The 1906 San Francisco Earthquake and a Century of Progress in Understanding Earthquakes and their Effects
2007 Emmons Lecture
Mary Lou Zoback, Vice President for Earthquake Risk Applications, RMS (Risk Management Solutions)
Abstract: The 1906 M=7.9 earthquake on the San Andreas Fault marked the birth of modern earthquake science in the U. S. For the first time, an earthquake was interpreted as a recurring phenomena along an active fault. The trace of the San Andreas fault was mapped throughout California and offset on the 300 km long surface rupture documented. Comprehensive study of seismic intensity showed the strongest shaking occurred in areas of “made land” (fill) and soft sediment, including China Basin and the present day Marina district, two San Francisco neighborhoods again heavily damaged in 1989. Damage to structures showed destruction was closely related to building design and construction — a painful lesson oft repeated around the world. Based on repeated surveying data and geologic evidence of recurrence, Henry Reid proposed the elastic rebound hypothesis — that earthquakes represent sudden release of elastic energy along a fault resulting from a cycle of slow strain accumulation produced by relative displacements of neighboring portions of the crust. This hypothesis, developed five decades before the plate tectonics revolution provides the basis for modern seismic hazard analysis. We now know that a repeat of the 1906 earthquake is only one of a wide number of future major damaging earthquakes likely to impact the Bay Area. Although we can not predict earthquakes, we can predict their likely effects and the most hazardous regions.
No meeting recorded
Evolution and Occurrence of Uranium Deposits Through Geologic Time
Sam Romberger, Colorado School of Mines
Abstract: The formation of economic concentrations of uranium is controlled by the contrasting behavior of its two principal oxidation states, the normally immobile reduced uranous U4+ ion, and the generally mobile oxidized U6+ as the uranyl ion UO22+. During primary igneous processes the incompatible behavior of U4+ results in concentration in late alkalic differentiates. Therefore, the mobility and concentration of uranium will be strongly dependent on the redox state of the earth’s crust. A major timeline during the evolution of the earth was about 2400 Ma when the crust of the earth changed from reduced to oxidized. Previous to that time, uranium was concentrated in paleoplacer deposits, exemplified by Elliot Lake. Following the oxygenation of the atmosphere and hydrosphere most economical deposits, i.e, unconformity-related, sandstone-hosted, vein-type, and breccia pipe, resulted from fluid transport of uranium as uranyl carbonate complexes and deposition by reduction; similar processes resulted in the subeconomic shale-hosted deposits. Differentiation within these types lies in the, mode of transport, nature of reductant, steepness of the redox gradient and host structure.
Even though some deposit types appear to be concentrated at certain unconformity horizons, i.e. Middle Proterozoic of northern Canada and Australia, there is not a consistent relationship between deposit type and age. Concentration is more related to the establishment of geologic circumstances that result in contrasting redox states across lithologic boundaries, whether it was oxidized siliciclastic units overlying carbonaceous metamorphic rocks in the Athabasca Basin of Saskatchewan and Pine Creek Geosyncline of Australia, or facies changes within fluvial environments in sandstone-hosted deposits. Age-specific occurrences are a result of geologic processes favorable for the proper combination of source of uranium, redox gradients, and structure to serve as conduits for fluid flow, from crosscutting faults and breccias in unconformity-related deposits, to permeable fluvial channels in sandstone-type deposits. Circumstances that could be particularly favorable for mineralization include the superposition of elevated carbon dioxide levels in the hydrosphere with increased alkalic volcanism and deposition of volcaniclastic debris.
The Tarkwaian Gold Deposits of West Africa – A Geologic Travelogue
Thom Fisher, Colorado School of Mines
Abstract: The Tarkwaian gold deposits of West Africa are second in gold production only to the famous Witwatersrand of South Africa. Such “paleoplacers” are widely distributed and are the largest gold deposits in the world and produce over 60 percent of the world’s gold supply. At Tarkwa, gold has been mined since at least the 4th and 5th Centuries B.C. when the Phoenicians and Carthaginians sailed the Gold Coast in search of gold and more. Demand for the precious metal created trans-Saharan trade routes which traversed such legendary and exotic places as Timbuktu and Wangara.
