Stratigraphy and Geomorphologic History of Holocene Dunefield near Millicent along the Southeastern Coast of South Australia

Abstract

Much of the coast of Australian Continent is fringed by Holocene dunefields. The coastal dunefield near Millicent, southeast of South Australia, is one of the long and broad Holocene dunefields of Australia. It occupies the outer edge of broad Quaternary barrier plain expanding about 100 km inland. The outermost barrier called Robe range (barrier) consists of 3 units of barriers whose uppermost unit is Holocene sediments covering lower 2 units of Pleistocene barriers (beach ridge-dune units). The arangement of coastal landforms expressed by the barrier and interbarrier lowland was established during the late Pleistocene. It is located in the Westerly zone characterized by the winter rainfall in the same manner as the southern parts both of east and west coasts of the continent. But the annual rainfall of 500 mm to 800 mm is less than those of the latter two coasts with annual rainfall greater than 800 mm. The authors discuss the history of dune building based on the stratigraphy of the Holocene sediments and compare it with those of some other coastal dunefields of Australia.
The Holocene dune sediments near Millicent are divided into four units, such as the early Holocene dune sand (from 8, 000 B. P. to 5, 500 B. P.), the mid-Holocene dune sand (5, 500 B.P. to 2, 700 B.P.), the late Holocene dune sand (2, 700 B.P. to 500 B.P.) and the Present drifting sand (since 500 B.P.). Each of them consists of calcareous sand and organic layers, being subdivided into layers. The marine sediments containing shell beds of 4 m or 5 m above the present sea level show the dates between 6, 000 B.P. and 4, 000 B.P.
Along the west coast of Lake Bonney, the early Holocene dune sand made a long-walled dune covering the flat interbarrier lowland, behind the outermost Pleistocene barrier which is now distributed in the sea as offshore submerged rocks. It suggests that the wave erosion destructing the Pleistocene barrier into submerged rocks supplied much of sand inland during the period of sea level rising. During the period from 6, 000 B.P. to 4, 000 B.P., the postgalcial marine transgression attained its maximun, and invaded behind the Pleistocene barrier at Rivoli Bay and the west coast of Lake Bonney. A number of small barriers (beach ridge-dunes) were formed around Rivoli Bay and Lake George. Wave cut benches were formed at the foot of Holocene long-walled dune which was precedingly made up inland along the west coast of Lake Bonney.
The mid-Holocene dune sand is divided into two subunits of dates from 5, 500 B.P. to 3, 500 B.P. and from 3, 500 B.P. to 2, 700 B.P., the latter of which intercalates many organic layers, showing frequent changes in mobility and stability. Both subunits show the older phase of parabolic dunes having established the framework of dune landforms of the study area. The dunes were formed during the period from end of the most humid phase in Holocene to the driest phase. Individual dunes are wider and higher than those of the younger phase of parabolic dunes and the dunefield of this phase is the most extensive as much as 5 km inland, showing strong wind and much supply of sand.
The late Holocene dune sand is also divided into two subunits of dates from 2, 700 B.P. to 1, 500 B.P. and from 1, 500 B.P. to 500 B.P., the latter of which intercalates many organic layers containing standing dead trees. Both subunits show the younger phase of parabolic dunes, built up during the period containing the dry phase around 3, 000 B.P., the humid phase around 2, 000B.P. and the following dry phase. They form narrow-body parabolic dunes similar to hairpin dunes on those of the older phase and are less than those of the older phase in width and height. Standing stumps of dead Casuarina trees buried in organic layers of the latter subunit show that the dunefield had been covered with Casuarina woodland before the accumulation of the latter subunit.