Dotsero and Manitou Formations, White River Plateau, Colorado, with Special Reference to Clinetop Algal Limestone Member of Dotsero Formation

Abstract

Recent field investigations in the White River Plateau of central northwestern Colorado have shown that the age of a sequence of limestone, dolomite, and limestone flat-pebble conglomerate is Late Cambrian and Early Ordovician rather than Late Cambrian only, as formerly designated. The thickness of the sequence ranges from 185 to 251 feet, and the rocks are widespread on the top and flanks of the White River Plateau. The entire sequence was formerly placed in the Dotsero dolomite. It is here divided into the Dotsero and Manitou formations. The Dotsero dolomite is redefined as the Dotsero formation, which includes only the lower part (Late Cambrian) of the former Dotsero dolomite. The upper part of the former Dotsero dolomite is here placed in the Manitou formation of Earl Ordovician age. The Dotsero and Manitou formations contain two members each. The upper member of the Dotsero formation, the Clinetop algal limestone, is a unit composed of algal limestone and limestone conglomerate, commonly 3-5 feet thick, that persists throughout the White River Plateau. It forms a conspicuous ledge of bone-white rock that has a pale lavender tint and a distinctive swirl pattern on its top surface. Fossils from both formations were collected by the writers in 1947 and 1948, and 23 collections were made by the writers and A. R. Palmer in 1951. Upper Cambrian fossils identified by Palmer in these collections consist of 1 species of alga, 2 species of sponges, 2 species of graptolites, 8 species of trilobites, and undeterminable fragments of pelmatozoans and of inarticul te and articulate brachiopods. The Lower Ordovician fauna consists of 1 species of graptolite, 4 species of brachiopods, 5 species of gastropods, 1 species of cephalopod, 13 species of trilobites, 5 species of conodonts, pelmatozoan fragments, and sponge spicules.