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Depositional Environment
General Statement
Determination of physical and chemical conditions in an ancient depositional environment depends not only upon stratigraphic and lithologic studies of the pertinent rock sections, but also upon ecological evidence furnished by fossils contained therein. Conversely, paleoecological interpretation of fossils demands not only a critical examination of organic remains but also detailed investigation of the enclosing strata. Despite the interrelationship between depositional environment and paleoecology, fundamental differences separate the two concepts. Therefore, in the following paragraphs I will discuss first the environmental conditions of Graneros deposition from the standpoint of stratigraphic variation of rock texture, structure, and composition. Next will follow an analysis of paleoecological implications of Graneros fossils, including further evidence bearing upon conditions in the depositional environment. Ideally, a paleoecological study should embody some combination of qualitative and quantitative techniques designed to extract a maximum of information from the incompletely preserved record of ancient biotopes. Because Graneros fossils are abundant at only a few localities and are mostly preserved as molds, paleoecological data are restricted mostly to observations pertaining to features of preservation and orientation, faunal and lithologic association, and stratigraphic distribution. Within the limits imposed by time and evolution, comparisons are drawn between some of the more common Graneros species and living representatives of the same genera or families.
Salinity
General consideration of stratigraphy and paleontology of the Dakota Formation and Graneros Shale focuses attention on two major factors bearing on salinity. The first of these is the change upward in the Dakota from plant-, fossil-, lignite-, and channel-sandstone-bearing beds to interbedded shales, carbonaceous shales, and evenly bedded sandstones, some of which contain marine fossils, and the marine character of the gradationally superjacent Graneros. The second factor is the lateral equivalence of the upper part of the Dakota in central Kansas to the lower part of the Graneros farther west. These relationships reflect an eastward transgression during which one would expect the depositional environment to change gradually from wholly nonmarine to marginal marine and ultimately to open marine conditions. The fluvial environment, represented most conspicuously by Dakota channel deposits, was one of fresh water; the marginal marine environment would have been brackish owing to discharge of streams; the open marine environment most nearly would have approached normal or nearly normal salinity as the retreating shoreline became even more remote from central Kansas and dilution by river discharge diminished.
In addition to inferences based on stratigraphy and the broad aspects of Graneros paleontology, upward change in clay-mineral content, as discussed below, suggests a gradual salinity increase during deposition of the uppermost Dakota and Graneros sediments. Trask (1937) showed the relationship between increase of salinity and increase of calcium carbonate content of marine sediments and noted (1937, P: 292) that "Sediments in areas where the salinity is less than 34 parts [per thousand] in general contain less than 5 percent of calcium carbonate. . . ." The near absence of calcareous rocks in the lower part of the Graneros and conspicuous development of such beds in the upper part of the formation, especially including skeletal limestone and calcareous shale, seemingly support other evidence for increase of salinity to normal or nearly normal as Graneros deposition proceeded. Stratigraphic distribution in uppermost Dakota and Graneros strata of certain invertebrate fossils, especially brachiopods, foraminifers, and ammonites, which are discussed in detail below, further strengthens my conclusion that salinity of the water changed gradually from brackish to normal or nearly normal during accumulation of the Graneros sediments in central Kansas.
A most fruitful area for future geochemical investigation of the Graneros Shale would be a study of trace element distribution in relation to salinity.
Turbulence and Depth of Water
A variety of stratigraphic, sedimentologic, and paleontologic evidence manifests the environmental turbulence that existed during much of the time of Graneros deposition. Shale in the lower part of the formation at many localities is streaked with irregular laminae and lenses of very fine sand and silt which impart an appearance of the kind that Hantzschel (1939, p. 202) and many other authors ascribed to rapidly changing currents and agitated waters (Fig. 5, A, B). The gentle cross-lamination and cleanly washed condition of many sandstone, siltstone, and skeletal limestone units that are distributed unevenly throughout the formation are ample evidence of extensive wave and current activity.
Many of these cross laminations are clearly related to migration of ripple structures, and perhaps all of them are. Some lenses and zones of lenses of cross-laminated sandstone lying near the middle of the formation, especially including the yellowish, silty variety of sandstone described above, are apparently starved current ripples (Fig. 6, B). Thin laminae and very thin lenses of cleanly washed silt or very fine sand, scattered throughout the formation, but especially abundant in the lower half of the Graneros, resulted from alternation between silty clay deposition in quiet water and gentle stirring of bottom sediments by waves and currents. Locally, at Locality 5, extensively cross-bedded sandstone with dips up to 24 degrees (Fig. 9, A) reflects stronger current action than do laminated and gently cross-laminated units. Throughout the lower part of the formation valves or molds of burrowing pelecypods are oriented parallel to the bedding where they were deposited following sporadic but thorough bottom-sediment reworking by currents that left no clams in situ. Stronger currents concentrated Callistina lamarensis in gently cross-laminated sand beds where most valves lie convex upward in the rock. In the Ostrea beloiti Assemblage Zone currents of similar strength concentrated shell debris as layers of calcareous sand composed largely of Inoceramus prisms, the central Kansas area of deposition by then being far from shore. Throughout the formation paired pelecypod valves are rare because of the current transport to which nearly all were subjected.
