A PRE-COLORADO RIVER NORTHERN GULF OF CALIFORNIA MARINE EMBAYMENT RECORDED BY TIDAL RHYTHMITES IN THE SOUTHERN BOUSE FORMATION

Ongoing debate over depositional paleoenvironments of the late Miocene to early Pliocene Bouse Formation obscures our understanding of the timing and magnitude of regional uplift, as well as the conditions and processes that were active during initiation and early evolution of the Colorado River. Here we present new sedimentologic and statistical evidence for a strong tidal influence on deposition of the southern Bouse Formation in the Blythe basin. Abundant tidal lithofacies include: (1) matted sandy lime mudstone with reed and grass imprints; (2) heterolithic lime mudstone, wackestone, packstone, and grainstone with flaser, wavy, and lenticular bedding; (3) cross-bedded sandy grainstone with common reactivation surfaces; and (4) barnacle-oncoid grainstone with sigmoidal bundles and complex bedding. Systematic alternation of relatively thick and thin layers strongly suggests a mixed predominantly semidiurnal tidal depositional environment, and Fourier analysis highlights the cyclic, non-random nature of thickness trends. These observations indicate a pre-Colorado River tidal setting for the southern Bouse Formation at the north end of the Gulf of California marine oblique-rift basin. Deposition at sea level requires at least ~330 m of regional uplift in the lower Colorado River and western Colorado Plateau region during the past ca. 5.0-5.5 my.