Pliocene sedimentation in a shallow, cool-water, estuarine gulf, Murray Basin, South Australia



The Pliocene Norwest Bend Formation is a well-preserved succession of terrestrial and shallow-marine deposits in the Murray Basin, South Australia. Sediments in this unit consist of two discrete terrigenous clastic-rich, decametre-scale sequences, or informal members, which record episodes of marine incursion during the Early and Late Pliocene respectively. The base of each sequence is a transgressive lag and/or strandline deposit that is transitional upwards into a highstand, subtidal, terrigenous clastic and cool-water carbonate sediment accumulation. The top of each sequence is incised by fluvial channels that are filled by river deposits which formed as relative sea-level fell and terrestrial environments prograded basinward. Sedimentological data suggest that gross stratigraphic architecture was primarily determined by glacioeustasy. Differences in sedimentary style between these two sequences, however, reflect a major climatic change that took place in southern Australia during the mid-Pliocene. The lower quartzose sand member is formed of siliciclastic sediment derived from prolonged, deep, subaerial weathering and contains a bivalve-dominated, cool-temperate, open-marine mollusc assemblage. These sediments accumulated under an equitable, relatively warm, humid climate. The Murray Basin during this time, because of high fluvial discharge, was a salt-wedge estuary with typical estuarine circulation. In contrast, the upper, oyster-rich member is typified by large monospecific oyster buildups that grew in restricted coastal environments. Strandline deposits contain a warm-temperate skeletal assemblage. Contemporaneous aeolian sediments accumulated under warm, semi-arid climatic conditions. Well-developed ferricrete, silcrete and calcrete horizons reflect cyclic conditions of rainwater infiltration and evaporation in the seasonally dry climate that typifies southern Australia today. Highly seasonal rainfall produced an estuary that fluctuated annually from being well to partially mixed. These Pliocene sediments support the notion that mollusc-rich facies are the signature of cool-water carbonate accumulations in inboard neritic environments. Unlike bryozoans that dominate the outer parts of Cenozoic cool-water carbonate shelves, molluscs evolved to exploit an array of coastal ecosystems with wide salinity variations and variable sedimentation rates.