Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopGeomorphological evidence for neotectonicactivity on the northern Lapstone StructuralComplex.ANDREW RAWSON 1 AND DAN CLARK 21. NSW DEPT. OF NATURAL RESOURCES, C/O FACULTY OF SCIENCE AND AGRICULTURE,CHARLES STURT UNIVERSITY, LEEDS PDE, ORANGE PO BOX 883 NSW 28002. GEOSCIENCE AUSTRALIA, GPO BOX 378, CANBERRA ACT 2601ABSTRACTThe results of morphometric analysis of drainage lines crossing the northern Lapstone StructuralComplex west of Sydney show that there are indicators of recent and possibly ongoing tectonicdeformation. Stream long-profiles oversteepen as they cross major faults, valley cross sectionsnarrow to ‘V-notches’ where streams enter the upthrown side of fault blocks, and several streamshave been partially dammed, forming swamps and lakes. These indicators are consistent with amodel of periodic deformation of the Kurrajong Fault System, near to the threshold conditions ofuplift/incision. The most pronounced example of tectonic modification of stream morphology isBurralow Creek, which has been partially dammed to form Burralow Swamp. Sandy sedimentsunderlying an extensive lacustrine clay deposit, potentially relating to the last fault movement,obtained Late Pleistocene ages.INTRODUCTIONThe Lapstone Structural Complex (LSC) consists of a series of related faults and folds bounding thewestern edge of the Cumberland Basin approximately 50 kilometres west of the Sydney CBD (Fig.1). It is one of the most prominent tectonic and topographic features of the Sydney Basin. Theeastern margin of the complex is an escarpment, rising some 200-400 metres above the surroundingplain, known as the Lapstone Monocline. The western edge consists of a series of overlapping enechelonfaults known as the Kurrajong Fault System (KFS, Pedram 1983, Branagan and Pedram,1990). The maximum vertical displacement of the Kurrajong Fault System is approximately 130metres at Cut Rock (near Kurrajong Heights), with progressively smaller displacements to the south.The LSC has undoubtedly had a long and complex history, and there seems little doubt that it isrelated in part to basement structures that were active during deposition of the Sydney Basinsediments (Qureshi, 1984, Harrington and Korsch, 1985, Branagan and Pedram, 1990; Pickett &Bishop, 1992). Several lines of evidence suggest a Cenozoic age for a significant portion of therelief generation across the LSC, including: a) Miocene basalts west of the LSC appear to haveerupted onto a low-relief pre-incision land-surface (Van Der Beek, 2001), and b) deeply weatheredand sometimes lateritised Tertiary Rickaby's Creek Gravels, which show an internal structureconsistent with deposition in a braided stream environment (Bishop & Hunter, 1990), occur on bothuplifted and downthrown sides of the LSC (Bishop, 1986; Branagan & Pedram, 1990). Opinions aredivided regarding whether this relief generation is temporally related to the deformation whichcrafted the LSC in its present form (e.g. Branagan & Pedram, 1990; Van Der Beek et al., 2001;Schmidt et al., 1995), or relates to passive erosive exhumation of a structure which formed largelyduring Mesozoic syn-depositional deformation (Pickett & Bishop, 1992). Apparent truncation of the18.8 Ma Green Scrub Basalt by the Kurrajong Fault (Crook, 1956, Wellman and McDouga11, 1974;Pedram, 1983; Branagan & Pedram, 1990) has been taken as evidence for the former, while the factthat Rickabys Creek gravels unconformably overlie Hawkesbury Sandstone on the face andupthrown side of the Lapstone Monocline, and Wianamatta Shale on the downthrown side, has been19
Seismic Hazard in SydneyProceedings of the one day workshopFigure 1: The northern Lapstone Structural Complex showing the east facing LapstoneMonocline and west-facing scarps relating to the en echelon faults of the Kurrajong FaultSystem (modified after Branagan & Pedram, 1990).used to argue for the latter (Pickett & Bishop, 1992). Paleomagnetic data showing that rotation ofstrata on the limb of the Lapstone Monocline occurred after the mid-Cretaceous (~90 Ma, Schmidtet al., 1995) and prior to 8 ± 5 Ma (Bishop et.al., 1982 with ages recalculated by Pillans, 2003)suggests that the Lapstone Monocline might have formed during rifting of the Tasman Sea (e.g.Schmidt et al., 1995), and was fully formed by the Pliocene. However, a spatial association betweenthe LSC and the historic catalogue of earthquake epicentres (Gibson, this volume), and theneotectonic indicators discussed herein, suggest the potential for ongoing deformation.20
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