Journal of Sedimentary Research, 2006, v. 76, 91–116 Research Article DOI: 10.2110/jsr.2006.06 GROWTH, DEMISE, AND DOLOMITIZATION OF MIOCENE CARBONATE PLATFORMS ON THE MARION PLATEAU, OFFSHORE NE AUSTRALIA S.N. EHRENBERG, 1 J.M. MCARTHUR, 2 AND M.F. THIRLWALL 3 1 Statoil, N-4035 Stavanger, Norway and Department of Geology, United Arab Emirates University, P.O. Box 17551, Al Ain, U.A.E. 2 University College London, WC1E 6BT London, U.K. 3 Royal Holloway University College London, TW20 0EX Surrey, U.K. e-mail: sne@uaeu.ac.ae ABSTRACT: Strontium-isotope stratigraphy has been used to examine the timing of depositional events and dolomitization in two Miocene carbonate platforms cored by Ocean Drilling Program (ODP) Leg 194, just seaward of the Great Barrier Reef. The results provide firm constraints for correlating surfaces and depositional stages between the two platforms and thereby relating seismic sequences previously defined in the off-platform sediments to the lithostratigraphic units described from cores in the seismically transparent platform-top sites. Oyster-bearing beds at the base of both platform successions yield early Oligocene ages (29–31 Ma), thus dating initial transgression of the Marion Plateau’s volcanic basement. There followed a period of slow accumulation of shallow-water grainstones rich in quartz and phosphate grains in late Oligocene time (29– 23 Ma; seismic Megasequence A). The main growth of the carbonate platforms took place in early to late Miocene time (23– 7 Ma), comprising five depositional sequences. The first four of these (seismic Megasequence B) are common to both platforms and terminated with a possible karst surface at 10.7 Ma. Different sedimentologic expression of this megasequence in the two platforms reflects contrasting progradational versus aggradational geometries in the locations studied. The final growth stage (seismic Megasequence C) occurred only in the southern platform and terminated at 6.9 Ma. Both platform-demise events (10.7 and 6.9 Ma) approximately coincide with falls in global sea level combined with longer-term trends of decreasing water temperature. Sr-isotope ages of dolostones increase with depositional age, and older dolostones in the southern platform have more coarsely crystalline and fabric-destructive textures than overlying younger dolostones. These relationships are consistent with dolomitization by normal seawater shortly after deposition and overprinting of multiple times of dolomite recrystallization and cementation in the deeper strata. INTRODUCTION The Sr-isotope method of dating marine minerals has been used to determine ages of samples recovered by ODP Leg 194 (January–March 2001) from two adjacent carbonate platforms on the Marion Plateau (Figs. 1, 2). This method is based on calibration of the changes in 87 Sr/ 86 Sr of global seawater through Cenozoic time (Burke et al. 1982; Veizer et al. 1997; Veizer et al. 1999; McArthur and Howarth 2004). Measurement of 87 Sr/ 86 Sr in a marine mineral, such as biogenic carbonate, allows determination of its age by comparison to a calibration curve of 87 Sr/ 86 Sr against time, assuming that the primary isotopic value has not been altered by diagenesis or contamination (Fig. 3). In neritic carbonates where planktonic foraminifera and nannofossils are typically absent, this dating method allows resolution and accuracy far exceeding that of conventional biostratigraphy. This study has two main themes of broad interest to carbonate science. One is the timing of inception, growth, and demise of the Marion carbonate platforms, where a basic question is the relationship of platform stratigraphy and depositional facies to global trends in sea level and climate. To this end, we provide a detailed review of the sedimentologic core descriptions of the platform-top sites cored during Leg 194 and relate these results to the seismic stratigraphic framework developed for the Marion Plateau. The other theme is the dolomitization of these platforms, which we address by comparing dolostone apparent Sr-isotope ages, stratigraphy, and petrography. Shipboard data and results from Leg 194 are available in Isern et al. (2001; 2002; 2004) and on the ODP website at http://www.odp.tamu.edu/ publications/194_IR/194ir.htm. Porosity–permeability relationships and petrography of the Marion platforms are described in Ehrenberg (2004) and Ehrenberg et al. (2004). METHODS Core recovery was commonly low in the platform-top lithologies of the Leg 194 sites studied (Fig. 4), which necessarily limits the degree of confidence that can be attached to stratigraphic interpretations based on core descriptions and petrography. Following standard ODP convention, depth values were assigned by assuming that the recovered core material represents continuous recovery through the upper part of each core barrel length (4.5–9.7 m, depending on equipment type). For the upper part of Site 1196 (0–261 mbsf), drilling of a second hole provided duplicate core coverage, and the upper part of the platform section in Site 1193 was cored in both a second and a third hole (35–129 and 35–63 mbsf, respectively). The Sr isotope dating method is based on the systematic change of the isotope ratio 87 Sr/ 86 Sr of global seawater through Phanerozoic time Copyright E 2006, SEPM (Society for Sedimentary Geology) 1527-1404/06/076-091/$03.00