ABSTRACT The Colorado River extensional corridor, which stretched by a factor of 2 in the Miocene, left a series of lowland basins and intervening bedrock ranges that, at the dawn of the Pliocene, were flooded by Colorado River water newly diverted from the Colorado Plateau through Grand Canyon. This water and subsequent sediment gave birth, through a series of overflowing lakes, to an integrated Colorado River flowing to the newly opened Gulf of California. Topock Gorge, which the river now follows between the Chemehuevi and Mohave Mountains, is a major focus of this field guide, as it very nicely exposes structural, stratigraphic, and magmatic aspects of the Miocene extensional corridor, a core complex, and detachment faults as well as a pre-Cenozoic batholith. Topock Gorge also is the inferred site of a paleodivide between early Pliocene basins of newly arrived Colorado River water. Overspilling of its upstream lake breached the divide and led the river southward. The Bouse Formation in this and other basins records the pre–river integration water bodies. Younger riverlaid deposits including the Bullhead Alluvium (Pliocene) and the Chemehuevi Formation (Pleistocene) record subsequent evolution of the Colorado River through a succession of aggradational and re-incision stages. Their stratigraphic record provides evidence of local basin deepening after river inception, but little deformation on a regional scale of the river valley in the last 4 m.y. except in the Lake Mead area. There, faults interrupt both the paleoriver grade and incision rates, and are interpreted to record 100’s of m of true uplift of the Colorado Plateau. Warren Hamilton’s insightful work beginning in the 1950s helped set the stage for interpretation of Mesozoic orogeny and Cenozoic extension in this region, as well as the record of the Bouse Formation.

Abstract Airborne dust suppression is of critical importance to military operations conducted in desert environments. Airborne dust is commonly generated in the desert by surface and near-surface operations during operational, testing, and training missions. Currently, there is no standardized procedure for testing dust suppressants, and the U.S. military lacks a specific test operations procedure (TOP) designed to provide realistic testing of the performance and durability of commercial products sold for dust abatement. The primary purpose of this study is to provide recommendations for the future development of a TOP for testing dust suppressants applied to desert soils. Recommendations were developed from the evaluation of a polyvinyl-based synthetic polymer as a dust suppressant, which was tested at four test intervals over a 19–week period in the late spring and summer of 2008 at the U.S. Army Yuma Proving Ground. The dust suppressant was applied at three separate test sites having different surface characteristics and soil properties ranging from loose, sandy gravel to gravelly sand, alluvial-fan soils to soft, sandy-silt, alluvial-plain soils. Each test site was subjected to a variety of traffic impact types consisting of an increasing number of cumulative passes by different vehicle types—including a low-flying helicopter, a light-weight armored tracked vehicle, and heavy-, medium-, and lightweight wheeled vehicles, plus pedestrian foot traffic. In addition to the sites of traffic impact, three types of control plots were concurrently tested to act as reference sites, as well as to evaluate the longevity of the suppressant, which included: disturbed and static (undisturbed) baseline plots and a static benchline plot. Surface soil and dust-suppressant physical properties were measured following each traffic impact in the form of shear strength and bearing capacity, plus dust-emission flux as measured by a Portable In Situ Wind ERosion Laboratory (PI-SWERL). Results from this study show that dust-emission flux and surface-strength measurements from a layout of control and traffic impact test plots provide a quantifiable and repeatable approach in measuring the efficacy of a dust suppressant for a TOP used by the U.S. Army.

The upper Miocene to lower Pliocene Bouse Formation in the lower Colorado River trough of the American Southwest was deposited in three basins—from north to south, the Mohave, Havasu, and Blythe Basins—that were formed by extensional faulting in the early to middle Miocene. Fossils of marine, brackish, and freshwater organisms in the Bouse Formation have been interpreted to indicate an estuarine environment associated with early opening of the nearby Gulf of California. Regional uplift since 5 Ma is required to position the estuarine Bouse Formation at present elevations as high as 555 m, where greater uplift is required in the north. We present a compilation of Bouse Formation elevations that is consistent with Bouse deposition in lakes, with an abrupt 225 m northward increase in maximum Bouse elevations at Topock gorge north of Lake Havasu. Within Blythe and Havasu Basins, maximum Bouse elevations are 330 m above sea level in three widely spaced areas and reveal no evidence of regional tilting. To the north in Mohave Basin, numerous Bouse outcrops above 480 m elevation include three widely spaced sites where the Bouse Formation is exposed at 536–555 m. Numerical simulations of initial Colorado River inflow to a sequence of closed basins along the lower Colorado River corridor model a history of lake filling, spilling, evaporation and salt concentration, and outflow-channel incision. The simulations support the plausibility of evaporative concentration of Colorado River water to seawater-level salinities in Blythe Basin and indicate that such salinities could have remained stable for as long as 20–30 k.y. We infer that fossil marine organisms in the Bouse Formation, restricted to the southern (Blythe) basin, reflect colonization of a salty lake by a small number of species that were transported by birds.

Distinctive far-traveled fluvial sediment of the lower Colorado River fills 20 paleovalleys now stranded by the river downstream of Grand Canyon as it crosses the Basin and Range Province. These sediments resulted from two or more aggradational episodes in Pliocene and Pleistocene times following initial incision during the early Pliocene. Areview of the stratigraphic evidence of major swings in river elevation over the last 5 m.y. from alternating degradation and aggradation episodes establishes a framework for understanding the incision and filling of the paleovalleys. The paleovalleys are found mostly along narrow bedrock canyon reaches of the river, where divides of bedrock or old deposits separate them from the modern river. The paleovalleys are interpreted to have stemmed from periods of aggradation that filled and broadened the river valley, burying low uplands in the canyon reaches into which later channel positions were entrenched during subsequent degradation episodes. The aggradation-degradation cycles resulted in the stranding of incised river valleys that range in elevation from near the modern river to 350 m above it.