Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopTectonic activity can significantly control stream patterns and behaviour (e.g. Schumm et al., 2000).One of the most sensitive indicators of change is the longitudinal profile and valley floor profile of astream (e.g. Bendefy et al., 1967; Zuchiewicz, 1979). In alluvial environments, changes in valleyslopes may be accommodated by changes in stream sinuosity. However, in areas of resistantalluvium or bedrock, which tend to confine the channel and retard meander shift and bank erosion,deposition in areas of reduced gradient and incision in areas of increased gradient may dominatestream response. Streams crossing the LSC pass over the Hawkesbury Sandstone and sandstonerocks of the Narrabeen Group, upon which only skeletal soils are typically developed (Jones &Clark, 1991; Bannerman, 1990). Hence, the latter condition might be expected to pertain.The degree to which disequilibrium is preserved consequent of uplift, and its longevity, dependsupon the relative rates of uplift and erosion (including knickpoint retreat, valley incision and plateaulowering rates). A reconstruction of the ca. 20 Ma palaeo-topography west of the Kurrajong Faultbased upon the elevation of the bases of a number of basalt flows suggests that the landscape had alow relief (predominantly
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
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Landslides in the Sydney Basin - Geoscience Australia

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