Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopFigure 4: Field photographs from WheenyGap Ridge. (a) potential slickenside on 45ºwest dipping sandstone bedding plane in thehanging wall fold of the Kurrajong Fault. (b)vertically sheared and fractured sandstoneon Wheeny Gap Ridge at the location wherea sub-vertical fault is inferred to cross theridge (see Fig. 3a for location). (c) theWheeny Gap Fault exposed in a cliff on thenorthern side of Wheeny Gap. Viewingposition and direction is indicated bysouthern red arrow in Fig. 5a.Figure 5: SPOT 5 images of Wheeny Gap. (a) northern extension of Wheeny Gap Fault isindicated by arrows. Displaced cliff face is circled. (b) same as a, but with fault traces inferredas per Fig. 3.37
Seismic Hazard in SydneyProceedings of the one day workshopWe consider the west-dipping sandstone bedding seen on Wheeny Gap Ridge to be analogous to thatfound in the hanging wall of the Wheeny Gap Fault and associate this zone with a steeply eastdippingKurrajong Fault (Figs. 3a, b). This is consistent with the 80º east dip measured on a 3-4 mwide fault zone interpreted to relate to the Kurrajong Fault at Cut Rock on the Bells Line of Road3.7 km to the south (Branagan and Pedram 1990). The spatial relationships between the Kurrajongand Wheeny Gap faults suggest that they merge less than 300 m below the level of Wheeny Creek.The structural relationships exposed in Wheeny Gap are consistent with the interpretation of seismicprofiles further south (Herbert 1989); the faults underlying the west facing scarps (e.g. Glenbrook,Kurrajong, Burralow) are steeply east-dipping reverse faults, which in places, such as Wheeny Gap,link into moderately steep west-dipping reverse faults with clear evidence for wrench displacement.WHEENY CREEK STREAM MORPHOLOGYHack (1973) showed that any stream which is in dynamic equilibrium (i.e. having sufficient slope toadequately transport supplied sediments) exhibits a concave upward long(itudinal) stream profilethat plots as a straight line on semi-logarithmic axes. Streams which flow over differing lithologiesplot as a series of straight line segments, with steeper segments corresponding to more resistantrocks. Disequilibrium may be demonstrated by a change in slope unrelated to lithology.The Wheeny Creek long profile demonstrates a grossly convex-upward form (Fig. 6). In detail, thisform apparently comprises two concave upward sections which join near to the mapped boundarybetween the Hawkesbury Sandstone and Narrabeen Group sandstone (Fig. 6b). The gross profileform might therefore be explained in terms of this lithological change. This interpretation issupported when the profile is plotted on semi-log axes (Fig. 6c), with straight line segments ofdiffering slope coinciding closely with the lithological boundaries. However, the slope relating tothe Narrabeen Group sandstone is steeper than that of the Hawkesbury Sandstone, which is counterintuitive given the finer bedding and abundance of shale in the Narrabeen Group.The digital elevation data contains insufficient detail to resolve with certainty any perturbation ofthe channel slope across the Kurrajong and Wheeny Gap faults. However, there is an indication of ajump in the profile in this area (Fig. 6b). Also, a small perturbation of the profile is associated withthe passage of the stream through eastern margin of the Lapstone Monocline (Fig. 6c).AN EVOLUTIONARY MODEL FOR THE LAPSTONE STRUCTURAL COMPLEXWe have constructed an evolutionary model for the Lapstone Structural Complex that attempts toreconcile the apparently contradictory evidence for the age of major deformation (Fig 7). Thecomputer program Trishear 4.5 (Allmendinger 1998; Zehnder and Allmendinger 2000;http://www.geo.cornell.edu/geology/faculty/RWA/FF_DL.html) was used to forward model asimple basin geometry subjected to faulting. Fault-related folds are modelled by the program usingtrishear kinematics for deformation at the tipline and fault-bend folding over trailing ramps. Growthstrata are entered manually into the developing model. The effects of erosion have beenschematically added to the finished model using Adobe Illustrator 11.The observed thickening of strata across the LSC (Bryan 1966; Branagan and Pedram 1990; Pickettand Bishop 1992; Australian Oil and Gas Corporation Ltd. 1966) is reproduced by introducing aneast-dipping syn-depositional normal fault into the model (Fig. 7a). Steep dips are developed instrata proximal to the fault tip but the near surface strata are only gently warped, consistent with thepalaeomagnetic results of Schmidt et al. (1995) who contend that significant rotation of strata postdatesthe mid Cretaceous. The large offset produced in the basement relating to the fault coincideswith a marked deepening of the basin east of the LSC (SRK, 2006).38
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