The Tarkwaian deposits are of Lower Proterozoic age (ca. ~2.1-1.9 Ga) and mainly quartz-pebble conglomeratic basin-fill occurring in inverted extensional half-grabens along the Ashanti Mineral Belt (“Tarkwa Syncline”), a northeast trending basin some 220km long by 40km wide in south-central Ghana. The Tarkwaian is composed of some 2500m of metamorphosed (greenschist facies) siliciclastic sediments and are divided into four formations, the Kawere Quartzite, the Banket, the Tarkwa Phyllite (fine-grained lake sediments), and the Huni “sandstone.” The 150m thick Banket is the primary gold-bearing unit, and its sediments are interpreted to be largely braided stream deposits and alluvial fans. Gold production is from as many as ten stratigraphic units composed of quartz-pebble conglomerates separated by finer-grained trough cross-bedded units and, often, intraformational unconformities. Gold distributions are intimately associated with sedimentary and stratigraphic features and paleocurrent data indicate that at least three different drainage systems filled the basin.
Almost all of the gold bearing conglomerates are associated with a single cluster of current directions, indicating a possible single source for the quartz-pebble sediments. Intervening finer-grained sediments appear to have been swept in by streams flowing somewhat parallel to the graben axes. The gold of the Tarkwaian is very fine-grained to flour sized, and was probably derived from exhalative and epithermal deposits originally formed in the underlying Birimian arc systems. The Tarkwaian rests unconformably on this older (ca. ~2.3 – 2.1 Ga) metamorphosed accretionary sequence of arc and backarc volcanics and sediments.
Since the introduction of “modern” mining techniques to the area by French, Dutch, and British interests in the 1880s, the Tarkwaian has produced over 14,000,000 troy ounces (435Mt) of gold. Today, the primary concessions on the Tarkwaian, held by Gold Fields Ghana, Ltd. produce upwards of 800,000 troy ounces per year. The mines are operated mainly by local peoples managed by a small cadre of British, South African, and occasionally American expatriates.
While this region continues to grow from its mineral wealth, many of its people still live in deep poverty. Nonetheless, the Ghanaian people are proud, hardworking folk and are some of the most hospitable people in the world, with a ready and willing smile for strangers. They have a thirst for knowledge fed by the many schools and universities established by the Ghanaian government since independence in 1964.
Please join me for this geological travelogue and introduction to the gold paleoplacers of Ghana. Along the way we’ll meet some of the locals, catch a glimpse of the wild life, and see some of the geography of Ghana and surrounds.
Introduction to Space Weather (for the non-scientist)
Joan Burkpile, National Center for Atmospheric Research
Abstract: Space Weather is the term used to describe disturbances in interplanetary space that can harm astronauts, damage satellites, generate aurora, and disrupt power grids, communication and navigation equipment. These disturbances are driven, primarily, by activity occurring in the sun’s atmosphere. I will describe how conditions in the solar atmosphere create activity that can propagate through interplanetary space and effect the environment at Earth and all the planets.
The Volcanoes of Colorado:
A Colorado Scientific Society Symposium Honoring Thomas A. Steven
Topics: All aspects of the volcanic history of Colorado
On May 19, 2007, at the University of Northern Colorado campus in Greeley, the Colorado Scientific Society sponsored a one-day symposium on the volcanic geology of Colorado. The symposium was dedicated to Tom Steven, USGS retired, whose work provided the foundation of our understanding of Colorado’s volcanic history.
Download a PDF with abstracts of the presentations.
A new look at old friends—The paleogeography of the ancestral Rocky Mountains of Colorado
Chuck Kluth, Director, Center of Research Excellence, Colorado School of Mines
Abstract: New data, combined with earlier data, indicate that previous interpretations of the geometry and timing of the classical Ancestral Rocky Mountains in the Colorado region are in need of revision. N-S stratigraphy of the Fountain Formation along the present Front Range suggests that the Fountain on lapped a broad NW-SE arch that began to develop in early Pennsylvanian time. Interbedded or subjacent marine rocks are preserved as far north as Lyons and as far south as Perry Park, Colorado. The interbedded marine rocks and the preservation of earlier Paleozoic rocks indicate that the Front Range was separated from a narrow, uplifted block in the Colorado Springs area, the Ute Pass Block. The southwestern margin of the Front Range was faulted and had approximately 6 kilometers of structural relief. In contrast, the NE side of the Front Range is now interpreted to have been a NE dip slope with only minor faulting. The presence of Pennsylvanian marine rocks constrains the Front Range to have had its northern plunge end at approximately the Colorado-Wyoming state line.
