Uncompahgre Uplift Research Articles

Stress Field Dynamics and Fault Slip Potential in the Paradox Basin

AbstractThe Paradox Basin, straddling Utah, Colorado, Arizona, and New Mexico is characterized by an intricate amalgamation of evaporites and clastic layers and is dominated by prominent salt walls and related subsurface structures. Our research offers a new examination of the stress distribution across the basin, deriving from continuous and discrete stress measurements conducted in boreholes in the region and focal mechanism analysis, emphasizing variations over salt structures. Integrating Coulomb failure criteria with probabilistic methods, we assess potential fault movements resulting from fluid pressure alterations. Our approach provides a comprehensive understanding of the Paradox Basin's state of stress, showing a continuous change of the maximum horizontal stress orientation from N‐S at the Wasatch Fault Zone to WNW‐ESE in the northern part of the Paradox Basin and to WSW‐ENE in the southern part of the basin. Further East, into the Colorado Plateau and the Uncompahgre Uplift, the SHmax orientation becomes E‐W. Decoding stress orientation dynamics has enabled critical insights into fault slip potential, especially in the basin's northern region. The salt wall faults are less likely to slip, and the Paradox Formation's evaporite and clastic rock sequence can serve as a potential low seismic risk target for carbon storage and hydrocarbon extraction.

Strain partitioning between ductile and brittle stratigraphy

The Sand Wash fault zone is a segmented and discontinuous fault system that strikes northwest to south east in the central part of the Uinta Basin. It is approximately 34 kilometers long with an uncommonly wide damage zone, typically 100 to 200 meters wide. Due to recent, rapid, and large-scale incision by the Green River and its tributaries, the Sand-Wash fault zone is well exposed in several closely spaced canyons. These canyon exposures allow mapping of the lateral relationships through panoramic photographs and surface kinematic descriptions. Most movement on the Sand Wash fault zone occurred in the late Eocene, but minor, more recent movement likely occurred. Evidence for fault timing includes strata-bound, syndepositional movement which occurred during Lake Uinta time (55 to 43 Ma BP) resulting in debris flows, slump blocks, and small (>150 meters diameter) sag basins filled with poorly organized sediments. After lithification, elongate grabens formed with up to 33.5 meters of horizontal extension. Two styles of deformation are present. Brittle rocks, such as sandstone and limestone beds, are intensely fractured and faulted, whereas clay and organic-rich rocks are largely unfractured and unfaulted, with variably folded beds that have experienced some layer-parallel slip. Laterally, deformation is distributed up to 100 meters from the fault core, which is uncommonly large for faults with short lengths and little displacement. Vertically, displacement is concentrated in brittle sandstone and carbonate beds and rare in clay and hydrocarbon-rich units, such as the Mahogany oil-shale zone of the Eocene Green River Formation. The Mahogany oil-shale zone mostly displays ductile flow (granular flow) commonly forming small décollements between overlying and underlying units. Vertical displacement on separate fault segments is generally less than 5 meters and decreases down section, dying out completely around the top of the Mahogany oil-shale zone. In this paper we show evidence for syndepositional deformation along the Sand Wash fault zone, strain partitioning along décollement surfaces, fault surfaces that experience multiple deformational phases, pop-up blocks, and graben development. We also show that deformation on the fault zone is related to extension above a neutral surface of a larger fold. This larger fold is associated with a basement-rooted fault zone that moved during Laramide tectonism as the Uncompahgre uplift developed. The Sand Wash fault zone appears to have many similarities to the larger, and more deeply buried, Duchesne fault zone 25 kilometers to the north, and the more deeply eroded Cedar Ridge fault zone located 30 kilometers to the south. The high-resolution fault model, developed herein, is thus a good proxy for other complex fault zones in the Uinta Basin. Our model will be useful to oil and gas operators as they develop horizontal wells across this and other complex fault zones in the basin.

Detrital zircon geochronology and provenance of the Middle to Late Jurassic Paradox Basin and Central Colorado trough: Paleogeographic implications for southwestern Laurentia

