Limestones in the Devonian of Ohio, USA.

Thin bedded limestones in upper part of photo = Delaware Limestone Thick-bedded limestones in lower part of photo = Columbus Limestone

The Delaware and Columbus Limestones are significant carbonate units in the Devonian of central and northern Ohio. They're actually part of a much more widespread sheet of Devonian carbonates that extends from New York State to the Midwest. The two units represents deposition in a subtropical, shallow-water, carbonate platform environment. The rocks are principally micritic limestones, fossiliferous wackestones, and fossiliferous packstones. Some chert nodules are present in the Columbus Limestone. Fossils are typical Paleozoic shallow marine invertebrates - favositid corals, rugose corals, stromatoporoids, brachiopods, crinoids, blastoids, bryozoans, trilobites, bivalves, gastropods, cephalopods, rostroconchs, and tentaculites. Microfossils include conodonts and charophyte oogonia. Other fossils in the Columbus Limestone include vertebrates (fish), land plants (rare), and trace fossils. Some fossil horizons in the Columbus Limestone are partially silicified.

The Columbus-Delaware contact is a prominent disconformity (= type II sequence boundary). Biostratigraphic studies show that one conodont biozone is missing at the contact contact in central Ohio, probably representing ~1 to 3 million years. The base of the Columbus Limestone (not seen in this photo) is a major, continent-wide unconformity representing the Tippecanoe-Kaskaskia megasequence boundary (= type I sequence boundary).

The boundary bed is a bentonite (= altered volcanic ash bed) that has been correlated with the Tioga Bentonite of New York State.

Stratigraphy: basal Delaware Limestone (Eifelian to Givetian Stages, Middle Devonian) over uppermost Columbus Limestone (Eifelian Stage, lower Middle Devonian)