A Carbonate Hardground from the Lower Silurian Laurel Limestone of Shelby County, Indiana (section authored by Ryan Hanson)

This section is dedicated to the Laurel Limestone found in Shelby County, Indiana. The hand sample of the carbonate hardground examined here is the top surface of the formation collected from Blue Ridge Stone Quarry near Waldron, Indiana. The Laurel Limestone is early Silurian in age and pale yellowish in color for both fresh and weathered surfaces. Visible in the hand sample are iron sulfide veins traversing throughout the rock. Upon examination in thin section the Laurel Limestone is noted to have a sparry calcite matrix and contain many skeletal grains. The skeletal grains consist of echinoderm fragments, crinoid ossicles, and ostracode valves. These grains are sorted and exhibit sub-rounded to rounded characteristics. Due to these features the most likely deposition environment for the Laurel Limestone is a shallow marine facies above the wave base. Folk and Dunham classification of the Laurel Limestone are applied as biosparite and grainstone.

Ryan Hanson, Senior at The College of Wooster. Ryan is from Chagrin Falls, Ohio.

Figure 1. This photograph shows echinoderm fragments and a crinoid ossicle located near the center. These skeletal fragments exhibit the single calcite crystal structure that is associated with the phylum Echinodermata. Field of view is 3.0mm at 40x magnification.

Figure 2. This photograph of the Laurel Limestone shows a barely distinguishable single ostracode valve in cross section surrounded by echinoderm fragments. Field of view is 3.0mm at 40x magnification.

Figure 3. This photograph of a thin section of the Laurel Limestone exhibits a classic example of a single ostracode valve in thin section. Fragments of echinoderms surround the ostracode valve. Field of view is 1.2mm at 100x magnification.

Figure 4. The photograph here supports the abundance of echinoderm fragments seen throughout this thin section of the Laurel Limestone. The fragments are surrounded by a sparry calcite matrix. Field of view is 3.0mm at 40x magnification.