AbstractMiddle to Upper Jurassic strata in the Paradox Basin and Central Colorado trough (CCT; southwestern United States) record a pronounced change in sediment dispersal from dominantly aeolian deposition with an Appalachian source (Entrada Sandstone) to dominantly fluvial deposition with a source in the Mogollon and/or Sevier orogenic highlands (Salt Wash Member of the Morrison Formation). An enigmatic abundance of Cambrian (ca. 527–519 Ma) grains at this provenance transition in the CCT at Escalante Canyon, Colorado, was recently suggested to reflect a local sediment source from the Ancestral Front Range, despite previous interpretations that local basement uplifts were largely buried by Middle to Late Jurassic time.This study aims to delineate spatial and temporal patterns in provenance of these Jurassic sandstones containing Cambrian grains within the Paradox Basin and CCT using sandstone petrography, detrital zircon U-Pb geochronology, and detrital zircon trace elemental and rare-earth elemental (REE) geochemistry. We report 7887 new U-Pb detrital zircon analyses from 31 sandstone samples collected within seven transects in western Colorado and eastern Utah. Three clusters of zircon ages are consistently present (1.53–1.3 Ga, 1.3–0.9 Ga, and 500–300 Ma) that are interpreted to reflect sources associated with the Appalachian orogen in southeastern Laurentia (mid-continent, Grenville, Appalachian, and peri-Gondwanan terranes). Ca. 540–500 Ma zircon grains are anomalously abundant locally in the uppermost Entrada Sandstone and Wanakah Formation but are either lacking or present in small fractions in the overlying Salt Wash and Tidwell Members of the Morrison Formation. A comparison of zircon REE geochemistry between Cambrian detrital zircon and igneous zircon from potential sources shows that these 540–500 Ma detrital zircon are primarily magmatic. Although variability in both detrital and igneous REE concentrations precludes definitive identification of provenance, several considerations suggest that distal sources from the Cambrian granitic and rhyolitic provinces of the Southern Oklahoma aulacogen is also likely, in addition to a proximal source identified in the McClure Mountain syenite of the Wet Mountains, Colorado. The abundance of Cambrian grains in samples from the central CCT, particularly in the Entrada Sandstone and Wanakah Formation, suggests northwesterly sediment transport within the CCT, with sediment sourced from Ancestral Rocky Mountains uplifts of the southern Wet Mountains and/or Amarillo-Wichita Mountains in southwestern Oklahoma. The lack of Cambrian grains within the Paradox Basin suggests that the Uncompahgre uplift (southwestern Colorado) acted as a barrier to sediment transport from the CCT.

Post-Depositional Fluid Flow in Jurassic Sandstones of the Uncompahgre Uplift: Insights From Magnetic Fabrics

The anisotropy of magnetic susceptibility (AMS) in sedimentary rocks results from depositional, diagenetic, syn- and post-sedimentary processes that affect magnetic grains. Some studies have also shown the potential role played by post-depositional fluid flow in detrital and carbonate formations. Here we present a new case study of Middle-Upper Jurassic sandstones where secondary iron oxides, precipitated from fluids that migrated through pores, give rise to the AMS. These sandstones are well exposed in the Uncompahgre Uplift region of the Central Colorado Trough, Colorado. The magnetic foliation of these undeformed, subhorizontal strata consistently strike NE-SW over a large distance with an average 45° dip to the SE. This steep AMS fabric is oblique with respect to the regional subhorizontal bedding and therefore does not reflect the primary sedimentary fabric. Also, outcrop-scale and microscopic observations show a lack of post-depositional plastic (undulose extinction) or pressure-solution (stylolites) deformation microstructures in these sandstones, hence precluding a tectonic origin. The combination of magnetic hysteresis, isothermal remanent magnetization, and thermal demagnetization of the natural remanent magnetization indicate that these rocks carry a chemical remanent magnetization born primarily by hematite and goethite. High-field magnetic hysteresis and electron microscopy indicate that detrital magnetite and authigenic hematite are the main contributors to the AMS. These results show that post-depositional iron remobilization through these porous sandstones took place due to the action of percolating fluids which may have started as early as Late Cretaceous along with the Uncompahgre Uplift. The AMS fabric of porous sandstones does not systematically represent depositional or deformation processes, and caution is urged in the interpretation of magnetic fabrics in these types of reservoir rock. Conversely, understanding these fabrics may advance our knowledge of fluid flow in porous sandstones and may have applications in hydrocarbon exploration.

Hierarchical scales of soft-sediment deformation in erg deposits, Lower Jurassic Navajo Sandstone, Moab area, Utah, U.S.A.

ABSTRACTExtensive soft-sediment deformation (SSD) of multiple expressions and scales record active and dynamic events and processes in erg deposits of the Lower Jurassic Navajo Sandstone near Moab, Utah. The erg deposits preserve depositional environments of eolian dune, interdune, fluvial, playa, lake, and spring. A large range of SSD features, from intact beds showing little deformation to pervasively disturbed beds, exist in many of these deposits. A simplified classification index captures the different scales of SSD in ascending order of deformation intensity: 1) mostly intact bedding with small-scale wavy or undulatory deformation structures within single beds; 2) dish and flame structures; 3) meter-scale, kinked, slumped, rolled, overturned, vertical, and detached contorted crossbedding, and associated centimeter- to meter-scale pipes; and 4) disruptive diapirs and laterally extensive massive sandstone. The SSD features of deformed crossbed sets, diapirs, and massive sandstone beds, are consistently juxtaposed, and are thus genetically linked.Although the Navajo Sandstone has been considered a classic example of an extensive dry eolian system, both individual and combinations of strata bounded SSD features exemplify dynamic deformation, liquefaction, and fluidization that took place at various times after deposition. The lowest degree of deformation, SSD 1, is largely attributed to autogenic––inherent to the eolian system––or local allogenic processes. Larger degrees of deformation, SSD 2–4, were more likely produced by allogenic, external-forcing processes from regional changes in climate and/or near-surface groundwater conditions originating from the Uncompahgre uplift, with the deformation triggered by some event(s). Possible significant ground motion could have led to large-scale disruption in the Navajo sand sea across kilometer-scale intervals. The Navajo example establishes valuable hierarchical relationships of processes and products for recognizing and interpreting SSD in other ancient and modern eolian systems. This has particular relevance to sedimentary discoveries on Mars, where SSD features are visible from remote sensing imagery and rover exploration.

