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Silicified Rock in the Ogallala Formation

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Samples of silicified Ogallala rock for petrographic study were selected from 27 localities in Kansas; from three localities in Texas--near Midland, Canyon, and Lamesa; and one sample each from a locality near Beecher Island, Colorado, from Gregory County, South Dakota, and from Holt County, Nebraska. Extensive use of thin sections was made; these were supplemented by chemical analyses and study of crushed fragments. The cementing material was found to consist, for the most part, of silica in the form of opal associated with a small amount of chalcedony.

The silicified rocks of the Ogallala formation are grouped, on the basis of their texture, color, degree of cementation, and predominant lithology of the original deposit, into two major categories--here termed quartzite and chert--and several special types of minor extent and importance.


The quartzite occurs in lentils ranging in size from a few feet to more than a half mile in lateral extent and from less than 1 foot to more than 15 feet in thickness. Where exposures are adequate, these lentils may be observed to occur in loose sand and gravel, locally interbedded with silt and clay; the adjacent clastic material generally contains no appreciable amount of calcium carbonate cement. Bedding planes are commonly obscure, except in a few exposures where cross bedding is observed. Joints are rare and the few that were noted in the field display thin incrustations of slilca on the joint surfaces. In many places Ogallala quartzite immediately overlies chalky shale of the Niobrara formation (Cretaceous), which, in such association, is generally at least partly silicified. In Rawlins County, Kansas, the quartzite occurs a few feet above the Pierre shale (Cretaceous).

The texture of the quartzite ranges from fine-grained sand to coarse poorly sorted conglomerate containing boulders that have a maximum diameter of more than 50 centimeters. Where large boulders are present, they are invariably composed of chalk or shale derived from the underlying Cretaceous formations. Cementation of the sand and gravel ranges from the nearly complete filling of all original pore spaces with silica to only partial filling of the pore spaces, producing a minutely cavernous appearance. A cavernous appearance on some exposed surfaces is also caused by the lower resistance to weathering of the included pebbles and boulders of partly silicified Niobrara chalk.

The Ogallala quartzite in the region north of Texas ranges in color from greenish gray to greenish brown to bright green. The weathered color is light gray or white and reddish-brown. In some places the presence of special features, such as abundant white feldspar in some of the quartzite of Rawlins County, Kansas, gives the rock a distinctive appearance. The several quartzite exposures examined in western Texas display a pink to gray-pink color.

The rock has a hardness very little below that of quartz, and breaks through, rather than around, the constituent grains, often displaying a subconchoidal fracture. The apparent specific gravity given in Table 1 ranges from 2.37 to 2.41 except for one sample from near Woodruff in Phillips County, Kansas, which has a specific gravity of 2.63.

The constituents of the quartzite were determined by the study of thin sections. The grains, as in the rest of the Ogallala, consist predominantly of quartz but include variable amounts and types of feldspar and also scattered fragments from igneous, metamorphic, and a few sedimentary rocks. In some thin sections much material of local derivation is present.

The sand grains are subangular to rounded and fine to coarse in texture. Where opalization is most complete the very fine fractions are absent and the material is generally well sorted as in the sample from Norton County shown in Plate 4, figure 1. The feldspars include microcline, plagioclase, and orthoclase (Pl. 4, fig. 2), and range in quantity from less than 10 percent to more than 40 percent of the grains. Some of the quartzite from Rawlins County (Pl. 5, fig. 4) is characterized by a large proportion of somewhat altered white feldspar and an almost complete absence of the pink feldspars which are found elsewhere. This rock is a true arkose. In quartzite from other localities, the feldspars are comparatively fresh or show only incipient alteration.

Plate 4--Photomicrographs of Ogallala quartzite and chert from Norton, Graham, and Rawlins Counties, Kansas. Fig. 1--Fine-grained quartzite showing opaline cement from SE cor. sec. 5, T. 5 S., R. 22 W., Norton County, Kansas; plane polarized light, X 25. Fig. 2--Same as 1; crossed nicols, X 25. Fig. 3--Coarse-grained quartzite from Cen. S. line, sec. 31, T. 7 S., R. 22 W., Graham County, Kansas, showing vugs lined with calcite in opaline cement; plane polarized light, X 35. Fig. 4--Chert, showing fine-grained calcium carbonate, opal, chalcedony, and calcite from NE sec. 16, T. 4 S., R. 36 W., Rawlins County, Kansas, plane polarized light, X 35. Fig. 5--Same as 4; crossed nicols, X 35. A larger version is available as an Acrobat PDF file.

