Type section and use of name in Indiana: The Backbone Limestone was named for the ridge called the Devil's Backbone Ridge along the Mississippi River north of Grand Tower, Jackson County, southwestern Illinois (Savage, 1920, p. 173). There in the type section in a quarry (SE¼SW¼SW¼ sec. 94, T. 10 S., R. 4 W.) the Backbone consists of less than 100 feet of light-colored coarse-grained thick-bedded fossiliferous limestone.
The term Backbone was later defined in the Illinois Basin, including in subsurface Indiana, by Collinson and others (1967, p. 941) and Becker (1974, fig. 11C and p. 28). Further refinement for Indiana was made when Becker and Droste (1978, fig. 9 and p. 4-5) recognized that Backbone-like deposits were in interfingered relationship with Grassy Knob-like rocks and in a complementary thickness, partial-facies relationship with Clear Creek-like deposits they also recognized that the clean coarse carbonate rocks at the top of what had been called the Bailey Limestone more appropriately belonged in the lowermost Backbone. In effect, therefore, an arbitrary vertical cutoff was defined between the two parts of the Grassy Knob facies relationship and placed along the farthest basinward extension of the lowest tongue of Backbone-like lithology. (See, for example, Becker and Droste, 1978, fig. 5, sec. GG'; also the map in Droste and Shaver, in preparation, fig. 8.) This cutoff placed in the Backbone most of the cherry rocks that for a short time had been called the Grassy Knob in Indiana. Droste and Shaver (in preparation, fig. 8) also recognized the same cutoff boundary as leering almost wholly beyond the state boundary.
Description: The Backbone is characteristically made up of light-colored medium- to coarse-grained, rather pure bioclastic limestone. In accord with the Indiana definition given above, however, the Backbone also has two prominent intervals of drab cherry dolomitic limestone and dolomitic chert that are overlain and underlain by coarse-grained Backbone limestones. Some glauconite is present. (See typical sections in Becker and Droste, 1978, and Droste and Shaver, in preparation.)
The Backbone conformably overlies the drab fine-grained carbonate rocks of the Bailey Limestone (Silurian) in the deeper part of the Illinois Basin and probably conformably and unconformably overlies reef and nonreef rocks of the Wabash Formation (Silurian) toward the basin margin. Similarly, the Backbone has conformable and unconformable relations with overlying rocks, the Clear Creek Chert (Lower Devonian) and the Jeffersonville Limestone (Middle Devonian) respectively. The boundary with the Clear Creek is placed where light-colored fine- to medium-grained, somewhat cherry rocks (Clear Creek) become dominant over the coarser and purer carbonate rocks below (Backbone).
In its approximately 10-county area of distribution in subsurface southwestern Indiana, the Backbone thickens southwestward from am erosional zero to a north-south elongate area of maximum thickness in the westernmost counties south of Vigo County. The Backbone also thickens southward along this area, so that the thickest deposits may reach 600 feet (183 m) in Posey County. Westward from this area thinning occurs basinward, probably because of a complementary relationship with the Clear Creek Chert, which generally onlaps and overlies the Backbone.
Correlation: The Backbone of Indiana extends to rocks of the same name in the Illinois and Kentucky parts of the Illinois Basin, but because of varying definitions in the Illinois Basin simple statements on correlation cannot be made. In the Indiana definition, lower Backbone rocks correlate (as facies) with the Grassy Knob of extreme southwestern Indiana and adjacent Illinois, and upper Backbone rocks probably have a partial complementary age relationship with lower Clear Creek rocks of Indiana and Illinois.
The lowermost Backbone beds, common to Indiana and Illinois (often called the upper Bailey in Illinois), have yielded the conodont Icriodus woschmidti (Collinson and others, 1967, p. 940), which indicates an earliest Devonian (Gedinnian) age (early Ulsterian, American standard). Other Lower Devonian fossils found in Illinois indicate ages as young as Emsian (late Ulsterian) for the Backbone of Indiana and correlative Illinois rocks. (See Collinson and Atherton, 1975, p. 109-119, and Shaver and others, 1985.)