"Seine" or "Coutchiching"?

Abstract

In the vicinity of Sapawe and Steep Rock lakes, Rainy River district, Ontario, is a large series of schistose pre-Cambrian sediments bordered on the north by basic vol-canics (Keewatin), and on the south by intrusive granite. They strike west to Rainy Lake and to the east gradually finger out into the granite. Geologists have assigned the sediments a stratigraphic position both below the Keewatin, as "Coutchiching," and above the Keewatin, as "Seine." This paper deals with the structure of the sediments along the Keewatin contact in the area first mentioned. Evidence, based on minor structures in the sediments and the major structures of the area, is presented to show that the contact has suffered faulting of the shear type, and that the apparent dip of the sediments under the Keewatin is not definite proof of their age. The larger structures, involving faults and discordant folding of the disputed sediments and the Steep Rock Lake sediments near by, are interpreted as due to one period of diastrophism related to the intrusion of "Algoman" granites. The data given favor a Seine age for the sediments, but since areal mapping is incomplete, the question is left open.