U-PB LA-ICPMS DETRITAL ZIRCON GEOCHRONOLOGY OF THE UPPER CRETACEOUS WINTON FORMATION, QUEENSLAND AUSTRALIA: IMPLICATIONS FOR REGIONAL TECTONICS, SEDIMENTARY PROVENANCE AND VERTEBRATE PALEONTOLOGY

This study utilizes detrital zircon geochronology to constrain the depositional age, tectonic setting, basin evolution, and stratigraphic context of the poorly understood Winton Formation, northeastern Australia. To develop better context for Australia’s most significant dinosaur-bearing succession, we analyzed 13 samples from different stratigraphic levels and key fossil locations throughout the Winton and underlying units (basin wide). Detrital zircon ages were obtained by U-Pb LA-ICPMS dating and the results were subjected to seven different metrics to interpret maximum depositional age. The results considerably improved upon earlier palynological age constraints, suggesting that most of the key vertebrate fossil localities in the Winton Formation are no older than earliest Turonian to latest Cenomanian (92-94 Ma). The most abundant detrital zircon population clusters between 92-115 Ma, suggesting that much of the volcanic-rich sediment that characterizes the Winton Formation were syndepositionally eroded from an active continental volcanic arc system located along the eastern margin of Australia (presumably the Whitsunday Volcanic Province). This volcanic arc activity was not a singular event; rather we identified near-continuous detrital zircon grain ages between 92-330 Ma, indicating intermittent arc volcanism along that eastern margin due to continuous slab subduction of the Phoenix/Pacific Plate under the eastward migrating Gondwana margin. A particularly interesting result is the identification of those Jurassic grain populations; previously a period not associated with significant arc magmatism. In addition to this, multiple populations have been identified from other, older easterly lying sources (330-900 Ma) including but not limited to the Macrossan, Anakie, Cape River, Greenvale and Georgetown Provinces. Small populations of apparently recycled Proterozoic and Archean grains are also variably present in each of the samples. Transport of non-recycled populations derived from easterly to northeasterly sources is interpreted to be a result of transverse fluvial systems flowing into westerly lying continental basins. These results provide significant context for understanding temporal emplacement and sources for fossil entombing sediments of the Winton Formation.