Structural features of the Keewatin volcanic rocks of western Quebec

Abstract

The Archean basal complex of the Noranda district, Quebec, consists chiefly of alternating belts of Keewatin rhyolite and andesite with subordinate pyroclastic breccia and stratified tuff. These rocks have been folded with increasing intensity from north to south into eastward-pitching anticlines and synclines. To the north, where the folding is moderate, the lava belts trend sinuously from north to south and dip 45 degrees or less to the east, whereas to the south, where the folding is intense, they trend east-west and are vertical or nearly vertical. The structure of the lavas has been determined from the following data: (1) the strike and dip of interbedded stratified tuff and chert; (2) the attitude of columnar jointing; (3) the flow contacts within the flow belts; (4) the flow belts; and (5) tops determined by the presence of: (a) zones of breccia along the upper margins of individual flows, (b) the rounded tops and flat bottoms of pillows of the “bun” type, (c) the rounded tops and V-shaped bottoms of pillows of the “balloon” type, and (d) cross-bedding in stratified tuff. Within an area of 45 square miles extending from Noranda northward an uninterrupted succession of volcanic rocks, whose average total thickness is about 25,000 feet, has been determined. It is proposed to call this succession the Abitibi series.