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The Lorrain Formation crops out in 3 areas between Sault Ste. Marie, Ontario, and Ville-Marie, Quebec--Bruce Mines, Whitefish Falls, and Cobalt. It conformably overlies the Gowganda Formation in each area; in the Cobalt area it also locally overlies Archean granite. The Lorrain is overlain by the Gordon Lake Formation.
In ascending order, the Lorrain Formation consists of arkose, subarkose, jasper-bearing orthoconglomerate, and orthoquartzite. In the Bruce Mines area the Lorrain is 8,300 ft thick and is divided into 5 members (A-E); in the eastern areas the Lorrain is only 5,000 ft thick and is divided into 3 members (lower, middle, and upper). In each area, mineralogic and textural maturity increases upward; grains in immature arkose at the base are subrounded and 44% feldspar, and grains in supermature orthoquartzite at the top are well rounded and 95-100% quartz. Chronologically, the depositional environments suggested are shallow-water marine, lacustrine, delta fringe, overshelved beach, and high-energy beach.
Currents flowing south and southeast deposited most of the sediments; however, in jasper-bearing conglomerate of the Bruce Mines area, cross-bedding and pebble composition indicate a 90° variance in transport direction, and an additional source on the east. Abundant potash feldspar and lack of metamorphic and sedimentary rock fragments in the lower members suggest a plutonic provenance. Subsequent deposition of jasper, chert, and quartz-rich detritus indicates erosion and reworking of Precambrian jaspilites and igneous source rocks similar to those now exposed in the Canadian shield.
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