Mio-Pliocene chronostratigraphy, basin morphology and paleodrainage relations derived from the Bidahochi Formation, Hopi and Navajo nations, northeastern Arizona

Analysis of the Bidahochi Formation is used to determine temporal and spatial relations on the southern Colorado Plateau during late Miocene to Pliocene time. ⁴⁰Ar/³⁹Ar analyses and ash-bed geochemical fingerprinting of interbedded felsic fallout ash beds from distal sources constrain the ages of the lower five members of the Bidahochi Formation to between 15.8 to 6.6 Ma, with the onset of sedimentation estimated at 16 Ma. The chronostratigraphic framework is used to delimit the lithostratigraphic relations of fine-grained deposits in order to define depositional margins of the basin. The Bidahochi basin extended across the present-day Little Colorado River valley and onto the northeast dipping Mogollon Slope forming a basin with an estimated depositional surface area greater than 30,000 km². The depositional environment for the fine-grained sediments of members 1-4 has traditionally been interpreted as lacustrine (Hopi Lake). Stratal accumulation rates range from 1.8-10 cm/ka for the period between 16-13.7 Ma and then decrease to 0.3 cm/ka from 13.7-6.6 Ma. Comparison with Lake Powell sedimentation rates show that the upper Colorado River would have filled Hopi Lake with sediment to the maximum thickness of the fine-grained portion of the Bidahochi Formation in about 80 ka instead of the 9 Ma represented by the deposits. These new data dispute the hypothesis that the ancestral upper Colorado River was the source of Bidahochi Formation sediment and question a large, extensive lacustrine system interpretation.