RADIOMETRIC AGE CONSTRAINTS ON THE ORDOVICIAN-SILURIAN BOUNDARY FROM EASTERN CANADA AND SWEDEN

The transition between the Ordovician and Silurian periods is marked by the glaciation of Gondwanaland, eustatic sea level fall, a perturbation to the global carbon cycle and one of the “Big Five” mass extinctions of the Phanerozoic Eon. Because it is associated with a large amplitude (~130 m) glacioeustatic sea level fall, the Ordovician-Silurian (O-S) boundary is often marked by hiatus and exposure in the cratonic record. Exceptions are found in comparatively deeper localities, including the Anticosti Basin of Quebec, Canada and the carbonate mounds of the Siljan ring district in Sweden. Anticosti Basin, in particular, has emerged as one of the most well-preserved and complete records of geochemical and paleobiological change across the O-S boundary.

Here we present new ages on ashfalls collected from the Ellis Bay Formation (Anticosti) and the Boda Formation (Sweden), dated using U-Pb geochronology (CA-ID-TIMS) on single zircons. There is a serious need for radiometric ages in this crucial part of the Paleozoic, especially since the development of EARTHTIME U-Pb techniques has improved accuracy and allowed for dating of single zircon crystals at the <0.1% precision level.

Four of the fifteen horizons we collected contain euhedral zircon populations that indicate a volcanic origin.

These data provide new age constraints on these important sections, and will allow for better calibration of the rates of Earth system change during the Ordovician-Silurian transition. We also demonstrate how these data can be easily incorporated into Macrostrat, a geologic database that describes the age and properties of rocks in the upper crust. This integration will not only expose the data to a wide audience of interested users, but can also be used to update Macrotstrat’s age model, thereby improving synthetic results and analyses derived from the database.