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Petrology of the Smoky Hill Member
Stratified Chalk
The Smoky Hill Member consists primarily of obscurely to well laminated, shaly weathering chalk that is mostly foraminiferal pelmicrite with packstone or, less commonly, wackestone texture (Figs. 32, 33). The pellets are much compacted, and thus present fusiform outlines in thin sections cut normal to bedding. These pellets are composed almost entirely of coccoliths, contain little evidence of interstitial cementation, have only modest indications of aggrading neomorphism, and show little or no etching of individual coccoliths (Fig. 34). In a sample of laminated chalk from Locality 13 the average maximum dimension of 100 pellets is 0.12 mm (Hattin, 1975b). Contrasting sharply with these pellets is the adjacent matrix, which comprises a heterogeneous mixture of coccoliths, crystals of secondary calcite, and noncarbonate particles (Fig. 35). Textural differences between pellets and matrix are consistent throughout the composite section, regardless of whether the chalk is laminated, bioturbated, granular, or organic-rich, and has been reported also from pellet-rich rocks of the Greenhorn Limestone and Alberta Shale (Hattin, 1975b).
Figure 32--Polished surface of well-laminated chalk from lowermost unit of section in Sec. 2, T. 14 S., R. 26 W., Gove County, Kansas. Light-colored specks are mostly fecal pellets. Note alternation of mostly thicker, lighter-colored, fecal-pellet-rich laminae and thinner, darker-colored, fecal-pellet-poor laminae. x 2.
Figure 33--Photomicrograph of laminated chalk from lower part of section in Sec. 2B, T. 15 S., R. 26 W., Gove County, Kansas, showing abundance of fecal pellets (arrows), tests of planktonic foraminifers, pyrite framboids in foram (P), and black organic matter (O). Note well-stratified grain fabric. Crossed Nicols, X 40.
Figure 34--Scanning electron micrograph of fecal pellet from laminated chalk, showing abundance of well-preserved coccoliths. Sample from lower part of exposure in Sec. 16, T. 14 S., R. 25 W., Trego County, Kansas. X 3000.
Figure 35--Scanning electron micrograph of matrix in laminated chalk, showing conspicuous amount of secondary calcite and a coccosphere. Same sample as in Figure 34. X 3000.
Planktonic foraminiferal tests are also ubiquitous in samples of stratified chalk, with usual maximum dimension in the range 0.3 to 0.4 mm. Most commonly, test walls are preserved with the radial structure essentially intact. In many specimens the inner and outer surfaces of the walls bear minute overgrowths of syntaxial calcite. Less common are foraminifers in which the test walls have been partly corroded or even removed entirely by dissolution. Test chambers are filled usually by one to a few blocky crystals of low-magnesium calcite, which does not manifest centripetal growth or enlargement. Rarely, a chamber may be floored with micrite, with overlying space filled by blocky calcite. Outer surfaces of large calcite crystals may bear the imprint of much smaller syntaxial calcite crystals, but more commonly bear casts of wall pores. Foraminiferal test chambers are generally globular but in most thin-sectioned samples some tests have been crushed by compaction (Fig. 36). Such tests are filled with fine, microsparry calcite (Fig. 36) or undistorted blocky calcite. Dark coloration of little-weathered chalk is owing largely to content of organic carbon, which comprises from 0.5 to 5.8 percent of analyzed samples, and to presence of finely crystalline pyrite. The latter occurs within the matrix and within foraminiferal chambers (Fig. 33), mostly as spherical framboids less than 40 µm in diameter (Figs. 33, 36), or less commonly as a skeletal replacement mineral. The organic matter is black, having the appearance of charcoal in reflected light, and occurs as angular, silt-sized grains and as wispy streaks (Fig. 33) or flakes that lie parallel to bedding. Beds richest in organic carbon have a characteristic brownish color. Framboidal pyrite occurs commonly within the wispy concentrations of organic matter. Association of pyrite and organic matter in these rocks indicates that reducing conditions prevailed in the chalk-forming muds. Most thin sections also contain wispy streaks of reddishcolored iron oxide, some of which is concentrated along incipient microstylolites (Fig. 36) or along borders of fecal pellets.
Figure 36--Photomicrograph of laminated chalk, showing crushed foraminifer (F) filled with fine microsparry calcite. Note pyrite framboids (P) in matrix, and concentrations of iron oxide along incipient microstylolites. Lower part of section exposed in Sec. 16, T. 15 S., R. 26 W., Gove County, Kansas. Plane-polarized light, x 80.
All samples of stratified chalk contain fish bones or scales, usually in amounts totaling less than one percent of the rock. In most samples, macroinvertebrate skeletal grains are sparse but include fragments and prisms derived from inoceramid valves and fragments of oyster valves. Although common in certain strata of the Kansas Greenhorn (Hattin, 1975c), calcispheres are rare in chalk samples from the Smoky Hill composite section.
Stratified chalks have strongly preferred orientation, parallel to bedding, of tabular grains such as compressed fecal pellets, fish scales, bivalve fragments, and organic flakes. Owing to compaction, the stratified rock fabric is deformed around larger biogenic grains. Lamination of well-stratified chalks is on a millimeter to centimeter scale, and is largely a result of vertical differences in fecal pellet abundance. In fact, the great abundance of fecal pellets along some bedding planes enhances fissility of these chalks. Thinner, darker-colored laminae (Fig. 32) are generally deficient in fecal pellets. Burrow structures, which are sparse in the well-stratified chalk, truncate laminae locally. Extensive weathering tends to diminish or obscure the visibility of primary laminations, but such rock still splits readily along original bedding planes.
