Where the lower Colorado River traverses the Basin and Range Province below the Grand Canyon, significant late Pleistocene aggradation and subsequent degradation of the river are indicated by luminescence, paleomagnetic, and U-series data and stratigraphy. Aggradational, finely bedded reddish mud, clay, and silt are underlain and overlain by cross-bedded to plane-bedded fine sand and silt. That sequence is commonly disconformably overlain by up to 15 m of coarse sand, rounded exotic gravel, and angular, locally derived gravel. Luminescence dates on the fine sediments range from ca. 40 ka to 70 ka, considering collective uncertainties. A section of fine-grained sediments over a vertical range of 15 m shows normal polarity magnetization and little apparent secular variation beyond dispersion that can be explained by compaction. Aggradation on large local tributaries such as Las Vegas Wash appears to have been coeval with that of the Colorado River. The upper limits of erosional remnants of the sequence define a steeper grade above the historical river, and these late Pleistocene deposits are greater than 100 m above the modern river north of 35°N. Terrace gravels inset below the upper limit of the aggradational sequence yield 230 Th dates that range from ca. 32 ka to 60 ka and indicate that degradation of the river system in this area closely followed aggradation. The thick sequence of rhythmically bedded mud and silt possibly indicates settings that were ponded laterally between valley slopes and levees of the aggrading river. Potential driving mechanisms for such aggradation and degradation include sediment-yield response to climate change, drought, fire, vegetation-ecosystem dynamics, glaciation, paleofloods, groundwater discharge, and building and destruction of natural dams produced by volcanism and landslides.

Full article available in PDF version.

Full article available in PDF version.

The structure of the northern Cottonwood Mountains, located in the Death Valley region of southeastern California, is dominated by a faulted, east-facing, monoclinal flexure developed in Paleozoic strata of the Cordilleran miogeocline. We interpret this flexure as a rollover or fault-bend fold above a listric normal fault, probably the north-ward continuation of the Tucki Mountain normal fault system exposed about 45 km south of the study area. Synorogenic Tertiary sediments in the Ubehebe basin, overlying the rollover at the northern end of the Cottonwood Mountains, record the inception of normal faulting and eastward tilting around a horizontal axis trending about N25°W. Paleozoic strata in the Dry Mountain block to the west are subhorizontal. Racetrack Valley, a north-trending topographic depression between the Dry Mountain block and the northern Cottonwood Mountains, is interpreted as a major graben that accommodates differential stratal rotations within the rollover. The juxtaposition of Mesozoic thrust plates by Tertiary normal faults has obscured the structural simplicity of the rollover. Structural and stratigraphic correlations indicate that strata exposed in the Dry Mountain block and northern Cottonwood Mountains are parts of a single Mesozoic thrust plate. This new evidence suggests that the Racetrack thrust of McAllister (1952) does not root in northern Racetrack Valley, contrary to previous interpretations. A previously unrecognized klippe and other parts of the Ubehebe thrust plate lie structurally above the northern Cottonwood Mountains. We suggest that normal faulting has downdropped a portion of the Ubehebe thrust plate into Racetrack Valley between strata of the Dry Mountain block and northern Cottonwood Mountains. Thus, the development of a major graben within the rollover has produced an apparent thrust relation along both sides of the Racetrack Valley block. We correlate the Racetrack Valley block and Ubehebe thrust plate with the Last Chance thrust plate of Stewart and others (1966). Sequentially deformed, palinspastic reconstructions of the rollover indicate that conjugate faults formed symmetrical grabens during development of the rollover. The grabens accommodated bending within the rollover and probably nucleated above areas of maximum curvature on the basal listric fault. Thus, the nucleation of progressively younger grabens during extension apparently migrated toward the breakaway within the hanging-wall block while remaining fixed relative to the bend in the basal fault plane. The pre-middle Pliocene(?) age and structural position of strata in the Ubehebe basin suggest that they are partially correlative with upper Miocene to lower Pliocene strata in the Nova basin, also located on the eastern margin of the Cottonwood Mountains structural block but adjacent to Tucki Mountain. The location of these basins in the hanging wall of the Tucki Mountain normal fault system suggests that they may be allochthonous parts of an earlier basin. Distinctive cobbles with no known local source are abundant in a relatively thin stratigraphic zone within the Ubehebe basin. We interpret these cobbles as reworked clasts derived from the Oligocene Titus Canyon Formation. This is consistent with structural and stratigraphic correlations that indicate about 70 km northwest-directed transport of the Cottonwood Mountains block from a position adjacent to the Funeral Mountains (Snow and Wernicke, 1988,1989; Snow, 1989; Snow and others, 1989).

Abstract The Banning fault site is located at the mouth of Cottonwood Canyon, at the eastern end of San Gorgonio Pass. San Gorgonio Pass is the low divide separating the high steep terrain of the Peninsular Ranges Province to the south, from the higher but less steep Transverse Ranges Province to the north. The San Gorgonio Pass area is reached by taking 1–10 east from the Los Angeles-San Bernardino-Riverside region toward Indio. Most places with spectacular exposures of the Banning fault are, unfortunately, on property with restricted access, but one location is readily accessible to the public in Cottonwood Canyon near the eastern end of the Banning fault. Cottonwood Canyon is reached by taking the Verbenia exit from 1–10 about 45 mi (72 km) east of Riverside and San Bernardino. Cross north over 1–10 and turn left on the first road, Tamarack. Proceed west 0.3 mi (0.5 km) on Tamarack to Cottonwood Canyon Road. Go north 1.8 mi (2.9 km) on Cottonwood Canyon Road to a sharp right turn. Park in a parking area to the north of the turn, which is at the mouth of Cottonwood Canyon. Part of Cottonwood Canyon Road is paved; the remainder is an improved dirt road easily passable by passenger cars and buses.