Anomalously Coarse-Grained Siliciclastic Sediments on Slope of Tiburon Rise, Western Atlantic

James F. Dolan

One surprising result of Ocean Drilling Project Leg 110 (Lesser Antilles forearc) was the discovery of extensive, anomalously coarse-grained Eocene-Oligocene siliciclastic deposits on the slope of the Tiburon Rise. These deposits are difficult to explain in terms of (1) mode of deposition and (2) timing of deposition with respect to sea level.

These sands and silts occur as 1-150-cm thick planar-laminated beds within a 200-m thick sequence of hemipelagic clay. The siliciclastic layers locally exhibit graded bedding and partial Bouma sequences. Their quartz-rich composition and the rare occurrence of microcline and glaucophane suggest derivation from South America.

Although these sediments probably reached the Tiburon Rise area as turbidites, their ultimate depositional mechanism is unclear. Despite local turbiditic textures, occurrence of the sediments on the slope of the rise, 400 m above the adjacent abyssal plain, suggests they are not predominantly turbidites. One possible explanation is transportation upslope by bottom currents. An alternative possibility is that during the Paleogene the rise was not a bathymetric high and that these are simply distal turbidites. However, interbedded calcilutite turbidites and small-scale slumps derived from higher on the rise demonstrate that paleoslopes existed during deposition of the siliciclastic sediments.

The relationship between timing of deposition and sea level is also problematic. Deposition of the anomalously coarse-grained sediments occurred during a period of presumed high sea level, whereas sediments deposited during the subsequent late Oligocene sea level lowstand consist of fine-grained hemipelagic sediment. Exactly the opposite transition is predicted by traditional models of deep-sea siliciclastic sedimentation.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.