(JBD & RHS)
Type area, principal reference section, and use of name: The name Vernon Fork Member was given by Droste and Shaver (1975a, p. 404-406) to the rocks often called the laminated beds. Laminated Zone, chalk beds, or fine-grained dolomite of the Jeffersonville Limestone. The type area was designated as the area near Vernon and North Vernon along Vernon Fork of the Muscatatuck River, Jennings County, Ind. A principal reference section was established in the Berry Materials Corp. quarry (old name is Paul Frank quarry) at the north edge of North Vernon (NE¼ sec. 34, T. 7 N., R. 8 E.). At this reference section the Vernon Fork Member, containing the Tioga Bentonite Bed, is 17 feet (5 m) thick and lies between the Paraspirifer acuminatus Zone (above) and the Amphipora Zone (below) of the Jeffersonville; the top of the Geneva Dolomite Member is about 6 feet (1.8 m) farther down. At the same time the Pendleton Sandstone Bed was assigned to the Vernon Fork.
Description: Three major kinds of dolomites are found in the Vernon Fork and are generally found in the same stratigraphic order. The characteristic lower variety shows evidence of cyclic sedimentation. A single cycle begins at its base with a medium- to light-gray very fine grained dolomite with rounded quartz grains, massive to wavy irregular thin bedding, and gradational color change into the upper part of the cycle. The upper dolomite of a cycle is medium yellowish brown to light yellowish brown and very fine grained. On outcrop the single full cycle is less than 10 feet (3 m) thick in cores the cycle approaches 20 feet (6 m) in thickness. Three to five complete and partial cycles are recognized.
A second variety of Vernon Fork dolomite is fine to medium grained, finely vuggy, and mostly brown. This variety is most common in the subsurface in western Indiana counties adjacent to Illinois. The third variety of carbonate rock of the Vernon Fork ranges from dolomite to almost pure limestone. Its most obvious single characteristic is its lithographic to sublithographic texture. These rocks are weakly to very strongly laminated and brecciated, and their striking appearance gave rise to their designation as the laminated beds of the Jeffersonville.
Both the top and bottom parts of the Vernon Fork are marked by concentrations of rounded and frosted sand grains. The basal part is sandier and where well developed is called the Pendleton Sandstone Bed.
The Vernon Fork conformably overlies the Geneva Dolomite Member of the same formation in some sections and Jeffersonville beds unnamed to member in other sections (rocks of the Amphipora Zone as noted above). It is overlain, possibly unconformably, by the North Vernon Limestone in central Indiana; in southeastern Indiana, however, it is overlain conformably by other Jeffersonville beds unnamed to member (rocks of the Paraspirifer acuminatus Zone).
The Vernon Fork has a roughly semicircular area of distribution in 30 or more counties of western central Indiana (Droste and Shaver, 1975a, fig. 9). In that area it ranges in thickness from an erosional edge along its northern and eastern limits and from a depositional zero (due to facies relationship) along its southern limit to more than 80 feet (24 m) in Vigo County along the Illinois state line.
Correlation: The Vernon Fork is a northern facies of the very fossiliferous Jeffersonville Limestone in the Falls of the Ohio area, specifically that part above the Coral and Amphipora Zones. The facies change cannot be described as occurring along a defined vertical cutoff because the uppermost Jeffersonville zone at the Falls (Paraspirifer acuminatus Zone) extends far north and overlies type Vernon Fork rocks. Therefore, where the youngest Vernon Fork rocks were deposited, they correlate with the highest Jeffersonville biozone.
The Vernon Fork, as described here and as summarized by Conkin and Conkin (1979b, p. 29), has a complex, variable correlative relationship with the named biozones at the Falls of the Ohio. But contrary to the opinion of Conkin and Conkin, which apparently confuses properly defined rock units and biostratigraphic units, the Vernon Fork is defined on the basis of a distinctive lithology, which, of course, can have, and does have, a geographically changing time value.
The upper Vernon Fork rocks also correlate closely with the Cranberry Marsh Member of the Detroit River Formation in northern Indiana, whereas lower Vernon Fork rocks correlate mostly with middle and lower (in part) Detroit River rocks. Probably these Middle Devonian rocks, in both northern and southern Indiana, that exhibit features generally ascribed to penesaline to hypersaline environments were deposited in the one depositional regime and later became separated by erosion before deposition of the overlapping Traverse Limestone and Traverse-equivalent rocks.
The Tioga Bentonite Bed in the Vernon Fork, as well as in many other units in the northeastern United States, helps to establish age relationships with the Onondaga Formation in the New York Devonian standard. Similarly, the association of the Vernon Fork Member with particularly the Paraspirifer acuminatus Zone of the Jeffersonvale suggests age relationships with the upper part of the Columbus Limestone of Ohio. (See Oliver, 1976, p. 19.)