Half-moon oolites are described from the Upper Cambrian Allentown Dolomite from Carpentersville, Warren County, New Jersey. In hand specimens, their peculiar aspect is due to an internal dividing line convex upward which separates them into upper light and lower dark portions. The microscope reveals that such an aspect actually results from the settling and deformation of the core and of sets of concentric rings toward the bottom of the outer shell of the oolites. These diagenetically modified bodies are considered to have been originally normal oolites consisting of a calcite core surrounded by alternating layers of calcite and anhydrite (or gypsum). Immediately after deposition and before completion of compaction, the sulfate layers were dissolved generating the internal deformation and settling of the insoluble parts. The cavities were later filled with clear secondary dolomite during the general dolomitization of the beds. Half-moon oolites are outstanding indicators of top and bottom relations, although a rare occurrence in the geological column.

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