THE GREAT ARC OF EASTERN HUDSON BAY, CANADA: PART OF THE LARGEST MULTIRINGED IMPACT BASIN ON EARTH?

Almost 40 years ago, Beals (1968) proposed an impact origin for the great eastern arc of Hudson Bay, which extends for 650 kilometres through an angle of 155 degrees and has a coherent circular raised rim on its landward side. A rift extends at right angles outwards on the southeastern side and within the arc, the basin is filled with Proterozoic sediments. The best fit circle has a radius of 230 kilometres and the arc deviates from this circle by less than 10 km along its entire length. More recently, Goodings and Brookfield (1992) noted that closing the James Bay rift aligns the Sutton ridge to form an arc of 240 degrees, or two-thirds of a circle. The remainder is cut by the younger circular northern Hudson Bay cratonic basin. Apart from impact, no other plausible explanation has been proposed for this great ring fracture (and another ring fracture may exist outside this one). But, because no definitive evidence of impact was found, little has been published on the Hudson Bay arc since 1968. Recent studies of multi-ringed basins on other planets, and of other old multi-ringed basin on Earth (e.g. Vredefort), provide criteria for re-investigation and re- interpretation of published reports. Along the Hudson Bay arc, bodies of pseudotachylite, monomict and exotic breccias are associated with faults, and overlying sediments may show evidence of re-worked impact melts. If investigations are positive, Hudson Bay arc would form part of the largest identified multi-ringed impact on Earth, with a minimum diameter of 450 kilometres. Beals, C. S., 1968. On the possibility of a catastrophic origin for the great arc of eastern Hudson Bay. In: Beals, C.S.(editor), Science, History and Hudson Bay, volume 2. Department of Energy Mines and Resources, Ottawa. p.985- 999. Goodings, C.R. & Brookfield, M.E., 1992. Proterozoic transcurrent movements along the Kapuskasing lineament (Superior Province, Canada) and their relationship to surrounding structures. Earth-Science Reviews, 32: 147-185.