Tertiary volcanic rocks of far N.W. Tasmania

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.