Crustal Structure of the South Florida Bank Derived from Ocean Bottom Seismometer Refraction Profiles (Final Report on Refraction Experiments that were part of the South Florida Bank Study)

In March 1982, six seismic refraction lines, 70 to 90 km long, were shot in the southeastern Gulf of Mexico using the advanced Texas digital ocean bottom seismometers. Five lines were on the South Florida bank region in water depths of less than one kilometer and one was in water depth of about 2.4 km off the northern coast of Cuba. After data reduction, first arrival picks were made and least-square lines were fitted to the picks to obtain the apparent velocities and intercept times for the layers. Using these values, flat-layer crustal models have been computed. The two most dam inant refractors have apparent velocities of 5.6-5.9 km/sec and 6.2-6.6 km/sec. The top of the 5.6-5.9 km/sec layer varies in depth from 2-5 km below the sea surface and is interpreted to represent the crystalline basement. Alternatively, this layer may constitute a carbonate section with velocities indistinguishable from crustal velocities. Basement rock, at a depth of 3.4 km, was overlain by various carbonate facies in a well in the Pinellas County arch. In the South Florida bank area, the deepest refractor observed has an apparent velocity of about 7.5 km/sec at a depth of 25 km. The absence of any interpretable mantle arrivals in these long refraction profiles on the platform suggests that the crust underlying the South Florida bank platform is continental in nature. Possible mantle arrivals were seen at the ends of the line off the northern coast of Cuba (apparent velocities and depths: 7.7 km/sec and 21 km at the northern end and 8.4 km/sec and 26 km at the southern end), suggesting a mantle that dips strongly to the south towards Cuba. Similar crustal thickness has been observed in a refraction profile just northwest of this line. This deep crust structure compliments the earlier shallow crust structures for this area.