Volcaniclastic sandstones from the Upper Cretaceous Horsethief Formation, Montana, are characterized by two distinct cement assemblages; (1) Corrensite + or - Calcite + or - Dolomite, which is restricted to samples from delta distributary channel and distributary mouth bar environments, and (2) Montmorillonite + or - Calcite, which characterizes bay-beach, crevasse splay, lagoon, barrier island, and shallow subtidal environments. Petrographic, microprobe, scanning electron microscope, and fluid inclusion analyses indicate the early stage origin of these assemblages as authigenic chemical precipitates. In Horsethief volcaniclastics, pore-space reduction by precipitation of the clay minerals corrensite and montmorillonite reflects the chemistry of early stage pore-fluids, and does not involve syn- or post-depositional introduction of detrital clays.

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