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Tightening the Belt: Paleomagnetic-stratigraphic constraints on deposition, correlation, and deformation of the Middle Proterozoic (ca. 1.4 Ga) Belt-Purcell Supergroup, United States and Canada

¹ 6300 Country Club Drive, Flagstaff, Arizona 86004, USA
² Geological Survey of Canada–Pacific, Sidney, British Columbia V8L 4B2, Canada
³ Petrel Robertson Consulting, Ltd., Calgary, Alberta T2P 3C4, Canada

The Belt-Purcell Supergroup crops out in Montana, Idaho, Washington, British Columbia, and Alberta. It is the largest and most intensively studied Middle Proterozoic succession in North America. A major paleomagnetic survey of the Belt-Purcell basin in Montana was carried out by the U.S. Geological Survey between 1975 and 1985, but only partial results have been published. We reanalyze these data and add new data from the Canadian part of the basin. Results come from >2700 samples (mostly red beds) from 93 localities, spanning 13 formations or members of formations. They typically exhibit stable magnetizations carried by hematite in detrital specularite grains and pigment. Coherent paleomagnetic directions have either southwest declinations and moderate positive inclinations (normal polarity) or northeast declinations and moderate negative inclinations (reverse polarity). These directions clearly pass fold and reversal tests. A stratigraphically coherent directional swing and a reproducible polarity zonation indicate that the stable magnetization is primary and was acquired during the course of deposition. U-Pb dates suggest an 50 m.y. duration of deposition of the upper part of the Belt-Purcell Supergroup. The limited amount of apparent polar wander and the few polarity reversals recorded in the Belt- Purcell deposits are similar to the rate of polar wander and a long interval of stable polarity observed for the middle Cretaceous. An apparent incompatibility between early Middle Proterozoic (Elsonian) paleomagnetic poles from different regions of North America is now resolved; the Belt- Purcell poles are middle Elsonian in age and form the southern extreme point on a hairpin loop of the 1500–1400 Ma part of a refined Elsonian apparent polar wander path.

Key Words: Apparent polar wander • Belt- Purcell Supergroup • Middle Proterozoic • North American Cordillera • paleomagnetism • red beds

This article has been cited by other articles:

				J.W. Sears Belt-Purcell Basin: Keystone of the Rocky Mountain fold-and-thrust belt, United States and Canada Geological Society of America Special Papers, January 1, 2007; 433(0): 147 - 166. [Abstract] [Full Text] [PDF]

				J. W. Sears and R. A. Price Tightening the Siberian connection to western Laurentia Geological Society of America Bulletin, August 1, 2003; 115(8): 943 - 953. [Abstract] [Full Text] [PDF]