Mega-Salt Basins Result From Outer Marginal Collapse at the Rift to Drift Transition

Abstract

The creation of mega-evaporite basins such as those along the margins of the Gulf of Mexico (GOM) and South Atlantic (SAT) suggests evaporite deposition at rates of several km in less than a few million years. These rapid rates were once assumed to require tectonic control, but deeply penetrating seismic data now show that these deposits rest above largely unfaulted “top-rift” or “base-salt” unconformities and hence are post-rift or extremely late syn-rift units. Thermal subsidence could be expected to control them, but the observed subsidence rates far exceed thermal rates. We use seismic, well, and regional stratigraphic data from the Gulf of Mexico, South Atlantic, Mediterranean and Red Sea to assess the relative merits and shortcomings of possible mechanisms for the deposition of thick “mega-salt” basins. These include; (1) out-of-sequence late rift-related faulting/subsidence, (2) progressive infilling of pre-existing sub-sea air-filled depressions, and (3) post-rift outer marginal collapse. Option 1 is not supported by regional structural mapping in the GoM or SAT. Option 2 is fails to address the lack of interbedded/interfingered basin rim clastics and salt in both basins. Option 3 avoids these pitfalls and explains the unfaulted depositional continuity of thin salt in the eastern GOM, reaching from the onlap limit as allowed by global sea level in the Appalachiacola Basin to the structural level of the oceanic crust at the continent-ocean transition, which was emplaced at 2.5–3 km sub-sea. While current arguments for option 3 in the South Atlantic are weaker than for the GoM, we highlight circumstantial evidence from seismic and other data that favours evaporite deposition at global sea level on this margin. We highlight additional observations such as strong, early basinward tilting of the base salt surface, and the onlap by earliest sediments atop oceanic crust onto deformed salt cored folds above the rifted margin that are best explained by an outer marginal collapse model. Finally, we summarise how the concept of outer marginal collapse, which occurs between the rift and drift stages of margin formation, impacts petroleum systems in general.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015