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Coastal sedimentary facies and foraminiferal biofacies of the St Kilda Formation at Port Gawler, South Australia

Transactions of the Royal Society of South Australia 109(3-4): 121-142
Coastal sedimentary facies and foraminiferal biofacies of the St Kilda Formation at Port Gawler, South Australia
The St Kilda Formation, within Gulf St. Vincent and surrounding coastal lands is redefined to include all Holocene sediments deposited under the influence of marine processes. Upper surfaces of the formation are not confined, but rather may be surfaces of active sedimentation. Port Gawler is an area where marginal marine sediments are accumulating under the baffling, trapping and binding actions of seagrasses, cyanobacterial mats, mangroves, samphires and saltbush. These plant communities occur in essentially discrete zones, successively adjacent and parallel to the tidal shore lines. Their influence causes progradation and aggradation of bioclastic carbonate-quartz sands and muds. Bivalves gastropods and foraminifera are abundant and contribute significantly to the carbonate component of sediment. Distribution of molluscs is closely related to plant communities, sediment type and period of tidal inundation. Thus a series of laterally adjacent sedimentary facies are recognised to be associated with the following sub-environments: subtidal and interidal seagrass meadows; mangrove woodland; tidal distributaries; cyanobacterial mats of the inner sand flat; high tide beach; supratidal lagoons or sabkhas, dunes and storm ridges; the estuary and associated backwaters of the Gawler River. Foraminiferal biofacies, based on selected sieved fractions of dense-liquid flotation concentrates are established for the various sedimentary environments. In two vibrocores, down-hole assemblages of foraminifera are similar to those of modern Port Gawler environments. Palaeoenvironmental interpretation of the cores, based on both lithology and assemblages of forminifera, shows that the earliest sediments of the Holocene transgression were essentially samphire muds and shelly storm debris. Later sedimentation, after sea level stablised, occurred through processes of progradation and aggradation, inextricably related to communities of seagrasses, mangroves, cyanobacterial mats and samphires.

Accession: 004979716

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