Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 252, part 2
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Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 252, part 2
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Next Page--References
The strata that bracket the Carboniferous-Permian boundary record sedimentation on a broad shallow marine shelf that sloped southward toward the Anadarko basin. The midcontinent epeiric sea had a limited connection to the open ocean to the southwest between the Amarillo-Wichita and Sierra Grande uplifts (McKee and Oriel, 1967; Hoy and Ridgeway, 2003). According to paleogeographic reconstructions (Scotese and McKerrow, 1990; Scotese, 2004; Blakey, 2005), the midcontinent was located in tropical to subtropical latitudes north of the equator and gradually migrated northward throughout Permian time. The narrow (∼100-km/62-mi) passage between the Amarillo-Wichita and the Sierra Grande uplifts shallowed from a basinal to restricted shelf environment between the Late Carboniferous and Early Permian (Dutton and Goldstein, 1988) during a declining migrating deformation of the foreland (Dorobek, 2004) as the Marathon-Ouachita thrust belt migrated southward into Texas (Dickinson and Lawton, 2003). Increasing restriction and shallowing of the passage due to tectonism and a trend of increasing aridity during the Wolfcampian contributed to the increasing presence of episodic evaporite accumulation during regressions. The seaway eventually became more shallow and restricted, resulting in the abrupt onset of extensive subaqueous evaporite accumulation during the Leonardian (Watney et al., 1988).
Wolfcampian (lowermost Permian) strata are cyclic in character and, like upper Carboniferous (Pennsylvanian) cyclothems, probably record glacio-eustatic fluctuations in sea level as well as paleoclimatic and tectonic influences (e.g., West et al., 1997). Although interpretations of exact depositional environments and water depths have varied somewhat between different studies conducted on the strata encompassing the Carboniferous-Permian boundary, all who have reported on these strata show that the rocks record relative sea-level fluctuations of different scales.
Early work studied the Red Eagle Limestone within the context of the cyclothem concept and defined the Red Eagle cyclothem to include the Johnson Shale, Red Eagle Limestone, and Roca Shale (e.g., McCrone, 1963). McCrone (1963) related the cyclothem facies to water-depth changes and interpreted water depths ranging from 0 to just over 60 ft (18 m), although others had interpreted deeper water for some facies (e.g. Elias, 1937). Mudge and Yochelson (1962) called the Red Eagle the typical Council Grove cyclothem. They interpreted the fusulinid facies of the Glenrock Limestone Member to reflect maximum water depth and the Bennett Shale Member and Howe Limestone Member to be regressive deposits, with the Howe being relatively shallow water. Other studies similarly interpreted the Glenrock limestone as the maximum transgressive unit and the Bennett shale as a restricted, nearshore facies (e.g., McCrone, 1963; Clark, 1989). However, as discussed below, other studies have re-interpreted the Bennett shale as the maximum transgressive unit.
More recent studies have evaluated the Red Eagle Limestone within the context of sequence stratigraphy, focusing on significant surfaces and facies in relation to relative sea-level position. Keairns (1995) termed the Red Eagle cyclothem the Red Eagle Sequence. Wardlaw, Boardman, et al. (2004) recognized the same Red Eagle Sequence as a 4th-order (0.1-1 m.y.) sequence that occurs within a 3rd-order (1-10 m.y.) sequence consisting of the entire Council Grove Group (designated the Council Grove Third Order Sequence). Olszewski and Patzkowsky (2003) also recognize the Red Eagle cyclothem as a 4th-order sequence that they termed Composite Sequence IV. All three of these studies place the basal sequence boundary of the Red Eagle Sequence (or Composite Sequence IV) at a subaerial exposure zone (stacked paleosols) within the Johnson Shale and an upper sequence boundary at a subaerial exposure zone (stacked paleosols) within the Roca Shale. These three studies also recognize higher frequency cyclicity (5th order; 0.01-0.1 m.y.) within the Red Eagle Sequence (or Composite Sequence IV).
