EXTENSIONAL EVOLUTION OF THE GULF OF MEXICO BASIN AND THE DEPOSITION OF TERTIARY EVAPORITES
H. Hugh Wilson*
*5929 Talbott Road, Lothian, MD. 20711, USA.
Current interpretations of the evolution of the Gulf of Mexico Basin are guided by the critical assumption that all the salt in this huge basin is Jurassic and time-equivalent to the Louann salt of the interior evaporite basins of Texas, Louisiana and Mississippi. This assumption forces the interpretation that extensive salt sheets that now reside within Eocene to Plio-Pleistocene stratigraphic levels are allochthonous, having been emplaced by several episodes of Jurassic salt piercement and horizontal flow as shallow salt sills or sub-aqueous salt glaciers, during Tertiary sedimentation.
This interpretation is contested from several geological viewpoints. The uncontaminated Louann salt differs substantially from salt of the coastal, offshore northern Gulf and Mexican Saline Basin, in all of which the salt carries multiple shaley layers and exotic material. The intra-salt deformation of offshore sheets appears from cores and seismic profiles to be much less severe than would be expected if it had been emplaced via several generations of deep piercement diapirs. The intra-salt shale laminations in the Belle Isle Salt Mine, Southern Louisiana, carry Oligocene fauna and are so closely conformable with primary salt layering that their contemporaneity with the salt is a valid conclusion. Because the Belle Isle Shale Zone is so similar to shaley zones encountered in offshore sheets, a contemporaneous origin for them is also implied. The mixed Tertiary fauna found within, and below salt sheets is so typical of sedimentary reworking that its origin from evaporite basin margins is a logical conclusion.
The Gulf of Mexico Basin is unique as the presumed depository of multiple allochthonous salt sheets. Salt piercements in the remote Kavir Basin of Iran have been proposed as an analogue for the emplacement of salt canopies, but there is no sub-surface control in the Kavir Basin and the canopy concept is not unanimously accepted. Furthuremore, the popular view that salt sheets were emplaced by submarine salt glaciers during successive episodes of piercement diapirism is doubted because of the complete absence of glaciers in todays halokinetically active basin. The preservation from solution of purported submarine salt extrusions lacks both experimental and credible geological support.
Angular sub-salt contacts of many offshore salt sheets in the northern continental slope have been interpreted as sheer or thrust contacts. However, thrusting seems unlikely in an overall extensional environment and it is here concluded that the angular contacts represent sub-salt unconformities and that the Cretaceous and older exotic blocks found in some piercement domes have been plucked from a base-salt unconformity surface.
There is much circum-Gulf evidence for extensional fracturing of the basement during the Tertiary period which, it is thought, supports the expectation of basement block faults, now hidden from view below thick Tertiary sediments; these faults governed sedimentation into depocenters as the basin expanded.
It is concluded that salt sheets at Eocene, Oligocene, Mio-Pliocene and Plio-Pleistocene levels are autochthonous, a conclusion that has evolved from critical geological observations in the Belle Isle Salt Mine.