Rocky Mountain Basins Research Articles

Geologic characteristics, exploration and production progress of shale oil and gas in the United States: An overview

We present a systematic summary of the geological characteristics, exploration and development history and current state of shale oil and gas in the United States. The hydrocarbon-rich shales in the major shale basins of the United States are mainly developed in six geological periods: Middle Ordovician, Middle–Late Devonian, Early Carboniferous (Middle–Late Mississippi), Early Permian, Late Jurassic, and Late Cretaceous (Cenomanian–Turonian). Depositional environments for these shales include intra-cratonic basins, foreland basins, and passive continental margins. Paleozoic hydrocarbon-rich shales are mainly developed in six basins, including the Appalachian Basin (Utica and Marcellus shales), Anadarko Basin (Woodford Shale), Williston Basin (Bakken Shale), Arkoma Basin (Fayetteville Shale), Fort Worth Basin (Barnett Shale), and the Wolfcamp and Leonardian Spraberry/Bone Springs shale plays of the Permian Basin. The Mesozoic hydrocarbon-rich shales are mainly developed on the margins of the Gulf of Mexico Basin (Haynesville and Eagle Ford) or in various Rocky Mountain basins (Niobrara Formation, mainly in the Denver and Powder River basins). The detailed analysis of shale plays reveals that the shales are different in facies and mineral components, and “shale reservoirs” are often not shale at all. The United States is abundant in shale oil and gas, with the in-place resources exceeding 0.246×1012 t and 290×1012 m3, respectively. Before the emergence of horizontal well hydraulic fracturing technology to kick off the “shale revolution”, the United States had experienced two decades of exploration and production practices, as well as theory and technology development. In 2007–2023, shale oil and gas production in the United States increased from approximately 11.2×104 tons of oil equivalent per day (toe/d) to over 300.0×104 toe/d. In 2017, the shale oil and gas production exceeded the conventional oil and gas production in the country. In 2023, the contribution from shale plays to the total U.S. oil and gas production remained above 60%. The development of shale oil and gas has largely been driven by improvements in drilling and completion technologies, with much of the recent effort focused on “cube development” or “co-development”. Other efforts to improve productivity and efficiency include refracturing, enhanced oil recovery, and drilling of “U-shaped” wells. Given the significant resources base and continued technological improvements, shale oil and gas production will continue to contribute significant volumes to total U.S. hydrocarbon production.

Modeling Patterns and Controls of Food Web Structure in Saline Wetlands of a Rocky Mountain Basin

Modeling Patterns and Controls of Food Web Structure in Saline Wetlands of a Rocky Mountain Basin

Stratigraphy and hydrocarbon resources of the San Juan Basin: Lessons for other basins, lessons from other basins

This paper examines the relationships between stratigraphy and hydrocarbon production from the San Juan Basin of New Mexico and Colorado. Abundant data and the long production history allow lessons to be learned, both from an exploration and development perspective, that can be applied in other basins. Conversely, as new play types and technologies are defined and developed elsewhere, the applicability of those tools in the San Juan Basin needs to be understood for well-informed exploration and development activities to continue. The San Juan Basin is a Latest Cretaceous – Tertiary (Paleogene) structure that contains rocks deposited from the Lower Paleozoic to the Tertiary, but only the Upper Cretaceous section has significant hydrocarbon, mostly gas, production. Herein I make the case for studying depositional systems, and the controls thereon (e.g., basin development, eustasy, sediment supply), because they are the first-order controls on whether a sedimentary basin can become a hydrocarbon province, or super basin as the San Juan Basin has recently been defined. Only in the Upper Cretaceous did a suitable combination of forcing mechanisms combine to form source and reservoir rocks, and repeated transgressive-regressive cycles of the Upper Cretaceous stacked multiple successions of source and reservoir rocks in a way that leads to stacked pay potential. Because of the types of depositional systems that could develop, the source rocks were primarily gas prone, like those of other Rocky Mountain basins. Oil-prone source rocks are present but primarily restricted to episodes of peak transgression. A lack of suitable trapping mechanisms helps to explain the relative dearth of conventional oil pools. Although gas production has dropped precipitously in the past decade, driven primarily by overabundance of gas supply associated with the shale-gas boom, the combination of horizontal drilling and multi-stage hydraulic fracturing is being applied to revive oil production from some unconventional stratigraphic targets with success.