Controls on the structural and stratigraphic evolution of the megaflap-bearing Sinbad Valley salt wall, NE Paradox Basin, SW Colorado

Abstract The interplay between sedimentation and salt rise around a diapir results in distinct geometries that can be used to determine the structural and stratigraphic history within a basin. Using new geologic mapping, measured stratigraphic sections, and subsurface interpretations of seismic and well logs, we describe circum-diapir stratal geometries and deformation at the Sinbad Valley salt wall in the proximal, northeastern Paradox Basin, southwest Colorado (USA). We interpret these geometries in the context of newly recognized halokinetic features and salt-associated deformation (megaflaps, counterregional faults, intrasalt inclusions), present a revised stratigraphic and salt tectonic history of Sinbad Valley diapir, and compare these proximal features to those at the distal Gypsum Valley diapir and infer local versus regional controls on their formation. The deposition of conglomerates within the Paradox Formation, now preserved as intrasalt inclusions in the center of Sinbad Valley, record early elevation of the Uncompahgre Uplift. Subsequent differential sedimentary loading resulted in initiation of passive diapirism during the late Pennsylvanian through the latest Triassic/Early Jurassic, facilitated by movement on a NE-dipping, listric, counterregional fault that extends for >22 km southeast of the diapir. Exposures of a steeply dipping stratal panel of late Pennsylvanian-aged Honaker Trail Formation along the southwestern flank of Sinbad Valley are interpreted as a megaflap, a preserved remnant of the diapir roof that was folded into a vertical position by drape-folding during passive salt rise. Significant lateral changes in the surface geometry and depositional facies of the megaflap define four structural domains that may result from a combination of radial faulting and varying degrees of folding via limb rotation or limb rotation with minor hinge migration. Using key differences between Sinbad Valley and Gypsum Valley salt walls in regard to the megaflap facies, timing of megaflap formation, and the presence of a Paradox Formation conglomeratic intrasalt inclusion, we conclude that salt wall position (i.e., proximal versus distal) within a basin influences the characteristics of some of these features, whereas the timing of other features (e.g., megaflap formation) appears to be similar throughout the basin suggesting a more regional control.

PALEOENVIRONMENTAL AND PALEOGEOGRAPHIC IMPLICATIONS OF PALEOSOLS AND ICHNOFOSSILS IN THE UPPER PENNSYLVANIAN HALGAITO FORMATION, SOUTHEASTERN UTAH

The Upper Pennsylvanian (Virgilian) Halgaito Formation (HF) is an ∼ 125–155-m-thick succession of carbonate and carbonate-cemented siliciclastic strata exposed along Cedar Mesa on the Colorado Plateau in southeastern Utah. Defining the stratigraphic standing of the HF has been problematic due to differing paleoenvironmental and paleogeographic interpretations. This stratigraphic confusion is likely because the HF and overlying Cedar Mesa Sandstone lie at the interface between the underlying, predominately marine carbonate Honaker Trail Formation and the overlying, alluvial-eolian Organ Rock Formation. This study uses a combined ichnological, paleopedological, and sedimentological approach to refine the paleoenvironmental and paleogeographic history of the HF. The lower, marine portion of the HF is predominately fossiliferous packstone and calcareous sandstone, containing abundant Scalichnus and Thalassinoides. Marine sandstone beds contain shallow rhizoliths and indicate a relative sea-level drop that was punctuated by at least four, small-scale transgressions. The upper, continental section contains predominately eolian siltstone with siliciclastic Entisols and mottled Inceptisols. These paleosols contain large rhizohaloes and calcareous rhizocretions that are commonly associated with abundant Naktodemasis bowni. The uppermost ∼ 40 m of strata are laminated and crossbedded siltstone that contain little to no paleopedogenic development and few ichnofossils. These eolian beds were likely sourced from the erosion of underlying marine strata of the Hermosa Group as the Elephant Canyon Seaway regressed northward during the Pennsylvanian. Thin, coarse-grained fluvial strata can be observed throughout the HF and were likely sourced from the highlands of the Uncompahgre Uplift.

Detrital Zircon Geochronology of Pennsylvanian-Permian Strata in Colorado: Evidence for Appalachian-Derived Sediment and Implications for the Timing of Ancestral Rocky Mountains Uplift

Pennsylvanian-Permian time in north-central Colorado corresponds with uplift of the Ancestral Front Range and deposition of the Fountain, Ingleside, and Lyons Formations along its flanks. In southwestern Colorado, deposition of the Molas and Hermosa Formations along the flanks of the Uncompahgre Highlands largely represents Pennsylvanian time. We present new detrital zircon U-Pb geochronology data for the Ingleside and Lyons Formations in north-central Colorado and the Molas and Hermosa Formations in southwestern Colorado to understand sediment provenance and dispersal patterns. We determined U-Pb ages using LA-ICPMS on 120-150 zircon grains from five sandstone samples collected from shallow marine and eolian facies within the Ingleside, Lyons, Molas, and Hermosa Formations. All sandstone samples display a mixed Laurentian derivation, with age populations that record local and distal sediment sources. All samples also contain between 5% and 10% concordant Paleozoic-age zircon grains ranging from 330–490 Ma, coinciding with high magmatic flux during the Taconic and Acadian orogenies in the Appalachian orogen. Ultimate derivation from the Appalachians are also interpreted for zircon age populations ranging from 500-750 Ma and 1000-1300 Ma that likely originated from Pan-African and Grenville terranes respectively. This study detects the earliest documented appearance of Paleozoic zircons along the northern Ancestral Front Range, corresponding to deposition of the lower Ingleside Formation. We compare our data along the Front Range to previous detrital zircon studies from the underlying Fountain Formation to conclude that the Fountain-Ingleside transition was accompanied by a decrease in locally sourced detrital zircons, most likely marking the cessation of Ancestral Front Range uplift. Conversely, deposition across the Molas-Hermosa contact in southwestern Colorado was accompanied by an increase in locally-sourced detrital zircon grains, most likely marking the initiation of the Uncompahgre uplift.

