Abstract: Examination of samples from shale outcrops on the periphery of the Utah Valley graben has revealed an unusual association of disseminated pyrophyllite with various clay minerals. Numerous samples were obtained from exposures in brick clay pits located in the Manning Canyon formation of Mississippian-Pennsylvanian age and from the Long Trail member of the Great Blue formation of Mississippian age. Associated clay minerals in these beds include illite, illite-montmorillonite mixed-layer clays, 7 Å chlorite, 14Å chlorite, kaolinite and sericite. Associated nonclay minerals include quartz, calcite, and small amounts of dolomite. Associated secondary minerals are calcite, aragonite, jarosite, gibbsito and gypsum.
Three possible explanations of genesis have been considered in the present study : deposition of detrital pyrophyllite, surface weathering under special conditions, and hydrothermal or pneumatolytic activity. The third alternative explanation, hydrothermal or pneumatolytic activity, is believed to be the most acceptable. It is hypothesized that magnesium, iron, and interlayer cations have been removed from some of the original 2 : 1 layer clays in the shale by solutions localized along fault zones.