Eastern Colorado Plateau Research Articles

Revisions to the Wanakah Formation in and around Ouray County, Colorado

A well-defined type section and consistent nomenclature are fundamental requirements for successful correlation of stratigraphic units and subsequent accurate paleogeographic reconstructions. The exact location of the original type section of the Middle Jurassic Wanakah Formation of the eastern Colorado Plateau was found in Ouray County, Colorado. Two nearby reference sections are described that supplement the lithologic descriptions in the original type section. These reference sections and the original type section constitute a “principal reference section” for the Wanakah Formation. Informal names “upper shale beds”, “marl member”, and “beds at Sawpit” have been used to describe the uppermost member of the Wanakah. These terms are replaced with the formal name, “Crooked Tree Member of the Wanakah Formation”, in the reference sections to avoid confusion, foster consistent usage, and facilitate litho-stratigraphic correlation.

Low‐Rate Faulting on the Margin of the Colorado Plateau and Rio Grande Rift in North‐Central New Mexico

AbstractIn northern New Mexico, seismic hazard outside of the Rio Grande Rift (RGR) is generally considered low due to limited historical seismicity and few mapped Quaternary faults. Although the region has a complex tectonic history, nearly all known faults are considered inactive because of a lack of faulted Quaternary deposits. Analysis of lidar and existing geologic and geomorphic mapping permits identification and characterization of four faults on the Colorado Plateau/RGR margin. The Gallina, Willow Creek, and East and West Brazos Peak faults are normal or dextral‐normal faults that strike NW‐SE with lengths from 15 to 50 km. The Willow Creek fault has scarp heights <0.5–6.4 m on five Quaternary surfaces, with displacement increasing with relative surface age. An ∼5‐m‐high scarp on the ∼250 ka Brazos basalt yields an ∼0.02 mm/yr minimum dip‐slip rate. Oblique slip is recorded by dextral offset (∼40 m) of a middle Quaternary (?) terrace riser, a linear and en‐echelon stepping fault pattern, and a consistent down‐to‐the‐northeast fault expression. The West Brazos Peak fault displaces late Quaternary (?) surfaces by <4 m down‐to‐the‐southwest. We interpret these faults are Laramide‐aged faults reactivated as normal or dextral‐oblique faults in the Quaternary, consistent with regional stress and geodetic data. Although the faults have low slip rates (<0.1 mm/yr), the fault lengths suggest they may rupture in M6‐7 earthquakes and may pose a previously unrecognized seismic hazard to the region. Dextral transtension within the eastern Colorado Plateau is consistent with geodetic strain rates and earthquake focal mechanisms.

Phylogenetic revision of the psammophilic Trogloderus LeConte (Coleoptera: Tenebrionidae), with biogeographic implications for the Intermountain Region.

The genus Trogloderus LeConte, 1879, which is restricted to dunes and sandy habitats in the western United States, is revised using morphological and molecular information. Six new species are described from desert regions: Trogloderus arcanus New Species (Lahontan Trough); Trogloderus kandai New Species (Owens Valley); Trogloderus major New Species (Mohave Desert); Trogloderus skillmani New Species (eastern Great Basin and Mohave Desert); Trogloderus verpus New Species (eastern Colorado Plateau); and Trogloderus warneri New Species (western Colorado Plateau). A molecular phylogeny is presented for the genus and used to infer its historical biogeography. The most recent common ancestor of Trogloderus is dated to 5.2 mya and is inferred to have inhabited the Colorado Plateau. Current species most likely arose during the mid-Pleistocene where the geographic features of the Lahontan Trough, Bouse Embayment and Kaibab Plateau were significant factors driving speciation.

Trace element partitioning and Lu–Hf isotope systematics in spinel peridotites from the Rio Grande Rift and Colorado Plateau: Towards improved age assessment of clinopyroxene Lu/Hf–176Hf/177Hf in SCLM peridotite

