Shallow-water microbialite-volcaniclastic facies association in the Cambro-Ordovician Mt Windsor Subprovince, Australia

Abstract

The Trooper Creek Formation is a mineralised submarine volcano-sedimentary sequence in the Cambro-Ordovician Seventy Mile Range Group, Queensland. Most of the Trooper Creek Formation accumulated in a below-storm-wave-base setting. However, microbialites and fossiliferous quartz-hematite plus or minus magnetite lenses provide evidence for local shoaling to above fairweather wavebase (typically 5-15 m). The microbialites comprise biogenic (oncolites, stromatolites) and volcanogenic (pumice, shards, crystal fragments) components. Microstructural elements of the bioherms and biostromes include upwardly branching stromatolites, which suggest that photosynthetic microorganisms were important in constructing the microbialites. Because the microbialites are restricted to a thin stratigraphic interval in the Trooper Creek area, shallow-water environments are interpreted to have been spatially and temporarily restricted. The circumstances that led to local shoaling are recorded by the enclosing volcanic and sedimentary lithofacies. The microbialites are hosted by felsic syneruptive pumiceous turbidites and water-settled fall deposits generated by explosive eruptions. The microbialite host rocks overlie a thick association (less than or equal to 300 m) of andesitic lithofacies that includes four main facies: coherent andesite and associated autoclastic breccia and peperite; graded andesitic scoria breccia (scoriaceous sediment gravity-flow deposits); fluidal clast-rich andesitic breccia (water-settled fall and sediment gravity-flow deposits); and cross-stratified andesitic sandstone and breccia (traction-current deposits). The latter three facies consist of poorly vesicular blocky fragments, scoriaceous clasts (10-90%), and up to 10% fluidally shaped clasts. The fluidal clasts are interpreted as volcanic bombs. Clast shapes and textures in the andesitic volcaniclastic facies association imply that fragmentation occurred through a combination of fire fountaining and Strombolian activity, and a large proportion of the pyroclasts disintegrated due to quenching and impacts. Rapid syneruptive, near-vent aggradation of bombs, scoria, and quench-fragmented clasts probably led to temporary shoaling, so that subsequent felsic volcaniclastic facies and microbialites were deposited in shallow water. When subsidence outpaced aggradation, the depositional setting at Trooper Creek returned to being relatively deep marine.