Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopBurralow SwampBurralow Swamp (Figs. 1, 5) is an exceptional fault angle depression of the Kurrajong Fault Systembecause it contains a considerable sediment accumulation which has both lateral and stratigraphiccomplexity. Seismic tests on the cleared ground bordering the Burralow Fault (the "PaddockSands") show up to 20 metres of consolidated sediment with seismic velocities less than thoseexpected for sandstone bedrock (Rawson, 1990). The swamp surface itself is far larger in extentthan any other basin of the Lapstone Structural Complex, with valley wide swamp sedimentation inthe upstream half of the basin. The downstream half of the swamp is inset into and onlaps thePaddock Sands unit, which appears to be a deep and coherent deposit most likely related tocolluviation at the toe of the Burralow Fault scarp, which forms its eastern margin.Figure 5: Plan of Burralow Swamp showing the distribution of sedimentary units at thesurface, and the locations of boreholes and coreholes from Rawson (1990).25
Seismic Hazard in SydneyProceedings of the one day workshopFigure 5: Plan of Burralow Swamp showing the distribution of sedimentary units at thesurface, and the locations of boreholes and coreholes from Rawson (1990).Drilling carried out on the downstream end of the valley floor has shown that although the presentsurficial sediments are dominated by sands, there is evidence of a persistent large swamp or lake inthe past. This is indicated by an extensive deposit of organic clay/silt sediment (denoted theTabaraga Unit, Rawson 1990) which locally reaches up to 6-7 metres thick and extends down todepths in excess of 10 metres (Fig. 6). The unit rapidly thins in a downstream direction and pinchesout against a steeply rising bedrock bar relating to the Burralow Fault. We contend that this depositis the result of tectonic rejuvenation whereby movement of the Burralow Fault has imposed anew, relatively higher, perched base-level leading to disruption of drainage. Sandy strataextending from the top of the fines to the surface are interpreted to have been deposited duringthe progressive restoration of the creek to pre-faulting conditions.Figure 6: Longitudinal section through Burralow swamp showing the distribution of the clayrichTabaraga Unit, inferred to be related to uplift of 6-7 m on the Burralow Fault.Timing of Sedimentation in Burralow SwampHaving established that sedimentation within Burralow Swamp has been markedly affected bydisplacement on the Burralow Fault, the age of sediments within it should reflect the onset of atleast the most recent phase of activity. Two radiocarbon ages were obtained from smallcharcoal samples from borehole B-C4 (Fig. 5; Rawson, 1990). The first, at a depth of 12.90metres, came from coarse sandy sediments beneath the organic clays/silts of the Tabaraga Unit.This should indicate the age of pre-deformation fluvial sedimentation. The age obtained was28200 (+900/-800) BP (SUA-2873). The second sample, from within the clayey unit at 8.80metres, was dated at 35400 (+2400 -1990) BP (SUA-2874). The inversion of the dates isproblematic, and might relate either to contamination of one or both of the samples, orredeposition of the top sample. The fact that both dates lie close to the limit of the datingtechnique also reduces the confidence that can be placed in them.Sedimentological evidence does, however, mitigate in favour of accepting the dates, or at least alate Pleistocene age for the sediments. Both dates came from samples of clean, uninduratedcharcoal, which had no obvious contamination (eg. coatings, recrystallisation, or evidence of26
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Landslides in the Sydney Basin - Geoscience Australia

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