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In the Oklahoma Panhandle, strata of Cenomanian-Early Turonian age contain invertebrate faunas, the study of which adds to the understanding of Western Interior biostratigraphy and furnishes important clues to the physical and chronologic relationships of Colorado and Kansas sections to those of north Texas. The section studied includes upper sandy beds of the Dakota Group; lower shale member, Thatcher Limestone Member, and upper shale member of the Graneros Shale; and the carbonate-rich Lincoln Member, Hartland Member, and Bridge Creek Limestone Member of the Greenhorn Formation. Stratigraphically and lithologically, the Graneros and Greenhorn of Cimarron County, Oklahoma, compare most closely with sections exposing the same units in westernmost Kansas; several bentonite and limestone beds are demonstrably common to the two areas. The Cimarron County section also compares closely with the Cenomanian-Lower Turonian section on the Model anticline of southeastern Colorado except that beds of limestone in the Bridge Creek Member are more brittle and less chalky in the latter area. Comparison of the Oklahoma section with that of north Texas confirms the general similarity of the two stratigraphic sequences but with marked diachrony of lithic equivalents in the lower part of the section due to the southeastward transgression during Cenomanian time. In Texas the Britton Member of the Eagle Ford Formation contains Ostrea beloiti Logan in association with a bentonite seam, which is believed correlative with the X bentonite of the Western Interior and which lies near the top of the Graneros Shale in western Oklahoma. Above this level, calcareous beds of the Britton Member reflect maximum transgression and equate chronologically to lower Greenhorn carbonates of Cimarron County.

Faunas of the Cimarron County section are a mixture of Western Interior and Gulf Coast taxa and include several cosmopolitan or intercontinentally distributed species. Uppermost strata of the Dakota Group yielded only possible worm and arthropod burrows. The lower shale member of the Graneros Shale is apparently barren of all but trace fossils. The Thatcher Limestone Member of the Graneros contains earliest representatives of Inoceramus rutherfordi Warren and is assigned a late Early to early Middle Cenomanian age. The lower part of the upper shale member of the Graneros is assigned to the upper part of the I. rutherfordi Range Zone, and the upper part is assigned to the Ostrea beloiti Range Zone, which is of Middle Cenomanian age. The upper shale member contains the first abundant foraminifera in the section, with benthonic forms predominating in the lower part and planktonic forms predominating in the upper part. Acanthoceratid ammonites used elsewhere to divide the upper Graneros into three widely traceable zones were not recorded in the Cimarron County section.

Biostratigraphic correlation and application of European stage terminology is readily accomplished in the Greenhorn Formation of Cimarron County. Correlation of the Lincoln Member is based on inoceramid bivalves that apparently represent the regional zone of Dunveganoceras pondi Haas. Nine species of planktonic foraminifera make their first appearance at the base of the Lincoln Member, but all are long-ranging forms. Ammonites, bivalves, and planktonic foraminifera are abundant in the Hartland Member and serve to define three local assemblage zones in the lower and middle parts of the member. The lowest zone is characterized by Inoceramus flavus pictoides Sornay and is assigned a Late Cenomanian age. The second and third Hartland assemblage zones are characterized by species of Phelopteria; associated bivalves and cephalopods suggest a Late Cenomanian age. Uppermost Hartland strata contain few recognizable macroinvertebrate fossils but contain a suite of typical Late Cenomanian foraminifera known from the Hartland of other areas.

The Bridge Creek Limestone Member of the Greenhorn Formation is divisible into three regionally widespread zones. The lowest of these is the well-known Sciponoceras gracile Zone, which contains a diverse assemblage of latest Cenomanian fossils and is recognized throughout the Western Interior region and northern Texas. In Oklahoma, the base of this zone marks the extinction level of a great majority of planktonic foraminifera that characterize the Lincoln and Hartland Members. The second assemblage zone marks the base of Turonian strata in Oklahoma and is characterized by Mytiloides opalensis (Böse), which corresponds to the zone of Watinoceras coloradoense Henderson in areas farther to the northwest. The uppermost part of the Oklahoma section is characterized by the zone of Mytiloides mytiloides (Mantell), which is equivalent to at least part of the M. labiatus zone of most American authors. The double-keeled foraminifer Margino-truncana difformis (Gandolfi) and Neobulimina sp. cf. N. albertensis (Stelck and Wall) first appear near the top of the M. opalensis Zone and thus coincide nearly with the base of the Turonian Stage as defined by the base of the M. opalensis Range Zone.

