Seismic Stratigraphy of the Ontong Java Plateau

Abstract

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 often reflected in the physical properties of the sediment, which in turn control the appearance of seismic reflection profiles. Seismic stratigraphy analyses, correlating eight reflector horizons to each drill site, have been conducted in an attempt to map stratigraphic data. Accurate correlation of seismic stratigraphic data to drilling results requires conversion of traveltime to depth in meters. Synthetic seismogram models, using shipboard physical properties data, have been generated in an attempt to provide this correlation.Physical properties, including laboratory-measured and well-log data, were collected from sites drilled during Deep Sea Drilling Project Legs 30 and 89, and Ocean Drilling Program Leg 130, on the top and flank of the Ontong Java Plateau. Laboratory-measured density is corrected to in-situ conditions by accounting for porosity rebound resulting from removal of the sediment from its overburden. The correction of laboratory-measured compressional velocity to in situ appears to be largely a function of increases in elastic moduli (especially shear rigidity) with depth of burial, more than a function of changes in temperature, pressure, or density (porosity rebound). Well-log velocity and density data for the ooze intervals were found to be greatly affected by drilling disturbance; hence, they were disregarded and replaced by lab data for these intervals.Velocity and density data were used to produce synthetic seismograms. Correlation of seismic reflection data with synthetic data, and hence with depth below seafloor, at each drill site shows that a single velocity-depth function exists for sediments on the top and flank of the Ontong Java Plateau. Seismic reflection profiles down the flank of the plateau undergo three significant changes: (1) a drastic thinning of the sediment column with depth, (2) changes in the echo-character of the profile (development of seismic facies), and (3) loss of continuous, coherent reflections. Sediments on the plateau top were largely deposited by pelagic processes, with little significant postdepositional or syndepositional modification. Sediments on the flank of the plateau are also pelagic, but they have been modified by faulting, erosion, and mass movement. These processes result in disrupted and incoherent reflectors, development of seismic facies, and redistribution of sediment on the flank of the plateau.Seismic stratigraphic analyses have shown that the sediment section decreases in thickness by as much as 65% between water depths of 2000 m water depth (at the top of the plateau) and 4000 m (near the base of the plateau). Thinning is attributed to increasing carbonate dissolution with depth. If this assumption is correct, then changes in the relative thicknesses of seismostratigraphic units a...

Intrabasement reflections of the Ontong Java Plateau: Implications for plateau construction

“…These sequences continue to Site 289/586, and sequence mE has been interpreted at ODP Sites 807, 1183, and 1186. The sequences are laterally continuous and mostly parallel in water depths less than $2800 m; this relationship is consistent with observations from the northern and northeastern OJP Mosher et al, 1993]. In this region, seafloor topography largely reflects the morphology of the top of basement, despite the sediment and sedimentary rock column being >1000 m thick .…”

“…Sediment and sedimentary rock characterized by reflections of low to high amplitude may be divided into three sequences: middle Miocene (mM), Miocene/Oligocene transition (M/O), and middle Eocene (mE) ( Figure 3). Similar high-amplitude reflections have been reported elsewhere on the OJP [e.g., Mosher et al, 1993]. At Site 806, near Site 289/586, synthetic seismograms were confined to the upper sedimentary section .…”

Chemical and mechanical processes during burial diagenesis of chalk: an interpretation based on specific surface data of deep‐sea sediments

“…The data are presented versus depth (above) and versus age (below). On the latter figures, the depth of the lysocline versus age are shown, as interpreted by Mosher et al (1993).…”

“…9. The data for Site 803 and Site 807 are plotted versus depth as well as versus age, the latter accompanied by a curve showing the interpreted level of the lysocline versus time (Mosher et al , 1993). The general level of intraparticle porosity is lower at Site 803 than at Site 807, corresponding to the results of Lind (1993b).…”

Paleoceanography of the Eastern Equatorial Pacific during the Neogene: Synthesis of Leg 138 Drilling Results

“…This carbonate low is expressed as a seismic reflector that can be traced over thousands of kilometers across the central equatorial Pacific (Mayer et al, , 1986. Mosher et al (1993) identified a reflector in the western equatorial Pacific that was tentatively correlated with this central equatorial reflector, and Bloomer et al (this volume) were able to link this central equatorial Pacific reflector with a similar reflector in the eastern equatorial Pacific, which they correlated with the 9.5-Ma "carbonate crash." Farrell et al (this volume) further explore the regional extent of the "carbonate crash" by rationalizing the age models and correlating the carbonate records (converted to mass accumulation rate) from a number of central equatorial Pacific sites (RC12-65, RC12-66, DSDP Site 574) with those from Leg 138.…”