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Ogallala Formation of Northern Kansas

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The Ogallala of northern Kansas meets all the stated requirements of a formation (Ashley and others, 1933). Although it contains a wide range of lithologic types, it is essentially a continuous blanket of alluvium that may be regarded as "homogeneous in its heterogeneity"; it is considered a conformable sequence, as the multitude of minor diastems it contains are typical of this environment of sedimentation; it is marked at the base by a profound unconformity easily recognizable in the field, and at the top by a distinctive bed and an unconformity; in age it is entirely within the late Tertiary Neogene (Miocene-Pliocene); and it is readily mappable on conventional scales. The subdivision of the formation, however, is not so clear-cut. As is true in any alluvial complex, continuous "marker beds" do not exist and the establishment of rock units that can be defined entirely on the basis of their lithology and that can be mapped by conventional field methods has not been possible.

At the type localities (in Nebraska) of the units with which we are concerned, relatively minor attention has been given to the problem of lithologic definition. In fact, the members of the Ogallala formation in the central Great Plains region are times-tratigraphic units that possess average lithologic characters that are sufficiently distinctive to distinguish them one from another in a general way, but which are not adequate to permit a sharp delineation of members in any particular section. The members become meaningful when we consider them to possess approximate age equivalence from place to place. It is in this sense that the members will be used in the following descriptions.

Thickness also is not a constant factor in the Ogallala, particularly in the lower and thicker two of the three members. Deposition of the Valentine member was confined entirely to erosional valleys, and most of the Ash Hollow member also was restricted by bedrock divides. Therefore, the great bulk of the formation (all except the relatively thin Kimball member) was controlled in its rate of deposition by particular factors that obtained in individual valley sources. The uppermost part of the Ash Hollow member and the Kimball member transgressed many, if not all, of the former divides of this region to form an essentially continuous plain of alluviation, and these parts of the formation present a much greater degree of consistency of thickness. This range in thickness is strikingly shown on Plate 1, where it is noted that the interval from the top of the Ash Hollow member down to the top of the Rawlins volcanic ash bed ranges from 50 feet in Wallace County to 175 feet in the type section of the member in western Nebraska.

As the deposition of at least the lower two-thirds of the Ogallala in northern Kansas was controlled by pre-existing valleys cut in Cretaceous rocks it is important to have at least a general knowledge of these earlier valley patterns. This is available primarily from subsurface data that have been presented in the form of a contour map by Merriam and Frye (1954). As the control, although fairly evenly distributed, is sparse, the picture available is of necessity generalized. A few conclusions are unmistakable from an examination of these data.

The general trend of the bedrock valleys is easterly. Major buried divides extend in an easterly direction from southern Wallace County and from southern Sherman County. The divide in southern Sherman County was of regional importance, standing more than 200 feet above major valley floors both north and south of it; it extended in a sinuous line toward the east, the crest passing through northeastern Logan County and northern Gove County; and it may have bifurcated, one branch extending into northern Thomas County. This divide clearly separated the depositional province in the northern two tiers of Kansas counties from the area to the south, during the deposition of all the Valentine and much of the Ash Hollow. Much, if not all, of the area of Kansas north of this divide was part of an integrated drainage pattern--but this valley system may have been isolated from the area of the Ash Hollow type locality farther north by an as yet unknown divide or divides in southern Nebraska. The pre-Ogallala topography in southern Logan and Gove Counties is not known because of the present wide erosional valley of Smoky Hill River; this area may have been part of a broad upland divide area on the Cretaceous surface. A minor bedrock valley extended eastward from southeastern Wallace County across the northern parts of Wichita, Scott, and Lane Counties. South of this minor valley, another bedrock divide served to isolate the region of southwestern Kansas as a depositional province until it was overlapped by upper Ash Hollow or perhaps Kimball sediments. As this study is concerned with northern Kansas, stratigraphy of deposits south of the province represented by this minor bedrock valley across northern Wichita, Scott, and Lane Counties will not be described.

It has been suggested that the Ogallala represents a series of coalescent alluvial fans of vast extent, sweeping outward from the Rocky Mountains. Such a hypothesis cannot be accepted, as it would imply progressive eastward stratigraphic overlap with little regard to the pre-existing topography. All available evidence runs counter to such an interpretation. Although some slight eastward overlap may exist near the eastern margin of the formation, the Ogallala is in the main composed of a series of valley fillings, overlapping laterally from the axes of the major drainageways onto the gentle erosional slopes of the valley sides. Sedimentation eventually culminated in the complete overlap of most if not all of these divides and the final development of a coalescent alluvial plain.

