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Saguache County Colorado
Alluvial fans offer a means to unravel the intricacies of landscape, tectonic, and climatic dynamics. This book and accompanying geologic map highlight alluvial fans and their deposits exemplified by a suite of debris-flow alluvial fans emanating from the Holocene-active western range front of the Sangre de Cristo Mountains in south-central Colorado. The link between morphologies of fan surfaces and the sedimentary facies of their deposits permits a basis for evolutionary process interpretation of debris-flow alluvial fan geomorphology. A grasp of these processes will help earth scientists better discern complexities between buried paleo-surfaces (intraformational progressive unconformities), surficial deformation, and landform development as recorded in debris-flow fan deposits in the sedimentary record.
The genesis of metamorphosed Paleoproterozoic massive sulphide occurrences in central Colorado: geological, mineralogical and sulphur isotope constraints
Early incubation and prolonged maturation of large ignimbrite magma bodies: Evidence from the Southern Rocky Mountain volcanic field, Colorado, USA
Geophysical expression of buried range-front embayment structure: Great Sand Dunes National Park, Rio Grande rift, Colorado
An ignimbrite caldera from the bottom up: Exhumed floor and fill of the resurgent Bonanza caldera, Southern Rocky Mountain volcanic field, Colorado
Expression of terrain and surface geology in high-resolution helicopter-borne gravity gradient (AGG) data: Examples from Great Sand Dunes National Park, Rio Grande Rift, Colorado
Consequences of flight height and line spacing on airborne (helicopter) gravity gradient resolution in the Great Sand Dunes National Park and Preserve, Colorado
Evolution of ancient Lake Alamosa and integration of the Rio Grande during the Pliocene and Pleistocene
From Pliocene to middle Pleistocene time, a large lake occupied most of the San Luis Valley above 2300 m elevation (7550 ft) in southern Colorado. This ancient lake accumulated sediments of the Alamosa Formation (Siebenthal, 1910), for which the lake is herein named. The existence of this lake was first postulated in 1822 and proven in 1910 from well logs. At its maximum extent of nearly 4000 km 2 , it was one of the largest high-altitude lakes in North America, similar to but larger than Lake Texcoco in the Valley of Mexico. Lake Alamosa persisted for ~3 m.y., expanding and contracting and filling the valley with sediment until ca. 430 ka, when it overtopped a low sill and cut a deep gorge through Oligocene volcanic rocks in the San Luis Hills and drained to the south. As the lake drained, nearly 100 km 3 (81 × 10 6 acre-ft or more) of water coursed southward and flowed into the Rio Grande, entering at what is now the mouth of the Red River. The key to this new interpretation is the discovery of ancient shoreline deposits, including spits, barrier bars, and lagoon deposits nestled among bays and in backwater positions on the northern margin of the San Luis Hills, southeast of Alamosa, Colorado. Alluvial and lacustrine sediment nearly filled the basin prior to the lake's overflow, which occurred ca. 430 ka as estimated from 3 He surface-exposure ages of 431 ± 6 ka and 439 ± 6 ka on a shoreline basalt boulder, and from strongly developed relict calcic soils on barrier bars and spits at 2330–2340 m (7645–7676 ft), which is the lake's highest shoreline elevation. Overtopping of the lake's hydrologic sill was probably driven by high lake levels at the close of marine oxygen-isotope stage (OIS) 12 (452–427 ka), one of the most extensive middle Pleistocene glacial episodes on the North American continent. Hydrologic modeling of stream inflow during full-glacial-maximum conditions suggests that Lake Alamosa could fill at modern precipitation amounts if the mean annual temperature were just 5 °C (10 °F) cooler, or could fill at modern temperatures with 1.5 times current mean annual precipitation. Thus, during pluvial epochs the lake would rise to successively higher levels owing to sedimentation; finally during OIS 12, the lake overflowed and spilled to the south. The integration of the upper (Colorado) and lower (New Mexico) reaches of the Rio Grande expanded the river's drainage basin by nearly 18,000 km 2 and added recharge areas in the high-altitude, glaciated San Juan Mountains, southern Sawatch Range, and northern Sangre de Cristo Mountains. This large increase in mountainous drainage influenced the river's dynamics downstream in New Mexico through down-cutting and lowering of water tables in the southern part of the San Luis Valley.
Abstract The Southern Rocky Mountain volcanic field contains widespread andesite and dacitic lavas erupted from central volcanoes; associated with these are ~26 regional ignimbrites (each 150–5000 km 3 ) emplaced from 37 to 23 Ma, source calderas as much as 75 km across, and subvolcanic plutons. Exposed plutons vary in composition and size from small roof-zone exposures of porphyritic andesite and dacite to batholith-scale granitoids. Calderas and plutons are enclosed by one of the largest-amplitude gravity lows in North America. The gravity low, interpreted as defining the extent of a largely concealed low-density silicic batholith complex, encloses the overall area of ignimbrite calderas, most of which lack individual geophysical expression. Initial ignimbrite eruptions from calderas aligned along the Sawatch Range at 37–34 Ma progressed southwestward, culminating in peak eruptions in the San Juan Mountains at 30–27 Ma. This field guide focuses on diverse features of previously little-studied ignimbrites and caldera sources in the northeastern San Juan region, which record critical temporal and compositional transitions in this distinctive eastern Cordilleran example of Andean-type continental-margin volcanism.
