The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash

The Gunflint Formation, a Paleoproterozoic chemical-clastic sedimentary assemblage outcropping to the immediate northwest of Lake Superior, became famous in 1954 as containing the oldest fossil assemblage known at that time. Older microfossils have since been discovered, but the Gunflint procaryotes remain one of the most diverse Precambrian fossil communities. The finding of possible multicellular organisms in correlative lithic units in Michigan has recently added to the need for an exact age of the Formation. Zircons were extracted from rainout and storm reworked volcanoclastic beds in the upper portion of the Gunflint Formation. A euhedral zircon population has yielded a 1878.3 +/- 1.3 million years BP U-Pb age, believed to be nearly synchronous with the depositional age. This not only provides a precise age for the community of organisms, but also strongly supports a back-arc extensional setting for the Animikie Basin, rather than a foreland trough.