Late Quaternary Evolution of Twofold Bay, Southern New South Wales.

The data collected for this report are the results of a detailed analysis of the texture and composition of onshore and offshore sediments within Twofold Bay, regional geomorphic mapping of the onshore Quaternary geology and a seismic survey (Uniboom) of the adjacent continental shelf between Merimbula Bay and Disaster Bay (alongshore distance of c.50km) and radiocarbon dating of selected barrier and estuarine deposits in Twofold Bay are presented. Seismic and geomorphic mapping data indicate that the bulk of the Quaternary sediments in the region are contained within barrier, estuarine and nearshore deposits at or adjacent to the present coast. A number of prograded and stationary barrier types impounding coastal lagoons and barrier estuaries are encountered on the open coast and within Twofold Bay. The Whale Beach barrier at the entrance to the Towamba River in Twofold Bay is an unusual example of a prograded barrier greatly modified by the erosive influences of floods and tidal currents and is not readily classified into any of the common barrier types encountered in Eastern Australia. Available radiocarbon dates indicate that these barrier and estuarine deposits are mid to late Holocene in age. Radiocarbon dates reported here for the Boydtown barrier, a prograded foredune ridge plain on the western shores of Twofold Bay, indicate a similar age but reveal a marked increase in the rate of barrier progradation from c.3000 yrs. B.P. to the present. A detailed examination of the texture and composition of a variety of onshore and offshore deposits in Twofold Bay and beach deposits on adjacent open coast confirmed the presence of a number of distinct sediment types. Although there are significant textural variations between the types, they contain similar mineral assemblages- predominantly quartz with variable amounts of feldspar (K~feldspar dominant) and lithics and heavy mineral assemblages with a mixture of ultrastable (zircon, rutile, tourmaline) and metastable (andalusite, kyanite, garnet, pyroxene, amphibole, epidote) mineral species. A multivariate statistical analysis of the light and heavy mineral assemblages indicates a localised increase in the proportions of feldspar and pyroxene-hornblende in barrier and nearshore sediments adjacent to the Towamba River entrance in Twofold Bay. A marked increase in the proportions of these minerals are also noted in the Boydtown barrier deposits from c.3000 yrs. B.P. to the present. A model of late Quaternary marine and terrestrial sedimentation for the region in general, and Twofold Bay in particular, is presented. Main features of the model include: 
1. Inundation of the narrow coastal valleys in the foothills of the Southern Highlands in the latter stages of the postglacial marine transgression (PMT) to create a highly embayed open ocean coastline and Twofold Bay. 
2. Significant marine sedimentation in coastal embayments following stabilisation of sea level at its present position c.6500 years ago with the majority of barrier and estuarine deposits formed in the early stages of the sea level stillstand (6500 to 3000 years ago) comprised of sediments derived from the adjacent continent during the PMT. 
3. A trend towards coastal recession in the latter stages of the stillstand (3000 years ago to present day) as the offshore supply of sand to the coast dwindles. Notable exceptions include barrier deposits on both the open ocean coast (Wonboyn Beach - Disaster Bay) and more protected marine environments within Twofold Bay (Boydtown Beach, Whale Beach, Fisheries Beach). In these areas, renewed coastline progradation is initiated by the supply of quartzose sediment from offshore via littoral drift to the north (Wonboyn Beach), the supply of feldspathic sediments from the Towamba River (Boydtown Beach, Whale Beach) and the supply of biogenic material (shell fragments) from a localised nearshore source (Fisheries Beach).

Tags: Oceans | Marine Sediments | Sedimentation
Organization: Australian Ocean Data Network