Deep Crustal Structure of Northwestern Gulf of Mexico

Yosio Nakamura, Dale S. Sawyer, F. Jeanne Shaub, Kevin MacKenzie, Jurgen Oberst

We conducted a large-offset seismic experiment in the northwestern Gulf of Mexico, offshore Texas, using large-capacity air guns and ocean-bottom seismographs in order to map the salt-disrupted structure of the thickly sedimented shelf and slope. We shot five lines, each approximately 90 km long with four or more instruments, over an area extending from mid-shelf to the continental rise. We analyzed and interpreted the acquired data using two-dimensional ray tracing to match the theoretical and observed arrival times of refracted and reflected waves, refining both shallow structures as well as the depths to the deeper refractors of initial models constructed with conventional methods.

The entire study area is covered with sediments whose thickness ranges from 13 to 15 km along the shelf and slope, thinning to about 11 km on the continental rise. The most prominent reflector in the sedimentary column, interpreted to be the middle Cretaceous unconformity (MCU), lies at a depth of 7-11 km. The seismic velocity in the sediment column above the MCU (1.7-1.9 km/sec near the sea floor increasing to 3.0-4.0 km/sec just above the MCU) is appropriate for clastic sediments. The velocity below the MCU (3.7-4.7 km/sec) suggests the presence of carbonate and possibly salt. The basement or the top of the crust, as inferred from higher velocity refracted arrivals, is found at a depth of 13-16 km. The observed basement relief shows a northwest-southeast-trending ridge that may cons itute a seaward extension of the San Marcos arch, and a trough that runs south of the ridge. The systematic increase in basement velocity from 5.0-5.2 km/sec in the north to 5.6-5.8 km/sec in the south represents a broad transition from continental to oceanic crust. The Moho is observed at a depth of 20 km under the continental rise and deepens toward the slope.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.