A Carbonate Hardground from the Dunleith Formation (Rivoli Member, Middle Ordovician) of Iowa (section authored by Andrea Martin)

The Dunleith Formation of the Rivoli member is Middle Ordovician in age. This carbonate hardground sample was found in Winneshiek county, Iowa. The weathered surface of the rock is yellowish gray in color. The fresh, or cut, surface of this rock is very light gray. In the handsample unidentifiable shell fragments are visible. Also seen are large areas of peloids that are separate from the rest of the rock matrix. The location of these peloids may indicate burrows of organisms. This sample is composed of a micritic matrix. The Folk classification name given to this rock is a pelbiomicrite. The Dunham name is a packstone. The micritic matrix indicates a lower energy depositional system. The shell fragments indicate that there was some water energy capable of weathering the shells present in the system.

Andrea Martin, Sophomore at The College of Wooster. Andrea is from Draper, Virginia.

Figure 1. This thin section shows several echinoderm fragments in a micrite matrix. Also visible in this view are some shell fragments, probably pieces of brachiopod shells. Width of view = 3.0 mm.

Figure 2. In this picture a brachiopod shell can be seen. There are also echinoderm fossils present. Throughout the view, unidentifiable shell fragments can be seen sitting in the micritic matrix. Width of view = 3.0 mm.

Figure 3. A section of peloids (the darker rounded objects in the top right of the image) can be viewed in this part of the thin section. These peloids are separated from the rest of the matrix in what was once probably a burrow of an organism. Width of view = 3.0 mm.

Figure 4. This is a picture of an acetate peel, which is a replica of the cut surface of the sample, not an actual thin section taken of the rock. This image is also showing a section of the peloids that can be found in several places throughout the rock. Width of view = 3.0 mm.