Composition, radioisotopic ages, and potential significance of an altered volcanic ash (bentonite) from the Upper Cretaceous Judith River Formation, Dinosaur Provincial Park, southern Alberta, Canada

Abstract

A detailed integrated stratigraphic, sedimentologic, petrographic, geochemical and radioisotopic study of a Judith River Formation bentonite (the Plateau Tuff [PT]) from Dinosaur Provincial Park, southeast Alberta, Canada was undertaken 1) to establish a precise radioisotopic age for the PT and associated vertebrate fossils, and determine the reproducibility of radioisotopic data from the deposit, 2) to elucidate the depositional and diagenetic history and possible provenance of the PT, and 3) to conduct a comparative analysis of the geochemical compositions of the PT and associated ashes to assess whether sufficient variation exists for individual bentonites to be identifiable on the basis of their geochemical ‘fingerprints’. The PT represents the diagenetically altered remains of a non-reworked, vitric-crystal ash deposited and preserved within a shallow, areally extensive floodplain lake. Two or more accumulation events extending over a brief time interval (hours to days?) resulted in its deposition. The original composition of the PT's parent magma was rhyodacitic to rhyolitic. Trace element discrimination indicates a volcanic-arc tectonic setting for its subaerial, Plinian eruptive center, which was most likely located within the Elkhorn Mountains Volcanics complex of southwestern Montana. Radioisotopic dating of biotite and sanidine phenocrysts was conducted using conventional bulk-fusion K-Ar (biotite) and laser-fusion 40Ar/39Ar (biotite and sanidine) methods. 40Ar/39Ar laser-fusion analysis of sanidine is considered more precise and yields a reference age of 76.11 ± 0.22 Ma for this ash. Although the 40Ar/39Ar biotite and conventional K-Ar biotite results are concordant to one another and to the 40Ar/39Ar sanidine results within ls analytical error, the biotites serve only as a check on the more reliable sanidine results. Geochemical ‘fingerprinting’ of the PT and related Judith River Formation bentonites within and adjacent to Dinosaur Provincial Park (using principal component and factor analysis of XRF data) shows that fresh PT samples and a sample of a possible laterally equivalent ash unit (CH3-83) are distinguishable from other stratigraphically discrete Dinosaur Provincial Park bentonites as well as from weathered and/or contaminated PT samples. Radioisotopic dates for the PT and other bentonites bracketing the fossiliferous exposures of the Judith River Formation in Dinosaur Provincial Park are up to four million years younger than those of ashes from portions of the formation in northern Montana. The PT is equivalent in age to a bentonite directly underlying a ‘new’ dinosaur egg and ‘baby’ locality in southwestern Alberta.