The tectonic events that controlled the late Tertiary history of the central Great Plains are not fully understood. Northern Kansas was probably subject to subaerial erosion from the beginning of Tertiary time until the beginning of Ogallala deposition, and erosion of the gently sloping Cretaceous divide areas must have continued during all but the latest stages of Ogallala deposition. Stream-laid sediments started to accumulate much earlier to the north in Nebraska, where a thick sequence of Oligocene and Miocene underlies the Ogallala (Lugn, 1939), and in the regional view it could be argued that there is a general overlap from northwestern Nebraska southward to the divide north of Arkansas River in western Kansas. There is, in general, a thinning of the several units southward across this region. This suggests either a differential downwarping of the Plains or a differential upwarping of the source areas to the west.

It is certain that erosion in the Rocky Mountains was coincident with Ogallala deposition in northern Kansas. The relief in the mountains, however, must have been more subdued than it was during at least some intervals of the Pleistocene, as the texture of the sediments reaching Kansas was in general finer. Furthermore, the change in texture through the Ogallala does not indicate a strong uplift followed by stability. Such a structural history should produce coarse materials in the base of the formation, overlain by progressively finer sediments upward, whereas the actual progression is from finer in the Valentine to coarser in the Ash Hollow and basal Kimball (Sidney) to much finer in the middle and upper Kimball. This sequence may indicate that during late Tertiary time there were differential structural movements of small magnitude at different times.

One item of Rocky Mountain history seems clearly inferable from plains stratigraphy; by the end of Ogallala deposition there was a period of relative stability with subdued relief in the mountains, and the streams were transporting eastward only fine sediments. It is judged that the major erosion surface of the Rocky Mountains (Atwood and Atwood, 1948) was continuous with the surface of alluviation at the top of the Ogallala. The character of the sediments and particularly the extensive accumulation of calcium carbonate at and below the surface of this alluvial plain indicate that relative stability continued for a significant period of time. This stability was interrupted at the approximate beginning of Pleistocene time by an episode of erosion throughout the plains (Elias, 1948; Frye, 1948) and at least part of the mountains, which is judged to have been produced by structural movements that also produced arching of the Ogallala (Smith, 1940) in northwestern Kansas. The earliest post-Ogallala sediments of the central Great Plains are coarser than those of the upper Ogallala, and Pleistocene materials subsequently transported eastward from the Rocky Mountains to western Kansas are considerably coarser than any of the Ogallala materials. Thus it is judged that the relief of the mountains and also the eastward gradients of the transporting streams were greater during the Pleistocene than during the deposition of the Ogallala; that the topography across the entire region in late Pliocene (Kimball) time was relatively well adjusted, and that minor uplifts occurred in the Front Range during the time of deposition of Ash Hollow and earliest Kimball (Sidney) sediments.

The details of the stratigraphy of the Valentine, Ash Hollow, and Kimball members in northern Kansas will be described with a minimum of reference to specific localities. For specific locality data the reader is referred to the 31 measured sections from 16 counties in northwestern Kansas included with this report and to previously published measured sections in several counties in this part of the state (Frye, 1945; Elias, 1931; Frye and Leonard, 1949; Frye and Swineford, 1946; Prescott, 1953).

Valentine Member

The Valentine member is not well exposed in northern Kansas and it is concluded that age equivalents of little more than the upper half of the type Valentine section of Cherry County, Nebraska, are exposed in this part of Kansas. Correlation of the Valentine in Kansas with the type section is based on somewhat altered, partly indurated lentils of volcanic ash in the type section, which are judged to be equivalent to the Calvert volcanic ash bed of northern Kansas, and on abundant fossil plant material, with some supplemental evidence from fossil vertebrates and mollusks. Although exposures of rocks correlated with the Valentine member were studied in Phillips, Rawlins, Rooks, Graham, Ellis, Trego, Ness, and Scott Counties, most of our knowledge of the Valentine member was obtained from exposures in Norton County. Here these beds are best exposed along the south side of Prairie Dog Creek valley northeast of Norton and along the south side of North Fork Solomon River, from Lenora eastward to Logan.