Weathered chalk has lost its original dark coloration, owing to oxidation of organic matter and pyrite. The weathered rock is various shades of pale yellow and orange, the result of iron oxide staining.
In the upper part of the Smoky Hill, especially at Localities 21 and 24, are numerous beds of relatively hard and brittle, spally to shaly weathering chalk that lacks evidence of bioturbation and is not obviously laminated, but which splits readily along original bedding planes. Where partially weathered this rock is harder, may form projecting ledges, and has the general character of chalky limestone. Just as in well-stratified chalk, the nonlaminated chalk is highly speckled by compacted fecal pellets and weathering produces the same coloration. Except for sparse oysters and rare inoceramids this variety of chalk contains few macroinvertebrate fossils. As seen in thin sections the nonlaminated chalk is very similar to laminated chalk, comprising foraminiferal pelmicrite with packstone or, less commonly, wackestone texture, and the principal constituents are the same (Fig. 37). The fecal pellets are greatly compacted, and many have etched borders. Foraminiferal test walls are mostly intact or only partly corroded, and chambers are mostly filled with one to a few blocky crystals of sparry calcite that lacks evidence of centripetal growth or enlargement. All samples contain foraminiferal tests that have been crushed by compaction. Dark coloration of this chalk is owing in part to relatively high content of organic matter, which, like that in stratified chalk, occurs as wisps or flakes and as angular, silt-sized grains. Also contributing to dark coloration is abundant framboidal pyrite, which occurs inside foraminiferal chambers as well as in the matrix. Elongated grains have strongly preferred orientation parallel to bedding planes, and serve to enhance fissility. Scanning electron micrography of pellet and matrix areas reveals the same microtextural differences recorded above for stratified chalk (Figs. 38, 39). The pellets show minimal evidence of aggrading neomorphism or interstitial cementation, whereas both features are better developed in the matrix. These nonbioturbated, nonlaminated chalk beds are related genetically to and are gradational lithologically with the stratified chalk, differing primarily in the lack of well-defined laminae in fresh exposures, paucity of macroinvertebrate body fossils, smaller number of planktonic foraminiferal tests, and smaller maximum size (0.03 mm) of foraminifers. This genetic and gradational relationship is emphasized by scattered occurrence of such chalk beds within the lower 130 m of the composite section, where most chalk beds are more or less laminated, and by scattered occurrence of laminated chalk beds in the upper part of the member.
Figure 37--Photomicrograph of nonlaminated, nonbioturbated chalk from lower part of section exposed in Sec. 21, T. 15 S., R. 32 W., Logan County, Kansas. Note sparry calcite fill of foraminifers, framboidal pyrite (P) inside foraminifer chambers, and fecal pellets (arrows). Plane-polarized light, X 40.
Figure 38--Scanning electron micrograph of fecal pellet from nonlaminated, nonbioturbated chalk in lower part of section exposed in Sec. 21, T. 15 S., R. 32 W., Logan County, Kansas, showing abundance of well-preserved coccoliths and only small amounts of secondary calcite. X 3000.
Figure 39--Scanning electron micrograph of matrix in nonlaminated, nonbioturbated chalk, showing smaller number of well-preserved coccoliths than in pellets (Fig. 38), and relatively large amount of secondary calcite. Same sample as in Figure 38. X 3000.
X-ray diffractometry of laminated and nonlaminated chalk samples indicates that the chalk is composed primarily of low-magnesium calcite. Quartz, in amounts usually less than five percent, was detected in all but one analyzed sample. Dolomite (four samples), gypsum (one sample), and smectite? (one sample) were detected only in trace quantities. Selected samples of both laminated and nonlaminated chalk samples were analyzed by chemical methods (Table 4). The lime is chiefly that of calcite. The silica is partly that of quartz and partly that of other silicate minerals (principally clays). The ferrous iron and most of the sulphur are attributed to pyrite, the ferric iron representing oxidized pyrite. Ubiquitous occurrence of vertebrate remains in the chalk is reflected by phosphate content of the samples. Reported carbon is organic carbon, which occurs in all little-weathered samples, and is indicative of a larger total quantity of organic matter. Among 31 samples selected so as to represent all but the uppermost four meters of the composite section, insoluble residue content ranges from 15.8 to 52.5 percent, averaging 31.4 percent (a= 10.4). Most of the residues consist mainly of quartz, kaolinite, and mixed layer illite-montmorillonite. Accessory minerals include pyrite, feldspar, alunite, and gypsum. Chemical and residue data show that most Smoky Hill strata are impure chalks, and that some beds are best termed chalky marl.
Table 4--Chemical analyses of nonbioturbated chalk samples from Smoky Hill Member of Niobrara Chalk. Samples are arranged stratigraphically, with oldest sample at top of table. Sample KN-23-26 contains the highest percentage of organic carbon and is from a distinctively dark-colored bed, which is a good stratigraphic marker. In sample codes, first number is the locality, as listed in the register of localities. Localities not listed in the register are KN-14 (SW Sec. 21, T. 15 S., R. 26 W., Gove Co., Kansas) and KN-2 (E2 Sec. 27, T. 12 S., R. 35 W., Logan Co., Kansas).
