Clay Minerals in Rocks of the Lower Part of the Oquirrh Formation, Utah

Abstract

An association between clay minerals and rock type is recognized in the rocks of an interlayered sequence of fossiliferous shaly, sandy bioclastic, and cherty limestone, dolomitic limestone, calcareous and dolomitic quartzite, and quartzite that form a portion of the lower part (Pennsylvanian age) of the Oquirrh formation in the northern Oquirrh Range, Utah. These sedimentary rocks are largely of clastic origin and range from fine- to coarse-grained. In part they are cross-bedded, and fossils are locally abundant. Calcite-rich rocks predominate (78 percent) over dolomite- and quartz-rich rocks (17 and 5 percent, respectively). Clay minerals constitute less than one percent in quartzite and range from one to five percent in some limestones.

Clay minerals and rock types are commonly observed in the following associations: Illite and chlorite occur in limestone; illite, chlorite, and mixed-layer clay occur in cherty, bioclastic, and sandy limestone and in calcareous quartzite; chlorite and illite commonly are present in dolomitic quartzite; chlorite, illite, and mixed-layer clay are found in dolomitic limestone; and kaolinite, illite and chlorite are typically present in quartzite. The mixed-layer clay has a lattice spacing of 29.4 Å that expands to 31.0 Å when glycolated and contracts irregularly to 13.2 Å when heated to 500 °C; the clay is assumed to consist of 14 Å chloritic and 15 Å montmorillonitic material. Chlorite or mixed-layer clay, or both, generally are associated with rocks containing dolomite.