Our study of Colorado Plateau and Rio Grande rift spinel peridotite xenoliths determined bulk Lu–Hf budgets and cpx, opx, and whole rock 176Hf/177Hf to evaluate the potential age significance of Lu/Hf–Hf isotope correlations in sub-continental lithospheric mantle (SCLM) derived xenoliths. The samples have fertilities (spinel Cr# range from 0.1 to 0.5), and equilibration temperatures (950–1050°C) that overlap with those of spinel peridotites commonly used in Lu–Hf dating studies. The Lu/Hf of clinopyroxene (cpx) and its associated whole rock (WR) are similar in fertile samples, but cpx has a lower Lu/Hf than that of the WR in refractory samples. Orthopyroxene (opx) has systematically higher Lu/Hf than cpx, but in many samples, the opx and cpx have identical 176Hf/177Hf, suggesting that the Hf isotope composition of these minerals is supported by the Lu/Hf of the WR rather than that of the individual minerals. Many opx fractions and whole rocks have less radiogenic 176Hf/177Hf than their corresponding cpx (but similar to that of the host magma), reflecting contamination. Because the 176Hf/177Hf of cpx is supported by the WR Lu/Hf, it is necessary to use the latter rather than the cpx Lu/Hf for isochron dating. However, because the actual WR Lu/Hf values are susceptible to secondary overprinting, we recommend using model WR Lu/Hf values (determined from mineral Lu and Hf concentrations and modal abundances) to construct external cpx isochrons. Model WR and measured cpx 176Hf/177Hf are correlated (r2=0.98) in a suite of eastern Colorado Plateau xenoliths and yield an apparent age of 1.2Ga (versus 1.5Ga when – incorrectly – using cpx Lu/Hf). Despite the good correlation between Lu/Hf and 176Hf/177Hf, we do not find correlations between indicators of melt depletion and Lu/Hf or 176Hf/177Hf. This suggests that Lu/Hf–176Hf/177Hf correlations are not the result of melt depletion. Instead, we propose that Lu/Hf–176Hf/177Hf correlations are the result of mixing between depleted and enriched components (e.g. metasomatism of depleted lithospheric mantle). We emphasize that additional data should be utilized (e.g. whole rock Lu–Hf budget, degree of metasomatism, opx 176Hf/177Hf) when interpreting apparent Lu–Hf isochrons for age significance.

Calciodelrioite, Ca(VO3)2(H2O)4, the Ca analogue of delrioite, Sr(VO3)2(H2O)4

AbstractCalciodelrioite, ideally Ca(VO3)2(H2O)4, is a new mineral (IMA 2012-031) from the uraniumvanadium deposits of the eastern Colorado Plateau in the USA. The type locality is the West Sunday mine, Slick Rock district, San Miguel County, Colorado. The new mineral occurs on fracture surfaces in corvusite- and montroseite-impregnated sandstone and forms as a result of the oxidative alteration of these phases. At the West Sunday mine, calciodelrioite is associated with celestine, gypsum, huemulite, metarossite, pascoite and rossite. The mineral occurs as transparent colourless needles, bundles of tan to brown needles and star bursts of nearly black broad blades composed of tightly intergrown needles. Crystals are elongate and striated parallel to [100], exhibiting the prismatic forms {001} and {011} and having terminations possibly composed of the forms {100} and {61}. The mineral is transparent and has a white streak, subadamantine lustre, Mohs hardness of about 2½, brittle tenacity, irregular to splintery fracture, one perfect cleavage on {001} and possibly one or more additional cleavages parallel to [100]. Calciodelrioite is soluble in water. The calculated density is 2.451 g cm– 3. It is optically biaxial (+) with α = 1.733(3), β = 1.775(3), γ = 1.825(3) (white light), 2Vmeas = 87.3(9)° and 2Vcalc = 87°. The optical orientation is X = b; Z ≈ a. No pleochroism was observed. Electronmicroprobe analyses of two calciodelrioite samples and type delrioite provided the empirical formulae (Ca0.88Sr0.07Na0.04K0.01)Σ1.00(V1.00O3)2(H2.01O)4, (Ca0.76Sr0.21Na0.01)Σ0.98(V1.00O3)2(H2.01O)4 and (Sr0.67Ca0.32)Σ0.99(V1.00O3)2(H2.00O)4, respectively. Calciodelrioite is monoclinic, I2/a, with unit-cell parameters a = 14.6389(10), b = 6.9591(4), c = 17.052(2) Å, β = 102.568(9)°, V = 1695.5(3) Å3 and Z = 8. The seven strongest lines in the X-ray powder diffraction pattern [listed as dobs Å (I)(hkl)] are as follows: 6.450(100)(011); 4.350(16)(013); 3.489(18)(020); 3.215(17)(022); 3.027(50)(multiple); 2.560(28)(15,413); 1.786(18)(028). In the structure of calciodelrioite (refined to R1 = 3.14% for 1216 Fo > 4σF), V5+O5 polyhedra link by sharing edges to form a zigzag divanadate [VO3] chain along a, similar to that in the structure of rossite. The chains are linked via bonds to Ca atoms, which also bond to H2O groups, yielding CaO3(H2O)6 polyhedra. The Ca polyhedra form a chain along b. Each of the two symmetrically independent VO5 polyhedra has two short vanadyl bonds and three long equatorial bonds. Calciodelrioite and delrioite are isostructural and are the endmembers of the series Ca(VO3)2(H2O)4–Sr(VO3)2(H2O)4. Calciodelrioite is dimorphous with rossite, which has a similar structure; however, the smaller 8-coordinate Ca site in rossite does not accommodate Sr.

