Abstract: Paleotectonic Implications of Subaqueous Flow Structures in Mesozoic/Cenozoic Caribbean Deep-Sea Sediments

Florentin J. M. R. Maurrasse

Studies of primary sedimentary structures in DSDP Leg 15 cores have shown that graded bedding, associated cross-bedding, and convolute lamination characterize the Upper Cretaceous sequences of sites 146, 150, and 153. Well-defined grading is most conspicuous in the graywacke type of hemipelagic layers containing basaltic ashes that are interbedded with pelagic facies of Coniacian and Santonian ages. Subsequent layers with sandy texture gradually diminish in frequency and magnitude toward the end of the Maestrichtian. The layers show considerable variation in thickness and commonly are ungraded; the principal coarse modal class remains nearly constant throughout and contains only radiolarians and planktonic foraminifers. The structures are believed to be the results of int rmittent and frequent turbidity flows from an incipient ridge which was volcanically active in its early stages. Initial crustal rupture and succeeding tectonic activity at the onset of the Beata Ridge are postulated as the main triggering factors and source of the pelagic turbidites. Textures and structures of the turbidites also suggest deposition in the proximity of the main axis of flow.

Well-defined cross-bedding and cross-lamination are associated only with major hiatuses and "hard grounds" at sites 148 (Aves Ridge; middle Miocene/Paleocene) and 151 (Beata Ridge; Paleocene/Santonian). In contrast to the preceding proximal turbidites, intersecting layers are confined only to the sequences immediately adjacent to the major hiatuses, which also characterize abrupt overall facies changes. It is postulated that the hiatuses and adjacent subaqueous flow structures were caused by strong bottom currents probably related to the east-west flow of an early Caribbean current. The action of bottom currents apparently ceased not only because of major shifts in the hydrodynamic regime of the area through time, but also because of significant vertical tectonic readjustments that to k place subsequently at both sites and led to present conditions.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC