The “Mother Salt” in the Gulf of Mexico: Constraints from Isostasy and Geophysical Observations

Imbert, Patrice¹, Yann Philippe² (1) TOTAL CSTJF, Pau, France (2) Total E&P USA, Inc, Houston, TX

Play assessment in salt basins implies, among other factors, a good understanding of the structural history of the basin. One particular issue of interest is the distribution of initial salt in the basin. This paper addresses the issue of “mother salt” in the Gulf of Mexico basin.

In the vicinity of the Florida escarpment, Seaward-Dipping Reflectors below the base of salt indicate that evaporite deposition took place there on actively spreading volcanic crust, suggesting that oceanic crust underlies at least a good part of the sub-salt domain.

The base of the mother salt, where it can be picked, indicates that the bottom of the salt basin(s) lay beneath the level of both extended crust landward and “normal” oceanic crust seaward, thus creating “salt troughs”.

In the model proposed, the Louann salt was deposited during oceanic spreading, the salt mass filling almost entirely the space created by the extension. The salt troughs observed represent the isostatic response of oceanic crust emplacement below thick salt, almost twice as dense as seawater. Once salt deposition ceased, the salt began to creep basinward and deflate, preventing the development of an axial “chasm”. Creeping lasted until the deflated salt was dammed behind the axial ridge of the nascent oceanic domain.

As a consequence, sediments directly overlying the salt will be extended without visible compressional counterpart on the margins of the basin; salt creep will take place directly on accreting crust, with no intervening sediment between salt and basement, no possible source rock in particular.