Early theories of origin of the spheroidal, calcite-cemented sandstone concretions in the Dakota formation have been reviewed by Bell (1901) and Schoewe (Schoewe et al., 1937, pp. 183-184). The concretions have been described as corals, as bunches of calcium carbonate-depositing algae, and as erratic boulders brought down to their present location by a continental ice sheet. Landes (1935), Schoewe (in Schoewe, et al., 1937, p. 184), and Williams and Lohman (in press) have attributed the concretions and other carbonate- and silica-cemented sandstones in the Dakota and Kiowa formations simply to deposition by percolating ground water rich in those minerals subsequent to the deposition of the sandstone. Plummer and Romary (1942, p. 336) studied the stratigraphic occurrence of the calcite-cemented sandstones in greater detail and concluded that "the presence of the 'quartzite' is due to the fact that the conditions favorable to cementation are distributed at definite horizons and that, whenever a sufficiently permeable bed occurs, a 'quartzite' is formed at this horizon." Many of the sandstones attributed by Tester (1931, p. 279) to a marine embayment in northeastern Nebraska are cemented with calcium carbonate similar to that in the Dakota formation of Kansas.
Scattered areas of coarsely crystalline carbonate cement are not uncommon in sandstones of other ages and other localities. Walther (1924, pp. 74-75) believes that calcite sand-crystals, such as those of Fountainebleau, and barite sand-crystals are products of insolation and evaporation in desert areas. Jukes (1872; quoted by Rogers and Reed, 1926) relates the Fountainebleau sandcrystals to overlying beds of freshwater limestone, from which water percolated downward. Graham (1930, p. 704) describes sand-calcite crystals in the Upper Cambrian Jordan sandstone of Minnesota, and notes that they occur for some distance below the Oneota dolomite.
Mathias (1931) describes calcareous sandstone concretions (similar in general appearance to those of "Rock City") in the Upper Cretaceous Fox Hills sandstone in Colorado. He believes that they were penecontemporaneous with deposition of the Fox Hills sandstone because delicate algal structures are preserved, and he suggests that the source of the calcium carbonate would have been the overlying Fox Hills sea. Ross, Miser, and Stephenson (1929, pp. 184-185) describe coarsely crystalline calcite-cemented concretions in tuffaceous Woodbine sand of Arkansas and note that they must have formed soon after deposition of the tuff, because the volcanic glass within the concretions was prevented from being altered to bentonite. Similar deductions concerning concretions in the Hambre sand (Miocene) of California were made by Bramlette (1941).
A discussion of the origin of carbonate- (and silica-) cemented sandstones in the Dakota and Kiowa formations involves consideration of the following features: the degree of etching of the grains, the percentage of space occupied by cement, the common association with cone-in-cone, the restriction of carbonate cement to certain zones in the Dakota and its prevalence in the Kiowa shale, and its common association with marine shells and glauconite. The time of deposition of the cementing material must be established, and also the sequence of cements, if more than one type occurs. The ferruginous cement, which was not examined in detail, is not unusual and hypotheses of its origin will not be discussed to any great extent in the present report. Precipitation of iron, transported as the bicarbonate, may take place anywhere if oxygen enters the waters and carbon dioxide can escape (Twenhofel, 1939, pp. 383-384). This precipitation in some instances may be related to the position of the water table.
Origin of etched grains
The etching of sand grains by carbonate-bearing waters, by calcite replacement, or by other causes, has been described by several writers. Ross, Miser, and Stephenson (1929, pp. 184-185) describe etched and corroded augite and orthoclase grains which have been partly replaced by calcite in the concretions of the Woodbine sand. They note that corroded augite also occurs outside the concretions and conclude that carbonate-bearing waters were also capable of corroding and etching augite. They observed that no minerals of igneous rocks except augite and orthoclase had been affected. Krynine (1940, p. 24) noted that calcite, dolomite, and ankerite-siderite had corroded and partly replaced quartz grains in the Third Bradford sand. Hatch, Rastall, and Black (1938, pp. 299-300) report that the quartz grains of many sandy limestones and dolomites are etched, and that sometimes the carbonate crystals have replaced the quartz along cracks passing through the grains, which then are reduced to corroded remnants. They indicate (p. 90) that alkaline solutions released from decaying feldspars are probably to some extent responsible for corrosion of detrital quartz. Storz (1928, pp. 67-78) also observes that the decomposition of feldspars causes etching of quartz grains. Quartz grains partly replaced by kaolinite have been described by several writers (Ries, et al., 1922, pp. 161162; Ross and Kerr, 1931) and this feature has been attributed to the action of sulfates derived from pyrite.
The mechanism of replacement of quartz and feldspar by calcite is a common one and, in the case of quartz, apparently reversible. No further light can be shed on it in this study. The thin sections show that the sand grains were etched in situ (Fig. 3). and one concludes, when aided also by microscopic examination of loose grains, that the sand when originally deposited was fairly well rounded, or at least not sharply angular. Loose sand collected from below the Kearny County quartzite shows a degree of etching similar to that in the calcite-cemented sands. Sand separated from clays in the Dakota and Kiowa formations is also etched. The only Dakota and Kiowa sands observed by me which were not affected by solvent action or replacement are cemented with silica, barite, or celestite; the sand in some quartzites, however, has been etched.
Fig. 3--Drawings from thin section of calcite-cemented sandstone from Lincoln quarry, showing etched and displaced grains.