Provenance Signals In the Piceance Creek Basin: Unroofing of the Sawatch Range and Extent of the Early Paleogene California River System (Colorado, U.S.A.)

Abstract: New and compiled U-Pb detrital zircon ages (n = 495) and sandstone petrographic compositions (n = 45) indicate a largely similar provenance throughout deposition of the Paleocene and lowermost Eocene Wasatch Formation in the Piceance Creek basin of northwest Colorado, U.S.A. Age spectra are dominated by Cretaceous- and Precambrian-age zircons from fluvial sand bodies composed of compositional litharenites and feldspathic litharenites. Our comparison with age spectra from other geologic formations exposed in surrounding Laramide uplifts indicates the Wasatch Formation is most similar to Upper Cretaceous fluvio-deltaic and marine strata of the Mesaverde Group, Mancos Shale, and their equivalents. Age spectra and paleodrainage directions from the lower Paleogene Wasatch and underlying Ohio Creek formations suggests recycling of these strata off the Sawatch and Uncompahgre uplifts from south and southeast of the basin. The data are inconsistent with previous hypotheses that Yavapai–Mazatzal crystalline basement in the Sawatch Range and Mesozoic eolianites in the Uncompahgre uplift contributed significant sediment to the basin. Based on these provenance insights and estimates of maximum depositional ages from detrital-zircon ages and biostratigraphy it appears that exposure and erosion of sedimentary strata of the Sawatch Range began between 65 and 60 Ma. This timing is significantly younger than the previously hypothesized ∼ 72 Ma unroofing of the crystalline basement in the uplift. Furthermore, we find no provenance evidence that sediment was transported across the Douglas Creek arch into the Piceance Creek basin from the Uinta basin. Thus, we infer this subdued Laramide structure was sufficient to block sediment dispersal from the paleo–California River system, which contemporaneously deposited ∼ 3000 km 3 of sediment, transported from the magmatic arc, in the Uinta basin.

Stratigraphy and fossil plants from the Cutler Formation (Late Paleozoic) and their paleoclimatic implications, eastern Paradox Basin, Colorado

Exposures of the late Paleozoic Cutler Formation, near the town of Gateway, Colorado, have traditionally been interpreted as the product of alluvial-fan deposition along the western flank of the Uncompahgre uplift and within the easternmost portion of the Paradox Basin. The Paradox Basin formed between the western margin of the Uncompahgre uplift, a segment of the Ancestral Rocky Mountains, and the western paleoshoreline of the North American portion of Pangea. This part of Pangea is commonly thought to have experienced semi-arid to arid conditions and warm temperatures during the Pennsylvanian and Permian. We present stratigraphic and fossil plant evidence in this paper to support prior interpretations that the Cutler near Gateway, Colorado, was deposited by alluvial fans that hosted localized wetland areas. Our findings are consistent with the results of prior studies that have suggested the climate in the area was warm, semi-arid, and ice-free at the time the plants described in this paper were living. 
 Plant fossils collected from the Cutler Formation came from two sites in The Palisade Wilderness Study Area (managed by the U.S. Department of the Interior, Bureau of Land Management) of western Colorado. The stratigraphic sections at the sites were composed mostly of pebble to cobble conglomerate and sandstone, but the fossil plants were mainly preserved in fine-grained intervals (fine-grained sandstone to siltstone). The preservation of plant fossils in the proximal Cutler Formation is remarkable because the surrounding sections consist mostly of conglomerate and sandstone interpreted as fluvial and debris-flow deposits. The fine-grained strata containing the plant horizons must have been deposited in a wet and protected setting, possibly a spring-fed abandoned channel on the alluvial fan. The plants and their surrounding sediment must have been rapidly buried in order to allow for long-term preservation of the fossils. It seems likely that vegetation was abundant in and adjacent to low-lying wet areas on the fan’s surface, based on the abundance of plant fossils found at the two sites. The fossil plant assemblage includes Calamites, Walchia, and Pecopteris. The flora are interpreted to have lived near the apex of the alluvial-fan system. These fossils suggest that warm and at least seasonally and locally wet conditions existed in the area during the time that the plants were growing. More arid conditions during the late Paleozoic are suggested by the characteristics of some of the time-equivalent and near time-equivalent rocks exposed to the west of the study area in the central Paradox Basin.

Use of Quartz Microtextural Analysis To Assess Possible Proglacial Deposition For the Pennsylvanian–Permian Cutler Formation (Colorado, U.S.A.)

