Abstracts – 2006

January 2006 – Emmons Lecture

” Kinematic and Erosional Histories of the Nepalese Himalayan Fold-Thrust Belt: Implications for Mid-Crustal Channel Flow”

P.G. DeCelles
Department of Geosciences, University of Arizona, Tucson, Arizona

Regional structural mapping, thermochronology, provenance analysis of foreland basin deposits, and incremental restoration of balanced regional cross sections of the Nepalese Himalayan fold-thrust belt provide the basis for an assessment of recently proposed mid-crustal channel flow (MCCF) models for Himalayan geodynamics. As currently articulated, MCCF requires southward extrusion of mid-crustal rocks from beneath the Tibetan Plateau to daylight at an erosional “porthole” sandwiched between the Main Central thrust (MCT) and South Tibetan detachment (STD). The model predicts that (1) the STD and MCT have comparable slip and do not join in the subsurface; (2) the rocks presently exposed in the Greater Himalayan zone would have been involved in ductile extrusion during Early Miocene time (although some versions of the model call for present-day channel flow); (3) incorporation of Indian rocks into the channel from below the MCT is a turbulent process, resulting in mixing and structural overturning; (4) Tibetan mid-crustal rocks have been involved in the channel since its inception; and (5) the present outcrop of Greater Himalayan rocks has served as the erosional porthole throughout development of the channel. These features are testable with existing geological data from the Nepalese portion of the fold-thrust belt.

Detrital U-Pb zircon ages, Nd-isotopic data, and conventional petrographic data from foreland basin deposits in Nepal show no evidence of erosion of Tibetan mid-crustal rocks from Eocene to modern times. Conspicuously absent are Cretaceous zircons, juvenile Nd, and volcanic lithic grains, all of which would be expected derivatives from middle crust of the Lhasa terrane in southern Tibet. In view of the fact that Greater Himalayan rocks were transported >150 km southward during Early Miocene time, this indicates that MCCF, if it exists in the Nepalese Himalaya, must be confined to Indian material. Although Greater Himalayan age zircons, Nd-isotopic signatures, and abundant plagioclase appear in the detrital record ca. 22-20 Ma, there is no evidence for erosion of Greater Himalayan medium- to high-grade metamorphic rocks until Late Miocene time (11 Ma). Structural data indicate that the Greater Himalayan rocks were emplaced by the MCT along an extensive regional flat upon essentially undeformed (but metamorphosed) lower Lesser Himalayan rocks, and the present position of the Greater Himalayan topographic front is not coincident with the southward extent of the original MCT sheet. The present, steeply northward-dipping surface outcrop pattern of Greater Himalayan rocks in central and western Nepal resulted from erosion into a regional scale antiformal duplex in Lesser Himalayan rocks that began to develop in Late Miocene time. Thus, the presently configured erosional porthole could not have begun to exist until the Late Miocene. The structural facing directions of Lesser Himalayan rocks below the MCT (the floor of the channel) are consistently upright and northward, precluding turbulent mixing along the base of the channel. A variety of datasets support the following kinematic history of the Himalayan fold-thrust belt: Eocene-Oligocene thrusting in the Tibetan Himalaya, accompanied by amphibolite-grade metamorphism in the underlying Greater Himalayan rocks; Early Miocene emplacement of the MCT sheet and contemporaneous, but much lesser magnitude, northward slip on the STD; Middle-Late Miocene (post-12 Ma) emplacement of the Ramgarh thrust sheet; Late Miocene growth of the Lesser Himalayan duplex; Pliocene-Pleistocene slip on the Main Boundary thrust; and Pleistocene-Recent slip on thrusts within the frontal Subhimalayan imbricate belt. Considered alongside the detrital unroofing record, this kinematic history is incompatible with large-scale involvement of Tibetan middle crust in channel flow as currently articulated in the literature. Features often considered as diagnostic of MCCF in the Himalaya (such as ductile shear strain and out of sequence thrusting) are equally compatible with critical taper models of thrust belt behavior. On the other hand, a modified MCCF model involving only Indian material south of the Indus suture zone since mid- to Late Miocene time may be compatible with geological data as well as conventional kinematic models for thrust belts.

 


February 2006

“The MIST Experiment on the Space Shuttle Columbia: Fighting Fire in Microgravity”

Dr. Angel Abbud-Madrid
Associate Director of the Center for Commercial Applications of Combustion in Space (CCACS), Colorado School of Mines

Increasing interest in fine water mists as fire suppressants has been driven by the search for an environmentally friendly replacement of harmful chemical fire-suppression agents and by the need to protect both water- and weight-sensitive areas with low requirements for total water flow. In addition, with NASA’s renewed emphasis on spacecraft fire safety in orbital and planetary manned missions, water mist has also been identified as one of the best candidates for putting out fires in space. Since a weightless environment allows the fundamental study of the flame/mist interaction by eliminating the distorting effects of buoyancy, the Water-Mist Fire Suppression Experiment (Mist) was flown on the STS-107 mission of the Space Shuttle Columbia on January, 2003 to take advantage of this unique environment.

