Passive Margin Architecture in the Atlantic Basins

Hermann Lebit¹ and Luke Jensen²
¹ ENI Petroleum, Houston, Texas 77002
² Shell International E&P, Houston, Texas 77056

Post-rift tectonic stresses are considered insignificant at passive margins assuming the underlying oceanic and continental lithosphere have thermo-mechanically reequilibrated since onset of seafloor spreading. Thus sedimentation input remains the major (gravitational) driving forces sculpturing the passive margin architecture. Two end-member modes are suggested for gravity-driven instabilities along these margins: (i) load-driven outflow of sediment sequences above a detachment horizon due to progressive sediment input from prograding delta wedge; and (ii) mass wasting processes dominate margins with moderate sedimentation, due to sustainable slope gradients resulting from basinward subsidence. These processes might become significantly amplified when evaporite layers add a salt (channel) flow component to the system. Prograding delta systems drive gravitational collapse by growth faulting in the shelf sections. Rapid burial of these sequences may trigger salt withdrawal and lateral fluid migration. Normal faults usually bottom out at basal décollements that translate into outboard contractional regimes, dominated by thin-skinned fold belts, which form prolific hydrocarbon provinces in the Atlantic Basin, including Gulf of Mexico. Sequential kinematics restoration along regional seismic sections demonstrate equilibrium between extension in the shelf region and corresponding shortening near the toe of the slope with bulk displacement rates one order of magnitude less than those in tectonic belts. However, localized dramatic flow rates (channel flow) are evidenced by rapid lateral salt extrusion (salt canopies). Mechanics of passive margin sequences is analogous to gravitational flow in ice sheets and allows to model displacement fields in sedimentary wedges.

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012