Previous investigations of the seismic stratigraphic architecture of the Marion Plateau have produced a model for the evolution of the carbonate platforms on this plateau (Davies et al., 1989; Davies, McKenzie, Palmer-Julson, et al., 1991; Pigram et al., 1992). These studies proposed that the geometric relationship between the Miocene carbonate platforms and their slopes can be used to estimate the magnitude of Miocene sea level changes (Pigram et al., 1992). In order to image these geometric relationships in more detail and to provide additional site survey information for potential ODP drill holes, a 1-month site survey cruise in April 1999 was funded by the Australian Geological Survey Organisation (AGSO). A high-resolution multichannel seismic grid consisting of 1700 km of seismic lines, gravity cores, dredges, and bottom photographs was acquired from the Franklin (FR 03/99; AGSO 209), operated by the Commonwealth and Scientific Industrial Research Organisation (Heck et al., 1999). Acquisition and processing parameters for the seismic site survey data are summarized in Table T1.

Seafloor sediment samples were collected using pipe dredges, chain dredges, a Van-Veen grab, and a 1-T gravity corer with 10-cm-diameter barrels. Seafloor photographs were taken using a Benthos deep-sea camera and flash mounted on the standard Franklin conductivity/temperature/depth recorder frame. Sediment samples document the strong influence of currents in the modern environment at the Leg 194 sites on this open plateau (Heck et al., 1999). The study area is characterized by subtle bathymetric changes as drift sedimentation in the Pliocene-Pleistocene leveled out most of the preexisting relief. The significance of currents is also documented by bottom photographs showing prominent current ripples at most Leg 194 sites (Fig. F6A, F6D). Strong seafloor currents also favor early submarine diagenesis as shown by the cemented worm tubes that were dredged at the location of Site 1194 and seen on the seafloor photographs (Fig. F6A). Sediments on the plateau consist of wackestone to packstone containing abundant planktonic foraminifers. Minor skeletal grains include bryozoans, scaphopods, solitary corals, sponge spicules, and pteropods. The top of the southern platform edifice is coated with a red stained submarine hardground surface that is encrusted with serpulids and bryozoans with patches of soft sediment that partly overlie this crust (Fig. F6B, F6C).

Seismic data collected during the site survey cruise provided excellent images of late Oligocene-Holocene sedimentation on the Marion Plateau. Following the nomenclature of Pigram (1993), four unconformity-bounded megasequences (A-D) are identified overlying the continental basement (Figs. F7, F8). These five seismic units can be mapped through most of the survey area. The following section describes the seismic facies, geometry, and the general seismic stratigraphic architecture of these megasequences. Seismic sequence boundaries separating megasequences are labeled in uppercase letters that represent the name of the two bounding megasequences (A/B, B/C, etc.). All major seismic stratigraphic elements can be recognized in Figures F7 and F8, which show two transects along which all Leg 194 sites are positioned. Figures F9A and F9B show lithostratigraphic columns, chronostratigraphic frameworks, and petrophysical signatures for the same two transects.

Megasequence D has variable thickness and is composed of latest Miocene to Holocene hemipelagic sediments. Megasequence D sediments were deposited under the influence of strong currents in thick sediment drifts characterized by numerous downlap surfaces defining individual drift packages (Fig. F7). The thickness of Megasequence D is largely controlled by the preexisting relief existing at the end of the late Miocene as strong currents moved the sediments over the plateau and filled existing topographic depressions. Sediment sequence geometry indicates a predominant progradation of drift units, and thus a current direction, from north to south.

In the southern portion of the study area, Megasequence C comprises the seismically opaque MP2/MP3 carbonate platform edifice. Originally, this platform was believed to be purely of late Miocene age, and thus would have been initiated on distal sediments of MP2 during the middle to late Miocene sea level lowstand (Pigram et al., 1992). Sites 1196 and 1199 were drilled into of this platform to test this hypothesis (Fig. F8).

Scattering of the seismic signal from the well-indurated MP3 platform top makes it difficult to determine the geometry and architecture within this platform. On the seismic data, the southeast margin of the platform displays a completely different geometry from the northwest margin. The northwestern margin is composed of a nondepositional escarpment separating deeper-water Megasequence C from platform equivalents. Megasequence C thickens toward the platform, providing evidence for accumulation of platform-derived carbonates in a talus at the base of the escarpment. Site 1198 was located to recover these sediments. In contrast, the southeastern margin (Site 1197) shows a thick prograding package consisting partly of Megasequence C sediments similar to those penetrated at Site 1198. At this site, the top of Megasequence C can be traced into the MP3. Previous work indicates that the MP3 carbonate platform was likely to have drowned near the end of the late Miocene, possibly in the earliest Pliocene (Pigram, 1993).

The deeper-water facies of Megasequence C, as drilled at Sites 1192 and 1195, is restricted toward the west by the topographic high of the paleo-MP2 platform margin on which Megasequence C downlaps and wedges out. Toward the east, Megasequence C thickens toward the plateau edge (Fig. F7).

Seismic Megasequence B is generally thicker toward the west, thinning eastward and southward. In the western part of the study area, Megasequence B is dominated by the early middle Miocene MP2 carbonate platform. The upper portion of this sequence is characterized by a generally transparent to chaotic seismic section with hummocky reflections representing a vertically aggrading carbonate platform (Fig. F7). Below these chaotic reflections, a sequence of moderately continuous reflections dipping to the east indicates an eastward prograding slope system over which the MP2 platform initiated. Reflections at the top of Megasequence B can be traced from the platform top, to the margin, and into the deeper water sequences. Close to the MP2 margin, the uppermost portion of Megasequence B is characterized by a near horizontal high-amplitude reflection pattern that can be interpreted as a downstepped carbonate platform during a phase of lower sea level. Further out on the plateau, Megasequence B is composed of a thick package of gently eastward-dipping reflections.

Megasequence A, the oldest depositional megasequence over basement, is generally limited to the eastern part of the plateau, over which there is only limited seismic coverage. This thin sequence is characterized by highly continuous reflections prograding westward over basement. Because Megasequence A overlies and infills basement irregularities, it is variable in thickness.

Acoustic basement is characterized by a high-amplitude reflection at the interface with overlying sediments and numerous diffractions caused by the irregular bedrock surface. In some places, distinct morphologic structures, such as narrow highs and depressions, can be recognized clearly even on unmigrated seismic profiles. In general, the basement surface occurs at a similar two-way traveltime with a slight northeastward dip toward the edge of the plateau where basement is downfaulted to the Cato Trough (Fig. F7).

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