LATE MIOCENE-PLIOCENE LAKE SPILLOVER OF THE VERDE VALLEY(?) LED TO A SUDDEN CHANGE FROM ENDORHEIC TO EXOREIC DEPOSITION WITHIN THE PAYSON BASIN, ARIZONA, USA

Only a handful of major rivers (the Colorado, Verde, Salt, Gila, and Rio Grande) cross the formerly closed basins of the Basin-and-Range province to discharge into the ocean. The Verde River, with a modern drainage area of 17,130 km², drains a large portion of the highlands of south-central Arizona. Evidence from the Payson basin indicates that the upstream portions of the watershed formed one or more large, structurally controlled lakes that overflowed, transporting large quantities of water and sediment southeastward into the still tectonically active Payson basin. Although this is not a new idea, recent detailed geologic mapping in the Payson basin reveals a sharp contact between older, locally derived (endorheic) basin-filling sediments and younger, externally derived (exoreic) sediments. The endorheic sediments contain angular, poorly sorted clasts of Early Proterozoic gabbro and tonalite, sourced from the nearby Payson ophiolite. In contrast, the exoreic deposits contain well sorted and well rounded clasts of Paleozoic limestone and sandstone, as well as purple quartzite and granophyre, sourced from far up-steam. The basal few meters is a mixture of both deposits and fines upwards into a basin-wide ledge of freshwater limestone that resembles a “bathtub ring” which is interpreted to represent initial and sudden infilling of the basin by a huge volume of water that allowed for long-term lacustrine conditions. Overlying the limestone is several hundred feet of well rounded exoreic conglomerate that appears to form an alluvial fan or delta in the northern Payson basin. This delta is overlain by vast quantities of quartz silt that fills much of the Payson basin and may have been derived from extensive up-stream bedrock exposures of the Permian Supai Formation and also from the Miocene-Pliocene Verde Formation that fills much of the Verde Valley. Accumulation of the silt eventually led to aggradational piracy which forced the proto-Verde River to spill over a low divide in the northern Mazatzal Mountains and flow southward into the Horseshoe basin. Ongoing work with cosmogenic dating will, hopefully, help constrain the age of both of these spillover events.