|Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 205-217|
Indiana University, Bloomington, Indiana
Colorado Group strata in Kansas record deposition during the first and part of the second Late Cretaceous marine sedimentational cycles. The lower, more complete cyclothem, is asymmetrical stratigraphically but nearly symmetrical lithologically and comprises seven phases including, in ascending order: (1) ferruginous sandstone, carbonaceous or sandy shale, and dark-gray shale, represented by the upper part of the Dakota Formation; (2) medium dark-gray noncalcareous silty or sandy shale with numerous sandstone beds represented by the lower part of the Graneros Shale; (3) medium dark-gray silty shale with calcareous sandstone, inoceramite beds and local septarian concretions, represented by the upper part of the Graneros Shale; (4) olive-black to olive-gray shaly chalk and olive-gray limestone that contain relatively little terrigenous detritus, represented by the Greenhorn Limestone and Fairport Chalk Member of the Carlile Shale; (5) gray noncalcareous silty shale with septarian concretions, similar to phase 3, represented by most of the Blue Hill Shale Member of the Carlile; (6) gray, locally concretionary, noncalcareous silty or sandy shale with thin sandstone beds, similar to phase 2, represented by the upper part of the Blue Hill; (7) siltstone and sandstone, commonly clayey, represented by the Codell Sandstone Member of the Carlile. Macroinvertebrates are abundant only in phases 2 through 5. Foraminifera are common in all phases of the cyclothem.
Stratigraphy, lithology, sedimentary structure, and fossils indicate successive change from marginal and nearshore, shallow, brackish-water deposition in phases 1 and 2 to far offshore deposition in deeper water of normal salinity in phase 4, and return to nearer shore, shallower water deposition through phases 5 to 7.
The contact between phases 3 and 4 is a regional diastem of increasing magnitude southwestward along the outcrop. The break resulted from nondeposition and sublevation rather than subaerial erosion. The contact between phase 7 and the Niobrara Chalk is a regional disconformity for which there is likewise no evidence of subaerial erosion.
Following deposition of the Codell Sandstone (phase 7), renewed transgression occurred, but deposition did not begin until the sea floor sank sufficiently to permit further accumulation of sediments. The transgressive phases of the second cyclothem are not recorded, and the first rocks above the Codell are offshore phase-4 Niobrara carbonates. Regressive terrigenous detritus of phase 5 is represented in gray concretionary shale of the overlying Pierre Shale of the Montana Group.
Stratigraphic literature bulges with articles devoted to description and interpretation of repetitive bundles of Carboniferous and Permian strata known as cyclothems. Among the relatively few papers on Mesozoic cyclicity that of Allen (1959) stands forth as an outstanding example of detailed elucidation of cyclothems in Cretaceous deposits. In America the work of Spieker (1949) and Young (1957) on cyclicity of the Cretaceous rocks of Utah has become widely known, and the regional cyclicity exhibited by Upper Cretaceous rocks throughout the Rocky Mountain region has been described recently by Weimer (1960). At the outset of stratigraphic and paleontologic studies of Upper Cretaceous strata in west-central Kansas six years ago, the writer was impressed immediately by the broadly cyclical pattern exhibited by rocks in the lower part of the Colorado Group. During subsequent field work particular attention has been focused on aspects of lithology, fauna, and paleoecology having bearing upon better understanding of this outstanding example of cyclothemic sedimentation. By stratigraphic, lithologic, and paleontologic analogy parts of another, similar cyclothem can be recognized in the upper part of the Colorado Group and lower part of the Montana Group.
The lower part of the Colorado Group in Kansas comprises the Graneros Shale, Greenhorn Limestone, and Carlile Shale which, together with the uppermost part of the Dakota Formation, furnish record of marine deposition during the first Late Cretaceous cycle of sedimentation in the eastern part of the Western Interior Sea. The succession is part of an asymmetrical cycle of sedimentary rocks, termed Greenhorn cyclothem by Hattin (1962, p. 124), consisting of seven marine depositional phases as numbered in Figure 1. Except for rather abrupt lateral lithologic variations in the upper part of the Dakota, the overall succession exhibits remarkable uniformity throughout the west-central Kansas outcrop.