| Sample No. | SiO2 | Al2O3 | Fe2O3 | FeO | TiO2 | CaO | MgO | P2O5 | S | SO3 | CO2 | C | H2O(-) | H2O (110°-200°C) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| KN-1-CCC | 13.2 | 5.55 | 0.46 | 1.14 | 0.18 | 40.4 | 0.70 | 0.083 | 0.15 | 0.64 | 30.5 | 2.1 | 1.32 | 0.62 |
| KN-17-1 | 16.9 | 6.36 | 0.62 | 1.14 | 0.32 | 37.7 | 0.99 | 0.071 | 0.48 | 0.30 | 28.1 | 0.5 | 1.29 | 0.58 |
| KN-17-5 | 22.3 | 8.66 | 0.87 | 1.29 | 0.38 | 32.3 | 1.41 | 0.061 | 0.53 | 1.14 | 27.1 | 2.7 | 1.50 | 0.49 |
| KN-14-7 | 18.1 | 7.23 | 0.70 | 1.14 | 0.39 | 31.2 | 0.96 | 0.088 | 0.55 | 0.38 | 25.2 | 1.6 | 1.38 | 0.50 |
| KN-12-3 | 10.5 | 4.37 | 0.09 | 1.14 | 0.16 | 43.6 | 0.95 | 0.088 | 0.22 | 0.44 | 33.4 | 2.2 | 0.78 | 0.27 |
| KN-12-15 | 18.4 | 6.79 | 0.62 | 1.21 | 0.30 | 35.5 | 1.24 | 0.11 | 0.30 | 0.46 | 26.5 | 3.2 | 1.28 | 0.48 |
| KN-13-3 | 14.6 | 5.84 | 0.02 | 1.46 | 0.22 | 38.6 | 1.31 | 0.10 | 0.63 | 0.86 | 30.2 | 2.0 | 0.49 | 0.26 |
| KN-13-5 | 12.4 | 4.71 | N.D. | 1.37 | 0.17 | 41.4 | 0.77 | 0.097 | 0.59 | 0.67 | 32.5 | 2.0 | 0.71 | 0.20 |
| KN-13-9 | 16.9 | 6.43 | 0.85 | 0.97 | 0.16 | 37.4 | 1.24 | 0.095 | 0.30 | 0.32 | 29.8 | 1.7 | 0.96 | 0.39 |
| KN-13-13 | 22.6 | 8.38 | 1.09 | 1.21 | 0.29 | 32.0 | 1.36 | 0.12 | 0.42 | 0.41 | 25.8 | 2.9 | 1.14 | 0.48 |
| KN-19-3 | 19.8 | 7.62 | 0.13 | 1.78 | 0.22 | 34.1 | 1.11 | 0.11 | 0.60 | 0.30 | 26.3 | 2.9 | 1.10 | 0.34 |
| KN-19-11 | 18.9 | 7.00 | 0.84 | 0.89 | 0.22 | 34.6 | 1.14 | 0.11 | 0.59 | 0.27 | 28.4 | 3.0 | 0.93 | 0.35 |
| KN-19-19 | 24.6 | 9.23 | 0.62 | 1.61 | 0.31 | 29.3 | 1.28 | 0.13 | 0.73 | 0.81 | 23.7 | 3.3 | 1.18 | 0.36 |
| KN-19-23 | 18.2 | 6.63 | 0.63 | 1.05 | 0.20 | 35.8 | 0.88 | 0.11 | 0.42 | 0.45 | 27.5 | 3.7 | 0.91 | 0.33 |
| KN-18-5 | 18.0 | 6.47 | 0.45 | 1.29 | 0.21 | 35.6 | 0.94 | 0.12 | 0.44 | 0.62 | 27.5 | 2.9 | 1.15 | 0.56 |
| KN-18-11 | 15.5 | 5.55 | 0.86 | 1.05 | 0.18 | 38.3 | 0.64 | 0.12 | 0.33 | 0.31 | 29.8 | 2.9 | 0.92 | 0.47 |
| KN-18-27 | 7.02 | 2.50 | 0.30 | 0.65 | 0.050 | 45.9 | 0.25 | 0.068 | 0.26 | 1.19 | 34.8 | 2.4 | 0.92 | 0.43 |
| KN-23-26 | 21.3 | 8.54 | 1.60 | 1.22 | 0.24 | 29.2 | 0.62 | 0.12 | 0.98 | 0.21 | 23.6 | 5.84 | 1.71 | 0.70 |
| KN-2-1 | 9.44 | 3.29 | 1.64 | 0.97 | 0.14 | 39.9 | 0.63 | 0.086 | 0.19 | 5.70 | 29.1 | 3.3 | 2.07 | 1.11 |
| KN-2-11 | 8.58 | 3.43 | 2.66 | 1.46 | 0.11 | 37.9 | 0.47 | 0.082 | 0.66 | 7.02 | 27.2 | 3.9 | 2.13 | 1.48 |
| KN-2-19 | 6.36 | 2.86 | 3.34 | 0.57 | 0.15 | 40.4 | 0.42 | 0.079 | N.D. | 6.78 | 29.0 | 3.3 | 1. 74 | 1.83 |
| KN-2-23 | 8.98 | 4.15 | 2.36 | 1.29 | 0.082 | 38.6 | 0.48 | 0.075 | N.D. | 6.80 | 27.6 | 3.9 | 2.73 | 0.97 |
| KN-21-1 | 5.06 | 2.18 | 3.45 | 0.97 | 0.057 | 42.0 | 0.32 | 0.088 | 1.74 | 6.41 | 30.5 | 3.5 | 2.56 | 0.52 |
| KN-21-37 | 10.7 | 4.65 | 3.37 | 1.29 | 0.13 | 37.5 | 0.45 | 0.10 | 2.64 | 3.31 | 27.6 | 2.9 | 1.77 | 0.59 |
Bioturbated and Granular Chalks
The Smoky Hill composite section contains numerous, irregularly distributed beds of relatively resistant, nonlaminated chalk, which forms light-colored (light olive gray to very light gray) bands on surfaces of little-weathered exposures (Fig. 15). In the lower 95 m of the member such beds contain clear evidence of bioturbation, including discrete, deposit-feeder burrows of the sort referred usually to Planolites Nicholson (Fig. 40) or Chondrites von Sternberg. These bioturbated intervals range in thickness from as little as one or two cm to as much as 30 or 40 cm. The thicker beds weather so as to produce prominent projecting ledges (Fig. 16). The thinner beds form minor shoulders on eroding slopes and produce a surface litter of brittle, light-colored chips. In some burrowed intervals bioturbation was sufficiently extensive to destroy all laminae; partially burrowed beds preserve some or most laminae. Complete gradation exists between nonburrowed and highly bioturbated beds of chalk (Fig. 41). Bioturbated chalks are mainly foraminiferal and pelletal micritic wackestones, but some of the rocks are packstones (Fig. 42). Because of bioturbation, fecal pellets are less abundant than in nonbioturbated chalk, and surviving pellets commonly have indistinct boundaries. Most fecal pellets are in the size range 0.1 to 0.15 mm. Qualitative