Study of fossil associations served to distinguish the following seven of the ecological assemblages that have been recognized elsewhere in the Western Interior by Kauffman:

1. An assemblage dominated by robust, byssate inoceramid and small ostreid bivalves is found in and adjacent to the Thatcher Limestone Member of the Graneros Shale and represents a high-energy, inner-shelf environment.

2. An assemblage dominated by large, flat, thin-shelled inoceramids is found in the upper part of the Graneros Shale and represents the quiet to gently agitated waters of the midshelf environment.

3. An assemblage dominated by robust, inequivalve, nonbyssate to slightly byssate inoceramids occurs in chalky shale, shaly chalk, calcarenite, and limestone beds of the Lincoln Member, Hartland Member, and Bridge Creek Limestone Member of the Greenhorn Formation and represents a diverse suite of offshore environments that range, depending on variations within this assemblage, from a quiet, foul-bottom situation to a gently agitated, normal-marine shelf.

4. An assemblage dominated by small, thin-shelled pectinids occurs in the lower and middle parts of the Hartland Member and represents relatively deep, quiet-water, offshore-shelf environments or mass-mortality concentrations on single bedding surfaces.

5. An assemblage dominated by subequivalve, slightly to moderately biconvex, nonbyssate to weakly byssate inoceramids occurs sparsely in the upper part of the Hartland and especially in the Bridge Creek Members and represents far offshore, deeper waters having gentle currents.

6. An offshore assemblage dominated by few ammonite taxa is found in the upper part of the Sciponoceras gracile Zone, in the lower part of the Bridge Creek, and represents deeper, quiet-water, open-marine conditions.

7. An offshore assemblage dominated by diverse ammonite taxa is found in the Hartland Member and represents an open-marine, outer-shelf environment.

Dakota, Graneros, and Greenhorn strata described in Part 1 of this paper represent most of the transgressive part of the Greenhorn depositional cycle. Uppermost Dakota strata represent the transition from a marginal-marine to a marine environment along a fluviodeltaic coastline. Sandy shales in the lower member of the Graneros represent the inner sublittoral environment of moderate current and wave action at depths probably not exceeding 30 m (100 ft). The Thatcher Member of the Graneros represents a zone of offshore wave dissipation and depths of probably 15 m (50 ft) or less. The thin, even-bedded shales and the fauna of the upper member of the Graneros Shale represent deeper, quiet-water, farther offshore shelf environments. These were situated beyond the distributional range of coarse terrigenous detritus and experienced only occasional current scour on the sea floor, with water depths probably exceeding 45 m (150 ft). Even-bedded carbonate-dominated strata of the Lincoln and Hartland Members of the Greenhorn Formation represent deposition in quiet water, below wave base and well beyond the transport range of most terrigenous sand and silt. These strata and their associated pelagic and benthonic fossils reflect increased distance to the eastward-migrating shoreline and water depths probably exceeding 60 m (200 ft). Upward increase in amount of bioturbation in the Greenhorn section suggests progressively slower, probably interrupted sedimentation of fine carbonate oozes. The Bridge Creek Member represents the maximum stage of transgression that is expressed in the Cimarron County section; the strata imply deposition far from shore, beyond the distributional range of significant volumes of terrigenous detritus other than clay. In this member, pronounced alternation of purer, burrow-mottled limestones and little-burrowed, laminated chalk beds represent variations in rate of carbonate mud production or terrigenous detrital influx—probably the latter. While lower beds of the Bridge Creek were being deposited, optimum conditions prevailed for development of the pelagic biota, especially including ammonites, foraminifera, and coccolitho-phores. During deposition of upper Bridge Creek beds, the abundance of ammonites and benthonic macroinvertebrates declined markedly.

In Part 2 of this paper, macroinvertebrates and foraminifera of the Cimarron County section are documented, and most species are described in detail. Included i n Part 2 are descriptions or discussion of 16 species of foraminifera, 10 species or species groups of ammonites, 5 species of gastropods, 9 species of noninoceramid bivalves, 13 species of inoceramid bivalves, and 1 species each of brachiopods, corals, and annelids. Trace fossils include acrothoracican cirripeds, Crossopodia, and several forms of burrow structures. New forms include the species Phelopteria minuta Kauffman and Powell and Ehtolium gregarium Kauffman and Powell. New subspecies described here include Inoceramus prefragilis stephensoni Kauffman and Powell, Inoceramus pictus gracilistriatus Kauffman and Powell, and Stomoha-mites simplex cimarronensis Kauffman and Powell.

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