These Valentine exposures lie in at least two completely separated depositional provinces. The localities in northern Scott and Ness Counties and extreme southern Trego County are in the minor bedrock valley that extends eastward from southern Wallace County, and the member is thin in this province. In many places it strongly reflects a local source and is characterized by bentonitic clays (e.g., measured section, Scott County), silts, and opaline-cemented Cretaceous chalk rubble (e.g., measured section, Ness County). Scarcity of fossil plant material and the absence of the Calvert volcanic ash bed and of fossil mollusks makes the establishment of even a thin Valentine section doubtful in most of this area; the stratigraphic relations of the overlying Ash Hollow member, however, provide a basis for the conclusion that the volume of the Valentine member in this province is small and that its lithology is not typical of the more extensive deposits of Valentine farther north in the state.

For an understanding of the Valentine member we turn to eastern Norton County and to subsurface data farther west. Eastern Norton County provides more data for drawing a distinction between the Valentine and Ash Hollow members than any other area in Kansas. As has been pointed out, such a distinction is, of necessity, arbitrary. In this area the Calvert ash bed (correlated with altered volcanic ash in the upper part of the type Valentine) and the Rawlins ash bed (correlated with an ash bed that occurs about three-fifths of the distance down from the top of the type Ash Hollow) occur in close proximity and in close stratigraphic association with fossil plant material and fossil snails. It seems clear that the boundary separating the two members must be drawn arbitrarily between the stratigraphic positions of these two ash beds and somewhat closer to the Calvert than to the Rawlins. This line is shown on correlated section A-A' of Plate 1.

In this area fossil snail faunas and assemblages of fossil plants have been collected short distances above and below this arbitrary boundary. A fossil molluscan fauna with close affinities to the Laverne assemblage of fossil mollusks (Leonard and Franzen, 1944) occurs near the base of the Almena section (Pl. 1; Fig. 3) in soft, diatomaceous limestone in stratigraphic proximity to the fossil grass, Stipidium commune, diagnostic of the Valentine member. The molluscan fauna includes Lymnaea lavernensis, Helisoma valens, H. goodrichi, Calipyrgula hibbardi, and C. senta, which are species that occupy a prominent place in the molluscan populations of the Laverne fauna. In the Ogallala formation these molluscan species are unknown in Ash Hollow deposits, or occur in the lowermost part of this member (Fig. 3, Almena section, V and AH). Krynitzkia coroniformis, a fossil seed diagnostic of the lowermost Ash Hollow, invariably occurs stratigraphically above zones that yield S. commune (Pl. 1; Fig. 4. Compare stratigraphic occurrence of S. commune and K. coroniformis, as shown on Fig. 4). Although it has been necessary at some localities to draw the line between the Valentine and Ash Hollow members arbitrarily, this has been necessitated, not by lack of suitable criteria per se, but only because of the failure to find at precise stratigraphic levels in a particular outcrop the evidence that would permit application of the criteria.

Some generalities about the character of the Valentine can be drawn from these exposures in Prairie Dog Creek and North Fork Solomon River valleys, augmented by subsurface data farther west. The member probably attains a thickness in excess of 100 feet in central Sherman County, northern Thomas County, northwestern Sheridan County, eastern Decatur County, and northwestern Norton County. In southern Norton County it is probably not more than 75 feet thick, and it thins eastward to approximately 30 feet in the exposures south of Edmond. The Valentine member is composed preponderantly of fine to medium, relatively well sorted, gray to greenish-gray feldspathic sand, containing disseminated calcium carbonate and tubules and irregular concretions of opal. Greenish-gray, olive-drab, and gray silts containing irregular nodules and streaks of calcium carbonate are common; greenish-gray to drab bentonitic clays are fairly common in northwestern Phillips County, are exposed at a few places in Norton County, and have been penetrated in test drilling in Thomas County and a few other places. Coarse feldspathic gravels are rare but at many of the places where they do occur in the member they are cemented in groups of large, lenticular masses by green opal (e.g., southeastern Norton County and southern Phillips County; Frye and Swineford, 1946), and because of their hardness have an exaggerated prominence in the outcrops. At a few places pebbles of Cretaceous chalk are prominent lithologic constituents, and soft limestone or marl lentils (e. g., Almena section) are known. Tints of red brown, so common in the overlying Ash Hollow member, are rare in the Valentine member.

