VARIATION OF FALLOUT EJECTA WITH DISTANCE FROM THE SUDBURY CRATER

Ejecta from the ~1850 Ma Sudbury impact event in Canada have been examined from variable distances in a vector west of the crater to determine variation in composition (e.g., abundance of devitrified impact-related glasses). Transitions in the ejecta reflect different regimes of fallout and direct emplacement from the impact, mixed with decreasingly visible local effects. Three sites, Connors Creek (CC) in Michigan (~450 km from crater), Pine River (PR) in Ontario (~650 km), and Colerain (CR) in Minnesota (~1000 km), were chosen as similar aquatic depositional sections at varying distances. These sites have been analysed from field measurements for the CC location and from core samples from the other sections. Samples have been examined petrographically, geochemically (XRF) and with Raman spectroscopy for mineral phase identification. All sites contain a significant portion of ejected impact glass, but the character of the glass changes with distance from the crater. All samples have prominent carbonate alteration that obscures original compositional patterns. The largest glass fragments are present at the CC site, with decreasing size towards the CR deposits. The CC site contains abundant detrital quartz and channel deposits, while the PR site contains limited detrital material and has only moderate channel deposits, and the CR site is almost free of detrital material with no visible channel deposits. This is consistent with decreasing depositional energy at farther distances from the impact site. The mineralogy and chemistry of the glasses is similar at the various locations. Various phyllosilicates, including phlogopite, chlorite, and sericite have replaced the glasses. Pyrite, sphalerite, and various Ti phases (rutile, anatase, brookite) are present within the glasses. Rare shocked quartz has been identified at the CC and PR sites, but is not yet found at the CR site. Distinct layers within the glass deposits are visible, particularly at the CC and PR sites, which have a clear stratigraphic relationship of layers of glass above and below a layer of accretionary lapilli. The CR site does not contain accretionary lapilli, but the presence of impact spherules in the 1000 µm size range suggests an evolution in plume generated fallout deposits, which may be related to the decrease in energy with distance from the impact.