The San Luis Highland is interpreted to have been a west dipping fault block with approximately 8 kilometers of structural relief on its eastern side and a gentle west dip-slope on the western side. It is interpreted to have been a separate uplift from the adjacent Uncompaghre Uplift to the west, during at least its early history, and possibly its entire history. The San Luis Highland was uplifted in early Pennsylvanian time and shed coarse sediments eastward and northward into the Central Colorado Trough, and more fine grained sediments westward into the Paradox Basin.
The Uncompaghre Uplift is interpreted to have been uplifted in late Pennsylvanian and early Permian time, after the deposition of the middle Pennsylvanian evaporites. The data show that the geometry of the Uncompaghre front, to the SE of the Utah/Colorado state line, contrasts to the single large fault in Utah, and is a stack of SW directed thrust faulted basement blocks. Distribution of synorogenic sediments derived from the Uncompaghre Uplift was largely by axial river systems. Loading by the sediments caused the underlying salt to move into salt walls that nucleated on basement faults. These basement faults formed between middle-late Mississippian and middle-early Pennsylvanian time. The basement faults are usually interpreted as normal faults, but there is evidence that at least some of them were reverse faults. The development of accommodation space for each minibasin between salt walls ended when the pre-salt and post-salt sections welded together, as the last of the salt moved from beneath the basin. The locus of deposition then moved to the SW, farther away from the Uncompaghre front, and a younger salt wall and minibasin formed. This process was repeated several times, with the result that the salt walls are progressively younger toward the SW. The coarse alluvial fan material was preserved and prograded away from the mountain front only after the locus of deposition moved to the SW and axial rivers no longer redistributed the erosional debris. The new interpretation of the geometry and timing of the Uncompaghre Uplift suggests that the Paradox salts and the Eagle Valley Evaporites were deposited in a continuous basin that existed across the site of the later Uncompaghre Uplift.
The Central Colorado Trough was a NW-SE basin located between the Ancestral Front Range and the San Luis Highland and Uncompaghre Uplift. The Trough appears to have been complexly faulted, and contained crustal slivers that were uplifted in a complicated pattern within the trough. These blocks and slivers included the Ute Pass, Wet Mountain/Hartsel Uplift, possibly the ancestral Sawatch Uplift, and unknown small uplifts known only from lithologies and paleocurrent data from their synorogenic sedimentary packages. Normal block faults and thrust faults are located in the Central Colorado Trough, although the details of their relationships to each other are not yet known.
There appears to have been almost no reactivation of Late Paleozoic Ancestral Rocky Mountain structures during the Late Cretaceous/Early Cenozoic Laramide Orogeny. Most of the younger structures cut across the earlier structures. Structures oriented almost normal to the Laramide regional stress, such as the Uncompaghre and San Luis fronts, the Ute Pass and Gore faults, were reactivated with movements that appear to be orders of magnitude less than the late Paleozoic movement. The Laramide Front Range formed in a N-S orientation that is oblique to the NW-SE orientation of the Ancestral Front Range, which might have been at almost right angles to the regional Laramide stress fields.
CO2 Sequestration Potential of Colorado
Genevieve Young, Colorado Geological Survey
Abstract: The Colorado Geological Survey is a participant in the Southwest Regional Partnership on Carbon Sequestration whose primary goal is to determine an optimum strategy for minimizing greenhouse gas intensity in the southwestern United States. The Southwest Partnership is a large, diverse group of expert organizations and individuals specializing in carbon sequestration science and engineering, as well as public policy and outreach.
In 2000, CO2 emissions were more than 92 million short tons in Colorado; of which 46 percent came from coal-fired power plants. These stationary point sources afford the possibility of capture and separation of CO2 for transport to, and storage at, nearby “geologic sinks”.
Although CO2 sink potential is widely distributed across the state, characterization efforts during Phase I (2003-2005) focused on seven “pilot study regions” defined on the basis of maximum diversity in potential sequestration options relatively close to large CO2 sources. Utilizing both geologic and mineralization options, carbon storage capacity within these regions is an estimated 720 billion short tons. With the availability of suitable technology, the pilot areas have the potential of providing a long-term storage solution. The highest CO2 sequestration capacity potential for Colorado lies within the oil, gas, coalbed, and saline aquifer reservoirs of the Denver, Cañon City Embayment, Piceance, and Sand Wash basins.
