Sequences and biofacies packages in the mid-Cenozoic Gambier Limestone, South Australia: reappraisal of foraminiferal evidence

Abstract

The mid to outer neritic carbonates of the Gambier Limestone (Upper Eocene to lower Middle Miocene) can be divided into seven units by using criteria of sequence stratigraphy and foraminiferal biofacies. The boundaries fall mainly on erosional surfaces, even though the temporal duration of these surfaces appears to be largely beyond the resolution of foraminiferal biostratigraphy. The Eocene/Oligocene contact is distinctively unconformable in several sections with at least part of the Upper Eocene sediments missing. Cren nodules, common to abundant in most sections, are associated with deep- or cool-water bentaic assemblages (> 10C-20C m and < 15°C), indicating good nutrient-rich bottom conditions probably influenced by the Antarctic Circumpolar Current beginning during the Early Oligocene. The mid-Oligocene fall in sea-level was probably coupled with a major local uplift that removed at least part of the Lower Oligocene, an event widely recorded in the Australian-New Zealand region. In areas weakly affected, this glacioeustatic lowstand is represented by chert-free limestone and grey to pink dolomites in some sections, with a poorly preserved assemblage comprising few planktonic and deep-water benthic species. Local unconformities separate regional unconformity-bounded or allostratigraphic packages of strata to represent third-order sequences. Although variations in local subsidence might have influenced accumulation space and sediment thickness, glacioeustatic influence on the packaging of the sequences and units of the Gambier Limestone was easily the more effective and concordant with the global patterns.