Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopStream ProfilesThe most obvious result upon examination of the longitudinal profiles (Fig. 2) is the remarkablesimilarity between them. Almost all streams exhibit a reach of low gradient immediately upstreamof the Kurrajong Fault System, and a corresponding over-steepened reach downstream. Theexception to this rule is the Grose River, and to a lesser extent Wheeny and Glenbrook creeks, whichappear to be largely unaffected by the fault. In most cases this disruption is entirely independent oflithology, and largely independent of catchment size. Creeks to the north of the Grose River exhibitsimilar drainage disruption. However, the picture here is complicated by lithological variation.Most importantly, the steeper reaches downstream of the fault correspond with the supposedly lessresistant rocks of the Narrabeen Group, in direct conflict with expectations (cf. Hack, 1960; 1973a).It could be argued therefore, that these anomalous over-steepened reaches indicate tectonic, and notlithological, control of the profiles in this area.Figure 2: Long profiles for selected easterly flowing streams passing through the northernLapstone Structural Complex. a) Burralow Creek, b) Blue Gum Swamp Creek.The Stream Gradient (SL) Index is defined as the change in elevation of a stream reach normalisedfor the exponential decay of elevation along a typical stream length (Fig. 3). It is particularlysensitive to anomalous changes in slope, and can therefore be used to determine whether long profileshape is controlled by lithology or by tectonic deformation. For all streams crossing the LapstoneStructural Complex, SL values were found to be at least an order of magnitude greater between theKFS and Lapstone Monocline than in upstream reaches. This cannot be sufficiently explained bylithological variation across the fault zone, and is indicative of recent or ongoing deformation.22