LATE CAINOZOIC CLAY DEPOSITS IN THE PORT-STEPHENS AREA, NEW-SOUTH-WALES

Two discrete deposits of clay more than 20 m thick have been mapped in an area west of Port Stephens estuary, north of Newcastle on the cent ral New South Wales coast. This area was formerly shown as Permian and Carboniferous rocks. The 'Karuah' clays are lake deposits that accumu lated in a ponded valley ca 20 m above present sea-level in the early Quaternary. They occur immediately up-valley of the 'Medowie' clays wh ich are altered acidic tuffs. These latter deposits are younger than C retaceous and, if they created the Karuah lake, they must be Late Tert iary or early Quaternary in age. Independent work on New South Wales z ircon ages suggests the existence of a relatively young hot spot magma source beneath eastern Australia near the latitude of Newcastle/Port Stephens at about 1.0 Ma. Although the Medowie clays apparently derive from rhyolitic acidic parent lava, the possibility of a Quaternary vo lcanic source cannot be ruled out.