The Mattagami Formation is an erosional remnant of unconsolidated sand, clays (commonly kaolinitic), lignite, and rare conglomerate, extending 180 km east-west and 70 km north-south. The maximum known thickness is 130 m and the formation thins to the north. Outcrops are rare and drill-hole data are limited regionally. Devonian carbonates and shales and Jurassic shales underlie the Mattagami Formation and thick Pleistocene tills and clays overlie it.

An east-west-trending post-Mesozoic fault forms the south boundary of the Mattagami with Archean gneisses. Small isolated Mesozoic outliers occur farther south. The Grand Rapids arch trends northwest from the southeast corner, which is defined by Precambrian inliers and outcrops of Middle Devonian carbonates.

Across 50 km west of the arch, the Mattagami Formation contains many clean quartz cross-bedded sands, which lack correlative beds between drill holes. On Adam Creek, sand cross-cutting dark-gray shales suggests laterally migrating streams eroding flood-plain deposits. Lignite beds are thin and discontinuous.

East of the arch, the Mattagami is mainly clay with minimal sand, and total thickness is much less than in the west. Two lignite beds comprise the Onakawana deposit (185 million tons proved, approximately 5,000 BTU dry basis) with a total thickness of 16 m and known lateral continuity of 12 km. These are flood-plain and associated swamp deposits.

The paleogeomorphic Grand Rapids high was a barrier for the great northwest-trending river system meandering across 50 km, and preventing the river's incursion in the flood-plain and swamp environment to the east. North of the Onakawana lignite deposits and parallel with them, the interpreted trend of the river system is a prime target for exploration drilling.

End_of_Article - Last_Page 643------------