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Ogallala Formation of Northern Kansas

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Northward from central western Texas and southeastern New Mexico, across western Oklahoma, eastern Colorado, western Kansas and Nebraska, and into South Dakota extends a dissected blanket of late Tertiary deposits. This region, approximately 800 by 400 miles in extent, is clearly the largest expanse of almost continuously exposed nonmarine Neogene (Miocene and Pliocene) sediments on the North American continent. Although the existence of this depositional province has been known to geologists since the early scientific expeditions (e.g., Meek and Hayden, 1862; King, 1878), and the fluviatile origin of most of these sediments has been generally accepted for more than half a century (Gilbert, 1896, p. 576; Haworth, 1897; Johnson, 1901), little effort has been directed toward study of details of the stratigraphy and correlation. It is the purpose of this report to present results of stratigraphic and paleontologic studies of the late Tertiary (Neogene) deposits in the northern half of Kansas.

Modern study of the Tertiary of western Kansas was begun by Elias (1931) primarily in Wallace County, but was not followed up, except for a brief reconnaissance of Rawlins and Decatur Counties (Elias, 1937), until geologists of the State and Federal Geological Surveys, in a cooperative program of ground-water investigations, undertook county by county studies in the region. The first of these county ground-water reports giving major consideration to the Ogallala formation in the northern area was that on Thomas County (Frye, 1945), which is underlain in its entirety by this formation beneath a thick blanket of Pleistocene sediments, but work on the Norton County area (Frye and A. R. Leonard, 1949; Frye and Swineford, 1946; Swineford and Frye, 1946), provided the first opportunity to make significant additions to stratigraphic data obtained by Elias. Other published groundwater reports that contain data on Ogallala stratigraphy in this region are those for Cheyenne (Prescott, 1953), Scott (Waite, 1947), Lane (Prescott, 1951), and Sherman (Prescott, 1953a) counties.

The upper Tertiary of northern Kansas is richly fossiliferous. Fossil vertebrates have been known from these deposits since the early scientific expeditions in the region, and some localities, as for example, those in the area tributary to Republican River in northwestern Kansas (Osborn, 1909, p. 80), have long been well known. The abundant fossil seeds (Elias, 1942), however, are of far more significance to the field stratigrapher and are to be found in virtually any good exposure of the Ogallala formation in northern Kansas. Fossil mollusks, although heretofore not described from this formation, occur through most of its stratigraphic span and provide an additional tool for correlation.

Prior to the work of Elias, the unconsolidated deposits of the region, although several hundred feet thick, were referred to in general terms as "Mortar beds", "Tertiary grit", or "Magnesia beds", and were not clearly subdivided into Tertiary and Quaternary portions. As early as 1866, Hawn (p. 101) recognized his "Bluff formation" as distinct from the general expanse of the Plains Tertiary, and in 1885 Hay subdivided the post-Cretaceous deposits of Norton County into a lower unit that he considered to be of Miocene age (Ogallala of present classification) and an upper unit of silt that he called Pliocene (the Pleistocene Sanborn formation of present classification). When Haworth reviewed the Tertiary of western Kansas in 1897, however, he pointed out that although fossil remains showed clearly that, at least locally, some of these unconsolidated deposits were of Pleistocene age, they should all be regarded as part of a single formation ranging through a considerable span of time. Elias' work in Wallace County was the first to differentiate sharply the Pleistocene deposits from the Tertiary of the region. During the last decade, intensive studies of the stratigraphy of the Pleistocene deposits of Kansas (Frye and Leonard, 1952) have served to set them clearly apart from the Tertiary. Because much of the Pleistocene sediment is derived from the subjacent upper Tertiary of the Plains, the materials of the two ages have strong lithologic similarities.

The field work on which this report is based has included some time in northwestern Kansas each summer from 1942 through 1954. During many of these seasons, however, work on Tertiary stratigraphy was incidental to other projects centered primarily on Pleistocene geology and paleontology, petrography, or groundwater geology, and only during the seasons of 1952, 1953, and 1954 was field work directed primarily toward study of the stratigraphy of the Ogallala formation. Most of the sections were measured and fossils collected during those three seasons, although many data were gathered during re-examination of localities visited earlier.

We wish to acknowledge assistance given us in this work by members of the State and Federal Geological Surveys working in northern Kansas, particularly personnel of the cooperative ground-water program; by the paleontologists of the University of Kansas Museum of Natural History; and by the Nebraska Geological Survey and State Museum of the University of Nebraska.

