Depositional Facies of Leg 30, Deep Sea Drilling Project Sediment Cores

Abstract

Sediment cores from Leg 30 are subdivided into eight sedimentary facies. The vertical sequence of these facies is controlled by differences in tectonic setting from which the cores were obtained. Three sites (285, 286, 287) sampled marginal basins, whereas two sites (288, 289) were obtained from an equatorial plateau.The marginal basin facies at Sites 285 and 286 consist of basal sandy turbidites (facies 1) interbedded with mass flow and debris flow conglomerates (facies 2). Within the turbidite intervals, particularly at Site 286, an upward increase in flow intensity is suggested from the vertically upward increase in coarse-tail graded bedding which in turn is overlain by an interval of facies 2 conglomerates. Such coarsening-upward facies successions mimic the progradational model of submarine fan evolution documented elsewhere. These clastic facies are overlain by biogenic ooze (some reworked by turbidity currents) of facies 3 and they in turn are capped by abyssal red clays (facies 4). At Site 287 in the Coral Sea Basin, the vertical facies succession consists of basal biogenic oozes (facies 3) overlain by olive clays (facies 6) and capped by silty and clayey turbidites (facies 5) organized into graded cycles.The two sites on the Ontong-Java Plateau show a different facies evolution from a basal facies consisting of reworked and mixed volcanic ash and biogenic sediments and chert (facies 8) to an upper facies consisting of biogenic ooze only (facies 7). Reworking by both turbidity and bottom currents is responsible for the admixture of carbonate and volcanic material which are organized into a vertical sequence herein named the volcaniclastic-siltstone-pelagic-carbonate cycle. The cycle consists of a basal interval of graded, fresh volcanic ash which is overlain by a middle interval of biogenic limestone and chert containing parallel laminae, current ripples, and lenticular, flaser, and wavy bedding, and clay drapes. The clay drapes comprising parts of flaser beds consist of altered volcanic ash. The cycle is capped by a biogenic limestone interval. Such cycles are common to oceanic settings where biogenic sedimentation occurs near regions of active volcanism. The facies sequence at Sites 288 and 289 suggests tectonic quiescence for the Ontong-Java Plateau.The facies sequences in the marginal basins at Sites 285 (South Fiji Basin) and 286 (Hebrides Basin) suggest the following tectonicsedimentary history. During initial development of the marginal basins by sea-floor spreading, the basinal environment was characterized by high slope instability which favored deposition of sediments by turbidity currents. These turbidity currents built submarine fans which prograded toward the basin center as sea-floor spreading continued. As the basin widened, regional slope instability was reduced, and associated turbidity currents were characterized by reduced flow intensity. After spreading ceased, turbidity current activity was minimal, and biogenous sedimentation occurred followed by pelagic red clay depositi...