Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopThe Rickaby’s Creek Gravel is a fluvial unit disconformably overlying the Triassic rocks of theSydney Basin (e.g. David 1897; Branagan and Pedram 1990). Clast lithology, size and sedimentarystructures within the gravel point to a provenance identical to the present Warragamba-NepeanRiver system (Bishop 1986; Pickett and Bishop 1992). Near Penrith the gravels occur on theCumberland Plain, as well as on the face of the Lapstone Monocline and to its west as far asGlenbrook (e.g. Branagan and Pedram 1990). On the plain the gravels overlie Wianamatta Groupshales whereas they overlie Hawkesbury Sandstone in elevated regions (Chesnut 1982; Pickett andBishop 1992). This evidence casts doubt on the widely accepted view that the gravel originallyformed a continuous sheet that predates the monocline (e.g. David 1897), as shale has apparentlybeen stripped from elevated regions prior to deposition of the gravel (Pickett and Bishop 1992).Pickett and Bishop (1992) contend that this relationship is best explained if the monocline is an ‘old’structure (early Jurassic) that has been progressively exhumed during deposition of the Rickaby’sCreek Gravel, which is postulated to be a composite diachronous unit.STRUCTURAL RELATIONSHIPS EXPOSED IN WHEENY GAPA reconnaissance structural traverse was conducted down an easterly-trending ridge that approachesWheeny Gap from the west and crosses the trace of the Kurrajong Fault (Fig. 3). This unnamedridge is hereafter referred to as Wheeny Gap Ridge for ease of reference. The initial 300 m of thetraverse descends steeply through several cliff-lines formed in undeformed and horizontally-beddedblocky Hawkesbury Sandstone (Rh, Fig. 3). Thereafter the ridge flattens and is developed in poorlyexposed Narrabeen Group sandstone and shaley fine sandstone (Rn). At grid location279623mE/6292172mN (GDA94/MGA56) shaley fine sandstone was observed to dip 45-50ºtowards the west (Fig. 4a). Bedding at this location strikes roughly parallel to the Kurrajong Faultescarpment (~10º strike). A potential down-dip slickenside (~110º rake) was observed on onedipping bedding plane and is interpreted to reflect flexural slip (Fig. 4a). Within ~100 m to the eastthe sandstone is again sub-horizontally bedded. At grid reference 279691mE/6292198mN the ridgedescends into a 20 m wide saddle ~5 m below the ridge level. The sandstone within the saddlecontains sub-vertical (to steeply east-dipping) fractures and is sheared in several zones up to a metrewide (Fig. 4b). Fractures are open and are typically associated with pale clayey sand. We considerit probable that this saddle represents an eroded fracture zone, potentially relating to a steeplydipping fault inferred on the basis of a conspicuous ridge and associated drainage line on thesouthern side of the gap (Figs. 3b, 5).Beyond the saddle blocky cliff-forming sandstone is again encountered. The change in lithology isassociated with a rise in topography of ~10-15 m. Shaley fine sandstone intercalations suggest thatthese rocks belong to the Narrabeen Group. Fifty metres east of the saddle a relatively elevatedsubsidiary saddle occurs. The sandstone in this saddle is sub-vertically sheared with a 5 cm fracturespacing aligned with the Kurrajong Fault escarpment (~10º strike). Fractures are typically open andare variably filled with white clayey sand similar to that found further west (e.g. Fig. 4a).A reverse fault is visible on the northern cliff face of Wheeny Gap from the eastern end of the ridge,immediately before it drops steeply into Wheeny Gap (Fig. 4c). The fault, herein referred to as theWheeny Gap Fault, dips at between 45º and 50º to the west and appears from its intersection withthe topography to be north striking (Figs. 5a, b). The sandstone bedding in the hanging wall hasbeen dragged into the fault plane forming a monoclinal fold (Fig. 4c). Bedding dips approximately30º towards the east immediately above the fault plane, and is shallowly dipping to flat-lying on theeastern side (footwall) of the fault. The amplitude of the roll suggests a minimum of several tens ofmetres of displacement. A band of foliage several metres wide occupying the fault plane on the cliffface suggests a well-developed gouge or crush/fracture zone. The vertical displacement estimatefrom the monoclinal roll alone might therefore be a significant underestimate. This supposition is35