Geelong - Maude Area, Tertiary Stratigraphic Sites

Introduction

South of the Steiglitz plateau (just north of Maude), and extending to Geelong, the incised valleys of the Moorabool River and Sutherlands Creek expose a sequence of lavas and sedimentary rocks that shows an almost complete marine cycle of sedimentation commencing with a transgression in the Oligocene and terminating with a regression in the Pliocene. Outcrops occur in road cuttings, quarries, landslips on valley slopes, and o the coast of Corio Bay. The sediments include gravels, marine and terrestrial quartzose and calcareous sands, marine limestone, marl and clay, phosphatic nodules, and there is evidence of submarine as well as subaerial volcanism. The sediments are often richly fossiliferous enabling very detailed stratigraphic analysis and geological chronology of the transgressive - regressive sequence.

Bowler (1936) recognised six formations comprising the transgressive - regressive sequence but the rock nomenclature was revised by Abele (1976) to four formations. The lowermost unit, the Maude Formation, includes on volcanic and four sedimentary members and is overlain by the Batesford Limestone, the Fyansford Formation and the Moorabool Viaduct Sand. These are in turn covered by Newer Volcanic lava flows - hence the exposures of Tertiary are confined to the river valleys or the coast where the volcanics have been stripped back or incised.

The basal beds of the Maude Formation, exposed in the Maude area, are sands and gravels of the Sutherlands Creek Sand Member which rest on the steeply dipping Ordovician sedimentary bedrock. These are overlain by the first of the marine beds, the Lower Maude Limestone Member, although the limestone exposures are generally poor. The limestone is well bedded, dips very gently to the south, and although rich in skeletal material has few well preserved fossils. The limestone, which in places rests directly on the Ordovician, marks the inner limit of the marine transgression in the Maude area. The Lower Maude Limestone is covered by the Maude Basalt Member, a series of lava flows up to 30 m thick, which was erupted subaerially, but includes pillow lavas indicating that parts of the flows extended into a shallow sea. The lava was erupted ruing a brief recession of the sea and was followed by a further marine submergence and the deposition of The Upper Maude Limestone Member, a dense pink microcrystalline limestone with abundant shelly fossils.

In the south of the area, the basal beds are the Batesford Limestone, which rests directly on Palaeozoic granite at the Dog Rocks. The formation is well exposed at the Batesford quarry and along the Moorabool River near the quarry, and is a biocalcarenite consisting of broken skeletal remains of bryozoans, echinoides and bivalves. It accumulated in a clear water marine environment with a sandy sea floor. A deepening of the sea lead to a change in the depositional environment and the accumulation of the muddy sediments comprising the Fyansford Formation. These beds of silty clays and marls with occasional thin limestones, rest conformably on the Batesford Limestone but contain a different fossil assemblage. They outcrop in the Moorabool valley and on the coast at North Shore. The Fyansford Formation is overlain with slight angular unconformity by the Moorabool Viaduct Sand, a dominantly sandy formation which indicates a marked shallowing and final retreat of the sea from the area. There are also thin bands of phosphatic nodules commonly found at the base of the Moorabool Viaduct Sand. The Moorabool Viaduct Sand is widespread and occurs from north of Maude to Corio Bay.

Brief descriptions are given below of sites important in displaying the characteristics of the four formations.