Recent deep exploratory drilling indicates that the Interior Plains of Western Canada are underlain by rocks similar to those of the Canadian Shield. Samples from 100 wells in central and northern Alberta were examined petrographically, in an attempt to define the major Precambrian structural units of the area.

A division of the subsurface Precambrian of Alberta into two areas is made on the basis of eighteen potassium-argon age determinations. The boundary between the two areas extends from approximately Lat. 60° N., Long. 118° W., to Lat. 52° N., Long. 110° W. East of this boundary, ages of the order of 1,700 million years suggest that the subsurface Precambrian of northeastern Alberta is an extension of the Churchill geologic province. The dominant structural trend of this province is northeast-southwest. West of the boundary, age determinations are in the range of 1,200 to 1,500 million years. This geologic province, for which the name Peace River is suggested, appears to have an arcuate outline, striking north-south in northern Alberta, changing to northwest-southeast in central Alberta.

Petrographic data suggest that most of the Churchill province in northeastern Alberta is underlain by gneissic granitic rocks. The central part of the Peace River province is composed dominantly of gneissic adamellites and granodiorites, commonly with basic schists. The marginal phase is characterized by alkalic granitic rocks.

Aeromagnetic maps covering the south shore of Great Slave Lake are used to extend the major fault zone bounding the north side of Churchill province beneath the Paleozoic sedimentary rocks of the Alberta shelf.

The major Precambrian structural units are significant in relation to studies of Paleozoic sedimentation on the Alberta shelf. The low landmass that existed in western Alberta during Paleozoic time is believed due to the isostatic uplift of the eroded roots of the Peace River mountain belt. The concept of a younger Precambrian arcuate structure in western Alberta lends further support to the theory of continental growth by marginal accretion.

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