Figure 1--Graphic columnar section of Greenhorn cyclothem, Saline River Valley area (inset arrow), Kansas, showing prominent marker beds. Map shows outcrop of lower part of Colorado Group. A larger verison of this figure is available.
The first marine phase of the Greenhorn cyclothem includes upper Dakota marginal-marine strata that were deposited in a deltaic complex along the edge of the Western Interior Sea. Beds representative of both subaerial and submarine parts of the complex can be recognized. The dominant rocks in this part of the section are (1) more or less evenly bedded and generally highly ferruginous sandstone, (2) carbonaceous or sandy shale, and (3) dark-gray fissile shale. Clay-ironstone concretions are commonly associated with these beds, and lignite was observed within 5 feet of the top of the Dakota at one locality. Arenaceous foraminifers are common in some of the sandstone and shale beds, and marine mollusks are very abundant locally in sandstone beds.
The Dakota-Graneros contact is commonly transitional and differs greatly in aspect within short distances along the outcrop. The non-uniform position of the contact, as determined by reference to marker beds, reflects intertonguing of adjacent parts of the two units. In some localities the uppermost Dakota consists of thick-bedded sandstone that lacks conspicuous cross-bedding, in others consists of thin layers of evenly bedded sandstone that succeed dark-gray fissile shale or carbonaceous or sandy shale and local lignite and in still other localities is largely made up of dark-gray shale similar to that of the overlying Graneros Shale. Elsewhere the Graneros succeeds sandstone in the upper part of the Dakota through several feet of interbedded sandstone, siltstone, and silty shale (Fig. 2A). This stratal complex is interpreted as the record of deposition on the subaqueous topset plain of a delta. Beneath these beds are extensively cross-bedded, massive, ferruginous sandstone bodies, varicolored silt and clay units, and other sedimentary rocks that bear the stamp of fluvial and lacustrine deposition on the nonmarine part of the delta. During initial phases of marine deposition in the west-central Kansas outcrop area, the formerly nonmarine areas of deposition were blanketed by brackish-water deposits of shore lagoons, interdistributary bays, delta front platform, and salt marsh, represented today, respectively, by carbonaceous or silty shale, dark-gray fissile shale, evenly bedded sandstone, and lignite. Implication of upward succession through fluviatile, paludal, and lagoonal deposits to marine deposits cannot be mistaken, but unequivocal features of a beach environment are lacking. Beach sediments were probably reworked during transgression and are now represented by the more or less evenly bedded sandstones that generally lie at or near the top of the Dakota Formation. Such sands are commonly gently cross laminated, locally ripple marked, and contain worm burrows and castings. Invertebrate fossils in phase 1 include Lingula, here and there in a nearly upright position, the arenaceous foraminifers Trochammina, Ammobaculites, and Reophax, and, locally, large numbers of thick-shelled marine mollusks including gastropods, mussels, and clams. The mollusks consist of well preserved molds and the clam valves are commonly paired suggesting little transport before burial. Paucity and nature of marine fossils, size and abundance of plant debris, dominantly sandy texture, and sedimentary structures in phase-1 strata suggest that deposition of marine sediments in the upper part of the Dakota occurred at or directly adj acent to sea level in water of low salinity. The boundary between phases 1 and 2 is coincident with the top of the Dakota Formation which is marked by one or more of the following (1) highly limonitic sandstone, commonly. containing spherulitic molds, (2) layers of concretionary clay ironstone, and (3) chunks of carbonized plant debris.
Figure 2--Uppermost Dakota and lower Graneros strata. A, Transitional strata at top of Dakota, phase 1, sec. 25, T. 22 5., R. 22 W., Hodgeman Co.; B, Lower part of Graneros Shale, phase 2, sec. 35, T. 12 5., R. 14 W., Russell Co.; C, Fish-bone conglomerate, phase 2, sec. 3,T. 13 5., R. 11 W., Russell Co., X 2. A larger verison of this figure is available.