Five black and white photomicrographs

The material of local origin consists of fragments of shale or chalky shale, pieces of pelecypod shell, and a small amount of bone and foraminiferal tests. Plate 5, figure 1, is a thin section of basal Ogallala quartzite from northeastern Ness County showing a large shell fragment believed to be from the prismatic layer in the shell of a mollusk of the genus Inoceramus (Boggild, 1930, p. 262). This figure also shows several foraminifers and small pieces of chalky shale from the Niobrara which appear black in the photomicrograph. The foraminifers and the large shell fragment are composed of calcite. The pieces of chalky shale have been partly silicified but nevertheless contain a large amount of calcium carbonate. Many thin sections and hand specimens show penetration of sand grains into the chalky shale. Most of the foraminiferal tests in the quartzite are filled with optically continuous calcite. Tests in the underlying partly silicified Niobrara chalk (Pl. 5, fig. 2) have fillings of chalcedony or chalcedony and calcite, but the tests themselves, as in the quartzite, are calcareous. A large Niobrara boulder in basal Ogallala quartzite capping a hill 2 miles northwest of Logan has a silicified rind ranging in thickness from one-fourth to one-half inch. Plate 5, figure 3, shows the sharp contact between silicified (light-colored) and unsilicified parts of the boulder. Opalization of the basal sand and gravel in this area is incomplete and spotty.

Plate 5--Photomicrographs of silicified rock and Ogallala "mortar bed" from Ness, Phillips, and Rawlins Counties, Kansas. Fig. 1--Quartzite, showing shell fragment, foraminifers, and chalky shale from the underlying Niobrara formation; Cen. W. side, sec. 1, T. 16 S., R. 21 W., Ness County, Kansas; plane polarized light, X 35. Fig. 2--Partly silicified Niobrara chalky shale from below the quartzite shown in 1; plane polarized light, X 35. Fig. 3--Contact between silicified rind and unsilicified interior of boulder of Niobrara chalky shale included within the basal Ogallala quartzite; in sec. 29, T. 4 S., R. 20 W., Phillips County, Kansas; plane polarized light, X 25. Fig. 4--Quartzite, showing opaline cement and weathered feldspar grains; from NW sec. 29, T. 2 S., R. 32 W., Rawlins County, Kansas; plane polarized light, X 10. Fig. 5--Ogallala silty sand cemented with calcium carbonate; from sec. 19, T. 3 S., R. 19 W., Phillips County, Kansas; plane polarized light, X 25. A larger version is available as an Acrobat PDF file.

Five black and white photomicrographs

Quartzites containing large fragments of Cretaceous rock are not limited to the basal part of the Ogallala; they have been found higher than 70 feet above the base on Sugar Loaf Mound and on a butte in sec. 17, T. 5 S., R. 20 W., 4 miles southwest of Logan in Phillips County.

Several degrees of opalization are observed. In many specimens opal completely fills the interstices between the grains, and the rock is dense and tough, showing an absorption (Table 1) as low as 1 percent. Other specimens contain small vugs in the opal, some of which are lined with colorless chalcedony or calcite. Plate 4, figure 3, shows vugs lined with small calcite crystals.

Quartzite from the lower part of the Ogallala formation of northern Nebraska (SW cor. NW sec. 30, T. 32 N., R. 11 W., Holt County) and from the base of the Ogallala in southern South Dakota (southeastern edge of Dallas in western Gregory County) is similar in appearance to the green quartzite of Kansas (Pl. 6, fig. 5). Thin sections of the pink quartzite from Midland and Canyon, Texas (Pl. 6, figs. 1, 2, and 3), have only small areas and streaks of opaline cement, and much of the cementing material is partly calcareous. In the Texas samples the sand is fine-grained and contains some silt. Plate 6, figures 1 and 2, shows an area of chalcedony surrounded by finely banded opal. This feature is more closely related to the lithology of the Ogallala chert than to that of the green quartzite of Kansas, Nebraska, and South Dakota. The Texas quartzite should perhaps be regarded as intermediate in several characteristics between the chert and the green quartzite.

