DISTAL EJECTA FROM THE 1850 MA SUDBURY IMPACT IN THE LAKE SUPERIOR IRON RANGES

A distinctive breccia layer containing fragments of ejecta from an impact crater has been found in or near five of the iron ranges in the Lake Superior region in a 70,000 km2 area. The ejecta include shock-metamorphosed quartz grains as well as abundant particles of devitrified and altered glass, probably formed from impact-generated melt. Radiometric ages of stratigraphically bounding volcanic units bracket the age of the breccia between 1878 and 1836 Ma indicating that it is likely distal ejecta from the 1850 Ma Sudbury impact crater. The crater was about 600 km east of the center of the presently known ejecta field. The ejecta layer is an ultra-precise time marker and provides an unprecedented opportunity to correlate events in the geographically separated iron ranges of the Lake Superior region. The uppermost beds of the Biwabik Iron-Formation in the Mesabi iron range, the Riverton Iron-Formation in the Iron River-Crystal Falls district, and the Bijiki Iron-Formation in Dead River Basin are overlain directly by the ejecta-bearing breccia so are precisely time correlative. However, in the Gunflint iron range, a cryptic hiatus separates the upper beds of the Gunflint Iron-Formation from the ejecta-bearing bed suggesting uplift and emergence of that region prior to the impact. Likewise in the Gogebic iron range a disconformity separates the Ironwood Iron-Formation from the overlying Tyler Formation. The ejecta-bearing breccia lies above 100 m of argillite and siltstone of the lower part of the Tyler. In the Marquette iron range, deposition of the Negaunee Iron-Formation also probably ended considerably before the impact as a result of regional uplift. Thus, deposition of some of the major iron-formations of the region (Gunflint, Ironwood, Negaunee) had ended before the time of impact, but deposition of others (Biwabik, Riverton, Bijiki) seems to have persisted precisely until the impact, but not beyond. In the latter cases, the iron-formations are succeeded directly by the ejecta-bearing breccia, which is overlain conformably by black shale. Impact-related events may have terminated iron-formation deposition in the Lake Superior region by instantaneously completing the already begun transition from iron-formation to black shale deposition.