The experiment was designed, fabricated, assembled, and tested at the Center for Commercial Applications of Combustion in Space (CCACS), a NASA Research Partnership Center located at the Colorado School of Mines (CSM) in Golden, CO.

Mist was then integrated into the Combustion Module-2 (CM-2) at the NASA Glenn Research Center from where it was later transported and placed in the payload bay of the Space Shuttle.

The objective of Mist was to study the effects of droplet size and water concentration on the burning velocity of a propagating, premixed propane-air flame inside a cylindrical tube. Changes of the flame speed and shape were used as the measure of flame suppression efficacy. Thirty-two tests were conducted with four different fuel-air mixtures, two droplet sizes, and a variety of water loadings. All tests were conducted during microgravity periods (10-6 g) and over 90% of the information gathered in orbit was downlinked to Earth in the form of sensor and video-image data prior to the tragic end of the STS-107 mission. In addition, sensor data from a complete test point were recovered from a flash memory unit found among the debris of the Orbiter several months after the accident.

This talk will describe the project objectives, the development of the experiment at CSM, the operation of the experiment from the NASA JSC Mission Control Center in Houston in January 2003, the results obtained from the mission, and the impact of the research project on the design of the next generation of fire suppression systems on Earth and in space.


March 2006

“Late Cretaceous Subsidence in Wyoming”

Dag Nummedal
Colorado Energy Research Institute, Colorado School of Mines

The Farallon plate convergence with the western margin of North America during the late Cretaceous directly controlled rates and patterns of subsidence across the Rocky Mountains and Great Plains, through three linked mechanisms: 1) dynamic subsidence related to mantle convection above the subsiding slab – this subsidence mechanism operated on a wavelength of a few 1000s of miles and was in-phase along strike across most region, 2) flexural subsidence in the retroarc foreland basin landward of the Sevier orogenic belt – operating on a wavelength of less than 200 miles and probably asynchronous along strike, 3) dynamic subsidence or uplift related to plate convergence rate and subduction angle – in control of the temporal distribution of basement involved (Laramie) tectonism.

Quantitative modeling of subsidence induced by Sevier-belt flexure allows this component to be subtracted from the total subsidence across the region. One such detailed separation exercise has been performed across southern Wyoming, demonstrating that the Sevier-belt flexure influenced only the western parts of the Greater Green River basin, and that late Cretaceous subsidence from there eastward to Iowa was dominantly a product of dynamic subsidence. 3D modeling of the flexural forebulge in response to Sevier and Wind River thrusting demonstrates that this tectonic feature migrated southeastward in response to shortening on the Wyoming-Idaho salient of the Sevier thrust and the Wind River thrust, but rarely extended much farther east than the (tectonically younger) Rock Springs uplift.

This quantitative subsidence reconstruction reveals that most of the late Cretaceous Western Interior Seaway lay well to the east of the Sevier foreland basin; a finding that also is supported by mapping the forebulge as a zone of thin strata throughout the region.

Dr. Scott Kieffer from the Colorado School of Mines was a member of the scientific team mobilized by the U.S. Earthquake Engineering Research Institute to study geotechnical aspects of the Ken Chuetsu Earthquake. Dr. Kieffer will discuss some of the novel aspects of this earthquake, and their relation to extensive geotechnical failures.

 


“USGS project FRAME (Framing research in support of adative management of ecosystems)”

Christine Turner, George Leavesley, Richard Zirbes, and Roland Viger
U. S. Geological Survey

George San Miguel, National Park Service, Mesa Verde National Park
Jim Chew, USDA Forest Service, Missoula, Montana
William Romme, Department of Forest, Rangeland, and Watershed Stewardship, CSU
Lisa Floyd-Hanna, Prescott College, Arizona
Mark Miller, National Park Service, Kanab, Utah
Neil Cobb, Merriam-Powell Center for the Environment, Northern Arizona University
Kirsten Ironside, Northern Arizona University

Project FRAME is transforming the way that science is linked to natural resource management decision-making on federal lands. It does so at a time when federal land managers are called upon to make science-based decisions and to optimize the management of multiple resources under increased public scrutiny. Federal land managers need an adaptive management framework to accommodate changing conditions through the use of the appropriate science and consensus-building processes.

The FRAME project strategy is to couple the adaptive capabilities of the USGS Modular Modeling System (MMS) with accepted principles of collaboration. Our approach is to collaboratively engage the resource managers, modelers, and scientists in framing the science issues and in developing the appropriate science models to address the natural resource management issues. Through a multidisciplinary USGS project that includes partners from other agencies (NPS, BLM, BIA, and USFS), universities and research institutes, we have been focusing our initial efforts on natural resource and fire-management issues at Mesa Verde National Park. The principal models initially being used are the PRMS (Precipitation-Runoff Modeling System), and the SIMPPLLE model (SIMulating Patterns and Processes at Landscape Scales). Also being incorporated are results from a newly developed empirical sedimentation model related to post-fire runoff and erosion. Through the collaborative modeling effort at Mesa Verde, we have now developed a transportable methodology for collaboratively modeling integrated science for adaptive, multi-objective resource management that is applicable across a wide range of ecosystems.

