The Ogallala formation consists of fluviatile, late Tertiary (Neogene) deposits that underlie the surface that forms the High Plains. Ranging up to 300 feet in thickness, these deposits extend from Texas to South Dakota in a broad belt several hundred miles wide. Contrary to prevalent concepts, the Ogallala was not deposited as a series of broad, overlapping fans from the source in the Front Range of the Rocky Mountains; rather, alluviation from source areas in the Rocky Mountains was controlled by relatively broad and shallow, eastward-trending erosional valleys cut in Cretaceous bedrock.
Three members are recognized in the Ogallala formation in northwestern Kansas, the region to which this report is restricted. Each of these members has been correlated with type sections in Nebraska on the basis of data derived from lithologic comparisons, petrographically distinctive volcanic ash beds, and characteristic assemblages of fossil plant remains. In addition to these sources of comparative data, fossil molluscan faunules, previously undescribed anywhere in the Ogallala, have been utilized in correlations within northwestern Kansas. Assemblages of fossil vertebrates, although recognized as important in referring parts of the Ogallala formation in northwestern Kansas to the standard time scale, have not been utilized for correlations within the region under study, because of the localized geographical and stratigraphical occurrence of these faunas.
The Valentine member, the first to be deposited in northwestern Kansas, was confined entirely to valleys cut in Cretaceous bedrock, where it was subsequently buried by Ash Hollow and Kimball sediments. The Kansas outcrops are correlated with the type of the Valentine in Nebraska on the basis of petrographically similar volcanic ash together with other lithologic similarities, the presence within the rocks of the fossil grass seed, Stipidium commune, regarded as diagnostic of this segment of the Ogallala, and by the occurrence of a distinctive fossil molluscan assemblage. In general, the Valentine is composed of medium to fine, greenish to pale-tan sands, silt, and clay, bentonite, and lentils of volcanic ash, opal-cemented sand and gravel, and soft, diatomaceous marls, which in places contain external molds of gastropods. Because of its deposition in the bottom of bedrock valleys below subsequently deposited Ash Hollow and Kimball sediments, the Valentine is poorly and locally exposed. Its geographic extent in Kansas. is known from subsurface data.
The Ash Hollow member overlies the Valentine, overlapping laterally the gentle slopes of bedrock valleys in which it was deposited, and in many places transgressing former divides. The Ash Hollow is lithologically heterogeneous, being composed of a disorderly. assemblage of stratified and massive beds, composed of gray and pink to reddish-brown sand, silt, gravel, and clay, together with many lentils of volcanic ash and fossiliferous limestone and marl. In general this member is composed of coarser clastics than is the Valentine. The Ash Hollow is richly fossiliferous. It contains all the best-known fossil vertebrate assemblages occurring in the Ogallala of northwestern Kansas, previously referred to in this report, a series of distinctive assemblages of fossil seeds comprising three floral zones, and distinctive though local fossil molluscan faunules. Of the three members of the Ogallala formation, the Ash Hollow is the most conspicuously exposed at the surface in northwestern Kansas. This phenomenon is related to the facts that the Ash Hollow overlies the Valentine, which it generally completely covers; the overlying Kimball, which is generally thin, has been seriously attacked by, and locally completely removed by erosion; other overlying resistant beds to protect the Ash Hollow from erosion are almost completely lacking; and cementation by calcium carbonate and dispersed opal has produced many relatively resistant beds, which stand out conspicuously as bluffs or ledges wherever the Ash Hollow has been dissected.
The Kimball member, although averaging scarcely more than 20 feet in thickness, is the most widely distributed member of the Ogallala formation. At the time of its deposition, not only were the Cretaceous bedrock valleys filled, but most major divides had been transgressed by the Ash Hollow, so that the local factors that formerly controlled alluviation were no longer operative. Kimball sediments spread as a broad sheet over the underlying Ash Hollow without confinement to valleys, a distribution that tends to give a misleading impression of the volume of sediments contained within it. Locally, coarse gravel forms the base of the Kimball, but most of the sediments are gray, fine to medium sand, silt, and some clay, all heavily infiltrated with calcium carbonate, and capped by a layer of dense, nodular pinkish to grayish-pink or white limestone ranging from 6 inches to 3 feet in thickness. This limestone has been called the "Algal limestone" because of a fossil alga reported to occur within it; it is otherwise nonfossiliferous, and does not even contain diatoms, which should occur with other algae. The "Algal limestone" is an important stratigraphic marker, whatever its origin, because there is no doubt that it occurs at the stratigraphic top of the Kimball member and of the Ogallala formation.
Ten petrographically distinctive volcanic ash lentils have been described from the Ogallala formation in northwestern Kansas; six of them, because of their distinctive petrographic character, stratigraphic significance, and relatively wide distribution, have been given formal bed names; four other ash falls known to occur in this area have little stratigraphic significance and have not been named. The Valentine contains the Calvert bed, correlated with an altered volcanic ash in the type section of the Valentine; the other nine volcanic ash falls occur in the Ash Hollow. The Rawlins bed has been correlated with volcanic ash in the type section of the Ash Hollow in Nebraska; the Fort Wallace, Dellvale, Reager, and Reamsville beds have been used, together with other data, in correlations within the Ash Hollow in northwestern Kansas.
Fossil molluscan assemblages are known from eight stratigraphically important localities in the Ogallala of northwestern Kansas; these are distributed through Valentine, Ash Hollow, and Kimball rocks. The molluscan faunas undergo gradual progressive changes without marked faunal discordances, but faunal characteristics vary sufficiently with vertical distribution of molluscan assemblages to allow recognition of Valentine, Ash Hollow, and Kimball members.
Fossil seeds, widely dispersed geographically and abundantly represented in stratigraphically variable populations, are without question the most important stratigraphic tool among the several kinds of organic remains preserved in the Ogallala. Fossil floras, based on fossilized hulls of grasses, and nutlets and fruits of various herbs and a single tree, are listed from 27 stratigraphically significant localities in the Ogallala formation of northwestern Kansas. Assemblages of fossil plant remains permit positive identification of Valentine, Ash Hollow, and Kimball members, and make it possible to subdivide the Ash Hollow member into lower, middle, and upper stratigraphic units.
Kansas Geological Survey, Geology
Placed on web Aug. 4, 2011; originally published March 1956.
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