Geometry and Origin of Oolite Bodies in the Ste. Genevieve Limestone (Mississippian) in the Illinois Basin

Abstract

Oolitic limestones constitute 22 percent of the Ste. Genevieve and Paoli Limestones (Mississippian) in seven cores equally spaced along the length of outcrop in southern Indiana; which lies on the eastern margin of the Illinois Basin. Similar oolitic limestones are found in equivalent stratigraphic formations in Kentucky and Illinois. Isopach mapping and analysis of 889 crossbedding measurements in the Illinois Basin indicate that paleoslope during deposition of these oolitic limestones was to the southwest, as it was earlier during deposition of carbonate sands of the Salem Limestone (Mississippian) and later during deposition of most of the late Paleozoic sands. An elongate lenticular Ste. Genevieve oolite body about 2 miles (3.2 km) wide, more than 4 miles (6.4 km) long, and about 25 feet (7.6 m) thick, partly exposed in a quarry near Orleans, Ind., has been found to be oriented almost perpendicular to the paleoslope of the basin, as determined by isopach mapping and measurement of crossbedding. Synthesis of the environment of deposition based on lithologic relationships, geometry, and internal features indicates that the oolite body is a marine sand belt that has some features much like those forming today on the west edge of the Great Bahama Bank. Other Ste. Genevieve oolite bodies in the subsurface of the Illinois Basin, which are similar in shape and size to the oolite body at Orleans, are believed to have had a similar origin. Although predicting the location of oolite bodies of this type in the Illinois Basin is difficult, isopach mapping and crossbedding analysis may help in determining the orientation of these bodies near the outcrop. Based on 81 thin sections from the oolitic limestone lithology near Orleans, the average grain size is 1.47ø (.36 mm) and the average sorting is 0.90ø. Best sorting is 0.50ø, which is similar to that reported for other ancient oolitic limestones, but which is somewhat poorer than that found in modern oolites. Grain distribution for the oolite body at Orleans is generally negatively skewed near the base of the body, but in the upper part ranges from about 0 to +O.50. The oolitic limestone at Orleans is suited for many industrial purposes because its calcium and magnesium carbonate content averages more than 99 percent by weight. The apparent porosity of the center part of the oolite body is 3.38 percent, but the porosity of the exterior rind, averaging 1.25 percent, is much less. The relatively high apparent-porosity zone in the center part of the oolite body appears to be similar to porosity zones in oolite bodies that are found in the subsurface of the Illinois Basin and that are excellent reservoirs for petroleum.