Phase 2, represented by the lower half of the Graneros Shale, consists chiefly of dark-gray silty shale that is irregularly interlaminated with silt and fine sand and contains a few to several layers of thin-bedded, well-sorted, mostly noncalcareous sandstone (Fig. 2B). The sandstones are commonly cross laminated, sparsely ripple marked and dominantly composed of fine to very fine quartz grains. Calcareous sandstone occurs locally in this phase and clayey calcareous septarian concretions have been observed at one locality. Carbonized plant debris is common throughout phase 2; the size and abundance of such particles varying according to the grain size of the enclosing unit. Conglomerates of fish and mollusk remains and a mollusk-shell coquina occur locally (Fig. 2C).
Marine fossils are common in phase 2, including thick-shelled mollusks such as Callistina and Parmicorbula as well as Exogyra, the brachiopods Lingula and Discinisca, and arenaceous foraminifers, particularly Ammobaculites, Trochammina, Reophax, and Verneuilinoides. Burrows are common in phase-2 sandstones but are usually isolated rather than interpenetrating hence suggesting relatively rapid sedimentation.
Bandy and Arnal (1960, p. 1924) have indicated that foraminiferal populations rich in arenaceous species, and including the genus Ammobaculites, are indicative of brackish waters in modern marginal environments. In addition, modern Lingula commonly thrives near mouths of rivers or in other environments of low salinity (Hatai, 1940). The absence of ammonites and rarity of planktonic foraminifers also suggests brackish water. I believe that the environment of lower Graneros (phase 2) deposition was brackish probably owing to river discharge. The abundance of sand and carbonaceous plant debris suggests genetic relationship to the deltaic complex mentioned above. Sedimentary structures suggest shallow, agitated water and occasional storm waves that stirred bottom sediments deeply to produce lenses of lag conglomerate and coquina. The abundance of fossils is evidence that the nearshore phase-2 environment was well oxygenated; therefore, rapid sedimentation rather than inhospitable environment was responsible for preservation of laminated sediments.
Phase 3 comprises dark-gray silty shale that commonly includes thin calcareous sandstone beds in the lower part and Inoceramus-prism and shell-fragment biosparite ("inoceramite") beds in the upper part. The sandstone units are mostly thinner and less numerous than in phase 2 and are commonly thinly laminated and gently cross bedded to cross laminated (Fig. 3A). The calcareous sandstone beds are oscillation ripple marked locally with crests as little as 0.2 foot apart (Fig. 3B). Thin to very thin beds or zones of lenses of sandy to clayey siltstone are also common locally in the lower part of phase 3. Such beds are usually cross laminated and ripple marked and the structure of the lenses suggests that they are starved current ripples. Lenses of conglomerate, including shell fragments and fish remains, and similar to conglomerates of phase 2, are present locally in the lower part of phase 3. Shale of this phase is weakly calcareous in one fresh, very deep cut and is locally calcareous at the top elsewhere. Particles of carbonaceous plant material are smaller and less abundant than in phase 2. Septarian concretions are common locally along the Smoky Hill River Valley (Fig. 3C).
Figure 3--Structures in upper part of Graneros Shale. A, Thinly cross-bedded calcareous sandstone, phase 3, sec. 19, T. 15 5., R. 15 W., Russell Co.; B, Ripple-marked calcareous sandstone, phase 3, sec. 20, T. 15 5., R. 15 W., Russell Co.; C, Calcareous septarian concretion, phase 3, sec. 29, T. 15 5., R. 10 W., Ellsworth Co. A larger verison of this figure is available.
Comparison of ripple-mark characteristics with data published by Inman (1957) leads me to the conclusion that the water was, at times, only a few feet in depth (30 feet or less) during deposition of the lower part of phase 3. The paucity of ripple marks in phase 2 is believed to be a consequence of even shallower water where sediment reworking by waves and currents tended to obliterate these structures before burial occurred. Structures such as cross beds, cross laminations, and ripple marks are best developed near the middle of the Graneros Shale and are sparse near the top. The latter condition seemingly reflects increased depth of deposition for the upper part of phase 3. The lower part of phase 3, like phase 2, was probably deposited close to effective wave base, at least in the central part of the outcrop. This conclusion is based on layers of lag conglomerate which are scattered through these parts of the section. The storm-generated conglomerates formed at times when waves reached deeper than usual and concentrated coarser fossil debris that was scattered through the bottom muds. Absence of such coarse lag conglomerates in the upper part of phase 3 is taken as further evidence of greater depth of deposition for those beds. The general upward decrease in quantity and grain size of terrigenous detritus through the Graneros reflects increasing distance from shore of present outcrop area during deposition of phases 2 and 3.
