Carbonate sedimentation, climate change and stratigraphic completeness on a Miocene cool-water epeiric ramp, Murray Basin, South Australia

Abstract

AbstractLower to Middle Miocene warm temperate-water carbonates of the Murray Supergroup, comprising both fossiliferous heterozoan and foraminifer photozoan facies, accumulated on a low-energy, marine, mesotrophic epeiric ramp in a broad, shallow, intracratonic basin. Depositional conditions ranged from temperate-water with abundant nutrients supplied from the surrounding hinterland under a cool, moist climate regime in the early Miocene, to warm-temperate and subtropical seas with reduced levels of trophic resources associated with an increasingly arid regional climate in the middle Miocene. The succession is separated into three, metre-scale, 1.25–2.75 Ma, transgressive-regressive sequences, or stratigraphic packages. Transgressive carbonates range from Lower Miocene, heterozoan, open-marine echinoidal and bryozoan facies to Middle Miocene, bryozoan-rich, open-marine foraminifer-photozoan facies. Regressive deposits are mainly heterozoan, varying from restricted nearshore marine clays to molluscan seagrass meadow facies, all of which are interpreted to represent highly mesotrophic water conditions. The turn-around from transgression to regression, the transition zone, is variably preserved depending upon the ability of the depositional system to track changes in falling sea-level and is critical to the completeness of the succession. Stratigraphic completeness dramatically increases from the Lower to Middle Miocene stratigraphic packages as rates of sediment production and accumulation increased in conjunction with higher water temperatures and climatic aridity set against a background of increasing eustatic amplitudes. The result is a greenhouse-style succession generated in an evolving icehouse world. This highlights the principle that while allogenic repetition is produced by eustatic forcing, the nature and completeness of the ‘cool-water’ carbonate stratigraphic record can vary significantly due to subtle changes in the nature of the carbonate factory, which are, in turn, determined by sea-water temperature and trophic resources related to climate.