Samples from eight outcrops along strike of the Cretaceous-age Marshalltown Formation show considerable variation in the proportions of different types of greensand pellet. The large, dark, very smooth ovoid pellets are invertebrate feces or pseudofeces, consisting of quartz grains and silt enswathed in apatite. Botryoidal glauconite pellets are formed by the agglomeration of smaller fecal pellets, or by expansion of single, originally ovoid pellets. Accordion-shaped glauconite pellets form by uniaxial expansion of feces with the constituent clay flakes in an original transverse preferred orientation. The breakup of accordion grains, promoted by the growth of interstitial minerals along the folia, produces micaceous glauconite grains. Pale-green grains are incompletely glauconitized fecal pellets. Very few glauconite pellets from this formation are recognizable as internal molds of foraminiferids. There is no evidence that quartz alters to glauconite, but it is possible that glauconitization-expansion may split quartz grains. There is evidence that glauconitization is syndepositional.

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