Although the Valentine member is in the main quite different lithologically from the other members of the Ogallala, the upward change is transitional rather than abrupt. It is judged that deposition was relatively slow and took place at a fairly uniform rate and that the water table remained near the surface during the period of deposition. This latter condition is indicated primarily by the ferrous state of the iron, which imparts the greenish, and olive-drab tints to the sediments, by the greater abundance of branchiate snails, by relative rarity of fossil Celtis willistoni, and by local alteration of volcanic glass to montmorillonite.

Because of the scarcity of adequate exposures, it has not been practicable to obtain detailed knowledge of the internal stratigraphy of the Valentine to the degree possible for the other two members. On the basis of all available data, however, we judge that the oldest Valentine beds in northern Kansas are not as old as the basal part of the type Valentine section in Cherry County, Nebraska.

Ash Hollow Member

Almost all of the Ogallala exposures observed in northern Kansas fall within the stratigraphic span of the Ash Hollow member. Unlike the Valentine member, which is restricted to bedrock valley areas, the Ash Hollow, at least its upper part, was originally deposited over most if not all of this region. The relatively thin overlying Kimball member has been removed by post-Ogallala erosion from much of northern Kansas. The early workers therefore referred particularly to the Ash Hollow as the "Mortar beds" and "Tertiary grit". Furthermore, this member displays a greater degree of uniformity across the region than does the Valentine, although it too is composed of contrasting lithologic types. Correlation within the member has been facilitated by a concentration of volcanic ash falls in its lower half and a profusion of fossil plant remains throughout.

As has been pointed out, definition of members of the Ogallala in northern Kansas is in part arbitrary. In the exposures in eastern Norton County the Valentine-Ash Hollow boundary is defined for our purposes as falling between the positions of the Calvert and Rawlins volcanic ash beds and marked by paleontological zones. The Valentine member contains the diagnostic Stipidium commune fossil seed zone and an assemblage of distinctive fossil mollusks; the basal Ash Hollow deposits are characterized by the Krynitzkia coroniformis fossil seed zone and by molluscan assemblages that lack such characteristic Valentine species as Calipyrgula hibbardi, C. senta, Helisoma goodrichi, and Lymnaea lavernensis. In general, molluscan faunal zones are slightly less well defined than fossil seed zones at the Valentine-Ash Hollow boundary, but are sufficiently characteristic to permit positive identification if a suitable assemblage of fossils is available. We feel certain that the genus Calipyrgula does not persist above the Valentine, but this does not necessarily mean that collections can be made in uppermost Valentine exposures; similarly, K. coroniformis is the first diagnostic seed to appear in Ash Hollow sediments in this area, but there is no assurance that fossiliferous beds bearing this seed will be found in lowermost Ash Hollow deposits at any particular locality. For this reason, at many places the Valentine-Ash Hollow boundary must be drawn arbitrarily somewhere between the noncontiguous fossiliferous zones or on the basis of the position of the Calvert and Rawlins volcanic ash beds.

Although the Ash Hollow member of northern Kansas is correlated with the type section in Nebraska by the Rawlins ash bed, it probably does not include all of the span of the type section (Pl. 1). The lowermost beds at the type Ash Hollow section may be equivalent in age to the uppermost beds of the type Valentine. It is our judgment that the Ash Hollow member of northern Kansas includes age equivalents of all but the basal one-fifth of the type section.

The upper limit of the Ash Hollow member locally coincides with a distinct lithologic change to coarse gravel. In Kansas usage, the Sidney gravels are classed as the basal unit of the Kimball, so the base of this coarse gravel, where it is present, serves to mark the boundary between the two members. This is strikingly illustrated at the type locality of the Ash Hollow in Nebraska (Pl. 1), and in northern Kansas a similar abrupt change in lithology is well displayed in measured sections in western Wallace County, central Rawlins County, northern Scott County, and elsewhere. At many places in northern Kansas, however, the Sidney gravel lentils are missing and the base of the Kimball is marked by a progressive upward decrease in grain size of the sediments and progressive increase in calcareous material. Locally (e.g., Norton County) a soft limestone or marl forms the base of the overlying Kimball member. Distinctive volcanic ash lentils have not been described from this part of the section and therefore in the areas where a lithologic basis cannot be used for distinguishing the top of the member we must turn to a paleontologic definition.

