Seismic stratigraphy of the western Venezuela Basin

Abstract

Multi-channel seismic reflection records from the central Venezuela Basin reveal planar reflections within the upper half of the oceanic crust as well as within the overlying sediment layer. The high resolution of the reflection data allows a division of the sediment section into three seismic intervals bounded by strong laterally persistent amplitude peaks which can be traced throughout much of the basin. The underlying crustal reflection zone is divided into two seismic intervals based on an angular relationship between the reflections which constitute each zone. Deep-sea drilling permits local determination of age and lithology of the seismic intervals of the sediment section and the top of the uppermost crustal interval. The lateral continuity of high-amplitude peaks that separate the sedimentary seismic intervals suggests that the boundaries are for the most part time-synchronous throughout the Venezuela Basin. Thinning and local pinch-outs of the deeper sedimentary seismic intervals in several parts of the basin contrast with the continuity in thickness of the shallower sedimentary seismic intervals. This suggests that the deeper intervals were affected more than shallower intervals by bottom currents which scoured and redistributed sediment from Cretaceous to Eocene time, but which became much weaker in post-Eocene or Early Oligocene time. Perhaps the weakening of bottom currents in Late Eocene or Early Oligocene was related to the development of the Lesser Antilles island arc and the isthmus of Panama in Eocene time. Planar seismic reflections within the high-velocity crust of the Venezuela Basin extend 1.5 sec below the base of the sediment section. These planar reflections extend tens of kilometers laterally. The upper interval of this crustal section consists of fairly level internal reflections compared to the lower interval with more steeply dipping reflections. The boundary between these two seismic intervals is not a distinct reflection, but rather a zone of apparent merging of the two intervals. The reflections of the lowest seismic interval become lost in the noise at a depth approximately equivalent to the top of the region in the lower Venezuela Basin crust with refraction velocities of 7.2 km/sec.