Some tectonic implications of sandstone petrofacies in the Coffs Harbour Association,New England Orogen,New South Wales

The Coffs Harbour Association, New England Orogen, consists of thick, monotonous units of Late Palaeozoic greywacke, laminated siltstone and mudstone, and massive argillite. The rocks of the association have a common provenance, being derived predominantly from a volcanic arc source consisting of mainly dacite, with minor andesite and rhyolite. The Coramba beds in the Coffs Harbour Block are divided into four petrofacies based on QFL data and the occurrence of detrital hornblende. Upwards, the petrofacies are: A—volcanolithic, B—feldspathic, C—horn‐blende‐feldspathic, D—hornblende‐volcanolithic. The petrofacies and vertical variation in non‐volcanic detritus indicate minor erosion and exposure of a non‐volcanic source, followed first by recommencement of volcanism, penecontemporaneously with sedimentation, then further erosion of the non‐volcanic source area. There was little temporal change in the character of volcanic detritus shed from the source area. Equivalents of the four petrofacies are recognised in other blocks of the association, although because of structural complexity, a complete A‐D sequence has not been found. The Coffs Harbour sandstones are similar to sands in modern ocean basins derived from an arc system of either continental margin or island arc type. The sandstones are not similar to recycled orogenic provenances, such as found in accretionary prisms or trench‐slope basins; the compositions suggest that the sandstones were deposited in either a forearc or backarc setting.