A Carbonate Hardground from the Lebanon Limestone (Middle Ordovician) of Tennessee (section authored by Kate Joynt)

The Lebanon Limestone formation is Middle Ordovician in age and can be found in Tennessee. The weathered surfaces of the hand sample are yellowish gray in color and the fresh surfaces are medium gray. Within the hand sample, one can see many fossils, especially mollusks and echinoderms, and intraclasts. The matrix of the Lebanon Limestone is sparry and toward the top of the rock is a thick layer of calcite. The thin section of the Lebanon Limestone shows a sparry matrix and grains made up of mostly echinoderm fossils, chrinoid stems, and rounded intraclasts. The grains are poorly sorted with about sixty-five percent of the grains being bryozoans, five percent intraclasts, ten percent chrinoid stems, and twenty percent well-rounded intraclasts. The Folk name for the Lebanon Limestone is Intrabiosparite and the Dunham name is Grainstone. The depositional environment of this rock was probably shallow marine. A high energy environment would be necessary to create an erosional force, which would cause the fossil fragments and intraclasts in the rock.

Kate Joynt, Junior at The College of Wooster. Kate is from Peoria, Illinois.

Figure 1. This photograph of the thin section of the Lebanon Limestone shows an echinoderm fossil in the center. The fossil is distinguished by its parallel borings on either side of the shell. It is surrounded by sparry calcite mixture, fossil fragments, and in the upper left corner is an intraclast. Width of View = 3.0mm (40x).

Figure 2. This photograph shows a large well rounded intraclast in the center. When looking at the thin section, several of these intraclasts appear. It is surrounded by sparry calcite matrix. Width of View = 3.0mm (40x)

Figure 3. This is a picture of a bryozoan fossil. Although bryozoan fossils were not extremely common in the rock, there are several good examples in the thin section. In the Lebanon Limestone there is a great variety of fossil fragments. Width of View = 3.0 mm (40x)

Figure 4. This picture shows the sparry calcite matrix, which is found throughout the rock. It surrounds all the fossils fragments and intraclasts in the rock. Width of View = 3.0mm (40x)