Paleozoic stratigraphy of the Sawatch Range, Colorado

Abstract

The Sawatch Range, a high and rugged mountain uplift in central Colorado, separates the Arkansas River valley from the plateau region to the west. It includes some of the highest peaks in Colorado, with summit elevations around 14,400 feet. Very little of the region lies below 9000 feet. The rocks range from pre-Cambrian to Recent but consist mainly of pre-Cambrian granite and metamorphics and Paleozoic sedimentary rocks. All are cut by Tertiary intrusives. The Paleozoic sedimentary rocks represent every period except the Silurian. The Cambrian system is represented by the Sawatch quartzite, which attains considerable thickness locally and can be roughly subdivided into four members. The Ordovician rocks include three formations—the Manitou dolomite, Harding sandstone, and Fremont limestone. The Manitou, consisting of more or less siliceous dolomite and dolomitic limestone, is widespread and varies greatly in thickness. The Harding sandstone, represented by quartzite in part of the area, is thin and found only along the southern end of the range and along Cement Creek. The Fremont limestone is predominantly fossiliferous dolomite and is found only in the southern part of the Sawarch Range province. Unconformities occur at the top of each of the Ordovician formations, and the formations thin or wedge out to the north. The Devonian sedimentary rocks included in the Chaffee formation are widespread and include the shaly or sandy Parting member below and the Dyer dolomite member above. The Devonian sediments are discussed at length as many new data are presented. The Leadville limestone of Mississippian age unconformably overlies the Chaffee and is an important ore-bearing formation at Aspen and in other mining districts along the range. Another unconformity separates the Leadville from the Pennsylvanian Weber (?) formation above. The latter generally consists of shales in the lower parts, above which is a series of limestones and shales, overlain by sandy shales or sandstones. These beds grade upward into a series of red sandstones, grits, and conglomerates with some interbedded limestones, known as the Maroon formation, which includes both Pennsylvanian and Permian deposits. At present the boundary between the Pennsylvanian and Permian beds of this region has not been definitely drawn on either lithologic composition or color. The lower beds contain abundant fossils, but fossils become scarcer at higher stratigraphic positions. The information available suggests that the Pennsylvanian series may be 2000 to 2500 feet thick. Devonian fish fragments were obtained from the lower member of the Chaffee including the basal layers. A coral fauna, obtained from the upper Leadville limestone at several localities, suggests the presence of some Mississippian beds younger than those hitherto reported. Structures suggesting growths of calcareous algae were noted in the Manitou and the Chaffee, and well-preserved fossil algae were obtained from the upper Leadville limestone and lower Weber (?) formation. The paper includes 24 measured sections.