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  Identification of a Fine "Tuff" Lamination in the Rouse Hill Siltstone Member of the Ashfield Shale, Sydney Basin, Australia and Its Implications

  The Sydney Metro is Australia’s largest public transport project, and historical and recent ground data is being collected and used at an unprecedented scale to create robust rock models for the proposed tunnel alignments. Geotechnical investigations target the Triassic Wianamatta Group, Mittagong Formation and Hawkesbury Sandstone of the Sydney Basin. Despite these geological units being well documented, new knowledge gained during the Sydney Metro projects is enabling a better understanding of the Wianamatta Group. In particularly, the Rouse Hill Siltstone Member is typically described as a homogenous siltstone unit at the base of the Wianamatta Group. However, this unit often includes a distinctive white “tuff layer” lamination, which is typically ignored in geological models. Data from 100+ historical and recent boreholes across Sydney indicates that, where present, this lamination is remarkably uniform across the central Sydney region and sits horizontally at approximately 3 m above the base of the Rouse Hill Siltstone Member. This marker horizon assisted in identifying vertical offsets in the strata caused by faulting. The characterisation and identification of this layer will greatly improve our understanding of the geology across Sydney and allow for efficient targeting of geological structure for future tunnelling projects in these units.

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