A Carbonate Hardground from the Great Oolite, White Limestone (Middle Jurassic, Bathonian) of Gloucestershire, England (section authored by Kirk Lapham)

The Great Oolite White Limestone is Middle Jurassic in age and is exposed near Gloucestershire, England, among other locations. The fresh surfaces of the hand sample are tan in color and weathered surfaces are grayish-tan in color. Intraclasts and shell fragments (mostly oysters) are visible in the hand sample. The matrix of the Great Oolite White Limestone is micritic and contains several biologic borings. The thin section of the Great Oolite White Limestone also shows a micritic matrix and grains composed mostly of bivalve shells and some rounded intraclasts. The grains are moderately sorted with ninety percent of the grains made up of replaced bivalve shells and ten percent made up of well-rounded intraclasts. Under the Folk classification the Great Oolite White Limestone is an intrabiomicrite and the under the Dunham classification it is a packstone. The depositional environment is shallow marine, above the wave base and within the photic zone. The fragmentation of the shells indicates an erosional energy source (waves) and the intraclasts also indicate waves as an erosional agent.

Kirk Lapham, Junior at The College of Wooster. Kirk is from Mason, Michigan. (We had to scramble the name on his shirt.)

Figure 1. This photograph of a thin section of the Great Oolite White Limestone shows a large, calcite replaced bivalve shell (center). Intraclasts are also visible to the left and the right of the bivalve. The intraclasts are larger than the other grains and well rounded. Many shell fragments can been seen through out the photograph, evidence of a high-energy depositional environment. Width of view = 3.0 mm (40x).

Figure 2. This photograph shows three large, well-rounded intraclasts. Shell fragments are also apparent throughout the thin section. Most of these shell fragments are bivalves, replaced by calcite. Several other smaller intraclasts are visible, which are also well rounded. The intraclasts are also evidence of a high-energy depositional environment. Width of view = 3.0 mm (40x).

Figure 3. This photograph shows a wide range of bivalve shell fragments, replaced with calcite. Two intraclasts are also included in the photograph. While the rock is grain supported, a micritic matrix (very fine-grained calcite) is also included throughout the rock. Width of view = 3.0 mm (40x).

Figure 4. This is a magnified photograph of a large mass of the micritic matrix. The fine-grained calcite fills in around the intraclasts and bivalve shell fragments and holds the rock together. Several smaller intraclasts and bivalve shell fragments are also visible throughout the photograph. Width of view = 1.2 mm (100x).