Mississippian System

Type and reference sections and use of name: The Spickert Knob Formation was named by Rexroad and Lane (1984) for exposures in Floyd County, Ind., near Spickert Knob along Spickert Knob Road and adjacent gullies to the northeast. This is Stockdale's section 16 (1931, p. 115), which is in the NE¼ sec. 21 and SE¼ sec. 16, T. 2 S., R. 6 E., and extends a few tens of yards into sec, 22, New Albany Quadrangle. The Spickert Knob is the precise equivalent of Stockdale's (1929a) Locust Point and Carwood Formations, which in practice could not be separated as mappable units and which form a single depositional unit. Three good reference sections are (1) the type section of the former Locust Point Formation along a secondary road ascending the Knobstone Escarpment at the Ohio River in the NW¼NE¼ sec. 13, T. 4 S., R. 5 E., Harrison County (Stockdale, 1931, p. 114); (2) a section east of Edwardsville along former Indiana 62 and the Southern Railroad and mainly in the NW¼ sec. 6, T. 3 S., R. 6 E., and NE¼ sec. 1, T. 3 S., R. 5 E. (Stockdale, 1931, p. 221); and (3) farther north, a section exposed along Indiana 135 near Millport in the NW¼ sec. 29, T. 4 N., R. 4 E., Washington County (Kammer, Ausich, and Lane, 1983, p. 62-65). (See "Borden Group" for a discussion of the obsolete names that have been applied to this stratigraphic interval.)

Description: The Spickert Knob Formation consists of rocks between the New Providence Shale below and the Edwardsville Formation above. Both boundaries are conformable and to some degree have intertonguing relationships. The formation represents delta-slope deposits. It is dominated by siltstone but includes abundant silty shale, some sandstone, and minor amounts of limestone. The formation tends to be more shaly and argillaceous in the lower part and to be dominated by massive siltstone in the upper part, but it is a complex of discontinuous lenses that were deposited as shifting and coalescing delta lobes and sublobes. Ironstone nodules and geodes are irregularly distributed in the formation.

In its type area the Spickert Knob Formation ranges between about 228 and 300 feet (69 and 92 m) in thickness. It maintains approximately this range in most of the outcrop area, although its thickness is subject to the vagaries characteristic of deltaic deposition. The Spickert Knob was truncated by erosion before Pennsylvanian sedimentation, however, in the area of Warren County. Southwestward from the outcrop belt, the formation thins away from the source area of the deltaic sediments and is not recognizable separately from the New Providence Shale and the Edwardsville Formation in the subsurface of southwestern Indiana.

Correlation: The Spickert Knob Formation is approximately equivalent to the Nancy Member and the Holtsclaw Siltstone Member of the Borden Formation in north-central Kentucky. A variety of faunal elements (for example, brachiopods, crinoids, and cephalopods) in the formation have ranges that include or fall within the time-stratigraphic limits of the Keokuk Limestone of the Mississippi Valley (Kammer, Ausich, and Lane, 1983). Because the conodonts recovered from the New Providence Shale below (Rexroad and Scott, 1964) and the Floyds Knob Limestone Member of the Edwardsville Formation above (Gates and Rexroad, 1970 Whitehead, 1978) are of Keokuk-depositional age, the Spickert Knob, therefore, must also be equivalent to part of the Keokuk Limestone.