Stratigraphy and Petrology of the Morrison Formation (Jurassic) of the Canon City, Colorado, Area

Abstract

ABSTRACT Within the Canon City area, the Morrison Formation is predominantly claystone (71%) with interbedded sandstone (20%), siltstone, (7%), and thin carbonate (2%) units. Maximum thickness is approximately 107 meters (350 ft). The sequence conformably overlies the Jurassic Ralston Creek Formation in the eastern part of the area. At places in the western part it disconformably overlies the Ralston Creek Formation, but in other places, the Morrison lies nonconformably on the Pikes Peak Granite and Idaho Springs Formation. Disconformably overlying the Morrison is the Purgatoire Formation of Early Cretaceous age. Most sandstones within the Morrison are quartzarenites or subarkoses, with grain sizes generally fine to very fine. Calcite and microcrystalline quartz are the two major cements, with a paragenetic relationship among cementing materials of (1) microcrystalline quartz, (2) calcite, and (3) hematite. Composition and texture of the sandstones indicate that source areas of sedimentary rocks contributed most of the sediments. Acid igneous and metamorphic rocks, exposed mainly in the adjacent ancestral Rockies, contributed minor amounts of detritus. Illite, the most abundant of the Morrison clay minerals, was probably derived from pre-existing illite-rich sedimentary rocks; montmorillonite, the principal clay mineral in the middle Morrison, was formed from altered volcanic ash. Sediments accumulated on floodplains formed by numerous small meandering streams. Deposition of carbonates was in small lakes formed in low areas on the floodplains.