Abstract SEM analysis of quartz microtextures has been shown to be useful for the study of depositional environments in Quaternary strata, and more recently has been applied to ancient strata. Certain microtextures such as curved grooves, straight grooves, deep troughs, and mechanically upturned plates have been found to be distinctive of glacial processes in particular. SEM analysis of quartz microtextures from the proximal ( 3 km) facies of the Pennsylvanian–Permian Cutler Formation where exposed adjacent to the Uncompahgre uplift, along with qualitative (histograms), semiquantitative (ternary diagram), and quantitative (nMDS) techniques were used to assess the hypothesis that this unit may reflect, in part, proglacial deposition. The proximal lacustrine facies depositionally onlaps Unaweep Canyon, a modern feature hypothesized to be an exhumed valley formed initially by late Paleozoic glaciation. The proximal lacustrine facies exhibits an abundance of high-stress microtextures consistent with a glacial influence. Grains from the distal fluvial facies exhibit fewer high-stress microtextures and more percussion fractures, which are expected to increase with transport distance a priori, owing to more prolonged entrainment in the fluvial system. Multiple approaches to data analysis, including visual inspection of raw data histograms, semiquantitative classification of textures using ternary diagrams, as well as quantitative nonparametric multivariate ordination techniques (nMDS) produce consistent results that suggest that the proximal-most Cutler Formation may reflect proglacial deposition. Despite the abundance of precipitation (diagenetic) features inherent to sedimentary strata of this age, SEM microtextural analysis is a useful tool for the study of ancient alluvial systems where other climate indicators are generally lacking.

Provenance of a Permian erg on the western margin of Pangea: Depositional system of the Kungurian (late Leonardian) Castle Valley and White Rim sandstones and subjacent Cutler Group, Paradox Basin, Utah, USA

Consideration of petrographic and U-Pb provenance data and paleocurrent analysis of Kungurian (upper Leonardian) Cutler Group strata in the salt anticline province of the Paradox Basin of Utah demonstrates striking contrasts in composition and inferred sources of stratigraphically adjacent eolian and fluvial facies. Eolian strata, termed here the Castle Valley Sandstone, exposed in the Castle Valley northeast of Moab, Utah, and long correlated with the White Rim Sandstone, were deposited on the southwestern flank of a NW-trending diapiric salt wall. The eolian strata, which overlie red fluvial sandstone and conglomerate of the undifferentiated Cutler Formation, are as much as 183 m thick in outcrop and consist of two eolianite members separated by a thin sheet-flood deposit that contains pebbles derived from the salt wall and upturned conglomeratic strata adjacent to it. Both eolian and underlying fluvial deposits thin and onlap eastward onto the now-collapsed salt wall. Fluvial strata at Castle Valley and in exposures to the northeast were transported northwestward, parallel to the salt wall. Large-scale foresets in the lower eolianite member indicate dominant northeasterly wind directions (present coordinates) and transport directly away from the contemporary Uncompahgre uplift, whereas foresets in the upper member indicate variable northeasterly and northwesterly paleowinds. The eolian strata thus accumulated on the lee side of the salt wall, but sandstone composition and northwesterly wind components indicate net transport from the northwest, comparable with dominant southeastward sand transport, away from the Pangean shoreline, documented for the greater White Rim erg to the west and northwest. The NW and NE winds are both predicted by late Paleozoic atmospheric circulation models for western Pangea.

Stratigraphy and fossil plants from the Cutler Formation (Late Paleozoic) and their paleoclimatic implications, eastern Paradox Basin, Colorado

Exposures of the late Paleozoic Cutler Formation, near the town of Gateway, Colorado, have traditionally been interpreted as the product of alluvial-fan deposition along the western flank of the Uncompahgre uplift and within the easternmost portion of the Paradox Basin. The Paradox Basin formed between the western margin of the Uncompahgre uplift, a segment of the Ancestral Rocky Mountains, and the western paleoshoreline of the North American portion of Pangea. This part of Pangea is commonly thought to have experienced semi-arid to arid conditions and warm temperatures during the Pennsylvanian and Permian. We present stratigraphic and fossil plant evidence in this paper to support prior interpretations that the Cutler near Gateway, Colorado, was deposited by alluvial fans that hosted localized wetland areas. Our findings are consistent with the results of prior studies that have suggested the climate in the area was warm, semi-arid, and ice-free at the time the plants described in this paper were living. Plant fossils collected from the Cutler Formation came from two sites in The Palisade Wilderness Study Area (managed by the U.S. Department of the Interior, Bureau of Land Management) of western Colorado. The stratigraphic sections at the sites were composed mostly of pebble to cobble conglomerate and sandstone, but the fossil plants were mainly preserved in fine-grained intervals (fine-grained sandstone to siltstone). The preservation of plant fossils in the proximal Cutler Formation is remarkable because the surrounding sections consist mostly of conglomerate and sandstone interpreted as fluvial and debris-flow deposits. The fine-grained strata containing the plant horizons must have been deposited in a wet and protected setting, possibly a spring-fed abandoned channel on the alluvial fan. The plants and their surrounding sediment must have been rapidly buried in order to allow for long-term preservation of the fossils. It seems likely that vegetation was abundant in and adjacent to low-lying wet areas on the fan’s surface, based on the abundance of plant fossils found at the two sites. The fossil plant assemblage includes Calamites, Walchia, and Pecopteris. The flora are interpreted to have lived near the apex of the alluvial-fan system. These fossils suggest that warm and at least seasonally and locally wet conditions existed in the area during the time that the plants were growing. More arid conditions during the late Paleozoic are suggested by the characteristics of some of the time-equivalent and near time-equivalent rocks exposed to the west of the study area in the central Paradox Basin.

