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Biostratigraphy
Western Interior Standard Zonation
Intensive study of Cretaceous macro invertebrate fossils has brought biostratigraphic subdivision of U.S. Western Interior strata to a high degree of refinement. In a classic contribution to Western Interior stratigraphy, Cobban and Reeside (1952) established a zonal scheme that has wide application in marine beds of this richly fossiliferous region. The standard zonal sequence has been revised several times since that date, the more important contributions to Niobrara biostratigraphic refinement being those by Cobban and others (1962), Scott and Cobban (1962), Cobban (1962, 1964, p. 24, 1969), Scott and Cobban (1964, p. 24), Gill and Cobban (1966, p. 35), and Kauffman (1975, 1977, p. 82, 83). The standard Western Interior zonation, embracing strata of the Niobrara Chalk (= Niobrara Formation of Colorado), is shown in Table 2.
Table 2--Standard Western Interior biostratigraphic zones for the Niobrara and equivalent strata.
| Stages | Zonal Indices | Associated Species* | |
|---|---|---|---|
| Campanian | Lower (part) | Haresiceras natronense | Scaphites hippocrepis (fine-ribbed form) |
| Haresiceras placentiforme | Scaphites hippocrepis (coarse-ribbed form) | ||
| Haresiceras montanaense | Scaphites hippocrepis (coarse-ribbed form) | ||
| Santonian | Upper | Desmoscaphites bassleri | Haresiceras mancosense (late form) |
| Desmoscaphites erdmanni | Haresiceras mancosense (early form) | ||
| Clioscaphites novimexicanus | |||
| Middle | Clioscaphites choteauensis | Inoceramus (Endocostea) balticus* | |
| Clioscaphites vermiformis | Inoceramus cordiformis* | ||
| Clioscaphites saxitonianus | Inoceramus (Cladoceramus) undulatoplicatus* | ||
| Lower | Scaphites depressus | Protexanues shoshonensis, Scaphites binneyi, Inoceramus stantoni* | |
| Coniacian | Upper | Scaphites ventricosus | Inoceramus (Volviceramus) grandis* |
| Lower | Scaphites preventricosus | Inoceramus deformis | |
| Inoceramus erectus (late form) | |||
| Barroisiceras, Peroniceras | Inoceramus erectus s.s. | ||
| Turonian | Upper part | Scaphites coruensis | Prionocyclus quadratus |
| Inoceramus aff. I. perplexus | |||
| * Associated species marked* are not restricted to the zone. | |||
Chronologic Comparison of Pueblo, Colorado, and Kansas Sections
Both base and top of the Niobrara are regionally diachronous (e.g., Cobban, 1964; Hattin, 1975a) so that different zonal indices occur in lowermost and uppermost beds, depending upon geographic location. In the Pueblo, Colorado, section, Scott and Cobban (1964) recorded Inoceramus aff. I. perplexus (uppermost Turonian) in basal beds of the formation, whereas in central Kansas the basal beds contain Inoceramus deformis (Scaphites depressus Range Zone--Lower Coniacian) and in northeastern Nebraska the basal beds contain I. (Volviceramus) grandis (Hattin, 1975a). The base of the formation therefore ascends chronologically from southwest to northeast. Similarly, the top of the Niobrara ascends chronologically in a southeastward direction from central Montana and north-central Wyoming to the Black Hills area and beyond (Cobban, 1964), the uppermost beds climbing in that region from the zone of Desmoscaphites erdmanni (lower Upper Santonian) to the zone of Haresiceras placentiforme (Lower Campanian). Physical evidence demonstrates that the Niobrara-Pierre contact also ascends chronologically from Pueblo, Colorado, to the Smoky Hill River outcrop. At Pueblo, the unnamed transition member of the lower Pierre Shale contains a bentonite seam, which in Kansas lies within chalk of the Smoky Hill, well below the Niobrara-Pierre contact. Furthermore, beds equivalent to the uppermost Niobrara at Pueblo (upper chalk unit of Scott and Cobban, 1964) lie 12.8 m (42 ft) below the Niobrara-Pierre contact in the Smoky Hill composite section of Kansas.
At Pueblo (Scott and Cobban, 1964) the Fort Hays-Smoky Hill contact lies within the zone of Inoceramus deformis but in the Smoky Hill composite section this contact lies approximately at the base of the I. (Volviceramus) grandis Range Zone, which is separated from that of I. deformis by the zones of Inoceramus (Cremnoceramus) browni and Inoceramus koeneni. This places the contact close to the Lower/Upper Coniacian boundary. Sixty-five km (40 mi) east of Pueblo, Haresiceras placentiforme occurs near the top of the Smoky Hill Member (Scott and Cobban, 1964, p. 20), which indicates an Early Campanian age. The upper chalk unit of Scott and Cobban (1964, p. 21) and overlying unnamed transition member of the Pierre Shale have not yielded diagnostic fossils in southeastern Colorado, nor have equivalent parts of the Smoky Hill Member in Kansas, but these beds are also assignable to the Lower Campanian because in both areas Lower Campanian fossils are known in slightly younger strata of the Pierre.
Biostratigraphy of Smoky Hill Member in Kansas
Except for smooth molds of Baculites, ammonites have been recorded only sparingly in the Kansas Smoky Hill. Of the standard zonal indices only Clioscaphites vermiformis and C. choteauensis have been identified positively, and the former is known only from a few imperfect specimens. Comparison of the Kansas section with the Western Interior biostratigraphic standard is thus based largely on the sequence of inoceramid bivalve species, most of which are well preserved and relatively common. A listing of the most useful Kansas zone fossils, together with associated species, is presented in Table 3. Known ranges of Smoky Hill macroinvertebrate species are depicted in Figure 31. Species recorded in the field during the course of this study are illustrated in Plates 2-9.
