--> Detrital Zircon U-Pb Geochronology of the Blackhawk-Castlegate Succession, Book Cliffs and Wasatch Plateau, Utah

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Detrital Zircon U-Pb Geochronology of the Blackhawk-Castlegate Succession, Book Cliffs and Wasatch Plateau, Utah

Abstract

The Blackhawk Formation and Castlegate Sandstone are Campanian fluvial-deltaic and shoreline deposits within the Sevier foreland-basin fill along the Wasatch Plateau and Book Cliffs in Utah. Existing age constraints on the Blackhawk and Castlegate are based on correlation to ammonite zones in downdip mudstones, which are themselves constrained by radiometric dating of volcanic ash beds, and are therefore dependent on correlation methods and models. This study examines the Blackhawk-Castlegate succession with the following objectives: (i) develop an independent geochronological framework of maximum depositional ages through U-Pb dating of volcanogenic detrital zircons (DZs); (ii) test a hypothesis that very fine sands and coarse silts yield a more robust population of maximum depositional ages from volcanogenic DZs that approximate true depositional age than medium to fine sands; and (iii) revisit geochronological interpretations and deposition rate calculations of the Blackhawk-Castlegate succession in light of new data. Preliminary U-Pb dating of DZs in the uppermost Blackhawk produced U-Pb ages as young as 76.0 ± 2.8 Ma, with a calculated maximum depositional age (MDA) of 77.7 ± 1.6 Ma (n=7), and, in the lower Castlegate, produced U-Pb ages as young as 73.0 ± 4.0 Ma with a calculated MDA of 75.9 ± 2.0 Ma (n=13). In both cases, these dates are significantly younger than previous age models, and raise questions concerning the temporal significance of the classic Castlegate Sequence Boundary and relationships with basin-evolution models. U-Pb dating of DZs in the uppermost Blackhawk to test the facies dependence hypothesis produced no young (< 80 Ma) grains, but can aid in reconstructing paleodrainage patterns. Results of this study will contribute to future quantitative analyses of this heavily-studied foreland basin fill.