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.

The Tamar Trough, an Early Palaeogene fault structure, contains sedimentary beds and interleaved basaltic fl ows that infi ll the structure along its 70 km length. These infi lls represent a complex interplay between sedimentation, channel erosion, eruptive dislocations, and even 'out of trough' diyersions of the ancestral Tamar drainage. Several areas of resistant basalt fl ows remain in the south, upper, middle and lower Tamar reaches. Although some palynological control was known, radiometric dating of previously untested basalts now allows close integration and age-pegging for observed palynological biozones. The K-Ar and Ar-Ar ages of the basalt bodies indicate eruptive events at 47, 33-37 and 25 Ma, correlating with Proteacidites asperopo!us-Ma!vacipo!!is diversus, Nothofagites asperus and I'roteacidites tu bercu!atus biozone age sedimentary beds respectively. Basanite, alkali basalt and hawaiite fl ows dominate basalt lithology with lesser olivine nephelinite, transitional olivine basalt, olivine tholeiite and quartz tholeiite. Basalt geochemistry suggests derivation from different degrees of partial mantle melting (from 7 to 35%), with alkaline and tholeiitic basalts being derived from separate source regions. Most alkaline basalts have high-µ (HIMU) related trace element signatures, which are absent in the tholeiitic rocks. A basalt plug on the trough margin at Loira gave a Jurassic age and has Jura%ic dolerite-like geochemistry. The Tamar sequence suggests that the initial fl uvio-lacustrine and later channel-fi ll sedimentation from 65(?) to 24(?) Ma was then punctuated in places by periods of alkaline volcanism between 47 to 33(?) Ma, and alkaline and tholeiitic volcanism between 33 to 24(?) Ma. No Neogene fossils are known, so this later period was probably one of net erosion. These contrasting quiet sedimentary and more volcanic intervals are related here to a tectonic model that involves northerly drift of Victorian and Tasmanian lithosphere over several former Tasman metasomatised mantle plume sources.