The limited impact of Quaternary sea level rise on the Shoalhaven River, southeastern Australia
A sedimentological and geochronological approach is taken to determine the impact of the Holocene sea level rise on the Shoalhaven River, southeastern New South Wales. Sea level peaked at + 1 to 1.5 m by ∼7.8 ka, before declining slowly to present elevation at 2 ka. Whilst the coastal response to these fluctuations is well established, the question remains as to how far upstream these influences are recorded in the sedimentary record. Empirical evidence of fluvial impact of sea level rise lags behind well constrained information about the magnitude and timing of these events. Southeastern Australia, being tectonically stable and lacking glaciation throughout the Quaternary, is an ideal location to obtain evidence for fluvial responses to sea level rise. Base level is a key parameter to which the fluvial system adjusts, thus relative changes in sea level force a response from river systems. In order to maintain equilibrium, channels adjust their planform in response to base level change. However planform adjustment is restricted by the confinement of many coastal rivers in southeastern Australia. Whilst estuarine facies were identified at Wogomia, 35km from the current mouth of the Shoalhaven River, they appear to be reworked deposits from higher in the system, although they have not been identified beyond the bedrock confines immediately upstream. Shell fragments, dated at 7.38 ± 0.045 kcal yr BP, were retrieved from ∼2 m below the current low flow water level with at least 4 m of similar sediment above, and 8 m below, overlying Pleistocene basement. Rapid accretion associated with delta progradation in this confined setting is suggested as the mechanism for the deposition of thick beds (> 12.5 m) of estuarine material at Wogomia.
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