Influence of Magmatism and Tectonics on Sedimentation in an Extensional Lake Basin: The Upper Devonian Bunga Beds, Boyd Volcanic Complex, South-Eastern Australia

The Bunga Beds outlier of the Upper Devonian Boyd Volcanic Complex, south-eastern Australia, preserves a shallow- to relatively deep-water terrigenous sedimentary rock succession, associated syn-depositional rhyolites, minor basalts, and a variety of volcaniclastic facies, all interpreted to have formed in an extensional lake basin. Basin-margin facies associations are dominated by grey mudstones, with interbedded terrigenous and volcaniclastic sandstones and conglomerates, but facies intervals dominated by tractional sedimentary structures are limited. Local horizons are significantly bioturbated. These facies associations are interpreted as fan-delta (southern association) and delta (northern association) successions. They display initial abrupt (southern association) and gradual (northern association) upward fining representing trangression, then coarsening (representing progradation of deltas) and finally fining (deepening). They pass upwards and laterally into the deep-water basin-centre facies association, which is dominated by a classic turbidite facies association, including deep-water, black, pyritic shales, graded turbidites, slump and slide deposits and debris-flow deposits. The rhyolites were high-level, syn-depositional cryptodomes and partly emergent basin-floor domes; minor dykes also occur. The basalts are exclusively dykes and irregular intrusions, some with quench-fragmented and peperitic margins. Two groups of rhyolites are identified: a crystal-poor (< 12% crystals) group, which is commonly quench-fragmented, and a relatively crystal-rich variety (12-40% crystals), which is not quench-fragmented. Massive, unbedded, jigsaw-fit to clast-rotated hyaloclastite and auto-breccia are common facies of the rhyolites, and peperite margins also occur. Because bedded, resedi-mented, dome margin autoclastic deposits are a minor facies, the majority of the rhyolites were shallow, syndepositional cryptodomes. Dome-top intralacustrine explosive activity produced two preserved, bedded, dome-top tuff cone successions dominated by diffusely bedded, water-settled fall, crystal-tuffs, and pumice lapilli-stones. The provenance of the basin-fill succession is complex. Sandstones and conglomerates in both the basin-margin and basin-centre facies associations contain dominantly basement-derived detritus, including plutonic and vein quartz, and metasedimentary lithic fragments, representing an extralacustrine terrigenous epiclastic provenance. Varying fractions of variably reworked volcanic detritus, including quartz, plagio-clase and lithic debris, are mixed with the terrigenous sediments and represent an extralacustrine volcanic epiclastic provenance. One resedimented facies, however, consists of cuspate shards, and another of basaltic scoria suggesting an extralacustrine syn-eruptive resedimented pyroclastic provenance. Hyaloclastites, autobreccias and lava-associated peperites are attributed to an intralacustrine volcanic autoclastic provenance, whereas the intrusion-associated peperites have an intralacustrine syn-depositional intrusive autoclastic provenance. The tuff and pumice cone facies have an intralacustrine pyroclastic provenance. Resedimented autoclastic facies have an intralacustrine syn-eruptive resedimented autoclastic provenance, whereas resedi-mented pyroclastic debris has an intralacustrine syn-eruptive resedimented pyroclastic provenance.