The top of the Ash Hollow member is marked by the decline and disappearance of Biorbia fossilia, the occurrence of such seeds as Stipidium tubus, Berrichloa maxima, and Panicum elegans, and the absence of Prolithospermum johnsoni and Berrichloa minuta, which are restricted to the overlying Kimball sediments. Uppermost Ash Hollow deposits are characterized by the last occurrences of fossil mollusks characteristic of the Laverne fauna; fossil molluscan assemblages in the Kimball contain only far-ranging molluscan species of little stratigraphic significance, together with species such as Promenetus blancoensis and Lymnaea macella, characteristic of earliest Pleistocene molluscan faunas. In the uppermost fossiliferous limestone (base of Kimball member) at the Almena section the molluscan fauna (Fig. 3) contains a gastropod that is probably Gyraulus enautus, another characteristic early Pleistocene species, but identification from plastic casts is not conclusive. Again, the actual boundary between the Ash Hollow and Kimball members must be drawn arbitrarily at many places, not because suitable criteria are lacking, but because the material evidence may be lacking at precisely desired levels.

It is generally true that in this region of Kansas the Kimball member is commonly 35 to 45 feet thick, so where the "Algal limestone" is present the top of the Ash Hollow member may be located approximately by vertical measurement.

The local thickness of the Ash Hollow member ranges between wide limits, partly because of its overlap onto Cretaceous rocks. Where the full stratigraphic span of the member is present, however, there is a general thickening northward from west-central Kansas to west-central Nebraska. An average thickness for the member in Wallace County is approximately 75 feet, whereas in Norton County, near the Nebraska state line, it is at least 130 feet, and in the North Platte valley in Nebraska the Ash Hollow ranges to 200 feet in thickness.

The lower part of the Ash Hollow is restricted to the same major bedrock valleys that controlled the deposition of the Valentine member, but as alluviation progressively overlapped the valley sides, this member occupies a progressively larger area (Pl. 1; Fig. 2). The upper part of the member transgressed most, if not all, former divides and formed a virtually continuous coalescent sheet of alluvial material throughout northwestern Kansas and southwestern Nebraska.

The fact that the lower part of the member was restricted to pre-existing valleys indicates that the trend of the depositing streams continued to be generally eastward. The presence of thin Kimball deposits resting directly on Cretaceous Greenhorn limestone uplands in central Kansas is evidence that Ash Hollow deposits did not form a general alluvial plain eastward from Smith, Osborne, and Russell Counties, but were here restricted to valleys. It is judged, however, that some Ash Hollow sediments were deposited in. the bedrock valleys some distance eastward from the area of general distribution, because of the presence of local remnants of this member in northeastern McPherson County. Here, Ash Hollow beds constitute the lower part of the unit formerly called "Delmore formation" (Williams and Lohman, 1949; Hibbard, 1952), which is the easternmost known occurrence of the Ogallala formation in Kansas. Although even here much of the sediment indicates a Rocky Mountain source, a sizable part of the clastic materials included in the Ogallala of McPherson County was probably derived from the Cretaceous rocks of central Kansas.

The McPherson County deposits probably indicate the easternmost extent of eastward-trending Ogallala drainage; they may have accumulated in a major southward-trending valley that was in general aligned with the strike of the Permian bedrock and located on the belt of shales at the western limit of the long gentle dip-slopes from the crest of the Flint Hills. The fact that streams originating farther west did not cross the Flint Hills in Ogallala time is demonstrated by the nature of the deposits within the Flint Hills and farther east in the Osage Cuesta Plains (Frye and Leonard, 1952; Frye, 1955). Late Tertiary deposits in this region consist entirely of materials typical of the bedrock east of the McPherson County localities, and contrast strongly in lithology with the Ogallala to the west. Therefore, the eastward-trending Ogallala streams must have joined a major stream flowing southward out of the region and toward Oklahoma. Although present evidence is not conclusive, it indicates that relatively short tributaries to this major south-flowing Ogallala drainageway joined it from the east, down the dip-slope of the Permian cherty limestones of the Flint Hills. The term Ogallala has not been applied to the Tertiary chert gravels of the Flint Hills region--even though some of them are probably of the same age as the Ogallala--primarily because lithologically these chert gravels contrast strongly with typical Ogallala and because they represent a distinct and independent depositional province.

