Landslides in the Sydney Basin - Geoscience Australia

More documents

Recommendations

Info

Seismic Hazard in SydneyProceedings of the one day workshophumic acids) (Colin Murray-Wallace, verbal communication). The rest of the sediments withinB-C4 also exhibited no iron induration, regardless of sediment type. This is unusual, consideringthat deep weathering profiles of only late Pleistocene age have been identified in alluvium onthe nearby Nepean floodplain (Young et.al., 1987). Furthermore, the sediments were notcemented in any way, were not compacted, and there was no evidence of secondarycrystallisation (diagenesis). In short, no indicator could be found to suggest that these sedimentswere Tertiary in age.Eight samples were selected for pollen analysis from the Tabaraga Unit in boreholes B-C3 andB-C4 (Rawson, 1990). All of the samples indicate a Eucalyptus and Casuarina dominatedsclerophyllous forest with a heath understorey. Significantly, this forest is seen to be dry,having few ferns and virtually no rainforest component. Nothofagus pollen is also noticeablyabsent. This pattern is essentially modern in character i.e. definitely Late Pleistocene -Holocene, with no Tertiary or early Pleistocene evident (J. Dodson, personal communication).The presence of some `small spined' Asteraceae grains perhaps indicates a pre-Holocene age forthe sediments, as this particular taxon became extinct during the latest Pleistocene (cf. D'Costaet.al., 1989).The sediments within the basin show gross characteristics consistent with a rapid change in flowregime caused by a rise in local base level at the fault, followed by a gradual re-establishment ofequilibrium conditions. The dating results and sedimentology indicate that this change in flowregime occurred during the late Pleistocene.DISCUSSIONBoth the morphometric analysis of regional drainage and sedimentological evidence from fault angledepressions imply a significant influence of the Lapstone Structural Complex on landscapeevolution in the Lower Blue Mountains. The majority of stream reaches downstream of the KFSexhibit marked oversteepening which, for the most part, cannot be explained by lithological control.It is postulated therefore that the majority of streams have not had sufficient stream power to keeppace with uplift focused on the KFS and Lapstone Monocline. The incision rate estimated for theGrose River (
Seismic Hazard in SydneyProceedings of the one day workshopThere is an apparent conflict between this evidence for recent activity and the palaeomagnetic dataindicating that folding related to the Lapstone Monocline occurred prior to 8 ± 5 Ma (Bishop et.al.,1982 with ages recalculated by Pillans, 2003). Given that the Kurrajong Fault System and theLapstone Monocline form part of the same structure (Herbert, 1989; Fergusson, this volume),deformation on the one might be expected to be accompanied by deformation on the other. Onepossibility, which is testable by careful field observation, is that the blind fault underlying theLapstone Monocline (see Gibson, this volume) reached the surface in the Pliocene and deformationsince then has been accrued by sliding along the fault rather than by folding. Alternatively, therecent active period may have been the first since the late Tertiary (cf. Branagan & Pedram, 1990) orMesozoic (cf. Pickett & Bishop, 1992; Schmidt et al., 1992).Given the very low uplift rates postulated for the LSC, passive exhumation (cf. Pickett and Bishop1992) is likely to have played a significant role in the development of the contemporary landscape.In particular, the fact that the Tertiary Rickabys Creek Gravels overly Wianamatta Group shales onthe Cumberland Plain, and Hawkesbury Sandstone on the face and top of the Lapstone Monocline,implies that erosional lowering of the landsurface had stripped shale from many areas west of thesyn-depositional warp (proto-Lapstone Monocline) by the time of gravel deposition, but not to theextent that significant relief was generated [as indicated by structure within the gravels suggesting abraided stream depositional environment, Bishop & Hunter (1990)]. However, we demonstrate asignificant tectonic imprint in the geomorphology of streams crossing the LSC, as distinct from thelithologic control that would be expected to dominate in a scenario only involving passiveexhumation. Additional research, guided by new erosion rate data obtained on streams withcatchment areas significantly smaller than the Grose River, is required to determine definitivelywhether this tectonic imprint relates only to the last active period identified in the sediments ofBurralow Swamp, or to periodic activity throughout the late Tertiary.Intuitively, the prominent ‘V-notch’ valley profiles of streams between the Kurrajong Fault Systemand the Lapstone Monocline suggest a recent tectonic history extending beyond the last activeperiod. Furthermore, it is tempting to speculate that the apparent associations between the LSC andMiocene volcanics (Pedram, 1983; Branagan and Pedram, 1990; Van Der Beek et al., 2001) relate tolate Tertiary development of the complex, synchronous with the establishment of the current crustalstress field, and a pulse of tectonic activity recorded in all southeast Australian basins (Dickinson etal., 2002; Sandiford et al., 2004). A terrestrial analogue to this pulse of deformation is seen in theLake George Fault, which has accumulated up to 200 m of throw since the late Miocene (Abel,1985).CONCLUSIONSThe results of morphometric analysis of drainage lines crossing the northern Lapstone StructuralComplex show indicators of geologically recent, and potentially ongoing tectonic deformation.Stream long-profiles oversteepen as they cross major faults, valley cross sections narrow to ‘Vnotches’where streams enter the upthrown side of fault blocks, and several streams have beenpartially dammed, forming swamps and lakes. While these indicators are consistent with a model ofperiodic (perhaps temporally clustered) deformation of the Kurrajong Fault System, near to thethreshold conditions of uplift/incision, additional erosion rate data is required to determine therelative contribution of passive exhumation of syn-depositional basin structure in forming thepresent landscape.The important question to ask relating to seismic hazard is whether faults of the LSC are still withinan active period, or whether we are now in a quiescent period that might last many hundreds ofthousands to a million years or more (cf. Crone et al., 2003). Earthquake epicenters located at depthto the west of the LSC (Gibson, this volume) potentially indicate that the most recent active periodhas not yet finished.28
Page 5 and 6: Seismic Hazard in SydneyProceedings
Page 31: Seismic Hazard in SydneyProceedings
Page 65: Seismic Hazard in SydneyProceedings
Page 82 and 83:
Seismic Hazard in SydneyProceedings
Page 84 and 85:
Page 86 and 87:
Page 88 and 89:
Page 90 and 91:
Page 92 and 93:
Page 94 and 95:
Page 96 and 97:
Page 98 and 99:
Page 100 and 101:
Page 102 and 103:
Page 104 and 105:
Page 106 and 107:
Page 108 and 109:
Page 110 and 111:
Page 112 and 113:
Page 114 and 115:
Page 116 and 117:
Page 118 and 119:
Page 120:
show all

Landslides in the Sydney Basin - Geoscience Australia

Create successful ePaper yourself

Delete template?

Save as template?