Ocean Drilling Program, United States

Two Neogene carbonate platforms on the Marion Plateau, off northeastern Australia, were drilled during Ocean Drilling Program Leg 194. Approximately 510 m of mixed shelf and slope sediments were penetrated at Site 1193, and approximately 672 m of shallow-water platform sediments were penetrated at Site 1196. Shallow-water carbonates at Site 1193 were dominated by bryozoans. At Site 1196, coralline red algae dominated, though stony corals were common to abundant in some intervals. Larger benthic foraminifers (LBF) were abundant at both sites, with common genera including Amphistegina, Cycloclypeus, Lepidocyclina, Miogypsina, and Operculina. Five LBF assemblages were distinguished using thin sections cut from recovered cores. Assemblage A was found in mixed carbonate-terrigenous clastic neritic facies (<100 m) associated with basement flooding. Operculina complanata was common in terrigenous mud-rich facies, whereas Lepidocyclina spp. with relatively primitive, nonstellate morphologies dominated in more carbonate-rich facies. These taxa are characteristic of larger foraminiferal (LF) Associations 1-5, indicative of latest Oligocene to early Miocene ages. Nannofossil dates are similar ( approximately 18-24 Ma) for the units immediately above basement. Depositional environments ranged from deltaic muds with oyster and Operculina shells to a striking interval of in situ, large, flat Lepidocyclina badjirraensis at Site 1198, indicating a deep oligophotic ( approximately 100-150 m) paleoenvironment. Assemblage B was found in platform facies at Sites 1193 and 1196 and in corresponding slope sediments at Sites 1197 and 1198. This assemblage is characterized by predominantly nonstellate lepidocyclinids and ovate miogypsinids. Units containing this assemblage are interpreted to be late early Miocene ( approximately 16-18 Ma) and indicate deposition in a range of euphotic paleodepths, as interpreted from LBF morphologies. Assemblage C, found at Site 1196 (183-326 meters below seafloor), is characterized by Flosculinella, Austrotrillina, soritids, and smaller miliolid foraminifers. This LF 8 association is considered middle Miocene ( approximately 13.3-15.2 Ma), an assumption consistent with nannofossil dates from the interval. The assemblage and associated sediments indicate shallow (<12 m), somewhat restricted platform conditions, including seagrass meadows. Assemblage D was found in platform facies at Sites 1193, 1194, and 1196 and in corresponding slope sediments at Sites 1197 and 1198. This assemblage is characterized by the transition from mixed concentric and stellate lepidocyclinid morphologies to predominantly stellate forms, as well as by vermiform miogypsinids. Units containing this assemblage are interpreted to be middle Miocene ( approximately 11.9-16.4 Ma). At Sites 1193 and 1194, the middle Miocene regression is recorded by changes in foraminiferal taxa and LBF morphologies. Assemblage E was found in upper platform facies at Site 1196, in corresponding periplatform sediments at Sites 1197 and 1198, and also in transported sediments at Sites 1193 and 1194.