Norman J. Page Randolph Koski 1973 <p>The metasedimentary rocks of Precambrian age below the&nbsp;base of the Stillwater Complex, southwestern Montana, contain a&nbsp;metamorphosed diamictite similar to diamictite in strata of late&nbsp;Precambrian and Early Cambrian age in Canada and the Western United&nbsp;States. The diamictite consists of a quartz-cordierite matrix containing&nbsp;an average 10-15 percent of rock fragments diverse in shape, size,&nbsp;angularity, lithologic types, and textures. The diamictite unit is poorly&nbsp;sorted, crops out discontinuously for 14 miles, and locally has a crude&nbsp;layering containing dropstones. Its origin is debatable, but the rock is&nbsp;suggestive of glacial marine deposition. The minimum age of the&nbsp;diamictite of the complex is 2,750 m.y. from U-Pb determination on&nbsp;zircon of intrusive quartz monzonite, the maximum 3,140 m.y. as&nbsp;determined on zircon from metasedimentary rocks interbedded with&nbsp;the diamictite.</p> application/pdf en U.S. Geological Survey A Precambrian diamictite below the base of the Stillwater Complex, southwestern Montana article