Provenance of Pennsylvanian–Permian sedimentary rocks associated with the Ancestral Rocky Mountains orogeny in southwestern Laurentia: Implications for continental-scale Laurentian sediment transport systems

AbstractThe Ancestral Rocky Mountains system consists of a series of basement-cored uplifts and associated sedimentary basins that formed in southwestern Laurentia during Early Pennsylvanian–middle Permian time. This system was originally recognized by aprons of coarse, arkosic sandstone and conglomerate within the Paradox, Eagle, and Denver Basins, which surround the Front Range and Uncompahgre basement uplifts. However, substantial portions of Ancestral Rocky Mountain–adjacent basins are filled with carbonate or fine-grained quartzose material that is distinct from proximal arkosic rocks, and detrital zircon data from basins adjacent to the Ancestral Rocky Mountains have been interpreted to indicate that a substantial proportion of their clastic sediment was sourced from the Appalachian and/or Arctic orogenic belts and transported over long distances across Laurentia into Ancestral Rocky Mountain basins. In this study, we present new U-Pb detrital zircon data from 72 samples from strata within the Denver Basin, Eagle Basin, Paradox Basin, northern Arizona shelf, Pedregosa Basin, and Keeler–Lone Pine Basin spanning ∼50 m.y. and compare these to published data from 241 samples from across Laurentia. Traditional visual comparison and inverse modeling methods map sediment transport pathways within the Ancestral Rocky Mountains system and indicate that proximal basins were filled with detritus eroded from nearby basement uplifts, whereas distal portions of these basins were filled with a mix of local sediment and sediment derived from marginal Laurentian sources including the Arctic Ellesmerian orogen and possibly the northern Appalachian orogen. This sediment was transported to southwestern Laurentia via a ca. 2,000-km-long longshore and aeolian system analogous to the modern Namibian coast. Deformation of the Ancestral Rocky Mountains slowed in Permian time, reducing basinal accommodation and allowing marginal clastic sources to overwhelm the system.

Facies- to sandbody-scale heterogeneity in a tight-gas fluvial reservoir analog: Blackhawk Formation, Wasatch Plateau, Utah, USA

Using photomosaics and measured sections, this outcrop study characterizes facies- to sandbody-scale heterogeneity in the fluvial and coastal-plain deposits of the Blackhawk Formation of the Wasatch Plateau, Utah, USA, as an outcrop analog for the fluvial tight-gas reservoirs of the adjacent greater western Rocky Mountain basins as well as for conventional fluvial reservoirs elsewhere. Analysis on eight contiguous, vertical cliff-faces comprising both depositional-dip- and -strike-oriented segments provides field-validation and calibration of the entire range of fluvial heterogeneity, where: 1) large-scale heterogeneity (10's of m vertically and 100's of m laterally) is associated with stacking of channelized fluvial sandbodies encased within coastal-plain fines, 2) intermediate-scale heterogeneity (1's of m vertically and 10's of m laterally) is related to type and distribution of architectural elements like bar-accretion and crevasse-splay units within individual sandbodies, and 3) small-scale heterogeneity (10's of cm vertically and 1's of m laterally) is attributed to facies spatial variability within individual architectural elements.At a reservoir-scale (∼6 km strike-transect), impact of these heterogeneities has resulted in potential stratigraphic compartmentalization in varied patterns and scales within and among three zones, which have similar lateral extents. Distinct vertical or lateral compartmentalization, contrasting net-to-gross pattern, width-constraint by either large- or intermediate-scale heterogeneity, disparity in communication between principal reservoir compartments by intermediate-scale heterogeneity, and reservoir-quality segregation to barrier styles rendered by small-scale heterogeneity are documented in an array of trends. These intriguing trends are challenging to correlate across the reservoir-scale dataset, contributing to multiple, analogous exploration and production uncertainties. For improved tight-gas exploration and production strategy of the western Rocky Mountain basins, study results were also used in developing potential predictive tools: 1) thickness threshold of individual channelized sandbody favoring multiple well intersection, 2) aspect ratio in performing probabilistic sandbody-width estimation, and 3) prediction of sandbody amalgamation using underlying coal thickness.

