ABSTRACT

Diverse geochemical data on mineral compositions and abundances, coupled with stratigraphic data on mineral distributions, indicate that the difference in color between the Upper Ordovician red Juniata and underlying drab (gray-green) Bald Eagle Formations in central Pennsylvania is of secondary, diagenetic origin. The color boundary separating the two sandstone units cuts across lithofacies boundaries, and varies more than 700 feet in vertical position. Red hematite pigment consistently occurs between grains at points of grain contact in red rocks, whereas drab clay-mineral pigment does not consistently occupy this textural site in drab rocks. At least some of both pigments has been secondarily generated; drab-pigment generation occurred later than red-pigment generation. Opaque-oxid grains are of identical composition in both drab and red rocks, but are much less abundant in drab rocks. Chlorite is more abundant and more iron-rich in matrices of drab sandstones than in those of red sandstones. Drab rocks contain less total iron and less ferric iron than red rocks. These and other data suggest that red pigment was diagenetically removed from the lower portions of an initially all-red sequence by aqueous reduction and dissolution, with generation of iron-rich clay-mineral phases stable in a saturated environment.