A Carbonate Hardground from the Castle Hayne Limestone (Eocene) of New Hanover, North Carolina (section authored by Tom Johnson)

The Castle Hayne Limestone, which is exposed in New Hanover, North Carolina, is Eocene in age. The weathered surfaces of the hand sample are a dark greenish gray color while the fresh surfaces are medium dark gray in color. A sparry matrix is visible throughout the hand sample, along with fossils of both bryozoans and bivalve borings. The thin section of the Castle Hayne Limestone also shows a sparry matrix and many colorful grains of sand. The grains of the sample vary in roundness from being angular through sub rounded and are also poorly sorted. Most of the thin section specimen is composed of quartz grains, around fifty-five percent, with thirty percent of the grains are made up of bivalve shells, intraclasts, and bryozoans, while the final fifteen percent consists of little amounts of haematite, biotite, and glauconite. This specimen, by using the Folk classification system, is known as a sandy biointrasparite, and by using the Dunham classification system, it is called a packstone. The depositional environment of the Castle Hayne Limestone is in the open ocean, above wave base. The intraclasts, bivalve shells, and bryozoan fossils indicate that waves acted on the formation of the Castle Hayne Limestone.

Tom Johnson, Junior at The College of Wooster. Tom is from Wilmerding, Pennsylvania.

Figure 1. This is a photograph of an acetate peel of the Castle Hayne Limestone, which shows a large stick-shaped bryozoan running horizontal through the picture. The photograph also shows the characteristic inclined cellular structure and regular, fibrous walls. Width of view = 3.0 mm (40x).

Figure 2. Here is a photograph of a bivalve shell (running from top left to bottom right), that has been replaced by calcite. The calcite is from a dissolution precipitation mechanism, which destroyed all relict internal shell structure. Colorful quartz grains enveloped in a sparry matrix surround the shell. Width of view = 3.0 mm (40x).

Figure 3. In this photograph of a thin section of the Castle Hayne Limestone, a large fecal pellet is the focal point. It shows the common rounded, rod-shaped outline of fecal pellets. Again, a sparry matrix surrounds quartz grains along with the brown pellet, which runs from the bottom left corner to the top right corner of the picture. Width of view = 3.0 mm (40x).

Figure 4. This photograph shows an enlarged view of a wonderfully preserved calcite spar overgrowth. The overgrowth formed on a bivalve shell and kept growing until it formed into what you see running horizontal in the center of the picture. The growth of the calcite into the sparry matrix is also visible in this slide. Width of view = 1.2 mm (100x).