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Valley-fill alluvium deposited from ca. A.D. 1400 to 1880 is widespread in tributaries of the Paria River and is largely coincident with the Little Ice Age epoch of global climate variability. Previous work showed that alluvium of this age is a mappable stratigraphic unit in many of the larger alluvial valleys of the southern Colorado Plateau. The alluvium is bounded by two disconformities resulting from prehistoric and historic arroyo cutting at ca. A.D. 1200–1400 and 1860–1910, respectively. The fill forms a terrace in the axial valleys of major through-flowing streams. This terrace and underlying deposits are continuous and interfinger with sediment in numerous small tributary valleys that head at the base of...
Extensional reactivation of thrust faults north of the Uinta Mountains in Utah and Wyoming is accompanied by Quaternary surface faulting, tilting, and monoclinal folding. The 40-km-long Bear River fault zone consists of down-to- the-west, right-stepping, en echelon scarps, each about 3.0 to 3.5 km in length, striking N20°W to N20°E. Scarp-derived colluvial deposits record two Holocene surface ruptures with net vertical tectonic displacements ranging from <1 to >5 m per event (West, 1989, in press). Radiocarbon ages indicate surface faulting events occurred at about 4.6 ka and 2.4 ka. The length of surface rupture and net slip per event in the Bear River fault zone imply a seismic potential comparable to the Lost...
Hydrograph variability and channel morphology influence rates of fluvial bedrock incision, but little data exist on these controls in natural channels. Through field monitoring we demonstrate that (1) short-term bedrock channel incision can be rapid, (2) sustained floods with smaller peak discharges can be more erosive than flash floods with higher peak discharges, due to changes in bed alluviation, and (3) bedrock channel morphology varies with local bed slope and controls the spatial distribution of erosion. We present a three-year record of flow depths and bedrock erosion for a human-perturbed channel reach that drains the Henry Mountains of Utah, USA. Starting from a small and steep ([~]30% slope), engineered...
he Green River is one of the principal tributaries in the Colorado River basin and drains 44,700 m2 in Wyoming, Colorado, and Utah. Since October 1962, flows of the Green River have been regulated by Flaming Gorge Reservoir, which is located 412 river miles upstream from its confluence with the Colorado River. Mean annual runoff has not been affected by the reservoir. The duration of the relatively large discharges that transport most of the annual sediment load, however, has decreased significantly. As a result, the mean annual sediment discharge has decreased by 54% to 3.21 � 106 tons from 6.92 � 106 tons at the Jensen gage located 105 river miles downstream from the reservoir and by 48% to 8.83 � 106 tons from...
During the first 40 years of the twentieth century, erosion was the dominant geomorphic process affecting the morphology of the Little Colorado River channel. The discharge regimen was one of frequent large floods and high annual discharge that created a wide sandy channel free of vegetation. In the 1940s and early 1950s, average annual precipitation declined, reducing annual discharge to about 57% of that of the preceding period as well as reducing the frequency of large floods. The channel adjusted to the new hydrologic regimen by reducing its width. Parts of the channel were frequently dry, and riparian vegetation, primarily nonnative salt cedar, became established on the higher channel surfaces. Precipitation...
The warmest global temperatures of the Cenozoic Era occurred in early Eocene time, following a warming trend that started in late Paleocene time. The greater Green River Basin of southwestern Wyoming is one of the best areas in the Rocky Mountains for paleobotanical investigation of the Paleocene-Eocene climatic transition. Intensive sampling has resulted in the recovery of an estimated 189 species of plant macrofossils from the Tiffanian, Clarkforkian, Wasatchian, and Bridgerian land mammal "ages." The leaf morphologies and taxonomic affinities of these fossils were used in combination with other indicators to evaluate Paleocene-Eocene climates. Following cool humid conditions in the Tiffanian, the Clarkforkian...
Estimates of surface extension in the southern Basin and Range province and transition into the Colorado Plateau range from a few percent to several hundred percent locally, yet the crustal thickness varies perhaps only 10-15 km across these provinces. Within the southern Basin and Range and the metamorphic core complex belt, extremely extended crust is directly juxtaposed against equally thick (or thinner) crust that underwent far milder extension. Unless preextension crustal thickness varied dramatically over a short distance, the crust must have maintained its thickness during extension, through mechanisms that involve crustal flow and magmatism. We employ a 300-km-long profile of seismic refraction and coincident...