contrasts with nonbioturbated chalk include presence of less-compacted fecal pellets, generally smaller number of crushed foraminiferal tests, greater abundance of foraminiferal fragments, occurrence (sparse) of foraminiferal tests that have breached walls and a filling of micrite/microsparite, more common alteration of foraminiferal walls, usually smaller amount of optically visible organic matter, and paucity in many samples of wispy iron oxide streaks. Especially apparent in the most highly bioturbated rocks is the lack or near lack of bed-parallel orientation among elongated and tabular grains (Fig. 42). Minute pyrite framboids, or their oxidized counterparts, occur commonly inside foraminiferal chambers, but tend to be less abundant in the adjacent matrix than in nonbioturbated chalk. Bioturbated chalk fecal pellets are composed mostly of coccoliths, but overgrowths of neomorphic calcite are more common than in nonbioturbated chalk pellets (Fig. 43). As compared to non bioturbated chalk, the matrix of bioturbated chalk contains generally greater evidence of interstitial cementation and neomorphism (Fig. 44). Microtextural differences between laminated and bioturbated chalk apparently resulted largely from burrowing activity in the latter. This conclusion is supported by evidence from bioturbated limestones of the underlying Fort Hays Member, which also contains more secondary calcite than the laminated chalk deposits (Fig. 45). In degree of alteration by neomorphic overgrowths on coccoliths and amount of interstitial cement, complete gradation exists between soft, laminated chalks and the hardest, most thoroughly bioturbated chalky limestones. This relationship is true also for the older Greenhorn Limestone of Kansas (Hattin, 1971, 1975c). In massive, chalky limestones of the Fort Hays and in chalky limestones of the Greenhorn, all of which are highly bioturbated, surviving fecal pellets and macro invertebrate body fossils are little flattened by compaction, suggesting that the bioturbated beds became lithified earlier than the nonbioturbated chalks. Greater purity of chalk-forming muds and better circulation of interstitial fluids during burrowing activity played important roles in early lithification of bioturbated beds, as suggested also by Bathurst (1971, p. 402) for the Bioturbation Chalk of Northern Ireland, by Bromley (1975, p. 409) for chalk hardgrounds, and by Milliman (1966, p. 966) for deep-sea pelagic oozes. Bioturbated beds of the Smoky Hill Member are less pure than chalky limestones of the Greenhorn and Fort Hays. Accordingly, the Smoky Hill beds are less well cemented and suffered greater compaction than did chalky limestones of the two older Kansas units.
Figure 40--Sample of bioturbated chalk from lower part of exposure in Sec. 16, T. 15 S., R. 26 W., Gove County, Kansas, showing sediment-filled burrow structures of kind usually attributed to Planolites. X 2.
Figure 41--Polished surface, cut normal to bedding, of partly laminated, partly bioturbated chalk from lower part of exposure in Sec. 29, T. 15 S., R. 26 W., Gove County, Kansas. Note stratification differences between bioturbated and non bioturbated parts of sample. Bar scale = 1 cm.
Figure 42--Photomicrograph of bioturbated chalk from middle part of section exposed in Sec. 2, T. 14 S., R. 26 W., Gove County, Kansas. Note lack of stratified grain fabric, less compacted fecal pellets (arrows) and framboidal pyrite (P) inside foraminifer tests. Plane-polarized light, X 40.
Figure 43--Scanning electron micrograph of fecal pellet in bioturbated chalk sample from lower part of section exposed in Sec. 16, T. 15 S., R. 26 W., Gove County, Kansas. Note neomorphic calcite overgrowths on coccoliths. X 3000.
Figure 44--Scanning electron micrograph of matrix in sample of bioturbated chalk, showing large amounts of secondary calcite cement. Same sample as in Figure 43. X 3000.
Figure 45--Scanning electron micrograph of bioturbated chalk sample from upper part of Fort Hays Limestone Member, Sec. 24, T. 15 S., R. 25 W., Trego County, Kansas. Note extensive development of secondary calcite as overgrowths on coccoliths and as interstitial cement. X 3000.