Plate 6--Photomicrographs of silicified Ogallala from Texas, Colorado, and South Dakota. Fig. 1--Fine-grained quartzite showing opaline and chalcedonic cement from near Midland, Texas; plane polarized light, X 25. Fig. 2--Same as 1; crossed nicols, X 25. Fig. 3--Fine-grained Ogallala quartzite from southwest of Canyon, Texas; plane polarized light, X 25. Fig. 4--Chert from near Beecher Island, Colorado; plane polarized light, X 25. Fig. 5--Quartzite from the southeast edge of Dallas, Gregory County, South Dakota; plane polarized light, X 25. A larger version is available as an Acrobat PDF file.

Five black and white photomicrographs

The use of the term quartzite,, as applied to this particular rock, is admittedly not consistent with some of the definitions that been proposed for this term (Holmes, 19201, p. 194: Prisson and Knopf, 1926, p. 381.; Dana, 1932, p. 386; Grout, 1932, p. 386; Allen, 1936, p. 38; Hatch, Rastall, and Black, 1938, p. 104), however its use in this sense is allowed by other definitions (Chamberlin and MacClintock, 1934, p. 24; Sellards and Eaker, 1934, p. 239; Milner, 1940, p. 370). It seems to us advisable to use quartzite as designation of this particular lithology rather than some lesser known or more cumbersome name, because of its use in earlier literature (Hicks, 1888; Barbour, 1903, p. 163; Condra, 1908, pp. 18-23; 1908a, p. 49; Landes and Keroher, 1942, pp. 284, 306), by the residents of the area, and by many of the engineers and contractors who have used the rock for construction purposes.


The rock within the Ogallala formation here referred to as chert occurs as irregular areas of uneven or spotty silicification of "caliche" or "marl" that contain various amounts of silt or sand. Smith (1940, p. 46) has described an exposure of this chert in western Clark County as follows:

It forms a prominent white ledge, visible for a considerable distance. The chert is very brittle and easily shattered by the hammer, forming irregular, hackly fragments; seemingly it is thoroughly traversed by incipient fractures. The color ranges from white on the weathered surface to light gray on a fresh surface, and shows some dark mottlings. The rock is megascopically opaque, but contains scattered and irregular clots and veinlets of translucent, opaline silica. The veinlets are locally so prominent as to give-the rock a brecciated appearance.

This description is equally applicable to the extensive chert deposits that occur in southwestern Rawlins County and elsewhere. However, the percentage of silica varies within wide limits and at some localities the silica consists merely of irregular spots scattered through an otherwise calcareous deposit. Such minor amounts of silicification occur widely throughout the area of Ogallala outcrop and are typical of the deposits immediately below the "Algal limestone" (Elias, 1931, p. 136).

A specific gravity of 2.21, which is about 0.20 below the average for the Ogallala quartzite, was determined for the one chert sample tested.

The major constituents of the chert, as determined by the study of thin sections, are opal, chalcedony, and very fine-grained crystalline calcium carbonate, with minor amounts of quartz and feldspar grains and secondary calcite. Some samples contain more than 50 percent very fine-grained calcium carbonate mingled with an undetermined amount of disseminated silica (Pl. 6, fig. 4), whereas other samples consist predominantly of silica in the form of opal and chalcedony. A layer of opal is invariably found marginal to the very fine-grained calcium carbonate. Vugs in the opal are commonly found to be lined with chalcedony, which in a few thin sections is in turn lined with crystalline calcite. Plate 4, figures 4 and 5, shows this relationship in a sample from Rawlins County. A similar relationship is observed in a sample of partly silicified "Algal limestone" from near Lamesa, Texas, but the secondary crystalline calcite is optically continuous and completely fills the central part of the vug. All stages of this sequence, including the association of opal alone with the fine-textured calcium carbonate, may be observed. Grains of quartz sand occur in all parts of the rock except in the zones of chalcedony and secondary crystalline calcite.

Although use of the term chert as applied to the above-described variety of rocks may be questioned, it is believed to accord with definitions and usage prevalent in the literature (Twenhof el, 1939, pp. 367-368).

Miscellaneous Types

Hydrous silica is present not only in the two important lithologic types that have been described but also in several other well-indurated rocks more or less related to them. It has been observed by us to occur in minor amounts in many samples of Ogallala rock not associated with quartzite or chert. Irregular masses (up to 8 inches in long diameter) of dense, cream-colored, waxy or resinous opal were found on the east side of sec. 5, T. 2 S., R. 17 W., northeastern Phillips County, Kansas, in a lenticular bed composed largely of calcium carbonate several feet above a green quartzite. This material contains vugs lined or filled with the more common translucent opal and some chalcedony, and on the outside consists of dull white porous silica. A thin section shows a uniform texture, a high degree of opacity, and very few clastic grains. The rock is brittle and breaks easily with pronounced conchoidal fracture into small splinters.