Syn-sedimentary salt diapirism as a control on fluvial-system evolution: an example from the proximal Permian Cutler Group, SE Utah, USA

Loading of subsurface salt during accumulation of fluvial strata can result in halokinesis and the growth of salt pillows, walls and diapirs. Such movement may eventually result in the formation of salt-walled mini-basins, whose style of architectural infill may be used to infer both the relative rates of salt-wall growth and sedimentation and the nature of the fluvial-system response to salt movement. The Salt Anticline Region of the Paradox Basin of SE Utah comprises a series of elongate salt-walled mini-basins, arranged in a NW-trending array. The bulk of salt movement occurred during deposition of the Permian Cutler Group, a wedge of predominantly quartzo-feldspathic clastic strata comprising sediment derived from the Uncompahgre Uplift to the NE. The sedimentary architecture of selected mini-basin fills has been determined at high resolution through outcrop study. Mini-basin centres are characterized by multi-storey fluvial channel elements arranged into stacked channel complexes, with only limited preservation of overbank elements. At mini-basin margins, thick successions of fluvial overbank and sheet-like elements dominate in rim-syncline depocentres adjacent to salt walls; many such accumulations are unconformably overlain by single-storey fluvial channel elements that accumulated during episodes of salt-wall breaching. The absence of gypsum clasts suggests that sediment influx was high, preventing syn-sedimentary surface exposure of salt. Instead, fluvial breaching of salt-generated topography reworked previously deposited sediments of the Cutler Group atop growing salt walls. Palaeocurrent data indicate that fluvial palaeoflow to the SW early in the history of basin infill was subsequently diverted to the W and ultimately to the NW as the salt walls grew to form topographic barriers. Late-stage retreat of the Cutler fluvial system coincided with construction and accumulation of an aeolian system, recording a period of heightened climatic aridity. Aeolian sediments are preserved in the lees of some salt walls, demonstrating that halokinesis played a complex role in the differential trapping of sediment.

Provenance evidence for major post–early Pennsylvanian dextral slip on the Picuris-Pecos fault, northern New Mexico

The Picuris-Pecos fault is a major strike-slip fault in northern New Mexico (USA) that exhibits ∼37 km of dextral separation of Proterozoic lithotypes and structures. The timing of dextral slip has been controversial due largely to a lack of definitive piercing points of Phanerozoic age. The Picuris-Pecos fault formed the western boundary of the late Paleozoic Taos trough. A distinctive metasedimentary terrane that shed detritus into the western Taos trough was exposed on the Uncompahgre uplift west of the fault during the early to middle Pennsylvanian. We use the distribution of metasedimentary clasts and the age of monazite grains within clasts from conglomeratic strata of the western Taos trough to determine the paleolocation of the southern boundary of this metasedimentary terrane during the middle Pennsylvanian (Desmoinesian), and thereby quantify the subsequent separation on the fault. The rematching of detrital petrofacies with source terranes in the adjacent uplift requires ∼40–50 km of dextral separation on the Picuris-Pecos fault since the early Desmoinesian. This exceeds the present ∼37 km dextral separation of Proterozoic features by the fault, and thus implies that an ∼3–13 km sinistral separation existed on the fault in the early Desmoinesian. The ∼40–50 km of post–early Desmoinesian dextral separation on the Picuris-Pecos fault is the result of slip that accumulated late in the Ancestral Rocky Mountain deformation and/or during the Laramide orogeny.

Evolution of salt structures in the northern Paradox Basin: controls on evaporite deposition, salt wall growth and supra-salt stratigraphic architecture

The northern Paradox Basin evolved during the Late Pennsylvanian-Permian as an immobile foreland basin, the result of flexural subsidence in the footwall of the growing Uncompahgre Ancestral Rocky Mountain thick-skinned uplift. During the Atokan-Desmoinesian ( ~313-306 Ma) fluctuating glacio-eustatic sea levels deposited an ~2500m thick sequence of evaporites (Paradox Formation) in the foreland basin, interfingering with coarse clastics in the foredeep and carbonates around the basin margins. The cyclic deposition of the evaporites produced a repetitive sequence of primarily halite, with minor clastics, organic shales and anhydrite. Sediment loading of the evaporites subsequently produced a series of salt walls and minibasins, through the process of passive diapirism or downbuilding. Faults at the top Mississippian level localised the development of linear salt walls (up to 4500m high) along aNW-SE trend. A crosscutting NE-SW structural trend was also important in controlling the evaporite facies and the abrupt termination of the salt walls. Seismic, well and field data define the proximal Cutler Group (Permian) as a basinward prograding sequence derived from the growing Uncompahgre uplift that drove salt basinwards (towards the southwest), triggering the growth of the salt walls. Sequential structural restorations indicate that the most proximal salt walls evolved earlier than the more distal ones. The successive development of salt-withdrawal minibasins associated with each growing salt wall implies that parts of the Cutler Group in one minibasin may have no chronostratigraphic equivalent in other minibasins. Localised changes in along-strike salt wall growth and evolution were critical in the development of facies and thickness variations in the late Pennsylvanian to Triassic stratigraphic sequences in the flanking minibasins. Salt was probably at or very close to the surface during the downbuilding process leading to localised thinning, deposition of diapir-derived detritus and rapid facies changes in sequences adjacent to the salt wall structures.

