The Frasnian-Famennian extinction event in a stable cratonic shelf setting: Trout River, Northwest Territories, Canada

Abstract

The Frasnian-Famennian (F-F) extinction event occurs in platformal carbonates along the Trout River, N.W.T. Paleogeographically the site was about 150 km east of the western cratonic margin. Faunal evidence, based primarily on corals, and sedimentation rates of passive cratonic margins suggest that the late Frasnian Redknife and Kakisa Formations include both the Upper rhenana and the linguiformis conodont zones. The high biodiversity of the benthic fauna during the late Frasnian decreases sharply at the F-F boundary and recovers only gradually during the early Famennian. This trend is also expressed by the profile of δ 13C values. Microfacies analysis has identified two minor hiatuses at the F-F boundary. The lower break is represented by an unconformity at the upper contact of the latest Frasnian Kakisa Formation and is associated with surficial karstification and brecciation of the underlying biostromal carbonates. Anomalous trace element concentrations at the unconformity are interpreted as possible fallout material related to a bolide impact. This hiatus probably corresponds to the Lower triangularis 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 sponges, 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 conditions and/or a habitat ravaged by a catastrophic event—common characteristics after a global extinction event. Conodont evidence suggests that this brief depositional interval corresponds to the Middle triangularis Zone. The upper hiatus probably straddles the contact of the Middle and Upper triangularis zones. Evidence for this break are angular fragments of the wackestone-sandstone facies in the basal Trout River Formation. Conodonts from the basal siliciclastics of the Trout River Formation assign the lower part of this unit to the Upper triangularis Zone. In Alberta, 900 km to the south, similar platformal carbonates show a longer hiatus (the entire triangularis Zone) at the F-F contact, whereas slope sediments indicate continuous sedimentation with a sharp drop in biodiversity and a temporary proliferation of primitive organisms at the F-F boundary.