In the same section, and below the bed containing the "resin" opal, is a bed of sandy silt containing large elongated pale-buff or cream-colored mammillary or colloform concretions,, 10 to 14 inches long, of sandy to argillaceous silt cemented with opaline silica (Pl. 7, fig. 3). These are very similar to the green quartzite except for their silty character and the general absence of green coloration, although a few of the concretions have pale green centers.

Plate 7--Photomicrographs of silicified Ogallala from Phillips County, Kansas. Fig. 1--Silicified sandy silt, showing zonation of silicification; from the NW sec. 24, T. 5 S., R. 19 W., Phillips County, Kansas; plane polarized light, X 10. Fig. 2--Same as 1; crossed nicols, X 10. Fig. 3--Concretions in silty sand, showing opaline cement; from sec. 5, T. 2 S., R. 17 W., Phillips County, Kansas; plane polarized light, X 25. Fig. 4--Part of an irregular mass of chert included within fine-grained quartzite, from the NE sec. 18, T. 5 S., R. 19 W., Phillips County, Kansas; plane polarized light, X 10. Fig. 5--Same as 4; crossed nicols, X 10. A larger version is available as an Acrobat PDF file.

Five black and white photomicrographs

Another type of silicified rock occurs immediately below the uppermost quartzite near the measured section in sec. 24, T. 5 S., R. 19 W., 4.5 miles south of Speed in Phillips County. This consists of a bed 20 inches to 3 feet thick, composed of fine sandy silt cemented with opaline silica and some calcium carbonate into a dense rock of uneven hardness. Much of the opal is concentrated in small irregularly shaped green spots which are surrounded with alternating rings of greenish-gray and green material. Plate 7, figures 1 and 2, are photomicrographs of a thin section cut through one of the green spots. Differential weathering causes the green centers to protrude from the surface, whereas one or more of the outer circles may be sharply indented. Some of the green centers are encircled by three or more green rings representing a total diameter of more than 1.5 inches. Calcium carbonate concentrated in some of the outer greenish-gray circles is apparently a factor in differential weathering. This rock seems to be similar to the contraction spheroids in very calcareous porcelanites described by Taliaferro (1934, p. 205).

Associated stratigraphically with the green quartzite is still another variety of rock that in some respects is very similar lithologically to the pink Ogallala quartzite from Texas. It is dense, green, fine-grained material characterized by small areas (less than 1 inch in diameter) of white or colorless silica, which may also occur as the lining of small vugs. The green rock itself shows some areas consisting predominantly of opaline silica cement and others of mixed silica and fine-grained calcium carbonate. This rock occurs abundantly in at least three localities in Kansas--in sec. 7, T. 2 S., R. 17 W., northeastern Phillips County, where it grades laterally into green quartzite; capping a long butte 75 feet above brownish-green basal Ogallala quartzite in the NE sec. 18, T. 5 S., R. 19 W., southern Phillips County (Pl. 7, figs. 4 and 5); and capping Twin Mounds in the SE sec. 4, T. 9 S., R. 17 W. in eastern Rooks County, where it is also fairly high within the lower Ogallala. Plate 7, figures 4 and 5, shows part of one of the areas of colorless silica. It is chert or very fine crypto crystalline chalcedony surrounded by opal (which is white by reflected light) and set in green opaline and calcareous siltstone.

Reference has been made to silicified Niobrara chalk associated with silicified areas within the Ogallala. Landes and Keroher (1942, p. 306) have described this rock in some detail. A sample of silicified Niobrara chalk from northeastern Ness County (Pl. 5, fig. 2) was digested in dilute hydrochloric acid and was found to be 78 percent soluble by weight. The residue did not disintegrate, but was a white porous coherent mass consisting almost entirely of silica that had been evenly disseminated throughout the chalk.


Discussion of the origin of silicified Tertiary deposits in the Great Plains region, as follows, deals mainly with the green quartzite of Kansas because our knowledge of the geographic extent and stratigraphic position of this rock is much more adequate than that concerning the Ogallala chert or the pink quartzite of Texas.

