INSIGHTS INTO IMPACT PROCESSES FROM MORPHOLOGY OF GLASSY CLASTS OF THE ONAPING FORMATION, SUDBURY, ONTARIO

While the origin of the 1.85 Ga old Sudbury Structure, Ontario, Canada, was once controversial, evidence supporting an impact by an asteroid or comet has grown since the discovery of shatter cones and shock metamorphic features some 40 years ago. It is now widely accepted that the Sudbury Igneous Complex represents an impact melt (wholly or partially) and that the overlying Onaping Formation represents fallback-flowback breccias associated with the impact. However, our understanding of impact processes is still limited. From the perspective that the shapes of melt particles reflect the processes that form them, this study focuses on the morphology of once-glassy (now completely recrystallized) clasts still preserved within the Onaping Fm. The Onaping Fm. is composed of four members (Basal, Gray, Green, and Black), all of which are glass-rich breccias (suevite) and include clasts with shock metamorphic features. Here we document the morphology of vitric clasts from samples collected within the Black member, near Onaping Falls off of Highway 144. The clasts are supported by a carbon-rich, fine grained matrix, typically range in size from 0.1 to 10 mm, and exhibit a variety of textures including fluid-form features, flow banding, round vesicles, and stretched vesicles. The clasts show no signs of rounding, and blocky, angular, and splintery shapes suggest some form of quench fragmentation. Because of their morphological similarity to volcanic glass shards, comparison with volcanic clasts produced by well-documented styles of eruption can offer insight into melt characteristics, and processes of fragmentation and transport during the impact. At this point, the comparison is only qualitative. However, additional samples will allow us to make quantitative comparisons in the future using image-analysis techniques based on fractal geometry. Such a study may be especially useful if we can obtain samples from similar suevites found at other large impact sites such as the Vredefort structure in South Africa and the Ries structure in Germany.