Glauconitic Mica in the Morrison Formation in Colorado

Abstract

Glauconitic mica occurs as interstitial clay in a bed of fine-grained sandstone, 15 feet thick, in the upper part of the Brushy Basin shale member of the Morrison formation of Late Jurassic age on Blue and Lone Tree Mesas near Uravan, Montrose County, Colorado. This glauconitic mica, similar to other material ordinarily called glauconite, is interesting because the Morrison formation is generally regarded as nonmarine in origin, whereas glauconite commonly forms in a marine environment.

The identifying properties of the glauconitic mica are: strong 001 d spacing, 9.97A; very weak 002, 5.01A; strong 003, 3.34A; 060, 1.51A; green color, pleochroic; X and Z indices, approximately 1.578 and 1.600; SiO₂ 49 percent, Al₂O₃ 18 percent, Fe₂O₃ 13 percent, FeO 1.3 percent, MgO 2.8 percent, K₂O 7.8 percent, TiO₂ 1.1 percent, ignition loss 6 percent, others approximately 1 percent.

The glauconitic sandstone is underlain by variegated mudstones about 480 feet thick, which contain montmorillonite (with very sparse relicts of shards), illite, quartz, and also, in several layers, scanty analcime. The Burro Canyon formation of Early Cretaceous age overlies the glauconitic sandstone.

This paper discusses alternative origins of the glauconitic mica possible within the framework of an appropriate chemical environment other than, and apart from, one typically marine.