Geological Note: Hummocky cross‐stratification from the Boxvale Sandstone Member in the northern Surat Basin, Queensland

Abstract

The Boxvale Sandstone Member (BSM) in the upper part of the Early Jurassic Evergreen Formation, in the northern Surat Basin, consists of up to 90 m of interbedded mudrocks and sandstones, with minor coal and ironstone. The unit was interpreted by Exon (1976) as representing a marine transgression over a coastal plain environment. The upper part of the BSM consists of a 10-40 m thick coarsening-upward sequence and contains several very well-sorted quartzose sandstone beds with trace fossils and low-angle cross-bedding. Mollan et al (1972) suggested that the physical and biogenic sedimentary structures, good primary porosity and concentrations of heavy minerals within these sandstones pointed towards deposition in a lacustrine shoreline setting. Exon (1976) suggested that the presence of acritarchs was more consistent with a marine origin, and Gerrits (1985) considered these beds to represent sediment accumulation in marine shoreface and offshore environments. All evidence gathered to date is ambiguous in that it does not allow discrimination between possible marine or lacustrine environments of deposition. Recent fieldwork has resulted in the recognition of sedimentary structures, including hummocky cross-stratification, which have a bearing on this problem. Hummocky crossstratification has not previously been identified in the Evergreen Formation.