Extensive search was made for usable directional structures but unfortunately most of those found were in float. In beds likely to contain directional structures, the bedding surfaces are generally so thickly encrusted with jarosite, limonite, or gypsum that recognition of any structures present is difficult at best. A few reliable measurements, mostly from sections in the Smoky Hill River and Saline River valleys, were obtained from cross beds, tool marks, current ripples, and one ripple mark showing parting lineation. Flow direction in nearly all of these lies between N 60°W and N 60°E. Additional work is needed on this aspect of Graneros sedimentation.
Bone beds and lenses of coquinoidal limestone occurring at many places in the formation were produced by brief periods of turbulence that was greater than usual for the area. Possibly during occasional storms, sediments on the sea floor were scoured more deeply than usual, resulting in concentration, in lenses and layers, of coarse organic debris that originally was scattered widely through the sediments.
Well-preserved ripple marks are most common near the middle part of the Graneros and scarce elsewhere in the formation. Because some nearly symmetrical ripple marks in the Graneros are known from internal structure to be of current origin, the discrimination between current and oscillation types is therefore difficult in nearly symmetrical ripple marks with poorly preserved internal structures. The only clear-cut examples of oscillation ripple marks that I have found lie in a single unit near the middle of the formation at Localities 5 (Fig. 9, B), 7, and 32. Wave lengths of these ripples are 0.2, 0.3, and 0.3 foot, respectively. Comparison of these limited data with data published by Inman (1957) would suggest water less than 70 feet deep, but verification of this conclusion necessitates further measurements. Asymmetrical profile, or association of ripple marks with cross-laminated structure, or both, suffice to identify most of the remaining Graneros ripple marks as current-formed.
Kruit (1955, p. 97) believed that preservation of ripple marks in shallow-water marine deposits is dependent upon alternations in the supply of mud, silt, and sand. Despite apparent satisfaction of these conditions during deposition of the Graneros, I believe that the paucity of ripple marks in roughly the lowest third of the formation is primarily a consequence of nearshore, shallow-water turbulence. Above a critical current velocity current ripples are destroyed (Bucher, 1919, p. 165). Above a critical orbital velocity, usually occurring in water less than 30 feet deep, wave-generated ripples are planed off (Inman, 1957, p. 31). During deposition of the lower part of the Graneros these critical velocities were frequently exceeded; indeed, the sediments were often extensively reworked, and such structures tended to be obliterated by waves and currents before burial occurred. On the other hand, paucity of ripple marks near the top of the formation indicates deeper, more quiet water conditions. In addition to lithologic evidence cited above, this conclusion is supported by distribution of epibionts in Graneros strata. More generally turbulent conditions during deposition of the lower part of the formation contributed to the scarcity of epibionts in these beds. Occurrences of large numbers of epibionts locally in the upper part of the formation indicates less frequent disruption of bottom sediments, most likely owing to greater water depth. Foraminiferal evidence for progressively increasing depth during Graneros deposition is discussed below.
Rate of Deposition
Rate of Graneros deposition cannot be determined quantitatively at this time, but several lines of evidence bear on the relative rate of accumulation of sediment in the formation. As discussed in more detail elsewhere in this report, the Graneros is believed to represent a depositional environment roughly similar to that of the pro-delta silty clay and offshore clay of the Mississippi Delta region. In that region the rate of deposition was determined by Scruton (1956, p. 45) to be most rapid near the mouths of major distributaries and to be inversely related to clay content of sediment for the first few miles off the mouths of these distributaries. By analogy the relatively pure clays in the upper part of the Graneros were deposited more slowly than the more sandy and more silty beds in the lower part of the formation. Cross-laminated sandstone units in the lower part of the Graneros, although reworked by currents before burial, were probably deposited rapidly during flood stages of the source stream(s). The relatively high quantity of carbonized plant debris in many of these sandstone units supports this interpretation. Similarly, in a report on the Rhone Delta, van Straaten (1959, p. 208) noted that during flood stage, coarse-grained sediment is carried in larger quantities and farther seaward than during low-water river stages. Decrease in abundance of coarse-grained terrigenous detritus upward in the Graneros Shale is evidence of a progressive increase in distance from shore of the sediments in the central Kansas area during accumulation of the Graneros and of an ever-decreasing rate of sedimentation.
Greater abundance of kaolinite in the uppermost part of the Dakota and lower part of the Graneros, owing to rapid flocculation in the nearer shore environment, suggests that deposition of stream-transported clay was probably faster during deposition of this part of the formation.