Marker Unit 10 is the most prominent example of partially bioturbated rock in the composite section. This unit, which is 6.7 m thick at Locality 18, comprises tough, partially stratified chalk with petrologic features intermediate between those of non bioturbated and highly bioturbated chalk (Fig. 17). The rock is a foraminiferal pelmicrite with packstone texture, and contains scattered burrow structures, some of which contain fragmented tests of planktonic foraminifers. This chalk is mostly similar to the laminated and nonlaminated chalks, but parts that have been burrowed are texturally like typical bioturbated chalks.
In the upper half of the composite section most light-colored, tough, relatively resistant beds of chalk have a rather granular appearance (Fig. 14). In thin sections, fecal pellets and silt-sized grains of black organic matter are aligned essentially parallel to bedding planes, but lesser compaction of fecal pellets, smaller number of crushed foraminifers, and paucity of wispy organic matter and iron oxide have produced a less well stratified appearance than is typical for non bioturbated chalk. In natural exposures, burrow structures were recorded only sparingly in the granular chalk beds. Thin sections contain a few ovoid burrow structures, which contain micrite or microspar and are about 0.5 mm in diameter. Thin sections also reveal sparse, scattered, minute ovoid bodies of grayish to brownish color, which may also be filled burrows. Thus, beds of granular, non stratified, or only poorly stratified chalk appear to have resulted from bioturbation, but apparently were burrowed by organisms that were smaller and less abundant than those that burrowed the more obviously bioturbated chalks. These granular beds range in thickness from a few centimeter to as much as 0.76 m, and, because of superior resistance to erosion, form caprocks in chalk badlands. The granular chalks are mainly foraminiferal and pelletal micrites, with wackestone texture (Fig. 46); a few are foraminiferal pelmicrite packstones. Except for the nature of the burrow-like features, the petrographic character of granular chalk differs in no major way from that of the bioturbated chalk.
Figure 46--Photomicrograph of granular chalk from upper part of exposure in Sec. 12, T. 15 S., R. 32 W., Logan County, Kansas. Sample from light-colored bed above dark, organic-rich bed illustrated in Figure 14. Note poor definition of stratification, burrow-like structure (B), and fecal pellets (arrows). Plane-polarized light, X 40.
X-ray diffractograms of bioturbated and granular chalks are nearly identical. Low-magnesium calcite is the predominant mineral component, as in laminated chalk, and trace to moderate amounts of quartz occur in all analyzed samples. Where recorded in this section the quartz occurs as widely scattered grains of angular silt. Traces of kaolinite (one sample) and gypsum (two samples) are the only other minerals detected by X-ray methods.
A few samples of bioturbated and granular chalk were analyzed chemically (Table 5). These rocks have a higher content of CaCO3 than is normal for nonbioturbated chalk, which observation is supported by insoluble residue data. Among 18 samples of bioturbated and granular chalk digested in formic acid, the average residue is 22.5 percent (σ = 8.2).
Table 5--Chemical analyses of bioturbated and granular chalk samples from Smoky Hill Member of Niobrara Chalk. Samples are arranged stratigraphically with oldest sample at top of table. *Sample of chalky limestone that has suffered little compaction but lacks obvious burrow structures.
| SAMPLE No. | SiO2 | Al2O3 | Fe2O3 | FeO | TiO2 | CaO | MgO | P2O5 | S | SO3 | CO2 | C | H2O(-) | H2O (110°-200°C) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| KN-18-13 | 7.06 | 2.54 | 0.41 | 0.65 | 0.022 | 45.3 | 0.33 | 0.065 | 0.096 | 2.81 | 34.2 | 2.3 | 1.32 | 0.57 |
| KN-18-19 | 7.42 | 2.47 | 0.47 | 0.49 | 0.075 | 45.0 | 0.44 | 0.062 | 0.24 | I. 73 | 34.3 | 2.8 | 1.14 | 0.54 |
| KN-21-15* | 1.24 | 0.69 | 0.27 | 0.81 | 0.017 | 52.0 | 0.22 | 0.044 | 0.51 | 1.27 | 40.6 | 1.7 | 0.53 | 0.08 |
| KN-21-27 | 1.96 | 0.98 | 0.92 | 0.59 | N.D. | 49.4 | 0.24 | 0.049 | N.D. | 3.56 | 37.1 | 1.4 | 1.45 | 0.41 |
Skeletal Limestone
In the Smoky Hill composite section, I recorded no well-washed biosparite or biosparrudite such as characterizes parts of the older Greenhorn and Fairport (Fig. 6) chalky units. However, small, very thin lenses of biomicrite and biomicrudite composed mostly of fragmented inoceramid valves are scattered sparingly through the section (Fig. 47). Isolated occurrences and presence of a micritic matrix suggest that such lenses formed by in situ disintegration of bivalves under quiet-water bottom conditions, which characterized nearly all of Smoky Hill deposition.
Figure 47--Photomicrograph of biomicrudite from lower part of section exposed in Sec. 1, T. 14 S., R. 26 W., Gove County, Kansas. Interstitial micrite (dark) between prisms and fragments of inoceramids suggests essentially in situ accumulation of disintegrated bivalve debris. Plane-polarized light, X 40.