Influence of tectonics, sedimentation and aqueous flow cycles on the origin of global groundwater arsenic: Paradigms from three continents

Arsenic (As) is a trace element in the Earth’s crust. However, its presence in elevated concentrations in groundwaters of major aquifers around the world raises concern about its primary source(s). A close look at the global distribution of known As enriched areas reveals an intriguing systematic pattern, where most of the As enriched aquifers are parts of large sedimentary basins adjoining major orogenic belts, suggesting the existence of a large-scale geological process. Many of these sedimentary basins may be tectonically regarded as foreland basins that developed by lithospheric flexure at the time of mountain building processes (orogenesis) along convergent plate boundaries. Arsenic enrichment in the groundwater of these foreland basin aquifers may be ultimately sourced to crustal evolution processes related to plate tectonics, along with transportation of As-enriched magmatic rocks from the depth to surficial deposits, which subsequently release or mobilize the As to groundwater under conducive surficial biogeochemical processes. Circulating As-laden hydrothermal fluids may also be derived from magmas and form part of the discharge in the surficial hot springs in arc environments. These hydrothermal-genic deposits in orogenic belts may also act as the primary provenance for the As-laden sediments that are transported into foreland basins by wind, glacial erosion, and/or streams. Ultimately, the As-laden foreland sediments serve as modern-day aquifers, where the sediments release As into groundwater by water–rock interaction during various biogeochemical processes under conducive hydrogeochemical conditions. The significance of this hypothesis is that it proposes the existence of a common primary source for globally dispersed geogenic As-enriched aquifers, that have been so far mostly studied as individual occurrences. The proposed hypothesis can explain the widespread presence of As in areas as diverse as the Indus–Ganges–Brahmaputra basin (Himalayan orogen), the Chaco-Pampean basin (Andean orogen), Rocky mountain basin (Western Cordilleran orogen), New England and northeastern USA (the Applachian orogen).

Estimating source regions for snowmelt runoff in a Rocky Mountain basin: tests of a data‐based conceptual modeling approach

AbstractIn many mountain basins, river discharge measurements are located far away from runoff source areas. This study tests whether a basic snowmelt runoff conceptual model can be used to estimate relative contributions of different elevation zones to basin‐scale discharge in the Cache la Poudre, a snowmelt‐dominated Rocky Mountain river. Model tests evaluate scenarios that vary model configuration, input variables, and parameter values to determine how these factors affect discharge simulation and the distribution of runoff generation with elevation. Results show that the model simulates basin discharge well (NSCE and R >0.90) when input precipitation and temperature are distributed with different lapse rates, with a rain‐snow threshold parameter between 0 and 3.3 °C, and with a melt rate parameter between 2 and 4 mm °C−1 d−1 because these variables and parameters can have compensating interactions with each other and with the runoff coefficient parameter. Only the hydrograph recession parameter can be uniquely defined with this model structure. These non‐unique model scenarios with different configurations, input variables, and parameter values all indicate that the majority of basin discharge comes from elevations above 2900 m, or less than 25% of the basin total area, with a steep increase in runoff generation above 2600 m. However, the simulations produce unrealistically low runoff ratios for elevations above 3000 m, highlighting the need for additional measurements of snow and discharge at under‐sampled elevations to evaluate the accuracy of simulated snow and runoff patterns. Copyright © 2013 John Wiley & Sons, Ltd.