In the upper assemblage zone, Ostrea beloiti is preserved locally in clusters attached to large specimens of Inoceramus and one such cluster from Locality 15 involves at least two generations of Ostrea on a large Inoceramus. The time involved for growth of the Inoceramus and establishment of two or more generations of O. beloiti was apparently quite long and is further evidence that deposition of the upper part of the Graneros was relatively slow. No such occurrences of successive generations of sessile benthos were noted in the lower part of the formation, probably as the result of more rapid deposition, unstable substrate, or both.
Burrows of various sorts occur sparingly throughout the Graneros Shale and are most common in jarosite-cemented silty sandstone beds and lenses (Fig. 7, A, B, C) that lie in the lower two-thirds of the formation, generally in the Callistina lamarensis Assemblage Zone. In beds containing these structures, laminations are generally not disturbed, indicating, as suggested by Middlemiss (1962, p. 36), that accumulation and burial of the burrow-containing beds was too rapid to permit multiple penetration. Notable exceptions to the usual kind of burrowing are found in a clayey siltstone unit lying near the base of the Graneros at Locality 23, where laminae were distorted by activity of a highly mobile infauna (Fig. 7, D). Rarity of burrows in the upper part of the Graneros, where relatively slow deposition seems to have been the rule, and where thin cross-laminations are well preserved in skeletal limestone beds, may demonstrate paucity, of a mobile infauna of wormlike creatures in this part of the formation. This view is strengthened by the fact that, unlike the lower part of the Graneros Shale, the upper assemblage zone is almost lacking in shell-bearing, burrowing macro invertebrates.
Bottom Conditions
During Graneros deposition the sea bottom consisted chiefly of soft sediment made up mostly of clay and silt. The limited number of benthonic species in both Graneros assemblage zones may be largely attributable to the nature of this substrate. Thorson (1957, p. 466) has contrasted hard- and soft-bottom areas of the sea floor and notes lack of numerous "microlandscapes" and relatively small number of species in the latter. Such factors as lack of shelter and small quantity of fixed vegetation serve to reduce drastically the epifauna of the soft-bottom community and especially to limit the development of a sessile benthos. The development of the benthos in soft-bottom areas is particularly hampered if the sediments are unstable. The lower Breton Sound and pro-delta slope depositional environments in the east Mississippi Delta region that have been described by Parker (1956, p. 321) have a very fine silty clay to clayey silt bottom and support a limited macro invertebrate assemblage of only 20 species. Parker believed that high turbidity and fine fluid bottom may have hindered filter feeding of some animals and hindered larval settling of others in this area. The discussion of this area by Scruton (1956, p. 38) under the heading "pro-delta silty clays" fits closely the description of at least the lower part of the Graneros Shale. Despite prevalence of a soft sea bottom throughout Graneros deposition the benthonic faunas in the lower and upper parts of the formation differ markedly and must be attributed largely to factors other than the nature of the substrate. Changes in salinity, turbulence, and rate of deposition that occurred during accumulation of Graneros sediments probably contributed to differences in the benthos.
Paleoecology
Foraminifers
The upward change in foraminiferal content from dominantly arenaceous benthonic forms in the lower part of the Graneros to almost exclusively planktonic forms in the uppermost part of the formation is an expression of the same changes of environmental conditions that are reflected in lithology and in distribution of macroinvertebrates.
The Kiowa Shale (Lower Cretaceous) of Kansas, which is lithologically similar to the Graneros, contains a foraminiferal fauna that is dominated by arenaceous forms, among which those belonging to the genera Ammobaculites and Trochammina are most abundantly represented. Loeblich and Tappan (1950, P: 3) concluded that the chief factor controlling distribution of this assemblage was salinity, with a prevalence of brackish water. In a regional study of foraminifers from the Graneros Shale, Eicher (1965, p. 888) found that arenaceous forms dominated the samples, and he concluded that the formation was deposited in waters that were generally of less than normal salinity. The occurrence of large percentages of arenaceous foraminifers in some areas of brackish water in modern seas was mentioned by Bandy and Arnal (1960, p. 1924). They note also the frequent occurrence of abundant arenaceous genera with simple interiors, including Ammobaculites, in shallow brackish water but point out (p. 1925) that environmental interpretation based on this criterion must be supported by other evidence. Bandy and Arnal (1960, p. 1926) suggest that absence of calcareous species in an assemblage may indicate only that these have been dissolved following deposition. That this mechanism is not responsible for absence of calcareous benthonic foraminifers in the Graneros is proved by occurrence of both calcareous planktonic and arenaceous species in some samples, even at the base of the Graneros, locally.