On the south side of Plum Creek, Gove County, in SE Sec. 1, T. 14 S., R. 31 W., the chalk lying approximately midway between Marker Units 21 and 22 contains a single lens of fine-grained, pale yellowish brown skeletal limestone. When freshly broken, the rock emits a strong hydrocarbon odor. The lens is approximately 3.2 m (10.5 ft) wide and 2 to 3 cm (0.07-0.1 ft) thick, with protuberances on the lower surface that expand the thickness to 5.5 cm (0.18 ft) locally. This lens has been thoroughly bioturbated, as evident in plentiful casts of simple, horizontal burrows, and lacks internal stratification. Principal grain types, listed in order of decreasing abundance, are Inoceramus debris, tests of planktonic foraminifers, fish bones and scales, fragments of small oysters, and angular quartz sand (sparse). The rock is grain supported and bound mostly by micrite, and is therefore a biomicritic packstone. Inoceramid remains consist mostly of isolated calcitic prisms and scattered small fragments of the prismatic shell layer. Foram tests are intact or broken, and the walls are either well preserved or somewhat altered. Foram chambers are filled by one to a few blocky crystals of sparry calcite, which shows no evidence of centripetal growth or crystal enlargement. Sparry calcite occurs also as minute overgrowths on some foraminifer tests, as the filling in void spaces within bone fragments, and as local interstitial fill between allochems. Minute pyrite framboids, or the oxidized equivalent thereof, are common within foram chambers and occur sparingly within the micritic matrix.
In or adjacent to the zone of Clioscaphites choteauensis are very sparse, thin to very thin lenses (maximum ca. 5 cm thick) of hard, brittle limestone composed chiefly of remains of the free-swimming crinoid Uintacrinus socialis. Both articulated and disarticulated crinoidal material has been distorted by compaction. Abundant microstylolites, which occur along grain boundaries, are evidence of extensive pressure solution at grain-to-grain contacts (Fig. 48). Crinoid skeletal parts have been converted almost entirely to coarse microspar (Fig. 49), and the original high-magnesium calcite has been altered to low-magnesium calcite, which is the only mineral I detected by X-ray methods. Alteration of the skeletal elements to microsparite has been attributed by Neugebauer (1978a) to the effects of partial dissolution, and is an example of crystal diminution.
Figure 48--Photomicrograph of Uintacrinus-rich limestone from middle part of Smoky Hill Member in Sec. 27, T. 14 S., R. 32 W., Logan County, Kansas. Note fusiform shape of compactionally distorted crinoidal elements (arrows), and microstylolites (dark, crinkly lines) along grain-to-grain contacts. Plane-polarized light, X 40.
Figure 49--Scanning electron micrograph of crinoid skeletal element, showing recrystallization of original unit crystal to microsparite. Specimen from Sec. 27, T. 14 S., R. 32 W., Logan County, Kansas. X 3000.
Diagenesis
Although the Smoky Hill chalk is relatively soft and friable, especially in weathered exposures, all samples contain evidence of post-depositional alteration. The most obvious diagenetic feature is compaction, which is evident in the shape of crinoidal elements (Fig. 48); flattened valves of inoceramids, bakevelliids, and some oysters; deformation of inoceramid valves around their epizoic oysters; and flattened molds of cephalopods (Baculites, Clioscaphites, collignoniceratids) and bivalves (Lucina). Despite theoretically great resistance of spherically shaped structures to compaction, in many specimens of planktonic foraminifers the globular chambers have been crushed by compaction (Fig. 36). Compaction is also manifest where rock fabric arches over or bends downward beneath large allochems, such as oysters or inoceramid fragments (Fig. 50). Fecal pellets also have been compacted, most especially in the nonbioturbated chalk, and present conspicuously fusiform or elongate-elliptical outlines as viewed in hand specimens and thin sections (e.g., Figs. 33, 37). In all nonbioturbated rocks the preferred orientation of tabular grains is owing in part to the grain attitude at the time of deposition, in part to rotation of grains in response to compactional stresses, and in part to compactional flattening of the grains. Although compaction in carbonate grainstones is not common, and the lack of compaction in some Cretaceous chalks has been noted (e.g., Mapstone, 1975, p. 609; Kennedy and Garrison, 1975, p. 342; Mimran, 1978), compaction in chalk is actually a widespread phenomenon (e.g., Hattin, 1962, 1975c; Schlanger and Douglas, 1974; Kennedy and Garrison, 1975; Garrison and Kennedy, 1977). Scholle and Cloud (1977, p. 257) note that all chalk is subject to mechanical and chemical compaction, the principal mechanisms being the stress of overburden, tectonics, and pore-fluid pressure. Only the first of these was significant in compaction of the Smoky Hill carbonate muds because the area is tectonically stable and burial depths never reached sufficient depths for pressure solution to have become pervasive.
Figure 50--Photomicrograph of partially bioturbated chalk from Marker Unit 10 in Sec. 1, T. 14 S., R. 26 W., Gove County, Kansas, showing deformation of grain fabric around an oyster valve. Plane-polarized light, X 40.
In intervals of bioturbated chalk the p{imary fabric was homogenized by burrowing organisms, most of which were apparently worms. Their sediment-ingesting activities also caused fragmentation of foraminiferal tests, clustered fragments of which are preserved locally within burrow fill.
