STRONTIUM ISOTOPE STRATIGRAPHY OF THE COMANCHEAN SERIES IN NORTH TEXAS

Pioneering studies in north Texas and adjacent Oklahoma played an important role in the paleontologic and stratigraphic development of the Cretaceous Period in North America. We have used 129 strontium isotope analyses from fossils and rocks collected from 43 sites to define the "middle" Cretaceous path of seawater ⁸⁷Sr/⁸⁶Sr during Comanchean time. The section represents an estimated duration of 19.3 Ma, from late Aptian through middle Cenomanian; a time interval lacking paleomagnetic reversals for refined correlation. The ⁸⁷Sr/⁸⁶Sr of seawater rises from 0.707318 ± 13 in the basal Glen Rose Formation to reach a peak of 0.707522 ± 10 in the Walnut Clay, then slowly declines to 0.707468 ± 6 in the middle Grayson Marl. The upper Grayson Marl and lower Buda Limestone drop to 0.707421 ± 9 then rise to near 0.707473 in middle Buda time. The ⁸⁷Sr/⁸⁶Sr of the basal Woodbine Formation in the overlying Gulfian Series is within error of the middle Buda value. Thirty of the analyzed samples yielded ratios outside error of the mean determined for corresponding stratigraphic units. Whole rock limestones and spar fillings of shells were inconsistently more radiogenic than the best shell material and all but two aberrant samples could be identified using appearance and trace element content. Isotopic results from high quality shells with low Mn and Fe concentrations are remarkably consistent. The agreement of results from samples collected at a single site with results from geographically removed sites in the same formation interval is strong evidence that these samples retain the original open marine ratio of Comanchean seawater. The composite data set contributes to a sparsely studied and enigmatic portion of the Cretaceous ⁸⁷Sr/⁸⁶Sr seawater evolution curve. Although minor strontium isotope variation characterizes much of Comanchean deposition, refined correlation opportunities exist for the Glen Rose rise and upper Grayson-lower Buda fall in ⁸⁷Sr/⁸⁶Sr.