The key to commercial-scale geological CO2 sequestration: Displaced fluid management

The Wyoming State Geological Survey has completed a thorough inventory and prioritization of all Wyoming stratigraphic units and geologic sites capable of sequestering commercial quantities of CO2 (5–15 Mt CO2/year). This multi-year study identified the Paleozoic Tensleep/Weber Sandstone and Madison Limestone (and stratigraphic equivalent units) as the leading clastic and carbonate reservoir candidates for commercial-scale geological CO2 sequestration in Wyoming. This conclusion was based on unit thickness, overlying low permeability lithofacies, reservoir storage and continuity properties, regional distribution patterns, formation fluid chemistry characteristics, and preliminary fluid-flow modeling. This study also identified the Rock Springs Uplift in southwestern Wyoming as the most promising geological CO2 sequestration site in Wyoming and probably in any Rocky Mountain basin. The results of the WSGS CO2 geological sequestration inventory led the agency and colleagues at the UW School of Energy Resources Carbon Management Institute (CMI) to collect available geologic, petrophysical, geochemical, and geophysical data on the Rock Springs Uplift, and to build a regional 3-D geologic framework model of the Uplift. From the results of these tasks and using the FutureGen protocol, the WSGS showed that on the Rock Springs Uplift, the Weber Sandstone has sufficient pore space to sequester 18 billion tons (Gt) of CO2, and the Madison Limestone has sufficient pore space to sequester 8 Gt of CO2.

Sedimentary response to orogenic exhumation in the northern Rocky Mountain Basin and Range province, Flint Creek basin, west-central Montana

Middle Eocene through Upper Miocene sedimentary and volcanic rocks of the Flint Creek basin in western Montana accumulated during a period of significant paleoclimatic change and extension across the northern Rocky Mountain Basin and Range province. Gravity modelling, borehole data, and geologic mapping from the Flint Creek basin indicate that subsidence was focused along an extensionally reactivated Sevier thrust fault, which accommodated up to 800 m of basin fill while relaying stress between the dextral transtensional Lewis and Clark lineament to the north and the Anaconda core complex to the south. Northwesterly paleocurrent indicators, foliated metamorphic lithics, 64 Ma (40Ar/39Ar) muscovite grains, and 76 Ma (U–Pb) zircons in a ca. 27 Ma arkosic sandstone are consistent with Oligocene exhumation and erosion of the Anaconda core complex. The core complex and volcanic and magmatic rocks in its hangingwall created an important drainage divide during the Paleogene shedding detritus to the NNW and ESE. Following a major period of Early Miocene tectonism and erosion, regional drainage networks were reorganized such that paleoflow in the Flint Creek basin flowed east into an internally drained saline lake system. Renewed tectonism during Middle to Late Miocene time reestablished a west-directed drainage that is recorded by fluvial strata within a Late Miocene paleovalley. These tectonic reorganizations and associated drainage divide explain observed discrepancies in provenance studies across the province. Regional correlation of unconformities and lithofacies mapping in the Flint Creek basin suggest that localized tectonism and relative base level fluctuations controlled lithostratigraphic architecture.

