NEW GEOLOGIC MAPPING DOCUMENTS THE INCEPTION AND EVOLUTION OF THE LOWER COLORADO RIVER IN CHEMEHUEVI VALLEY, ARIZONA AND CALIFORNIA

Valleys in the lower Colorado River corridor between Grand Canyon and the Gulf of California contain good exposures of the 'LOCO group', the key strata that record the river's c. 5 Myr evolution. Recently completed geologic mapping in Chemehuevi Valley (CHV) near Lake Havasu City, AZ, is part of an effort to develop an accurate and meaningful portrayal of the LOCO group's full extent at a regional scale. Mapping results include geologic observations that corroborate and augment evidence from detailed geologic mapping elsewhere in the river corridor.

Map units in CHV include rocks and features that record major tectonic extension; unroofing and denudation of crustal rocks; and coeval volcanic activity, fluvial processes, and co-seismic mass wasting. As extension waned in CHV and throughout the corridor, a landscape of disconnected, mountain-flanked desert valleys remained. The river abruptly arrived c. 5 Ma into the valleys in a cascading process of lake-overflow that culminated in full integration of the river to the Gulf.

Each member of the LOCO group is present in CHV. Deposition began with lake-overflow, recorded by carbonate and siliciclastic facies of the Bouse Formation. The river then experienced a series of voluminous, progressively smaller aggradation events, as recorded by: the c. 4 Ma, >200 m Bullhead Alluvium; the c. 70-100 ka, ~100 m Chemehuevi Formation; and the c. <10 ka, <50 m 'Blythe alluvium'. The first is likely a singular response to integration, and the latter two are records of major environmental change. Major unconformities in the fluvial sequences document several periods of deep incision following each aggradation episode, sometimes interrupted by temporary phases of equilibrium and(or) aggradation. There are interspersed, thin scarp-bounded alluvial terraces on the valley flanks including the proposed c. 0.5-1 Ma (??) 'Palo Verde alluvium' and the ca. 70 to 10 ka 'Riverside terrace alluvium' that record brief pauses during incision. Efforts to develop tighter age-constraints on the LOCO group and to identify more members are underway.