General Aspect of the Formation

The Ogallala formation of northern Kansas is a heterogeneous complex of elastic deposits. The thickness of the formation ranges from more than 300 feet to less than 3 feet; the texture ranges from coarse gravel containing pebbles as much as 3 inches in long diameter to clay; and the sorting ranges from good to poor. Cementing material, not everywhere present, includes tough opal, disseminated white opal, and various amounts of calcium carbonate. Colors of the deposits are dark to pale green, pink, reddish brown, tan, buff, pastel grays, and ash gray. Lentils of volcanic ash, marl or marly limestone, and bentonite contrast with the predominant stream-laid clastics. Throughout this heterogeneous assortment of sediments there is virtually no distinctive bed that can be traced appreciable distances in the field. The formation's topographic expression includes flat uplands, gentle erosional slopes, and sharp cliffs.

A striking characteristic of the coarser materials in the Ogallala is their lack of uniformity in the reflection of local sources. In general, the coarse materials indicate a source in the Rocky Mountain region to the west, but in some places the gravels are predominantly of Cretaceous chalk typical of the adjacent bedrock and at others a smaller but significant percentage of shale, chalk, or sandstone pebbles may be observed mixed with a larger portion of typical Rocky Mountain materials.

The surface on which the Ogallala rests is a subaerial erosion surface of gentle declivities (Merriam and Frye, 1954) cut on shales, chalky shales, chalky limestones, and locally sandstones of Cretaceous age. This surface slopes gently eastward and has an erosional relief (normal to the direction of major drainage) of somewhat less than 300 feet. The topography of this buried surface reflects the position of relatively resistant units in the Cretaceous bedrock (e.g., Fort Hays limestone, Greenhorn limestone). The Ogallala formation in northwestern Kansas dips eastward at the average rate of approximately 15 feet per mile and is arched along an axis extending eastward from central Sherman County (Smith, 1940). Regional dips to the north and to the south from this axis average 1 1/2 or 2 feet per mile, but are not uniform.

The Ogallala is locally very well exposed in bluffs along major valleys and tributary canyons, but large areas of flat High Plains surface, because they are blanketed by upper Pleistocene loesses, present no usable exposures of the formation. As a result, detailed stratigraphic studies, including the collection of fossil plant material, fossil mollusks, and samples of volcanic ash for petrographic study (Swineford, Frye, and Leonard, 1955), were concentrated along the lower portions of valleys tributary to Republican River (Cheyenne, Rawlins, Decatur, Norton, and Phillips counties), the digitate eastern edge of the outcrop area (Smith, Rooks, Ellis, Trego, and Ness counties), and the margins of the Smoky Hill River valley (Wallace, Logan, Scott, and Gove counties). In the extensive flat High Plains upland areas, typified by Sherman and Thomas counties and adjacent areas, knowledge of the Ogallala formation is of necessity derived principally from the logs of test holes drilled as a part of the cooperative Federal and State ground-water program.

Definitions and Type Localities

The stratigraphic classification of the Neogene deposits of northern Kansas is based entirely on subdivisions and type localities in Nebraska (Moore and others, 1951). The name Ogallala formation (but spelled Ogalalla at that time) was introduced by Darton in 1899. His original definition is in part as follows (Darton, 1899, p. 734): "Extending from Kansas and Colorado far into Nebraska there is a calcareous formation of late Tertiary age to which I wish to apply the distinctive name Ogalalla formation . . . ." And (p. 735), "In its typical development the Ogalalla formation is a calcareous grit or soft limestone containing a greater or less amount of intermixed clay and sand, with pebbles of various kinds sprinkled through it locally, and a basal bed of conglomerate at many localities . . . . The pebbles it contains comprise many crystalline rocks, which appear to have come from the Rocky Mountains." And further (p. 741), "The general thickness of the Ogalalla formation varies from 150 to 300 feet, the greater amount being along the Wyoming line in the northwestern corner of Kimball County . . . . As before explained, it is in the main, the extension of the 'Tertiary grit', 'Magnesia', or 'Mortar beds' of the Kansas geologists."

There is little doubt as to which deposits Darton intended to include in this formation, although in 1899 and a subsequent report (Darton, 1905, p. 178) he failed to designate a type section or locality, or to define clearly the span of the unit. He partly corrected this omission in 1920 (Darton, 1920, p. 6) when he wrote concerning the Syracuse-Lakin, Kansas, area: "The Ogalalla formation is believed to be a stratigraphic unit and to be continuous from the type locality near Ogalalla station in western Nebraska . . . . It is believed that the bones of Pleistocene age found in some places are in local deposits of later age that overlie the true Ogalalla, which appears more likely to have been laid down in Pliocene and late Miocene time."

Thus, in 1920, Darton designated a type area and clearly indicated that he did not intend to include the younger, Pleistocene deposits within the formation, in contrast to Haworth's (1897) contention that they were indistinguishable.

