HR: 17:00h
AN: OS44A-05    [Abstracts]
TI: Isotopic Stage 3 Deposition and Stage 2 Erosion of a Clinoform in the Gulf of Papua: Regional Tectonics Versus Eustatic Sea-Level Change
AU: * Milliman, J D
EM: milliman@vims.edu
AF: College of William & Mary, School of Marine Science, Glouster Point, VA 23062
AU: driscoll, n W
EM: ndriscoll@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093
AU: Slingerland, R
EM: sling@geosc.psu.edu
AF: Penn State University, Department of Geosciences, University Park, PA 16802
AU: Babcock, J
EM: jbabcock@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093
AU: Walsh, J P
EM: walshj@mail.ecu.edu
AF: East Carolina University, Department of Geology and Coastal Resources, Geenville, NC 27858
AB: Preliminary analysis of 21 piston cores, 24 gravity cores, 1300 nmi of towed CHIRP and more than 2500 nmi of hull-mounted CHIRP seismic data in the Gulf of Papua (GoP) shows that beyond the modern clinoform lies an older, partially eroded clinoform comprising the middle shelf. Approximately 30-40 m of generally sub-parallel reflectors lie beneath a relatively hard acoustic surface and above a set of obliquely prograding reflectors. Based on stratal geometry, their stiff character when cored, and their present elevation, we deduce that this mid-shelf clinoform prograded two-thirds of the way across the pre-existing shelf in response to a relative sea-level rise during Stage 3, when the rate of tectonic subsidence outpaced the rate of eustatic sea-level fall. Deflection of the transgressive surface from horizontal yields a differential subsidence of about 1 to 2 mm/yr from the peripheral bulge in the south to near the basin depocenter in the north. On the basis of the Chappel and Shackleton eustatic sea level curve, eustatic sea level fell from ~-40 to ~-80 m between 45,000 and 25,000 years BP. During this period, tectonic subsidence is interpreted to have outpaced eustasy to create the observed 30-40 m of accommodation. In the central Gulf mid-shelf region, northeast of the Fly River mouth, this clinoform has been dissected by six major valleys ranging in width from 10 to 15 km, cut into the sub-parallel strata and separated from one another by "mesas" that stand 10-20 m higher than the valleys. The mesas also dip gently to the north, presumably reflecting regional subsidence. Cut into both the valleys and mesas are numerous channels, mostly less than ~200 m wide but as much as 30-40 m deep. We interpret this erosion to be the product of rapid eustatic fall commencing ~25,000 years BP (Stage 2) that lowered sea level to -125 m. As eustatic sea level rose from -120 to +3 m about 7,000 years BP, the former mid-shelf river channels within the river valley were partially filled, presumably by fluvial and estuarine facies. The transgression was so rapid, however, that the valleys themselves remained unfilled except for minimal input from eroding interfluves. During this rise the river mouths backstepped landward and carbonate sediments began to be deposited on the outer shelf, blanketing the former alluvial plain.
DE: 4219 Continental shelf processes
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting