Marine geology of Port Phillip, Victoria

The marine geology of Port Phillip is described in detail, based on data fr om seismic profiling, vibro-coring and grab sampling, Three major unconsoli dated facies can be distinguished: sands and muddy sands peripheral to the present coastline, muds covering the major central region, and channel fill s of muds and sands. The first two facies units result from an increase in wave sorting towards the coast, reworking of Tertiary and Quaternary sandst one outcrops around the coast, and a dominant mud supply from river sources into the central area. The distribution and thicknesses of the unconsolida ted facies have been augmented by a shallow-seismic program that reveals th e thicknesses of the modern sediments overlying an older surface comprised of consolidated clays and sandy clays of Pleistocene or older age. In centr al Port Phillip, muds and sands up to 27 m-thick have infilled Pleistocene channels cut into underlying consolidated units. Sediments immediately abov e the channel bases show characteristic seismic patterns of fluvial deposit ion, The presence of peat deposits together with gas phenomena in the water column suggest organic breakdown of channel-fill deposits is releasing met hane into the bay waters. Outside the channel areas, carbon-14 dating indic ates that the unconsolidated sediments largely post-date the last glaciatio n sea-level rise (<6500 a BP). with an early Holocene period of rapid depos ition, similar to other Australian estuaries. Stratigraphic and depositiona l considerations suggest that the undated channel-fill sequences correlate with the formation of cemented quartz-carbonate aeolianite and barrier sand s on the Nepean Peninsula at the southern end of Port Phillip. Previous the rmoluminescence dating of the aeolianites suggests that channel-fill sequen ces B, C and D may have been deposited as fluvial and estuarine infills ove r the period between 57 and 8 ka. The eroded surface on the underlying cons olidated sediments is probably the same 118 ka age as a disconformity withi n the Nepean aeolianites. Further estuarine and aeolianite facies extend be low the disconformity to 60 m below sea-level, and may extend the Quaternar y depositional record to ca 810 ka. Pliocene and older Tertiary units progr essively subcrop below the Quaternary northwards up the bay.