Honours Thesis Abstract
PLIO-PLEISTOCENE SEDIMENTS OF THE MURRAY BASIN
Laura Gow
University of Melbourne, Victoria
2006 AIG Honours Bursary Winner
The Kanawinka Escarpment extends across the entire Murray Basin from the Mount
Lofty-Flinders Ranges to Portland. Initially identified as a series of relay faults, this
structure has more recently been interpreted as a coastal erosion feature. Two
stratigraphic units in the Portland region, the Werrikoo Limestone and Bridgewater
Formation were deposited on the escarpment. The origin of the escarpment can
therefore be deduced by obtaining ages and depositional environments for these two
units.
Several outcrops of the Werrikoo Limestone and Bridgewater Formation were
stratigraphically measured, sampled and photographed in the Portland-Casterton area.
Stratigraphic sections were produced for each locality and correlations made between
sections.
Foraminifera were used to provide ages, while the lithology and
palaeontology assisted in determining depositional environment.
The Bridgewater Formation was deposited in coastal aeolian conditions, and the
Werrikoo Limestone was deposited in open to shallow marine, estuarine and intertidal
conditions. An age of 1.66-0.73 Ma has been assigned to the Bridgewater Formation
and a Plio-Pleistocene age between 2.22 and 0.73 Ma can be applied to the Werrikoo
Limestone.
The occurrence of planktonic Globorotalia puncticuloides and
Globorotalia truncatulinoides further constrain these age ranges.
The development of Lake Bungunnia, a giant lake produced by tectonic damning of the
Murray River, appears to have influenced the coastal environment during the period 3.50.7 Ma. A consequence of lake formation was a lack of sediment supply to the coast,
which in turn produced erosional conditions. This erosion produced the Kanawinka
Escarpment during the Plio-Pleistocene and provided the sediment necessary for the
deposition of open marine Werrikoo Limestone. Deposition of the Bridgewater
Formation was possibly influenced by the breaching of Lake Bungunnia. The
combination of decreasing sea level and an influx of sediments resulted in the exposure
of the previously deposited Werrikoo Limestone, thus providing a platform for
deposition of the shoreface sediments.
The discovery of the Werrikoo Limestone and the interpretation of the Kanawinka
Escarpment east of Portland provide strong evidence for the palaeoshoreline origin of
the escarpment.