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AAPG Rocky Mountain Section Meeting

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Ages of the Dakota Sandstone and the position of the Albian-Cenomanian (Lower-Upper Cretaceous) Boundary in Western Interior North America

Abstract

Cobban had a pithy characterization for the Dakota Sandstone interval in the Rockies−it was "tricky". Perhaps it was because the stratigraphic record surrounding these rocks is non-marine, and his classical biostratigraphic tools were strictly marine. Or, it may have been a comment on discontinuity in the stratigraphic record below the base of his mid-Cenomanian Western Interior Seaway ammonite zonation. Nevertheless, these sparsely fossiliferous rocks are important because they mark the initial marine flooding events onto the North American craton at the beginning of the Upper Cretaceous Western Interior Basin and are widespread oil and gas source rocks and reservoirs, as well as significant aquifers. Rocky Mountain Section – AAPG: 2019 Annual Meeting 40 High-resolution 40Ar/39Ar sanidine and U/Pb zircon ages demonstrate that rocks correlated broadly as the Dakota Sandstone were deposited over nearly 5 Ma extending from the late Albian (uppermost early Cretaceous) into the early Cenomanian stage (basal late Cretaceous). Improved isotope geochronology precision allows discrimination of bentonite units erupted <100 ka apart. Adoption of common ages for standard materials permit direct correlation using ages derived from both Ar and U/Pb isotopic systems. 40Ar/39Ar ages from the Black Hills in Wyoming confirm a 5 Ma gap between the Albian Skull Creek Shale (104.87 Ma) and 99.58-99.49 Ma ages on mid/upper Newcastle Sandstone. A similar age of 104.69 Ma by U/Pb zircon is obtained for Skull Creek Shale beneath the Dakota 'J' Sandstone at Dinosaur Ridge. Overlying Kassler sandstone U/Pb ages range from 104.02- 103.92 Ma, close to the Skull Creek. Upper Thermopolis Shale (101.36 Ma) at Greybull, and Muddy Sandstone bentonite at Maverick Springs Dome (101.23 Ma) are >2 Ma younger than the Kassler, and 1.5 Ma older than Newcastle Sandstone. Ages on Shell Creek shale from Bighorn (100.07 Ma) and Wind River (99.62 Ma) basins bracket the current 100 Ma age of the Albian- Cenomanian boundary. Cenomanian Mowry Shale ranges from 99.30 to 97.52 Ma in Wyoming and Montana. Clayspur bentonite atop the Mowry is 2.5 Ma younger than the Albian-Cenomanian boundary, despite historic correlation of this marker (Fish Scales) as boundary between Lower and Upper Cretaceous in the Western Interior. These ages suggest more transgressive-regressive events, and considerably more time at unconformities between late Albian-early Cenomanian sequences than heretofore recognized.