LATE PALEOZOIC CONODONT SEQUENCE BIOSTRATIGRAPHY OF WESTERN PANGEA: SUCCESSES AND SETBACKS

Several Late Paleozoic cyclothems of Midcontinent North America are characterized by unique and often abundant core shale conodont faunas permit recognition of individual cycles. Application of the Midcontinent conodont sequence biostratigraphic timescale to basins of western North America, however, has yielded only mixed results. In the thicker, carbonate-dominated western sections, a Midcontinent-type core shale is poorly developed or absent and a corresponding conodont fauna more difficult to obtain. Conodont faunas that are recovered from the strongly cyclical carbonates generally come from shallower-water facies than the core shale and faunal differences owing to ecology make identification of the faunas of individual Midcontinent cycles problematic.

At least nine Midcontinent cycles can be recognized in Moscovian and Kasimovian strata of the Paradox basin (eastern Utah) on the basis of their constituent conodonts, thereby enabling correlation of several other cycles by extrapolation. The late Moscovian-Asselian sea level history of the eastern Ely basin (eastern Nevada) has been greatly improved through comparison with the Midcontinent. Rapid sedimentation and low conodont yields complicate correlations in the Oquirrh basin (northern Utah). The exquisitely exposed Moscovian-Kasimovian section at Arrow Canyon (Bird Spring trough, southern Nevada) contains only a few conodont-bearing horizons and these are dominated by generalized idiognathodids. Conodont faunas in Moscovian-Asselian strata in the Pedregosa basin in southwest New Mexico (Horquilla Limestone) are comparable to selected Midcontinent faunas and permit recognition of the Carboniferous-Permian boundary. Deep-water carbonate turbidites of the Keeler Canyon basin of southeastern California yield poorly preserved conodont elements that permit only tentative correlation to the Midcontinent.

This preliminary research suggests that a modified scheme of conodont sequence biostratigraphic units may need to be developed for the western United States, which can then be tied to Midcontinent sequence biostratigraphic timescale as the conodont faunas permit.