Relationship between the Montgomery Lake and Hurwitz groups, and stratigraphic revision of the lower Hurwitz Group, District of Keewatin

Recent detailed mapping has resulted in a possible solution to a long-standing stratigraphic controversy as to whether the contact between the Montgomery Lake Group (age uncertain) and the Hurwitz Group (Paleoproterozoic) is an unconformity. In its type area, the Montgomery Lake Group consists of mainly immature litharenite and lithic wacke, and is separated from underlying Archean greenstones and the overlying Hurwitz Group by angular unconformities. In the Padlei belt, a unit previously referred to as ''Montgomery Lake sedimentary rocks,'' ''Montgomery Lake Group,'' or ''map unit X'' consists of mainly submature to mature subarkose to quartz arenite, with interbeds of pyritic (auriferous) and nonpyritic (locally red) quartz pebble conglomerate. These rocks differ from the Montgomery Lake Group in its type area, but bear strong lithologic and stratigraphic resemblence to rocks in the lower Hurwitz Group. Furthermore, interfingering between map unit X and Hurwitz Group lithotypes suggests that map unit X is an integral part of the Hurwitz Group and that its upper contact is transitional. We formally propose that map unit X be called the ''Noomut Formation,'' a new unit at the base of the Hurwitz Group. In the absence of an unconformity in its lower part, the lower Hurwitz Group provides a continous record of the time during which free oxygen was sustained in Earth's atmosphere, and of paleoclimatic changes that may be related to the breakup of an earliest Paleoproterozoic supercontinent.