Fossils in phase 3 include Ostrea beloiti, Inoceramus rutherfordi, Plesiacanthoceras amphibolum, and Borissiakoceras reesidei. In the middle part of the Graneros Shale in the approximate position of transition from phase 2 to phase 3, Reophax is the dominant arenaceous foraminifer and a few planktonic forms are present. In phase 3 the foraminiferal assemblage is dominated by Reophax and the probably planktonic forms Heterohelix and Hedbergella, the last being dominant locally near the top of the formation.
The upward increase in numbers of planktonic foraminifers and appearance of ammonites suggests water of near-normal salinity for upper beds of the Graneros (phase 3). The greater abundance of thin-shelled Inoceramus in this phase may reflect preference for water of higher salinity but could be related to greater water depth and decreased turbulence. Trask (1937) showed long ago the relationship between increase of salinity and increase in calcium carbonate content of sediments. The general increase in calcium carbonate content of the Graneros Shale in phase 3, i.e., calcareous shale, calcareous sandstone, inoceramite, is related to gradual increase in salinity during deposition of this phase and seemingly supports the fossil evidence for high salinity.
Phase 4 includes chiefly nonterrigenous calcareous strata of the Greenhorn Limestone and Fairport Chalk Member of the Carlile Shale. The major rock variety, shaly chalk, is distributed throughout the section, but this phase can be subdivided into three parts based upon the kinds and proportions of other calcareous rocks. (1) Olive-black, laminated, speckled shaly chalk with thin layers of olive-gray burrowed chalk, light-colored chalky limestone, very thin lenses of foraminiferal limestone, and, especially near the base, beds, lenses, and zones of lenses of inoceramite, represented by the Lincoln and Hartland Members of the Greenhorn (Fig. 4A). Limestone beds at the base of this part of phase 4 are generally cross-laminated or very thinly cross-bedded inoceramite with local conglomerate of fish remains, shell fragments, bentonite pebbles and coprolites. Macroinvertebrates in the lower part of phase 4 include Inoceramus pictus, ranging nearly throughout; a large species of Dunveganoceras, a small Exogyra, Ostrea beloiti, and Exogyra columbella in the basal inoceramites; a few impressions of small ammonites (Borrissiakoceras?) in the Lincoln Member; and Cerithiopsis n. sp., ?Sciponoceras gracile, and sparse impressions of coiled ammonites in the Hartland Member. Calcareous planktonic foraminifers are abundant throughout. (2) Olive-black, laminated, speckled shaly chalk interbedded with beds of olive-gray (usually weathered very pale orange) chalky limestone in the lower part (Jetmore Member) and nodular and concretionary layers and a few beds of olive-gray chalky limestone in the upper part (Pfeifer Member of Greenhorn and lower part of Fairport Member of Carlile (Fig. 4B). Shell fragments are scattered throughout the section. This part of phase 4 is characterized by Inoceramus labiatus, which ranges throughout; Watinoceras and other coiled ammonites in the lower part; abundant baculitid impressions and Ostrea n. sp. in the middle and upper portions; and Inoceramus cuvieri and Collignoniceras woollgari in the upper part. Inoceramus shells are profuse in shaly chalk and some limestone beds near the top of the Jetmore Member. Weathered surfaces of Inoceramus shells and shell-fragment lenses are dotted with specimens of calcareous foraminifers. Many specimens of Inoceramus are preserved with valves articulated but open. Worm? burrows are common to abundant in several limestone beds in the lower half of the Jetmore Member; some are broad and flattened, some small, cylindrical, and calcite-filled. (3) Olive-gray laminated speckled shaly chalk with beds of olive-gray chalky limestone in the lower few feet and scattered beds of olive-gray marly chalk in the upper part. The middle and upper parts contain scattered very thin beds alid lenses of inoceramite. The uppermost beds are less calcareous and more clayey than those below. This part of phase 4 is represented by all but the lowest few feet of the Fairport Member of the Carlile Shale (Fig. 4C).