Space between grains
The packing of the grains in. the carbonate-cemented sandstones and in some of the silica-cemented sandstones is extremely poor, and very few of the grains are in contact with each other. Hadding (1929, pp. 19-20, 204) writes that this condition in calcareous sandstones is due to the deposition of calcareous mud along with the sand grains, followed by recrystallization of the matrix. Two other causes for the wide spacing of the grains are apparent in thin sections from the Kiowa and Dakota formations, and it is therefore not necessary to postulate the deposition of a pure calcareous mud. The most obvious of these factors is the replacement of the outer parts of the grains by calcite. In some large areas of cement, outlines or "ghosts" of grains which have been completely replaced may be observed, and it is conceivable that the process of replacement might continue until calcite alone remains. The second factor, a quantitatively minor one, is the tendency of calcite to force particles apart during the course of crystallization. This is shown in thin sections of sandstone where the grains have been forced apart along incipient fractures and the fragments scattered and rotated so that they are no longer in optical continuity with each other (Fig. 3).
Sequence of cements
As has been previously noted, quartz and calcite cement are not observed together in many thin sections of sandstones in the Kiowa and Dakota formations. The thin section (Fig. 2) from Twin Mounds, described above, perhaps sheds more light on the sequence of cementation than do the other thin sections, but it is by no means certain that the same conditions prevailed in all the areas of silica-cemented sandstone in the two formations. Study of this thin section suggests the following events after deposition of the sand. (1) Introduction of calcite cement. This is indicated by the presence of calcite in a fracture across one quartz grain, the replacement of part of a feldspar grain, and the eccurrence of minute particles of calcite in the chert; by the etched appearance of many of the grains; and by the presence of grains which have been parted and rotated since deposition. The last two features are typical of the calcite-cemented sands and are not observed in most of the other sandstones, and are therefore thought to be fair evidence of the former presence of carbonate cement. (2) Introduction of silica cement replacing calcite. The chert is the earlier of the two types of silica cement present in this rock, for it occurs adjacent to the grains and in the fractured displaced grains. The quartz cement is later than the chert, for although it is optically continuous with the grains, there is in most parts of the thin section a thin layer of chert between the grain and the secondary quartz (Pl. 6D). The position of ferruginous material in the sequence is not clear. Some of it may be primary. Other parts, which occur in the chert as faint stains with rounded outlines, may be the remnants of decomposed or replaced feldspars.
In those thin sections of quartzite in which the sand grains are not etched, the evidence of replacement of carbonate is not clear, and the quartz or chert may be the primary cement. The only characteristic of these rocks which suggests that the latter is not the case is the relatively large amount of space between the grains. Chert and quartz seem to have been deposited simultaneously in several of the thin sections studied. Cayeux (1920) and Krynine (1941) describe primary quartz deposited on the sea floor during deposition of the sediment.
The position of barite in the sequence is in doubt. No grains embedded in the barite crystals (in the thin section from sec. 36, T. 14 S., R. 4 W., McPherson County) which had been fractured and rotated were noted. On the other hand, etching is present, but not to the degree that it is in the grains occurring in the adjacent calcite. The etching of the grains in the barite may have been effected (1) by ground-water solutions before cementation; (2) by incipient replacement of quartz and feldspar by calcite, followed shortly thereafter by replacement of the calcite by barite; or (3) by direct replacement of quartz and feldspar by barite.
The history of the dolomite cement is also unknown. It may be primary, but is generally considered to be a replacement of calcite. As the sand grains in the dolomite-cemented sandstones are less strikingly etched than those in calcite-cemented sandstones, the dolomite is judged to be either primary or to have replaced calcite shortly after the original cementation.
Time of cementation
Two lines of evidence point to the hypothesis that cementation with calcite was penecontemporaneous (Richardson, 1921) with deposition of the sand. The first of these is the apparent restriction of the calcite-cemented sandstone to certain horizons, particularly in the Dakota formation. If it were due merely to circulating ground waters long after deposition, its distribution would be more haphazard, and it would not be so commonly associated with glauconite. The second evidence is more direct, and consists of a thickening of the sand bodies at the local areas where they are cemented from top to bottom with calcium carbonate. At one place in the Kanopolis quarry (sec. 19, T. 16 S., R. 6 W., Ellsworth County) calcite-cemented sandstone forms a sag or bulge in a sandstone bed overlying dark-gray shale, and a thin bed of cone-in-cone curves around the base of the cemented sandstone at the contact. This suggests that consolidation of the sediments took place after cementation, as has been suggested by Shaub (1937, p. 344) for other localities.
Supporting evidence of penecontemporaneous origin of similar calcareous sandstone bodies has already been noted in the literature (Mathias, 1931; Ross, Miser, and Stephenson, 1929, pp. 184-185, Bramlette, 1941.)
Some of the silica cement seems to have replaced calcite cement at an early stage, but some may have been introduced at a much later time, as indicated by the advanced stage of etching of the sand grains in some samples. Barite and celestite cement were probably formed fairly soon after deposition of the sand.
Causes of localization of cement
The available evidence suggests that the calcite-cemented sandstone is related to a marine or near-marine environment. This evidence has been described earlier in the section on petrography. An immediate cause of deposition of the cement possibly is the mixing of fresh and marine waters, in sands at or immediately below the sea floor which are well sorted and permeable enough to allow the mixing to take place.
Kansas Geological Survey, Geology
Placed on web June 14, 2007; originally published Nov. 1947.
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