This remained the typical definition of the Ogallala formation until Elias (1931) and Hesse (1935) re-examined the exposures in the vicinity of Ogallala, Nebraska, proposed a type section on Feldt Ranch approximately two miles east of the town (SE sec. 33, T. 14 N., R. 38 W.), and described the vertebrate fossil fauna. Shortly thereafter, Lugn (1938; 1939) published the official classification of the Nebraska State Geological Survey, in which Ogallala was assigned the rank of group with four contained formations, in descending order, Kimball, Sidney, Ash Hollow, and Valentine. Although using Nebraska terms, the Kansas Geological Survey has classed Ogallala as a formation and recognized Kimball (including Sidney), Ash Hollow, and Valentine as members (Moore, Frye, and Jewett, 1944; Moore and others, 1951). Each of these three members has its own type description.

The Ash Hollow was first described by Engelmann (1876, p. 260) as follows: "They [rocky strata] continue along the South Fork, cropping out at intervals at one or the other side of the river, and were found most developed in Ash Hollow, where they attain a thickness of over 250 feet. This series is composed of an alternation of loose, finely sandy, and of harder rocky strata, the latter consisting of fine to coarse drift-sand, generally cemented by carbonate of lime, forming more or less calcareous sandstones, and gritty, very impure limestones . . . . Their age is, probably, the Pliocene-Tertiary; but I have no paleontological proof of it." Engelmann also recognized fossil seeds in these rocks (1876, p. 261). "In these rocks, near the flanks of Platte River, I found numerous fossilized seeds of the size of a small cherry stone, apparently related to the living genus Celtis, which have improperly been called Lithospermum . . . ."

Modern use of the term Ash Hollow dates from Lugn's classification (1938, p. 223) of the Nebraska Tertiary, in which he used this term in a sense almost identical to that of Engelmann. A year later he (Lugn, 1939) cited the same exposures in Ash Hollow canyon as the type section of the unit.

Deposits now included in the lowest, or Valentine, member of the Ogallala were first called "Valentine beds" by Barbour and Cook in 1917 (p. 173) from exposures east of the town of Valentine in Cherry County, Nebraska. This general area had earlier yielded collections of fossil vertebrates, but formal stratigraphic names had not been applied to the deposits, which were generally included within the "Loup River beds" of Meek and Hayden (1862). After the introduction of the term Valentine, it was used intermittently (e.g., Thorpe, 1922) until 1935 when Johnson restudied the stratigraphy of the area and redescribed this unit. In that same year Stirton and McGrew (1935) in a description of mammalian faunas proposed the names "Niobrara River" for the lowest beds, "Burge" for the intermediate beds, and "Valentine" for the uppermost beds, in this vicinity. This proposed subdivision radically changed the definition of Valentine as described by Johnson, and set off a controversy concerning the usage of the term Valentine (Colbert, 1938; Johnson, 1938; Lewis, 1938; Lugn, 1938; McGrew and Meade, 1938). Lugn (1938; 1939) presented the official position of the Nebraska State Geological Survey, which was, in effect, the acceptance of the Valentine as defined by Johnson in 1935. Johnson (1935, p. 467) had designated: "The type locality of the Valentine beds is on the south side of a drainage cut between the old and new railroad grades . . . in the NE 1/4 Sec. 17, T. 33 N., R. 27 W., Cherry County, Nebraska. It is the site of a vertebrate-fossil quarry which was discovered by Mr. J. B. Burnett in 1915."

At the time of his official endorsement of Johnson's usage of Valentine, Lugn (1938, p. 223) also stated that the beds exposed above the type section and termed the "cap rock bed" by Johnson should be included in the Ash Hollow formation of the official classification of the Nebraska Geological Survey. Although there has not been complete agreement among all workers concerning the usage of Valentine, since 1938 the definition proposed by Johnson has been official usage of the Kansas and Nebraska Geological Surveys and will be adhered to in this report.

The Kimball, or uppermost member of the Ogallala, was named and defined by Lugn in 1938 (p. 224) as follows: " . . . from its typical occurrence at the highest remnant levels of the High Plains in Kimball County, Nebraska. The thickness of the Kimball formation ranges from 30 to 40 feet when present in its full development; and it consists of silt, clay, fine sand more or less cemented with caliche, with one or two algal limestone beds at the very top." A year later Lugn (1939, p. 1262) restated the definition, modified to give a range in thickness of 25 to 50 feet.

In the reclassification of the Ogallala in Nebraska (Lugn, 1938; 1939) the Sidney gravel formation was defined as occurring stratigraphically between the Ash Hollow and Kimball. It was named from typical exposures at Sidney, Nebraska, and was described as a gravel unit 15 to 50 feet thick included in the upper part of the Biorbia fossilia seed zone. On the basis of those exposures where coarse gravel occurs at this stratigraphic position, we judge it to be a lenticular body of channel gravel that is not continuously traceable over long distances. The Kansas Geological Survey has not recognized this unit as of member rank but rather has included it as a local basal gravel phase of the Kimball member (Moore, Frye, and Jewett, 1944; Moore and others, 1951).

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Kansas Geological Survey, Geology
Placed on web Aug. 4, 2011; originally published March 1956.
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