Sandstone with opaline cement has been considered by most petrographers to be a very uncommon type of rock. A possible reason for this conclusion may well be the failure on the part of many geologists to recognize its character in the field or to consider the nature of siliceous cement, when observed, important enough to mention in reports. No previous writer, from 1888 to the present (Hicks, 1888; Todd, 1889; Haworth, 1897; Barbour, 1903; Condra, 1908, 1908a; Moss, 1932; Elias, 1937, p. 23; Landes and Keroher, 1942) describing the green Ogallala quartzite from Kansas, Nebraska, or South Dakota, has mentioned the opaline character of the cement. The literature on Tertiary deposits of Texas, on the other hand, is replete with discussions of the petrology and genesis of opaline sandstone (Goldman, 1915; Udden, Baker, and Böse, 1916; Dumble, 1918; Bailey, 1926; Plummer, 1932; Bowling and Wendler, 1933; Sellards and Baker, 1934), although a discussion of the quartzite in the Ogallala formation of Texas has not been found. Elsewhere, references to the occurrence of opaline cement seem to be rare; Pöhlmann (1886, p. 246) mentions an occurrence in Paraguay and Wanless (1923, p. 258) notes the presence of opaline cement in sandstone dikes in the White River beds of South Dakota.

Study of the origin of the Ogallala quartzite requires attention to several major problems: the source of the silica; the causes of its deposition in the form of opal and its localization in lentils; the relationship of the silicification to the sequence of geologic events during Pliocene and Pleistocene time; and the cause of the prevalent green color of the rock.

Source of silica--Mudge,, who in 1874 (pp. 115-116) first noted the presence of silicified Ogallala in western Kansas, believed that the silica might have been supplied by thermal springs or related phenomena. Penrose (1889, p. 88) believed that silicic acid contained in surface waters was the source of siliceous cement in the Fayette sandstone of Texas. Silica is a common constituent of many natural waters. It is noteworthy, however, as pointed out by Roy (1945), that the silica found in most ground waters has a low concentration and occurs in the form of a true solution rather than colloidal suspension. Inasmuch as the cement of the Ogallala quartzite is preponderantly opaline in character, the silica is probably in the colloidal form (Dana, 1932, p. 355; Hatch, Rastall, and Black, 1938, p. 199) and the waters from which the cementing material was deposited must have been heavily charged with colloidal silica in contrast to the low concentration of silica in true solution found in normal ground water. Thus an abundant and relatively near-by source of silica must have existed during the period of cementation so as to provide chemical character of the waters differing markedly from surface and ground waters associated with Ogallala deposits at the present time. The only adequate source of such silica seems to be the extensive deposits of volcanic ash that occur at a higher stratigraphic position within the Ogallala formation. Volcanic ash is known to occur widely in the younger Pleistocene strata also and may have constituted a source for the stratigraphically higher Ogallala chert. Ash deposits are considered by several workers (Rubey, 1928; Reed and Longneeker, 1932, p. 72; Price, 1933, pp. 506, 519; Smith, 1934; Murata, 1940; Frye, 1942, p. 101) to be the probable source of silica in some sediments, and volcanic ash is judged to have been the source of the opaline cementing material in the sandstones of the Catahoula, Fayette, and Oakville formations in Texas (Dumble, 1918, p. 147; Bailey, 1926, pp. 12-13; Plummer, 1932, p. 725). Relationships are particularly clear in the case of the Catahoula where opaline-cemented sand is interbedded with ash deposits and grades laterally into them. The most extensive deposits of volcanic ash in the Ogallala of Kansas occur stratigraphically higher than the quartzite. Since deposits overlying the quartzite beds have been removed by erosion at most places where the latter are found, the interpretation of origin of the opaline silica here given cannot be confirmed by finding ash beds or bentonitic clays derived from ash in the vicinity of quartzite deposits. The only unquestioned occurrence of bentonite in association with quartzite was found in the large quarry west of Woodruff in Phillips County.

Factors controlling deposition--The extreme localization of the quartzite as lentils in the Ogallala formation, commonly bounded by a sharp contact with uncemented sand, gravel, or silt, and the restriction of silicification in the Niobrara to the zone immediately below its contact with the Ogallala, seem to have an important bearing on the origin of the silica. It has been repeatedly demonstrated that colloidal silica is precipitated by such ions as calcium, magnesium, and bicarbonate in aqueous solution (Cox, Dean, and Gottschalk, 1916, pp. 7-12; Tarr, 1917, p. 436; Lovering, 1923, p. 536). If solutions heavily charged with silica are somewhat acid in nature, as implied by Roy (1945, p. 398), the required environment for precipitation of colloidal silica is produced where these solutions encounter an area of existing calcium carbonate cement (Lovering, 1923, p. 536). Such an origin of silica deposits has been suggested for the Gueydan formation (Catahoula tuff) by Bailey (1926, p. 57), the Oakville, Catahoula, and Lower Reynosa formations by Price (1933, pp. 505-506), and the Ogallala formation by Landes and Keroher (1942, p. 284). Accordingly, it seems probable that the opaline cement of the Ogallala quartzite was deposited as replacement of an earlier generation of calcium carbonate cement, and that the opal of the silicified Niobrara was formed as a partial replacement of the chalk.

