FIBROUS-VEIN QUARTZ PEBBLES IN THE SHARON FORMATION, NORTHEASTERN OHIO: AN INDICATOR OF A METAMORPHIC SOURCE TERRANE

The Pennsylvanian Sharon Formation in northeastern Ohio, basal unit of the Pottsville Group and correlated to the Olean Conglomerate of New York, is recognized as an orthoquartzite with quartz-pebble conglomerate layers. With over 96% silica content, it forms resistant scenic ledges and is economically mined for silica sand. Previous workers have reported the pebbles to be dominantly white “vein quartz” with minor rose and smoky quartz, chert/jasper, quartzite, sandstone, siltstone, and siliceous slate. Interpreted as an alluvial plain deposit last eroded from a sedimentary terrane, the original source of the quartz is still a mystery.

Following the odd-pebble collection of Bowen (1953), we surveyed several hundred pebbles (4-64 mm range, after Wentworth 1922) collected from pebble-rich conglomeratic layers of the Sharon Formation at Chapin Forest, Sidley Quarry, and Best Quarry in northeastern Ohio. Pebbles were collected from decomposed outcrop in the field and samples disaggregated in the laboratory, and then sorted into groups based on composition and texture using bright reflected light and a stereomicroscope. Representatives of each group were chosen and sliced into thin sections (n=55) to verify mineralogy and texture. Preliminary results have produced two new groups not previously reported in the literature: black-spotted white quartzite and fibrous-vein quartz.

The fibrous pebbles are common, but only make up 7% and 12% of both the white and rose quartz groups, respectively. In thin section, the fibrous pebbles display either uniform parallel fibers, stretched prismatic grains, or slender curved lenticular grains. All quartz grains, with the exception of those in some sedimentary clasts, show undulose extinction and grain boundary annealing. These deformed textures (fibrous, stretched, undulose) are typical of tectonic terranes. Although the previously reported “vein quartz” has been dominantly interpreted as igneous in origin, this evidence for a tectonized metamorphic source terrane supports Newberry’s (1873) original report that the pebbles were “portions of quartz vein . . . from some area where metamorphic, crystalline rocks have suffered erosion.”