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them further. In the Piper field, the accumulation of Coniacian and Santonian
mark only on the downthrowing side of a southern fault in places suggests low
energy. It is inconceivable that subsidence at the rate of 50,000 yrs/m could
maintain a balance between accumulation and non-accumulation of mark for 5
m.y.: this must be the net result. Hence we infer that sediment did accumu-
late from time to time on both sides of the fault (which was moving very
slowly indeed), but that erosion removed more -of this sediment from the up-
throwing block from time to time, rather than cutting into the older sediments.
The areas of erosion and truncation of older strata are probably limited to
the highs.
The present relief of an unconformity surface is not necessarily a good
guide to the contemporary relief of the surface. The transgression that com-
pletes the unconformity is necessarily diachronous, and accumulation early
in the lows tends to maintain a more subdued physiographic relief than that
of the erosion or non-accumulation surface; and much of the present relief
may have been created by subsequent differential subsidence.

Bass Strait oil and gas fields, Australia

In the southeast corner of mainland Australia, offshore from the state of
Victoria, lies the Gippsland basin (Fig. 13-10). In 1965, the first offshore

NEW SOUTH WALES
.-
\,
*d--l--w-*

S.E. AUSTRALIA
M ESOZOl C -TE RTI ARY BAS INS
, 500 KM
BASIN AREA HOBART
-
Fig. 13-10. The Mesozoic-Tertiary basins of south-east Australia. (Courtesy of Esso Aus-
tralia Ltd.)