Figure 4--Greenhorn and Fairport strata. A, Shaly chalk with thin lensing layers of inoceramite, top of Lincoln Member and basal Hartland Member, phase 4, sec. 18, T. 13 S., R. 12 W., Russell Co.; B, Chalky limestone beds and nodule in Jetmore Member. phase 4, sec. 7, T. 8 So, Ro 3 W., Cloud Co.; C, Shaly chalk with beds of marly chalk, Fairport Member of Carlile Shale, phase 4, sec. 21, T. 11 So, R. 17 W., Ellis Co. Contact with phase 5 is at base of rod. A larger verison of this figure is available.
The contact between phases 4 and 5 is sharp locally but is normally transitional upward through several inches to several feet of decreasingly calcareous shaly chalk and calcareous clay shale. Fossils in this portion of phase 4 include Inoceramus cuvieri, Inoceramus latus, and Ostrea n. sp. In the middle part Inoceramus cuvieri is very large and the valves are commonly covered with serpulids, Ostrea, barnacles, and bryozoans. Paired valves of Inoceramus are common throughout. Except for the bryozoans and serpulids this part of phase 4 is very similar to that represented by the Lincoln and Hartland Members of the Greenhorn Limestone. Basal beds of phase 4 are nearly everywhere in sharp lithologic contrast to strata of phase 3 and the two are separated nearly everywhere by a widespread diastem that increases in magnitude from northeast to southwest along the outcrop (Fig. 5A). Basal Greenhorn crossbedded inoceramite (Fig. 5B) and local conglomerate represents deposition on a sea floor that had earlier been raised to, and locally above, the marine base level of aggradation so that deposition was halted briefly over a wide area. That no appreciable regression occurred is evident in the fact that lowest phase-4 sediments represent deposition far from shore well beyond areas where terrigenous detritus dominated sedimentation. As the sea floor subsided, sediment accumulation became possible again, first in a zone of considerable wave activity, then under deeper- and quieter-water conditions during which time the typical Greenhorn beds accumulated. Locally, as in southern Hodgeman and northern Ford Counties, Kansas, a thick bentonite present near the top of the Graneros (phase 3) in all more northerly sections is lacking and at one exposure phase-4 beds rest only 4 feet above noncalcareous sandstone containing phase-2 mollusks. Bentonite that was reworked during this erosional interval was ultimately incorporated as pebbles into basal phase-4 limestone beds. Phase 4 represents the stage of maximum eastward transgression of the Western Interior Sea, the shoreline at that time lying perhaps scores of miles east of the present outcrop. The part of section containing the highest percentage of chalky limestone, i. e., Jetmore, Pfeifer, and basal part of Fairport, represents the climax of phase-4 deposition. Increase in quantity of terrigenous detritus near the top of phase-4 heralds beginnings of regression.
Figure 5--Features of basal Greenhorn and Blue Hill strata. A, Basal part of Greenhorn Limestone (phase 4) in sharp contact with clayey shales of phase 3, sec. 35, T. 12 S., R. 14 W., Russell Co.; B, Cross-bedded inoceramite at base of Greenhorn, phase 4, sec. 18, T. 13 S., R. 12 W., Russell Co.; C, Septarian concretion and shale, Blue Hill Member of Carlile Shale, phase 5, sec. 29, T. 11 S., R. 16 W., Ellis Co. A larger verison of this figure is available.
