A review of the Cenozoic palynostratigraphy of the River Valleys in Central and Western New South Wales.

Abstract

The palynology of sediments from the Murray, Murrumbidgee, Lachlan, Macquarie and Namoi River Valleys of the Western Slopes of New South Wales reveals remarkably similar patterns in the alluvium of all of the valleys. Mid Miocene and older palynofloras found on the flood plains are rarely (if ever) seen in the valleys where almost all of the palynofloras are placed in the late Miocene-Pliocene M. galeatus Zone. A few palynofloras of the Pleistocene T. pleistocenicus Zone are found at the top of the sequence. The alluvial fills of the palaeovalleys are similar also: in a basal late Miocene-Pliocene unit: the sands and gravels are almost entirely quartz whereas the upper unit of Pleistocene age has a variety of resistant rock types and only a minor quartz component. The alluvium of these river valleys is an important groundwater resource. In the mid Miocene, a time of high sea level, the rivers of the Western Slopes discharged into the flooded Murray Basin. Following major falls in sea level in the late Miocene, there was a basin-wide time of erosion/non-deposition and entrenchment of the river valleys. Denudation associated with this regression removed older sediments in the valleys and probably carved out the valley-in-valley structures. Tectonic events were probably small and only maintained the elevation of the Highlands. The palynofloras indicate a substantial change in the vegetation and climate over this time: from rainforest and a wet climate in the mid Miocene to eucalypt sclerophyll forest and a drier, more seasonal climate in the late Miocene-Pliocene to woodlands/grasslands and a much drier climate in the Pleistocene. Deposition of the basal quartz rich alluvial unit occurred under a high rainfall, high-energy regime whereas the upper unit was deposited under a drier climate and low energy regime. Eustasy was a major forcing factor in the Neogene, but by Pleistocene time, the Murray Basin had become isolated from the sea and the much drier climate had become the major forcing factor.