Initiation of the Western Interior foreland basin

In this issue of Geology, Fuentes et al. (2009, p. 379) present a new subsidence plot and age data from detrital zircons that help to constrain the date of initiation of the foreland basin in Montana (United States). They offer these data as evidence for the resolution of a controversy regarding the initiation of foreland basin sedimentation in the Western Interior. However, their discussion raises the question of what geological data constitute the defi nitive evidence of the foreland basin style of sedimentation, and therefore the criteria that should be used to defi ne its commencement. A useful defi nition of a foreland basin is that it is a depression that develops adjacent and parallel to a mountain belt. Foreland basins form because the mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric fl exure. At convergent plate margins, foreland basins develop behind the marginal arc (retroarc or retroforeland type; Jordan, 1995) or above the downgoing continental plate adjacent to a collision zone (peripheral or proforeland type; Miall, 1995). The downward fl exure is accompanied by the uplift of a forebulge in the basement, typically a few hundred kilometers out from the fold-thrust belt, and beyond that there is a shallow depression called the backbulge basin. The relationship between crustal loading and basin formation was fi rst articulated by Price (1973) with respect to the foreland basin of southern Alberta, and subsequent modeling of the Alberta Basin by Beaumont (1981) demonstrated the quantitative link between fl exural loading, the formation of a crustal depression, and its fi lling by syntectonic sedimentation. Jordan (1981) demonstrated another key criterion for the defi nition of the foreland basin style of subsidence and sedimentation: the distinctive isopach pattern of the basin fi ll, elongated parallel to the mountain front and asymmetric in cross section, with a depocenter located close to the contemporaneous fold-thrust belt, the uplift of which serves as the proximal sediment source. Cross (1986) used this criterion to distinguish foreland basin subsidence from that due to subcrustal mantle loading, which yields a much broader, wider pattern of basinal subsidence. The Western Interior basin (of which the Alberta basin and the Rocky Mountain basin are parts) was initiated as a retroarc basin as a result of a fi rst-order change in the plate kinematics of Pangea. This supercontinent, assembled by multiple plate convergence and suture between Ordovician and Permian time, began to fragment by rifting in the Triassic, and by early Middle Jurassic time, oceanic crust was forming off what is now the Atlantic coast of the United States. The North American continent began a long process of northwestward drift, which carried the plate some 70° of longitude westward, relative to the Pacifi c plate, and, some 40° of latitude northward until Paleocene time, followed by a change in trajectory and a subsequent southward drift of ~10° up to the present day (Engebretson et al., 1985). There are numerous indicators of a fi rst-order change in magmatic, tectonic subsidence and sedimentation patterns associated with this change in plate kinematics. The fi rst appearance of westerly-derived detritus from a rising arc or orogen has long been accepted as providing the timing of initiation of the Western Interior foreland basin. Prior to the establishment of the basin, the western continental margin was miogeoclinal in character, with sediment derived from easterly sources (the craton and the Canadian Shield). Ron Blakey and I (in Miall, 2008), designated the

Coalbed- and Shale-Gas Reservoirs

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Introduction Annual natural-gas production from coalbed- and shale-gas reservoirs in the US is approximately 2.7 Tscf, which represents 15% of total natural-gas production. Approximately 1.7 Tscf of this gas comes from more than 40,000 coalbed gas wells completed in at least 20 different basins. The remaining 1.0 Tscf comes from more than 40,000 shale gas wells completed in five primary basins. While the pace of coalbed-gas drilling is starting to slow, shale gas continues to be one of the hottest plays in the US, and drilling is expanding rapidly, especially in the south-central US (the Barnett shale and its equivalents), the Appalachian basin, and numerous Rocky Mountain basins. Outside the US, more than 40 countries have investigated the potential of coalbed gas, resulting in commercial projects in Australia, Canada, China, and India. No commercial shale-gas projects currently exist outside of the US, but work continues to identify both new shale-gas reservoirs and to add incremental shale-gas production in existing reservoirs. Given that worldwide coalbed-gas resources are estimated to exceed 9,000 Tscf and shale-gas resources are estimated to exceed 16,000 Tscf, it is clear that tremendous potential exists for future growth (Kawata and Fujita 2001). Reservoir Fundamentals Coals are sedimentary rocks containing more than 50 wt% organic matter, whereas shales contain less than 50 wt% organic matter. Methane is generated from the transformation of this organic matter by bacterial (biogenic gas) and geochemical (thermogenic gas) processes during burial. The gas is stored by multiple mechanisms including free gas in the micropores and sorbed gas on the internal surfaces of the organic matter. Nearly all coalbed gas is considered to be sorbed gas, whereas shale gas is a combination of sorbed gas and free gas. Coalbed-gas reservoirs contain an orthogonal fracture set called cleats that are oriented perpendicular to the bedding and provide the primary conduit for fluid flow. Gas diffuses from the matrix into the cleats and flows to the wellbore. In shale-gas reservoirs, gas is sometimes produced through more-permeable sand or silt layers interbedded with the shale, through natural fractures, or from the shale matrix itself. In some cases, natural fractures are healed by a mineral filling and must be forced open by hydraulic-fracture stimulation. It also is possible to have both shales and coals interbedded in a single reservoir, resulting in gas contributions from both lithologies.

