Clastic spring sediments: a tool for palaeoflood reconstruction?

Abstract

Clastic spring sediments, the product of groundwater transport by fissure or conduit flow, remain largely uninvestigated. Yet such sediments may be much more common than has generally been realised and examples within the stratigraphic record are likely to have been widely overlooked. This study investigates the potential of these deposits for preserving a history of past floods. Clastic spring sediments at Mount McKinlay Spring in the hot, arid Gammon Ranges of northeast South Australia were dated by 14 C analysis. Five samples of pollen grains taken from sediments interpreted on geomorphic grounds as postdating European contact (AD 1843) all yielded dates of AD 1955-1958. These are likely to be the product of sedimentation in March 1956 following the greatest single day's rainfall in the instrumental record. This event, with a return period of around 100 years, may have caused the undercutting of the banks of the creek downstream of the springs, the collapse of mature River Red Gums growing adjacent to the channel and the accumulation of a jam of logs across the creek. The spring deposits appear to represent solely the product of high-magnitude, long return-period floods. A series of older spring sediments at the site dating from the first millennium BC to the mid-18th century may therefore provide an opportunity to establish event magnitude and frequency, to determine return periods, and to assess the role of climatic change and human environmental impact on flood frequency. This approach may offer a valuable means of reconstructing the chronology of floods, particularly in locations affording few other sources of palaeoenvironmental information.