Resolving the Architecture of a Complex, Low-Accommodation Unit Using High-Resolution Sequence Stratigraphy and Ichnology: The Late Permian Freitag Formation in the Denison Trough, Queensland, Australia

The early Late Permian Freitag Formation comprises mudrocks, sandstones and thin conglomerates of shallow marine to coastal plain origin, formed along the western margin of the Bowen Basin, Queensland, Australia. The unit is up to 166 m thick in this area, thinning eastward (distally) and southward (laterally) into offshore deposits. The Freitag Formation is considered to be prospective for gas in the northern Denison Trough on the basis of several discoveries in channelized sandstone bodies, the origins and stratigraphic context of which are complex and not fully understood. A re-evaluation of the Freitag Formation, using outcrops, core data, and wireline logs has been completed, and the succession has been subdivided into four unconformity-bounded sequences. The unit, as a whole, can be described as a progradational-retrogradational sequence set, recording a long-term (∼1.5 m.y.) cycle of relative sea-level fall and rise, with shorter-term (?400 k.y.) cycles superimposed on this trend. Depositional facies include non-deltaic, nearshore marine deposits, subaqueous deltaic deposits, and coastal embayment/lagoon deposits. Distinction between these variants is based principally on ichnological criteria. Ichnological signatures throughout much of the formation and in most parts of the study area reflect the overall deltaic aspect of the succession. Key surfaces commonly contain palimpsest trace fossil suites, including many that clearly represent the Glossifungites Ichnofacies. The complex internal architecture is interpreted to be the result of repeated cycles of 10's of meters amplitude relative sea-level fluctuation, possibly related to ice volume fluctuations at the close of the late Paleozoic Gondwanan Ice Age. This revised model for the Freitag Formation suggests that there is an unaddressed potential for gas discoveries in channel sandstones at various levels within the formation.