OFR 151.pdf - CRC LEME

5.2.1 Late Jurassic
During the Late Jurassic and earliest Cretaceous, the continent lay between palaeolatitudes of
~45-85 0 S, with the South Coast of New South Wales being the part of the continent that was
closest to the geographic South Pole. Much of the palaeo-northern Australia was at a
palaeolatitude of 60 0 S or lower (Veevers et al. 1991). Climates are likely to have been
'continental' although most reconstructions are in fact predictions based on GCMs, e.g. Moore
et al. (1992). This model, which assumes topographic relief of up to 2 km elevation along the
palaeo-eastern margin, predicts annual temperature ranges of −22 0 C to +10 0 C at
palaeolatitude 70 0 S (Hobart) and –10 0 C to +20 0 C at palaeolatitude 45 0 S (Darwin). Annual
precipitation is estimated to have been in excess of 1000 mm. The model also predicts that
the palaeo-northwestern margin (palaeolatitude 40 0 S), which fronted onto the Neo-Tethys
Sea, was affected by storms.
5.2.2 Early Cretaceous
Patterns of climatic change during the Berriasian to Aptian [141-112 Ma] are complicated by
the fragmentation of East Gondwana, which allowed seaways to develop around the palaeonorthern
and western margins of the continent, and movement of Australia as a whole into
increasingly high palaeolatitudes, which will have increased the area subject to low
photoperiods during winter. For example, the region stretching from southern Queensland to
northeastern Tasmania lay south of 80 0 S during the Valanginian to Aptian [141-123.5 Ma]
and therefore will have been subject to three or more months of winter darkness. Conversely,
plants growing along the Western Australian coastline, then part of the palaeo-northern
margin at ~45 0 S, will have been unaffected by low photoperiods.
Marine flooding is likely to have provided new sources of moisture, tempering the continental
climatic regimes and the third major influence on Early Cretaceous climates was a series of
punctuated marine transgressions into the Perth and Carnarvon Basins (palaeo-northern
margin) and Carpentaria, Laura, Eromanga and Surat Basins in the palaeo-centre and southeast
of the continent). These transgressions began in the Valanginian and culminated in the
Late Aptian [119-114 Ma] when approximately one quarter of the continent was occupied by
epicontinental seas (cf. Immenhauser and Scott 1999). Any carbonate platforms developed
along the palaeo-northern margin facing the Neo-Tethys Ocean will have been drowned
during this prolonged ‘high stand’ (cf. Gotsch et al. 1993).
The epicontinental seaways in Australia were subject to winter freezing during the
Valanginian to Albian [112-99 Ma]. Geological evidence for sub-zero temperatures in the
Eromanga/Carpentaria Basin are supported by the GCM results, which predict a mean annual
temperature range of −18 0 C to +27 0 C in these basins and averages close to 0 ° C over much of
the continent (references in Frakes and Francis 1988, Frakes 1999). Sedimentary evidence
indicates storms were frequent (cf. Barron and Washington 1982). Frakes (1999) proposes
that the Cretaceous Period also included more temperate intervals, which were accentuated by
the effective northward movement of the continent. For example, by the end of the Albian
[112-97 Ma], Western Australia was located at palaeolatitudes of between 50-60 0 S and
oxygen isotope data indicate SSTs in the Carnarvon Basin had risen to between 7-11 0 C
(Stevens and Clayton 1971): Average SSTs in the Eromanga-Surat Seaway were ~12 0 C,
reaching up to 16 0 C where marine circulation was more restricted (Pirrie et al. 1995).
Conversely, water temperatures averaged 0 ° C (range from –5 0 to +5 0 C) in the Otway Basin at
a palaeolatitude of ~85 0 S within the developing Australo-Antarctic Rift System (Gregory et
al. 1989).
Regression during the late Middle to early Late Albian led to anoxic conditions within central
areas of the Eromanga-Carpentaria Seaway. Haig and Lynch (1993) note that changes in
relative sea level in northeastern Australia during the Albian are out-of-phase with eustatic
60