Frank Peel¹

(1) BHP Petroleum Americas, Houston, TX

ABSTRACT: Emplacement, Inflation and Folding of an Extensive Allochthonous Salt Sheet in the Late Mesozoic (Ultra-Deepwater Gulf of Mexico)

Recent seismic mapping has revealed that the deep salt layer, which forms the decollement for the Mississippi Fan Foldbelt (MFFB), is not the autochthonous Louann Salt. It is instead an extensive allochthonous sheet which was emplaced in the Late Jurassic and Early Cretaceous. This sheet advanced at least 15-20 miles downdip over a strike width of 100-150 miles. The base salt geometry shows ramp-flat geometry indicative of several pulses of salt flow.

Advance of the salt front finished during the Early Cretaceous, but downdip salt flow continued, resulting in inflation of the allochthonous sheet, coeval with the development of small-scale salt-withdrawal basins. This style changed during the mid-Cretaceous "MCU" hiatus, during which a fold belt developed along the whole of the frontal part of the allochthonous sheet. The MCU-age fold belt has different scale and geometry from the younger (Miocene) MFFB; it extends much further to the SW than the later belt.

Large-scale salt-withdrawal basins and salt pillows developed above the deep salt during the Oligocene-Miocene. Part (but not all) of the area was then caught up in the Miocene folding event which created the MFFB.

The final structure of each of the traps in the region is therefore a composite of several distinct processes, each of which has distinct scale and geometry. The Cretaceous-age folding and salt emplacement events are not driven by updip sedimentation, as is common for linked extension-compression systems. Instead, they are probably driven by tilting of the basin margin.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado