Table 4Table 4--Generalized section of the geologic formations in Grant, Haskell, and Stevens Counties, Kansas.
Classification of the Rexroad deposits as a member of the Ogallala formation previously has been judged appropriate on the grounds that (1) known occurrence of these fossil-bearing beds is geographically restricted to parts of Meade County, Kansas, (2) the lower boundary of the Rexroad deposits is poorly defined, (3) lithologic distinction between Rexroad and pre-Rexroad Ogallala deposits is slight or lacking, and (4) there is great difficulty generally in mapping Rexroad as a separate stratigraphic unit. Although a disconformity indicating break in sedimentation is inferred to exist in parts of Meade County at the base of the Rexroad, elsewhere in this region sedimentation of Rexroad on Ogallala was presumed to be conformable (Frye and Hibbard, 1941). Definition of the stratigraphic span of the Ogallala formation in southwestern Kansas properly is not based solely or even primarily on time concepts, in such fashion that it is presumed to exclude deposits interpreted to be younger than the so-called "Algal limestone" of northern Kansas-despite the fact that this limestone marks the top of the Ogallala in the north. Thus, it has seemed proper to apply the name Ogallala in a broadly inclusive manner to Tertiary terrestrial deposits of the western Kansas region in which neither lithologic nor structural grounds for significant subdivision in mapping is afforded. Inasmuch as the Rexroad (?) deposits of Seward County and areas farther west, which are reported to rest unconformably on Laverne deposits, are not established as true Rexroad, evidence is not yet available to show that an important break in sedimentation occurs at the base of the Rexroad; also, satisfactory grounds for recognizing Rexroad deposits outside of Meade County, Kansas, seem not yet to have been brought forward. Nevertheless, paleontological evidence emphasizes the lesser age of the Rexroad beds (very late Pliocene or early Pleistocene) as compared with pre-Rexroad Ogallala deposits elsewhere (early to late Pliocene). All things considered, it is judged desirable to revert to the original usage of Smith (1940) in which Rexroad deposits are treated as a distinct formation.
The question of recognizing Rexroad formation in Seward, Grant, Haskell, Stevens, and Morton counties is quite distinct from that of classifying these deposits as a formation or as a member. Evidence that Cenozoic deposits older than the Meade formation of this region belong to the Rexroad rather than the Ogallala seems not yet to have been obtained. Vertebrate remains from southeastern Seward County studied by Hibbard (1944, 1944a, and personal communication, June 14, 1946) indicate middle Pliocene age or at least a distinctly greater antiquity than the type Rexroad fauna; this refers to Hibbard's Saw Rock Canyon locality and vicinity. Leonard reports that fossil mollusk assemblages associated with the Saw Rock Canyon vertebrates are clearly differentiated from those of the type Rexroad. Rhinoceros remains found by Smith (1940, p. 74) near Arkalon in Seward County assuredly denote pre-Rexroad Ogallala, though mapped as Rexroad (Byrne, F.S., and McLaughlin, T.G., unpublished report). Accordingly, I judge that no secure grounds have yet been adduced for recognition of Rexroad outside of areas in Meade County, and it seems probable that all of the Rexroad (?) formation of this report comprises Ogallala deposits of pre-Rexroad age. Changes have been made in proof to indicate the questionable identification of Rexroad in Grant, Haskell, Stevens, Morton, and Seward Counties.
The outcrop areas of Rexroad (?) formation (probably Ogallala) have been omitted from the geologic map (Pl. 1); although small (see p. 118), these areas are not differentiated from the Meade formation.
** - The classification is that in use by the State Geological Survey of Kansas but not by the U.S. Geological Survey.
Kansas Geological Survey, Grant, Haskell, and Stevens Geohydrology|
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Web version May 2002. Original publication date July 1946.