Sedimentary Archives of Downstream Energy Fluctuations on the Kangaroo River of NSW

The primary aim of the present study was to develop a late Quaternary history of theKangarooRiver: something which had not been done before. The study examined how the river had responded to climatic fluctuations, why it had responded in a particular way and when these changes had occurred. Five sites were selected at key points along the length of the river. A Global Positioning System (GPS) was used to record precise locations. Surveys were conducted at all sites using Light Detection and Ranging (LiDAR) wherever possible. At alluvial sites, sediment was collected by augering or taking core samples. Collected material was later analysed for grain size using a Malvern Mastersizer 2000 and for mineralogy using X-ray diffraction (XRD). Cobble size was measured using the Wolman series. Samples from two sites were also subjected to Optically Stimulated Luminescence (OSL) to date past geomorphic events and a charcoal fragment from one of these sites was radio radiocarbon dated. Sedimentary results indicated that each site responded differently to fluctuating energy levels and that the response pattern was dictated by changing structural controls. The valley floor widens and narrows as it flows through harder and softer units, altering the dominant mode of flood plain accretion. Mineralogy of the terrace alluvium reflected the composition of the geological units through which the river has flowed. Five OSL dates were obtained from two alluvial sites and their ages were late Holocene ranging between ~ 3.5 – 1.6 ka. The one radiocarbon date was mid Holocene at ~ 5.9 ka and was thought to have been incorporated in the terrace following fluviatile remobilisation. The absence of Pleistocene sediments like those in adjacent coastal valleys suggests that the combination of a very small catchment size, low channel to floodplain width and high intensity rain events has limited the Kangaroo River’s sedimentary history. Because of the small range of these dates, it was difficult to conclude whether they represented the first deposition immediately following the Holocene Climatic Optimum or whether they were the product of local climatic events.