Stratigraphy and Sedimentation of Redbed Facies in Narrabeen Group of Sydney Basin, Australia

Abstract A study of a widespread redbed facies within the Narrabeen Group of the Sydney basin indicates that the material is a transported laterite that was deposited in a piedmont deltaic environment and was derived from two major source areas. These conclusions are based on paleocurrent data, mineralogic and petrographic data, and geometry and stratigraphy of the redbeds. The Triassic Narrabeen Group has a maximum thickness of 520 m and is dominated by clastic sediments that were deposited in a fluvial environment. The widespread redbeds within the group have a variable thickness. Redbeds of the western Sydney basin, previously uncorrelated with major redbed developments over the eastern part of the basin, are time-equivalent units and hence, redbed sedimentation covered an area of more than 13,000 sq km. These redbeds, which are present near the top of the group, range in thickness from 90 to 180 m and are defined as the “redbed facies.” The most significant redbed of this facies is the Bald Hill Claystone in the southern Sydney basin, which was found to correlate with the Wentworth Falls Claystone Member over the remainder of the basin; most of the data presented in this paper are centered around these units. Paleodrainage patterns within the “redbed facies” were determined on a basinwide scale and restricted to a vertical interval 30 m above and below the base of the Bald Hill Claystone and the Wentworth Falls Claystone Member. At a few widely spaced localities in the western Sydney basin, current data were compiled for complete Narrabeen Group sections. Paleodrainage direction in the section below the base of the Bald Hill Claystone and the Wentworth Falls Claystone Member is dominantly southeastward, with an abrupt change in direction to northeast above the base of the redbed units in the southern Sydney basin. A disconformity surface which separates the Bald Hill Claystone from the underlying units resulted when uplift in the south exposed large areas of volcanic rocks. Lateritization and transportation of this material by the newly developed northerly drainage system yielded the Bald Hill Claystone. Drainage in the northern area of the basin remained unchanged and was southeastward throughout Narrabeen Group sedimentation. From the central area of the basin where the two systems merged, drainage was eastward. Flint clays associated with the redbeds are distributed over three areas of the Sydney basin (the southeastern part, the Blue Mountains, and the Putty area), and occur at the same stratigraphic level. The oolitic flint clays of the southern region (the Garie Formation), which are thought to represent transported bauxitic material, were deposited rapidly on the flanks of exposed bauxitized areas on the eastern and southern margins of the basin. The other deposits, which are more massive and relatively devoid of oolites, represent floodplain deposits formed in low swampy depressions in the western and northern Sydney basin, by an internal drainage system.