BOOBYALLA SUB-BASIN : A CRETACEOUS ONSHORE EXTENSION OF THE SOUTHERN EDGE OF THE BASS BASIN

Abstract

Geophysical methods, including gravity, magnetics, refraction and reflection seismic, together with drilling have revealed the presence of a steep, fault-bounded trough of Cretaceous age in the Boobyalla Plains area, northeastern Tasmania. The trough is at least 500 m deep and contains the first proven Cretaceous sedimentary sequence from onshore Tasmania in the Boobyalla Sub-basin. The Boobyalla Sub-basin is the southeastern extremity of the Bass Basin and is bounded by faults having NW-SE, N-S and NE-SW trends.

These Late Cretaceous sediments consist of poorly sorted boulder conglomerate, often containing dolerite boulders several metres in diameter, pebble conglomerate and poorly sorted ferruginous sandstone. Clast lithologies are variable but reflect local derivation. Away from the trough margins the infilling sediments become finer grained with conglomerate becoming a less prominent part of the sequence. The coarse-grained sequences are interpreted to have been deposited rapidly in close proximity to a fault scarp.

The sediments are biostratigraphic equivalents of and represent a proximal (near-source) facies of the Eastern View Coal Measures, which were encountered in Durroon 1, drilled in the Bass Basin and some 60 km northwest of Boobyalla Plains. A minor volcanic episode probably affected this section of the Bass Basin about 100 Ma B.P. This may be related to tectonic disturbances and could be responsible for an unconformity at the base of the Eastern View Coal Measures indicated by offshore seismic information.