Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopanywhere in the whole Sydney Basin, as defined by geologists, and even beyond, can affect theinner region and its man-made structures.The region consists dominantly of sedimentary rocks deposited in the Permian and Triassic periods(about 260 – 200 million years before present (B.P.)). In the Sydney region the Triassic rocks areexposed. The underlying Permian rocks are exposed on the south coast (from Stanwell Park andsouth), to the north around Lake Macquarie to Newcastle and in the valleys of the western BlueMountains. In the Sydney region the Permian rocks are at about 900m deep. In a few places these‘solid’ Triassic and Permian rocks have a relatively thin cover (say 100m) of mostly unconsolidatedsediments of Pleistocene to Recent age (2 million BP), notably around Botany Bay, the PenrithLakes, and Maroota (to the north west).Igneous activity has occurred, but is not extensive. It consists of both intrusions and extrusions. Inthe inner Sydney region these range in age from about 200 Ma to 15Ma. Volcanic necks such as atHornsby, Dundas, Nortons and Bents Basins, the distinctive Prospect intrusion, and numerous dykeshave been mapped. The dykes appear to be commoner in the eastern (coastal) area. This might be amatter of better exposure on the coast than across the Cumberland Plain, although dykes arecertainly rare in the Blue Mountains.While the sedimentary succession is generally flat-lying, there is clear evidence of minor foldingduring deposition, and there is also post-depositional folding and faulting. Some of this postdepositionaldeformation can be attributed to the last phases of movement of the Hunter-BowenThrust System (Moelle and Branagan, 1987; Mills et al, 1989; Branagan, 2000a and 2000b), butother later activity occurred, mainly in the period 80 – 20 Ma.Even within the “inner Sydney Basin” we can recognise smaller basins and domes, which reflectthese deformations. The most obvious is the Botany Basin, which is also, incidentally, the site of thethickest succession of young sediments (Tertiary – Cainozoic). These sediments will react to seismicshaking quite differently to the older, consolidated rocks, as happened in the Newcastle regionduring the 1989 earthquake.Evidence indicates that both compression and tension have occurred during and after deposition, andthere appear to have been two long periods of tension – at the end of major deposition (LateTriassic), and Early Tertiary (sea-floor spreading), as indicated particularly by the dyke intrusions.POSSIBLE SITES OF SEISMIC ACTIVITYEmphasis has been given to the Lapstone Structural Complex as a likely site of strong seismicactivity (based on past seismic records. However it is pertinent to point out other structuralweaknesses that have been mapped in the Sydney region in the past thirty years. Although thesemostly show little or no signs of geologically recent activation, they should not be entirelydiscounted as sites of seismic activity. At the various sites mapped a variety of fault types canbe recognised. They include normal, steep reverse, overthrust, strike-slip and fault breccia(crushed rock) varieties. It is not unusual to find several different fault types in close proximity.This is not to be unexpected, as once a rock mass has been weakened by faulting it is quitelikely to be the locus of further faulting, and although one might expect similar movements ashad taken place previously, the weak material could easily deform in a new way. Although alittle removed from the “inner Sydney Basin”, the fault systems mapped just east of the MooneyMooney Creek Bridge on the F 4 Freeway, are a particularly good example of such deformation.Here reverse folding/faulting, normal and strike-slip faults have been recorded (Mills et al., op.cit.).8