Structural analysis of wavecut platform structures, Shellharbour, New South Wales

Lennox, Paul1, Goff, James1,2, Edwards, David1 & Coates, Ashlie1,3

1The University of New South Wales, Sydney, Sydney, Australia, 2School of Ocean and Earth Sciences, University of Southampton, Southampton, UK, 3Department of Industry, Innovation and Science, Canberra, Australia

Bedrock-sculptured features on the wavecut platform south of Shellharbour, NSW have been structurally assessed as some consider they have been formed by a southeast to northwest directed (mega)tsunami. These features have formed within the up to 120 m thick, Late Permian Upper Bumbo Latite unit of the Gerringong Volcanics.

The research focused on two subareas; the southern side of Bass Point (Bushranger Bay) where apparent s-forms due to tsunami-sculptured rock platforms expose so called “muschelbruche” having a relief of decimeters to several metres and Atcheson Rock one kilometre to the southwest. At Atcheson Rock the orientation of the channel and the so called vortex nearby; a semi-circular depression with raised internal feature were assessed with respect to the orientation of the local jointing, layering and flow foliation.

The muschelbruche vary in size from a few metres across to tens of metres across. The orientation of bedrock-sculptured features south of Bushranger Bay display various relationships to the flow foliation and jointing. In some cases the sides or raised back of the “muschelbruche” are aligned subparallel to either the flow foliation, joint strike or some combination. The long axis of the muschelbruche are not predominantly aligned northwest-southeast as expected if they were generated by a single (mega)tsunami from the southeast.

The cross-cutting ? Cenozoic dyke at Atcheson Rock weathers to a channel in the basalt and is along strike of a similar eroded dyke and channel on the coastline south of Bushranger Bay. The strike of this dyke is subparallel to the strike of flow foliation and the dominant jointing in the basalt.   Some assert that the canyon containing this dyke is not structurally controlled. Normal coastal erosion of this dyke has caused Atcheson Rock headland to almost be cut off from the mainland. The Cenozoic dyke has intruded subparallel to the dominant northeast to southwest striking joints and flow foliation in this region. Some consider that the vortex pit on the south side of Atcheson Rock was eroded by a tsunami from the southeast causing a whirlpool over 10m wide and characterised by a central plug of bedrock 5m high. The presence of a  rounded but north-south elongated depression in the basalt (“vortex”) can be explained by weathering related to the pervasive localised NNE-SSW and ~ NW-SE striking jointing, gently southeast-dipping layering in this locality but not the NE-SW or NW-SE striking flow foliation.

The orientation, scale and development of various coastal erosional features including channels, scour-like structures (muschelbruche) and depressions near Shellharbour, NSW can be explained through normal coastal processes without the necessity to invoke (mega)tsunami-related scouring from the southeast.


Biography

Paul Lennox has over thirty years experience teaching undergraduates and undertaking field-based map-based research in Eastern Australia. He has published papers on deformation in granites and their enclosing country rocks in the Lachlan Orogen   and  the  tectonic development of the Southern New England Orogen.

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