Late Cretaceous dysoxia in a southern high latitude siliciclastic succession, the Otway Basin, southeastern Australia

The warm greenhouse world of the Late Cretaceous created oceans that were poorly stratified latitudinally and vertically. Periodically these oceans experienced globally significant events where oxygen minimum zones enveloped the continental margins. Evidence of the effect of one of these Ocean Anoxic Events (OAEs) is preserved in the southern high latitude strata of the offshore Otway Basin in southeast Australia. During the Late Cretaceous, thick successions (up to 6 km) of mudstone-dominated deltaic to upper bathyal sediments (the Otway Delta) were deposited in an elongate inlet (ca. 500 km wide) between Antarctica and Australia located at least 70° S. The initial Turonian strata of this succession (the Waarre Formation) were deposited in upper delta plain to delta front conditions. The overlying late Turonian Flaxman Formation and basal Belfast Mudstone Formation preserve evidence of transgressive lower delta plain to prodelta conditions at inner to middle shelf depths. These units were subject to periodic dysoxia during deposition caused by intermittent freshwater input and deepening seas resulting in periods of thermohaline stratification and reduced bottom waters. Rapid subsidence from 89.3 Ma to 85.7 Ma created significant accommodation space leading to the seaward progradation of normal marine prodelta to upper bathyal mudstone-dominated facies at middle shelf to upper slope depths. After a period when the oxygen minimum zone contracted near the base of the Coniacian, upward-increasing dysoxia in the Belfast Mudstone Formation heralds the onset of Coniacian to Santonian dysoxic conditions. This event in the Otway Basin correlates to OAE 3, the last Ocean Anoxic Event of the warm Cretaceous before the onset of cooler conditions in the uppermost Cretaceous. The evidence suggests that, rather than confined to low latitude tropical areas, the effects of OAE 3 reached southern high latitude regions during the warm Late Cretaceous. The cessation of growth faulting after 85.7 Ma reduced accommodation space and delta front to prodelta facies prograded rapidly seaward. Hyposaline conditions and higher sedimentation rates due to delta front progradation and shallowing during this time caused the local extinction and dissolution of many of the calcareous benthic foraminiferal taxa of the Belfast Mudstone Formation.