The Fish Creek-Vallecito basin contains a 5.5-km-thick section of late Miocene to early Pleistocene sedimentary rocks exposed in the hanging wall of the West Salton detachment fault. These deposits preserve a high-fi delity record of late Cenozoic subsidence and basin fi lling that resulted from deformation in the San Andreas fault system of southern California. Existing and new paleomagnetic data, combined with new U-Pb zircon ages of two tuffs high in the section, show that the section ranges in age from ca. 8.0 ± 0.4 Ma at the base to ca. 0.95 Ma at the top. Geohistory analysis reveals: (1) moderate subsidence (0.46 mm/yr) from ca. 8.0 to 4.5 Ma; (2) rapid subsidence (2.1 mm/yr) from 4.5 to 3.1 Ma; (3) moderate subsidence (0.40 mm/yr) from 3.1 to 0.95 Ma; and (4) rapid uplift and erosion that has exhumed the section since ca. 1 Ma. Onset of sedimentation at ca. 8.0 ± 0.4 Ma records earliest extension or transtension in the area, possibly related to localization of the Pacifi c-North America plate boundary in the Salton Trough and Gulf of California. Alternatively, marine incursion at 6.3 Ma may be the earliest record of plate-boundary deformation in the Gulf of California-Salton Trough region. A thick interval higher in the section records progradation of the Colorado River delta into and across the basin starting ca. 4.9 Ma. Progradation continued during an abrupt increase in subsidence rate at 4.5 Ma, and fl uvial-deltaic conditions persisted for 1.4 m.y. during the rapid-subsidence phase, indicating that delta progradation was driven by a large increase in rate of sediment input from the Colorado River. Uplift and inversion of the basin starting ca.

Sedimentary rocks in the Borrego Badlands, Southern California, contain a record of Pleistocene crustal deformation during initiation and evolution of the San Jacinto fault zone. We used detailed geologic, stratigraphic, and paleomagnetic analysis to determine the age and geometry of the deposits and reconstruct the history of fault-controlled sedimentation in this area. The base of the ~300 to 500 m thick Ocotillo Formation is a paraconformity to abrupt conformable contact that records a brief hiatus followed by rapid progradation of coarse alluvial sediment over lacustrine facies of the Borrego Formation at 1.05 ± 0.03 Ma. This coincides with regional-scale progradation of Ocotillo Formation sand and gravel, and appears to record initiation of strike-slip faults in the southwestern Salton Trough at ca. 1.1 Ma. Thickness trends, clast compositions, paleocurrents, and distribution of paleosols provide evidence for initiation of the East Coyote Mountain fault at ca. 1.05 Ma, followed by onset of NNE-ward basin tilting obliquely toward the Santa Rosa segment of the Clark fault at ca. 1.0 Ma. Stratigraphic omission of the Ocotillo Formation and progressively older units southwest of the Coyote Creek fault beneath the Fonts Point Sandstone provides evidence that tilting to the northnortheast was related in part to growth of the San Felipe anticline during deposition of the Ocotillo Formation. Map and stratigraphic data suggest that the Coyote Creek fault in the western Borrego Badlands postdates Ocotillo deposition, and thus appears to have propagated southeast into the study area at ca. 0.6 Ma. The Fonts Point Sandstone is a thin, sheetlike alluvial deposit that records the end of deposition and onset of transpressive deformation in the Borrego Badlands. The base of the Fonts Point Sandstone changes from a conformable contact in a narrow belt southeast of the Inspiration Point fault, where it is dated at 0.6 ± 0.02 Ma, to an angular unconformity on the folded Ocotillo Formation northwest of the fault. The pattern of stratal truncation records initiation of the Inspiration Point fault at ca. 0.6 Ma. This coincides with a major structural reorganization in the San Jacinto fault zone that initiated the modern phase of north-south shortening and erosion in the southwestern Salton Trough.

