EVIDENCE FOR VERY SLOW ACCUMULATION OF CRETACEOUS (MIDDLE CAMPANIAN) ARDMORE BENTONITES AND LOWER SHARON SPRINGS FORMATION, PIERRE SHALE, EASTERN WYOMING

Closely-spaced radiometric dates from the Pierre Shale reference section near Red Bird, Wyoming suggest that the Middle Campanian Ardmore bentonite series and overlying organic-rich, bentonitic lower Sharon Springs Formation are more highly condensed than previous age models have indicated. In a blind test, preliminary ⁴⁰Ar/³⁹Ar age dates for the two lowest bentonite beds of the Ardmore series, the uppermost bentonite of the series 3m above the base, and a bentonite in the lower Sharon Springs 6m above the top of the Ardmore series yielded dates in the correct stratigraphic order that likely span 2.5 to 3 Ma. The dates from the lowermost two Ardmore bentonite beds are consistent with the published 80.54 ± 0.55 Ma age of the Ardmore. Published biozone boundaries at Red Bird place all four of the new ⁴⁰Ar/³⁹Ar ages within the Baculites obtusus ammonite zone, suggesting that biozone’s duration may be significantly longer than previously thought. The new radiometric dates fit within published, more widely-spaced Middle Campanian radiometric dates; remove a kink from a profile of Campanian Strontium isotopic ratios; and fit the stratigraphic position and age of the C33r/C33n geomagnetic reversal boundary reported by others in the Pierre Shale of South Dakota and Judith River Formation of Wyoming. Furthermore, the new ages and duration of the Ardmore series at Red Bird correspond well with ages of the entire Little Elkhorn Mountain volcanic series in western Montana. Very slow accumulation of the Ardmore series and lower Sharon Springs has implications for the precise absolute age of the geomagnetic reversal boundary; estimates of accumulation rates of Middle Campanian strata throughout the Western Interior Seaway; precise correlation of Middle Campanian strata that occur between marker fossil occurrences; and understanding of subsidence and paleo-water depth. For example, 2.5 my of subsidence during slow accumulation of the Ardmore and lower Sharon Springs contributed to development of water depths as great as 200m in the eastern Powder River Basin. This foreland basin axial trough was subsequently filled by subaqueous clinothems with prograding foresets of rapidly-deposited claystone that downlap and change facies into condensed bottomsets of clayshale rich in marine organic matter.