Hemipelagic Rocks at Bissex Hill, Barbados: Sedimentology, Geochemistry, and Depositional Environment

Abstract

ABSTRACT The island of Barbados exposes the crest of the Barbados accretionary prism of the Lesser Antilles forearc. The sedimentology of Lower Eocene hemipelagite was studied in three fault-bounded packets near Bissex Hill. Three facies were identified based on coherent stratigraphic successions. Facies 1 hemipelagite strata are well layered to massive and consist of radiolarite layers (T layers), matrix-supported radiolarian mudstone (F layers), layers with an intermediate abundance of radiolarian tests (N layers), mud-rich (M) layers containing less than 20% radiolarian tests, rare quartzose sandstone intervals, mud-chip breccia beds, and wispy, laminated, bioturbated units. Many T and N layers have parallel laminations and sharp basal contacts. Few T and N layers have coarse-tail grading, cour structures, and floating mud clasts. Most T and N layers were deposited by turbidity or bottom-water currents. F layers probably represent waning-flow current or suspended-load deposits. Wispy, laminated units are most likely bioturbated T, N, and F layers. Facies 2 strata are wispy, laminated terrigenous mudstones greater than 10 cm thick. Thorough bioturbation indicates slower rates of deposition relative to facies 1 strata. Facies 3 is mudstone composed of sparse radiolarian tests floating in a terrigenous-mud matrix. Facies 3 deposits are interpreted as extensively bioturbated sediment which, prior to reworking, contained fewer intercalated T layers. Bulk-rock chemistry indicates that the hemipelagites were derived from terrigenous and biogenic sources. Weight-percent Al2 UB>O3 correlates with the percentages of clay-sized material in T, N, and F layers. Low total organic carbon (bitumen-free; 0.06-0.83%) and Type III kerogens indicate an oxic depositional environment. The hemipelagic rocks were deposited in abyssal-plain or outer-trench-slope environments.