Doctor of Philosophy
Department of Geography
Nott, Jonathan Freeman, Cainozoic landform evolution on the Shoalhaven plain N.S.W. with special reference to the alluvial and aeolian history over the last 45 million years, Doctor of Philosophy thesis, Department of Geography, University of Wollongong, 1990. https://ro.uow.edu.au/theses/1384
An alluvial sheet, in places 130m thick and over 2000km2 in area, together with an extensive palaeovalley network and scattered source bordering and terrestrial sand dunes provide a history of drainage and landscape development in the middle and upper Shoalhaven catchment over the last 45 to 60 million years. This record tests the applicability, for this highland catchment, of models of longterm landscape evolution the passive margin of eastern Australia. It also provides a comparison to lake shore and slope deposits within neighbouring catchments upon which much of our understanding of Late Quaternary climate changes within southeast Australia is based.
This study shows that mid Oligocene basalt flows dammed the Shoalhaven River downstream of the eastward bend near Tallong to form a lake approximately 100km in length. In fact, within a relatively brief period of time Lake Tolwong formed twice; the second episode being considerably longer. Into Lake Tolwong were deposited two separate units of well laminated silts and clays, separated by a thin conglomeratic sequence representing a brief return to fluvial conditions. Each of the lacustrine units contains a well preserved spore and pollen assemblage, from cool temperate rainforest taxa, which can be equated with the Early to middle Oligocene lower Proteaceidites tuberculatus Zone of the Gippsland Basin. The second lacustrine unit reached a thickness of 60m before being partially stripped and fluvially reworked following the progradation of approximately 50m of coarse deltaic sediments. Basalt damming of the Endrick River and Limekiln Creek, eastern and western tributary networks within the Shoalhaven catchment, during the Middle Eocene also accounts for the accumulation of lacustrine sedimentary sequences in these valleys.
Deposition of these sedimentary sequences has occurred within an extensive palaeovalley network carved into the Shoalhaven Plain. These valleys were excavated into a landsurface which in places was deeply weathered down to at least 150m and, in other areas, capped with a thin sheet of well silicified, generally fine grained sands and shallow valley fills of well ferruginised coarse sands and gravels. The age of the palaeovalley network places a minimum age on the deep weathering and deposition of the high level duricrusted sediments throughout the catchment. This age must be at least very Early Tertiary. These Tertiary sedimentary sequences and palaeovalleys provide further evidence of the great antiquity of drainage patterns within the Shoalhaven catchment, well as the very slow rates of geomorphic change within a tectonically stable landscape that has experienced a stable and humid climate for much of its history. They also show that highland fluvial systems such as the Shoalhaven can evolve independently of external base level controls.
The Quaternary alluvial terraces of the upper catchment provide clues to the behaviour the Shoalhaven River during the Miocene and Pliocene for they show that upstream of the headward advancing Shoalhaven gorge the Shoalhaven River did not re-excavate its Oligocene alluvial fill until probably the Pleistocene. Throughout the Miocene, a period traditionally regarded to have been one dominated by planation to produce the Shoalhaven Plain, the river moved freely across its Oligocene sedimentary base reworking only the upper 10-20m of fill.
The Late Pleistocene saw several episodes of valley incision and aggradation. The upper five alluvial terraces at Larbet, near Braidwood are much thicker and contain considerably coarser sediments than the lowest terrace and contemporary floodplain. Thermoluminescence dates from the floodplain and four lowest terraces, spanning a period from 7-460ka, reveal that the Shoalhaven was a much more active stream prior to 57ka. The period 20-35ka, generally regarded as a time in which streams throughout southeastern Australia were active and receiving much sediment as a product of widespread slope instability, is registered as a relatively insignificant period of fluvial activity in the Shoalhaven valley. It is likely that the 14C chronologies of slope and shore deposits in neighbouring highland catchments have identified only the dwindling stages of a much wetter period prior to the last glacial maximum which, in the Shoalhaven catchment, culminated in dry, windy conditions and the development of source bordering and terrestrial dunes between 6-19ka.