Probable Triassic Rocks Along Eastern Front of Sangre de Cristo Mountains, South-Central Colorado

Abstract

Sedimentary rocks of Triassic age generally have been thought to be missing from outcrops in the Sangre de Cristo Mountains of Colorado where at most places the Entrada sandstone of Jurassic age rests unconformably on rocks that have been assigned to the Sangre de Cristo formation of Pennsylvanian and Permian age. Two units of probable Triassic age have been separated from the top of the Sangre de Cristo formation; one is here described and named the Johnson Gap formation. The other is correlated with the Lykins formation of central Colorado and in this report is referred to as the Lykins(?) formation. The Johnson Gap formation crops out for 8 mi. or more N. of the Colorado-New Mexico boundary, and is correlated with a part of the Dockum group of Triassic age of northern New Mexico. The Lykins(?) formation crops out in the area of Huerfano Park and locally along the eastern front of the Sangre de Cristo Mountains as far S. as the North Fork of the Purgatoire River. Fossil wood found near the top of the unit has been determined as Triassic(?) in age. The Johnson Gap formation and the Lykins(?) formation differ lithologically and probably are not correlative. The Johnson Gap consists mainly of gray silty conglomeratic limestone interbedded with gray and light red silty and siliceous limestone, red siliceous siltstone, greenish gray and brown plastic shale, and gray fine-grained quartzose sandstone. The Lykins(?) is made up of brownish red, purplish red, and buff sandstone, siltstone, and shale, and a few thin light gray limestone beds. It is probably older than the Johnson Gap formation in that the Lykins(?) formation seems to grade into the underlying Sangre de Cristo formation, whereas the Johnson Gap formation overlies the Sangre de Cristo unconformably. The striking lithologic similarity of the rocks of the Lykins(?) formation along the eastern front of the Sangre de Cristo Mountains to those of the most southerly exposures of the Lykins formation 20 mi. ENE. of Canon City, Colorado, suggests that the rocks once were probably continuous throughout S.-central Colorado, but were locally removed by erosion following a post-Triassic uplift in the general area of the Wet Mountains.