DISCOVERY OF A PLANT MACROFOSSIL FROM LOWER PERMIAN GLACIAL MARINE MUDSTONES IN TASMANIA: IMPLICATIONS FOR PALEOCLIMATE DURING DEPOSITION OF POST GLACIAL TASMANITE HORIZONS

Late Paleozoic glaciation of the Tasmania Basin in Southeastern Gondwana was followed by widespread deposition of marine mudstones, generally ascribed to the post-glacial rise in sea level and inundation of a still glaciated landscape. The presence of algal cyst (Tasmanites) bearing shales with dropstones has been interpreted to indicate a sea ice and iceberg dominated post glacial environment for this transition. We recently examined the stratigraphic section exposed in the NW portion of Tasmania, where the Wynyard Tillite is overlain by the marine Inglis Siltstone and interbedded Tasmanite oil shales. Previous geologic mapping (1968) has indicated the presence of plant macrofossils close to the base of the Inglis Siltstone, but the exact affinities and stratigraphic context have remained unknown. We discovered an intact leaf imprint and carbonized films within the Tasmanite shale, some ~20 m from the basal contact of the Inglis Siltstone (Early Permian age).

The leaf venation pattern reveals a pronounced parallel arrangement, distinctively different than either Glossopteris or Gangamopteris forms, the typical plant assemblages associated with the Lower Permian of SE Australia. Instead we assign the fossil plant fragment to the Cordaitales specifically the Cordaites. This is one of the earliest known occurrences for this plant genera in the region and its presence in marine sediments suggests an unusual paleoenviornment. Cordaites are generally associated with non-marine coal bearing strata in most Gondwana. Our observation suggests that marine deposition of ice rafted debris (found in the sample bearing the plant specimen), sea ice marginal productivity (Tasmanites), and freshwater influx with plant leaves were all taking place in NW Tasmania soon after recession of glacial ice. The stratigraphic position of this fossil plant with respect to the Rhacopteris flora below (interbedded within varved siltstones and tillites of the Wynyard Tillite) and the Gangamopteris dominated flora above (found in upper portions of the Quamby Mudstone and Inglis Siltstone) suggest a much more varied terrestrial flora during and following glaciation than has been previously presumed ie., therefore more temperate than polar, despite the high paleolatitude (~80o S) of the Tasmania basin at this time.