Stratigraphy and the Origin of Silica at Gal Hill, Green River Formation, Sanpete County, Utah: A Petrographic and Geochemical Study

Cherts occur alongside carbonate rocks in many different depositional contexts, however their exact modes of formation are not always understood. This study collected and examined chert, carbonate, stromatolite, and tuff samples from Gal Hill, an outcrop of the lacustrine carbonate Green River Formation in Central Utah, and constructed a stratigraphic column of the locality in order to determine depositional environment, a timeline for chert formation, and a relationship between dolomitization and volcanic eruptions at the Gal Hill locality through the use of x-ray diffraction and petrographic analysis. Cherts at Gal Hill were classified either as N-chert, which referred to nodular chert, or B-chert, which designated chert that precipitates along bedding planes in the case of silicified stromatolites. The strata of Gal Hill are dominated by a thinly laminated calcimudstone, leading to the interpretation of its depositional environment as a mudflat. The most prominent bed at Gal Hill is a 4-foot thick tuff bed from 11 to 15 feet in the stratigraphic column. Tuffs at Gal Hill were found to be generally porphyritic with euhedral biotite and plagioclase phenocrysts in a crushed glass matrix. Carbonate samples were found to contain high amounts of the clay mineral illite. This study concludes that silica at Gal Hill was sourced from a combination of smectite-illite transformations and volcanic ash weathering. N-Chert and B-chert were both found to often contain a mixture of microcrystalline quartz and mud. The silica mineral chalcedony was also found to occur in some Gal Hill chert samples. The presence of chalcedony indicates that silica at Gal Hill precipitated from solution and not from a gel. Silica at Gal Hill was also found to have precipitated while the carbonate sediment was still soft. Based on the presence of carbonate “ghosts” in many chert samples, much of the silica at Gal Hill was found to have formed through the replacement of original carbonate sediment. Evidence for a relationship between volcanic ash and dolomitization was inconclusive