Abstract

Accounts of epitaxial cements on pelmatozoan fragments assume a progressive reduction in numbers of sub-crystals, which in turn reduces boundary free energy. Pelmatozoan grainstones from Hadeland, Norway, show several departures from this model. In some a specific event, such as an abrupt change in water chemistry, increased the numbers of sub-crystals, which ranged from forms with planar faces, through those showing distorted faces and growth zones, to crystallites embayed by dissolution. Many ossicles show complete or partial dissolution, with subsequent growth of cements restoring optical continuity. The cement stratigraphy suggests that these features have a common cause, with local variations in concentration of some non-essential ion occurring against a background of regional chemical change. Such variation on a centimeter scale is thought to reflect near-surface diagenesis.

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