THE LAKE SUPERIOR AREA "EVENT LAYER": TESTING THE CONNECTION WITH THE 1850 MA SUDBURY IMPACT

The Sudbury structure, with its thick meltsheet precisely dated at 1850 Ma, is the largest partially preserved, deformed, impact crater in the known geological record. Estimates for the final crater diameter are ~300 km, based on scaling relationships and the diameter of the annular syncline that surrounds the large central uplift (Bleeker et al., 2024, ILSG abstract & refs.). Using Chicxulub as analog, the ejecta and fall-out layer of the Sudbury impact should be global, at least in places where ca. 1850 Ma stratigraphy is sufficiently preserved, although at distal localities this layer may be thin (cm scale) and difficult to recognize.

To date, only the distinct “event layer” in Paleoproterozoic rocks of the Lake Superior area has been postulated to represent the Sudbury ejecta and fall-out layer. Current constraints on this event layer, including Ir enrichment, are consistent with this hypothesis. Yet, a similar event layer has not (yet?) been recognized at other localities of ca. 1850 Ma stratigraphy, in Canada or beyond. Some of these localities are at comparable radial distances from a “ground zero” at Sudbury, e.g. Mistassini Basin. This has prompted us to further test the putative link between the Lake Superior event layer and the Sudbury impact.

First, we are refining the ages of felsic tuffs at the base of the overlying Rove Fm shales. Initial data suggest the first tuff layers are as old as 1842 Ma, thus tightening the permitted time interval for the event layer to 1842–1856 Ma. Second, the Peavy Pond granodiorite, which cuts the event layer stratigraphy south of Lake Superior, had a previous age estimate (SHRIMP) of 1850±11 Ma, thus potentially falsifying the link with Sudbury. New CA-TIMS zircon data show this granodiorite to be 1829±2 Ma, therefore allowing the link with Sudbury. Third and most definitive, we have searched for “tracers” in the Lake Superior event layer that can be uniquely tied to Sudbury. Shocked ca. 2460 Ma zircons, from the Copper Cliff Rhyolite and its subvolcanic intrusions, exposed in the central uplift and unique to the Sudbury area, provide such a tracer. In a large sample of the event layer from west of Thunder Bay, Ontario, our data identify a distinct population of 2460 Ma zircons, one with probable shock features. These zircons can only represent direct delivery of Sudbury ejecta. Where else can we identify the global Sudbury fall-out layer? Got to be there!