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A Precambrian diamictite below the base of the Stillwater Complex, southwestern Montana

May 1, 1973

The metasedimentary rocks of Precambrian age below the base of the Stillwater Complex, southwestern Montana, contain a metamorphosed diamictite similar to diamictite in strata of late Precambrian and Early Cambrian age in Canada and the Western United States. The diamictite consists of a quartz-cordierite matrix containing an average 10-15 percent of rock fragments diverse in shape, size, angularity, lithologic types, and textures. The diamictite unit is poorly sorted, crops out discontinuously for 14 miles, and locally has a crude layering containing dropstones. Its origin is debatable, but the rock is suggestive of glacial marine deposition. The minimum age of the diamictite of the complex is 2,750 m.y. from U-Pb determination on zircon of intrusive quartz monzonite, the maximum 3,140 m.y. as determined on zircon from metasedimentary rocks interbedded with the diamictite.

Citation Information

Publication Year 1973
Title A Precambrian diamictite below the base of the Stillwater Complex, southwestern Montana
DOI
Authors Randolph Koski, Norman J. Page
Publication Type Article
Publication Subtype Journal Article
Series Title Journal of Research of the U.S. Geological Survey
Series Number
Index ID 70161756
Record Source USGS Publications Warehouse
USGS Organization