Fine to coarse feldspathic sand is the predominant clastic constituent of the Ash Hollow member. Although gravel and silt predominate locally in some zones, and a small percentage of silt is commonly present in the sand zones, their combined volume is only a fraction of the bulk of the sand. Clay is present in very small amount, but volcanic ash is estimated to constitute more than five percent of the lower half of the member. The coarse sand and gravel that forms the conspicuous ledges so characteristic of this member generally contains a large percentage of granitic materials. In the Valentine member there are local gravel beds derived from a nearby bedrock source, but such beds are relatively rare in the Ash Hollow. Sediments of the Ash Hollow have a greater degree of uniformity of source, and therefore of general lithology, than those of the Valentine.

As shown by the measured sections and Plate 1, there is little if any regular gradation of texture through the member-gravel and silt are known to occur locally at any stratigraphic position. A progressive change in color, however, seems clear. The underlying Valentine member is predominantly gray or greenish gray, and shades of pink and pinkish drab become prominent about at the arbitrary boundary between the two members. The color transition continues through the lower one-third to one-half of the Ash Hollow member, hues of red becoming more prominent upward except where they are masked by a large percentage of calcium carbonate cement. In the upper half of the member, shades of green are absent, and where the color is not masked by the gray of the cement, shades of red and tan are everywhere present. Locally the reddish zones have the appearance of the B horizons of a surface soil (e.g., beds 8 and 9 of Scott County State Lake section), but at many places where such an origin cannot clearly be demonstrated the color may be due to weathering in the zone of aeration above the water table.

Much of the contrast in appearance between the Ash Hollow and Valentine members may be caused by a lowering of the water table and resultant different effect of surface weathering during the relatively slow deposition of the Ash Hollow member. The generally poorer sorting and lack of cross bedding in the Ash Hollow materials suggest rapid but intermittent sedimentation, with little opportunity for reworking, each deposit remaining for relatively long periods in a near-surface position thus allowing some mixing by surface processes. The general lack of any apparent bedding in zones many feet thick suggests some alteration of the sediments before they were deeply buried. The form of some of the calcium carbonate cemented zones (Pl. 2B, 3A) is strongly suggestive of soil caliche accumulation. As some of these "caliche" zones are separated by only a few feet of noncalcareous silty sand, it is judged that, locally, increments of sedimentation 6 feet thick or thicker accumulated rapidly and were preceded and followed by periods of surface stability sufficiently long to permit the development of deep, mature lime-accumulating soils containing caliche zones several feet thick. This type of calcareous material was used as a natural mortar for sod houses by the earliest settlers of the region; this use led to the early application of the term "Mortar beds" to the Ogallala formation.

This mode of origin cannot be called upon to account for all the calcareous cement and carbonate rock of the Ash Hollow. There are at least two other general classes of calcareous beds in the member, namely, fresh-water limestones or marls, and sands and gravels cemented by the action of groundwater, in addition to nodules and tubules scattered throughout.

Fresh-water limestones are quantitatively rare and have been studied most in Norton County. Their abundant contained diatoms and locally abundant fossil snails or snail molds leave little doubt that they accumulated in relatively shallow ponds of quiet water. In some places they contain a significant quantity of volcanic ash shards and an open network of opal.

Lenticular bodies of sand and gravel, many of them crossbedded, hence judged to be stream-channel deposits (Pl. 3B, C), have been observed at virtually all stratigraphic positions in the member. They are more common in the upper half of the member but are not everywhere present, even in the upper part. These lentils are generally the coarsest materials in the section. They are firmly cemented with calcium carbonate and form prominent bluffs. As they commonly lack association with reddish noncalcareous B horizons, they are interpreted as being the result of cementation below water-table, although the calcium carbonate may have been concentrated as a result of surface conditions.

A prominent constituent of the cement in the Ash Hollow, although less important quantitatively than the calcium carbonate, is opal. The silica may have been derived from volcanic ash lentils, from weathering of feldspathic sand, or locally from accumulations of diatoms. It is widely distributed through the cemented zones of the member as open "sponge-work" and as minute tubules. Some of the unusual weathering effects in Ogallala rocks may have been influenced by this small increment of opal cement.

