Drilling Strategy | Table of Contents
SCIENTIFIC OBJECTIVES
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 fluctuations, much of the information on
the relationship between sea-level and depositional facies is qualitative.
Extensive seismic surveys over the Marion Plateau have enabled the description of the various
depositional sequences found on the plateau (Figs. 2, 10). These data, along with sedimentological
information from Leg 133 cores and dredge samples, have been used to develop a growth model for
carbonate platform phases on the Marion Plateau and the relationship of these platforms to sea-level
change (Fig. 3). Coring and downhole measurements during Leg 194 will address the following
objectives:
Magnitude of the N12-N14 second-order eustatic sea-level fall
- Although difficult, determining the magnitude of eustatic sea-level fluctuations is
significant not only for the establishment of a Phanerozoic sea-level curve but also for
understanding the stratigraphic response of continental margin sediments to sea-level
forcing. Leg 194 coring provides an opportunity to determine the absolute magnitude of
one of the major Cenozoic sea-level falls.
Oligocene-Pliocene, third-order sea-level fluctuations
- Coring will recover an Oligocene-Pliocene sea-level record preserved in the carbonate
platform growth phases of the Marion Plateau. Seismically, this record appears to include a
complete third-order event stratigraphy from 30 to 4 Ma within the second-order sea-level
falls that dominate the sequence stratigraphic framework of the Marion Plateau. Analyses
of these variations, and the higher order fluctuations contained within them, will provide
information on the timing and influence of sea level on the carbonate growth phases of the
Marion Plateau.
Sedimentary facies change and development of sequence stratigraphic units controlled by sea
level changes
- Leg 194 coring will recover 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. Leg 194 coring will enable detailed analysis of the cumulative result of these different
sedimentological responses to sea-level forcing.
Impact of potential sequence of temperate to subtropical carbonates
- It is likely that the MP3 platform is at least partially composed of temperate to subtropical
carbonates. Thus, the results of this coring will rely on and extend the results of Leg 182
(cool-water carbonates of the Great Australian Bight). Stable isotopic data from Leg 133 Site
811 showed that during the late Miocene, sea-surface temperatures (SST) 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. Sea-surface
temperatures 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 sub-tropical
bioassemblage. If MP3 is indeed composed of cooler water fauna, the depth of platform
initiation probably would be deeper than for purely tropical carbonate. Should the early
growth phases of MP3 be temperate in nature, documenting the transition of these cooler
water forms into the tropical forms existing for most of the MP3 growth phase will be an
important outcome of Leg 194.
Fluid flow and diagenesis within pure carbonate and mixed siliciclastic/carbonate depositional
environments
- The mechanisms, rates, and distributions of fluid transport through carbonate platforms and
reef structures are critical to the understanding of diagenetic processes (Buddemeier and
Oberdorfer, 1986) and for the geochemical cycling of many elements. Fluid movements
have the ability to chemically alter the mineralogic composition of the sediment by
converting 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 et al., 1991) and the
carbonate platforms of northeast Australia (McKenzie et al., 1993; Davies, McKenzie,
Palmer-Julson, et al., 1991). Recent studies have shown that carbonate sediments off
northeast Australia were dolomitized by multigenerational fluids flowing through the
platforms (McKenzie et al., 1993). The movements of fluids through the Queensland Plateau
were demonstrated by using 87Sr/86Sr isotopic ratios and the Sr composition of interstitial
waters (Elderfield et al., 1993).
- Fluid flow can also alter the sedimentary structure, permeability, and porosity of the 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 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 tropical South Pacific and its influence
on carbonate platform development.
- Leg 194 coring will enable the investigation of paleoceanographic variations in the western
Coral Sea and will provide the opportunity to correlate these variations with changes in sea
level. Changes in paleocirculation in the study area have been modified both by the
movement of continental fragments resulting from local rifting events and by the northward
movement of the Indo-Australian Plate and its collision with the Asian Plate. Northward
movement of the Indo-Australian Plate caused significant variations in climate because of
movement across climatic boundaries. These changes, in addition to global climatic
variations, had dramatic influences on the depositional environments in the Coral Sea, which
today are dominated by tropical carbonates. An important paleoceanographic objective is to
determine the effects of changes in paleocirculation and climate on the development of
carbonate platforms and reefs off northeast Australia.