Linked Aspects of Nonmarine Cretaceous–Tertiary Boundary Events

Here I attempt to weave together a few strands of knowledge about the Cretaceous–Tertiary asteroid collision and its biological effects. The sheer enormity of kinetic energy involved in the collision and the immediate conversion of the energy to infrared radiation upon worldwide reentry of suborbitally lofted debris is not adequately appreciated by most paleontologists who deal with nonmarine organisms. Consideration of the physics of such a collision leads to the conclusion that there should have been no live Paleocene nonavian dinosaurs, but a view that nonavian dinosaurs survived into the Paleocene has been accepted by some students on the basis of “Paleocene pollen” found stratigraphically beneath specimens of nonavian dinosaurs. However, investigation of the biostratigraphic basis of dating “Paleocene” dinosaurs points to flaws in palynological dating of Late Cretaceous and early Paleocene strata in Rocky Mountain basins of deposition. The argument for “Paleocene” pollen occurring with or stratigraphically beneath non-reworked nonavian dinosaurs is not acceptable because palynological zone P1 and most or all of zone P2 occur in Upper Cretaceous strata in the Hell's Half Acre area of central Wyoming where the palynological scheme of zonation originated, not in Paleocene strata. Although said to conflict, palynological occurrences are actually compatible with results from dinosaurs. Finding P1 and/or P2 pollen stratigraphically beneath nonavian dinosaur remains does not push those dinosaurs up into Tertiary strata. Populations of sheltering organisms, provided that they were of small enough individuals, would have survived the K–T calamity easily. However, manifestations of Mayr's “founder effect” would have altered the equilibria of genetic composition of populations forever. “Molecular clocks” would have temporarily run fast, giving the impression that phylogenetic branch points lie deeper in the Cretaceous than was actually the case.

Paleoenvironmental and Carbon-Oxygen Isotope Record of Middle Cambrian Carbonates (La Laja Formation) in the Argentine Precordillera

Abstract The La Laja Formation (Early to Middle Cambrian) is one of the oldest units exposed at the base of the lower Paleozoic carbonate platform of the Argentina Precordillera. This is a key unit regarding the hypothesis of the Precordillera as a Laurentia-derived allochthonous terrane currently located in the south-central Andes. According to the faunal affinity and stratigraphic development of the thick Cambrian carbonate bank, the Argentine Precordillera would have been attached to Laurentia. The La Laja Formation contrasts with the rest of the overlying units of the Cambro-Ordovician carbonate platform by being partly mixed carbonates–siliciclastics. This dominantly shallow subtidal unit is internally arranged into several Grand Cycles indicating a complex environmental mosaic, probably with local depocenters related to variable subsidence. This unit records the stabilization of the rifted margin of the Precordillera terrane, prior to the broadening of the carbonate sedimentation during the passive-margin drifting stage. A high-resolution δ13C and δ18O isotope study, in concert with a detailed paleoenvironmental analysis, was carried out to better understand both environmental and chronostratigraphic evolution of the La Laja Formation. Three δ13C positive excursions were recorded; the first one at the Glossopleura biozone within the Soldano Member, the second beginning at the base of the Rivadavia Member, and the third during deposition of the Las Torres Member. Comparisons with other Middle Cambrian curves, in the Precordillera and elsewhere (Rocky Mountains and Great Basin, U.S.A., the western Hunan Province in south China, and the Amadeus, Georgina, and Daly basins in Australia) suggest a global control on these excursions. Mechanisms to produce these positive excursions could be related to high bio-productivity and increased burial of Corg (organic carbon) produced by high nutrient influx to the ocean associated with a relative sea-level fall. Local environmental controls could have in part altered the original isotopic signal.