Eruptive and noneruptive calderas, northeastern San Juan Mountains, Colorado: Where did the ignimbrites come from?
Magnetic stratigraphy of the Eocene-Oligocene floral transition in western North America
Eocene and Oligocene floras of the western United States show a climatic deterioration from warmer conditions to much cooler and drier conditions. Recent 40 Ar/ 39 Ar dates and magnetic stratigraphy have greatly improved their correlation. In this study, the uppermost Eocene Antero Formation, Colorado, is entirely reversed in polarity, and is correlated with late Chron C13r, based on 40 Ar/ 39 Ar dates of 33.77–33.89 Ma. The early Oligocene Pitch-Pinnacle flora of Colorado is within rocks of normal polarity, and best correlated with Chron C12n (30.5–31.0 Ma), based on 40 Ar/ 39 Ar dates of 32.9–29.0 Ma (although correlation with Chron C11n is also possible). The late Oligocene ( 40 Ar/ 39 Ar dated 26.26–26.92 Ma) Creede flora of southwestern Colorado is correlated with Chron C8r. The early Oligocene ( 40 Ar/ 39 Ar dated at 31.5 Ma) Granger Canyon flora in the Warner Mountains, near Cedarville, northeastern California, is correlated with Chron C12r. These results are compiled with previously published dates and magnetic stratigraphy of the Eugene-Fisher floral sequence in western Oregon, the Bridge Creek floras in central Oregon, other floras in the Warner Mountains of northeast California, and the Florissant flora of central Colorado. In Colorado, the climatic change seems to have occurred between the Florissant and Antero floras, and is dated between 33.89 and 34.07 Ma, or latest Eocene in age, although the Pitch-Pinnacle flora suggests that the deterioration was less severe and took place in the early Oligocene. In northeast California, the dating is not as precise, so the climatic change could have occurred between 31.5 and 34.0 Ma (probably early Oligocene). In western Oregon (Eugene and Fisher Formations), the change occurs between the early Oligocene Goshen flora (33.4 Ma) and the early Oligocene Rujada flora (31.5 Ma). In the John Day region of Oregon, it occurs before the oldest Bridge Creek flora, dated at 33.62 Ma (right after the Eocene-Oligocene boundary). Thus, only two of these four floral sequences (Eugene, Oregon, and Cedarville, California) clearly show the early Oligocene climatic change consistent with that documented in the global marine record, whereas the climatic change was seemingly abrupt in the late Eocene in Colorado between 33.89 and 34.07 Ma, and also sometime during the late Eocene (before 33.62 Ma) in central Oregon.
MAZZETTIITE, Ag 3 HgPbSbTe 5 , A NEW MINERAL SPECIES FROM FINDLEY GULCH, SAGUACHE COUNTY, COLORADO, USA
Syndepositional thrust-related deformation and sedimentation in an Ancestral Rocky Mountains basin, Central Colorado trough, Colorado, USA
Probabilistic assessment of volcanic hazard to radioactive waste repositories in Japan: Intersection by a dike from a nearby composite volcano
Paleoclimate and paleoelevation of the Oligocene Pitch-Pinnacle flora, Sawatch Range, Colorado
U-Pb geochronology of the Proterozoic volcano-plutonic terrane in the Gunnison and Salida areas, Colorado
Early Proterozoic supracrustal rocks near Gunnison and Salida, Colorado, include sequences of tholeiitic metabasalt, metarhyolite to metadacite, and interbedded volcaniclastic turbidite. These rocks were intruded by synchronous gabbroic sheets, complexly folded and metamorphosed in upper greenschist to upper amphibolite facies, and intruded by plutons ranging from quartz diorite to granite. U-Pb ages of zircons show that an early period of volcanism in the Gunnison area occurred from 1,770 to 1,760 Ma and was followed by emplacement of plutons from 1,755 to 1,750 Ma. A younger sequence of volcanic rocks was formed in both the Gunnison and Salida areas between 1,740 and 1,730 Ma. In the Gunnison area these rocks were intruded by major plutons from 1,725 to 1,714 Ma. Near Howard, Colorado, southeast of Salida, metarhyolite yielding ages of 1,713 and 1,668 Ma is believed to be part of the younger sequence. Late, post-tectonic granite plutons were emplaced in both areas from 1,700 to 1,670 Ma. The age data, petrography, and geochemistry of these rocks indicates that they are part of a broad belt of juvenile, arc-related terranes, exposed from southern California across Arizona, New Mexico, Colorado, and known in the subsurface as far east as western Missouri, that was accreted to the southern edge of the continent during the Early Proterozoic.