Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopFigure 6: (a) Hillshaded and colour draped DEM (LPI 25 m) of the northern LSC showingWheeny Gap, the Lapstone Monocline, the prominent west facing scarp relating to theKurrajong Fault, and Burralow Swamp. (b) topographic profiles constructed along the linesmarked in part (a) with faulting and stratigraphy superposed as per Fig 3c. (b) topographicprofiles in part (b) marked on semi-log axis to highlight streambed disequilibrium. Streamlongitudinal profiles are shown in pink.The landscape is then subject to erosion, and develops a slight easterly slope as beds of WianamattaGroup shales with progressively greater dips are exhumed (Fig. 7b). We speculate that the scarpparallel (longitudinal) course of the Nepean River may have developed when a proto-river, flowingin an easterly direction down this developing slope, was captured by the developing rift shoulderrelating to the opening of the Tasman Sea (ca. 95–100 Ma, Colwell et al. 1993).39
Seismic Hazard in SydneyProceedings of the one day workshopBetween 15 Ma and 20 Ma (e.g. Pulford 1997; Pickett 1984; van der Beek et al. 2001) extensivesub-horizontal sheets of basalt were extruded onto this gently dipping, low relief surface (Fig. 7b).The observation that these basalts overly both Wianamatta Group shales and Hawkesbury/Narrabeensandstones (Bryan 1966) suggests that erosional lowering of the landsurface had stripped shale frommany areas west of the syn-depositional warp by the time of basaltic volcanism, but not to the extentthat significant relief was generated (cf Pickett and Bishop 1992). Subsequent erosion strips shalefrom the face and most of the crest of the syn-depositional warp (Fig. 7c). Rickaby’s Creek Gravelsare deposited on a sandstone substrate on the crest and face. The Nepean River is entrenched whereit flows across exhumed sandstone (e.g. Nepean Gorge and Bents Basin).A change in the crustal stress regime, perhaps between 10 and 5 Ma (e.g. Sandiford et al. 2004),initiates reactivation of a low-angle west-dipping basement fault causing uplift across the LSC andthe propagation of faults into Sydney Basin strata. Refraction of one of these faults as it propagatesinto the basin results in the formation of a prominent east facing monocline (the LapstoneMonocline) and west facing scarps (the Kurrajong/Glenbrook fault system) (Fig. 7d). Benches ofRickaby’s Creek Gravel are progressively stranded on the crest and face of the monocline as itgrows. The entrenched meanders of the Nepean River deeply incise as the uplift proceeds. Majorknickpoints retreat westward across the LSC forming deep gorges (e.g. the Grose River, WheenyCreek). Almost all the shale is stripped from west of the LSC as these knickpoints migrate to thewest. Relicts of shale are preserved beneath basalt caps.Figure 7: Cartoons depicting our proposed model for the evolution of the Lapstone StructuralComplex looking north at an E-W section through Penrith (see text for explanation). Thethickness of lithological units is schematic. Axes units approximate kilometres. Note that byrefracting a low-angle (30º) basement fault to 50º as it propagates into the basin the geometricfeatures seen in Fig. 3c and 6b can be accurately reproduced (ie. sharp west-facing scarp 2km west of the east facing monocline). The structural geometry in frame A may have formed asearly as the Jurassic (Pickett and Bishop, 1992).40
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Landslides in the Sydney Basin - Geoscience Australia

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