Permeability, Pore Pressure, and Leakoff-Type Distributions in Rocky Mountain Basins

Summary The permeability, pore pressure, and leakoff type interpreted from more than 1,200 diagnostic fracture-injection/falloff tests were collected in a database and statistically evaluated for four Rocky Mountain basins. The statistical analysis includes the range of observed permeability and pore pressure and the fracture leakoff type distribution. The analysis reveals that pressure-dependent leakoff, fracture-tip extension during shut-in, and fracture-height recession during shut-in are the most common leakoff types. Overall, pressure-dependent leakoff, which can be indicative of highly productive fractured reservoirs, is the most common leakoff type in all Rocky Mountain basins. The analysis also shows orders-of-magnitude variation in gas permeability within all basins, with observed gas permeability ranging from less than 0.001 to greater than 0.10 md.

Inversion Breakup in Small Rocky Mountain and Alpine Basins

Comparisons are made between the postsunrise breakup of temperature inversions in two similar closed basins in very different climate settings, one in the eastern Alps and one in the Rocky Mountains. The small, high-altitude, limestone sinkholes have both experienced extreme temperature minima below −50°C and both develop strong nighttime inversions. On undisturbed clear nights, temperature inversions reach to 120-m heights in both sinkholes but are much stronger in the drier Rocky Mountain basin (24 vs 13 K). Inversion destruction takes place 2.6–3 h after sunrise in these basins and is accomplished primarily by subsidence warming associated with the removal of air from the base of the inversion by the upslope flows that develop over heated sidewalls. A conceptual model of this destruction is presented, emphasizing the asymmetry of the boundary layer development around the basin and the effects of solar shading by the surrounding ridgeline. Differences in inversion strengths and postsunrise heating rates between the two basins are caused by differences in the surface energy budget, with drier soil and a higher sensible heat flux in the Rocky Mountain sinkhole. Inversions in the small basins break up more quickly following sunrise than for previously studied valleys. The pattern of inversion breakup in the non-snow-covered basins is the same as that reported in snow-covered Colorado valleys. The similar breakup patterns in valleys and basins suggest that along-valley wind systems play no role in the breakups, since the small basins have no along-valley wind system.

Syndepositional thrust-related deformation and sedimentation in an Ancestral Rocky Mountains basin, Central Colorado trough, Colorado, USA

Pennsylvanian–Permian synorogenic deposits (Minturn and Sangre de Cristo Formations) of the Central Colorado trough record an interplay of deformation and sedimentation in an Ancestral Rocky Mountains basin. The Central Colorado trough was a north-trending basin bordered by basement-involved highlands of the Uncompahgre uplift on the west and the Ancestral Front Range and Apishapa uplift on the east. Stratigraphic data show that the Central Colorado trough was an asymmetric basin in which coarse-grained sediments were deposited adjacent to the Sand Creek–Crestone thrust fault system of the Uncompahgre uplift. These deposits pinch out eastward against the Apishapa uplift along the eastern margin of the basin. Lithofacies analysis shows that the Central Colorado trough was filled by fan-delta, fluvial-delta, and turbidite deposits of the Middle Pennsylvanian Minturn Formation and by alluvial-fan, braided-stream, and meandering-stream deposits of the Upper Pennsylvanian–Permian Sangre de Cristo Formation. Geologic mapping has identified three syndepositional structures in the strata of the Central Colorado trough that indicate Pennsylvanian–Permian shortening: (1) the Gibson Peak growth syncline in the footwall of the Crestone thrust fault, which formed by syndepositional rotation of the Crestone Conglomerate Member of the Sangre de Cristo Formation during thrust displacement; (2) the Sand Creek thrust fault, which cuts the lower part of the Crestone Conglomerate Member but is covered by younger deposits of the Crestone Conglomerate Member; and (3) an intraformational angular unconformity in the Sangre de Cristo Formation that separates folded strata from overlying less deformed strata. All three structures indicate general east-west shortening during deposition. We interpret the Central Colorado trough as a flexural basin on the basis of syndepositional thrust-related structures, basin asymmetry, and lithofacies distribution. Displacement on the east-verging Sand Creek–Crestone thrust fault system appears to have controlled uplift of the central part of the Uncompahgre uplift and also subsidence in the adjacent basin. The Apishapa uplift, located along the eastern margin of the basin, is interpreted as a possible flexural forebulge related to crustal loading of thrust sheets along the western margin of the Central Colorado trough. Geologic mapping, subsurface data, and identification of syndepositional structures has also led to a better understanding of post-Paleozoic deformation of Pennsylvanian–Permian strata of the Central Colorado trough and the structural evolution of the present Sangre de Cristo Mountains.

