Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan

• Published in: Geology (Boulder) 2007-09, Vol.35 (9), p.827-830
• Main Authors: Pufahl, Peir K, Hiatt, Eric E, Stanley, Clifford R, Morrow, Jared R, Nelson, Gabriel J, Edwards, Cole T
• Format: Article
• Language: English
• Subjects: Baraga Group; chronostratigraphy; clasts; coastal environment; cores; depositional environment; ejecta; geochemistry; Great Lakes region; impacts; intertidal environment; iridium; key beds; Marine; marker beds; Marquette County Michigan; metals; metamorphic rocks; metamorphism; metasedimentary rocks; Michigan; Michigan Upper Peninsula; microstructure; North America; platinum group; Precambrian; Proterozoic; reworking; rhodium; rock, sediment, soil; sea-level changes; shock metamorphism; Stratigraphy; Sudbury ejecta; tsunamis; United States; upper Precambrian
• ISSN: 0091-7613
• DOI: 10.1130/G23751A.1

An ejecta layer produced by the Sudbury impact event ca. 1850 Ma occurs within the Baraga Group of northern Michigan and provides an excellent record of impact-related depositional processes. This newly discovered, ∼2-4-m-thick horizon accumulated in a peritidal environment during a minor sea-level lowstand that punctuated a period of marine transgression. Common ejecta clasts include shock-metamorphosed quartz grains, splash-form melt spherules and tektites, accretionary lapilli, and glassy shards, suggesting sedimentation near the terminus of the continuous ejecta blanket. Sedimentologic and geochemical data indicate that primary fallout from a turbulent ejecta cloud was reworked to varying degrees by an impact-generated tsunami wave train. Observed platinum group element anomalies (Ir, Rh, and Ru) within the Sudbury ejecta horizon are sufficient to suggest that the impactor was a meteorite. Documenting and interpreting the detailed characteristics of the Sudbury ejecta horizon in Michigan have yielded a fingerprint to identify this chronostratigraphic marker in other Paleoproterozoic basins. For the first time a foundation exists to assess the consequences of the Sudbury impact on Precambrian ocean chemistry and early life.