DISCOVERY OF THE SUDBURY IMPACT LAYER IN MICHIGAN AND ITS POTENTIAL SIGNIFICANCE

We have found an ejecta layer in northern Michigan produced by the 1850 Ma Sudbury impact event. The localities are 500 to 700 km from the crater center. The stratigraphic position and character of the layer are similar to previously reported ejecta at more distal localities in Ontario and Minnesota. In Michigan the layer is near the base of the Baraga Group, a marine sedimentary sequence of siliciclastic rocks, iron-formation, carbonaceous black shale, and turbidites. The layer was deposited at depths ranging from tidal at some localities to below wave base at others. It occurs in several lithofacies and ranges from 0-30 m thick, indicating varied processes of deposition and reworking. It consists of polymict breccias in which chert fragments are nearly ubiquitous and commonly are the dominant type of clast. Chert fragments range up to 2 m diameter. The impact layer commonly lies on iron-formation or ferruginous chert, which is the likely source for these fragments. Other clast types include a variety of volcanic and sedimentary rocks and rare granitic clasts. Concentrically zoned accretionary lapilli as large as 2 cm diameter are prominent at several localities. The matrix of the layer is commonly highly replaced by carbonate minerals and less commonly by fine-grained silica, which destroyed much of the finer texture. Nevertheless, altered devitrified glass, as relict shards, spheres, and vesiculated particles from 0.5 to 1 mm diameter, are evident in many specimens. Well-rounded sand-sized quartz grains are common and constititute from a few percent to as much as fifty percent of the layer. These grains, as well as multiple lapilli beds at some localities, suggest reworking and mixing by submarine processes, possibly including impact-generated tsunamis. A small percentage of quartz grains from some localities have planar zones of inclusions, rarely in multiple sets, that we interpret as relict shock-induced planar deformation features. Work is continuing to trace the impact layer through the Lake Superior region to understand the consequences of the Sudbury impact well outside of the crater. The layer also provides a unique ultra-precise time marker that will allow precise reconstruction of sedimentary facies relationships within the classic Lake Superior iron ranges.