An important example of slope instability in thickly bedded Carboniferous sandstones in the New England Fold Belt Geology of Eastern New South Wales is described and analysed. The zone of instability affects an area of 300m by 100m on an undeveloped hillside of gentle to moderately steep topography. Interesting morphological features of this style of sliding are described. The mechanism of movement is found to involve sliding of large, joint-bounded blocks of gently dipping, thickly bedded sandstone on thin siltstone inter-bed lenses. The role of the two sets of sub-orthogonal, primary non-diastrophic joints in forming and releasing the blocks is recognised. Block movements are found to be only partly governed by the interaction between the topography and the dip of the strata. An explanation of the additional influence of water pressures in joints on the kinematics of the sliding blocks is provided. The important roles of site clearing and anomalous climatic conditions in triggering the slide are also discussed.
Slope instability in the Permian and Triassic geologies of the Sydney Basin of eastern Australia is well known and well documented. Of these 38 reports, none were specific to the Carboniferous or Devonian geologies of the New England fold belt. It is the purpose of this paper to introduce the phenomenon of slope instability in mildly folded sedimentary sequences, such as the Carboniferous strata of the New England fold belt, by describing and analysing an impressive instability feature of considerable significance. The feature is a translational block slide in thickly bedded, dipping, non-marine sandstones. It is described in extensive detail in an article prepared for a local conference on engineering geology in the Hunter Valley (Fityus et al., 1998), and it is summarised here to promote its exposure to the wider geotechnical community.
