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Oolitic grainstone, part of a sand shoal near the edge of a 330 million year old sea. Spaces between ooids* are filled with calcite and iron-oxide cement (arrow points to iron oxide layer). Ooid at the bottom grew around a fragment of fenestrate bryozoan. This rock formed the substrate for a small microbial mound.

Article in the Journal of Paleontology: DOI: https://doi.org/10.1017/jpa.2019.103

*Notice the concentric bands and radiating elements within the ooid cortexes. Most ooids nucleate around carbonate particles that roll around in high-energy tropical marine environments. Similar ooids form today in the Bahamas and other places.