Regional and climatic controls on seasonal dust deposition in the southwestern U.S.

Vertical dust deposition rates (dust flux) are a complex response to the interaction of seasonal precipitation, wind, changes in plant cover and land use, dust source type, and local vs. distant dust emission in the southwestern U.S. Seasonal dust flux in the Mojave–southern Great Basin (MSGB) deserts, measured from 1999 to 2008, is similar in summer–fall and winter–spring, and antecedent precipitation tends to suppress dust flux in winter–spring. In contrast, dust flux in the eastern Colorado Plateau (ECP) region is much larger in summer–fall than in winter–spring, and twice as large as in the MSGB. ECP dust is related to wind speed, and in the winter–spring to antecedent moisture. Higher summer dust flux in the ECP is likely due to gustier winds and runoff during monsoonal storms when temperature is also higher. Source types in the MSGB and land use in the ECP have important effects on seasonal dust flux. In the MSGB, wet playas produce salt-rich dust during wetter seasons, whereas antecedent and current moisture suppress dust emission from alluvial and dry-playa sources during winter–spring. In the ECP under drought conditions, dust flux at a grazed-and-plowed site increased greatly, and also increased at three annualized, previously grazed sites. Dust fluxes remained relatively consistent at ungrazed and currently grazed sites that have maintained perennial vegetation cover. Under predicted scenarios of future climate change, these results suggest that an increase in summer storms may increase dust flux in both areas, but resultant effects will depend on source type, land use, and vegetation cover.

Recycling detrital zircons: A case study from the Cretaceous Bisbee Group of southern Arizona

Statistically indistinguishable U-Pb age spectra for detrital zircon populations from the quartzose petrofacies of the fluvial Lower Cretaceous Bisbee Group (southeastern Arizona) and from quartzose eolianites of the Middle to Upper Jurassic San Rafael Group (eastern Colorado Plateau) support the inference that quartzose detritus was reworked into the Bisbee basin of the border rift belt from older eolianites exposed along the uplifted rift shoulder to the north. The comparative data indicate that age spectra of detrital zircons can be reproduced faithfully during sediment recycling, and the possibility of zircon recycling should be considered for many provenance interpretations based on U-Pb ages of detrital zircons. Analogous relations between U-Pb age spectra of detrital zircons in the Lower Cretaceous basal McCoy Mountains Formation (California) and in Lower to Middle Jurassic eolianites of the western Colorado Plateau to the north suggest similar reworking of detritus from sedimentary cover of the rift shoulder into the keel of the rift belt farther west.

Upper mantle structure beneath the eastern Colorado Plateau and Rio Grande rift revealed by Bouguer gravity, seismic velocities, and xenolith data

The lithospheric architecture of the Colorado Plateau–Rio Grande rift transition zone reflects a narrow corridor of active tectonism and magmatism between the tectonically stable Colorado Plateau and the stable interior of North America. In this paper we analyze Bouguer gravity data, crustal thickness variations, xenolith data, and seismic wave speed structure to argue that the Rio Grande rift and southeastern Colorado Plateau is underlain by a low‐density upper mantle province that does not trend along upper crustal tectonic boundaries but, rather, is correlated with regions of middle to late Tertiary magmatism. We model uppermost mantle density contrasts using Bouguer gravity along five parallel profiles across the Colorado Plateau and validate our models by converting modeled density contrasts to seismic velocity variations for comparison with regional seismic data, as well as comparing modeled densities to calculated densities from xenoliths. We find that upper crustal features and crustal thickness variations do not explain long‐wavelength (>200 km) variations in Bouguer gravity across the Colorado Plateau–Rio Grande rift. A large 200–440 km wide region of low uppermost mantle density is modeled beneath the eastern Colorado Plateau margin, Jemez Lineament, Rio Grande rift, and parts of the westernmost Great Plains. This region has a density contrast of −90 to −30 kg/m3 relative to the adjacent Colorado Plateau and Great Plains provinces and coincides with previously imaged regions of low upper mantle seismic velocities. In situ densities inferred from xenolith data in the Colorado Plateau support the modeled upper mantle density variations and support the idea that the eastern and western margins of the plateau have been significantly modified by metasomatism. We suggest that the region of low upper mantle density likely reflects some combination of temperature variations, partial melt contents, and compositional variations related to Tertiary magmatic activity and/or thinned lithosphere. The relationship of this zone of modified upper mantle to extension in the Rio Grande rift remains enigmatic, however, particularly given the lack of spatial correlation between the trends of upper mantle structures and surficial extensional structures.

