Document Type

Article

Publication Date

1960

Abstract

The East Lake Creek area is in the northwest part of central Colorado, west of the Continental Divide. The area consists of about 40 square miles of Paleozoic sedimentary rocks which dip off the extreme north end of the Sawatch Range.

The medium-grained, nearly white Upper Cambrian Sawatch quartzite is about 190 feet thick, and grades into brown, sandy dolomite of the Peerless formation which is 80 to 100 feet thick. The Lower Ordovician Manitou dolomite rests unconformably on the Peerless, and is dark gray and medium to coarsely crystalline. The Manitou is absent east of East Lake Creek, but is up to 40 feet in thickness to the west. It is separated by an angular unconformity from the Middle Ordovician Harding quartzite. The Harding is a nearly white, medium-grained orthoquartzite with some grayish yellow green sandstone lenses, and is up to 20 feet in thickness.

A prominent unconformity separates the Ordovician sediments from the Upper Devonian Chaffee formation. The white, vitreous, coarse-grained Parting quartzite member is about 75 feet thick. It differs from the quartzites of the Sawatch and Harding by its coarseness and more angular grains. The upper member of the Chaffee formation is the Dyer dolomite, which is uniform, finely crystalline, light gray, and approximately 100 feet thick. The Mississippian Leadville limestone disconformably overlies the Dyer, and is a gray, lithographic limestone, varying from 70 to 120 feet in thickness. The base of the Leadville is a grayish brown, dolomitic sandstone locally called the Gilman member.

The Belden formation of Early Pennsylvanian age disconformably overlies the Leadville. It is about 400 feet thick, and is composed of alternating beds of dark gray, aphanitic limestone and gray to black, fissile shale. It is highly fossiliferous, while the older sediments yielded no fossils. Its fauna includes abundant specimens belonging to the genera Amphissites, Millerella, Climacammina, Lophophyllidium, Stereostylus, Fenestrellina, Rhombopora, Chonetes, Marginifera, Dictyoclostus, Spirifer, Composita, Echinocrinus, Stylophycus, and several others which were not identified. Abundant Millerella occur in a zone 100 feet thick above the middle of the formation. The presence of the Millerella, and the absence of more advanced fusulinids has led to the conclusion that at least this part of the Belden is of Morrowan age.

The Belden grades into the Minturn formation which contains about 1000 feet of gray and brown, micaceous siltstones, fine to coarse gray sandstones, and thin partings of gray shale. This lower zone is overlain by several hundred feet of gray, massive beds of gypsum and anhydrite, with thin beds of dolomite, siltstone, and shale.

Two reverse faults and one anticline trend about N45E across the southern part of the area; the faults have stratigraphic throws of 1200 to 1500 feet and can be traced for about 8 miles, and the anticline can be traced for about 5 miles. The structures all appear to be the result of a local compressive force applied to the sediments during the uplift of the Sawatch Range.

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