New data on the geology and geochronology of the area south of Tooraweenah, New South Wales

Abstract

New mapping of the Tooraweenah-Bearbong region, northeast of Gilgandra in north-central New South Wales, has been assisted by recently acquired geophysical and other remotely sensed data from the Brigalow Belt South Bioregion audit. Geological units in the region are mainly units from the Mesozoic Surat Basin and overlying Miocene Warrumbungle Volcanics. Revision to previous mapping has been possible and new geological units have been recognised. Radiometric imagery, in particular, has facilitated mapping and interpretation, especially of regolith material. New K-Ar geochronological data have confirmed the presence of Jurassic mafic igneous rocks (Garrawilla Volcanics and Glenrowan Intrusives) and, further, that trachyte in the Dilly hills area is contemporaneous with magmatic activity that formed the Warrumbungle volcano in the Middle Miocene. Petrological data have confirmed the dominantly mafic alkaline compositions of the Garrawilla Volcanics and Glenrowan Intrusives. The data indicate a largely unfractionated mantle source for these units and are consistent with an intracontinental extensional tectonic setting for their emplacement. Certain kaolinite clayrocks in the Purlawaugh Formation have volcanic fragmental protoliths, probably related to underlying Garrawilla Volcanics. The widespread Pilliga Sandstone displays development of a ferruginised palaeosurface, possibly related to other extensive Late Cretaceous to early Tertiary palaeosurfaces in eastern Australia. There has been subsequent erosion of the palaeosurface, as well as formation of the Warrumbungle volcano in the Miocene. Lavas and small intrusions related to the southwestern margin of the Warrumbungle volcano are present, and range from hawaiite to quartz trachyte in composition. The Dilly trachyte has a composition intermediate between mafic trachytes and strongly fractionated peralkaline trachytes in the main part of the Warrumbungle volcano. During and after formation of the volcano, outwash deposits of trachyte clast-dominated conglomerate were formed. The conglomerate predated development of the current landscape with extensive cover of transported regolith and fluvial systems. Erosion has led to base lowering of at least 100 m-300 m since the Middle Miocene within and adjacent to the Warrumbungle volcano.