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The ancestral Murray River on the Lacepede Shelf, southern Australia: Late Quaternary migrations of a major river outlet and strandline development

Peter J. Hill1*, Patrick De Deckker,2, Chris von der Borch1 and  C.olin V. Murray-Wallace3

1 formerly as Visiting Fellow of the Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200
2 Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200
3 School of Earth & Environmental Sciences, University of Wollongong, NSW 2522, Australia.

Figure 1. 3-D oblique view, looking north, over the Lacepede Shelf region and mapped Murray River palaeochannels (LGM channel in brown). Onshore topography is draped by Landsat 7 imagery. The area marked by a 'V' (in the southeast corner) contains outcrops of Newer Volcanics that were active around Mt Gambier during the Quaternary, and possibly were responsible for the semi-regional uplift that led to the preservation of the Pleistocene strandlines on the Coorong-Mt Gambier coastal plain to the NNW. The dashed yellow line separates the Mio-Pliocene and Pleistocene strandlines and marks the northwest extension of the Kanawinka escarpment. The wide continental shelf (light brown/fawn area) dips very gently seaward to a depth of about 200 m at the shelf edge, beyond which the continental slope, deeply incised by canyons, descends steeply to a flat abyssal plain at 5200 m. During major glaciations in the Pleistocene, the Lacepede Shelf was an expansive coastal plain across which the Murray River flowed southward from the present Lakes Alexandrina/Albert, then roughly WSW and into the head of Sprigg Canyon. The river flowed directly south at times during early lowstands.

 

The Murray River drains the 1.06 million km2 Murray-Darling Basin, and discharges into the sea at the Murray Mouth in southeast South Australia. The outlet faces the 180-km wide Lacepede Shelf and forms part of a wave-dominated beach barrier/lagoon complex, the largest of its type on the Australian coast. Global glacial cycles during the Pleistocene produced lowered sea levels and exposure of much of the Lacepede Shelf, with the palaeoshoreline advancing out to the present edge of the continental shelf during glacial maxima. Mapping and sediment sampling of the Lacepede Shelf in 2006 and 2007 allowed the ancient course of the Murray River on the shelf to be traced and studied for the first time, revealing a 200-km long system of ancient infilled channels and lagoons.

The main system of anastomosing Pleistocene channels begins southeast of the present Murray mouth, off Lakes Alexandrina and Albert, runs southward initially but then veers WSW across the central Lacepede Shelf before heading southwest directly south of central Kangaroo Island, and splaying into the head of Sprigg Canyon at the shelf edge. The LGM channel starts near the current Murray mouth and forms part of this channel system. An earlier channel system ran a more direct, shorter 150-km SSW path from off Lakes Alexandrina/Albert to the shelf edge.

The Lacepede Shelf is founded on a platform of gently folded, shallow-marine Miocene carbonates. The top of the platform is a prominent regional flat-lying erosional unconformity with local karstic relief in the order of 10 m. Unconsolidated sediment cover (?Pliocene-Quaternary) is mostly relatively thin or absent. However, a depocentre (Lacepede Basin), commonly 6-10 m thick, underlies the central Lacepede Shelf and that part of the shelf directly south of Lakes Alexandrina/Albert. This basin comprises estuarine, lagoonal/lacustrine and fluvial facies of the palaeo Murray River, including channel fill and point-bar deposits.

Sediment drifts and residual palaeo dune fields are common components of the young sediments on the shelf. A Holocene yellow/red fine quartz sand less than a metre thick, likely to have formed from reworked LGM aeolian sheets/dunes as well as lagoonal/lacustrine sediments, is the predominant sediment type at the surface of the central and northern Lacepede Basin. Palaeochannels or palaeovalleys of the Murray River incised into the Miocene carbonate platform are typically 10-20 m deep, are 450-1000 m wide and contain up to 25 m of sedimentary fill and cover.


Note: This work will appear in the first issue of the Australian Journal of Earth Sciences in 2009