BIDAHOCHI OR BUST: TIME TO FILL AND SPILL THE BEANS ON THIS FORMATION’S IMPORTANCE TO UNDERSTANDING THE COLORADO RIVER AND GRAND CANYON

Active tectonics in western North America ponded much of the Cenozoic landscape. The Green River Formation (Lake Uinta) and Great Salt Lake (Lake Bonneville) exemplify these basins. One process explains how a closed basin transitions into an open fluvial system. Basins fill and spill across their sills. Seven basins filled and spilled along the lower Colorado River (LOCO), at least three for the Gila River, and at least one each for Arizona’s Salt and Verde Rivers. We explore whether two to three basins filled and spilled along the upper Colorado River (UPCO) before arriving in LOCO by ~5.2 Ma. The Bidahochi Formation sits upstream of Grand Canyon and records closed basin sedimentation until <6.1 Ma. The upper Bidahochi preserves a species increase, a spike in Sr87/Sr86, and an overall boost in sedimentation rates starting ~7 Ma. Further upstream, the upper Colorado River incised from a position 1,500 m above the modern river after 10 Ma, the Green River exited Browns Park after 8 Ma, and San Juan Mountains derived gravels deposited atop Black Mesa <9 Ma, all events that support UPCO’s late Miocene arrival into the Bidahochi basin. With UPCO’s arrival into Bidahochi basin, quartz-dominated sands prograde across a preexisting ephemeral lake plain and interfinger with clay rich beds toward the basin center. Synchronously, maar volcanism and associated lava flows at Hopi Buttes erupt into the lake/playa system. We propose steady state deposition between aggrading sub-aqueous nearshore fine sands and the deeper lake clays, a balanced-filled lake system. As example, Shonto maar preserves over 300 m of green clay and fish fossils related to modern Colorado River species. The maar’s eruptive surface is projected to have been 200 m above the modern surface. As such, lake sediments buried the surrounding landscape to an elevation of roughly 2,060 m, including the known maars and associated lava flows in the Hopi Buttes volcanic field. Carbonate-cemented fine sands, sourced from the Chuska Mountains, crossed what is today Chinle Wash onto Balakai Mesa. They deposited into an aggrading lake plain up to and above an elevation of 2,220 m, only ~50 m from the modern sill height of the Kaibab Plateau. Between 6.1-5.2 Ma, Bidahochi basin fills and spills across the sill, connecting UPCO with LOCO via Grand Canyon.