Quaternary sandy deposits and canyons on the Venezuelan margin and south Barbados accretionary prism

Abstract

The geomorphological and tectonic setting, distribution and origin of Late Quaternary sandy deposits on the southern termination of the Barbados accretionary prism and the adjacent abyssal plain have been investigated using Seabeam and 3.5 kHz echosounding data, and lithological and mineralogical analyses of 28 Kullenberg cores. Quaternary sands on this prism are deposited in piggyback basins. They are transported along three major canyons, the morphology and trend of which are highly variable and which are tectonically controlled. The narrow and relatively shallow canyon sections are in some ways like the major distributary channels of deep sea fans, but they are true canyons in terms of the sedimentary processes taking place, namely active erosion associated with the continuous uplift of tectonic structures and deposition of only slightly elevated sediment levees. The sand is partly trapped on basin floors as flat sediment sheets composed of thinly and thickly bedded fine grained sandy turbidites. High deposition rates occurred during the last period of low sea levels. The sands originated from the Orinoco-Trinidad shelf, as inferred from geographical considerations and clay and heavy mineral provenance data (predominently kaolinite, illite, hornblende and epidote). Sands of the abyssal plain settled on a deep sea fan system that trends north-south, parallel to the toe of the prism. Most of the sediments are introduced at the southern end of the system where they are delivered by the three major canyons which merge, on the fan, in a northward braided channel system. This material consists of sandy turbidites that originated from the Orinoco-Trinidad margin. The grain size of the sands is similar to or coarser than the grain size of the prism sands. Sand components are identical and preclude supply from the Amazon river. The sedimentation rate, during periods of low sea level, seems to be lower. A significant percentage of smectite in the clay fraction suggests that a minor part of the material may be derived from the Demerara margin, located closer to the main smectite source, i.e. the Amazon delta. These results emphasize the difference between the sand distribution on active and passive margins.