Chronostratigraphy of the Late Turonian System, Powder River Basin, WY: insights from integrated U–Pb geochronology, core-to-outcrop stratigraphy, and biostratigraphy

Upper Turonian strata of the U.S. Western Interior Basin contain an anomalous number of shelf-isolated marine sandstones theorized to represent unconformity-bound depositional remnants. High-precision geochronologic constraint is crucial to resolving enigmatic lithostratigraphic correlations of these units. Upper Turonian strata atop the Mid-Turonian Unconformity in the Powder River Basin, Wyoming are investigated through study of shelf-isolated marine sandstones and shales of the Wall Creek Member of the Frontier Formation and the Turner Member of the Carlile Shale. Four new high-precision CA-ID-TIMS U–Pb ages, 41 whole cores, and 19 measured outcrop sections are correlated across the basin using compiled ammonite zones and digital well logs. A bentonite collected 4 m below the base of the Wall Creek sandstone in the northwestern part of the basin yields an age 93.98 ± 0.07 Ma, documenting a hiatus of ~3 Ma at the sharp base of the deltaic sandstones containing the Late Turonian ammonite S. whitfieldi. Two bentonites collected from the middle section of the Turner in the northeastern and south-central basin yield ages of 90.67 ± 0.09 and 90.645 ± 0.016 Ma, respectively, correlatable to the S. whtitfieldi ammonite zone. A bentonite from the overlying Sage Breaks Member of the Carlile Shale from the south-central part of the basin yields an age of 88.567 ± 0.015 Ma, placing a significant Turonian–Coniacian hiatus atop the Turner. These ages suggest that the lower Turner, containing the P. macombi ammonite zone, was deposited during a widespread shoreline progradation atop the Mid Turonian Unconformity and equivalent strata are not preserved in the Wall Creek Member to the west. Consequently, only the middle and upper Turner Member are coeval with the Wall Creek Member, as both units display lower progradational, then retrogradational stacking of coastal to offshore sediments within the S. whitfieldi to P. germari ammonite zones.