Muddy bottom conditions and more rapid sedimentation may have been a factor in restricting the lower Graneros benthonic microfauna to a few species of arenaceous foraminifers. However, except for Reophax minuta, the number of arenaceous forms decreases upward in the Graneros; yet the rocks, although indicating generally muddy bottom conditions, suggest progressively slower deposition in gradually deepening water. The decrease in arenaceous foraminifers is not compensated for by appearance of other benthonic genera; therefore, changing bottom conditions seem unlikely as a main factor controlling distribution of benthonic foraminifers in the Graneros Shale. Increasing depth, however, may have played an important ecologic role.
Stainforth (1952) related certain assemblages of large arenaceous foraminifers to environments of turbidity-controlled deposition, and Tappan (1962, p. 124, 127) has suggested such control for some of the Alaskan Cretaceous microfaunas. Although Graneros arenaceous foraminifers mostly belong to families characteristic of such turbidity-controlled environments, none are particularly large; in fact, such species as Reophax minuta, R. pepperensis, and Verneuilinoides perplexus are quite small. Nonetheless, the water in which much of the Graneros was deposited must have been kept quite turbid by constant discharge of fine-grained detritus from the major stream system( s) that existed to the east or northeast. Turbidity can reduce the production of food used by foraminifers, and this has the greatest effect upon calcareous species (Tappan, 1962, p. 124). This mechanism would possibly explain the lack of calcareous benthonic foraminifers in the Graneros Shale but does not account for the decrease upward in the section of arenaceous forms. Eicher (1965, p. 889) related the sporadic occurrence of calcareous benthonic forms in the Graneros of Colorado to rare occasions when salinity was high enough for a sufficiently long time, so as to permit invasion of the area by such species. For central Kansas, at least, this interpretation seems unlikely because beds in the upper part of the Graneros that contain good indicators of normal salinity, such as ammonites or predominance of species of planktonic foraminifers, are apparently devoid of calcareous benthonic species.
Planktonic foraminifers are generally regarded as indicative of open seas and normal salinity (Smith, 1955, p. 147; Tappan, 1962, p. 125; Phleger, 1964, p. 34). The scattered occurrence of planktonic foraminifers in the lower part of the Graneros and their dominance near the top is acceptable evidence of gradually increasing salinity as Graneros deposition proceeded. Smith (1955, p. 147) wrote that the absolute abundance of planktonic foraminifers is inversely related to distance from their source area and directly related to depth. If this criterion is valid generally, then Graneros sediments dominated by planktonic foraminifers were deposited farthest offshore and in the deepest water. Studies of foraminifers in the Gulf of Mexico have shown that planktonic species are essentially lacking in water less than 70 feet deep (Lowman, 1949, fig. 13), 70 to 100 feet (Bandy, 1956, p. 192), or 160 feet (Phleger, 1951, p. 67), depending upon the area investigated. If anything can be drawn from existing foraminiferal distribution in the continental shelf area nearest to Kansas, the appearance in abundance of planktonic foraminifers above the middle of the Graneros may indicate depths of 70 feet or more for that part of the formation. The benthonic rnacroinvertebrates that occur in the upper part of the Graneros are proof that the deepest waters were well areated, so lack of oxygen is not the reason for lack of benthonic foraminifers in this part of the section.
In conclusion, I suggest that salinity was the dominant factor controlling foraminiferal distribution in the Graneros Shale. Turbidity is the best explanation for absence of calcareous benthonic species, and increasing depth probably accounts in part for lack of arenaceous species in the uppermost part of the formation in the central and northern parts of the outcrop. Assemblages dominated by several species of arenaceous foraminifers accumulated in shallow-water, nearshore areas where seawater was diluted by discharge of nearby streams. Planktonic tests were occasionally washed by currents into these nearshore areas. As deposition proceeded, salinity increased and Reophax minuta, probably more nearly euryhaline than the other arenaceous species, became dominant. Toward the close of Graneros deposition in central Kansas, planktonic foraminifers became the dominant forms in an open epeiric sea in which salinity, and/or depth, was greater than could be tolerated by arenaceous species that characterized the nearer shore, shallower water sites of deposition.
Brachiopods
Basal beds of the Graneros Shale contain abundant Lingula and abundant Discinisca at Localities 13 and 3, respectively. The lingulids are associated with rare pectenoid pelecypods; the discinids occur with sparse Inoceramus and possibly a Lingula. Some of the Lingulas are nearly erect in the sediment and the entire assemblage may be essentially in situ. Although nearly all of the Discinisca remains are brachial valves, these include a wide range in size, are well preserved, and thus may not have suffered extensive transport. According to Hatai (1940, p. 175), modern species of Lingula commonly flourish near the mouths of large rivers or in brackish-water environments. Actually, Lingula can tolerate a wide salinity range and is known to have lived in seawater so diluted by fresh water as to be fatal to other marine animals (Craig, 1952; Ferguson, 1963).