Reinterpretation of a voluminous regional ash-flow sheet (Masonic Park Tuff) as two separate tuff sheets of similar phenocryst-rich dacite erupted from separate source calderas has important implications for evolution of the multicyclic Platoro caldera complex and for caldera-forming processes generally. Masonic Park Tuff in central parts of the San Juan field, including the type area, was erupted from a concealed source at 28.6 Ma, but widespread tuff previously mapped as Masonic Park Tuff in the southeastern San Juan Mountains is the product of the youngest large-volume eruption of the Platoro caldera complex at 28.4 Ma. This large unit, newly named the "Chiquito Peak Tuff," is the last-erupted tuff of the Treasure...
A stratigraphic and pollen analytical study of a thick sequence of alluvium at Chaco Canyon National Monument has provided a detailed history of late Quaternary alluviation, channel trenching, vegetation, and climate for the eastern Colorado Plateau. The alluvium at Chaco Canyon consists of five informal units, each separated by an erosional unconformity: Fajada (late Pleistocene), Gallo (7000 to 2400 B.P.), Chaco (2200 to 850 B.P.), Post-Bonito (600 B.P. to 1860 A.D.), and Historic (1935 to present). The Fajada paleosol occurs throughout northwestern New Mexico and is radiocarbon dated as older than 6,700 yr. Pollen analysis of the four younger alluvial units provides evidence for a mid-Holocene interval of aridity...
A zone of high-angle normal faults trends eastward across southeastern Utah and southwestern Colorado. Locally these faults bound huge blocks of downthrown sedimentary strata to form grabens of considerable length. Two of these grabens cross the Abajo Mountains, one of a group of laccolithic centers that dot the Colorado Plateau. Evidence from prior work suggested that the age of one of these grabens?and, by inference, of the other grabens?was the age of the uplift of the Abajo Mountains. New evidence, including two dikes along one of the faults that bound a graben and a laccolith wholly confined to a graben, suggests that the grabens were in existence before the Abajo Mountains were formed. Published in Geological...
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Detailed studies of 11 segmented normal faults from five grabens in the Needles District of Canyonlands National Park, Utah, demonstrate key processes in the growth, linkage, and evolution of grabens. Field observations and stereophotogrammetry reveal a ubiquitous asymmetry in cross-sectional geometry, based on distinct map patterns of graben-bounding faults, rollover anticlines with attendant joint dilation, footwall uplift with joint closure, and spoon-shaped graben floors. Master and antithetic faults across Devils Lane and several other grabens are defined quantitatively by the displacement distributions along the faults. Two-dimensional shape parameters that characterize the displacement profiles indicate that...
Heat-flow, temperature gradient, and gravity data are analyzed along five profiles crossing from the Colorado Plateau or Great Plains to the high topographic areas of the Southern Rocky Mountains. These profiles cross large, negative, Bouguer gravity anomalies associated with the high topography in the region, and are within the area studied by two recent seismic investigations. The heatflow and gravity profiles typically cross from maximum to minimum values in the region, allowing an FFT (fast Fourier transform) algorithm to be applied to the data so that high frequencies can be filtered. The resulting smoothed data can be fitted by the first few terms of a sine series to estimate mid-depths of crustal and upper...
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Great thicknesses of eolian dune deposits of early Oligocene age crop out in the Chuska Mountains of northwestern New Mexico-Arizona (as much as 535 m thick) and in the Mogollon-Datil volcanic field of western New Mexico-Arizona (as much as 300 m thick). 40Ar/39Ar ages of intercalated volcanic rocks indicate eolian deposition in these areas was approximately synchronous, with eolian accumulation beginning regionally at ca. 33.5 Ma and ending at ca. 27 Ma. Probable eolian sandstone of Oligocene age 483 m thick is also present in the subsurface of the Albuquerque Basin of the Rio Grande rift. The beginning of eolian deposition on the Colorado Plateau corresponds closely to the beginning of eolian (loessic) deposition...
Fieldwork within a series of mesoscale grabens in southeast Utah has revealed a particularly well-exposed system of interlinked extensional faults. A series of down-faulted grabens are developed within a 460-m-thick brittle layer of upper Paleozoic sandstone and shale, which overlies a ductile layer with a high gypsum content. All the major grabens consist of two or more overlapping elements, which are composed of fault segments. These segments may be hard-linked (fault surfaces are joined) or soft-linked (fault surfaces are isolated, but linked by ductile strain of the rock volume between them) in map view. Relay structures are defined as zones connecting the footwalls and hanging walls of overlapping fault segments...