Depositional environments were mostly quiet during accumulation of phase-4 sediments as evidenced by numerous paired valves of Inoceramus, excellent preservation of many pedunculate barnacles, thin-shelled Ostrea, and Inoceramus labiatus, uniform bedding of most deposits, and preservation of laminae virtually throughout the shaly chalk section. Sedimentation rate was apparently relatively slow as suggested by gigantic inocerami that locally are host to more than one generation of epizons and as further indicated by multiple penetrations by burrowing organisms in single layers scattered here and there through the section. However, lenses of inoceramite and shell fragments, isolated Inoceramus fragments, and overturned specimens of adult I. cuvieri having large epizoal growth of Ostrea on exteriors of both valves prove that storm waves disrupted the sediments occasionally, leaving behind a shambles of shell material to be swept by currents into lenses and thin layers. The abundance of ammonites and planktonic forams throughout the section and high percentage of CaCO3 suggests water of normal salinity.
Sedimentary rocks of phase 5 include predominantly dark-gray noncalcareous slightly silty shale that contains numerous clay-ironstone and calcareous septarian concretions, many of which are fossiliferous (Fig. 5C). This phase is represented by approximately the lower four-fifths of the Blue Hill Member of the Carlile Shale and is similar lithologically and, to a certain extent, paleontologically to phase 3. The dominant fossils are Collignoniceras hyatti, Scaphites carlilensis, and Inoceramus flaccidus, together with Proplacenticeras, sparse Ostrea, locally common to abundant Yoldia and Lucina, and a few species of gastropods including Bellifusus and Tessarolax. Foraminifera in this phase are a mixture of arenaceous and mostly planktonic calcareous forms, as in phase 3. The shift to noncarbonate sedimentation in phase 5 reflects influx of large quantities of terrigenous clay and silt during regression, but the fossil assemblage suggests deposition in water that was of normal or nearly normal salinity throughout. Evidence of wave and current activity is rare in phase 5 possibly because the watersaturated mud could not support structures produced by mechanical agitation. Local alignment of pelecypods and an isolated occurrence of fish-tooth conglomerate that shows preferred orientation furnish some evidence of current activity. The reason for lack of inoceramite beds, so common in the analogous transgressive phase 3, is not clear. The supply of detritus during regression seemingly would have been sufficient to keep pace with subsidence and Inoceramus was present in considerable numbers, at least locally. But fossil preservation is largely excellent, specimens show no signs of transport and large I. cuvieri near the base of the Blue Hill Member are apparently intact. Deposition of phase 5 sediments occurred under quiet conditions chiefly below effective wave base.
Phase 6 includes medium dark to dark-gray, locally concretionary, noncalcareous silty and finely sandy shale that contains thin to very thin beds of soft, generally weakly consolidated sandstone. Sand, silt, and clay are commonly interlensed and interlaminated in these beds. Small lenses of the coarser detritus impart a mottled appearance to the fresh rock, and the shale is less flaky than that below. This phase is transitional with phases 5 and 7, is represented by the upper part of the Blue Hill Member, and is similar lithologically to phase 2 of the cyclothem (Fig. 6A). Macroinvertebrates are very sparse in upper Blue Hill rocks but foraminifers, particularly arenaceous forms, are common. Phase 6 locally contains a bed of laminated to gently crosslaminated calcareous siltstone or very finegrained sandstone. At one locality calcareous siltstone in phase 6 has oscillation ripple marks with crests only 0.2 foot apart suggesting very shallow water. The larger quantity of sandstone and greater proportion of arenaceous foraminifers in phase 6 suggest approach of the retreating shoreline and perhaps somewhat brackish water.
Figure 6--Upper Carlile and Niobrara strata. A, Sandy strata in upper part of Blue Hill Member, phase 6 (below top of rod), overlain by Codell Member, phase 7, sec. 3, T. 11 S., R. 17 W., Ellis Co.; B, Contact (top of rod) between Codell Member of Carlile Shale (phase 7) and Niobrara Chalk (phase 4 of second cyclothem), sec. 3, T. 11 S., R. 17 W., Ellis Co.; C, Contact between Fort Hays and Smoky Hill Members of Niobrara Chalk (both phase 4 of second cyclothem), sec. 25, T. 14 S., R. 25 W., Trego Co. A larger verison of this figure is available.
