Year

1992

Degree Name

Doctor of Philosophy

Department

Department of Geology

Abstract

The non-marine ?Late Jurassic to Early Cretaceous Casterton Formation and overlying Early Cretaceous Otway Group are the oldest known rocks in the Otway Basin, one of several extensional basins that developed along the rifting southern margin of Australia. The two units were deposited in grabens and half-grabens on pre-Mesozoic basement and are currently known mainly from the subsurface.

Seven sedimentary lithofacies comprise the Casterton Formation and overlying Otway Group. The sedimentary environments represented by these lithofacies comprise a range of alluvial settings. The Casterton Formation comprises Lithofacies I, which is predominantly lacustrine, and Lithofacies II, alluvial fan/valley fill. In the overlying Otway Group, the Pretty Hill and Geltwood Beach Formations consist of Lithofacies III, a fluvial sequence including both channel tract and overbank deposits. Lithofacies IV to VII, corresponding to the Eumeralla Formation, are widespread and may be a basis for stratigraphic subdivision of the formation. In turn, they consist of low energy fluvial and swamp deposits, freshwater lacustrine facies, high energy fluvial braidplain and fan deposits and lower energy braidplain deposits, which record the influence on the basin, initially, of a second phase of rifting and subsequently, of pyroclastic volcanism.

Palaeocurrent data indicate derivation of much of the volcaniclastic material from intra-basinal sources towards the axis of the rift. Isopach and structural data point to a major palaeovolcano southwest of Cape Otway at a triple junction within the rift. Petrofacies studies indicate another source volcano in the northeast of the basin.

Three sandstone petrofacies in the Casterton Formation and Otway Group were derived from four sources: Palaeozoic granitic and metasedimentary basement rocks, Permian sediments, Jurassic basalt and Cretaceous andesitic/dacitic pyroclastic rocks. Initially, sediments were deposited in discrete depocentres. The quartz-rich Pretty Hill petrofacies was derived mainly from weathered granite and quartz-rich Permian sediments and is known from only two locations; the Crayfish petrofacies is more varied and widespread, with relatively quartz-rich and lithic-rich constituents. The quartz-rich consitutent resembles the Pretty Hill petrofacies and was probably derived from the same sources. The onset of pyroclastic volcanism in the mid-Aptian subsequently dominated the sediment supply. Epiclastic volcanic detritus is a major constituent of the Eumeralla petrofacies in which local incursions of basement-derived material can be linked to adjacent structurally elevated areas.

Two fluvial depositional systems were distinguished in Eumeralla Formation outcrops, corresponding to the upper two lithofacies comprising this formation. Both systems were characterized by high stage fluvial flow. The older System A consisted of alluvial fan(s) and/or braidplain developed adjacent to an uplifted basement block and may have evolved from an earlier lacustrine system, possibly part of Lithofacies V. System B comprised an extensive braidplain radiating from a palaeovolcano. Debris flow and hyperconcentrated stream flow facies mark the transition between pyroclastic and fluvial deposition, and are the most proximal facies exposed. Low energy fluvial deposition with swamps (Lithofacies VII) occurred more distally. Soils developed on inactive parts of the braidplain.

The depositional models, lithofacies and petrofacies analyses profoundly change the perception of the development of the Otway rift basin. Only the first phase of Otway Basin development resembles conventional rift basin models, which invoke extra-basinal filling of grabens and half-grabens. During a second phase of rifting, major intra-basinal volcanism at the rift axis and in particular at a triple junction within the rift, resulted in the basin's filling from its centre, with drainage towards the margins.

Petroleum prospectivity of the Otway Group offshore is downgraded due to the presence of palaeovolcanoes. Onshore, the Pretty Hill petrofacies, a favoured drilling target, is shown to be highly localized in its occurrence. Lacustrine facies along the basin margin may be the best potential petroleum source rocks.

Comments

Accompanying charts can be consulted with the hard copy of the thesis in the Archives Collection, call no. is 551.770994/1

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