Tertiary volcanic rocks of far N.W. Tasmania

Abstract

The Tertiary volcanic rocks of far north-west Tasmania comprise tuffs, pillow-basalts and breccias, entrail basalts, and massive basalts, and eruptions appear to have occurred at several intervals from Lower to Upper Tertiary times. Sea level fluctuations have been important in determining the form assumed by the flows. The lavas were predominantly saturated black glass olivine basalts with one extrusion of undersaturated olivine alkali basalt. The oldest of the volcanics is a widespread formation of sub-aqueously deposited tuffs. These were followed, after a period of erosion, by massive basalts, including a basal zone of entrail lava, which were probably erupted during an Upper PalaeoceneUpper Eocene marine regression. Extensive submarine eruptions followed, resulting in the formation of large cones composed predominantly of pillow breccias. These were probably formed during a marine transgression in the Upper EoceneUpper Oligocene. A period of erosion, probably subaerial, dissected these cones, and limestones were later deposited on their eroded flanks during a major marine transgression in the Miocene. A final volcanic phase, probably during an Upper Miocene-Pliocene marine regression, saw widespread eruptions of massive basalts, some of which filled valleys eroded in the older volcanics and sediments. The magmatic history of the eruptions in this area appears to be significantly different from that of the Cainozoic volcanics of Victoria.