The Bald Eagle and Juniata Formations are thick Upper Ordovician units which comprise much of the Taconic clastic wedge in central Pennsylvania. Large parts of these formations consist of a homogeneous subgraywacke-sandstone and conglomerate lithofacies of fluvial origin. The Bald Eagle (lower) part of this lithofacies is greenish gray (drab), whereas the Juniata (upper) part is dark red, with thin drab layers. Because drab and red parts of this lithofacies have identical sedimentologic histories, X-ray diffraction investigations of sandstone-matrix clay minerals were undertaken to establish possible differences in abundance and in octahedral-layer cation content of clay-mineral species between the drab and red beds.

The major clay phases present are illite and chlorite. Consistent variations in the ratios illite/chlorite (determined by peak area) and Fe⁺²/Mg in chlorite (determined by structure factors) occur between adjacent drab and red rocks. The illite/chlorite ratio is lower in drab than in red beds, and the Fe/Mg ratio is higher in drab-bed than in red-bed chlorite. Statistically, these ratios are inversely correlative, and suggest that the present clay-mineral distribution is not of depositional origin but is a result of diagenetic modifications of a detrital clay suite. Drab-bed chlorite commonly occurs as coatings between two generations of silica cement, which suggests diagenetic generation of chlorite in drab beds rather than secondary destruction of preexisting

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chlorite in red beds. Reduction and dissolution of hematite pigment in red beds may have supplied the necessary iron.

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