Mires in the Dark: High Latitude Coals in the Jurassic Walloon Subgroup of the Surat Basin, Australia

Abstract

The Jurassic Walloon Subgroup in the Surat Basin is home to Australia's largest coal seam gas play, with 28,613 PJ of proven and probable reserves. Unlike many other coal seam gas plays, the Walloon coals are thin and discontinuous which creates a problem for correlation and effective exploration. Individual coal beds are <0.4 m thick and are rarely correlatable over areas >10 km². Previously thought to have been deposited during the Callovian in a nonmarine intracratonic basin, newly acquired ID-TIMS dates on zircons from volcanic tuffs in coals indicate deposition in the Oxfordian (between 162 and 158 Ma) and a basin subsidence rate of approximately 60 m/Myr that is more similar to subsidence rates of foreland basins. Plate tectonic reconstructions place the Surat Basin at high latitude (>75°S). Although that suggests the area would have experienced about three months of continual darkness each year, there is no evidence of ice formation at the time of deposition of the Walloon Subgroup and the global warming trend through the Oxfordian may have allowed mild temperatures year round allowing the development of mires at high latitudes. The restriction of primary organic productivity to the summer months would lead to relatively slow rates of peat accumulation in Walloon mires: rates that may have been substantially slower than the relatively fast subsidence rates. The thin coal seams may be explained by the relatively frequent drowning of mires and inundation by clastic sediments. Other Mesozoic high latitude coals, including those from the Cretaceous in Alaska and Antarctica, share similar characteristics to the Walloons Subgroup including a high inertinite content and thin, discontinuous coal beds.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015