The internal stratigraphy of the member is relatively simple and nondescript. In natural exposures the caliche and cemented zones simulate bedding, but such false beds are not traceable for more than a few hundred yards at most. Stratigraphic placement within the member cannot be made on the basis of general lithology alone, and stratigraphic sequence can be used for this purpose only at localities where there is exposed an exceptionally thick series of deposits. Molluscan faunas are known from several stratigraphic positions within the member, but their geographic distribution is not adequate to permit their general use for local detailed work. In study of the internal stratigraphy of the Ash Hollow, five distinctive named volcanic ash falls and abundant fossil plant material have been the most useful features. Assemblages of fossil seeds, which allow subdivision of the Ash Hollow into lower, middle, and upper portions, are discussed in detail elsewhere in this report (see also, Fig. 3, 5).

Kimball Member

The Kimball member is the uppermost and thinnest of the three subdivisions of the Ogallala formation. Isolated remnants of the uppermost beds of the member (particularly the "Algal limestone") on the Cretaceous uplands of Russell, Lincoln, Osborne, Mitchell, Jewell, and Cloud Counties give this member by far the most widespread geographic distribution, but complete sections of Kimball member resting on Ash Hollow beds and capped by "Algal limestone" are exposed in only a few counties. The member is known primarily from its exposures in the south valley wall of Smoky Hill River valley in Wallace County and eastward (note measured sections), and in Sherman, Cheyenne, and Rawlins Counties. Elsewhere in northern Kansas a full span of Kimball is present locally (e.g., measured section, Logan County; northwest of Brewster, Thomas County; southwest of Lenora, Norton County), but the member is generally absent or represented by its lower part only (e.g., Almena section, Norton County).

Although the distribution of the member is now greatly reduced, it is judged that the Kimball was deposited over a larger area than any other part of the Ogallala formation. Its original extent is reconstructed from scattered erosional remnants of the member; some rest on Ash Hollow deposits, but more consist of "Algal limestone" resting on a sequence of a few feet of predominantly local silt, sand, and gravel, which in turn rests on Cretaceous rocks. Remnants of the Kimball member are known to occur in 31 counties in the northern half of Kansas, and have been observed in every county westward from Jewell, Osborne, Lincoln, and Ellsworth. The Kimball was the culmination of Ogallala alluviation, and overlapped most if not all of the former bedrock divides. Its surface extended as a vast coalescent plain of regional alluviation. During Kimball deposition the former bedrock valleys were no longer effective in controlling the position or direction of flow of major streams, which became free to shift in response to load, volume, and gradient. Such an environment eliminated formerly important local factors of control and thus the Kimball has a greater degree of uniformity, both in lithology and thickness, than the lower members throughout its extent from west-central Nebraska to west-central Kansas.

The stratigraphic span of the Kimball is more distinct and more easily determined than that of either of the other two members. Its top is the stratigraphic top of the formation, marked at many places by the "Algal limestone", and where lentils of Sidney gravel are present the base of the member can be drawn at the base of the gravels. The base of the member can be drawn fairly closely on the basis of plant fossils and fossil molluscan assemblages. The occurrence of the fossil plants Prolithospermum johnsoni, Berrichloa minuta, and B. maxima in the same assemblage is an accurate guide to the Kimball member. Fossil molluscan assemblages that contain Promenetus blancoensis and Lymnaea macella, but lack such species as Helisoma valens, H. goodrichi, Pseudosuccinea columella, and of course all characteristic Valentine species, characterize Kimball deposits. Molluscan faunal assemblages and fossil seed floras are as well defined across the Ash Hollow-Kimball boundary as they are across the Valentine-Ash Hollow boundary.

The Kimball member in northern Kansas, where a full section is present resting on Ash Hollow beds, has an average thickness of about 30 feet, but the measured sections included here show a range from 44 feet (Cheyenne County) to less than 25 feet (Ellis and Wallace Counties). In the belt of Greenhorn limestone (Cretaceous) outcrop in north-central Kansas, as little as 2 1/2 feet of Kimball, including the "Algal limestone" at the top, rests directly on bedrock.

Except for the basal Sidney gravel lentils, the Kimball member is predominantly fine textured and richly calcareous. Feldspathic medium to fine sands and silts predominate. In zones where the calcium carbonate content is low, these beds are reddish tan to red brown, but the calcareous material commonly masks the hues of red, giving the entire member an ash-gray color. The calcium carbonate content of the member commonly increases upward, the upper one-half to one-third of the member consisting of impure, relatively soft limestone locally capped by the "Algal limestone" bed. In many exposures this upper silty, sandy limestone shows a platy structure in the upper part, grading downward through a somewhat massive zone into a prismatic to nodular zone 8 to 10 feet below the top. This downward gradation of structure is well illustrated in the exposures south of Dighton, Lane County, shown in Plate 2A.