Hot Fan or Cold Outwash? Hypothesized Proglacial Deposition in the Upper Paleozoic Cutler Formation, Western Tropical Pangea

Abstract The Permo-Pennsylvanian Cutler Formation in the proximal Paradox basin (western equatorial Pangea) has long been interpreted as low-latitude alluvial-fan deposits shed from the Uncompahgre uplift during the Ancestral Rocky Mountains orogeny. Here, we employ sedimentologic analysis of the Cutler strata within 10 km of its exposed source to hypothesize the alternative interpretation of deposition in a tropical proglacial–periglacial system. At this proximal location, the Cutler Formation onlaps Precambrian basement and dips gently southwestward, such that the section youngs distally. Along the onlap contact, crudely bedded and subhorizontal granule to cobble conglomerate of traction-flow origin is angularly juxtaposed atop inclined (15–30°) boulder diamictite and granule conglomerate of mass-flow origin (inferred subaqueous mass flows). The steeper dips form a semi-radial pattern that, together with facies evidence, suggest a subaqueous fan near this location. Immediately basinward (1–3 km from the onlap) is a gently dipping (≤ 10°) assemblage of cobble diamictite, granule conglomerate, sandstone, and mudstone of inferred subaqueous mass-flow origin. We propose that these associations collectively record topset, foreset and proximal bottomset deposits, respectively, of a lacustrine Gilbert-type delta. The slightly younger Cutler Formation exposed from 3 to 10 km from the onlap contains subhorizontal (≤ 4–7°) granule conglomerate and subordinate sandstone deposited by hyperconcentrated flood flows and traction flow in a fluvial system characterized by frequent high-discharge flow. Scattered siltstone units form a distinct assemblage reflecting silt genesis and eolian sorting (loess deposits), but with common overprinting by lacustrine and fluvial processes. We propose that these facies document the presence of generally abundant water. However, chemical and mineralogical immaturity record relatively minimal chemical weathering most compatible with a cold and wet system. Lateral to the inferred topset exposure, the Cutler strata fill a paleotrough within the Precambrian basement and contain abundant lonestones interpreted to record ice rafting, suggesting that the lacustrine system was, at times, ice-contact. Abundant faceted clasts and loess are also consistent with a glacial influence. Ultimately, the onlap of the Cutler strata onto the Precambrian basement of ancestral Unaweep Canyon, which has elsewhere been hypothesized to be a late Paleozoic valley of glacial origin, establishes a potential link between the two. We propose that, as an alternative to the long-held view of an alluvial fan in a hot (tropical) climate, the facies and climate indicators of the proximal Cutler system record a proglacial to periglacial, largely lacustrine–fluvial system related to tropical glaciation of the Uncompahgre uplift during the late Paleozoic. The proximity of marine and paralic facies to the Cutler “fan” system further implies that such glaciation resulted from cold temperatures, rather than high elevations, thus indicating a globally cool planet during the time recorded by the study section. If valid, this hypothesis raises the possibility that upper Paleozoic strata in other, uplift-proximal equatorial systems record anomalously cold conditions, and perhaps merit reassessment.

Pennsylvanian sinistral faults along the southwest boundary of the Uncompahgre uplift, Ancestral Rocky Mountains, Colorado

Resolution of the large-scale kinematics of the Pennsylvanian-Permian Ancestral Rocky Mountains requires definition of sense of slip and time of movement on specific faults within the system of fault-bounded basins and uplifts (e.g., Paradox basin, Uncompahgre uplift). Along an east-west–striking segment of the boundary between the Paradox basin and Uncompahgre uplift, a system of steep faults defines a horst and graben system, which includes the Grenadier fault block. Positive flower structures (anticlines) along both the bounding (Coal Bank Pass and Molas Creek faults) and internal (Snowdon fault) faults of the Grenadier fault block indicate strike-slip displacement, and oblique subsidiary folds show sinistral slip. On both sides of the Snowdon fault, stratigraphic thinning of parts of the Pennsylvanian Hermosa Group toward the fault documents synsedimentary growth of the positive flower structure, suggesting Pennsylvanian sinistral slip along this part of the Ancestral Rocky Mountains. Sinistral strike-slip along one east-west–striking segment of the fault system between the Paradox basin and Uncompahgre uplift is compatible with reverse slip on northwest-striking segments, demonstrating that a regional assembly of data to define sense of slip and time of movement on specific faults will better constrain regional-scale models for kinematics and mechanics of Ancestral Rocky Mountains deformation.

