File:Black chert nodule (Delaware Limestone, Middle Devonian; Emerald Parkway roadcut, Dublin, Ohio, USA) 2 (42189051211).jpg

The Delaware Limestone is a significant carbonate unit in the Devonian of central and northern Ohio. It's actually part of a much more widespread sheet of Devonian carbonates that extends from New York State to the Midwest. The Delaware Limestone represents deposition in a subtropical, shallow-water, carbonate platform environment. The rocks are principally micritic limestones, fossiliferous wackestones, and fossiliferous packstones. Fossils are typical Paleozoic shallow marine invertebrates.

The thin-bedded Delaware Limestone is underlain by the thick-bedded Columbus Limestone (also Devonian). The contact is a prominent disconformity (a type II sequence boundary). Biostratigraphic studies have shown that one conodont biozone is missing at the Columbus-Delaware contact in central Ohio, probably representing ~1 to 3 million years. The Delaware Limestone is overlain by soft gray clayshales of the Olentangy Shale (lower Upper Devonian).

Seen here is a black chert nodule hosted in gray limestone. Chert is a microcrystalline to cryptocrystalline, quartzose sedimentary rock. It can occur as discrete nodules or masses having various shapes or can occur as bedded chert. Chert forms diagenetically (= post-depositional) by chertification as water-rich fluids charged with dissolved silica migrate through and partially replace sedimentary rocks - usually limestone. Chert can also be biogenic in origin, as buried siliceous organic skeletons (e.g., hexactinellid sponges, radiolarians, etc.) are dissolved and the silica reprecipitates nearby.

Stratigraphy: lower Delaware Limestone, Eifelian Stage, lower Middle Devonian