GEOCHRONOLOGY OF THE GASKIERS GLACIATION

The Neoproterozoic stratigraphy of the Avalon terrane in Newfoundland includes glacigenic diamictites of the Gaskiers Formation, which are thought to capture a possible Snowball glaciation event. The Snowball Earth hypothesis requires a several million year duration for glaciation in order for atmospheric CO₂ from volcanic outgassing to reach high enough levels to trigger deglaciation, allowing potential Snowball events to be tested using geochronological constraints. The abundance of interbedded volcanic tuffs in the Ediacaran marine succession of Avalon provides a unique opportunity for radioisotopic geochronology. However, complexities in regional stratigraphy and open-system behavior in the U-Pb isotopic system have made it difficult to determine the spatial and temporal extent of the Gaskiers glaciation on timescales necessary to test the Snowball hypothesis. Recent recognition of the Trinity diamictite on the nearby Bonavista Peninsula, along with advances in the high-precision chemical abrasion thermal ionization mass spectrometry technique for U-Pb zircon geochronology, have allowed for new precise and accurate age constraints on the glacial event. Analyses of volcanic tuff samples stratigraphically below, within, and above the diamictite facies and evidence for ice constrain the duration of the Gaskiers glaciation locally to < 1.86 Myr on the Avalon Peninsula and < 710 kyr on the Bonavista Peninsula, using internal error. With the additional constraint that deglaciation was roughly synchronous across the peninsulas, the glacial event represented by the Trinity diamictite has a maximum duration less than 340 kyr. The short duration of this event makes it unlikely to have been a Snowball event.