ABSTRACT

Petrographic examination of a composite Cordilleran geosynclinal sedimentary section in Western Wyoming reveals striking vertical changes in mineralogy. Mica, cement, and matrix exhibit systematic upward changes in composition, but not in abundance. Feldspar is rare except in certain Cretaceous sandstones. Lithic fragments increase in abundance upward and exhibit orderly changes in composition. Paleozoic sandstones are rich in harder lithic clasts (quartzite, orthoquartzite, chert); younger sandstones (Mesozoic and Tertiary) contain softer fragments (carbonate, shale, siltstone).

Systematic changes in mineralogy and lithology support the concept of cyclical sedimentation in geosynclines. Specific changes in composition reinforce (and modify) published conclusions about dispersal systems, source areas, and tectonic activity within the Cordilleran region. Vertical contrasts in petrology reflect orderly, progressive shifts in tectonic activity towards the depositional basin, and imply a general ordering of geosynclinal tectonism in time and space relative to particular geographic areas.