Fossil Content of Local Limestone

The fossil content of local limestone shows the diversity of life in the late Ordovician period.

Bryozoans and brachiopods, nearly equal in abundance, form about 60% of the allochems in local limestone.

assorted brachiopods and bryozoa fossils

Crinoids form about 20% and trilobites nearly 10% of the allochems.

coral, gastropod, trilobite, and mollusc fossils

According to detailed studies by Martin* (1975), brachiopods and trilobites decrease appreciably in abundance upward in the Cincinnatian Series. In contrast, algae, ostrocodes, and corals, which are uncommon throughout most of the sequence, are very common in the upper part. Dolomite forms less than 1% of the limestones of the lower part of the sequence, but more than 5% of those in the upper part.

* Martin, W.D., 1975. The petrology of a composite vertical section of the Cincinnatian Series limestones (Upper Ordovician) of southwestern Ohio, southeastern Indiana, and northern Kentucky: Jour. Sed. Petrology, v 45, no. 4, pp. 907 -925.

Photomicrographs of local limestone samples showing fossil fragments (Bc=brachiopod, Br=bryozoan, O=ostracode, T=trilobite) and matrix (M=microspar, P=pseudospar). Both intact and fragmented, but recognizable shells and skeletons are known as allochems. Finer-grained mud-sized particles initially lithify to micrite and can later recrystallize to microspar or pseudospar. Adapted from Martin, 1975.