The Southwest Partnership is currently conducting three pilots for Phase II (2005-2009) of the U.S. DOE/NETL Carbon Capture and Storage Program. These include one pilot each in the San Juan Basin, the Paradox Basin, and the Permian Basin. The Colorado Geological Survey is taking a key role in the design, implementation, and analysis of the San Juan Basin Fruitland coal pilot because of expertise in coalbed methane development.
Field-scale Measurement of Water Table Profiles in a Drained Slope
COOK, Diana I. 1; SANTI, Paul M. 1; SHORT, Richard D. 2; and HIGGINS, Jerry D. 1
(1) Dept Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401, email@example.com, (2) Blackhawk Geologic Hazard Abatement District, 4125 Blackhawk Plaza Circle, Danville, CA 94506
Abstract: In 2003, Crenshaw developed a method for calculating the average water table height between horizontal drains in a slope. The method relies on drain flow rates, slope geometry, and soil hydraulic conductivity. The corrugated shape of water table profiles between drains and the departure of the water table surface from the drain near its uphill end were verified by Crenshaw using laboratory-scale physical modeling and computer modeling. However, field verification of Crenshaw’s findings was not conducted at the time. During the summer of 2007, a study program was conducted seeking to confirm Crenshaw’s findings using field-scale physical modeling at the Blackhawk Geologic Hazard Abatement District (GHAD) test site in Danville, California. The Blackhawk GHAD test site is composed of a 2H:1V concrete slope 30 feet long and 12 feet wide, simulating impermeable bedrock conditions. The concrete is embedded with 5 perforated pipes which cross the slope laterally. These 5 pipes are connected to a water system and are used to simulate base flow recharge. For this study, the concrete was covered with compacted layers of a lean clay representing local materials known to be susceptible to slope failures. Two wick drains were installed during construction at a spacing of 8 feet. A total of 50 standpipe piezometers were installed in the slope in order to measure water table profiles both between and along the two drains. Measurements were taken during both recharge and drawdown events. The test was repeated with a locally representative clayey gravel/sand soil. Field test results generally confirm the findings of Crenshaw (2003), with some localized variations in the water table profiles. The variations are most likely due to factors such as inhomogeneity of soil properties and/or compaction, the development of preferential pathways within the slope during recharge, clogging of piezometers, or differential water infiltration in the slope due to a loss of pressure along the perforated recharge pipes.
Petrology and Geochemistry of Tannurjeh Porphry Cu-Au, North-eastern Iran
SAADAT, Saeed, Department of Geological Sciences, University of Colorado, Boulder, CO, firstname.lastname@example.org;
KARIMPOUR, Mohammad Hassan, Geological Sciences, CU- Boulder;
Ferdowsi Mashhad, Boulder, CO 80309;
AJAYEBI, Kimiya, Islamic AZAD University,Tehran, Iran; and
STERN, Charles, Geological Sciences, University of Colorado, Boulder, CO 80309
Abstract: Tannurjeh the first porphyry Cu-Au system was discovered in Eastern Iran and is located within volcanic-plutonic of Kashmar- Khaf belt. Eocene rhyolite, dacite, rhyodacite and minor andesite are intruded by monzonite, quartz monzonite, diorite and quartz diorite porphyry (Oligo-Miocene). Most of them have small exposure. Hydrothermal breccias (rich in goethite & Au) with circular exposure are being recognized in some portions of the system. Intrusive rocks are meta-aluminous, medium to high-K series, I-type, Na2O > K2O and indicate a continental arc setting. Unaltered intrusive rocks have magnetic susceptibility > 200 ×10-5 SI. Both ASTER and ground checking revealed very broad altered zones characteristic of Cu- Au porphyry systems. Such as propylitic, sericitic, argillic, alunite and vuggy silica that are well developed in the area. The silicified zone covers a large area. Pyrite and minor chalcopyrite occur both as disseminated and fracture filling, but no stockwork.
Au, Cu, Zn, Pb and As, show anomaly in stream sediments and in rocks. Due to strong oxidation, most of sulfides are being oxidized and copper is being leached and secondary argillic zone is formed. Secondary iron oxides (mainly goethite) vary between 1 to %10.
Morphology of the intrusive, alteration types and geochemical anomalies indicate that present exposure is high level of porphyry system. High pyrite content, broad zone of silicification, confining the copper with potassic zone at depth, and high and very broad zone of Au anomaly make Tannurjeh to be new type of porphyry Cu-Au deposit.
