Mississippian System

Type section and history of name: The Popcorn Sandstone Bed was named by Swann (1963, p. 32-33) for 7 feet (2.1 m) of fine-grained calcareous sandstone at the base of the Paoli Limestone in Malott's (1952, p. 80) Popcorn Spring section in the SE¼SE¼SW¼ sec. 5, T. 6 N., R. 2 W., Lawrence County. Malott described this as "Aux Vases sandstone" and also described as Aux Vases 4.5 feet (1.4 m) of calcareous green unctuous shale exposed along a bluff near the junction of Popcorn Creek and Indian Creek (SW¼NW¼SE¼ sec. 18, T. 6 N., R. 2 W.), less than 2 miles (3.2 km) west of the Popcorn Spring section (Malott, 1952, p. 81).

When this partly clastic unit at the base of the Paoli Limestone was first recognized as an important and widely traceable unit on the Indiana outcrop, it was tentatively correlated with the Aux Vases Sandstone of Missouri and Illinois (Malott and Esarey, 1940, unnumbered plate). Later the unit was more fully described, and the correlation was more positively asserted (Malott, 1946, p. 325 Malott, Esarey, and Bieberman, 1948 Malott, 1952). On the basis of subsurface studies, however, Swann and Atherton (1948) and Pinsak (1957) indicated that the Aux Vases of Malott properly correlates with the base of the Shetlerville Limestone Member of Illinois rather than with the somewhat older Aux Vases Sandstone. Gray, Jenkins, and Weidman (1960, p. 49-50), therefore, abandoned the name Aux Vases for surface usage in Indiana and considered this unit as a part of the Paoli Limestone. Adoption here of the term Popcorn Member provides an appropriate and useful name for this unit, which marks the base of the Chesterian Series in Indiana.

Description: Along the outcrop in Indiana the Popcorn Member is a few inches to 13 feet (0.1 to 4.0 m) thick, but it generally ranges from 1 to 3 feet (0.3 to 0.9 m) in thickness. In some places the Popcorn is entirely calcareous sandstone, and in others it is entirely calcareous green shale, but in most places along the outcrop mixed lithologies prevail. Argillaceous and arenaceous limestones are probably the dominant lithologies, but in places they are interbedded with green calcareous shale or fine-grained calcareous sandstone. A variety of limestone textures is common, including detrital, micritic; and oolitic in places silty dolomite and dolomitic limestone are present. Bedding varies from nearly horizontal parallel orientation where the finer grained shale and siltstone are dominant to crossbedding in coarser grained quartzose sandstone and impure carbonate sandstone. The Popcorn is similar to other impure carbonate units in the Paoli and the Ste. Genevieve, which may make identification difficult, but it can be distinguished from the shale and sandstone of the Bethel Formation, which are not calcareous.

The Popcorn Member underlies the Shetlerville Member of the Paoli Limestone and overlies the Ste. Genevieve Limestone. Although it is missing from a few places, the Popcorn Member is widespread in the outcrop area of the Paoli Limestone from Crawford and Harrison Counties on the Ohio River northward to Putnam County, where it is disconformably truncated by the Mansfield Formation (Pennsylvanian). It is clearly recognizable in cores near the outcrop, but deeper in the subsurface it is less certainly known. According to some opinion, it is within the so-called "lower Renault" of common subsurface usage and is difficult to recognize because it is thin (Pinsak, 1957 Swann, 1963).

Correlation: Although the Popcorn Member marks the base of the Chesterian Series in Indiana, it is not widely recognized elsewhere in the Illinois Basin. According to Swann (1963), correlative rocks are found in the basal part of the Shetlerville Limestone Member of the standard Chesterian section.