Interaction between salt and mobile shale in the East Breaks foldbelt, northwestern Gulf of Mexico

Abstract

The East Breaks foldbelt comprises a series of dominantly slope-parallel folds and thrusts detached on a mostly evacuated salt canopy on the northern Gulf of Mexico slope. The canopy was emplaced near the end of the Eocene. The foldbelt formed mostly in the Oligocene, although minor amounts of shortening continued through the Miocene. We attribute the anomalous trend of structures in the foldbelt to convergent gravity gliding produced by the bend in topography in the northwest corner of the Gulf of Mexico. We suggest that convergent deformation was localized in the East Breaks foldbelt because the area comprised a shale-dominated slope between two sand-prone slope aprons during the Oligocene. These aprons were stronger than the shalier East Breaks area in between, allowing convergent deformation to be partitioned into weak shales.The fact that the Oligocene sequence in the area is shale prone means that shortening was accommodated on multiple detachment levels with fishtail thrusts, as well as in mobile shales, which occur alongside salt diapirs sourced from the canopy. The coincidence of salt structures and mobile shales allows a comparison of their behavior during shortening. Mobile-shale deformation is most intense near a minimally shortened salt massif, leading us to conclude that salt behaved more competently than mobile shale during shortening. This conclusion is counter to the general assumption that salt, where present, is the weakest part of any geologic system.