Keywords: Tannurjeh, porphyry Cu-Au, Iran, breccias
Longevity of Acid Rock Drainage (ARD): Mineralogical and chemical comparison of mine-waste piles and post-glacial talus rock producing acidic solutions
Jessica Duggan, Colorado State University
Abstract: Significant environmental problems are caused by acid solutions generated naturally and anthropogenically from rocks. Pyrite, a common accessory mineral in epithermal and porphyry deposits, oxidizes under atmospheric conditions and contributes to the environments acidity. This project documents the mineralogy and chemical characteristics of originally pyritic material exposed to oxidizing environments for different time periods, in order to determine natural and anthropogenic acid production rates, and quantify these rates over orders of magnitude longer time frames than have been previously investigated in laboratories. Two acid producing sites with similar mineralogy and climate, but orders of magnitude different exposure time, are located in major mineralized districts of the Colorado Rocky Mountains. The Chattanooga acid iron fen, located in the Silverton caldera of southwestern Colorado, is fed by natural acid generating, post-glacial (~10,000 year) talus slope, while series of abandoned 100 year-old mines and sulfide-rich waste piles in the Sugar Loaf mining district, on the western edge of the historical Leadville, CO mining district generate acidic solutions. Field mineral distribution, microscopic textures, chemical characteristics, and chemical mass balances of these materials exposed to atmospheric conditions are determined to quantify acid production. Important rate-controlling factors to consider for acid generating environments are field scale sulfide dissolution mechanisms, secondary mineral reactions, grain-size, rock-pile volumes, climate, hydrologic parameters, biologic catalysts, and vegetation. The mineralogy and chemical components of rocks from these sites are compared to ecologic, water quality, and net acid production data collected by Yager et al., 2005, Chimner, R., per. comm. 2006, and Dee, K., per. comm. 2007.
Late Pleistocene Equilibrium Line Altitude Trends and Precipitation Distribution in the Sangre de Cristo Range, Colorado
Kurt A. Refsnider, Keith A. Brugger, Eric M. Leonard, James P. McCalpin, Phil Armstrong
Abstract: Using primarily field observations, we reconstructed the late Pleistocene extents and equilibrium line altitudes (ELAs) of 31 glaciers across the Sangre de Cristo Range of southern Colorado. Reconstructed ELAs range from 3,185 m for the Hunts Glacier at the northeast end of the range to 3,707 m for the Willow Glacier on the west side near the town of Crestone. In general, reconstructed ELAs are 100-150 m lower for the east side of the range for glaciers at similar latitudes. Along strike, reconstructed ELAs on both sides of the range increase to a maximum near the highest topography in the vicinity of Crestone and the Blanca Massif and then decrease slightly into the Culebra Range toward the Colorado-New Mexico border.
Modern precipitation varies considerably across the range, and precipitation gradients between the extremely dry San Luis Valley and the crest of the range are very steep. Meteorological stations show that the east side of the range likely receives ~35% more precipitation during the winter months than any other part of the range, but given the lack of stations at higher elevations, we cannot accurately determine vertical precipitation gradients for most of the range. To circumvent this limitation, we regressed the PRISM (Parameter-elevation Regressions on Independent Slopes Model) precipitation data against topographic data, resulting in a basic precipitation-elevation relationship. Normalized residuals for mean winter precipitation show a 10-30% enhancement in precipitation across the east side of the range and a deficit of similar magnitude on the west side of the range.
Areas of enhanced and deficient precipitation correspond closely with lower and higher reconstructed ELAs, respectively. The similarities between the reconstructed ELA trends and the modern precipitation distribution suggests that during the late Pleistocene, winter precipitation from upslope storms delivering moisture from the southeast exerted a strong positive influence on the mass balances of glaciers on the east side of the range. Storms delivering Pacific-derived moisture were, like today, probably not the dominant source of precipitation for the Sangre de Cristo Range. Such a late Pleistocene precipitation distribution may also in part explain the dramatic geomorphic differences between valleys on opposite sides of the range.
Late Paleozoic Uplifts in Colorado, Utah, Wyoming, New Mexico, and Adjacent Areas
Dr. William Nesse, University of Northern Colorado
Abstract: The location of “Ancestral Rockies” uplifts that developed in the Pennsylvanian is defined by the distribution of sedimentary cover on Precambrian Basement as documented by geologic mapping and oil well data. Areas of major Pennsylvanian uplift which shed clastic debris into adjacent basins are approximated by areas where Permian and younger sediments rest on basement. Areas with preserved Mississippian and older sediments must be in adjacent basins. Areas with Pennsylvanian sediments on basement are marginal to the uplifts and are either areas of pre-Pennsylvanian non-deposition or erosion, or areas where Pennsylvanian uplift was sufficient to erode early Paleozoic rocks before being buried by sediments derived from the uplifts.