Boardman et al. (1995), Boardman et al. (1998), Boardman (1999), and Wardlaw, Boardman, et al. (2004) used facies and biotic relationships, including water-depth-dependent microfossil biofacies analysis, to interpret depositional history and construct 5th-order onshore-to-offshore relative sea-level curves. For the Red Eagle Sequence (or Composite Sequence IV), the marine transgressive surface above the basal sequence boundary (stacked paleosols in the Johnson Shale; Miller, 1994) occurs in the upper part of the Johnson Shale and characteristically is recognized as a thin intraclastic packstone (Boardman, 1999). Transgression continued through deposition of the Glenrock limestone into the Bennett shale. The Glenrock limestone is interpreted as a shallow-water carbonate consisting mostly of sparsely fossiliferous intraclastic packstone to grainstone, with the uppermost portion being a fossil-rich wackestone to packstone characterized by abundant fusulinids (along with coated grains, gastropods, brachiopods, and bivalve debris) and large Thalassinoides-like, or Diplocraterion-type burrows filled with dark-gray to black shale, presumably from the overlying Bennett shale (Miller, 1994; Muehleisen, 1994; Boardman et al., 1998). The contact of the Glenrock limestone with the Bennett shale is sharp (and irregular in places). Boardman et al. (1998) interpreted the contact as a major flooding surface and possible marine hardground, although they do not provide any evidence for a hardground interpretation.
The basal part of the Bennett shale contains the greatest diversity of conodonts (the Streptognathodus biofacies of Boardman et al., 1995) and is interpreted as representing a marine condensed section deposited in an open marine mid-shelf setting. This condensed section at the base of the Bennett shale and a second condensed section, approximately 0.3 m above in the Bennett shale (also consisting of the Streptognathodus biofacies), represent maximum transgression. In addition to the abundant conodonts, orbiculoid brachiopods, ammonoids, and fish debris are found in the condensed sections. After deposition of the basal condensed section, a minor relative fall in sea level is interpreted for deposition of the limestone unit of the Bennett shale prior to subsequent rise and deposition of the second condensed section (Boardman et al., 1995). At Tuttle Creek the limestone unit of the Bennett shale contains abundant remains of fish teeth and orbiculoid brachiopods (Boardman et al., 1998). According to Boardman et al. (1998), the Bennett changes facies from dark shales in northern and central Kansas to dominantly fossiliferous carbonates in southern Kansas, with the two condensed sections still identifiable as a thin, shaly, glauconitic, fossiliferous wackestone with abundant phosphatized mollusks and abundant Streptognathodus conodonts (lower condensed section) or a thin, highly fossiliferous shale with abundant Streptognathodus conodonts (upper condensed section). For Boardman et al. (1995), Keairns (1995), Boardman et al. (1998), Boardman (1999), and Wardlaw, Boardman, et al. (2004), the two condensed sections in the Bennett shale represent the deepest water conditions (maximum transgression) within the Red Eagle Sequence, and for Wardlaw, Boardman, et al. (2004) the deepest water (maximum transgression) for the entire Council Grove Third Order Sequence. This interpretation differs from earlier work that interpreted the Bennett shale as restricted, nearshore facies (Mudge and Yochelson, 1962; McCrone, 1963; Clark, 1989).
The maximum transgression in the Bennett shale is followed by Howe limestone regressive deposits consisting of foraminiferal grainstones that grade (shallow) upwards to coated-grain grainstones and stromatolite facies. Shapiro and West (1999) interpret the Howe stromatolites at Tuttle Creek Lake Spillway as being deposited in a shallow subtidal to intertidal open marine setting during an episodic regression, and coeval stromatolites in Lyon County as suggestive of marginal marine to supratidal settings. Regression culminated with deposition of the Roca Shale containing well-developed green to red blocky mudstone; stacked paleosols (Miller, 1994; Rankey and Farr, 1997) form the upper sequence boundary of the Red Eagle Sequence (or Composite Sequence IV). Thin carbonate units interbedded with the paleosols are interpreted as being deposited in marginal marine environments and are shown as minor flooding events on the 5th-order sea-level curve for the Roca Shale (Boardman et al., 1995; Boardman et al.,1998; Miller, 1994; Miller et al., 1996; Rankey and Farr, 1997).
In addition to the relative sea-level changes that likely reflect a eustatic signature, at least to some extent, the Carboniferous-Permian boundary and associated strata reflect other important broad-scale geologic events and changes. West et al. (1997) noted that the boundary reflects a rather abrupt change in the lithologic character of cyclothemic deposition that can be at least partially attributed to climatic change associated with tectonically driven paleogeographic changes (e.g. northward paleolatitudinal shift of the midcontinent) and final assembly of Pangea. Climatically, the Wolfcampian seems to have been a time of major transition from generally wetter conditions in the Virgilian to significantly drier in the Leonardian and Guadalupian. The apparent establishment of a Northern Hemisphere monsoonal circulation by latest Carboniferous to Early Permian, coupled with increasingly more restricted epeiric seaways along the western equatorial Pangea, led to a transition from seasonal humid (Virgilian), to dry (Wolfcampian), to nonseasonal dry (Leonardian) (Tabor and Montanez, 2002).
