64 New Insights into the Development and Evolution of the Colorado River System from Geologic Mapping (Posters)

Thursday, 17 May 2018
8:30 AM - 4:30 PM
Outdoor Tents

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

Paper number: 64-5
Program: Theme Sessions
Time: 8:30 AM – 4:30 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 5

doi: 10.1130/abs/2018RM-313481

© Copyright 2018 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.