Shortening a mixed salt and mobile shale system: A case study from East Breaks, northwest Gulf of Mexico

Abstract

The coexistence of two low-shear strength layers in a continental margin, such as salt and shales, conditions the resulting structural style and also constitutes a challenge for seismic imaging and energy resource exploration. We have analyzed the three-dimensional structure of a mixed salt-shale system in the western Gulf of Mexico, in the East Breaks foldbelt, by interpreting a depth-migrated seismic dataset. Overlying the allochthonous Sigsbee canopy and pierced by isolated salt diapirs, the Oligocene shale-prone sequences were shortened during the Miocene through pervasive deformations, developing a major suprasalt detachment system associated with duplexes below and different fold and thrust systems above. Oligocene shales locally reach critical state and flow, resulting in: (1) inflated shale-cored detachment anticlines, with stacking of allochthonous mobile shale domains; (2) detached lift-off anticlines with shale bulbs and “fishtail” thrusted welds; (2) allochthonous shale sheets at the upper tip of thrusts, fed by fluidized material migrating along associated fault zones; (3) hourglass shale diapirs, and; (4) “Christmas trees” and vertical mud pipes connected to small mud volcanoes. The salt diapirs deformed simultaneously as did the shales. Salt accommodated shortening by localized sub-vertical welds that isolated them from the allochthonous Sigsbee canopy, and by weld thrusts formed by fault-bounded blocks with highly sheared salt and with mobile shales. Mobile shale structures wrap around the squeezed salt diapirs, presenting a unique pattern that has not yet been documented in experimental models with precursor salt diapirs deformed under contraction. Mixed salt–shale systems are a new challenge for both structural analysis and experimental models, as well as for seismic model building in situations where there is a high dispersion of seismic energy due to the unique properties of mobile shales.