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

The Murray River drains the 1.06 × 10⁶ 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 paleoshoreline 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 west-southwest 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 Last Glacial Maximum 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 south-southwest path from off Lakes Alexandrina and 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 and Albert. This basin comprises estuarine, lagoonal/ lacustrine and fluvial facies of the paleo Murray River, including channel-fill and point-bar deposits. Sediment drifts and residual paleo dunefields 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 Last Glacial Maximum eolian sheets/dunes as well as lagoonal/lacustrine sediments, is the predominant sediment type at the surface of the central and northern Lacepede Basin. Paleochannels or paleovalleys of the Murray River incised into the Miocene carbonate platform are typically 10-20 m deep, 450-1000 m wide and contain up to 25 m of sedimentary fill and cover.