Provenance Analysis of the Bouse Formation, Lower Colorado River from Detrital Zircon (U-Th)/Pb Geochronology

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Motz, Samantha L.
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The timing and mechanism of Colorado River integration from the Grand Canyon to the Gulf of California have long been debated. Early research proposed river integration developed “bottom-up” due to Pliocene marine incursion and regional uplift. However, mapping, stratigraphy, and geochemical analyses of early Colorado River deposits instead support a “top-down” integration by progressive filling of lake basins connected by the Colorado River. Key to this debate are interpretations of the depositional environment of the Pliocene Bouse Formation. Here we present a new dataset of detrital zircon (U-Th)/Pb geochronology (n = 1774 single-grain ages) to explore the sedimentary provenance of sand horizons in the Bouse Formation. Our results span 13 Bouse samples from four sub-basins in the lower Colorado River corridor: Mohave, Chemehuevi, Parker, and Cibola. Additional samples of underlying Pyramid gravel and modern sediment from the Colorado River, Bill Williams River, and Silver Creek are presented for comparison. Except for three samples from the Mohave sub-basin, statistical comparison of grain-age populations illustrates that the Bouse Formation has a non-local provenance consistent with a large drainage area comparable to the modern Colorado River. The excepted samples reflect derivation from local source rocks. Within the Bouse Formation’s stratigraphy, grain-age populations do not vary. Still, inter-sub-basins vary geographically, which we attribute to the progressive admixture of zircons from local source rocks and tributaries. Overall, our provenance analysis is consistent with the deposition of Bouse sand horizons as delta-front turbidities originating from a river with a well-mixed and lithologically diverse sediment load. Exceptional samples from the Mohave sub-basin may be explained by interbedding of transverse fan-deltas from local tributaries. Our analysis does not support the deposition of the Bouse Formation in separated and locally sourced lake systems. Instead, it promotes deposition by a single, high-discharge river rapidly progressing southward, integrating previously separated sub-basins.
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