THE SUDBURY IMPACT LAYER IN THE WESTERN LAKE SUPERIOR REGION

At approximately 1850 Ma, an impactor in excess of 10 km in diameter struck the area that is now Sudbury, Canada, producing one of the 3 largest known terrestrial craters. Fall-back breccia at Sudbury is well known; however, the more distant ejecta blanket was only recently discovered in the western Lake Superior region. Known as the Sudbury impact layer (SIL), it occurs at and near the stratigraphic top of the Paleoproterozoic (ca. 1878 Ma) Gunflint Iron Formation. We define the SIL to include: 1) autochthonous material interpreted to be seismically folded and shattered iron-formation, chert, and carbonate rock (breccia); overlain by a few meters of 2) strata composed largely of allochthonous material derived in part from target rocks (ejecta); and 3) irregular layers that appear to be conglomeratic mixtures of locally and distally derived material. The SIL deposits exhibit extreme lateral variability in thickness and stratigraphy. In the Thunder Bay area (650 km from target), thick boulder-rich successions filled depressions, and higher areas contain thinner, finer-grained debris with more material derived from target rock. This records earthquake-induced deformation of subaerially exposed, brittle substrate. By contrast, seismic deformation in the uppermost iron-formation layers at Gunflint Lake (750 km from target) produced both ductile (soft-sediment) and brittle features in close proximity. This indicates incomplete lithification and implies a shallow submarine environment in the Gunflint Lake area at the time of impact. A base surge swept across this debris-mantled terrain, moving local material from high to low areas and depositing accretionary lapilli, ash pellets, and what is now devitrified glass. Subsequent fluvial reworking at Thunder Bay and tsunami reworking at Gunflint Lake modified the upper units locally. This was followed by sub-aerial exposure throughout the region, resulting in lithification and diagenetic modification. At ca. 1835 Ma, marine transgression buried the SIL beneath black shale of the Rove Formation. The stratigraphic arrangement of facies in the SIL has important temporal implications for modeling processes of deformation and deposition during impacts on Earth, which may also be applicable to understanding the products of impacts on extraterrestrial bodies.