Geoscience Reference
In-Depth Information
been subject to erosion for many hundreds of millions of years, in contrast to the
much younger landscapes to the south and east. For example, the Cretaceous and
Cenozoic rocks in the Lake Eyre Basin have a denudational history of only a few tens
of millions of years.
Until about 130 Ma ago, Australia, Antarctica and Greater India were joined
together as part of eastern Gondwanaland, a supercontinent that also once comprised
Africa and South America (Veevers,
2000a
;Veevers,
2000b
). Towards the start of the
Cretaceous (around 130 Ma), a rift formed between Australia-Antarctica and Greater
India, and the Indian Ocean began to develop. Late Carboniferous to Jurassic uparch-
ing of the eventual rift margins preceded this rifting, leading to drainage reversal.
Rifting was accompanied by volcanic activity in the Perth and Carnarvon basins,
with an incursion of shallow seas, followed by deeper waters as the rift margins
collapsed during the Late Cretaceous. The sea floor spreading rate began to increase
around 80 Ma ago, reaching up to 17.5 cm/year, and it remained rapid until the late
Palaeocene, around 53Ma ago (Johnson et al.,
1976
;Veevers,
2000a
;Veevers,
2000b
).
A rift also began to form during the Palaeocene between Antarctica and Australia,
which at first spread apart quite slowly (about 4 mm/year) (Veevers,
2000a
). Slow
northward drift of Australia continued until around 45Ma ago, when the rate increased
by an order of magnitude to 6-7 cm/year (Veevers,
2000a
;Veevers,
2000b
;McGowran
et al.,
2004
). Late Eocene marine limestones were laid down in the Eucla Basin and
onto the Precambrian shield (Veevers,
1984
;Veevers,
2000a
; Clarke et al.,
2003
).
The Southern Ocean was developing progressively, with the opening of the Tasman
Gateway at around 34 Ma and of the Drake Passage between Antarctica and South
America around 30Ma ago (Doake,
1977
; Livermore et al.,
2005
). Antarctic glaciation
was well underway by around 34Ma (McGowran et al.,
2004
), as a result of the change
in ocean currents around Antarctica (
Figure 22.8
).
At about the same time that rifting was taking place along the southern continental
margin of Australia, rifting was also occurring along the eastern edge of the continent,
culminating in the separation of the Campbell Plateau, New Zealand and the Lord
Howe Rise from Australia, as well as the formation of the Tasman Sea (Jones and
Roots,
1974
; Roots,
1975
;Veevers,
1984
;Veevers,
2000a
;Veevers,
2000b
).
After fully separating from Antarctica around 45 Ma ago, Australia has moved
north into dry, subtropical latitudes at a mean rate of 5-6 cm/year, resulting in the
birth of the Southern Ocean between Australia and Antarctica. The opening of the
Drake Passage led to the establishment of the circum-Antarctic ocean current, which
is driven by the prevailing westerly winds. As a result, Antarctica became thermally
isolated from warmer ocean waters to the north, and rapid cooling ensued. In the
Southern Ocean, the changing isotopic composition of both planktonic and benthic
foraminifera indicates major cooling of deep ocean water as well as surface water
(Shackleton and Kennett,
1975
;McGowranetal.,
2004
). Cumulative ice build-up in
