Petrology and Paragenesis of the Ordovician Manitou Formation Along the Front Range of Colorado

Abstract

ABSTRACT Field and laboratory studies of the Manitou Formation (Lower Ordovician) along the southern Front Range of Colorado indicate that in this area the formation originated as an allochemical limestone. Deposition occurred in an environment of moderate to low energy that was interrupted occasionally by brief periods of high energy. Biomicrite, intramicrite, and micrite comprise, in order of decreasing abundance, the major lithologies of the original limestone. This section studies show evidence of five post-depositional alterations of the sediment: (1) Grain growth recrystallization of microcrystalline calcite to sparry calcite mosaics. (2) Dolomitization of selected horizons in the northern localities and rather complete dolomitization in the more southern localities. (3) Silicification ranging in degree from partial void filling to complete silicification that produced dense chert beds. (4)Calcitization of chert and dolomite. (5) Oxidation of iron-bearing minerals, especially glauconite and ankeritic dolomite, to limonite and hematite. Glauconite replacement of calcite and dolomite and dolomite replacement of chert and glauconite can be demonstrated but are sufficiently rare that they are not considered as major diagenetic events. The occurrence of extensive void-filling quartz and recrystallized microcrystalline calcite suggest that lithification of the original sediment probably was complete after the stage of silicification. Early partial lithification is suggested by the presence of intraclasts at several horizons.