Miocene to Pleistocene sedimentation at ODP Site 133-823

More than 2000 turbidite, debris-flow, and slump deposits recovered at Site 823 record the history of the Queensland Trough since the middle Miocene and provide new insights about turbidites, debris flow, and slump deposits (herein termed gravity deposits). Changes in the composition and nature of gravity deposits through time can be related to tectonic movements, fluctuations in eustatic sea level, and sedimentological factors. The Queensland Trough is a long, relatively narrow, structural depression that formed as a result of Cretaceous to Tertiary rifting of the northeastern Australia continental margin. Thus, tectonics established the geometry of this marginal basin, and its steep slopes set the stage for repeated slope failures. Seismic data indicate that renewed faulting, subsidence, and associated tectonic tilting occurred during the early late Miocene (continuing into the early Pliocene), resulting in unstable slopes that were prone to slope failures and to generation of gravity deposits. Tectonic subsidence, together with a second-order eustatic highstand, resulted in platform drowning during the late Miocene. The composition of turbidites reflects their origin and provides insights about the nature of sedimentation on adjacent shelf areas. During relative highstands and times of platform drowning, planktonic foraminifers were reworked from slopes and/or drowned shelves and were redeposited in turbidites. During relative lowstands, quartz and other terrigenous sediment was shed into the basin. Quartzose turbidites and clay-rich hemipelagic muds also can record increased supply of terrigenous sediment from mainland Australia. Limestone fragments were eroded from carbonate platforms until the drowned platforms were buried under hemipelagic sediments following the late Miocene drowning event. Bioclastic grains and neritic foraminifers were reworked from neritic shelves during relative lowstands. During the late Pliocene (2.6 Ma), the increased abundance of bioclasts and quartz in turbidites signaled the shallowing and rejuvenation of the northeastern Australia continental shelf. However, a one-for-one relationship cannot be recognized between eustatic sea-level fluctuations and any single sedimentologic parameter. Perhaps, tectonism and sedimentological factors along the Queensland Trough played an equally important role in generating gravity deposits. Turbidites and other gravity deposits (such as those at Site 823) do not necessarily represent submarine fan deposits, particularly if they are composed of hemipelagic sediments reworked from drowned platforms and slopes. When shelves are drowned and terrigenous sediment is not directly supplied by nearby rivers/point sources, muddy terrigenous sediments blanket the entire slope and basin, rather than forming localized fans. Slope failures affect the entire slope, rather than localized submarine canyons. Slopes may become destabilized as a result of tectonic activity, inherent sediment weaknesses, and/or during relative sea-level lowstands. For this reason, sediment deposits in this setting reflect tectonic and eustatic events that caused slope instabilities, rather than migration of different submarine fan facies.

Grain-size, carbona and calcium carbonate at DSDP Leg 58 Holes

Sand-silt-clay distribution was determined on 10-cm**3 sediment samples collected at the time the cores were split and described. The sediment classification used here is that of Shepard (1954), with the sand, silt, and clay boundaries based on the Wentworth (1922) scale. Thus the sand, silt, and clay fractions are composed of particles whose diameters are 2000 to 62.5 µm, 62.5 to 3.91 µm, and less than 3.91 µm, respectively. This classification is applied without regard to sediment type and origin; therefore, the sediment names used in this table may differ from those used elsewhere in this volume; e.g., a silt composed of nannofossils may be called a nannofossil ooze in a site chapter.

Organic carbon and nitrogen at DSDP Leg 58 Holes

The sediments penetrated on Leg 58 of the Deep Sea Drilling Project in the Philippine Sea represent long periods of geologic time during which depositional conditions apparently remained very constant. Organic carbon and nitrogen contents of the sediments decrease with increasing depth of burial, before leveling off at minimum values of about 0.05 to 0.10 per cent and 0.01 per cent, respectively. The depth at which the minimum values are reached varies from site to site, but ages of sediments corresponding to the minima are all about 5 m.y. We infer that slow bacterial diagenesis is responsible for the gradual depletion of organic carbon and nitrogen. It is likely that the rate of bacterial metabolism is controlled by the rate of diffusion of electron acceptors within the sediments. These results suggest that bacterial ecosystems in deep-water sediments play a much more important role in diagenesis than has previously been thought.