Debrisites from the Sudbury impact event in Ontario, north of Lake Superior, and a new age constraint: Are they base-surge deposits or tsunami deposits?

Eight outcrops of chaotic debrisite containing ejecta from the 1850 Ma Sudbury impact event have been identified in and near the city of Thunder Bay, Ontario, 650 km west of the center of the Sudbury crater. Ejecta features include devitrified vesicular impact glass, spherules, accretionary lapilli, microtektites and tektites, and shocked quartz grains containing relict planar features including planar deformation features (PDFs) and planar fractures. The original volume of ejecta has been significantly reduced by carbonate replacement and recrystallization, so that today it only makes up ∼20% of the debrisite volume. Volumetrically, the primary component of the debrisite is ripped-up clasts of the local bedrock, including carbonate grainstones, stromatolites, and chert of the 1878 Ma Gunflint Formation. These boulder-to coarsesand-sized clasts commonly fine upward, in marked contrast to the chaotic nature of the remainder of the debrisite. Seven of the eight sites have had the upper portion of the impact layer removed by glaciation. The eighth site shows a complete stratigraphic section from the Gunflint Formation, through the ejecta bearing layer, and into the overlying 1832 Ma Rove Formation. Zircons extracted from one of the ejecta sites, and probably derived from Gunflint tuffaceous material entrained by the turbulent flow, reveal U-Pb ages clustered around 1856 Ma. The sequence of events deduced from these outcrops begins with mafic volcanic ash being deposited and reworked in a carbonate-dominated, nearshore environment that supported microbial mat growth and stromatolites. During subsequent subaerial exposure, blocky, meteoric calcite cements formed in this material. Upon impact, earthquakes fractured this bedrock as well as underlying Gunflint Formation cherts and carbonates. Impact-generated base surges, or less likely, tsunamis, stripped the area of loose sediment and ground up the Gunflint carbonates into fine-sand-to fine-gravel-sized clasts. This debris was mixed with the ejecta, along with ripped-up, fractured Gunflint chert-carbonate clasts, before being deposited as a chaotic, laterally variable layer. In an ensuing period of subaerial exposure lasting <18 m. y., weathering, erosive reworking, and cementation modified the deposits. The Animikie Ocean then transgressed the area, depositing the overlying Rove Formation carbonaceous shale. © 2010 The Geological Society of America. All rights reserved.