Occurrence of inarticulates at Localities 13 and 3, to the near exclusion of other macroinvertebrates, is taken as evidence of marine water of very low salinity. At higher levels in the Graneros Shale brachiopods occur sparsely, either alone and possibly representing similar conditions of low salinity, or with other macroinvertebrates and probably reflecting salinity increased sufficiently to support other forms of benthonic marine life. In a recent review of Lingula ecology, Ferguson (1963, p. 670) noted that the genus is presently restricted to tropical and subtropical waters between latitudes 30°S and 41°N and cites Hatai's personal opinion that minimum temperature requirements for Lingula are about the same as that for corals, namely 18°C. Hatai (1940, p. 175) reported a bathymetric range of 0 to 23 fathoms for modern Lingula and these figures have been accepted generally by other workers. Ferguson (1963, p. 670) reasoned that since Lingula flourishes in the presence of a rich food supply, the genus would ". . . flourish in the tidal zone and for a short distance below low tide." The long-ranging, little-changing Lingula, so obviously successful in its ecologic niche, has probably experienced similar ecological controls since the early part of its history, Cretaceous time being no exception. Lingula was apparently not "flourishing" in areas of Graneros deposition, so one or more environmental factors may have been less than optimum. Bottom conditions were eminently satisfactory; supporting evidence for shallow depth, low salinity, and adequate food and oxygen supplies are discussed elsewhere in this report. Tentatively I have concluded that the general paucity of Lingula in an otherwise suitable environment is owing either to temperature below the optimum or to lack of preservation.Concerning living species of Discinisca, Muir-Wood (1928, p. 469) stated that all are shallow-water tropical marine forms that occur generally at depths ranging from 0 to 20 fathoms. She noted that large, thin-shelled, concentrically lamellar forms lacking radial sculpture, including two species that live along the east edge of the Pacific Ocean, are, attached to valves of their fellows or to valves of living Mytilus in water of 5 to 9 fathoms. Hertlein and Grant (1944, p. 33) emended the depth range of one of these species to 0 to 9 or 10 fathoms. The Graneros discinids closely resemble these modern non-radially sculptured species and by analogy are believed to indicate water no deeper than 10 fathoms. At Localities 6 and 14 I found 11 total of four Discinisca molds that are attached to molds of clams; three are attached to Callistina and one to Parmicorbula? Both clams were burrowers but now lie parallel to bedding, thus attachment and growth of Discinisca occurred after these clams had been excavated by sea-floor scour and demonstrates that sedimentation was, on rare occasions, slow enough to permit such epifaunal growth.
Pelecypods
Paleoecological significance of Callistina lamarensis is difficult to interpret not only because the genus is extinct but also because the family Veneridae to which it is assigned is one of the largest and most ecologically diversified groups of living pelecypods. Modern venerids are distributed bathymetrically from the beach environment to depths of several thousand feet and around North America have been reported from Arctic to equatorial waters. Some species have great latitudinal range. Most North American venerid species prefer water of normal or of nearly normal salinity but some species are tolerant of brackish water and some live in hypersaline lagoons. Different species of the same genera show wide range in habitat. The venerids are shallow burrowers and seemingly prefer sandy rather than mud bottoms but many deeper water species have been dredged alive from mud. From the foregoing one can see the danger inherent in any direct ecological comparison of extinct and living venerid species, and the even greater hazard if the ancient species belongs to an extinct genus.
Callistina lamarensis is distributed essentially throughout the lower part of the Graneros Shale and is therefore interpreted as indigenous to the attendant depositional environment despite evidence of displacement discussed below. Except in the coquinoidal limestone lens at Locality 5, shells and molds of Callistina everywhere lie parallel to bedding of the enclosing shale or sandstone. Such occurrence suggests that Callistina lived on the surface of accumulating sediment rather than buried in the bottom mud, but knowledge that modern venerids are shallow burrowers leads me to conclude otherwise. Lenslike distribution of fine sand and silt and cross lamination of sandstone and siltstone beds in the lower part of the Graneros manifest sediment reworking by wave and current action that in turn account for the present orientation of the C. lamarensis valves. Depth of reworking need not have been great because this species, like certain modern venerids, may not have been completely buried while in its burrow. Even so, proposed reworking of the sea floor must have been very general, because I observed no specimens in situ. Transport suffered by exhumed valves varied considerably. Molds in shale and in silt lenses are complete and have sharply defined growth ridges, and articulated valves are preserved in shale at Locality 14. Molds in sandstone beds are commonly of broken valves, but many are complete and growth ridges are undamaged. The large number of Callistina seen in sandstone beds at a few exposures likewise accumulated as a result of bottom scour, probably during occasional periods of greater than usual current activity. Selective orientation of valves at these places is accepted as proof of current action; valve counts for five slabs are given in Table 8. Callistina valves in the coquinoidal limestone lens at Locality 5 range from small fragments to valves that are paired. All specimens lack signs of wear and were probably not transported far before being heaped together on the sea floor.
