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Importance of Determining the Magnitude of Eustatic Sea-Level Variations
Measuring the amplitude and timing of eustatic sea-level fluctuations is essential both for the establishment of an accurate eustatic sea-level curve for the Phanerozoic and for the accurate interpretation of sediment sequences on continental margins. Several attempts have been made to determine the amplitude of glacioeustatic fluctuations, including passive-margin sequence stratigraphy (Vail et al., 1977; Vail and Hardenbol, 1979; Haq et al., 1987); modeling of sedimentary depositional regimes (Watts and Thorne, 1984); calibration of the oxygen isotope curve (Majors and Mathews, 1983; Miller et al., 1987; Williams, 1988); and analysis of the depositional history of carbonate sediments on atolls (Schlanger and Premoli-Silva, 1986; Halley and Ludwig, 1987; Moore et al., 1987; Lincoln and Schlanger, 1987, 1991). These analyses yield a wide range of results, and although the different independent data sets often agree with regard to the timing of sea-level events, significant differences between estimates for the magnitude of sea-level fluctuations remain. The establishment of a eustatic sea-level curve has major implications for global stratigraphic correlation and basin analysis, and defining the amplitude of such a curve remains one of the major challenges in sea-level research (COSOD II, 1987; Sahagian and Watts, 1991; JOIDES Planning Committee, 1996). The excellent record of Miocene sea-level fluctuations preserved in the carbonate platforms of the Marion Plateau, southern Coral Sea (Figs. 1, 2), provides an ideal opportunity to test sea-level models and quantify the magnitude of eustatic variations.

Growth Phases of the Marion Plateau and their Record of Sea-Level Variations
Carbonate platforms and their slopes are sensitive indicators of sea-level variations, as they predominantly record growth during sea-level highstands and shutdown during sea-level lowstands. Sampling through carbonate platforms records sea-level effects in a "dipstick" fashion. On the other hand, sediments on platform margins and slopes record sea-level variations as alternations of shallowing and deepening sequences. The geometric relationships between the carbonate platforms and adjacent slope sediments of the Marion Plateau have been clearly imaged by seismic data, enabling the investigation, correlation, and dating of sediment sequences. The information recovered from Leg 194 sites (Fig. 2) will provide an independent basis for development and assessment of the global sea-level curve.

The lower-middle Miocene MP2 platform appears to have formed as a series of transgressive and highstand system tracts (Figs. 3, 4). Five highstand events are recorded by MP2 (MP2a - MP2e), providing a record of third-order sea-level variations. Only MP2e was sampled during Leg 133, and thus the age at which the other events occurred is not known (Davies, McKenzie, and Palmer Julson, 1991). Newly acquired site survey seismic data indicates that MP2 prograded over its former slope sediments. These pulses of progradation are strongly controlled by sea-level fluctuations, as was shown for the progradation of the Great Bahama Bank (Eberli and Ginsburg, 1989; Eberli, Swart, and Malone, et al., 1997).

The upper Miocene MP3 platform began to form during a lowstand on the outer slope sediments of MP2. The MP3 phase subsequently evolved into a series of highstand systems tracts but remained structurally lower than the top of MP2 for most of its history (Fig. 3). The upper Miocene MP3 platform records four sea-level cycles (MP3a-MP3d; Fig. 4). The sea-level rise during MP3d corresponds to the last phase of platform growth. This rapid sea-level rise, in conjunction with other environmental factors, resulted in the drowning of most of the MP3 platform (Pigram et al., 1992).

At present, it is difficult to compare the growth phases seismically imaged within the MP2 and MP3 platforms to global events, as the exact timing of their development will not be known until the sequences are cored. In addition, the internal structure of the MP3 platform, whose top is exposed with a nondepositional hardground at the seafloor, shows no internal seismic structure because of the nonpenetrating seismic signal and the probably well-cemented and homogeneous lithology. However, the recovery and dating of sediments from these sequences will provide important information on Miocene sea-level events and their influence on continental margin sedimentation. Data from these sediments may also be used in conjunction with other "sea-level" legs cored as part of the Ocean Drilling Program (ODP) global sea-level strategy.

