The stratigraphy of the alluvial sediments filling the Macquarie River valley, New South Wales, was investigated to test models of landscape evolution of the rifted margin Eastern Highlands of Australia. In the neighbouring Lachlan River valley, the stratigraphy of Neogene sediments has been incorporated into a model proposing denudation and episodic passive (denudational isostatic) uplift of the highlands throughout the Neogene. In this study, we aimed to test the regional extent of this model and the influence of sediment supply on the stratigraphic record and its interpretation. The sediments of the Macquarie River valley show three major changes in the depositional regime. First, erosion of the valley basement in the Late Miocene was followed by (and possibly synchronous with) deposition of a basal clay and sand unit in the Late Miocene. Second, the valley was filled with a wedge of sediment containing abundant gravel and sand in the Pliocene; and third, the older gravelly unit was eroded and reworked in the Quaternary, and a sandy clay unit, which forms the modern floodplain, was deposited. Deformation of the Middle Miocene basalt‐filled valley provides good evidence for substantial uplift of the highlands in the Middle–Late Miocene, synchronous with or shortly after widespread volcanism, but continuing no later than the Late Miocene. The Pleistocene incision is best accounted for by climate change to drier conditions, triggering a sediment‐starved response by the Macquarie River, rather than further uplift. While valley incision and subsequent infilling can be viewed as a response to tectonism, the internal sedimentology and stratigraphy of the sediments appears to respond to fluctuations in sediment supply brought about by extrinsic climate change and intrinsic catchment lithology. The first‐order tectonic events are not synchronous with uplift in the Lachlan valley and are restricted to relatively local spatial scales. However, the secondary climatic forcing has a broader regional expression.
The rainfall‐runoff events following five fires that occurred within a 40‐year period in eucalypt forests of the Nattai catchment, southeastern Australia, were investigated to quantify the postwildfire hydrological response and to provide context for lower than expected erosion and sediment transport rates measured after wildfires in 2001. Daily rainfall and hourly instantaneous discharge records were used to examine rainfall‐runoff events in two gauged subcatchments (>100 km 2 ) for up to 3 years after fire and compared with nonfire periods. Radar imagery, available from 2001, was used to determine the intensity and duration of rainfall events. Wildfires in the study catchment appear to have no detectable impact on surface runoff at the large catchment scale, regardless of fire severity, extent or time after fire. Instead, the magnitude of postfire runoff is related to the characteristics of rainfall after fire. Rainfall is highly variable in terms of annual totals and the number, size, and type of events. Rainfall events that cause substantial surface runoff are characterized by moderate‐high intensity falls lasting one or more days (≥1 year average recurrence interval). These are triggered by synoptic‐scale weather patterns, which do not reliably occur in the postfire window and are independent of broad‐scale climate dominated by the El Niño–Southern Oscillation (ENSO). This study highlights the importance of considering the characteristics of rainfall, as well as local factors, in interpreting the postfire hydrological response.
The denudational history of the Macquarie River valley, flowing westerly off the Eastern Highlands in the Central-West of New South Wales, was investigated to provide further evidence for highland evolution. The study area is the bedrock-confined alluvial valley, extending from Burrendong Dam to Narromine, at the margin of the highlands and the Darling Riverine Plain (Figure 1). The catchment is formed of Palaeozoic rocks of the Lachlan Fold Belt south east of Wellington and Mesozoic sediments of the Oxley Basin to the north and north west as far as Narromine. Neogene sediments fill the alluvial valley and Riverine Plain (Martin 1991, Watkins & Meakin 1996). Mid Miocene basalt flows (11.6 – 13.9 Ma) overlying alluvial gravels are found within the study area at Mookerawa and Brocklehurst (Wellman & McDougall 1974, Dulhunty 1973).
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