Profiler-Sonobuoy Measurements in Colombia and Venezuela Basins, Caribbean Sea

Abstract

Interval velocities, refraction velocities, and layer depths from 38 airgun-sonobuoy profiles made along a west-east traverse of the Colombia and Venezuela basins at about 15°N are presented in a schematic structure section, which includes details from single-channel reflection-profiler records converted to depths. The seismic data are correlated with earlier reflection and refraction surveys and with the results of JOIDES drilling. Two prominent subbottom reflectors, Horizon A^Prime and Horizon B^Prime, are traced across each basin. Interval and refraction velocities in the material beneath B^Prime in the Venezuela basin range between 3.5 and 5.7 km/sec, but most are in the range 4.5 to 5.3 km/sec. These values are in close agreement with published laboratory measurements of sonic velocities made on DSDP cores of basalts from Horizon B^Prime. It is therefore unlikely that the material below B^Prime can contain large amounts of sediment unless the sediment has a velocity near that of basalt. The discontinuous reflectors below B^Prime are thought to be thin layers of interbedded ash or sediment. It is argued that the Venezuela basin has normal oceanic that has been depressed about 3 km by the addition of flood basalts, and that the velocity structure of typical Caribbean crust (6.2 on 7.3 km/sec material) does not exist in the Colombia and Venezuela basins, except as arbitrary divisions in a scattering of crustal and basement velocities that range between 4.0 and 7.5 km/sec, as they do in other ocean basins. It is suggested that the upper interface of primordial basement coincides with the deepest reflector below B^Prime. Therefore, it is likely that the Late Cretaceous basalts and dolerites drilled at the depth of Horizon B^Prime do not represent primordial crust, but reflect the last major igneous event in the history of development of the Caribbean Sea.