Abstract

Bimineralic ooids consist of an inner core and an outer cortex of dissimilar calcium carbonate polymorphs. Similar bimineralic ooids have been described from the Proterozoic in Montana, the Cambrian in Newfoundland, the Mississippian in Arkansas, the Pennsylvanian in Kansas, the Jurassic in the Gulf Coast, the Pleistocene on the Florida Shelf, and the Recent in Baffin Bay, Texas. In all these examples the bimineralic ooids preferentially consist of an inner core of calcite (high or low Mg) with radial and/or radial-concentric structure, and an outer cortex of aragonite with concentric structure. After diagenesis the ooids will have an inner core with radial and/or radial concentric structure with microspar (if originally Mg calcite) or without microspar (if originally calcite), and an outer cortex of mosaic calcite (originally aragonite).

In the Pennsylvanian Canyon Formation the majority of the ooids either are represented by oomoldic porosity, or are composed of mosaic calcite (if they were originally aragonitic) or microspar (originally Mg calcite). Canyon bimineralic ooids are rare, and consist of an inner core of mosaic calcite (aragonitic ooid) with an outer cortex of radial-concentric structure (calcitic ooid), a reversal of the pattern seen in the prior documented cases. A model for diverse types of ooids has been proposed for the Cambrian Port-au-Port ooids in Newfoundland in which aragonitic ooids form in the intertidal zone and calcitic ooids form seaward (subtidal). In the transition zone there are aragonitic and calcitic ooids plus minor bimineralic ooids.