POTASSIUM-ARGON DATES OF UPPER CRETACEOUS ASH FALLS, ALBERTA, CANADA*

Calibrating the zenith of dinosaur diversity in the Campanian of the Western Interior Basin by CA-ID-TIMS U-Pb geochronology

The spectacular fossil fauna and flora preserved in the Upper Cretaceous terrestrial strata of North America's Western Interior Basin record an exceptional peak in the diversification of fossil vertebrates in the Campanian, which has been termed the 'zenith of dinosaur diversity'. The wide latitudinal distribution of rocks and fossils that represent this episode, spanning from northern Mexico to the northern slopes of Alaska, provides a unique opportunity to gain insights into dinosaur paleoecology and to address outstanding questions regarding faunal provinciality in connection to paleogeography and climate. Whereas reliable basin-wide correlations are fundamental to investigations of this sort, three decades of radioisotope geochronology of various vintages and limited compatibility has complicated correlation of distant fossil-bearing successions and given rise to contradictory paleobiogeographic and evolutionary hypotheses. Here we present new U-Pb geochronology by the CA-ID-TIMS method for 16 stratigraphically well constrained bentonite beds, ranging in age from 82.419 ± 0.074 Ma to 73.496 ± 0.039 Ma (2σ internal uncertainties), and the resulting Bayesian age models for six key fossil-bearing formations over a 1600 km latitudinal distance from northwest New Mexico, USA to southern Alberta, Canada. Our high-resolution chronostratigraphic framework for the upper Campanian of the Western Interior Basin reveals that despite their contrasting depositional settings and basin evolution histories, significant age overlap exists between the main fossil-bearing intervals of the Kaiparowits Formation (southern Utah), Judith River Formation (central Montana), Two Medicine Formation (western Montana) and Dinosaur Park Formation (southern Alberta). Pending more extensive paleontologic collecting that would allow more rigorous faunal analyses, our results support a first-order connection between paleoecologic and fossil diversities and help overcome the chronostratigraphic ambiguities that have impeded the testing of proposed models of latitudinal provinciality of dinosaur taxa during the Campanian.

The Cretaceous period

Fifty significant items from the large body of radiometric data on the Cretaceous period are reviewed and the following dates are acceptedas a basis for discussion: Jurassic-Cretaceous (Volgian-Ryazanian) boundary: 136 m.y.; Lower Cretaceous-Upper Cretaceous (Albian-Cenomanian) boundary: 100 m.y.; Upper Cretaceous-Tertiary (Maestrichtian-Danian) boundary: 64 m.y. A precise scale of geochronology for this period is impeded by inadequate stratigraphical dating of igneous events, unreliability of results from sedimentary sources (glauconite), and lack of international agreement on the limits of the Cretaceous System.

Glauconite dates from the Upper Jurassic and Lower Cretaceous

Few dates are available from stratigraphically well-documented igneous materials of late Jurassic and early Cretaceous age. Glauconite dates therefore have a special interest despite the geochronological limitations of the mineral. Evidence from the Albian and Cenomanian, moreover, shows that some Mesozoic glauconites give results only slightly lower than the probable correct dates.

New glauconite determinations from sedimentary rocks near the Jurassic–Cretaceous boundary include four English dates close to 130 m.y. Those from Greenland and Russia are lower, ranging between 107 and 117 m.y. Published information concerning comparable stratigraphical horizons also includes low values and several near 130 m.y.

Our results suggest that the Jurassic–Cretaceous boundary (sensu borealis) is a little older than 130 m.y. They are thus consistent with the figure of 135 m.y. assigned to the boundary by Holmes and Kulp.