Abstract

The vertical sequences of sedimentary facies in three southwest Pacific marginal basins are controlled by sea-floor spreading. The basal sediments at Site 285 (South Fiji Basin) and Site 286 (Hebrides Basin) are turbidites that coarsen upward into debris-flow conglomerates because of submarine-fan progradation. The clastic facies are overlain by biogenic sediments and are capped by pelagic red clays. At Site 287 (Coral Sea Basin), the vertical sequence is partly reversed, consisting of basal biogenic sediments overlain by olive clays that were produced by turbid suspension and are capped by silty-clayey turbidites organized into graded cycles.

The following tectonic-sedimentary events are suggested. During initial rifting of the marginal basins, the overlying blanket of oceanic sediment was slumped onto the basin floor. The basin environment was characterized by high slope instability during early stages of basinal sea-floor spreading, which favored deposition of coarser turbidites and debris-flow sediments on fans prograding basinward. As the basin widened during later stages of spreading, regional slope gradients declined, and associated turbidity currents were characterized by reduced flow intensity. After spreading appeared to have ceased, pelagic oceanic sedimentation occurred, followed by pelagic red clays once the basin floor subsided below the calcite compensation depth. Higher sediment accumulation rates are characteristic of the early stage of sea-floor spreading, intermediate sediment accumulation rates are characteristic of the late stage of spreading, and low accumulation rates are characteristic of the pelagic phase. The reversed facies sequence at Site 287 owes its origin to deposition of a clastic wedge of turbidites associated with later vertical uplifts along the basin boundary. Sediment accumulation rates increased during this resurgent tectonic phase. Such resurgent tectonic phases may repeat the facies sequences observed at Sites 285 and 286.

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