STRONTIUM ISOTOPIC ANALYSIS OF TEPHRA: A CASE STUDY FROM THE WESTERN INTERIOR SEAWAY

Bentonite, a claystone authigenically formed from devitrified volcanic ash, retains specific chemical signatures from the hydrologic system in which the clay was precipitated. Radiogenic isotopes preserved in bentonite layers (e.g. Sr) have not been analyzed in detail, and could be used to infer oceanic vs volcanic input based on the strontium isotopic ratio. This study analyzes the ⁸⁷Sr/⁸⁶Sr data for a suite of Cenomanian (Late Cretaceous) bentonites from the Western Interior Seaway deposits of southwestern South Dakota, and shows strontium signatures of the volcanic material are not associated with oceanic strontium signatures.

The strontium isotope signature of ⁸⁷Sr/⁸⁶Sr was determined for 11 individual bentonite layers collected near Hot Springs, SD and dated to the Cenomanian (approx. 97.9 +-1.4 Ma). In addition, a series of calcitic fossil material (bivalves and ammonoids) were collected to test for evidence of meteoric diagenesis and to determine possible similarities between inorganic and biogenic strontium. XRD analysis confirmed the bentonite clay content to be nearly 95-100% dioctahedral montmorillonite. The isotopic analysis of carbonate material fell within a range of .707389 to .707539, with an average of .707444. The ⁸⁷Sr/⁸⁶Sr signature of the fossil material precisely matches the well-documented global strontium ratio during the Cenomanian, approximately .7074, demonstrating an absence of any significant diagenetic alteration.

After applying a rubidium correction for post-depositional radiogenic Sr, the ⁸⁷Sr/⁸⁶Sr values of sampled bentonites range from .70762 to .70782, the average being .70769. The average ⁸⁷Sr/⁸⁶Sr of the bentonites differ from the oceanic average ⁸⁷Sr/⁸⁶Sr by .0002 during the Cenomanian, a significant variation in terms of strontium content. Timing of the Cenomanian ash deposits coincide with the accretion and assimilation of exotic island arc terranes to the western border of Laurentia. It appears that the strontium values preserved in the bentonites are recording the radiogenic signatures of these magmatic events rather than ocean water, and therefore, could be used to reconstruct the timing of terrane accretion and associated magmatism along convergent margins.