West et al. (1997) noted that Wolfcampian strata developed during a period of extensive glaciation, similar to conditions during Carboniferous (Desmoinesian) deposition. Ross and Ross (1987) suggested that eustasy even increased during the early-to-mid Permian. West et al. (1997) postulated that differences in facies composition between Wolfcampian and Desmoinesian cyclic deposits, and an apparent shallowing-upward trend in Permian strata, could also have resulted from continental elevation changes due to thermal uplift (for example, insulating effects of the supercontinent [Anderson, 1982]) and changes in volume of mid-ocean ridges, both of which could have muted the effects of changing glacial ice volume. The migration of Marathon-Ouachita thrusting and foreland flexure to more distal sites in the early Permian, and further assembly of Pangea along the Ural suture zone eventually lead to formation of the supercontinent and increasing influence of these other tectonic processes. This scenario may have support as reflected in the 5th-order sea-level curves of Boardman et al. (1995), Boardman et al. (1998), Boardman (1999), and Wardlaw, Boardman, et al. (2004). Following the maximum transgression in the Bennett shale (believed to be the maximum flooding event for the early Permian according to Keairns, 1995), the curves show that magnitudes of sea-level fluctuations are less for cyclic deposits in the rest of the Council Grove and Chase Groups as compared to earlier deposited cyclic strata.
In summary, in addition to being a significant biostratigraphic boundary, the Carboniferous-Permian boundary and enclosing strata also have significance because they reflect important geologic events and changes that occurred on a regional and global scale.
The new Carboniferous-Permian system boundary in Kansas falls within a lithostratigraphic unit, the Red Eagle Limestone. The appropriateness of leaving it within the lithostratigraphic unit, or splitting the Red Eagle into two formations at the system boundary, was considered. In accordance with Article 22 of the North American Stratigraphic Code (2005), the boundary will be left within the Red Eagle Limestone.
The North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature, 2005, Article 22, p. 1,566 ) defines a lithostratigraphic unit as "a defined body of sedimentary, extrusive igneous, metasedimentary, or metavolcanic strata which is distinguished and delimited on the basis of lithic characteristics and stratigraphic position. A lithostratigraphic unit generally conforms to the Law of Superposition and commonly is stratified and tabular in form." Article 22, Remarks (e), "Independence from time concepts" (North American Commission on Stratigraphic Nomenclature, 2005, p. 1,566) also stated "The boundaries of most lithostratigraphic units are time independent, but some may be approximately synchronous. Inferred time-spans, however measured, play no part in differentiating or determining the boundaries of any lithostratigraphic unit." Article 49 of the Code (North American Commission on Stratigraphic Nomenclature, 2005, p. 1,574) defines a biostratigraphic unit as "...a body of rock defined or characterized by its fossil content." The basic unit in biostratigraphic classification is the biozone, of which there are several kinds. Article 49, Remark (c), "Independence from lithostratigraphic units" stated "Biostratigraphic units are based on criteria that differ fundamentally from those for lithostratigraphic units. Their boundaries may or may not coincide with the boundaries of lithostratigraphic units, but they bear no inherent relation to them."
Thus, it is not inappropriate to have a biostratigraphic boundary within a lithostratigraphic unit, whether it is a member, formation, or group, and therefore, it is unnecessary to change the boundaries of any of these lithostratigraphic units. In the case of the Carboniferous-Permian boundary in Kansas, that biostratigraphic boundary is at the base of the Bennett Shale Member of the Red Eagle Limestone within the Council Grove Group.
Gregory P. Wahlman and Scott Ritter provided careful, constructive reviews of the manuscript, and we are grateful for their time and expertise. Marla Adkins-Heljeson is thanked for editorial assistance, and Jennifer Sims is thanked for graphic arts support.
This paper was completed by current members of the Kansas Geological Survey's Stratigraphic Nomenclature Committee, which was re-established in July 2005, to address stratigraphic issues and establish formally accepted stratigraphic nomenclature for Kansas. The Stratigraphic Nomenclature Committee is the official arbiter of stratigraphic nomenclature and issues in Kansas, subject to review by the State Geologist.
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