Alteration of primary igneous minerals was studied in sedimentary volcanic deposits of the middle Eocene Aycross formation, Pitchfork formation, and early basic breccia of the Absaroka Range, Wyoming. Olivine, hypersthene, augite, hornblende, and plagioclase are altered to varying extents in these beds. Chlorite, clay, barite, calcite, and chalcedony are present in pseudomorphs after the primary igneous minerals. The sequence of deposition, shown above from earliest to latest, is indicated by replacement relations in pseudomorphs and by successive coatings and replacements in amygdules. A groundmass of recrystallized clay minerals and authigenic chlorite characterizes the beds in which alteration is most complete. Zeolite and koninckite occur as authigenic minerals in some of the sedimentary deposits. Alteration of igneous minerals to clay was accomplished by weathering in a warm, humid climate after the sediment was deposited; later replacements by barite, calcite, and chalcedony took place after burial. Different degrees of alteration are explained by differences in rate of burial and depth of the water table. This alteration of primary igneous minerals greatly affects the reliability of stratigraphic correlations based upon mineral analyses, and may also lead to misidentification of the original composition of the volcanic material.

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