Regional paleohydrologic and paleoclimatic settings of wetland/lacustrine depositional systems in the Morrison Formation (Upper Jurassic), Western Interior, USA

During deposition of the Upper Jurassic Morrison Formation, water that originated as precipitation in uplands to the west of the Western Interior depositional basin infiltrated regional aquifers that underlay the basin. This regional groundwater system delivered water into the otherwise dry continental interior basin where it discharged to form two major wetland/lacustrine successions. A freshwater carbonate wetland/lacustrine succession formed in the distal reaches of the basin, where regional groundwater discharged into the Denver–Julesburg Basin, which was a smaller structural basin within the more extensive Western Interior depositional basin. An alkaline–saline wetland/lacustrine complex (Lake T'oo'dichi') formed farther upstream, where shallower aquifers discharged into the San Juan/Paradox Basin, which was another small structural basin in the Western Interior depositional basin. These were both wetlands in the sense that groundwater was the major source of water. Input from surface and meteoric water was limited. In both basins, lacustrine conditions developed during episodes of increased input of surface water. Inclusion of wetlands in our interpretation of what had previously been considered largely lacustrine systems has important implications for paleohydrology and paleoclimatology. The distal carbonate wetland/lacustrine deposits are well developed in the Morrison Formation of east-central Colorado, occupying a stratigraphic interval that is equivalent to the “lower” Morrison but extends into the “upper” Morrison Formation. Sedimentologic, paleontologic, and isotopic evidence indicate that regional groundwater discharge maintained shallow, hydrologically open, well oxygenated, perennial carbonate wetlands and lakes despite the semi-arid climate. Wetland deposits include charophyte-rich wackestone and green mudstone. Lacustrine episodes, in which surface water input was significant, were times of carbonate and siliciclastic deposition in scarce deltaic and shoreline deposits. Marginal lacustrine deposits include ooid and skeletal packstone–grainstone, siltstone, and sandstone. Distal lacustrine units are skeletal mudstone–wackestone, microbialites, and laminated (siliciclastic) mudstone. Differentiation between wetlands and distal lacustrine units is not always possible. Palustrine features, Magadi-type chert (MTC), and evaporites record episodes of increased aridity and exposure. Farther upstream, during deposition of the upper part of the Brushy Basin Member, the ancestral Uncompahgre Uplift imposed a barrier to shallow, eastward-flowing groundwater that discharged into the San Juan/Paradox Basin on the upstream side of the uplift. This created the closed hydrologic setting necessary for development of an alkaline–saline wetland/lacustrine complex (“Lake” T'oo'dichi'). Silicic volcanic ash, delivered by prevailing winds from calderas west and southwest of the basin, contributed to the pore-water evolution in the sediments. A distinctive lateral hydrogeochemical gradient, reflecting increasing salinity and alkalinity in the pore waters, altered the ash to a variety of authigenic minerals that define concentric zones within the basin. The basinward progression of diagenetic mineral zones is smectite→clinoptilolite→analcime±potassium feldspar→albite. The groundwater-fed wetlands were shallow and frequently evaporated to dryness. Scarce laminated gray mudstone beds record distinct episodes of freshwater lacustrine deposition that resulted from intermittent streams that carried detritus well out into the basin.

A flexural model for the Paradox Basin: implications for the tectonics of the Ancestral Rocky Mountains

ABSTRACTThe Paradox Basin is a large (190 km × 265 km) asymmetric basin that developed along the southwestern flank of the basement‐involved Uncompahgre uplift in Utah and Colorado, USA during the Pennsylvanian–Permian Ancestral Rocky Mountain (ARM) orogenic event. Previously interpreted as a pull‐apart basin, the Paradox Basin more closely resembles intraforeland flexural basins such as those that developed between the basement‐cored uplifts of the Late Cretaceous–Eocene Laramide orogeny in the western interior USA. The shape, subsidence history, facies architecture, and structural relationships of the Uncompahgre–Paradox system are exemplary of typical ‘immobile’ foreland basin systems.Along the southwest‐vergent Uncompahgre thrust, ∼5 km of coarse‐grained syntectonic Desmoinesian–Wolfcampian (mid‐Pennsylvanian to early Permian; ∼310–260 Ma) sediments were shed from the Uncompahgre uplift by alluvial fans and reworked by aeolian‐modified fluvial megafan deposystems in the proximal Paradox Basin. The coeval rise of an uplift‐parallel barrier ∼200 km southwest of the Uncompahgre front restricted reflux from the open ocean south and west of the basin, and promoted deposition of thick evaporite‐shale and biohermal carbonate facies in the medial and distal submarine parts of the basin, respectively. Nearshore carbonate shoal and terrestrial siliciclastic deposystems overtopped the basin during the late stages of subsidence during the Missourian through Wolfcampian (∼300–260 Ma) as sediment flux outpaced the rate of generation of accommodation space. Reconstruction of an end‐Permian two‐dimensional basin profile from seismic, borehole, and outcrop data depicts the relationship of these deposystems to the differential accommodation space generated by Pennsylvanian–Permian subsidence, highlighting the similarities between the Paradox basin‐fill and that of other ancient and modern foreland basins. Flexural modeling of the restored basin profile indicates that the Paradox Basin can be described by flexural loading of a fully broken continental crust by a model Uncompahgre uplift and accompanying synorogenic sediments. Other thrust‐bounded basins of the ARM have similar basin profiles and facies architectures to those of the Paradox Basin, suggesting that many ARM basins may share a flexural geodynamic mechanism. Therefore, plate tectonic models that attempt to explain the development of ARM uplifts need to incorporate a mechanism for the widespread generation of flexural basins.