Oligocene-Miocene calcareous nannofossils from the Torquay Basin, Victoria, Australia

Abstract

Oligocene-Miocene rocks of the Torquay Basin contain at least 55 different species of calcareous nannofossils. These species allow zonation of an offshore subsurface section (Nerita No. 1 Well) and two onshore sections (near Torquay). The Angahook Formation and the base of the Jan Juc Formation in the Nerita No. 1 Well belong to Martini's (1971) NP 22 calcareous nannofossil Zone (early Oligocene). The NP 23 Zone could not be identified with certainty, but Zones NP 24/25 (late Oligocene) to NN 2 (early Miocene) were recognized in the Jan Juc Formation. The Oligocene-Miocene boundary is present between 122 m and 135 m below the sea floor at this site. The Puebla Formation, exposed at the sea floor, contains NN 5 spanning the early-middle Miocene boundary. Onshore along the Torquay coast, NP 24/25 (late Oligocene) is present in the Angahook Formation; the Jan Juc Formation at Bird Rock also contains NP 24/25, except the uppermost 2.5 m which belongs to NN 1. The NN 2 Zone begins at the contact between the Jan Juc and Puebla Formations and continues upward for at least 3 m. After a covered interval, NN 4 was identified 10 m above the contact. The Oligocene-Miocene boundary is placed at the base of the hard band capping Bird Rock in the type section of the Jan Juc Formation. Nannofossil assemblages in the Torquay sections suggest deposition in cool, shallow continental shelf waters. The assemblages in Nerita No. 1 Well also suggest cool shelf waters, although deeper than at Torquay.