EVALUATING MODELS FOR CRETACEOUS PALEODRAINAGE AND SEDIMENT ROUTING USING DETRITAL ZIRCON U-PB PROVENANCE AND GEOCHRONOLOGY IN THE COLORADO FRONT RANGE

The Late Jurassic Morrison Formation through the Aptian to Cenomanian Dakota Group is a succession of fluvial, deltaic, and shallow-marine strata deposited within the back-bulge of the Mesozoic Sevier retroarc foreland-basin system, now located in the present-day Colorado Front Range and US midcontinent. Multiple sediment-routing models have been proposed for North American paleodrainage during this time period. This study investigates the Late Jurassic through Campanian Morrison, Lytle, Plainview, and Dakota succession with the following objectives: (a) examine river system scale and fluvial style differences from alluvial architectures; (b) test previous paleodrainage models through detrital zircon (DZ) U-Pb geochronology and provenance and present a new reconstruction of sediment-dispersal patterns; and (c) develop the geochronological framework using DZ U-Pb maximum depositional ages (MDAs) that approximate true depositional age (TDA). Comparison of alluvial architectures and sedimentological characteristics from sites along the Colorado Front Range result in observed differences in fluvial styles represented by the Morrison, Lytle, Plainview, and Dakota formations. The Morrison Formation is interpreted to have been deposited by a net aggradational meandering fluvial system; the Lytle Formation is interpreted to represent a mixed bedrock-alluvial valley system, where the river is inherently degradational and confined within a broad incised valley; the Plainview Formation is interpreted to consist of mixed fluvial and near shore marine deposits, aggrading within the Lytle bedrock valleys; and the upper Dakota Formation is interpreted to represent a fluvial system that cut across delta plains represented by the lower Dakota during regression. Statistical analysis of DZ U-Pb populations of 22 samples collected for this study indicate clear differences between the interpreted west-derived Cretaceous Lytle and Dakota signatures of the Colorado Front Range, southwestern Kansas, and the Black Hills of South Dakota and the interpreted east-derived Dakota signatures of the further eastern US midcontinent in Kansas, Nebraska, and South Dakota. West-derived samples have DZ U-Pb signatures with Western Cordilleran magmatic arc grains present, which are distinctly absent in east-derived samples. Moreover, east-derived DZ U-Pb signatures are dominated by primary Appalachian-Grenville populations from the Appalachian-Ouachita Cordillera, whereas west-derived signatures are dominated by recycled Appalachian-Grenville populations from the Sevier fold and thrust belt, along with contributions from the Western Cordilleran magmatic arc, Mid-Continent, and Yavapai-Mazatzal source terrains. Mixing models and statistical comparisons were used to provide a new paleodrainage model that features convergence of east- and west-derived systems in the US midcontinent, which then flow north toward the Western Canada Sedimentary Basin and Boreal Sea. This fluvial system is interpreted as the south-to-north oriented axial system that deposited the Early Cretaceous McMurray Formation of the Alberta Oil Sands. DZ samples from the Late Jurassic Morrison Formation in Colorado produced MDAs that are slightly younger than ages previously published from sanidine grains in bentonites from Utah. These new MDAs indicate that Morrison deposition continued for another 1-2 Myrs longer than previously thought. DZ samples from the Early Cretaceous Lytle and age-equivalent formations did not produce multiple young volcanogenic grains to enable MDA calculation due to deposition during the Early Cretaceous magmatic lull. DZ samples from the Albian to Cenomanian Dakota Formation produced the first U-Pb ages for the Dakota in Colorado and MDAs consistent with published age estimates.

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