The axial trace of the Meander anticline, which has a cumulative length of 25.5 mi, coincides with the Colorado River in Canyonlands National Park. On the southeast side of the river, eight tributary canyons also contain anticlines. In all cases, the anticlines occupy canyons that are eroded between 400 and 1,800 ft into the Pennsylvanian-Permian section. A system of closely spaced grabens has developed in the Pennsylvanian-Permian rocks which comprise the plateaus adjacent to the deeper canyons. The grabens are most extensively developed southeast of the Colorado River, where they follow the strike of the Meander anticline for 17 mi and extend 9 mi into the plateau. The Pennsylvanian Paradox evaporites, consisting...
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Oxygen isotope values from lacustrine carbonate in the Laney Member of the Green River Formation (Wyoming) exhibit a sudden, basinwide, ~6{per thousand} upsection decrease in {delta}18O at ca. 49 Ma. 40Ar/39Ar geochronology constrains the duration of the isotopic shift to ?~200,000 years. This change coincides with a sudden change in lake type, from balanced-filled in the lower LaClede Bed to overfilled in the upper LaClede Bed, as well as an increase in the proportion of calcitic (>80% calcite out of total carbonate by X-ray diffraction [XRD]) samples from 32% to 73%. The {delta}18O shift is correlatable through several locations across the Greater Green River Basin, and also coincides with a previously observed...
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Isotopic and elemental records of authigenic calcite from lacustrine deposits in the intraforeland basins of Utah were analyzed in an effort to reconstruct the regional paleoclimate, paleohydrology, and paleotopography of the early Cenozoic central North American Cordillera. Isotopic profiles for Paleogene Lakes Uinta, Flagstaff, and Claron show relatively large oxygen isotopic shifts that are diachronous among basins with an ~7{per thousand} decrease in {delta}18Ocalcite values at ca. 45 Ma in Lake Flagstaff, an ~5{per thousand} decrease in {delta}18Ocalcite values between ca. 42 and 35 Ma in Lake Claron, and an ~6{per thousand} decrease in {delta}18Ocalcite values between ca. 44 and 43 Ma in Lake Uinta. We interpret...
A reasonable interpretation of the geologic history of the Colorado River in the Basin and Range province can be put together by considering the work done by several geologists in the Imperial Valley, California, the Parker–Blythe–Cibola area, California and Arizona, and, the Lake Mead and Hualapai Plateau areas, Arizona. In the Imperial Valley, the Imperial Formation, whose age has been interpreted variously as late Miocene or early Pleistocene but most commonly Pliocene, records a transgression of the marine waters of the Gulf of California and deposition of sediments very probably of Colorado River origin. These sediments contain reworked Late Cretaceous foraminifers derived from the Mancos Shale of the Colorado...
The Green River is the longest tributary of the Colorado River. Near the town of Green River, Utah, the Green River narrowed in two discrete phases during the twentieth century. The first phase of narrowing decreased average width by about 5% and occurred between about 1930 and 1940, when the magnitude of 2-yr flood, mean annual discharge, and effective discharge decreased by about 30%, 28%, and 37%, respectively. During this first phase of narrowing, saltcedar (Tamarisk spp.), an invading non-native tree, began to establish itself in the study area, but botanists of that time did not describe the tree as abundant. Channel width was stable in the 1940s and 1950s even though saltcedar were becoming already abundant...
The stratigraphy, provenance, and structural involvement of tectogenic sedimentary rocks along the southern and western margins of the Wind River Range in western Wyoming record a complex Laramide and later uplift history and identify the timing and position of individual faulting events. The chronology of these events is (1) main uplift of the range by displacement on the Wind River fault between 90 and 49 to 50 Ma, (2) the formation of reverse faults in the hanging wall of the Wind River fault ca. 50 to 51 Ma, (3) sagging of the toe of the Wind River fault into sedimentary fill of the Green River basin between 46 and 23 Ma, (4) uplift of the core of the range along reactivated Precambrian shear zones between 30...
map background search result map search result map Processes of faulting in jointed rocks of Canyonlands National Park, Utah The Chuska erg: Paleogeomorphic and paleoclimatic implications of an Oligocene sand sea on the Colorado Plateau Paleogene landscape evolution of the central North American Cordillera: Developing topography and hydrology in the Laramide foreland Geomorphic controls on lacustrine isotopic compositions: Evidence from the Laney Member, Green River Formation, Wyoming Processes of faulting in jointed rocks of Canyonlands National Park, Utah The Chuska erg: Paleogeomorphic and paleoclimatic implications of an Oligocene sand sea on the Colorado Plateau Geomorphic controls on lacustrine isotopic compositions: Evidence from the Laney Member, Green River Formation, Wyoming Paleogene landscape evolution of the central North American Cordillera: Developing topography and hydrology in the Laramide foreland