No abstract

The Colorado River in the southwestern U.S. provides an excellent natural laboratory for studying the origins of a continent-scale river system, because deposits that formed prior to and during river initiation are well exposed in the lower river valley and nearby basinal sink. This paper presents a synthesis of regional stratigraphy, sedimentology, and micropaleontology from the southern Bouse Formation and similar-age deposits in the western Salton Trough, which we use to interpret processes that controlled the birth and early evolution of the Colorado River. The southern Bouse Formation is divided into three laterally persistent members: basal carbonate, siliciclastic, and upper bioclastic members. Basal carbonate accumulated in a tidedominated marine embayment during a rise of relative sea level between ~6.3 and 5.4 Ma, prior to arrival of the Colorado River. The transition to green claystone records initial rapid influx of river water and its distal clay wash load into the subtidal marine embayment at ~5.4-5.3 Ma. This was followed by rapid southward progradation of the Colorado River delta, establishment of the earliest through-flowing river, and deposition of river-derived turbidites in the western Salton Trough (Wind Caves paleocanyon) between ~5.3 and 5.1 Ma. Early delta progradation was followed by regional shut-down of river sand output between ~5.1 and 4.8 Ma that resulted in deposition of marine clay in the Salton Trough, retreat of the delta, and re-flooding of the lower river valley by shallow marine water that deposited the Bouse upper bioclastic member. Resumption of sediment discharge at ~4.8 Ma drove massive progradation of fluvial-deltaic deposits back down the river valley into the northern Gulf and Salton Trough.These results provide evidence for a discontinuous, start-stop-start history of sand output during initiation of the Colorado River that is not predicted by existing models for this system. The underlying controls on punctuated sediment discharge are assessed by comparing the depositional chronology to the record of global sea-level change. The lower Colorado River Valley and Salton Trough experienced marine transgression during a gradual fall in global sea level between ~6.3 and 5.5 Ma, implicating tectonic subsidence as the main driver of latest Miocene relative sea-level rise. A major fall of global sea level at 5.3Ma outpaced subsidence and drove regional delta progradation, earliest flushing of Colorado River sand into the northern Gulf of California, and erosion of Bouse basal carbonate and siliciclastic members. The lower Colorado River valley was re-flooded by shallow marine waters during smaller changes in global sea level ~ 5.1-4.8 Ma, after the river first ran through it, which requires a mechanism to stop delivery of sand to the lower river valley. We propose that tectonically controlled subsidence along the lower Colorado River, upstream of the southern Bouse study area, temporarily trapped sediment and stopped delivery of sand to the lower river valley and nort...

We examine the Pleistocene tectonic reorganization of the Pacific-North American plate boundary in the Salton Trough of southern California with an integrated approach that includes basin analysis, magnetostratigraphy, and geologic mapping of upper Pliocene to Pleistocene sedimentary rocks in the San Felipe Hills. These deposits preserve the earliest sedimentary record of movement on the San Felipe and San Jacinto fault zones that replaced and deactivated the late Cenozoic West Salton detachment fault. Sandstone and mudstone of the Brawley Formation accumulated between ∼1.1 and ∼0.6-0.5 Ma in a delta on the margin of an arid Pleistocene lake, which received sediment from alluvial fans of the Ocotillo Formation to the west-southwest. Our analysis indicates that the Ocotillo and Brawley formations prograded abruptly to the east-northeast across a former mud-dominated perennial lake (Borrego Formation) at ∼1.1 Ma in response to initiation of the dextral-oblique San Felipe fault zone. The ∼25-km-long San Felipe anticline initiated at about the same time and produced an intrabasinal basement-cored high within the San Felipe-Borrego basin that is recorded by progressive unconformities on its north and south limbs. A disconformity at the base of the Brawley Formation in the eastern San Felipe Hills probably records initiation and early blind slip at the southeast tip of the Clark strand of the San Jacinto fault zone. Our data are consistent with abrupt and nearly synchronous inception of the San Jacinto and San Felipe fault zones southwest of the southern San Andreas fault in the early Pleistocene during a pronounced southwestward broadening of the San Andreas fault zone. The current contractional geometry of the San Jacinto fault zone developed after ∼0.5-0.6 Ma during a second, less significant change in structural style.