Insights into the evolution of the uppermost continental mantle from xenolith localities on and near the Colorado Plateau and regional comparisons

The interplay between the evolution of mantle and crust has been investigated by textural and electron probe analysis of Cr‐diopside spinel peridotite xenoliths from six localities and by comparisons to a database of mineral compositions. The six localities are on a 300‐km transect from the Colorado Plateau into the adjoining Rio Grande rift. Typical xenolith populations from the Colorado Plateau province have magnesian olivine (average Fo90.5) and Ti and Na‐poor clinopyroxene. In contrast, typical xenolith populations from the Rio Grande rift and the Basin and Range are characterized by more Fe‐rich olivine (average Fo89.6) and clinopyroxene with a range of Ti and Na (average 0.44 wt% TiO2). The contrast between the xenoliths from these adjacent provinces documents that the Colorado Plateau has a depleted mantle root that helps to stabilize it. At Mount Taylor, however, a locality high on a composite volcano within the eastern Colorado Plateau, most xenoliths are not depleted, and the Fe‐rich olivine (Fo86–Fo88) in them may characterize mantle affected by silicate melt metasomatism at high melt/rock ratios. Low‐Al orthopyroxene occurs at two of the six transect localities, and part of a large porphyroblast contains only 0.13 wt% Al2O3; because of the unusual compositions and textures, water‐rock reactions are inferred to have been important in parts of the refractory continental mantle. Zonation of Ca and Al in orthopyroxene provides constraints on the temperature histories of the unusual pyroxenes. Equilibration temperatures for xenoliths from Lunar Crater, Nevada, average 1270°C, much higher than the average of 1012°C for others from the Basin and Range; Lunar Crater xenoliths may be samples of a plume or asthenosphere emplaced recently into the uppermost mantle.

Implications of Jurassic, Cretaceous, and Proterozoic piercing lines for Laramide oblique-slip faulting in New Mexico and rotation of the Colorado Plateau

Research Article| June 01, 1999 Implications of Jurassic, Cretaceous, and Proterozoic piercing lines for Laramide oblique-slip faulting in New Mexico and rotation of the Colorado Plateau Steven M. Cather Steven M. Cather 1New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico 87801 Search for other works by this author on: GSW Google Scholar Author and Article Information Steven M. Cather 1New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico 87801 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1999) 111 (6): 849–868. https://doi.org/10.1130/0016-7606(1999)111<0849:IOJCAP>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Steven M. Cather; Implications of Jurassic, Cretaceous, and Proterozoic piercing lines for Laramide oblique-slip faulting in New Mexico and rotation of the Colorado Plateau. GSA Bulletin 1999;; 111 (6): 849–868. doi: https://doi.org/10.1130/0016-7606(1999)111<0849:IOJCAP>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Regional isopach patterns and pinch-out data for nine stratigraphic units of Jurassic and Cretaceous age are either permissive or supportive of significant dextral slip of Laramide age along the eastern Colorado Plateau boundary in New Mexico. The best-constrained dextral offset estimates are for the Sand Hill–Nacimiento fault system (20–33 km) and the previously published 13 km estimate for the Defiance monocline, which together yield a cumulative offset of 33–46 km. Mesozoic stratigraphic constraints for other Laramide fault systems are less precise, and typically provide only maximum limits for possible dextral offsets because of widely spaced control points and broad areas of Tertiary erosion. These less precise constraints allow as much as 40–60 km of Laramide dextral slip along what is now the Rio Grande rift and as much as 110 km across the entire breadth of the Laramide deformed zone in central and northern New Mexico.Well-documented dextral offsets of Proterozoic lithologies and structures across the Tusas-Picuris fault system (15 km) and Picuris-Pecos fault (37 km) probably represent minimum Laramide displacements because of the need to account for sinistral components related to other deformations. These displacements, when combined with dextral offsets along Sand Hill–Nacimiento and Defiance structures, yield a minimum dextral offset of ∼85 km for Laramide structures in northern New Mexico. This minimum dextral offset is approximately equivalent to the amount of Laramide crustal shortening on and northward of the Colorado Plateau, a result that argues against nearby Euler pole locations for Laramide rotation of the Colorado Plateau relative to cratonic North America. Geological constraints allow ∼0° to 3° of Laramide clockwise rotation of the Colorado Plateau. Additional clockwise plateau rotation during late Cenozoic development of the Rio Grande rift was 1° to 1.5°. Geological constraints thus indicate that clockwise rotation of the Colorado Plateau from combined Laramide and Rio Grande rift deformations was between about 1° and 4.5°. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Crustal thickness variations across the Colorado Rocky Mountains from teleseismic receiver functions

