Origin of Chert in Permian System in Southwestern Utah and Northwestern Arizona: ABSTRACT

Abstract

Within the Permian System in southwestern Utah and northwestern Arizona, the Brady Canyon Member of the Toroweap Formation and the Fossil Mountain and Harrisburg Members of the Kaibab Formation contain five different forms of chert. These chert forms provide information about the origin and emplacement of chert in the Permian System. The forms present are: rounded chert nodules, ribbon chert, silicified burrows, disseminated chert, and massive chert that grades into limestone. Sources of chert are attributed to upwelling of deep bottom waters, silica-derived from freshwater mixing with saline water in deltaic complexes, and precipitation of silica through biological processes. Examination of fossiliferous rounded chert nodules, silicified burrows, ribbon chert, disseminat d chert, and stringers of chert, indicate that deposition was the result of secondary solution moving through areas with greater porosity. Disseminated chert, found in the Fossil Mountain Member, was deposited in the areas where the porosity was greatest. Massive chert in the Harrisburg Member formed at the water table where dissolution of the limestone occurred. This chert layer was then exposed to erosion. Chert emplacement occurred following the partial dolomitization of the limestone early in the diagenetic history. Some chert appears to have been deposited as secondary cement in the carbonate rock following cementation and dolomitization. Chert horizons in the upper part of the Kaibab Formation suggest that chert may have developed much later during an erosional cycle. This is suppo ted by colloform structures and gradation from a massive chert down to limestone. End_of_Article - Last_Page 525------------