Page 401 - Petroleum Geology

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(Fig. 16-11). In the Otway basin, the post-unconformity transgression accu-
mulated Oligo-Miocene calcareous sediments, and there is a mid-Cretaceous
unconformity within a Cretaceous transgressive-regressive cycle, with marine
mudstones in places on the unconformity and above. The Bass and Gippsland
basins have much the same stratigraphy and style except that the Cretaceous
in non-marine, with coal measures throughout.
The whole of the north-western, western and southern margin of Australia
has a similar geological style although the events tended to be earlier in the
north-west and progressively later anticlockwise round the continent. It seems
reasonable to interpret the record of western Australia as the consequences of
processes that resulted in the separation of India (or whichever continent lay
to the north-west) and the record of the south coast as the consequences of
processes that led to the separation of Antarctica - separation beginning in
the north-west and progressing anticlockwise to the south-east. Tasmania
then appears to be a rift remnant (pace Tasmanians!), with the Otway, Bass
and Gippsland basins lying between it and the mainland. It is no doubt geol-
ogically significant that the petroleum provinces are at the ends of this margin,
with little in the middle, but the reasons for this are not yet understood.
On the opposite side of the world lies the North Sea. The development of
the North Sea basin (Ziegler, 1975,1981; Kent, 1975a, b, 1977) is remark-
ably similar to that of the north-west shelf of Australia, with but two modifi-

1 S.E. AUSTRALIA YS N I ~,j

MESOZOIC-TERTIARY BASINS M A

Fig. 16-11. The Otway, Bass and Gippsland basins of south-east Australia. (Courtesy of
Esso Australia Ltd.)