The facies and paleoenvironmental evolution of Miocene carbonate platforms from northeast Australia were mostly known through scattered exploration wells (see synthesis of Davies et al., 1989) and Ocean Drilling Program (ODP) Leg 133 drilling of the Queensland and North Marion Plateaus (Davies, McKenzie, Palmer-Julson, et al., 1991; Betzler and Chaproničre, 1993; Brachert et al., 1993; Chaproničre and Betzler, 1993; Isern et al., 1993; Martín and Braga, 1993; Martín et al., 1993; McKenzie and Davies, 1993; Betzler et al., 1993, 1995; Betzler, 1997). ODP Leg 194 drilling operations were performed on two new carbonate platforms located north and south of the Marion Plateau (Fig. F1). Some of the objectives were to achieve a better understanding of carbonate platform development in the region with respect to climate, sea level fluctuations, and paleoceanography and to calculate the second-order magnitude of the late middle Miocene (N12–N14) sea level fall (Shipboard Scientific Party, 2002a, 2002b).
The Southern Marion Platform (SMP) is an isolated platform of the Marion Plateau, which is a deeper extension of the Australian continental margin (Davies et al., 1989) (Fig. F2). Before Leg 194, the SMP was mainly known through seismic analyses. It was considered late Miocene in age and nucleated on the slope sediments of the Northern Marion Platform after the late middle Miocene sea level fall (Pigram et al., 1992; Pigram, 1993; Liu et al., 1998). The Shipboard Scientific Party (2002a, 2002b) provided new detailed information about the SMP. Through several seismic transects (Fig. F1), they found evidence of an asymmetrical pattern for the SMP architecture with an escarpment-like margin on the northwestern side and a thick package of prograding clinoforms on the southeastern margin (Fig. F2). During Leg 194 drilling, hemipelagic and periplatform deposits on both margins were recovered from Sites 1197 and 1198 (~665 m thick and ~515 m thick, respectively), whereas a ~660-m-thick succession of shallow-water carbonates on top of the platform (Figs. F1, F2, F3) was penetrated at Sites 1196 and 1199. Underlying acoustic basement was reached and consists of either highly altered basalts (Sites 1197 and 1198) or phosphate-rich sands (Site 1196). The whole deposits were addressed to the seismic regional megasequences (A–D) of Pigram (1993), separated by sequence boundaries (Fig. F2). Megasequences B and C, both Miocene in age, contain the SMP (Fig. F2). In the Marion Plateau, the Megasequence B/C and C/D boundaries were further dated through seismic correlation to ~11.0 Ma and ~7.2 Ma, respectively (Shipboard Scientific Party, 2002a). The platform deposits were subdivided into four lithostratigraphic units at Sites 1196 and 1199. Site 1196 is situated 20 km east of the Great Barrier Reef at 304.2 meters below sea level (mbsl) and Site 1199 is situated 5 km northeast of Site 1196 at 315.9 mbsl (Fig. F1). Both sites were dated from early Miocene to Pliocene based on the determination of large benthic foraminifers, planktonic foraminifers, and calcareous nannofossils. The substrate was defined as a fifth lithostratigraphic unit, most likely latest Oligocene in age. At least three phases of growth, exposure, and dolomitization were recognized within the platform and partly correlated to the megasequences of Pigram (1993) (Fig. F3). The oldest phase is mostly early Miocene in age and is represented by a ~250-m-thick succession of dolostones with rhodoliths and corals preserved as ghosts (Units III–IV). The two youngest phases are early(?) to middle Miocene and most likely late Miocene in age, respectively, and consist of ~410-m-thick interbedded rhodalgal, coral, and porcellaneous foraminiferal (dolomitic) limestones (Units I–II). During the last two phases, the SMP was interpreted as asymmetrical with a flat-topped, reef-rimmed western margin and an eastern margin evolving from a distally steepened to a more homoclinal ramp (Shipboard Scientific Party, 2002a) (Figs. F2, F3) The lithologic boundary between the last two phases was ascribed to a surface exposure observed at the top of Subunit ID at Sites 1196 and 1199 (Fig. F3). A low-amplitude, low-frequency reflection at ~110–130 meters below seafloor (mbsf) underneath the top of the SMP was considered the corresponding seismic boundary. The reflection was correlated to the Megasequence B/C seismic boundary and partly related to the late middle Miocene (N12–N14) sea level fall (Fig. F2). The top of the platform is marked both by an exposure surface and by an overlying hardground surface a few centimeters thick. The hardground surface is represented by planktonic wackestone deposits and infillings dated to Pliocene with a maximum age of 3.2 Ma and by laminated ferromanganese crust (Shipboard Scientific Party 2002a, 2002b). It is assumed that the end of the youngest platform growth phase coincides with the Megasequence C/D seismic boundary and the drowning of the SMP (Fig. F3).
Our study focuses on the two youngest platform growth phases of the SMP and concerns the 410-m-thick upper part of the platform drilled at Sites 1196 (Holes 1196A and 1196B) and 1199. This part of the platform is the least dolomitized of the whole succession and is middle (early?) to late Miocene in age (Shipboard Scientific Party, 2002a). The aim of this study is