Foraminiferal biofacies and palaeoenvironmental evolution of an Oligo-Miocene cool-water carbonate succession in the Otway Basin, southeast Australia

This multidisciplinary study integrates facies studies and foraminiferal an alyses to assess the palaeoenvironmental evolution of an Oligocene to Mioce ne cool-water carbonate succession in the Otway Basin, southeastern Austral ia. The cool-water carbonate succession in the Otway Basin records signals relating to the evolution of the Southern Ocean throughout the Cenozoic. Th e strata are correlated with the relative coastal onlap curve of Haq et al. (1988) and several sequences can be identified in three formations. The Ea rly Oligocene Narrawaturk Formation (TA 4.5) comprises near the base high-e nergy, inner shelf biofacies (lowstand systems tracts) and up-section to lo wer energy mid- to outer shelf marls (TST and maximum flooding surfaces) wi th storm events and/or minor shallowing intervals. Foraminiferal reworking and post-depositional dolomitization occurs at the top of this unit. The La te Oligocene Clifton Formation (TB 1.1 and TB 1.2.) was deposited in a rela tively high-energy inner to mid-shelf environment. The base of this unit pr eserves evidence of a shift in biofacies that correlates to a major sea-lev el fall at the Mid/Late Oligocene boundary coincident with a major ice adva nce in Antarctica, and correlates with other Mid-Oligocene unconformities w orld-wide. The Late Oligocene Gellibrand Marl Formation (TB 1.2 and TB 1.3) began with low-energy outer shelf cherty marry biofacies (TST and MFS) fol lowed by mid- to outer shelf calcisiltites (HST). High-energy mid- to outer shelf conditions were established after an hiatus in the Late Oligocene. A relative sea-level rise at the base of the Lower Miocene (TB 1.5 and TB2.1 ) led to the deposition of lower energy outer shelf cherty marls, Four biofacies with distinctive foraminiferal faunas are distinguished. (1) Grey mid- to outer shelf low-energy bryozoal marls with infaunal foraminif era and high plankton values. Two foraminiferal assemblages occur: lagenids and Uvigerina are common in the Narrawaturk marls, whereas bolivinids and Astrononion occur in the Gellibrand marls. The Faunal variation in the marl s may relate to changes in nutrient supply, anoxia, the presence or absence of organic material and/or changes in depth. (2) Chalky packstone facies w ith a high epifaunal content were deposited in oligotrophic inner to mid-sh elf palaeoenvironments subject to intermittent reworking. (3) Bryozoan-poor inner to outer shelf foraminiferal packstones and grainstones facies enric hed in epifaunal forms. (4) Well-sorted coarse-grained regular echinoid and bryozoan-rich packstones to grainstones. Infaunal taxa are absent in this facies, where most preserved foraminifera are robust spherical to discoidal forms. The facies were deposited in inner to mid-shelf palaeoenvironments where reworking by intense wave action (either above normal wavebase or by storms) winnowed out all smaller foraminifera. The stratigraphic and palaeoenvironmental utility of the Cenozoic foraminif era studied is improved greatly by facies analyses. Similar integrated stud ies will lead to better correlations and palaeoenvironmental interpretation s of southeastern Australian sequences and equivalent successions in the so uthern hemisphere.