To determine the sea-level event stratigraphy on the Marion Plateau through drilling, it will be necessary to establish

1. The depositional history of the Miocene carbonate platforms (Fig. 3) of the Marion Plateau by
2. The amplitude of the middle Miocene (N14-N12) sea-level fall by determining
To establish the magnitude of the sea-level fall that led to the formation of the lowstand MP3 platform, it is first necessary to determine the paleowater depth of the top of the lower-middle Miocene MP2 platform (Fig. 3). Leg 133 coring (Sites 816 and 826) showed that the top of this platform consisted of a tropical reefal assemblage deposited in water depths no greater than 20 m (Davies, McKenzie, Palmer-Julson, et al., 1991). This depth defines the approximate point from which sea level began to fall (Pigram et al., 1992). Sampling evidence indicates that the top of MP2 has been subjected to subaerial exposure. The dissolution and erosion that is likely to have resulted from exposure would have made the present-day top of MP2 lower than it was originally, introducing an error in determining the highest position of sea level immediately before the fall. The extent of erosion is difficult to quantify, but it can be expected that the loss would be small because the high diagenetic potential of these tropical carbonates would tend to create a carbonate pavement that would be difficult to erode. Any sediment loss from the top of MP2 will result in the underestimation of the true amplitude of sea-level fall (Pigram et al., 1993).

The low-point of the sea-level fall is defined by the paleowater depth at the time the first sediments of the upper Miocene lowstand MP3 platform were deposited (Pigram et al., 1992; Fig. 2). The MP3 platform was not sampled during Leg 133 drilling, and therefore the biofacies that compose this platform can only be inferred seismically. The seismic characteristics of MP3 are poorly imaged and difficult to assess, but appear to have both "tropical" (vertically accreted) and "temperate" (mound like) signatures. The presence of cooler water fauna would indicate that the depth of platform initiation was deeper than that for purely tropical carbonate. Without sampling the MP3 platform, we can only speculate on the paleowater depth of the MP3 formation. Three possible scenarios are

  • If MP3 is entirely tropical, its formation depth was likely to be ~20-25 m, resulting in a N12-N14 sea-level fall of 185-190 m. This eustatic change is greater than other estimates for this time interval (30-90 m, Miller et al., 1987; >100 m, Vail and Hardenbol, 1979).
  • If MP3 is subtropical in composition, the depth of initiation could have been ~50-70 m. Thus, the N12-N14 sea-level fall would be ~135-155 m.
  • It is also possible that sea level fell below the level on which MP3 was established, thus also affecting the estimate of sea-level change, but no seismic evidence supports this conclusion (Pigram et al., 1992).

    The Influence of Subsidence on Sea-Level Magnitudes
    The inability to remove tectonic subsidence effects from relative sea-level signatures has hindered the quantification of eustatic sea-level variations in many areas. However, a sea-level shift that occurs between two sites of equal tectonic subsidence will provide an accurate record of the magnitude of eustatic change.

    For the difference between the top of MP2 and the initiation of MP3 to be an accurate measure of the N12-N14 eustatic fall, it is necessary to demonstrate that there is no differential subsidence along the drilling transect. There are two lines of evidence to support this. First, the Marion Plateau is not structurally compartmentalized and therefore behaves as a single structural entity (Symonds et al., 1988). Seismic lines between the proposed sites show that there are no structural elements, such as faults, between the sites that could cause them to have relative differential subsidence. Second, because the Marion Plateau basement surface is planated with minimal dip to the northeast, depths to basement surface contours can be considered isosubsidence lines (Fig. 5). The eight proposed sites are all near the 1-s basement contour, indicating a minimal basement gradient between the sites and thus negligible differential subsidence.

    Calibration of eustatic sea-level variations can only be realistically estimated on slowly subsiding, structurally well-understood margins where an accurate tectonic subsidence history can be established and where sites of equal tectonic subsidence, which have both the highstand and the lowstand history preserved, can be located. The advantage of such areas is that although falling sea level follows the slow tectonic subsidence of the platform, the relative depth change recorded between two sites is self-correcting because they both subside by the same amount. For the predicted middle Miocene Marion Plateau subsidence rates, the increase in water depth at both sites as a result of tectonic subsidence (<10 m) is an order of magnitude less than the eustatic sea-level change over the same interval. The tectonic component of sea-level change is therefore within the error of paleowater depth estimates achievable in these sediments.

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