Conclusion that the siliceous cement of the quartzite originated by replacement of calcareous cement is supported by the physical resemblance of hard lentils seen in outcrops of the Ogallala, some showing sand and gravel cemented by calcium carbonate and others having an opaline siliceous cement. Pebbles of silicified Niobrara chalk may be observed in loose noncalcareous sand and gravel only a few feet below some of the quartzite lentils. The fact that such chalk pebbles were silicified after deposition is demonstrated by incorporation of sand grains in peripheral portions of the pebbles. Study of thin sections shows that small grains of calcium carbonate are included locally within the opal cement. The silicification of the Niobrara chalk, and particularly the development of chert deposits at several stratigraphic positions within the Ogallala, is observed to have been incomplete and gradational within the dominantly calcium carbonate rock.

The chalcedony linings of vugs may be the result of a change in chemical character of the depositing waters. Evidently such linings of chalcedony were deposited after the large mass of opaline cement was introduced into the rock, and the absence of sand grains in the linings indicates that the chalcedony microcrystals grew outward into cavities. Plate 5, figure 5, is a photomicrograph of a thin section of "mortar bed" showing no replacement by silica. The general similarity between the textures and types of vugs of this rock to those of the Ogallala quartzite is apparent. As silica in dilute solutions is believed by some to be crystalloid (Roy, 1945), the relationships of the chalcedony vug linings suggest that following deposition of opaline silica concentration of silica in Ogallala waters decreased possibly because an advanced stage in the leaching of ash deposits had been reached, because the water table had declined, or because ash beds serving as source of the silica had been removed by erosion. These conclusions are strengthened by the presence in some vugs of crystalline calcite as a band deposited on the chalcedony, which seems to indicate a return to normal ground-water conditions.

Age of silicification--Although it is impossible to establish conclusively from available evidence the time at which the silica was introduced into the Ogallala quartzite, some data indicate the following sequence of events. During deposition of the Ogallala sediments the regional water table must have stood relatively near the surface (Frye, 1945a). As has been pointed out, the quartzite invariably occurs stratigraphically low in the Ogallala, and although the volcanic ash is also low, it is believed to occur higher in the section than the quartzite. Shortly after accumulation of the ash, these deposits would be below the water table and subject to hydration and leaching, so that heavily charged siliceous waters would move downward and outward. That leaching and subsequent precipitation of the opal occurred while the ash was below water level rather than later is suggested by the green color of the cement, which is caused by the presence of ferrous iron. Ferrous iron (Reiche, 1945, p. 24) is believed to be readily adsorbed by silica gel, which may account for the intensification of green coloration in the opal cement of the quartzite when compared with the pale green color of some other lower Ogallala deposits. This iron would probably have been altered to the ferric state if the ash had been exposed to weathering above the water table (Reiche, 1945, p. 32). The inferred sequence of conditions is also indicated by the presence of bentonitic rather than kaolinitic altered volcanic ash in the Woodruff quarry (Ross and Hendricks, 1945, pp. 60, 66). It is possible that an erosional disconformity occurs within the Ogallala of northwestern Kansas, and if this is true, the silicification of the quartzite probably preceded the erosion interval. The presence of a small area of unsilicified limestone surmounting the quartzite cap of Sugar Loaf Mound (Pl. 2A) suggests that erosion had reduced the land surface to such level after the silicification that produced the quartzite and before the final stage of Ogallala deposition.

Some of the scattered chert deposits in the Ogallala occur at a high stratigraphic position within the formation, and since the only obvious source of this silica is early Pleistocene volcanic ash beds, the date of the silicification must have been later than early Pleistocene. It is to be noted that these younger chert deposits lack the distinctive green color that characterizes the quartzite.

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Kansas Geological Survey, Geology
Placed on web Aug. 20, 2007; originally published July 1946.
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