Applications of conodonts in resolving Pennsylvanian-Permian stratigraphic problems in north-central Texas, New Mexico, Colorado, and Kansas
Pennsylvanian-Permian strata of midcontinent North America comprise numerous cyclothems that are the result of repetitive sea-level changes that produced multiple alternations of marine and non marine facies. Placement of chronostratigraphic boundaries within cyclothems is problematic because the variations in sea-level do not allow a continuous representation of marine facies. This creates gaps in the faunal (and floral) record across potential series and stage boundary intervals. Conodont faunas were collected from localities in north-central Texas, Kansas, Oklahoma, and the Sacramento Mountains of New Mexico. These collections were analyzed for biostratigraphically significant faunas that could delineate, with greater resolution, the Late Pennsylvanian Missourian-Virgilian Boundary. Size criteria, in particular the use of adults not juveniles for index species is addressed, determining that indeed size does matter, and that adult forms are the only platforms that should be considered as biostratigraphically significant.
Two new morphotypes of Streptognathodus, Streptognathodus sp. C and Streptognathodus sp. D were identified on the basis of carina length and termination relative to platform length. These new morphotypes combined with existing species Streptognathodus firmus, Streptognathodus pawhuskaensis, and Idiognathus simulator, were sufficient to develop diagnostic faunal zones across the boundary interval. Correlations were based on first appearances of three streptognathodid species, and then a second level of confidence obtained with percentages of species in successive faunas.
The newly developed zonations was successfully used to correlate the Stanton, South Bend, and Iatan cycles of midcontinent Kansas with the Upper Winchell, Placid- Ranger dual cycle, and Colony Creek of north-central Texas. The new zonation was extended into New Mexico permitting correlation of the Missourian-Virgilian boundary strata (South Bend cycle) near cycle 2C-5a (of Raatz). The majority of Dry Canyon samples were stratigraphically above the boundary, as evidenced by the presence of the conodont I. lohatulus, and two forms of S. virgilicus that resemble the new morphotypes S. sp C and S. sp. D. It appears as though the trend of a shortening carina, that was useful at the Missourian-Virgilian boundary may 'reset' and repeat itself higher in the Virgilian.
In Central Colorado, conodont collections from the Kerber and Sharpsdale formations (Morrowan- Atokan) were assessed for possible biostratigraphic correlation to other regions of Colorado. The carbonates of the upper Kerber and lower Sharpsdale are latest Morrowan based on the conodont data. Recommendations are to sample the Central Colorado equivalent, the Belden Shale for additional, and hopefully, diagnostic conodont faunas.