The terminal phase of the first late Cretaceous cyclothem includes the light olive-gray, locally calcareous, commonly clayey siltstone and sandstone of the Codell Member of the Carlile Shale (Fig. 6B) which is transitional with phase 6 but in sharp lithologic contrast and abrupt stratigraphic contact with chalky limestone of the overlying Niobrara. The coarser terrigenous detritus reflects nearness of the shoreline and records the maximum stage of regression that is expressed in the west-central Kansas outcrop. This phase is similar lithologically to the thick sandstones of phase 1, but lacks plant debris and is nowhere highly limonitic. As in regressional phases 5 and 6, evidence of wave and current activity is sparse but includes rare gentle cross lamination, rounded and polished fish bones and teeth, and local clay pebbles. Macrofossils are sparse, as in much of phase 1, but worm burrows, arenaceous foraminifers, fish remains, and fecal pellets are common.
The contact between beds of the first cyclothem and the overlying Niobrara, like that between phases 3 and 4, described above, is at a regional break. The Niobrara Chalk lies with knife-sharp contact on the Codell Sandstone at most localities and lithologic evidence of hiatus is supported by lack of at least three major Carlile fossil zones that are represented elsewhere (Cobban and Reeside, 1952). This break has been termed a disconformity by some workers, but the time value of the hiatus is less in Colorado and Wyoming than in Kansas. No evidence of subaerial exposure is apparent, so the break probably has resulted from lack of deposition and sublevation rather than subaerial erosion. During this halt in sedimentation, renewed transgression was taking place, probably owing to subsidence of areas to the east, but the sea floor in west-central Kansas was in a zone of sediment transport rather than of deposition so the early phases of transgressional sedimentation are not represented. When the sea floor in the latter area had subsided sufficiently to permit sediments to accumulate, the shoreline was again far to the east, and west-central Kansas was beyond the depositional range of significant quantities of terrigenous detritus. Hence the first sediments to be deposited above the Codell are offshore phase-4 carbonates of the second cycle of late Cretaceous marine sedimentation (Fig. 1).
Strata constituting the second cyclothem are assigned to the Niobrara Chalk and Pierre Shale. Chalky limestone of the Fort Hays Limestone Member of the Niobrara is analogous to the Jetmore-Pfeifer (middle subphase of phase 4) parts of the Greenhorn cyclothem. The light olive-gray (usually weathered pale yellowish-gray to nearly white) chalky limestone is characterized by various robust species of Inoceramus, which have no direct counterparts in the Greenhorn, Ostrea congesta, and Gryphaea sp. The limestone is abundantly burrowed, with both chalk- and coarsely crystalline calcite-filled varieties as in the Greenhorn cyclothem. The olive-gray laminated speckled shaly chalk of the Smoky Hill Chalk Member is comparable to the Fairport part of the Greenhorn cyclothem and is replete with very large specimens of Inoceramus, many of which are heavily encrusted with Ostrea congesta, sparse serpulids and barnacles, rare crinoids, and a host of vertebrate remains. These beds represent the terminal subphase of phase 4 of the Niobrara cyclothem (Fig. 6C). Dark-gray concretionary noncalcareous shale of the Pierre, representing phase 5, rests conformably upon the Niobrara and marks influx of abundant terrigenous detritus during regressional deposition in the second or Niobrara cyclothem of the late Cretaceous in west-central Kansas.
Stratigraphy, sedimentary structures, texture, composition, and fossils in the Greenhorn cyclothem indicate successive change from delta-topset deposition in phase 1 to nearshore deposition in shallow brackish water in phase 2 (delta foreset) to progressively farther offshore deposition in water of increased salinity through phase 3 (delta bottomset), with maximum transgression and greatest distance from shore during accumulation of phase 4. Regression is recorded chiefly in the increased quantity and grain size of terrigenous detritus upward from the uppermost part of phase 4 through phases 5 to 7. By analogy with ecological distribution of modern animal groups, I conclude that phases 3 through 5 were deposited in water of salinity nearest to normal. Parts of a second, similar cyclothem are recognized in the Niobrara and Pierre, but the transgressive half of this cycle is not represented in west-central Kansas.
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