Interactions between Allochthonous Salt Deformation, Minibasin Geometry, and Stratigraphic Fill along the Louisiana Upper Slope

Mark G. Rowan and Paul Weimer

Slope minibasins in Ewing Bank and northern Green Canyon contain discrete packages of stratigraphic fill. To study the interactions between salt deformation and sedimentation, we have systematically described 40 minibasins in the area based on the three-dimensional external form of the stratal packages, which are classified as: layers - tabular bodies with constant thickness; wedges - packages that thicken in one direction; troughs - elongated bodies that thicken towards the axis; and bowls - packages that are elliptical in map view and that thicken in all directions towards the center.

We observe consistent patterns of vertical succession of these packages, and relate them to the origin and evolution of underlying allochthonous salt. Some minibasins display either numerous N-thickening wedges overlain by a layer, or a trough overlain by a S-thickening wedge and then a layer. These correspond to "roho" and stepped counter-regional end-member models of salt sheets. In contrast, other minibasins contain bowl-shaped packages at depth, overlain variously by N- or S-thickening wedges and layers. These represent the displacement of salt from bulb-shaped diapirs that initially formed part of a salt-stock canopy. The timing of transition from bowls to wedges or layers in different minibasins ranges from 4.2 Ma to 0.7 Ma, and probably marks the time of salt weld formation ben ath minibasin centers.

There appears to be no simple relationship between sedimentation and the distribution and ages of the various tectonostratigraphic packages. Instead first-order controls on facies development and sand content may be the regional slope gradient (distance from the shelf margin), the location of major deltas, and the volume of clastic input to the entire system. The local sea-floor topography defined by salt and minibasin morphologies provides only second-order effects superimposed on the regional pattern.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California