A biostratigraphy of the Late Permian and Triassic of the Sydney basin

Abstract

The stratigraphic framework of the Late Permian to Triassic sequences which comprise the upper coal measure successions, the Narrabeen Group, the Hawkesbury Sandstone, Wianamatta Group and equivalent strata in the Sydney Basin is outlined together with a brief summary of the available biostratigraphic information. The vertical distribution of microfossils from 252 samples, arranged in 14 sample sequences, is illustrated. Taxonomic treatment of 119 species comprising 74 genera of spores and pollen is undertaken and 19 species of microplankton are briefly described. Nineteen species and three genera are proposed as new taxa. On the basis of the vertical distribution and relative prominence of the described palynomorphs five assemblage zones are delineated. These are in descending order, 1. The Faloisporites Assemblage Zone (four zonules identified) 2. The Protohaploxypinus samoiloviohii Assemblage Zone 3. The Lunatisporites pelluoidus Assemblage Zone 4. The Protohaploxypinus retioulatus Assemblage Zone (two zonules identified) 5. The Dulhuntyispora Assemblage Zone. The Dulhuntyispora Assemblage Zone occupies the coal measure sequences below the base of the Narrabeen Group, The junction of the Dulhuntyispora Assemblage Zone and the Protohaploxypinus retioulatus Assemblage Zone is coincident with the base of the Narrabeen Group, the latter zone occurring in the lower part of the Narrabeen Group. The Lunatisporites pelluoidus Assemblage Zone and the Protohaploxypinus samoiloviohii Assemblage Zone, which exhibit essentially homotaxial microfloras, occupy the central portion of the Narrabeen Group succession, The Faloisporites Assemblage Zone extends from the upper portion of the Narrabeen Group, through the Hawkesbury Sandstone to the top of the Wianamatta Group, However, the unatisporites pelluoidus Assemblage has not been encountered in the northwest portion of the basin. The microfloral sequence of the Sydney Basin compares closely with other Gondwana occurrences, particularly those of Western Pakistan and Madagascar. The individual assemblages are examined in terms of equivalent microfloras from a number of phytogeographic provinces. Environmental synthesis suggests that the base of the Narrabeen Group coincides with a substantial erosional hiatus. The environment of deposition of the sediments above the hiatus is interpreted as brackish or shallow marine on the basis of the geometry of the basal sequence and the occurrence of a diverse and abundant acritarch assemblage, Evidence of the co-existence of the floras represented by the Lunatisporites pelluoidus Assemblage, the Protohaploxypinus samoilovichii Assemblage and the Faloisporites Assemblage is illustrated and an interpretation of the environments of the individual floral communities is attempted. Despite meagre data, an erosional Hiatus is indicated between the top of the Narrabeen Group and the Hawkesbury Sandstone. The contemporaneous deposition of the Hawkesbury Sandstone and the Ashfield Shale is discussed, The similarity of the microfloras of these formations, together with their mutually adjacent depositional areas is interpreted as supporting the concept of a tidal delta-barrier bar origin for the Hawkesbury Sandstone.