THE FRASNIAN FAMENNIAN EXTINCTION EVENT IN A STABLE CRATONIC SHELF SETTING - TROUT RIVER, NORTHWEST-TERRITORIES, CANADA

The Frasnian-Famennian (F-F) extinction event occurs in platformal car bonates along the Trout River, N.W.T. Paleogeographically the site was about 150 km east of the western cratonic margin. Faunal evidence, ba sed primarily on corals, and sedimentation rates of passive cratonic m argins suggest that the late Frasnian Redknife and Kakisa Formations i nclude both the Upper rhenana and the linguiformis conodont zones. The high biodiversity of the benthic fauna during the late Frasnian decre ases sharply at the F-F boundary and recovers only gradually during th e early Famennian. This trend is also expressed by the profile of delt aC-13 values. Microfacies analysis has identified two minor hiatuses a l the F-F boundary. The lower break is represented by an unconformity at the upper contact of the latest Frasnian Kakisa Formation and is as sociated with surficial karstification and brecciation of the underlyi ng biostromal carbonates. Anomalous trace element concentrations at th e unconformity are interpreted as possible fallout material related to a bolide impact. This hiatus probably corresponds to the Lower triang ularis Zone, because no diagnostic conodonts of that time interval are present. The lower and upper hiatuses are separated by the deposition of a wackestone-sandstone facies with an impoverished fauna of sponge s. calcareous algae and some foraminifera, preserved only in neptunian dykes in the upper Kakisa and as fragments in the overlying Famennian Trout River Formation. The depauperate fauna suggests hostile conditi ons and/or a habitat ravaged by a catastrophic event-common characteri stics after a global extinction event. Conodont evidence suggests that this brief depositional interval corresponds to the Middle triangular is Zone. The upper hiatus probably straddles the contact of the Middle and Upper triangularis zones. Evidence for this break are angular fra gments of the wackestone-sandstone facies in the basal Trout River For mation. Conodonts from the basal siliciclastics of the Trout River For mation assign the lower part of this unit to the Upper triangularis Zo ne. In Alberta, 900 km to the south, similar platformal carbonates sho w a longer hiatus (the entire triangularis Zone) at the F-F contact, w hereas slope sediments indicate continuous sedimentation with a sharp drop in biodiversity and a temporary proliferation of primitive organi sms at the F-F boundary.