CONSTRAINING LATE TONIAN CLIMATE THROUGH U-PB GEOCHRONOLOGY OF THE GLACIGENIC KONNAROCK FORMATION FROM SOUTHWEST VIRGINIA, USA

Geological evidence such as the abundance of platform carbonates in Tonian stratigraphic sequences suggests that Earth was a “greenhouse” world prior to the Cryogenian global glaciations. Here we present recently published U-Pb zircon ages for rhyolite flows within glacially influenced lacustrine sediments of the Konnarock Formation in southwest Virginia, USA, that challenge that interpretation. The Konnarock Formation, consisting of laminated mud and siltstones with outsize clasts of Grenvillian granite-gneiss and putative drops stones, as well as massive diamictite, has an unconformable relationship with Laurentian basement and is itself unconformably overlain by the Ediacaran Chilhowee Group. Three rhyolite bodies within Konnarock Formation stratigraphy were sampled for U-Pb geochronology. Their ages range between ca. 752.6 and 751.3 Ma and obey stratigraphic order, strengthening the argument that these felsic rocks represent extrusive volcanic domes that constrain the age of glacially influenced lacustrine sedimentation in eastern Laurentia. The Tonian paleomagnetic record for Laurentia shows that this continent was positioned broadly in the tropics during deposition of the Konnarock Formation, presumably supporting the idea of a late Tonian (ca. 750 Ma) Snowball Earth. However, continuous and non-glacial temporally correlated stratigraphic records from western Laurentia preclude this possibility.

We therefore explore the climate implications of tropical glaciers outside of a Snowball Earth scenario in the context of paleoaltitude as lacustrine sedimentation does not have the prerequisite of deposition below sea level. We use three climate scenarios: 1) a Cretaceous-like “greenhouse”, 2) a modern/last glacial maximum (LGM) “icehouse” and 3) a hypothetical climate state colder than the LGM to interpret what the glacigenic Konnarock Formation means for global climate at the time of its deposition. Given the expected elevations for continental rift mountains and likelihood of preservation in the rock record we find the most parsimonious explanation to be that the Konnarock Formation was deposited at an altitude between 1 and 3.5 km in a global climate as cold-or colder than- the LGM. This prediction implies the climate transition to the Sturtian Snowball was not as extreme as previously imaged.