High-precision U-Pb zircon ID-TIMS dating of two regionally extensive bentonites: Cenomanian Stage, Western Canada Foreland Basin

The stratigraphy of Cretaceous rocks in the Western Canada Foreland Basin is well constrained by dense bore-hole 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 (40)Ar/(39)Ar ages from correlative United States samples, supporting the observation that (40)Ar/(39)Ar ages may systematically underestimate ages of Cretaceous bentonites by similar to 1%, as suggested by other recent studies. The X bentonite immediately precedes a major late middle Cenomanian eustatic transgression that inundated the Dunvegan delta complex. The new ages for the X and Bighorn River bentonites indicate an average minimum subsidence rate of similar to 0.27 mm/year in the most proximal part of the Cenomanian foredeep in northeast British Columbia during this time. The new age of the Bighorn River Bentonite, coupled with the orbitally tuned time scale of Sageman et al. (2006), suggests a Cenomanian-Turonian boundary age of 94.12 +/- 0.13 Ma.