Abstract

Samples from 13 stratigraphic sections in Oklahoma, Texas, Utah, and Nevada were studied in conjunction with an investigation of conodonts in Late Mississippian (Chesterian) and Early Pennsylvanian (Morrowan) strata in the western United States. The formations studied include the Barnett, Chainman, Indian Springs, Soapstone and Pitkin, which are entirely or in part Chesterian in age, and the Marble Falls, Bird Spring, Ely, Moleen, Round Valley, Hale and Bloyd Formations, which are entirely or in part Morrowan in age.

Data pertaining to the placement of the Mississippian-Pennsylvanian and lower and upper Carboniferous boundaries are reviewed; it is concluded that the Mississippian-Pennsylvanian boundary, based on the occurrence of the conodont genus Rhachistognathus, occurs within the lower part of the western European Homoceras (H1) Goniatite Zone.

Several broadly occurring but seemingly thin conodont biostratigraphic zones believed to be useful for intercontinental correlations are established for Late Mississippian (Chesterian) and Early Pennsylvanian (Morrowan) rocks. These are: (1) the Rhachistognathus Zone, believed to be essentially equivalent to the Homoceras (H1 and H2) Goniatite Zone, and which falls in strata of both Chesterian (Late Mississippian) and Morrowan (Early Pennsylvanian) age; (2) the Streptognathodus expansus-S. suberectus Zone, believed to be equivalent to the upper part of the G1 and possibly to the lower part of the G2 goniatite zones; (3) the Idiognathodus humerus-1. sinuosis Zone, believed to be equivalent to the lower part of the G2 Goniatite Zone; and (4) the Streptognathodus parvus-Adetognathus spathus Zone, believed to be equivalent to the upper part of the G2 Goniatite Zone. The latter three zones occur in strata of middle to late Morrowan age. The Kladognathodus-Gavusgnathus naviculus Zone, the Adetognathus unicornis Zone, the Gnathodus girtyi simplex Zone, the “Idiognathoides” aff. “I.” nodulifera and /or “Streptognathodus” noduliferus-ldiognathoides convexa zones (= Declinognathodus noduliferus Zone of this paper) and “Gnathodus” basslen zones, established by earlier authors, are all or in part recognized in the western United States.

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