HYDROTHERMALLY ALTERED JURASSIC NAVAJO SANDSTONE OF SOUTHWESTERN UTAH

Local areas of extremely hard, silica-cemented Jurassic Navajo Sandstone occur in the northern part of the Red Mountain Wilderness Area, south of the Veyo Cinder Cone, and north and west of Snow Canyon State Park in southwestern Utah. The normally porous, eolian Navajo Sandstone contains zones of heavily altered, well-indurated sandstone or quartzite observed in ridges, at elevations up to 150 meters above the surroundings. Initial mapping indicates that most of these well-cemented zones are in elongate N-S trends, about 1 to 1.5 km long, consistent with Basin and Range faulting and regional joint patterns. The main Snow Canyon follows one of these major fault zones, which branches near its northern end, with a terminus close to Veyo Cinder Cone. Observations of quartzite in the Navajo occur in the ridges directly west of this major fault or joint (expressed aerially as a distinct lineament).

Within the resistant Navajo ridges, there are decimeter- to meter-scale lenses of the hard quartzite within less cemented host rock. Fresh samples of the quartzite are pale-yellowish brown to light gray in color, with 1-3 mm thick contact surfaces of granular sandstone that is a pale orange to pinkish gray color. In the absence of any evidence of metamorphism, the unusually high amount of silica cement in the quartzite (with some remnant quartz grains) is interpreted as a diagenetic phenomenon. Regional volcanic activity may have provided silica-rich hydrothermal fluids that permeated porous Navajo Sandstone close to fault conduits. The closest vent – the Veyo Cinder Cone (0.69 Ma) – is part of the Pine Valley Volcanic Field of that area. However, no preserved basalt flows overlie the Navajo Sandstone ridges of the study area; instead very young basalt flows occur in paleotopographic lows, implying the silica cementation predated the flows. Some areas show an abundance of ethnographic flakes of the quartzite demonstrating the utility of this lithology in native cultures.