The use of the terms Ancestral Rocky Mountains, Ancestral Front Range, and Ancestral Uncompaghre is rooted (Lee, 1918, Ver Wiebe, 1930) in the incorrect presumption that Laramide and Pennsylvanian uplifts are coincident and represent areas of continuous topographic and structural relief from the Paleozoic to the present. Continued use of Laramide/Modern structural terms for Pennsylvanian features is inappropriate and confusing. It is suggested that the terms Arapahoe uplift and Ute uplift replace Ancestral Front Range, and Ancestral Uncompaghre respectively. The term Anasazi uplifts is suggested as an alternative to Ancestral Rocky Mountains.
Based on the criteria outlined above, the Arapahoe, Apishapa, Ute, Sierra Grande, Pedernal, and Zuni/Defiance uplifts can be documented. The Arapahoe and Apishapa Uplifts extend NNW from SE of Pueblo diagonally across the modern Front Range, Middle and North Parks, and Park Range and a short distance into Wyoming. The Ute uplift forms a dog-leg shape in SW Colorado and adjacent areas in New Mexico and Utah. The oldest sediments to extend continuously across the Arapahoe and Ute uplifts are Jurassic. The Sierra Grande, and Pedernal uplifts are irregular shaped areas in NE and south central New Mexico respectively. The Zuni/Defiance uplift is a roughly circular area with two segments straddling the New Mexico-Arizona border. Permian sediments rest on basement nearly continuously on the Sierra Grande, Pedernal, and Zuni/Defiance uplifts.
A number of features have been identified in the literature as Pennsylvanian uplifts, but which are either platform areas within basins, or did not exist. Platforms are areas where the water depth shoaled so that deposition was restricted. Mallory’s (1966) Pathfinder uplift in Wyoming is a distal portion of the gentle north flank of the Arapahoe uplift. It is a platform area and includes areas where both early Paleozoic and Pennsylvanian sediments rest on basement and is not a major structural feature. An area in western Wyoming is often shown as an uplift, but this too, is a platform area with nearly continuous coverage of Pennsylvanian sediments resting on lower Paleozoic sediments. The Piute/Emery platform area in central Utah has been re-defined based on the available oil well data to be an area which lacks Pennsylvanian sediments but where older sediment is preserved. The Florida Island and Roosevelt areas in southwest and eastern New Mexico respectively are small platform areas, but not significant uplifts. The “Ancestral Sawatch Range,” presumed to have once occupied the site of the modern Sawatch Range did not exist. Mississippian and older sediments are preserved around the flanks and on top of the Sawatch Range indicating that the area was within the Pennsylvanian Central Colorado Trough depositional basin in the Pennsylvanian.
Depositional basins adjacent to the uplifts received up to 20,000 feet of clastic and evaporite sediments. These include the Eagle Basin/Central Colorado Trough, Paradox Basin, Dalhart Basin, Rowe-Mora/Taos Basin, Tucumcari Basin, and Orogrande basin. The Pennsylvanian basin in eastern Colorado is usually incorrectly referred to as the Denver Basin, but the Denver Basin is a Laramide construct. It is suggested that this basin, which extends to the SE to join with the Anadarko Basin, be referred to as the Pueblo Basin. The Central Colorado Trough is often shown in the literature extending continuously into New Mexico and connecting with the Rowe-Mora basin. Data from this study, combined with a reinterpretation of isopach data presented by McKee and Crosby (1975), show that the Cimarron Arch separates the Rowe-Mora basin from the basin in Colorado. Further, it appears that the Central Colorado Trough was divided into two segments. The northern portion is often referred to as the Eagle Basin. It is suggested that the southern portion, roughly centered on the modern Sangre de Cristo Range, be referred to as the Cuchara Basin.
Data from this study and reinterpretation of the Pennsylvanian isopach data from McKee and Crosby (1975) establish the presence of a more-or-less continuous Pennsylvanian structural arch that includes the Florida platform, Zuni Defiance uplift, and Piute/Emery platform, extending from southern New Mexico, across northeast Arizona and into west central Utah. This structural arch served as the western or southwestern boundary for the Orogrande, Rowe-Mora, and Paradox basins.