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Leg 194

Sea-Level Magnitudes and Variations Recorded by Continental Margin Sequences on the Marion Plateau, Northeast Australia

Establishment of an accurate eustatic sea-level curve for the Phanerozoic has important implications for the accurate interpretation of sediment sequences on continental margins, and for global stratigraphic correlation and basin analysis. Although passive margin sequence stratigraphy, modeling of sedimentary depositional regimes, and oxygen isotope stratigraphy have advanced the understanding of cause and effect of eustatic sea-level variations, the relationship between sea-level change and depositional facies is still qualitative and determining the amplitude of eustatic sea level changes remains a major challenge. The proposed drilling on the Marion Plateau in the western Coral Sea provides a superb opportunity to address these objectives, which present a critical component of the ODP Long Range Plan. The proposed program will be a significant complement to previous sea-level legs (Bahamas Leg 166, New Jersey Margin Leg 174A, Great Australian Bight Leg 182) and build on achievements from earlier ODP drilling in the region (Leg 133).

Cretaceous rifting in the western Coral Sea formed continental fragments that are now capped by carbonate platforms. One of these fragments, the Marion Plateau, provides ideal "dipstick" drilling targets to determine the magnitude of sea-level change and its influence on the geometry of the adjacent, mixed carbonate-siliciclastic sediment sequences. The proposed sites are essentially located along a single strike line without intervening structural elements, and subsidence rates were small during the Miocene. The stratigraphic relationship between an early to middle Miocene second-order highstand carbonate platform complex (MP2) and a late Miocene second-order lowstand platform complex (MP3) allows direct determination of the magnitude of the middle Miocene N12-N14 sea-level fall, one of the largest in Cenozoic history. The Marion Plateau also records an excellent Miocene sea-level record, which is likely to include a complete third order event stratigraphy between 30 and 4 Ma.

The proposed transect includes seven primary sites to obtain complete sediment sections of the MP2 and MP3 platform phases (two sites), and proximal (two sites) and distal (three sites) slope facies to provide the stratigraphic framework. Water depths for the sites vary between 293 and 363 m, and penetrations to basement are 570 to 720 m depth. All holes will be wireline-logged. The final site locations and operational plan will be developed at the precruise meeting at ODP/TAMU (spring/summer 2000), using the latest site survey data and accurate coring and logging time estimates.

Drilling Plan

The objectives of the investigation will be accomplished by drilling 8 sites extending from shallow water across the platform edge and onto the slope. This transect will make it possible to date the shallow water carbonates by correlating them into the deeper section with planktonic microfossils using seismic data.

Standard logs (gamma, density, resistivity, porosity) combined with detailed sonic and WST logs will assist in defining correlation horizons in the stratigraphic sections.

Leg Prop # Description Site Prop Lat. Prop Long. Prop Water Depth Prop. Total Depth Prop
194 510 Marion Plat./Coral Sea CS-01A 19°55.6'S 151°36.2'E 354 694
  Full 3 lats/longs were CS-02A 19°49.8'S 151°54.7'E 363 626
    in 510 full-3 CS-03A 20°48.0'S 152°17.7'E 318 600
      CS-04A 20°55.7'S 152°37.8'E 319 610
      CS-05A 20°58.1'S 152°44.6'E 309 570
      CS-06A 20°58.6'S 152°46.1'E 293 720
      CS-07A 21°03.7'S 153°01.6'E 326 610
      CS-08A 21°04.3'S 153°03.2'E 326 580

Staff Scientist: Peter Blum

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