292-12 Chronostratigraphy of the Late Turonian System, Powder River Basin, WY: paleogeographic insights from integrated U-Pb geochronology, core-to-outcrop stratigraphy, and biostratigraphy

Abstract:

Upper Turonian strata of the U.S. Western Interior Basin contain an anomalous number of  unconformity-bound marine sandstones interpreted as remnants of broader depositional systems. High precision geochronologic constraint is crucial to resolving enigmatic stratigraphic correlations of these units. Upper Turonian strata atop the composite Mid-Turonian Unconformity (MTU) in the Powder River Basin (PRB), Wyoming are investigated through study of shallow marine sandstones and shales of the Wall Creek Member of the Frontier Formation and the Turner Member of the Carlile Shale. Five new high-precision CA-ID-TIMS U-Pb zircon ages, 41 whole cores, and 19 measured outcrop sections were correlated across the basin using compiled ammonite zones and digital well logs. A bentonite collected below the sharp base of the Wall Creek Member containing the Late Turonian ammonite Scaphites whitfieldi in the northwestern part of the basin yields a Cenomanian age 93.98 ± 0.07 Ma, documenting a hiatus of ~4 Ma. A bentonite collected in concretionary mudstones below the Wall Creek sandstone in the southwestern margin of the PRB, mapped as the Cenomanian-age Belle Fourche Member by previous authors yields an Upper Turonian age of 90.872 ± 0.013 Ma, correlative to the zone of S. warreni and documenting the preservation of older strata in the region than previously mapped. A bentonitic mudstone below the uppermost sandstone unit in the S. warreni zone in the eastern PRB yields a single-crystal age of 90.670 ± 0.09 Ma, suggesting a conformable relationship with the overlying S. whitfieldi unit in the Turner in the central PRB, which yields an age of 90.645 ± 0.016 Ma. Where a complete Upper Turonian succession is preserved, strata display five progradational episodes within an overall retrogradational sequence. Both the Wall Creek and Turner Members display identical stacking of nearshore to offshore sediments from S. whitfieldi to P. germari, although an additional unit consisting of S. warreni and P. macombi-age strata is only present in the easternmost PRB. Successively younger strata of both the Wall Creek and Turner members onlap a convex MTU towards a maximum hiatus along an apparent northwest-southeast-trending feature in the central PRB, likely created by differences in accommodation created by temporary forebulge-style lithospheric flexure in the backbulge during a documented 80 km eastward transfer from the Willard-Paris-Meade to the Crawford Thrust in the Sevier fold-and-thrust.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.