Variations in crustal thickness from the Great Plains of Kansas, across the Colorado Rocky Mountains, and into the eastern Colorado Plateau are determined by receiver function analysis of broadband teleseismic P waveforms recorded during the 1992 Rocky Mountain Front Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) experiment. The receiver functions are calculated using a time domain deconvolution approach and are interpreted in terms of a single crustal layer, with thickness determined by a grid‐search comparison of observed receiver functions with synthetics. The average crustal thicknesses determined by these methods are Kansas Great Plains, 43.8±0.4 km; Colorado Great Plains, 49.9±1.2 km; Colorado Rocky Mountains, 50.1±1.3 km; and northeast Colorado Plateau, 43.1±0.9 at latitudes of 38°–40°N. The main variations in crustal thickness that we observe are between the Kansas Great Plains and the Colorado Great Plains and between the Rocky Mountains and the Colorado Plateau. There is not a significant crustal thickness difference between the Colorado Great Plains and the Colorado Rocky Mountains. Together with gravity data and mass balance calculations, these results are incompatible with the hypothesis that the compensation of the Rocky Mountains relative to the Great Plains is accommodated purely by an Airy‐type crustal root or any other mechanism that restricts compensation solely to the crust and requires significant support for the excess topography of the Rocky Mountains to come from the mantle. Models with a rigid elastic plate may match receiver function estimates of crustal thickness but underpredict the amplitude of the gravity low over the Rockies. Our favored model includes lateral variations in crustal velocities obtained from refraction studies and crustal thickness variations constrained by the receiver functions. These models indicate that there is a profound transition in mantle density structure near the eastern range front.

The active southwest margin of the Colorado Plateau: Uplift of mantle origin

During Cenozoic time, the Colorado Plateau was raised about 2 km above sea level. The most-recent and best-documented uplift of the plateau (;1 km) has been concentratedatitssouthwestmarginbetween6 and 1 Ma, whereas the eastern Colorado Plateau may have been at high elevations sinceEocenetime.Tobetterunderstandthe recent tectonic activity at the southwest marginoftheColoradoPlateau,wecompile detailed crustal thickness and density informationfromseismicandgravitydatafor a region that includes northwest Arizona and the southern tip of Nevada. This information is used to isolate the mantle contribution to uplift. Wefind that there is relatively low density mantle underlying the southernmarginoftheplateauinnorthwest Arizona, which could result from about 60‐80 km of thinning of the dense mantle lithosphere combined with about 100 &Cof heating through a 100-km-thick mantle layer. The available estimates from earthquake-source seismology in or near the studyareaarecompatiblewiththisestimate of lithospheric thinning. We speculate that uplift may result from subduction-related thinning of the continental lithosphere.

The western margin of the Rio Grande Rift in northern New Mexico: An aborted boundary?