No abstract

Sedimentary and volcanic rocks in the Blue Mountains province (BMP) of northeastern Oregon preserve a well studied record of Triassic-Jurassic magmatism, basin evolution, and terrane accretion. Terranes of the BMP represent two magmatic arcs (Wallowa and Olds Ferry terranes), an intervening oceanic subduction and accretionary complex (Baker terrane), and a complex thick succession of sedimentary rocks commonly known as the Izee terrane. We divide volcanic and sedimentary rocks into two regionally correlative, unconformity-bounded megasequences: (1) MS-1, Late Triassic to Early Jurassic deposits that change up section from (1a) older volcanic and volcaniclastic deposits of the Wallowa and Olds Ferry arcs to (1b) marine turbidites, shale, and argillite with chert-clast conglomerate and olistostromes derived from the emergent Baker terrane; and (2) MS-2, Early to Late Jurassic marine deposits that overlap older rocks and structures and record ϳ20 to 40 m.y. of deep crustal subsidence in a large marine basin. Many of the known stratigraphic relationships in the Blue Mountains cannot be explained using the existing model for a Middle Triassic to Late Jurassic west-facing, non-collisional volcanic arc and forearc basin. We propose a new tectonic model for the BMP based on prior studies and comparison to modern analogues, which includes: (1) Middle Triassic magmatism in the Wallowa and Olds Ferry arcs during subduction and progressive closure of an ocean basin; (2) Late Triassic collision between facing accretionary wedges of the Wallowa and Olds Ferry arcs, and growth of marine basins on both sides of the emergent Baker terrane thrust belt; (3) Early to Middle Jurassic terrane-continent collision which resulted in closure of a wide back-arc basin, crustal thickening and loading in Nevada, and growth of a large marine collisional basin in the BMP; and (4) Late Jurassic thrusting, regional shortening, and final accretion of the basin and underlying terranes to western North America. This analysis suggests that collisional tectonics may have played a significant role in plate interactions that drove Triassic-Jurassic crustal thickening, mountain building, and basin development in the western North American Cordillera. introduction 1175 collisional tectonics in the Blue Mountains province, northeastern Oregon

We applied multiple geochemical tracers ( 87 Sr/ 86 Sr, [Sr], d 13 C, and d 18 O) to waters and carbonates of the lower Colorado River system to evaluate its paleohydrology over the past 12 Ma. Modern springs in Grand Canyon reflect mixing of deeply derived (endogenic) fluids with meteoric (epigenic) recharge. Travertine (<1 Ma) and speleothems (2-4 Ma) yield 87 Sr/ 86 Sr and d 13 C and d 18 O values that overlap with associated water values, providing justification for use of carbonates as a proxy for the waters from which they were deposited. The Hualapai Limestone (12-6 Ma) and Bouse Formation (5.6-4.8 Ma) record paleohydrology immediately prior to and during integration of the Colorado River. The Hualapai Limestone was deposited from 12 Ma (new ash age) to 6 Ma; carbonates thicken eastward to ~210 m toward the Grand Wash fault, suggesting that deposition was synchronous with fault slip. A fanningdip geometry is suggested by correlation of ashes between subbasins using tephrochronology. New detrital-zircon ages are consistent with the "Muddy Creek constraint," which posits that Grand Wash Trough was internally drained prior to 6 Ma, with limited or no Colorado Plateau detritus, and that Grand Wash basin was sedimentologically distinct from Gregg and Temple basins until after 6 Ma. New isotopic data from Hualapai Limestone of Grand Wash basin show values and ranges of 87 Sr/ 86 Sr, d 13 C, and d 18 O that are similar to Grand Canyon springs and travertines, suggesting a long-lived springfed lake/marsh system sourced from western Colorado Plateau groundwater. Progressive up-section decrease in 87 Sr/ 86 Sr and d 13 C and increase in d 18 O in the uppermost 50 m of the Hualapai Limestone indicate an increase in meteoric water relative to endogenic inputs, which we interpret to record progressively increased input of high-elevation Colorado Plateau groundwater from ca. 8 to 6 Ma. Grand Wash, Hualapai, Gregg, and Temple basins, although potentially connected by groundwater, were hydrochemically distinct basins before ca. 6 Ma. The 87 Sr/ 86 Sr, d 13 C, and d 18 O chemostratigraphic trends are compatible with a model for downward integration of Hualapai basins by groundwater sapping and lake spillover.The Bouse Limestone (5.6-4.8 Ma) was also deposited in several hydrochemically distinct basins separated by bedrock divides. Northern Bouse basins (Cottonwood, Mojave, Havasu) have carbonate chemistry that is nonmarine. The 87 Sr/ 86 Sr data suggest that water in these basins was derived from mixing of high-87 Sr/ 86 Sr Lake Hualapai waters with lower-87 Sr/ 86 Sr, first-arriving "Colorado River" waters. Covariation trends of d 13 C and d 18 O suggest that newly integrated Grand Wash, Gregg, and Temple basin waters were integrated downward to the Cottonwood and Mojave basins at ca. 5-6 Ma. Southern, potentially younger Bouse basins are distinct hydrochemically from each other, which suggests incomplete mixing during continued downward integration of internally drained basins. Bouse carbonates display a southward trend ...