Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopFigure 3: Stream Gradient Index (SL) for selected easterly flowing streams passing throughthe northern Lapstone Structural Complex. a) Burralow Creek, b) Blue Gum Swamp CreekValley Cross SectionsThe valley cross-sections of 13 of the streams crossing the KFS show a marked difference as thestreams pass into the deformed area (Fig. 4). The three largest streams (i.e. Wheeny Creek, GroseRiver and Glenbrook Creek) appear to be largely unaffected. Two representative examples aregiven to illustrate changes in valley shape. Those of Burralow Creek are shown in Fig. 4a, whilenearby Morgan Creek cross sections appear in Fig. 4b.The cross-sections of both Burralow Creek and Morgan Creek exhibit a wide valley floor on theupstream (downthrown) side of the fault, and V-shaped profiles immediately downstream of thefault. This is consistent with uplift between the KFS and the Lapstone Monocline (cf. Bull andMcFadden, 1977). Downstream of the Kurrajong fault-line vertical incision appears to be outpacinglateral erosion, whereas the upstream side is controlled by the local base-level produced by the fault,therefore lateral erosion is dominant.FAULT ANGLE DEPRESSIONSAn overwhelming result of the long profile analysis is the fact that drainage modification is evidentfor all streams with catchment sizes up to and including that of Glenbrook Creek. This modificationis usually in the form of a marked reduction in stream gradient upstream of fault zones. Fieldevidence shows that this reduction in gradient is coincident with many swamps and lakes (eg.23
Seismic Hazard in SydneyProceedings of the one day workshopMountain Lagoon, Burralow Swamp, Blue Gum Swamp, Shaws Creek Swamp and WarrimooSwamp). Other smaller unnamed swamps are also juxtaposed with the Kurrajong Fault System.Figure 4: Valley cross sections across selected easterly flowing streams passing through thenorthern Lapstone Structural Complex. a) Burralow Creek, b) Morgans Creek.Smaller BasinsMost of the smaller basins investigated were found to have a marked paucity of sedimentaccumulation at the fault, the exceptions being Burralow Swamp, Blue Gum Swamp, and possiblyShaw's Creek Swamp. Morgan Creek Swamp was found to contain no more than 3 metres ofsediment at the fault, Winmalee Creek only 5 metres, and Warrimoo Creek only about 3.5 metres.Sediments are generally organic fine to medium sands, with minimal clay or silt fraction. In eachcase the maximum sediment depths were found in localised regions adjacent to a very narrowswamp. Away from the swamp axes, bedrock depth decreases sharply and sandy surface sedimentsare considered to be primarily slope wash deposits. Wide, flat floored basins first identified from airphotos as large accumulations of sediment are in fact largely bedrock controlled.These reaches of low gradient are considered to be caused by the imposition of a local base level atthe fault, and it seems that the only effect of fault displacement is to maintain this perched base leveland thus hinder incision upstream. Sedimentation could only have taken place immediatelyfollowing major uplift events, whereas incision into the uplifted zone would act against theaccumulation of sediments. The lack of sediments in the fault angle depressions, combined with the"neotectonic" features of the basin morphology suggests a long term average condition whereincision largely matches uplift. This is an indication of slow and persistent uplift along theKurrajong Fault System.24
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Landslides in the Sydney Basin - Geoscience Australia

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