Date of this Version
Outcrop of the oldest Cretaceous sequence in the Giralia Anticline and the Giralia No. 1 well, penetrating the same sequence, are described and biostratigraphically assessed in detail. The Cretaceous rocks lie on an erosion surface cut into Permian strata. A 10 m thick basal sand unit, the Birdrong Sandstone, is overlain by 56 m of carbonaceous siltstone-mudstone (Muderong Shale). The Birdrong Sandstone in the anticline belongs to the Muderongia australis Zone of late Hauterivian-Barremian age, as does the lower part of the Muderong Shale. The age of the upper Muderong Shale is uncertain, as is the age of a 10 m thick sandstone unit (probable Windalia Sand Member) which separates the Muderong Shale from the late Aptian Windalia Radiolarite. Abundant fossil conifer wood, much of it with Teredolites borings, is present in outcrop referred to the upper part of the Birdrong Sandstone. Scattered ichthyosauran bones, probable plesiosauran remains, and rare ammonites also are present at this level. A growth-ring analysis of the wood suggests that-sa seasonal humid mesothermal climate prevailed in the region. Changes in sediment composition, palynomorph assemblages, and foraminiferal biofacies reflect retrogradation of marine facies during deposition of the Birdrong Sandstone and lowermost Muderong Shale, followed by aggradation through most of the Muderong Shale with maximum water depths less than 50 m. Within the sequence, the Birdrong Sandstone and the lowermost Muderong Shale represent a transgressive systems tract, whereas most of the Muderong Shale belongs to a highstand systems tract. The sequence reflects a transgressive pulse that was part of the progressive submergence of vast areas of the Australian continent during the Early Cretaceous. This late HauterivianBarremian transgressive pulse is recognised in widely separated basins and may represent a synchronous continent-wide sea-level rise.