Discovery of probable ice‐rafting in the Late Mesozoic of the Northern Territory and Queensland

Abstract

Isolated rounded clasts up to small boulder size have been discovered in fine‐grained strata of Jurassic to Cretaceous age in the eastern Northern Territory and northwestern Queensland, between Borroloola and Burketown. The dominantly silty strata have previously been assigned to the Mullaman Beds and the Gilbert River Formation and fossil evidence indicates a partial late Aptian age but their full age range is uncertain. These sedimentary sequences were mainly deposited on the shallow marine marginal shelf of the southwestern Carpentaria Basin. The mechanism of transport of the clasts into basin sediments is considered to have been rafting by ice, since fossil driftwood remains are small and there is no evidence of rafting in floating tree roots, or of lateral emplacement by swift currents. In all respects these deposits resemble clast‐bearing mudstone of approximately the same age as those in the Eromanga Basin in South Australia and New South Wales, which also are considered to have been ice‐rafted. At the time of deposition the localities would have been situated at palaeolatitudes of 55–65°S. Strongly seasonal climates are inferred for this part of Australia in the Late Jurassic‐Early Cretaceous.