Table 8--Valve orientation of Callistina lamarensis.
| Sample | Valves convex upward |
Valves concave upward |
|---|---|---|
| KG-5-F | 47 | 0 |
| KG-20-J | 78 | 8 |
| KG-28-F | 70 | 3 |
| KG-40-M | 93 | 7 |
| KG-41-C | 45 | 5 |
Although Callistina valves at times were scattered widely on the sea floor, none bear evidence of attachment of sessile benthonic forms such as serpulid worms, bryozoans, and cirripeds that are so abundant in some parts of the Kansas Upper Cretaceous. I found one Exogyra that had been attached to Callistina and three Callistina valves to which Discinisca was attached. I believe the paucity of epifaunal elements to have been the result of highly turbid water and generally unstable substrate which virtually prohibited development of an epifauna during deposition of beds in the C. lamarensis Assemblage Zone.
Callistina lamarensis furnishes no direct evidence regarding ancient salinity. The species occurs locally in uppermost Dakota strata that represent the shore zone of the Western Interior Sea and ranges locally upward into the Ostrea beloiti Assemblage Zone that I consider to represent water of normal or near-normal salinity. Callistina lamarensis is most abundant in association with strata and fossils believed to represent water of less than normal salinity and the species may have preferred this environment, but the Dakota and O. beloiti Zone occurrences of C. lamarensis suggest that this clam tolerated a wide range of salinity.
The chief occurrences of Exogyra columbella are in the coquinoidal limestone lens at Locality 5 and sandstone beds in which specimens have been concentrated by current action. All valves that I collected in the lens, save one pair, are disarticulated and the right valves are sparse. The turbulence that produced the shell-bearing lens caused separation of the valves and concentrated the left ones. Near the base of the formation at Locality 37, valves are also disarticulated, but right valves are common and several left valves lie with the concave side up. Apparently these specimens were moved little, if at all, from the site of growth.
Attachment scars are plainly visible on a few specimens of Exogyra and lie on the outer surface of the hooked beak. The angle between the plane of attachment and the plane of commissure ranges from 39 to 120? degrees in five specimens. The left valve was oriented beak downward and ventral margin uppermost. The nature of the substratum could be determined for only one specimen, the beak area of which bears the clear impression of Callistina. During early Graneros deposition few large objects on the sea floor, other than Callistina, were available to serve as a substratum for Exogyra. The impression of the umbonal region of one specimen on the umbonal region of another is preserved on a few left valves and was caused by crowded living conditions in the sparse habitable areas of the sea floor. At no place did I find evidence of banklike development of Exogyra where younger generations could have grown upon shells of the old. The footholds gained occasionally by this species were evidently ephemeral. The small size and thin shells of most Graneros specimens of Exogyra columbella suggest a short life span.
Like most other pelecypods in the Callistina lamarensis Assemblage Zone, Exogyra columbella has a limited stratigraphic distribution, including only the uppermost part of the Dakota Formation and lower part of the Graneros Shale. Although E. columbella has been collected elsewhere in rocks as young as the upper part of the Greenhorn, I have seen this form only sparingly in the Greenhorn of Kansas, and then only in skeletal limestone beds lying at or near the base of the formation. The stratigraphic distribution of this species in central Kansas may reflect an ecologic requirement of relatively high energy levels in the depositional environment. Because of the usual occurrence of E. columbella in sandy rocks of the Mancos Shale, Pike (1947, p. 21) believed that the species inhabited a nearshore environment and, although occupying a very limited vertical range locally, might not be a precise indicator of age. For species of Exogyra having a very small shell, Jourdy (1924, p. 35) postulated a precarious life in an environment characterized by shallow, turbid water and considerable current activity. The conclusions of Pike and Jourdy are entirely compatible with lithologic and stratigraphic evidence bearing on the depositional environment of Graneros beds containing E. columbella.
Recent North American clams of the genus Corbula are characteristic of the warmer waters adjacent to the Continent. Corbula is distributed from the littoral zone to depths of several hundred fathoms and some species, such as Corbula swiftiana Adams, extend through a bathymetric range of 2500 feet or more. Most records that I have examined would suggest that corbulids have a preference for a mud bottom, but the genus has been found living in silt, sand, and gravel as well. Keen (1958, p. 208) notes that corbulids are common in the intertidal zone where specimens are attached by a byssus in gravel or under rocks. The European species Corbula gibba Olivi burrows in muddy, gravelly sand where it lies with the posterior margin at or just below the sediment surface and is attached to a shell or stone by a single byssal thread (Yonge, 1964, p. 362). Although most American species of Corbula prefer water of normal salinity, some, such as the Gulf of Mexico species Corbula contracta Say, apparently tolerate at least moderately brackish water (Parker, 1956, P: 326), and Keen (1958, p. 208) mentions one aberrant Central American species that occupies mangrove swamps and areas near mouths of streams. Consideration of the wide variety of habitats occupied by modern species of Corbula, the wide bathymetric range of some species, and the wide variety of substrate or sediment types inhabited by others, renders the paleoecological interpretation of an extinct species of corbulids difficult, let alone an extinct genus of that family, which can only be compared with modern forms.
