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Oligo–Miocene seagrass-influenced carbonate sedimentation along a temperate marine palaeoarchipelago, Padthaway Ridge, South Australia

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Abstract

Oligo–Miocene carbonates associated with the Padthaway Ridge form the southern margin of the Murray Basin, South Australia. The carbonates are a thin, somewhat condensed succession of echinoid and bryozoan-rich limestones that record accumulation in the complex of islands and seaways and progressive burial of the Ridge through time. The rocks are grainy to muddy bioclastic packstones, grainstones and floatstones, composed of infaunal echinoderms, bryozoans, coralline algae and benthic foraminifera, with lesser contributions from molluscs and serpulid worms. Locally as much as half of these skeletal components are Fe-stained, relict grains that imbue the lithologies with a conspicuous yellow to orange hue. This variably lithified succession is partitioned into metre-scale, firmground-bounded and hardground-bounded beds textured by extensive Thalassinoides burrows. Dominant lithologies are interpreted as temperate seagrass facies. Limestones contain attributes indicative of both seagrass-dominated palaeoenvironments and carbonate production and accumulation on unconsolidated, barren sandflat palaeoenvironments. Together these two depositional systems are thought to have generated a single multigenerational, amalgamated facies recording sedimentation within a complex temperate seagrass environment. Limestones overlying the Padthaway Ridge reflect a gradually warming climate, increasing water temperature and decreasing nutrient content, within the framework of a ridge gradually being buried in sediment. This succession from cool–temperate to warm–temperate to subtropical through time permits recognition of the relative influence of changing oceanography on a seagrass-dominated shallow inter-island sea floor. Criteria are proposed herein to enable future recognition of similar temperate seagrass facies in Cenozoic limestones elsewhere.

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