The northwestern margin of the Espanola basin, part of the Rio Grande rift in northern New Mexico, is characterized by a zone >17 km wide of oblique-slip faults that offset upper Paleozoic and Mesozoic strata of the eastern Colorado Plateau from Eocene and younger sedimentary rocks of the rift. Along this margin, a reasonably complete section of pre- and synrift Tertiary sediments is exposed. Combined interpretations of seismic reflection, seismic refraction, gravity, and geologic data acquired along a profile perpendicular to this boundary define the geometry of faulting, possible rotation of sedimentary units, and stratigraphy of rift fill. Vertical separation on the westernmost major fault, assumed to be the bounding fault between the rift and the Colorado Plateau, is Although Tertiary units are progressively faulted downward toward the axis of the rift, depth to inferred Precambrian crystalline rocks becomes shallower and the stratigraphic thickness of the intervening Paleozoic and Mesozoic units decreases toward the axis. We interpret pinching out of these units toward the east as erosional thinning on the western flanks of the Laramide-age Sangre de Cristo/Brazos geanticline, which underlay much of the present rift basin. Imprecise age constraints suggest that faulting of the rift margin began 10-7 Ma, but was not active after 7 Ma. Extension was apparently transferred to the Embudo fault zone, which remained active until at least 2.5 Ma and possibly into Quaternary time. The Embudo transfer zone effectively decoupled the Abiquiu embayment from the main Espanola basin. Thus the boundary at Abiquiu preserves an early stage in the formation of the rift boundary. The shift in activity may have resulted from a change in regional stress field, or from increasing magnitude of strain, or both. The change in locus of extension reflects a narrowing of rift basins through time and an integration of main bounding structures between adjacent basins. Although we are uncertain whether the Abiquiu region, which uniquely preserves an early stage of deformation, is representative of other areas of continental extension, our results indicate that the initial formation of rift basins may occur as high-angle, planar normal faults distributed over a broad zone. No evidence from seismic data or from rotation of beds exists to indicate that faults become listric with depth, which is compatible with the small amount of extension (3.5%) inferred at this boundary.

Jurassic Lake T'oo'dichi': A large alkaline, saline lake, Morrison Formation, eastern Colorado Plateau

A large alkaline, saline lake, Lake T9oo9dichi9, occupied the entire eastern part of the Colorado Plateau region during deposition of the Brushy Basin Member of the Upper Jurassic Morrison Formation. The lake extended from near the site of Albuquerque, New Mexico, to near the site of Grand Junction, Colorado, and occupied a region that encompassed the San Juan and ancestral Paradox basins, making it the largest ancient alkaline, saline lake known. The lake was typically shallow and frequently evaporated to dryness, although the water table was probably never far beneath the surface. Intermittent streams carried detritus from source areas to the west and southwest far out into the lake basin, a reflection of a low depositional gradient and the ephemeral nature of the lake. Prevailing westerly winds carried silicic volcanic ash to the lake basin from an arc region to the west and southwest. A lateral hydrogeochemical gradient, characterized by increasing salinity and alkalinity from the margin of the lake to the center, developed in pore waters of the lacustrine sediments and resulted in alteration of the ash to a variety of authigenic minerals that define concentric zones within the lake basin. The basinward progression of diagenetic mineral zones is smectite → clinoptilolite → analcime ± potassium feldspar → albite. A concomitant lateral zonation of mixed-layer illite/smectite developed, from highly smectitic in the outermost zones to highly illitic in the central zone. Authigenic albite and illitic mixed-layer clay, minerals previously thought to require elevated temperatures for formation, crystallized during early diagenesis at syndepositional temperatures in the lacus trine sediments of Lake T9oo9dichi9. This study presents evidence for a syndepositional origin for both of these minerals. These findings have important implications in terms of limiting the use of these minerals as geothermometers. Recognition of alkaline, saline-lake deposits in the Morrison Formation significantly alters interpretations of depositional environments of this formation, and it also has important implications for paleoclimatic interpretation. Late Jurassic climate was apparently much more and than had previously been thought. In fact, sedimentologic evidence suggests that the lake basin was typically dry for extended periods and enjoyed only brief wet intervals. This conclusion has important consequences for environmental interpretation of the habitat that was favorable for large herbivorous dinosaurs, which thrived in the Late Jurassic.

