The late Miocene–earliest Pleistocene Calivil Formation in central Victoria: coarse-grained alluvial deposition following tectonism

The Calivil Formation is an extensive coarse-grained alluvial formation deposited within narrow valleys across the central Victorian highlands. Palynological and stratigraphic evidence indicates that it is late Miocene in age; deposition continued into the late Pliocene or Pleistocene in some areas. Older palynological ages previously attributed to the Calivil Formation most likely represent either unrecognised or reworked Olney Formation sediments. The Calivil Formation dominantly comprises pebbly sandstones, sometimes trough cross-bedded and very occasionally planar-laminated, along with poorly sorted, matrix- or clast-supported pebble conglomerates, all deposited during flood flows. Occasional matrix-supported pebble-boulder conglomerates represent debris flows, and minor mudstones were mostly deposited in pools and abandoned channels. This lithofacies assemblage suggests that deposition occurred primarily within sandy braided streams during floods, which typically had, based on hydraulic calculations, average velocities of 2.1–2.5 ms−1, reflecting periodic intense storms impacting a dry, sparsely vegetated late Miocene landscape affected by regular fire. Calivil Formation sediments were also deposited on small deltas and perhaps in coastal lagoons along the shoreline at the maximum extent of the late Miocene–Pliocene transgression that inundated much of the Murray Basin. Calivil Formation deposition probably began in response to tectonism (Kosciuszko Uplift) during the late Miocene, as demonstrated by >134 m of incision prior to its deposition near the Great Divide; this exceeded any Cenozoic sea-level fall. Across the central Victorian highlands, the Calivil Formation occurs at lower elevations than the older, overall coarser-grained White Hills Gravel, which was deposited in broad valleys, probably in the latest Paleocene to earliest Eocene. Incision owing to the Kosciuszko Uplift reworked the White Hills Gravel into narrow valleys containing the Calivil Formation. Plio–Pleistocene alluvial gravel deposition similar to the Calivil Formation also affected southern Victoria.