The geology of Darwin Crater, western Tasmania, Australia

Darwin glass is a siliceous impact glass found in a 400 km2 strewn field near Mt Darwin, western Tasmania, Australia. It has been dated by Ar–Ar methods at 816 ± 7 ka. A 1.2 km diameter circular depression, named Darwin Crater (42°18.39′S, 145°39.41′E), is the assumed source crater for the glass. Darwin Crater is situated in a remote rain forested valley developed within Siluro–Devonian quartzite and slate (Eldon Group). Earlier geophysical investigations demonstrated that the structure is an almost circular bowl-shaped sediment-filled basin. This paper provides the first detailed description of the geology of Darwin Crater. The centre of the crater has been penetrated by two drill cores, the deeper to a maximum depth of ∼ 230 m. The drill cores intersected fine-grained lacustrine sediments (∼ 60 m thick) overlying poorly sorted coarser crater-fill deposits. The pre-lacustrine crater-fill stratigraphy comprises an uppermost polymict breccia (∼ 40 m thick) of angular quartz and country rock, which contains very rare (≪ 1%) fresh glass fragments (Crater-fill Facies A). Beneath the polymict breccia facies, the drill core intersected monomict sandy breccias of angular quartz (Crater-fill Facies B), and a complicated package of deformed slate clasts (Crater-fill Facies C). Quartz grains in the crater-fill samples contain abundant irregular fractures. In some of the most deformed quartz grains, sub-planar fractures define zones of alternating extinction that superficially resemble twinning. Kinked micas are also present. While the deformation observed in clasts of the crater-fill facies is far greater than in rocks cropping out around the crater, no diagnostic shock indicators, such as planar deformation features (PDF's) in quartz, were observed. If the crater is of impact origin, as seems likely due to the close association with Darwin glass, this is another example of a simple crater where diagnostic shock indicators appear to be absent, preventing confirmation of an impact origin by petrographic analysis of crater-fill samples alone.