Graneros corbulids are best known and most abundant in the coquinoidal limestone lens at Locality 5 where right and left valves occur in a ratio of nearly 3 to 1 and are in an excellent state of preservation. One set of paired valves was collected from the coquinoidal limestone. Seemingly, many of these specimens, like Callistina, Yoldia?, and Nuculana in the same lens, probably were not transported far before burial. Parmicorbula? hillensis var. was observed at a few other localities in shale that I believe represents the original mud-bottom habitat preferred by the living clam. The gaping posterior end of the right valve of this species is indicative of a well-developed siphon, hence a burrowing habit, but no specimens were observed in the life position. Like the associated clams, the present orientation of Parmicorbula? valves is further evidence that the sea floor was reworked sufficiently often and deeply enough to displace all of these shallow-burrowing pelecypods. Lingula, which today is a relatively deep-burrowing form, alone among Graneros endobenthonic forms, has been discovered in a position suggesting in situ preservation.
Nuculanid clams in the Graneros, including the genera Nuculana and Yoldia?, are represented in Recent oceans by species of the same two genera. In North America Yoldia is distributed from the Arctic Ocean to the Gulf of Mexico, including waters off both the east and west coasts of the conterminal United States. According to Maury (1920, p. 8), all species of Yoldia in the Gulf of Mexico are deep-water forms because the genus favors cold water and is typical of the Arctic and Antarctic regions. Off the California coast Yoldia is typically a deepwater form, but in Atlantic waters Yoldia occurs in shallow water as far south as Cape Hatteras and just below the low tide mark as far south as southern New Jersey (Abbott, 1954). Some species tolerate enormous bathymetric ranges. The genus characteristically occurs in water of normal salinity but is known locally from polyhaline water as, for example, in Long Island Sound where Yoldia limatula lives in waters having a maximum salinity of 29.2%0 (Sanders, 1956, p. 398). Yoldia is a vagrant, shallow-burrowing form and prefers mud bottoms. Individuals of Y.? subacuta var. in the Graneros were probably not as rare as my observations would suggest. Yoldia? shells are preserved at only one locality; nuculanid molds from a few other localities could belong to either Nuculana or Yoldia?, but positive identification is impossible. Valves of these genera are small and could be overlooked easily despite our intensive laminaby-lamina fossil search at many localities. The generally muddy bottom that existed during Graneros deposition is compatible with knowledge of modern Yoldia ecology. However, none of the Graneros nuculanid molds was oriented as in life, so these clams, like Callistina, apparently suffered pre-burial transport, also.
Like Yoldia, Nuculana lives today in all water bodies surrounding North America through a bathymetric range from low tide mark to the bathyal zone and prefers a mud bottom. In her book on tropical west American seashells, Keen (1958, p. 17) notes that Nuculana is characteristically a northern genus that prefers cold water. Nonetheless, some species have been observed in very shallow water in the tropics. Although most American species of Nuculana occur in offshore waters of normal salinity, some inhabit somewhat brackish water in the Gulf Coast Region (Parker, 1956, 1959, 1960).
Modern representatives of nuculanid genera found in the Graneros Shale occupy such a wide range of habitats that direct environmental comparison with the extinct species is hazardous. Nuculanid clams comprise only a small part of the Callistina lamarensis assemblage and for this reason are of little paleoecologic value.
The virtual restriction of Ostrea to the upper to to 15 feet of the Graneros might be taken as evidence of differential preservation because most of the calcareous beds in which oysters are abundant and well preserved lie in that portion of the section. However, noncalcareous shale units lying between calcareous beds contain identifiable Ostrea at several places and noncalcareous sandstone in the lower half of the formation contains Ostrea shells in at least one section (Loc. 18). Thus the paucity of Ostrea in noncalcareous sandstone and shale in the lower half of the formation is not owing to post-depositional solution but rather to some ecologic factor or factors. The stratigraphic and geographic distribution of these oysters essentially parallels that of ammonites, abundant planktonic foraminifers, and calcareous beds, all of which suggest water of normal salinity. I conclude, therefore, that the environmental factor having greatest influence in controlling distribution of Ostrea beloiti was salinity. This control may not have been direct, however. A fragment of an Inoceramus shell, or at least the impression of this genus, is discernable on the attachment surfaces of all larger specimens of lower valves of these oysters. Second-generation oysters commonly gained footholds on the valves of older specimens, but none of the oysters are attached to the shells of any other kinds of organisms. The only substrate generally suited to the development of Ostrea beds thus seems to have been Inoceramus valves. Where Inoceramus could not live, Ostrea did not become established. Inoceramus does occur sparingly in the lower part of the Graneros at Localities 3 and 14 but the molds observed at these localities are of a different species than that to which upper Graneros oysters were attached. Inoceramus fragments occur in calcareous sandstone 5 feet above the base of the Graneros at Locality 7, and shell fragments possibly belonging to Ostrea were also observed in this bed, but the ecologic relationship of these sparse and fragmentary remains is unknown.
