Applications of conodonts in resolving Pennsylvanian-Permian stratigraphic problems in north-central Texas, New Mexico, Colorado, and Kansas.

摘要

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, the Sacramento Mountains of New Mexico, and Central Colorado. 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 faunas across the boundary interval. Correlations were based on first appearances of the four Streptognathodid species, and then a second level of confidence obtained with percentages of faunal distribution.; North central Texas and Midcontinent Kansas conodont biostratigraphy, based on the newly developed zonations were able to successfully correlate the Stanton, South Bend, and Iatan cycles of the Midcontinent with the Upper Winchell, Placid-Ranger dual cycle, and Colony Creek of north central Texas. The correlation, and new zonation was extended into New Mexico resulting in correlations of the Upper Winchell, Placid-Ranger, and Colony Creek with the K, I3, and I2 cycles of the Beeman and Holder Formations in New Mexico. The new zonation and correlations to New Mexico enforce the age of significant phylloidal mound building as Missourian.; In Central Colorado, conodont collections from the Kerber and Sharpsdale formations (Morrowan-Atokan) were assessed for possible biostratigraphic correlation to other regions of Colorado. Sampled intervals did yield conodont elements, but none that could be successfully correlated with comparable biostratigraphic data. Recommendations are to sample the Central Colorado equivalent, the Belden Shale for additional, and hopefully, diagnostic conodont faunas.