Abstract

Uncertainty over depositional paleoenvironments of the upper Miocene to lower 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 integration and early evolution of the Colorado River (western United States). This paper presents new sedimentologic and quantitative evidence for a strong tidal influence on deposition of the basal carbonate member of the southern Bouse Formation. Lithofacies with tidal sedimentary structures include lime mudstone with reed and grass imprints; heterolithic facies with horizontal, flaser, wavy, and lenticular bedding; and cross-bedded grainstone with sigmoidal bundles, reactivation surfaces, and complex bedding. Systematic alternation of relatively thick and thin layers records a predominantly semidiurnal mixed-tidal depositional environment, and Fourier analysis highlights the cyclic, non-random nature of layer-thickness trends. These observations provide evidence that the southern Bouse Formation accumulated in a pre–Colorado River tide-dominated marine setting at the north end of the Gulf of California. Deposition at sea level requires ∼330 m of uplift in the lower Colorado River and western Colorado Plateau region during the past ∼5.0 m.y.

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