Significance of past and recent heat-flow and radioactivity studies in the Southern Rocky Mountains region

As reported in The Geological Society of America Bulletin of 1950, Francis Birch's innovative heat-flow research in the Colorado Front Range introduced frequently used terrain correction and thermal-conductivity measurement methods. That report also presented the first empirical evidence for a positive correlation between above-normal flux and radioactive roots in an isostatically compensated mountain area with a thick crust, an observation that strongly influenced continental heat-flow studies for the next 18 years. Birch's Front Range study clearly showed that reliable continental heat-flow research requires knowledge of bedrock radioactivity. This concept reached an unprecedented level of acceptance in 1968, when linear relations between heat flow and radiogenic heat production were discovered for three contrasting provinces in the United States. Subsequently, such lines have been central to virtually all heat-flow and radioactivity research on land. Heat-flow (Q) data for 139 boreholes provide new detail on thermal regimes in the Southern Rocky Mountains and bordering areas, as do radiogenic heat-production (A) data for 60 locales. Interpretations also emphasize reduced and residual heat-flow values; reduced values correspond to the intercepts of regional Q-A lines, whereas a residual value is the flux at a locale after the probable effect of near-surface heat production has been subtracted. The observed Q-A lines demonstrate that the Front Range and other easterly frontal ranges of the Southern Rocky Mountains in Colorado and northern New Mexico are characterized by a reduced flux (54-58 mWm-2) that is dramatically higher than that (∼27 mWm-2) in the Wyoming Basin-Southern Rocky Mountains area in southeastern Wyoming. Because the transitions between these provinces are narrow (≤50-60 km), sources in the upper crust must explain some of the contrasting reduced heat-flow values. In southeastern Wyoming, normal heat flow in Archean and early Proterozoic basement terranes probably reflects deep erosion that produced a thin (∼7 km) near-surface granitic layer that overlies a low-radioactivity lower crust. In the Colorado Front Range, Proterozoic, Mesozoic, and Cenozoic silicic rocks with relatively enriched radiogenic heat could comprise a 20- to 25-km-thick granitic layer in the upper crust that produces a large part of the above-normal reduced and residual flux. Here, partial melting of deep protolithic rocks in late Mesozoic and Cenozoic times could have produced a lower crust with low radiogenic heat production. By these views, Birch's heat flow-radioactive mountain root model of the Colorado Front Range is confirmed if a large part of the topography is isostatically compensated by low-density pre-Miocene crystal-line masses in the upper crust. Background reduced heat flow in Colorado parts of the Southern Rocky Mountains and the eastern Colorado Plateau is high (54-68 mWm-2). Zones of unusually high residual flux (88-118 mWm-2) occur in the Rio Grande rift zone in the environs of the Colorado mineral belt in the Leadville-northern Sawatch Range region, eastern parts of the San Juan Mountains in southern Colorado, and in the Park Range-Mountain Parks area near the Colorado-Wyoming border. The flux in these areas implies unrealistically high equilibrium temperatures near the crustmantle boundary, and the 50- to 60-km-wide borders of the Leadville-northern Sawatch Range residual heat-flow anomaly must be caused by sources in the upper crust. There-fore, young (10- to 1-m.y. -old) intrusions in a late Cenozoic rhyolitic complex in the upper crust are preferred to explain gravity lows, late Cenozoic uplift and igneous activity, and the high residual flux in the Leadville-northern Sawatch Range area. Similar models may apply elsewhere in the northern rift zone. If this interpretation is correct, magmatic thickening of the crust, not extensional-subsidence mechanisms, probably explains late Cenozoic uplift and extension of the northern Rio Grande rift-Southern Rocky Mountains system. Because very high regional flux in the northern rift zone and the San Juan Mountains areas implies above-liquidus equilibrium temperatures in the lower crust and upper mantle, serious inconsistencies arise when steady-state thermal models are examined relative to volumes of late Tertiary volcanism, uniform crustal thickness, and the absence of unusually low P-wave velocities in the mantle in the Southern Rocky Mountains.

Late Quaternary sedimentation and paleoecologic history of Chaco Canyon, New Mexico

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 that occurs later than recognized elsewhere in the Southwest. Around 5,800 yr ago both pinyon and ponderosa forests were rapidly diminished by a decrease in precipitation. Maximum reduction of the pine forests and woodlands and the greatest aridity of climate persisted from 5600 to 2400 B.P. Ponderosa forests in the mountains adjacent to Chaco Canyon had begun to expand their range by 2200 B.P., and the major expansion of pinyon woodlands in the Chaco area began about 850 yr ago. The climate at Chaco Canyon when the Basketmaker and Pueblo peoples lived there about 600 to 1150 A.D. was drier and warmer than it is today. The ponderosa pine logs used in pueblo building were obtained from the mountains east, south, or west of Chaco Canyon. Abandonment of the pueblos coincided with the incision of the valley floor by the post-Bonito channel system.