The Carswell structure in the western Athabasca Basin, northern Saskatchewan (Canada), has previously been interpreted as an eroded impact structure with a minimum diameter of ~36 km, the outer margin of which is broadly defined by an outer ring of sediments composed of the only algal reefs observed in the Athabasca Group, the Carswell Formation. This ring surrounds an 18-km-wide uplifted basement core composed of gneiss units of Archean to Paleoproterozoic age that display shatter cones, planar deformation features (PDFs), pseudotachylyte veins, and impact melts and breccias (Cluff melt sheet, Cluff breccias) indicating that pressures and temperatures locally exceeded 60 GPa and 1500 °C. Detailed analysis of the basement–Athabasca Group contact from field and drill core samples indicates that shock features are not present in the Athabasca Group sediments in direct contact with the highly shocked basement gneisses. This pattern is inconsistent with a post–Athabasca Group age for the impact. Moreover, our study has revealed PDF-bearing quartz grains in basal units of the Athabasca Group 14 km south of the southern edge of the basement core (outside the estimated outer ring). The new proposed model suggests that the impact event is of pre-Athabasca (Proterozoic) age and that it produced a multiring structure that controlled the paleogeography of the Athabasca Group units in the western part of the basin. The model is well supported by basin analysis and gravity data. The Carswell Formation is the result of algal reefs building on peak-ring–related seamounts at the end of Athabasca Group deposition. The overturned bedding observed locally adjacent to the basement core is interpreted as the result of gravity-driven readjustment of the central uplift.

Abstract Ultrahigh pressure (UHP) rocks are extraordinary rocks that recorded extreme metamorphic conditions. Their study has been an exciting part of geosciences and in the last few years has emerged as a distinct discipline. Only few UHP terranes have been recognised worldwide so far and this only over the last ca. 20 years. They occur within major continental collision belts, predominantly in Eurasia and Africa (see e.g. Ernst & Liou, 2000 ; Liou, 2000 ; Carswell & Compagnoni, 2003 for a general review). Even though UHP rocks have been subject to intense studies in the last two decades, many questions regarding their formation and evolution remain open. Geochronology can assist this search for information in providing the absolute time of their formation and several other parameters regarding their evolution.