Opaline chert is characteristically associated with the Kimball member, but has an erratic geographic distribution. In northern Kansas it has its maximum development in the Kimball in the exposures along the south side of Little Beaver Creek, south of McDonald, Rawlins County. In this area the silicified zone is as much as 12 feet thick and extensive (Frye and Swineford, 1946). Generally, silicification of the Kimball member is less extensive and less uniform. In the measured sections in Cheyenne and Wallace Counties, silicification is restricted to thin zones of opal (including minor quantities of chert) distributed discontinuously through the member; at other localities silica is present as an open spongework throughout the deposit. Chemical analysis (Frye and Swineford, 1946) shows that even in the thick, massive Rawlins County deposit silica constitutes only slightly more than half of the rock and that calcium carbonate is an important constituent.

The opal and chert of the Kimball member are judged to be secondary replacements of the calcium carbonate, although the source of the silica is not known. Volcanic ash deposited directly upon the surface of the formation is a possible source, or silicates may have become available from the weathering of feldspathic sand and silt in the upper part of the Kimball. In contrast to the opal-cemented sands and gravels of the Valentine member, the color, type of replacement, association of clay minerals, and stratigraphic relations all suggest that the opal of the Kimball member was deposited by downward-percolating waters in the vadose zone, but derived from a source on or near the alluvial surface at the top of the formation, rather than from contained volcanic ash lentils by groundwater circulation.

The top of the Kimball member-and of the Ogallala formation-is marked at many places by the "Algal limestone" bed. As has been pointed out, the origin of this bed has been a subject of debate since it was described by Elias in 1931. It is a hard, generally fractured, irregular limestone that ranges in thickness from less than 1 foot to about 3 feet. It is marked by concentric wavy laminae and pisolitic structures generally showing an alternation of pinkish-tan and gray colors; it contains grains of quartz and feldspar and locally granitic and other igneous pebbles; its upper surface is generally pitted and uneven where exposed; and Elias (1931) has described the fossil alga Chlorellopsis bradleyi as occurring within it. In strong contrast to the freshwater limestones or marls that occur from the lower part of the Kimball member downward into the Valentine member, it has never yielded remains or indications of fossil mollusks or of diatoms. In some exposures it has a sharp contact at the base, but in others (Pl. 2A) it seems to be gradational with the platy impure limestones below it. The general appearance and lithology of the bed is the same where it caps a section of 250 feet of Ogallala deposits and where it is separated from Cretaceous sediments by less than 5 feet of locally derived clastic material. There is no other bed within the Cenozoic sediments of northern Kansas that even approximately resembles the "Algal limestone" in appearance.

Whatever its origin may have been, it seems undeniable that this bed is genetically related to the surface of the plain of alluviation that marked the end of Ogallala deposition in the region, and that the plain on which it formed maintained surface stability for a significant interval of time. The described presence of a fossil alga is seemingly the only line of evidence that demands formation of the bed in a body of water; whereas the general relations to the other Kimball sediments below, lack of other organic remains, encrusting relationship over the undulating Cretaceous uplands of north-central Kansas, and the secondary solution effects might be explained as phenomena produced by weathering on a relatively stable surface during a long interval of time.

In northern Kansas the post-Ogallala history includes a long interval of surface exposure of the remnants of the alluvial plain surface that is the top of the Ogallala. Throughout the region this surface marks the end of upward accumulation of alluvial deposits, and where Pleistocene materials rest upon it they are loess or locally dune sand. Stream-laid Pleistocene deposits of the region are all confined to terrace or valley flat positions and were formed after dissection of the Ogallala plain had begun. Furthermore, none of the eolian Pleistocene deposits found resting on the stratigraphic upper surface of the formation are older than Illinoian and most are of Wisconsinan age. The dissection of the Ogallala surface by major streams probably began in Nebraskan time, but large areas (e.g., Sherman County) are still virtually unmodified by erosion. Thus, the surface of the alluvial plain that marked the end of Ogallala deposition existed under conditions of erosional-depositional equilibrium for a relatively long interval of time, affected only by surficial processes of weathering.

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