Development of a snowfall–snowmelt routine for mountainous terrain for the soil water assessment tool (SWAT)

The soil water assessment tool (SWAT) is a hydrologic model originally developed to evaluate water resources in large agricultural basins. SWAT was not designed to model heterogeneous mountain basins typical of the western United States, and as a result, has performed poorly when applied to mountainous locations. The intent of this study was to increase the versatility of SWAT by developing the capability to simulate hydrology of a non-agricultural mountainous region with a large snowmelt component. A western Wyoming basin, representative of Rocky Mountain basins, was selected to evaluate model performance, identify governing hydrologic processes, and improve the snowmelt routine. An initial evaluation of SWAT performance indicated an inability of the model to represent snowmelt processes. Based on simulation results and field observations, algorithms were developed which use elevation bands to distribute temperature and precipitation with elevation. Additional routines which control snowpack temperature, meltwater production, and areal snow coverage were designed to simulate the influence of season and elevation on the evolution of basin snowpack. The development of the new snowmelt algorithms improved the average annual Nash–Sutcliffe R 2 correlation between simulated and observed Wind River streamflow from an initial value of −0.70 to +0.86.

ABSTRACT: Insights into Processes of Continental Extension from Field Studies in the Northern and Rocky Mountain Basin and Range Province

ABSTRACT: Insights into Processes of Continental Extension from Field Studies in the Northern and Rocky Mountain Basin and Range Province

Vegetation dynamics and primary production in saline, lacustrine wetlands of a Rocky Mountain basin

In lacustrine wetlands of the semiarid Laramie Basin (Wyoming, USA), we investigated whether dominance by Chara spp. versus dominance by Potamogeton pectinatus alters the amount and form of organic carbon available to consumers. In these two wetland states, we compared relative biomass and production of different producer types (macrophytes, epiphyton, epipelon, and phytoplankton). Based on chlorophyll a measurements, light profiles, and a model of primary production, differences in relative biomass and production of different algal types depended mainly on combined effects of macrophyte growth form and water depth. Canopy density (kg m −3) of Chara spp. was at least twice that of P. pectinatus (1.3–3 versus 0.16–0.52 kg m −3), and Chara habitats also supported a much higher biomass of epiphyton than did P. pectinatus (80–224 versus 9–24 mg Chl a m −2). However, the lower canopy density of P. pectinatus, by providing better light conditions for phytoplankton and epipelon, led to similar total algal production. Models estimated 66–133 g C m −2 assimilated by algae in Chara habitats and 66–105 g C m −2 assimilated by algae in P. pectinatus habitats. Differences in the form of algal production might have trophic correlates. In Chara habitats where most algal production is epiphytic, earlier studies showed that macroinvertebrate consumers were mostly scrapers and epiphytic deposit-feeders (snails and amphipods). In P. pectinatus habitats where phytoplankton and epipelon are more important, consumers were mostly filter-feeders and often benthic deposit-feeders (cladocerans, copepods, and chironomid larvae). Despite these taxonomic differences, total invertebrate biomass between states was similar and may reflect similarities in total algal production. Our results indicate that the state shift in vegetation among saline, irrigation-driven wetlands is associated with major changes in relative production of different algal types, despite continued coverage of submersed macrophytes.

99/00250 Developmental geology of coalbed methane from shallow to deep in Rocky Mountain basins and in Cook Inlet-Matanuska basin, Maska, U.S.A. and Canada

99/00250 Developmental geology of coalbed methane from shallow to deep in Rocky Mountain basins and in Cook Inlet-Matanuska basin, Maska, U.S.A. and Canada