In general, Smoky Hill chalk beds are not well cemented, which explains the friability of even the little-weathered chalk. Nonetheless, most samples examined by scanning electron microscopy contain evidence of calcitic overgrowths on coccoliths and of interstitial cement consisting of minute calcite rhombs within the chalk matrix (Figs. 35, 39, 44). This calcite reflects inception of the lithification process, which, if sufficiently extended, would have resulted in gradual obliteration of primary microtextural features. In the older Greenhorn Limestone, highly bioturbated beds of chalky limestone did undergo more extensive neomorphism and cementation, and now consist primarily of microsparry calcite (Hattin, 1971, 1975c). With few exceptions the matrix of Smoky Hill chalk samples contains greater abundance of secondary calcite than do the fecal pellets, which implies that the pellets were protected somehow from diagenetic alteration, perhaps because of an original mucilaginous coating. Neugebauer (1974) suggests that during chalk lithification cement precipitation on coccoliths is retarded because of their small size or geometric shape, but notes (Neugebauer, 1975a) the common occurrence of overgrowths on coccoliths of the Kansas chalk. Adelseck and others (1973) and Roth and Berger (1975) determined that during diagenesis the smallest < lµm) coccoliths may be dissolved selectively, whereas secondary calcite overgrowths commonly develop on the larger ones. In addition to overgrowths on coccoliths, syntaxial overgrowths have been recorded also in foraminifera (Neugebauer, 1975b; this paper), crinoidal limestone (Neugebauer, 1978a), and inoceramid prisms (Neugebauer, 1978b) of the Smoky Hill chalk.
In Smoky Hill chalks, just as in the older Greenhorn and Fairport chalks (Hattin, 1962, 1975c), much secondary calcite was precipitated within planktonic foraminiferal chambers, usually as only one to a few large crystals (Fig. 51). This calcite may fill most or nearly all of the original void space of the chamber or, rarely, the space above a floor of geopetal micrite. None of this chamber-filling blocky calcite contains evidence of centripetal growth or crystal-size increase. Similar sparry calcite has been reported in chambers of modern foraminifers from deep-sea chalk (Schlanger and others, 1976, p. 167). Some Smoky Hill thin sections contain a few foraminifers whose chambers are filled with micrite and/or microsparite that represents infiltration of carbonate ooze through fractured or breached test walls. Severely crushed foraminiferal tests are usually filled with microsparite (Fig. 36), but a few are filled with blocky calcite. Presence of large sparry crystals in most Smoky Hill foraminifers reflects slow growth, and is believed to have occurred during burial diagenesis (see Folk, 1974, p. 47). If the blocky calcite had crystallized during early diagenesis, as the minute syntaxial overgrowths apparently did, fewer foraminifera would have been crushed by compaction. Among partially crushed foraminifers are many in which remaining void space is filled with blocky crystals of sparry calcite, which shows no evidence of strain and was therefore emplaced after compaction of the tests.
Figure 51--Scanning electron micrograph of planktonic foraminifer test, showing large crystals of low-magnesium calcite that nearly fill the chamber. Chambers containing only one crystal (very common) are usually filled completely. Sample from lower part of section exposed in Sec. 16, T. 14 S., R. 25 W., Trego County, Kansas. X 600.
One possible source of early diagenetic calcite is the metastable aragonite of bivalves and cephalopods. Several authors (e.g., Bathurst, 1971, p. 442; Neugebauer, 1973, p. 224; Kennedy and Garrison, 1975, p. 370; Scholle, 1977, p. 991) have mentioned this possibility, but have generally considered such a source as minor on the grounds that aragonitic skeletal remains were rare in chalk-forming muds. In addition, Kennedy and Garrison (1975, p. 370) and Hancock (1975, p. 512) state that cementation of the hardest English chalks preceded skeletal aragonite dissolution because molds of aragonitic skeletons are preserved therein. Contrary to such arguments, the Smoky Hill chalks almost nowhere preserve skeletal aragonite, and dissolution of such remains occurred while the muds were still relatively soft because, without exception, potential void spaces left by removal of aragonite have been closed by compaction. Finally, the abundance and distribution of inoceramids alone imply that substantial amounts of secondary calcite could have been derived from aragonitic skeletal sources. One must remember further that Smoky Hill chalks are not well cemented, and only minor volumes of CaCO3 were required to effect the observed degrees of neomorphism and cementation. As Wise (1977, p. 718) has noted, wherever aragonitic skeletons do occur in chalk-forming muds' 'they should be considered as important potential donors of carbonate for cementation."
An additional source of secondary calcite in Smoky Hill chalks is that derived from pressure solution (= solution transfer) during burial diagenesis-a mechanism of chalk cementation favored by many workers (e.g., Neugebauer, 1974; Mapstone, 1975, p. 611; Hancock, 1975, p. 524; Scholle, 1977; Garrison and Kennedy, 1977, p. 130). Thin sections of many Smoky Hill chalk samples contain numerous incipient microstylolites, which occur especially along margins of fecal pellets but also occur sparingly along grain-to-grain contacts. Correlation between abundance of well-developed microstylolites and degree of cementation is illustrated by the Uintacrinus limestone (Fig. 48). However, well-developed microstylolites are not common in the Smoky Hill, nor are incipient microstylolites usually developed on a large scale. The Smoky Hill lacks such features as the clay pellicles reported by Haakansson and others (1974, p. 224) or the flaser chalks described by Garrison and Kennedy (1977). Additionally, the maximum overburden in Kansas seemingly did not reach the 1000-m thickness regarded by Neugebauer (1974, p. 156) as necessary to the generation of large amounts of cement through pressure solution. Collectively, the evidence suggests that pressure solution does not account for a large volume of secondary calcite in Smoky Hill chalks.
Adelseck and others (1973, p. 2760) and Roth and Berger (1975, p. 92) have suggested that the smallest coccoliths tend to be dissolved preferentially, and supply the calcium carbonate necessary for overgrowths on large coccoliths. Most Smoky Hill coccoliths show little evidence of etching, thus contrasting markedly with etched coccoliths of the North Pacific oozes reported by Matter and others (1975). Etching of coccoliths appears to have been an insignificant source of secondary calcite in Smoky Hill chalks.
