Abstract:
The flank of the Ontong Java Plateau, a large deep water carbonate plateau in the western equatorial Pacific, is an ideal location for studying responses of carbonate sedimentation to the effects of changing paleoceanographic conditions. These carbonate responses are reflected in the physical properties of the sediment, which in tram control the appearance of seismic reflection profiles; hence, seismic profiles should contain a record of paleoceanographic changes. Reflection profiles have been interpreted in combination with results from 2 DSDP and 5 ODP drill sites from the top and flank of the plateau. The sediment column is over 1000 m thick at the top of the plateau, consisting of mainly pelagic sediments. Below 2800 m water depth, the sediment column commences to thin and sediment-mass failures (faulting, slumping) are apparent, although sedimentation is still primarily pelagic. Sediment column thinning is believed to be related to increasing carbonate dissolution below the lysocline. Comparison of the seismic stratigraphy with a synthetic stratigraphy, computed by modelling carbonate deposition and dissolution, indicate the following: (1) the plateau has likely maintained its present bathymetry throughout the Neogene, by balancing subsidence with sediment accumulation; (2) sediment winnowing is apparent on the top of the plateau, and mass wasting is apparent on the flank; both of which are most evident during the Miocene; (3) thinning of the sediment column can be explained, entirely, by carbonate dissolution, with reasonable estimates of the dissolution gradient Synthetic seismograms, using physical properties dam, have been generated to accurately correlate seismic proffles to drilling results, and show that the velocity structure of the sediment column, on the first order, is a function of the depth below seafloor, regardless of the depth of deposition, age, or state of induration of the sediment. Changes in interparticle (between grains) porosity; which in turn are likely a function of changing sedimentation rates, are the likely causes of reflections in seismic profiles. A significant change in sedimentation rates at all drill sites corresponds with a change in the appearance of seismic reflection profiles, especially on the flank of the plateau, at about 9.5 Ma. This change corresponds with a major reorganisation of the Pacific Ocean water masses and circulation patterns resulting from the advance of Antarctic glaciation, intensification of North Atlantic Deep Water, and restriction of the Indo-Pacific Seaway.