeson, John Snedden 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 poorly known, 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, and on interpretation of ages of overlying seismic horizons that downlap onto oceanic crust. Backstripping shows that if water depths were too deep during salt deposition, 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.  As SDRs form at the transition to sea floor spreading and salt directly overlies the SDRs, we suggest that salt was deposited at the time that sea floo0r spreading commenced.  Salt then flowed over the ridge axis, submerging the spreading center under salt making what we term ‘intrusive oceanic crust’.  Finally, as salt thinned across the widening basin, the ridge axis broke through the salt and normal sea floor spreading continued.  This final breakthrough resulted in the formation of outer basement ramps.  Salt flowage also resulted in re-distribution of Tithonian source rock on onto crust younger than Tithonian.