Tests of Smoky Hill planktonic foraminifers exhibit a wide range of diagenetic effects. Some preserve details of original microstructure, whereas others have been recrystallized. Many have minute secondary overgrowths of syntaxial calcite, whereas others have been corroded, even to the point where test walls have been largely destroyed. The timing of processes that altered these foraminifers is uncertain but it is apparent that some were donors of calcite for secondary mineralization, and some were the objects of secondary calcite overgrowth. This problem is worthy of further study. Neugebauer (1975b) has described in great detail the processes of test alteration in Niobrara foraminifers and concludes that the extensive overgrowths on wall crystals resulted from the large surface areas presented for diagenetic reactions. Neugebauer (1978a,b) has shown also that the calcite of echinoderms and the prismatic layer of inoceramids from the Niobrara chalk show effects of partial dissolution. However, the small quantities of CaCO3 from these sources seems to have been consumed in cementation of the Uintacrinus limestone and development of calcite overgrowths on Inoceramus prisms rather than in cementation of adjacent chalk. These skeletal sources therefore were apparently of small significance as compared to aragonitic sources of CaCO3.
That bioturbation had an influence on the degree of chalk cementation is now well documented (Hattin, 1971, 1975c, 1981). Kansas chalks that are most highly bioturbated are those that have undergone the greatest amount of diagenetic alteration and contain evidence--fossils preserved in-the-round, noncompressed burrows and fecal pellets--for precompactional lithification. Except for a few beds within the Fort Hays Member, none of these better-lithified chalks contains flasers (see Garrison and Kennedy, 1977), microstylolites, or other compelling evidence of pressure solution. Carbonate cement for these early-lithified units is judged to have come partly from aragonitic skeletal remains, but most is the result of improved circulation of CaCO3-saturated water during bioturbation.
In summary, several possible mechanisms may account for observed calcite overgrowths, interstitial cement, and void-filling calcite in Smoky Hill chalks. In answer to the question, "Are these sources sufficient to furnish the needed amounts of secondary calcite," I remind the reader that these rocks are poorly cemented, and large volumes of secondary calcite were not needed to produce the observed effects.
Diagenetic framboids of pyrite, ranging from 5 to 40 µ.m, are nearly ubiquitous in Smoky Hill chalks, and are most abundant in non bioturbated strata. Presence of conspicuous quantities of pyrite is clear evidence of interstitial reducing conditions, which are in harmony with occurrences in the chalks of appreciable amounts of organic matter. Smaller quantities of both pyrite and organic matter in the more extensively bioturbated beds suggest that burrowing activity circulated oxygenated water through the sediment and thereby retarded development of a reducing microenvironment within the carbonate mud. Despite effects of bioturbation, reducing microenvironments were maintained within foraminiferal chambers, where framboids are approximately as common as in nonbioturbated chalks, as well as locally within the adjacent matrix. Organic carbon content of nonbioturbated Smoky Hill chalks ranges from 0.5 to 5.8 percent, averaging 2.8 percent for 25 samples (Table 4), whereas four samples of bioturbated and granular chalk have organic carbon content ranging from 1.4 to 2.8 percent, averaging 2.0 percent. The difference between the two groups of rock samples is smaller than that reported by Hattin (1971) for the bioturbated and nonbioturbated Greenhorn chalks and is explained by the smaller amount of bioturbation in most burrowed beds of the Smoky Hill.
Smoky Hill organic matter occurs as black, silt-sized grains and as wispy streaks or flakes in which small amounts of pyrite are common. The reddish-colored streaks' of iron oxide that occur most commonly in non bioturbated chalks have similar size and shape as the organic wisps, and may be their oxidized equivalent. Samples of yellow- or orange-colored chalk contain only the reddish-colored wisps, the pyrite and organic matter having been oxidized during weathering.
A postdepositional feature recorded primarily in the upper half of the composite section consists of closely spaced, wispy crinkles (Fig. 52), which occur in mostly thin zones within beds of dark-colored, nonlaminated chalk. The crinkles are irregular fractures, lying roughly parallel to stratification, and are filled with sparry selenite. Although these crinkles have the appearance of chalk flasers (Garrison and Kennedy, 1977), concentrations of clay and other insoluble minerals are lacking, so the features are not the result of pressure solution. Presence of gypsum suggests that these crinkles are an epigenetic phenomenon that developed by near-surface evaporation and precipitation from sulphate-rich waters during weathering of the section. I have recorded selenite in chambers of planktonic foraminifers occurring in weathered chalk samples, so the presence of gypsum in the crinkly fractures does appear related to weathering. The weathering hypothesis requires testing by comparison of the surface phenomena with unweathered chalk from equivalent beds in the nearby subsurface, where the crinkles presumably would be absent.
Figure 52--Polished surface of chalk, cut normal to bedding, showing gypsum-filled wispy crinkles. These structures lack insoluble dctr irus and iron oxide and were not produced by pressure solution during burial diagenesis. Sample from middle part of exposure in Sec. 20, T. 15 S., R. 32 W., Logan County, Kansas. Bar scale = 1 cm.
The Smoky Hill chalk manifests a wide variety of diagenetic features, none of which was sufficient to produce hard, well-cemented chalk. Hardgrounds, such as those recorded commonly in European chalk deposits, are lacking.
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Kansas Geological Survey, Geology
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