Eustatic and palaeoenvironmental assessment of the mid-Cretaceous Bathurst Island Group of the Money Shoals Platform, northern Australia

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Abstract The Cretaceous System of the Money Shoals Platform consists of the Bathurst Island Group which is extensively developed on the continental borderland of northern Australia. The platform experienced a simple post-Cretaceous tectonic history and in its southern part the Bathurst Island Group is nonconformable on Precambrian basement, an ideal circumstance for evaluating eustacy and palaeoenvironmental succession. The group records a long-term sea-level cycle of late Aptian-mid-Turonian age. Its lowest part represents a transgressive systems tract developed on a pediment surface of very low local and regional relief. Compositionally mature glauconitic sandstone and radiolarian mudstone are characteristic facies, indicative of terrigenous sediment starvation for the shallow transgressive sea which received wind-driven, plankton-bearing currents from the shelf edge 400 km to the north. A district-scale nodular phosphorite horizon and associated highly condensed section capping the transgressive suite represents the culmination of deepening. It is succeeded by a shelfal mudstone succession comprising the bulk of the Bathurst Island Group and interpreted as a highstand systems tract. Clay mineralogy of this and the underlying radiolarian mudstone is uniform and dominated by smectite/mixed layer species of poor crystallinity. The clay assemblage is attributed to a pedigenic origin associated with weathering of moderate intensity on a poorly drained, low relief landscape. The upper part of the group comprises bioturbated glauconitic and quartzose sandstone displaying sedimentary evidence of high-energy depositional conditions consistent with regression. It is viewed as a prograding/aggrading, shelf margin systems tract which accumulated under circumstances of declining accommodation. A disconformity at the top of the group, cut on strata of mid-Turonian age, completed the sedimentary cycle. Calculated sedimentation rates indicate a major change in detrital sediment supply broadly coincident with the passage from transgressive to highstand systems tract and unrelated to eustacy. The change is attributed to mid-Cretaceous landscape rejuvenation. It is also reflected in the late Albian-Cenomanian sedimentary record of the epicontinental Great Artesian Basin where enhanced sediment supply likewise overprinted and obscured the eustatic signature. Mid-Cretaceous landscape rejuvenation reflected in the sedimentary record broadly correlates with evidence of uplift in eastern and southern Australia now recognised from fission track studies. It also correlates with a widely recognised palaeomagnetic overprint attributed to a mid-Cretaceous thermal event affecting the crust of southern and eastern Australia. Although short-term (third order) sea level cycles are not apparent for the Bathurst Island Group, three eustatic episodes related to global patterns can be recognised: late Aptian transgressive onlap, early Albian maximum flooding and late Turonian regression.