In most beds where Ostrea is abundant, the specimens are not oriented uniformly, the attached valve commonly lying with the lower surface uppermost. At Locality 15, one cluster of oysters was in an upright position on the paired valves of a large Inoceramus and the assemblage may be in the position of growth; however, other oysters in the same bed are overturned. All observed Ostrea valves are disarticulated, but many remain unbroken. The common occurrence of these relatively well-preserved oysters in skeletal limestone, clean calcareous sandstone, and some conglomeratic beds suggests that disruption of shell beds and disarticulation of valves occurred during periods of turbulence (probably storms) when the sea floor was stirred by wave or current activity which concentrated well-washed coarser detritus and shell debris. The small size of Ostrea biostromes in the Graneros is apparently a consequence of sporadic turbulence. In several respects Ostrea beloiti is much like O. congesta of higher parts of the Colorado Group in Kansas, that is, in growing upon Inoceramus, in having a lower valve with large attachment area and nearly upright outer wall, in possessing a relatively thin shell, and in growing, where crowded, in low-lying clusters. In general form Ostrea beloiti is not unlike O. equestris Say, a modern American high-salinity oyster.
Identifiable specimens of Inoceramus rutherfordi associated with Ostrea beloiti are common at only a few localities; therefore, little can be said concerning ecology of the latter except for their possible control over distribution of Ostrea and, because of similar stratigraphic occurrence, the relatively high salinity that was apparently necessary for their growth. Indeed, Vokes (1948, p. 128) believed that Inoceramus usually inhabited water of normal salinity. The fragmentary condition of Inoceramus remains in most skeletal limestone beds can be attributed to shell fragility as well as to occasionally turbulent environmental conditions. Beds of skeletal limestone in which the grains consist largely of isolated prisms of Inoceramus attest to the delicate nature of the valves. Oyster valves are commonly whole in limestone beds where scarcely a fragment of Inoceramus is large enough to be detected by the naked eye.
Ammonites
The restriction of ammonites to the upper assemblage zone of the Graneros Shale is more likely due to environmental control than to lack of preservation of such fossils in the lower part of the formation. The evidence of foraminiferal and calcareous-rock distribution, discussed above, indicates that salinity of the Western Interior Sea was increasing as Graneros deposition proceeded. The stratigraphic distribution of ammonites is similar to that of planktonic foraminifers and calcareous rocks and seems, therefore, to be also closely related to salinity. Scott (1940, p. 308) has noted the preference of modern cephalopods for water of normal salinity. Ammonites are most common in the Smoky Hill and Saline River areas of the Graneros outcrop and calcareous rocks are also most in evidence there. I observed calcareous foraminifers lower in the section here than elsewhere. Increase of salinity during the Graneros transgression may have occurred somewhat earlier here than in adjacent areas, with resulting development of more fully marine conditions as the upper part of the formation was being deposited. This conclusion excludes the Ford-Hodgeman county area where any sediments of the Ostrea beloiti Assemblage Zone were removed by erosion prior to inception of the Greenhorn deposition.
Cephalopod remains are common to abundant in only three of the sections I studied. At Localities 32 and 33, where specimens are in coquinoidal limestone lenses, the association of fragmentary fish remains, whole and broken oyster shells, Inoceramus shells and prisms, gastropods, quartz sand grains, and the broken condition of many ammonite conchs is evidence that the fossils were concentrated by storm activity when bottom sediments were deeply disturbed, fine sediments were set in suspension, and assorted coarse debris accumulated through the agency of wave or current action. Strongly sculptured conchs of Plesiacanthoceras amphibolum may be an adaptation to the occasionally turbulent kind of environment that produced the several shell and pebble beds that lie in the upper 15 feet of the Graneros.
The host of ammonite remains collected at Locality 43 are mostly well-preserved molds that lie parallel to the bedding at several horizons. A bedding plane on one shale block contains at least 19 specimens of Borissiakoceras reesidei and one of Plesiacanthoceras amphibolum in an area of 16 square inches. Diameter of the B. reesidei molds ranges from 3.8 to 19 mm. Concentration of these ammonites by gentle current action is manifest in the scattered assortment of shell fragments, fish scales, bones, and coprolites that lie on the same bedding plane in rock that otherwise consists mostly of homogeneous clayey shale.
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Kansas Geological Survey, Geology
Placed on web Dec, 15, 2014; originally published December 1965.
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