Natural Resources, School of
The Sidney Gravel and Kimball Formation; Supposed Parts of the Ogallala Group (Neogene), Are Not Objectively Mappable Units
Date of this Version
In the 1930•s. G. L. Lueninghoener and A. L. Lugn prepared geologic maps of several counties. including Kimball and Cheyenne in the southern Nebraska Panhandle. for the Nebraska Geological Survey. Rock units designated as the Kimball Formation and the underlying Sidney Gravel were shown on these maps. Studies by Swinehart (1974). Breyer (1975. 1981). and Diffendal (1985) demonstrated that these units could not be defined in several areas in western Nebraska. Results of this study show clearly that the Kimball Formation and Sidney Gravel cannot be traced for more than a few kilometers from their type areas.
The Kimball Formation does not have a base that can be traced with confidence away from th~ type area south of Kimball. No gravel sheet (Sidney Gravel) occurs at this locality beneath the Kimball. Furthermore. at least the lower two of the three gravel lithologies supposedly typical of the formation occur repeatedly throughout the Ogallala sequence.
The discontinuous remnant sand and gravel at the type exposure of the Sidney Gravel have the characteristics of the material originally described by Lugn (l939a). To the north. across an intermittent stream. the lower of two sand and gravel bed sequences separated by caliche sandstones is at the same elevation as the type Sidney and could be the same unit. Farther to the north and east of the type exposure. several sand and gravel bed sequences are laterally and vertically discontinuous. Each one could be equivalent to the original Sidney. but none can be traced back to the type area. Complex geometries and possible post-Ogallala deformation also make correlations questionable.
Arbitrarily designating rocks as part of the Kimball Formation if they occur high in a local section. as Lueninghoener and Lugn did. is poor practice. Designating any sand and gravel in the Ogallala sequence as the Sidney or always assigning the stratigraphically highest sand and gravel observed at each exposure in the southern Panhandle to the Sidney has resulted in miscorrelations. The formations as originally defined are not mappable. Usage of the terms should be abandoned. and the term Ash Hollow Formation should be used in their place.
Published by Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas 78713.