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The scientific themes of Leg
194 are discussed below:
of the second-order eustatic sea level fall of Zones N12-N14: The Marion Plateau provides ideal
targets to address the causes, magnitudes, and effects of sea level change on continental margin
sediments. Although one of the fundamental controls on the nature and geometry of continental margin
deposition is sea level change, much of the information on the relationship between sea level and
depositional facies is qualitative. Determining the magnitude of eustatic sea level fluctuations is
also significant for the calibration of a Phanerozoic sea level curve.
of carbonate platforms in a current-dominated environment: Slope sedimentation adjacent to most
carbonate platforms is controlled by the prevailing wind direction with the windward side of the
platform being relatively sediment starved and the leeward side having higher depositional rates. On
the other hand, carbonate platforms and slope sediments of the Marion Plateau are controlled by strong
seafloor currents. These currents determine the morphology and growth potential of the platforms as
well as the location and amount of sediment transported from the platform top. The results of Leg 194
drilling, along with available seismic data, will enable the characterization of these current
dominated carbonate platform systems.
of subtropical carbonate platforms: The carbonate platforms of the Marion Plateau are dominated by
subtropical and cool subtropical carbonates. The results of Leg 194 coring will bridge the gap between
the cool-water carbonates sampled during Leg 182 (Great Australian Bight: Cenozoic cool-water
carbonates) and the tropical to subtropical carbonates recovered during Leg 166 (Bahamas Transect).
Stable isotopic data from Leg 133 Site 811 on the more northerly Queensland Plateau showed that during
the late Miocene, regional SSTs were
cool (~20°-22°C), as were global sea-surface temperatures (Isern et al., 1996). Given the more southerly location of the MP3 platform with respect to Site 811 (Queensland Plateau), temperatures
were probably similar to, if not cooler than, those over the Queensland Plateau. SSTs at or below
20°C would not prevent tropical coral growth but would make it more likely that the MP3 platform was constructed of a "cooler," more subtropical bioassemblage. Documenting the transition
of cooler water biota into warmer water forms will be an important outcome of Leg 194.
change and development of sequence stratigraphic units controlled by sea level changes in a mixed
carbonate and siliciclastic sediment system: Leg 194 coring recovered a detailed record of carbonate
and siliciclastic sediment facies variations resulting from mixing of sediment from carbonate banks on
the Marion Plateau and the continental margin of Australia. In general, carbonate sediment export
increases during sea level highstands as carbonate banks have additional accommodation space for
increased growth of carbonate-producing organisms. On the other hand, terrigenous sedimentation
generally decreases during highstands because of the elevated erosional base level. Detailed analysis
of the cumulative result of these different sedimentological responses to sea level forcing will be an
important result of Leg 194 drilling.
Oligocene-Pliocene third-order sea level fluctuations: The Oligocene-Pliocene sea level record
preserved in the carbonate platform growth phases of the Marion Plateau includes a third-order event
stratigraphy within the second-order sea level falls that dominate the sequence stratigraphic
framework. Analyses of these variations, and the higher order fluctuations contained within them,
provide information on the timing and influence of sea level on the carbonate growth phases and
sedimentation of the Marion Plateau.
and causes of fluid flow within pure carbonate and mixed siliciclastic/carbonate depositional
environments: Determining the mechanism and rate of fluid transport through carbonate platforms and
reef structures is critical to understanding diagenetic processes (Buddemeier and Oberdorfer, 1986)
and the geochemical cycling of many elements. Fluid movement has the ability to chemically alter the
mineralogic composition of the sediment by hastening the conversion of metastable minerals such as
high-Mg calcite and aragonite to more stable calcite and dolomite (Mullins et al., 1984; Simms, 1984).
Alteration of carbonate sediments to dolomite has been significant in both the Bahamas (Varenkamp,
1991) and the carbonate platforms of northeast Australia (McKenzie et al., 1993; Davies, McKenzie,
Palmer-Julson, et al., 1991). Studies using 87Sr/86Sr isotopic ratios have shown that carbonate sediments off northeast
Australia were dolomitized by multigenerational fluids flowing through the platforms (McKenzie et al.,
1993). Fluid flow can also alter sedimentary structure, permeability, and porosity of a carbonate
deposit, thus having important effects on flow pathways and reservoir potential. The existence of
fluid flow has been described in tropical carbonate platforms such as the Great Bahama Bank and the
Queensland Plateau (Eberli, Swart, Malone, et al., 1997; Elderfield et al., 1993) and also in
temperate-water carbonates (Feary, Hine, Malone, et al., 2000). However, the mechanisms causing this
flow are neither well documented nor understood.
- Role of
climatic and paleoceanographic change in the subtropical South Pacific and its influence on carbonate
platform development: In addition to sea level fluctuations, paleoceanographic variations in the
western Coral Sea have significantly affected the development of carbonate platforms and reefs off
northeast Australia. Paleocirculation has been modified both by the movement of continental fragments
resulting from local rifting events and by the northward movement of the Indo-Australian plate.
Northward movement of the Indo-Australian plate also resulted in significant environmental variations
in climate because of movement across climatic boundaries. These changes, in addition to global
climatic variations, influenced the depositional environments in the Coral Sea, which today are
dominated by tropical carbonates.
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