Pre-Cenozoic geology of the Latrobe Valley Area—onshore Gippsland Basin, S.E. Australia

Abstract

In 2010–2011, a well on the uplifted northern edge of the Latrobe Valley (Yallourn North-1A) cored a 550 m section of mostly arenaceous sediments from the Lower Cretaceous Tyers River Subgroup. A follow-up core-hole (Yallourn Power-1) aimed at extending the Tyers River Subgroup section some 5 km south into the Latrobe Valley instead encountered Paleozoic basement rocks immediately below Cenozoic coal measures. From a re-examination of earlier coal and groundwater bore results, and new interpretations from gravity, seismic and magneto-telluric (MT) surveys, there is a significant area of Paleozoic basement rock that may underlie the whole northern Latrobe Valley area. The uplifted Yallourn North Lower Cretaceous sediments are a separate basin entity herein named the Monash trough. It appears they are separate from the main Lower Cretaceous Strzelecki Group Basin sediments on the southern side of the Latrobe Valley. Attributes of the Monash trough may underlie the main Strzelecki Basin, but this remains to be substantiated by further drilling. The intervening subcrop of Paleozoic basement rocks is herein named the Glengarry basement block. It shows characteristic gravity, MT and seismic features covering some 200 km2 of the northern Latrobe Valley area. The boundary between the Glengarry basement block and Strzelecki Basin approximates to the Princes Highway. It is uncertain whether structural separation of the Monash trough from the main Strzelecki Basin always existed, or whether uplift and stripping of Cretaceous rocks over the Glengarry basement block occurred in post-Cretaceous but pre-Cenozoic times. Comparative rank and maturity indices indicate a greater depth of burial of the Glengarry basement block than what exists today, whereas less stripping and loss of section have occurred to the Monash trough. Cretaceous sediments of the Tyers River Subgroup (Rintouls Creek Formation, Tyers Conglomerate) in the Monash trough are dominated by mudstones, siltstones with lesser quartzose sandstones, conglomerates and thin coals. The sediments are over 300 m thick and are conformably overlain by 100 m of volcaniclastic sediments typical of the main Strzelecki Group, in turn overlain by nearly 100 m of Cenozoic coal measures. New detailed spore–pollen dating of Yallourn North-1A cores indicates that all Cretaceous sediments in the Monash trough are Barremian in age. This revises the traditional Neocomian age assigned to the formation. High total organic carbon levels in the 100 m-thick mudstones of the Locmany Member in the Rintouls Creek Formation constitute a mature petroleum source rock worthy of future hydrocarbon exploration.