Speaker: Dr. John W. Snedden, Senior Research Scientist, Director, Gulf Basin Depositional Synthesis Project, Institute for Geophysics, Jackson School of Geosciences.
John W. Snedden, Ian O. Norton, Michael R. Hudec, Abdulah Eljalafi, and Frank Peel, The University of Texas at Austin.
The presence of Jurassic age salt in the Gulf of Mexico has been known for almost 60 years, originally recognized as two separate salt bodies. Reconstruction of the original saline basin as it appeared prior to sea floor spreading has been difficult to carry out due to subsequent allochthonous salt movement that obscures stratal relationships at depth. However, a new approach to seismic mapping of the Louann salt using its tectonostratigraphic character allows recognition of distinct Louann facies transitions from deep basin to onshore lapouts. Halite-dominated Louann is more ductile, often seismically dim, and readily facilitates fault detachment. Halite likely formed in the deep basin, with coeval anhydrite forming in shallow water sabhkas and evaporitic lagoons. Anhydrite-dominated sections are seismically and structurally distinct with high amplitude/continuity and a less ductile character. A zone of mixed seismic response that we infer to be interbedded halite and anhydrite separates these seismic facies.
Our new paleogeographic reconstruction of the Louann salt has been developed on the basis of this seismic facies mapping, in combination with new plate tectonic models. 87/86Sr ratios from interior salt basins indicate a proxy age of 170-ma when matched against the global strontium seawater curve. While a 170-ma age for the Louann salt is 7-8 ma earlier than previous estimates, this occurs during a phase when various plates are in closer proximity and thus conditions are more conducive to restriction and evaporation.
This reconstruction also allows evaluation of two contrasting hypotheses regarding source of the original seawater that fed this saline giant in the deep Gulf of Mexico basin. Our restoration shows narrow gateways to the Atlantic Ocean, from the deep Louann basin through the Florida straits, Northern Cuba and the Bahamas. The chain of narrow basins connecting the Atlantic to the Gulf may have acted to deplete incoming seawater of all but Na and CL, evidenced by the remarkably pure Louann halite in the deep basin. The mapped configuration of narrow passages in the eastern Gulf of Mexico and dominance of basin marginal anhydrite in Mexico (versus halite elsewhere) also leads one to question the conventional model of a marine connection between the Louann basin and the Pacific Ocean. Pacific affinity macrofauna, originally a critical data point supporting the Pacific seawater entry model, are now known to be younger than the Louann salt.
Salt is a critical component of the prolific Gulf of Mexico petroleum system, setting up traps, providing top seal, and mitigating heat flow so that Mesozoic source rocks generate later in the basin burial history. Understanding the original distribution of Louann salt is also essential to basin modeling and structural restorations.
Biography: Dr. John W. Snedden is Senior Research Scientist and Director of the Gulf Basin Depositional Synthesis Project at the Institute for Geophysics, University of Texas at Austin. He received degrees from Trinity University (San Antonio), Texas A&M University (College Station), and Louisiana State University (Baton Rouge). With multiple domestic and international assignments, he worked for Mobil and ExxonMobil for over 25 years in research, exploration, development, and production prior to joining UT. Recent research has focused on the Gulf of Basin. He has served as Vice-President of GCS-SEPM and Secretary-Treasurer of SEPM. John has won the SEPM Excellence in Oral Presentation award, GCAGS Journal Best Paper Award and AAPG’s A.I. Levorsen Best Paper Award. John was selected as one of 50 people to be presented as Heritage of the Petroleum Geologist honorees, at the upcoming American Association of Petroleum Geologists Convention in Houston, Texas, April 2-5th.