Table 3--Biostratigraphic zonation of the Smoky Hill Member in Kansas.
| Standard Zonation | Kansas Zonation | Associated Species | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Haresiceras natronense | Inoceramus (Endocostea) balticus s.l. | I. (Platyceramus) platinus Baculites sp. (smooth) |
||||||||
| Haresiceras placentiforme | ||||||||||
| Haresiceras montanaense | ||||||||||
| Desmoscaphites bassleri | ||||||||||
| Desmoscaphites erdmanni | ||||||||||
| Clioscaphites choteauensis | Clioscaphites choteauensis | Baculites sp. (smooth) Phelopteria sp. I. (Platyceramus) platinus I. (Endocostea) balticus s.l. |
||||||||
| Clioscaphites vermiformis | Clioscaphites vermiformis | Baculites sp. (smooth) Inoceramus (Platyceramus) platinus |
||||||||
| Clioscaphites saxitonianus | Inoceramus (Cladoceramus) undulaioplicatus | I. stantoni I. (Platyceramus) cycloides I. (Platyceramus) platinus s.l. |
||||||||
| Scaphites depressus | Inoceramus (Volviceramus) grandis | I. (Platyceramus) platinus s.l. | ||||||||
| Scaphites ventricosus | ||||||||||
Figure 31--Species ranges of macro invertebrate body fossils in the Smoky Hill type area. Only species for which sound stratigraphic evidence is available are plotted on this chart. Dotted parts of ranges designate intervals for which no specimens were recorded. Dashed ranges or parts of ranges indicate uncertainty regarding exact extent of range. Ranges of Baculites sp. cf. B. codyensis, Beoahites sp. B, and upper part of Lucina sp. range are based on Miller (1968). Range of Uintacrinus socialis is based on statements by Williston (1897) and Springer (1901). Extension of Stramentum haworthi range above Marker Unit 13 is based on specimens in pink chalk, from upper part of member, in collection of Sternberg Memorial Museum, Fort Hays Kansas State University. All other data are based on observations by the author. Range for Pseudoperna congesta embraces all small epizoic oysters recorded by the author; very small numbers of specimens have been attributed to two other species (Miller, 1968), for which stratigraphic data are not available.
Apparent absence of eight scaphite zonal indices in Kansas could be owing to (1) incomplete field data, (2) lack of preservation, or (3) lack of scaphites in western Kansas during much of Smoky Hill deposition. The Smoky Hill Member has been the object of fossil searches for generations, but only Clioscaphites choteauensis has been recorded reliably in previous literature (Miller, 1968, p. 45). The single specimen (now lost) of Clioscaphites vermiformis reported by Morrow (1931) was from the Monument Rocks exposure in Gove County. Beds at that locality are in the zone of Clioscaphites choteauensis, to which species Morrow's specimen probably belonged. I have visited repeatedly exposures that are the basis for the Smoky Hill composite section, and have tried systematically to examine bedding surfaces through all measured units. The poor record of scaphites thus seems not to be the result of poor collecting. In Kansas Clioscaphites choteauensis is not only common through several meters of strata that represent its zone but is also preserved excellently (Pl. 8, 6). In contrast, C. vermiformis, whilst reasonably well preserved, is much less common than C. choteauensis. Paucity of the former thus seems related more to originally small numbers of specimens than to poor preservation. The excellent preservation of Baculites molds in many intervals between the base of the C. choteauensis zone and Marker Unit 22 demonstrates the preservability of originally aragonitic ammonites at least in some strata where scaphites might be expected. The general absence of scaphites is, therefore, not attributed to lack of preservation. From stratigraphic evidence now available, I conclude tentatively that conditions in the water column were favorable for scaphitid ammonites only during deposition of the S. vermiformis and C. choteauensis zones, and that near absence of scaphitids, and indeed most other ammonites, through most of the Smoky Hill, is owing to environmental factors in the water column. It must be noted, however, that the Smoky Hill lacks the sort of hard limestone beds and concretions in which scaphites are so commonly preserved in the Western Interior, and that even the common species C. choteauensis was not recorded in Kansas until 1958 (Miller). Further search may yet serve to turn up some of the other zonal species of scaphites.
Not shown on the range chart (Fig. 31) or on Plates 2-9 are many named macroinvertebrate species for which little or no stratigraphic data are available. Most of these forms are described and illustrated in the work of Miller (1968) and include:
| Ostrea exogyroides Logan | Ostrea rugosa Logan | Pecten bonneri Miller | Spinaptychus sternbergi Fischer and Fay | Actinocamax sp. aff. A. laevigatus Arkhangelsky | Actinocamax sternbergi Jeletzky | Actinocamax walkeri Jeletzky | Tusoteuthis longa Logan | Niobrarateuthis bonneri Miller | Niobrarateuthis walkeri Green | Calantica (Titanolepas) martini Withers | Serpula intrica White | Serpula tenuicarinata Meek & Hayden |
All of the above-named species are rare, some are known from only a single specimen, and for some the locality data are unknown.
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Kansas Geological Survey, Geology
Placed on web Feb. 20, 2015; originally published Dec. 1982.
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