Abstract

Ooids in grainstones from the Spring Hill Member of the Pennsylvanian Plattsburg Limestone in southeastern Kansas were precipitated as both aragonite and calcite. Either phase predominated in different outcrops, in different units within an individual outcrop, in different grains within an individual sample, and in different portions of an individual ooid cortex. Selective diagenetic dissolution of aragonite cortices gave rise to a variety of fabrics reported from many other politic limestones. These include oomoldic porosity, sparry cortices, strings of spastolithic (deformed) and elephantine (trunk-to-tail) connected ooids, and displaced nuclei. These and other reported occurrences of fabrics which record the former presence of aragonite are not evenly distributed through Phanerozoic time. Rather, they predominate during times of general continental emergence. While this distribution supports the postulations of Mackenzie and Pigott (1981) and Sandberg (1983) that the dominant mineralogy of ancient marine limestones may have corresponded closely to positions of global sea level, fabrics of the Plattsburg Limestone demonstrate that evaluating changes in ooid compositions with geologic time is not a simple "either aragonite or calcite" proposition.

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