A Carbonate Hardground from the Point Pleasant Limestone (Middle Ordovician) of Bracken County, Kentucky (section authored by Michael Miller)

The Point Pleasant Limestone is Middle Ordovician in age and exposed / collected in Bracken county Kentucky. The hand sample ranges in color from medium dark gray (fresh surface) to yellowish gray (weathered surface). Brachiopods and echinoderms are visible by the unaided eye in the hand sample matrix. This matrix is biopelsparite with grains that are coated and abraded. These grains include shell fragments, peloids, and mud/silt. Biologic borings are found in the upper portion of this specimen, they have been filled with mud and silt and are very distinct from the typical spary matrix. The grains, as seen in the thin section, are moderately sorted and show signs of abrasion and are also coated. Many of the shell fragments are rounded and the majority of peloids are well rounded, this suggests a high-energy depositional environment. Under the Folk classification the Point Pleasant Limestone is a biopelmicrite and the under the Dunham classification it is a packstone. The proposed depositional environment is shallow marine within wave base because of the high incidence of rounding seen in the grains.

Michael Miller, Sophomore at The College of Wooster. Michael is from Louisville, Ohio.

Figure 1. This photograph of a thin section of the Point Pleasant Limestone shows the typical matrix of the sample. Bioclasts fragments including bivalve shells, echinoderms, and bryozoans can be found throughout the specimen. Peloids showing abrasion and rounding are also typical for this material. The large number of shell fragments and abraded peloids provides evidence of a high-energy depositional environment. Width of view = 3.0 mm (40x).

Figure 2. This photo shows the fill of the borings present in the upper portion of the sample. This matrix consists mainly of silt and mud. Width of view = 3.0 mm (40x).

Figure 3. This image shows the contact area between a boring and the matrix of the limestone. The typical shell fragments, some replaced with calcite, can also be seen throughout this image. Two peloids (center) are surrounded by a spary matrix and can be found throughout the specimen. Width of view = 3.0 mm (40x).

Figure 4. This is a photograph of the biopelsparite matrix. The spary matrix contains several peloids and fossil fragments. These include brachiopods (= 30%), echinoderms (= 30%), and a few trilobites (= 5%). The brachiopods are unique in that they have a coating of calcite outlining them. The echinoderms show micrite overgrowths and the identifiable trilobite fragments are of the thoracic portion. Width of view = 3.0 mm (40x).