Deep-water sedimentary bedforms in a mobile substrate terrain: examples from the central Gulf of Mexico basin

We have used high-resolution geophysical data to investigate depositional and erosional bedforms in two geomorphologic provinces of the deepwater central Gulf of Mexico Basin: (1) the Mad Dog and Atlantis areas in the Sigsbee Escarpment region and (2) the Holstein minibasin within the salt canopy in the slope. Multibeam bathymetry indicates that the seafloor relief in the study areas is highly irregular because it is influenced by the dynamic behavior of underlying salt bodies resulting in the development of diverse bathymetric features. Side-scan images reveal erosional furrows of different morphologies at the base of the Sigsbee Escarpment that are oriented subparallel to the escarpment. Wide and sinuous furrows overlie mass transport deposits (MTDs), whereas, in other areas along strike, narrow rectilinear furrows are found beneath MTDs. The furrow fields in the Sigsbee Escarpment are located within a large series of erosional features that are linked to the action of westward flowing bottom currents associated with topographic Rossby waves that manage to rework sediments at water depths up to 2000 m. The interaction between the bottom current flow and the seafloor is likely influenced by the MTD’s irregular top surface relief and lateral changes in the escarpment’s morphology resulting in the development of complex sinuous furrow morphologies. North of the escarpment, subbottom profiles indicate a series of buried sediment waves found in the southern rim of the Holstein minibasin. Sediment wave morphometry indicates wavelengths ranging from 116 to 339 m and wave heights between approximately 0.8 and 2.4 m. Sediment waves were likely formed by turbidity currents as they exited the minibasin. The vertical change in topographic relief from the minibasin to the salt high led to variations in flow thickness and flow velocity of turbidity currents passing over the minibasin’s open rim. Consequently, these changes in flow regime led to the formation of sediment waves.

Crustal architecture of the northwestern and central Gulf of Mexico from integrated geophysical analysis

The tectonic history of the Gulf of Mexico (GOM) is a subject for ongoing debate. The nature of the crust in the northwestern and central parts of the basin remains poorly understood. Joined interpretation of two 2D seismic cross sections — GUMBO1 and GUMBO2 — with potential fields (gravity and magnetics) constrained with available well data allows testing various hypotheses about the subsurface structures and crustal architecture in the study area. In the northwestern GOM, two contradicting hypotheses about the nature of the crust were tested — exhumed mantle versus a thinned and intruded continental crust resulted from magma-rich rifting. The nature of the crust was also investigated in the central GOM, where the disagreement in the location of the ocean-continent boundary (OCB) from various published tectonic models reaches 140 km (87 mi). The results suggest that the crust in the study area is stretched continental with multiple magmatic additions represented by dense and highly magnetic bodies with fast seismic velocities, presumably introduced during the magma-assisted rifting of the GOM. The contact between oceanic and continental domains, i.e., the OCB, is interpreted to be near the Sigsbee Escarpment for both modeled lines. The analysis does not support the presence of thick presalt sediments in the study area. This result questions the currently accepted tectonic reconstructions of the GOM as thick presalt deposits are imaged confidently by various seismic surveys along the western Yucatan margin, which is believed to be a conjugate for the study area. This apparent mismatch in distribution of the presalt sediments requires further investigation.

Coupling geomechanical modeling with seismic pressure prediction

We couple geomechanical modeling with seismic velocity to enhance the prediction of pressure and stresses in complex geologic settings. In these settings, pressure is controlled by mean and shear stresses rather than by only the vertical (overburden) stress. We estimate total mean and shear stresses from a geomechanical model. Effective mean and shear stresses are calculated from velocity using a relationship that we develop between velocity and these stresses. The pressure prediction process is iterated to attain convergence between the predicted pressure field and the one input in the geomechanical model. We also explicitly predict the full stress tensor. We apply our method along with the standard, vertical-effective-stress method to a salt basin beneath the Sigsbee Escarpment in the Mad Dog field, Gulf of Mexico. The methods are constrained to the same pressure data along a calibration well and are then used to predict pressure and stresses across the basin. We find that salt and basin bathymetry substantially perturb the stress field. The pressures predicted by the two methods differ the least at the calibration well and the most in areas where the total mean and shear stresses are the most different from those at the same burial depth at the calibration well. Our method is shown to predict pressures measured along a subsalt well better than the standard, vertical method. We calculate minimum stress and the drilling window along a vertical profile near salt and find that they significantly differ from the ones predicted by the standard, vertical method.

Deepwater Exmouth Plateau, North Carnarvon Basin: preliminary investigations into ridge and furrow features

Numerous fields of long, shallow subsurface linear ridge and furrow features were mapped during the interpretation of a 3D seismic dataset covering Hess Exploration Australia Pty Ltd’s WA–390–P deepwater Exmouth Plateau permit. These kilometre scale features are often slope parallel and have separations of between 100 to 400 metres between ridge crests. Heights range from the limit of seismic resolution up to approximately nine metres. Similar linear shallow subsurface features have been interpreted in the North Falkland Basin in Desire Petroleum’s Tranches C and D permits. Initial investigations suggest that these features appear similar to the Holocene and older mega furrows/palaeo-mega furrows identified along the lower slope/rise in the Gulf of Mexico, most notably along the base of the Sigsbee Escarpment, and along other continental slope/rise settings. Evidence of seabed and shallow sub-seabed sediment instability in the form of slumps and slides together with the effects of shallow sediment deformation and dewatering are also visible across the WA-390-P area. Ridge and furrow features from the deepwater Exmouth Plateau area are described in detail alongside examples from the North Falkland Basin. It is suggested that interaction between gravity driven downslope processes, sediment dewatering and alongslope sedimentary processes could be a possible mechanism of formation for these features. Horizontal and vertical delineation of these features can contribute towards regional understanding of subsurface sediment instability.