High-precision U–Pb zircon ID–TIMS dating of two regionally extensive bentonites: Cenomanian Stage, Western Canada Foreland BasinThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.
The stratigraphy of Cretaceous rocks in the Western Canada Foreland Basin is well constrained by dense borehole data that allow three-dimensional mapping of transgressive–regressive events, paleogeography, and subsidence patterns. However, it is difficult to estimate rates of change or to place events in a precise temporal framework because very few of the many bentonite beds have been dated using modern techniques. In this study, two bentonites, of at least regional extent, were dated using isotope dilution – thermal ionization mass spectrometry (ID–TIMS) U–Pb methods on chemically abraded zircon crystals. The older bentonite correlates with the “X” bentonite of the late middle Cenomanian and yields an age of 95.87 ± 0.10 Ma. The Bighorn River Bentonite, which lies just below the Cenomanian–Turonian boundary, yields an age of 94.29 ± 0.13 Ma and is interpreted to equate with the “B” bentonite and bed 80 at Pueblo, Colorado. Both bentonites dated here are older than the previously reported 40Ar/39Ar ages ...