Sulfate-mineral crusts from pyrite weathering and acid rock drainage in the Dakota Formation and Graneros Shale, Jefferson County, Nebraska

Pyrite weathering in the uppermost Dakota Formation and lower Graneros Shale (Cretaceous) near Fairbury, Nebraska, produces acid materials and soils (typically pH<6), acid rock drainage (pH<2.5), and surface crusts of gypsum and the hydrated Fe3+ and Al-sulfate minerals copiapite, alunogen, metahohmannite, coquimbite, paracoquimbite, and meta-alunogen. The Fe and Al minerals produced by acid-sulfate weathering are relatively rare (heretofore undocumented from Nebraska), and form in quantity only during episodes of dry weather lasting up to several weeks. The maturation of mineral crusts on the Dakota Formation involves the dehydration of hydrous Fe3+ and Al minerals. Precursor Fe2+ sulfates, as found in many other cases of pyrite weathering, were not observed in surface crusts during the study period. Surficial mineral crusts on the Graneros Shale consist only of gypsum with included clay minerals and quartz derived from the underlying shale. The processes described herein are noteworthy in a regional geochemical and hydrogeological context, and also by virtue of their apparent dependency on weather conditions. id-sulfate weathering sites described herein seem to be relatively unique in the surrounding region. Nonetheless, the study of these sites suggests that there are potentially widespread effects of natural acid-sulfate weathering associated with the Dakota Formation and other pyritiferous Cretaceous strata in the region, and that the localized effects of anthropogenically induced acid-sulfate weathering in roadcuts, quarries, and other excavations can present engineering problems. Acid-sulfate weathering is likely to have influenced the development of regional groundwater chemistry during the Quaternary and may create adverse engineering and environmental conditions, such as the retardation of vegetation cover, encouragement of subsoil gypsum growth, acceleration of shale slaking, and possibly even the corrosion of roadbed materials.