Geological and Shallow Salt Tectonic Setting of the Mad Dog and Atlantis Fields: Relationship Between Salt, Faults, and Seafloor Geomorphology

Abstract

Abstract Both the Mad Dog and Atlantis field development areas (southeast Green Canyon) extend across the Sigsbee Escarpment, the surface and seaward-most manifestation of mobile salt in the northern Gulf of Mexico. Understanding the shallow geologic setting of the Sigsbee Escarpment, and specifically the role of salt tectonics, is critical to evaluating the shallow geohazards of these fields. We combine exploration 3D seismic, high resolution 3D seismic, AUV bathymetry, AUV side scan, AUV sub-bottom profiling, piston cores, boreholes, and ROV observations to interpret the structural and geomorphic setting of the present-day Sigsbee Escarpment. These data sets can be combined to identify structural domains, and to illustrate their interaction over time. Bathymetric data show different seafloor textures across the area, indicating that portions of the Sigsbee have evolved differently in the recent geologic past. We show that these differences are due to a combination of salt morphology, supra-salt stratigraphy, and slumping. There are two primary modes of slope failure on the escarpment face, shallow-seated, small scale slumping, and deeper-seated amphitheatre-shaped failures. We distinguish among fault systems and slope failures of different origin and relate these differences to seafloor geomorphic provinces and variation in the geometry and movement history of salt. The supra-salt section along this portion of the Sigsbee Escarpment contains evidence for active deformation in the form of normal faults with seafloor offset. In the study area, all of the faults on and above the escarpment are extensional, even though the salt is translating seaward and there are some buckle folds above the salt sheet indicating regional contractional strain. Although numerous models of propagating salt sheets have been published, we find that the Atlantis and Mad Dog data fit the "salt glacier" or "tank tread" model best. These models imply that the salt is flowing under gravitational forcing from above, that the supra-salt section is not in compression, and that the basal traction of the salt sheet leads to the supra-salt section flowing over the "tank tread" of the escarpment front. Both normal faults in the supra-salt section and seaward dipping beds above the frontal salt monocline can provide pre-existing and preferential failure planes for slumping at the escarpment front. These dip-slope conditions control the slumping in the shallow-seated slope failure portions of the escarpment at both the southwest Mad Dog and northeast Atlantis field areas. In the amphitheatre-shaped deep seated slump regions of central/northeast Mad Dog and southwest Atlantis, a regional seismic reflector (Horizon 25), interpreted to be a sand-rich unit, projects to the base of the slump headscarps. The orphology of the deep seated slumps - steep headwalls, relatively flat bases, and linear side walls - suggests that they are forming from internally driven failure. In situ pore pressure measurements support this model, and field work from the Caspian supports a model whereby exposure of the sand horizon at the escarpment face leads to its draining, and the creation of an effective seal to the overpressured section below.