Abstract

The Campanian Blackhawk Formation and Castlegate Sandstone are widely exposed along the Wasatch Plateau and Book Cliffs in east-central Utah, USA, and represent alluvial–deltaic and shoreline deposits in the Sevier foreland basin along the western margins of the Western Interior Seaway. This study presents detrital-zircon U-Pb ages (DZ U-Pb) from 31 samples in the Blackhawk–Castlegate succession so as to examine paleodrainage areas and sediment routing, the influence of autogenic processes on the stratigraphic record, and the timing of deposition.

DZ U-Pb populations indicate the uppermost Blackhawk and lower Castlegate of the Wasatch Plateau represent multiple “parent” rivers with different drainage areas. However, there is no apparent change in provenance across the interpreted basal Castlegate unconformity, and the same parent rivers were responsible for deposition of the uppermost Blackhawk and lower Castlegate at each location. At the broader regional scale, paleodrainage systems included a series of smaller transverse fluvial systems that drained the proximal Sevier fold and thrust belt, as well as a longitudinal river system that flowed from the Mogollon Highlands in present-day central Arizona to the north-northeast. DZ U-Pb data from updip locations in the Wasatch Plateau provide faithful records of provenance for these parent systems, however downdip locations in the Book Cliffs record mixing of different parents, initially due to contributive patterns, where rivers with slightly different drainage areas merge in the downstream direction, then due to distributive patterns farther downdip on an alluvial–deltaic plain where backwater-mediated avulsions are common. Autogenic surface dynamics therefore produce DZ U-Pb daughter populations that complicate interpretations of tectonic and climatic forcing, and the influence of other external controls.

Published age estimates for the Blackhawk–Castlegate succession have relied on correlation of chronologically constrained shallow-marine strata to presumed updip alluvial–deltaic equivalents. However, the youngest population of DZ U-Pb ages in the uppermost Blackhawk and the lower Castlegate through Bluecastle Tongue provide independently constrained maximum depositional ages (MDAs). MDAs for the upper Blackhawk and lower Castlegate are up to 2 Myr younger than previous inferences, but the Bluecastle Tongue MDA is consistent with previous constraints: deposition of the upper Blackhawk, formation of the interpreted basal Castlegate unconformity, and deposition of the lower Castlegate through Bluecastle Tongue represents an ∼ 2 Myr period from ca. 77 to 75 Ma. The lack of provenance change across the basal Castlegate surface at any single location, combined with the short period of time it represents, suggests that this key sequence stratigraphic surface is not an unconformity but formed instead from autogenic scour by migratory channels.

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