Detrital Zircon U-Pb Geochronology and Paleodrainage Reconstruction of the Blackhawk-Castlegate Succession, Wasatch Plateau and Book Cliffs, 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. Long-standing 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) test a hypothesis that very fine sands and coarse silts yield a more robust population of maximum depositional ages (MDAs) from volcanogenic detrital zircons (DZs) that approximate true depositional age than medium to fine sands; (ii) develop an independent geochronological framework of MDAs through U-Pb dating of volcanogenic DZs; and (iii) propose a paleodrainage reconstruction for the Blackhawk-Castlegate succession in light of new data. MDAs calculated throughout the succession approximate the time of deposition for the upper Blackhawk (77.7 ± 1.7 Ma in Horse Canyon and 79.6 ± 1.7 Ma in Tusher Canyon), Lower Castlegate (75.8 ± 1.8 Ma in Horse Canyon), and potentially the Bluecastle Tongue (75.9 ± 1.9 Ma in Price Canyon). In the case of the upper Blackhawk and Lower Castlegate, the MDAs are both up to 2 Myrs younger than the age constraints determined by ammonite zone correlations, and raise questions concerning the temporal significance of the classic Castlegate Sequence Boundary and relationships with basin-evolution models. Previous studies indicate that Santonian drainages were dominated by an axial fluvial system with headwaters in the Mogollon Highlands, with lesser contributions from the Sevier fold-and-thrust belt and the various Cordilleran magmatic sources. The DZ U-Pb age data in this study is analyzed using multidimensional scaling, spatial relationships between clusters, and examination of specific peaks in age distributions. Regional sediment routing patterns appear to be quite similar during the Campanian deposition of the Blackhawk-Castlegate succession: there is evidence for an axial drainage flowing north from the Mogollon Highlands that passed by an area that is now Straight Canyon, intersected by transverse fluvial systems in the area that is now Price Canyon that drain the largely recycled strata exposed in the Sevier fold-and-thrust belt. The DZ U-Pb age data collected and used for this thesis are uploaded as three supplementary Excel files: KBH Data, BC Data Unbiased, and BC Data Biased. The first two files have the data used for multidimensional scaling and peak analysis of kernel density estimates, but lack some grains used for MDA calculations. The targeted population of n=50 grains for MDAs is included in the BC Data Biased file.