Magnetic and chemical stratigraphy for the Werribee Plains basaltic lava flow-field, Newer Volcanics Province, southeast Australia: implications for eruption frequency

Abstract

Data from diamond drillholes and geophysically-logged wells have been combined with surface data to produce a stratigraphy for the Werribee Plains lava field, in the easternmost part of the Cenozoic Newer Volcanics Province, southeastern Australia. Outcrop is scarce in the Newer Volcanics Province, hence nine diamond drillholes (up to 110 m deep) and 40 wells have provided a fresh perspective of the geology of this intraplate basaltic lava flow-field. Two to three samples from every flow unit identified in drillcore were demagnetised (175 samples). The primary remanent magnetisation was identified in the majority of the samples and the results were used to establish a magnetostratigraphy. Samples that are palaeomagnetically unstable were identified and discarded from the study. Genetically related eruption packages were correlated between drillholes along a cross-section, based upon age constraints (previously obtained K – Ar ages and a 40Ar/39Ar age obtained in this study), geochemistry, volcanic facies, secular variations in the magnetic field, and the palaeotopography. Stratigraphic information from surface geology was based on bulk-rock geochemistry, magnetic-polarity data, airborne-geophysical data, and geochronology. Samples from the eruption centres and surrounding lava flows were analysed and classified using the CIPW normative classification scheme as tholeiitic and alkalic. Within these rock types, major element abundances were explored to identify any unique chemical subtypes. A geochemically distinct tholeiitic type was identified. Based on combining the surface geology with the subsurface data, the products of 33 discrete eruptions have been identified. These eruptions occurred between ca 4.6 and 1.4 Ma (within the Matuyama, Gauss and Gilbert Chrons of the universal magnetic-polarity time-scale), yielding an average eruption frequency for the Werribee Plains of one eruption every 97 000 years.