DETRITAL ZIRCON (U-TH)/PB PROVENANCE GEOCHRONOLOGY OF THE EARLY PLIOCENE BOUSE FORMATION, LOWER COLORADO RIVER, USA

The timing and mechanism of Colorado River integration from Grand Canyon to the Gulf of California has long been a topic of debate. 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: a tufa with associated nearshore sands and gravels, micritic limestone, and thicker siliciclastic sequences exposed along the margins of the lower Colorado River valley.

Here we present a new dataset of detrital zircon (U-Th)/Pb geochronology (n = 1774 single-grain ages) to explore sedimentary provenance of sand horizons in the Bouse Formation. Our results span 14 Bouse samples from four sub-basins in the lower Colorado River corridor: Mohave, Chemehuevi, Parker, and Cibola. Additional samples of underlying Pyramid gravel, overlying Bullhead Alluvium, and modern sediment from the Colorado River, Bill Williams River, and Silver Creek are presented for comparison.

Except for 3 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, but inter-sub-basins vary geographically, which we attribute to progressive admixture of zircons from local source rocks and tributaries.

Overall, our provenance analysis is consistent with 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 deposition of the Bouse Formation in separated and locally sourced lake systems but instead supports deposition by a single, high-discharge river that rapidly progressed southward integrating previously separated sub-basins.