 

 


May 2006

“An Overview of the Diamond Industry”

Karin Hoal
Research Associate, Colorado School of Mines

In recent years, the diamond industry has gone through a number of significant changes affecting the production, distribution, marketing and retail aspects of the pipeline. De Beers’ pivotal role has changed, new players and regions have become increasingly significant, and issues such as conflict diamonds, synthetic diamonds, and diamonds for development now form a large part of marketing. This talk will review the geological concepts in diamond formation, and discuss exploration and mining methods currently employed. The recent developments in the industry are presented in terms of current and future industry trends, and how they may affect developing regions as well as consumers.

 


“3D Geologic Modeling and Fracture Interpretation of the Tensleep Sandstone, Alcova Anticline, Wyoming”

Nathaniel J. Gilbertson, Newfield Exploration Company, Denver, Colorado
and
Neil F. Hurley,Colorado School of Mines, Golden, Colorado
Alcova anticline is a Laramide-age structure on the southeast margin of the Wind River basin, central Wyoming. The Tensleep Sandstone is exposed at the core of the anticline. The North Platte River cuts across the axis of the anticline, resulting in two near-vertical walls of Tensleep Sandstone, approximately 500 m (1640 ft) wide, 100 m (330 ft) tall, separated by approximately 140 m (460 ft).

The purpose of this study is to: 1) determine the changes in fracture orientation and intensity across the Alcova anticline, 2) use the emerging technology of LiDAR to aid in the quantification of fracture orientation and intensity in an outcrop setting, 3) characterize fractures at Alcova anticline in a way that will allow the data to be used in a fractured reservoir flow model of analogous structures and, 4) complete a revised geologic map of the Alcova anticline and vicinity.

LiDAR is a laser-scanning technique that provides high-resolution (1-2 cm, 0.4-0.8 in) topography of outcrop surfaces. The LiDAR survey at Alcova anticline contains sufficient data points to resolve fracture planes =1 m2 (11 ft2) in area. LiDAR analysis has provided height, strike, and spacing data for 575 fractures with a trace length greater than 5 m (16 ft) for both outcrops. LiDAR data interpretation of fracture planes at Alcova anticline results in orientation and spacing values consistent with those measured in the field at Alcova anticline. There are 3 major and 1 minor fracture sets. Fracture height and spacing values fit simple power-law distributions.

One result of this study is a new geologic map of the Alcova area, with formation contacts constrained by GPS (global positioning system). A set of 14 balanced serial cross sections, constrained by the field map, were used to construct a 3D geologic model of the Tensleep sandstone. The model was restored using a flexural-slip unfolding algorithm. This model was tested for geometric attributes, such as dip magnitude, dip direction, rate of dip change (simple curvature), and Gaussian curvature. Strain was tracked during the restoration process. Rate of dip change (simple curvature) was found to have the greatest correlation to the location of tectonically produced fractures. Areas of elevated strain correspond directly to field-mapped transverse faults at high angles to the main thrusts.

The Tensleep Sandstone has been identified as a test candidate for carbon dioxide sequestration at the Teapot Dome oil field, 90 km (50 mi) northeast of Alcova anticline. The results of the LiDAR analysis, the 3D geologic model, and field observations at Alcova anticline provide a set of input parameters for a fractured reservoir model of the Tensleep Sandstone at Teapot Dome field. The input parameters are fracture set orientation, fracture set height, fracture set aspect ratio, fracture set spacing, and the distribution of the fracture sets over the structure.

 


September 2006

“The role of lake levels in oil shale distribution”

Yuval Bartov
Research Assistant Professor, Colorado School of Mines

The oil shale richness and distribution in the Piceance Creek basin is not uniform and is mostly developed at the central part of the basin. Towards the margins there is a significant reduction of richness and thickness of the organic rich beds. This trend is controlled by several processes, some of which are also applicable in marine settings: rate of organic production, rate of consumption (oxidation and bacterial) and rate of dilution by the inorganic sedimentation.

However, lake systems have another control that most often plays a most important role – lake levels. Lake levels control water depth, location of depositional environments, distribution of sediment volume and lake salinity.

The depositional history of the Green River Fm illustrates the above model. Numerous lake level changes controlled the location of the lake shorelines. After an initial highstand, the lake regressed, and deeper water was restricted to the central part of the basin. During periods of low lake level, the water depth was too shallow on the lake margins for long term stratification, which is required for the anoxic state in the lower part of the water column. Yet the algal bloom persisted, as indicated by the rich oil shale deposits in the center of the basin. After the low lake level phase, water chemistry changed and salinity increased. Soon after, prominent oil shale beds were deposited throughout the basin. These beds are related both to the high lake levels and creation of brines in the lake, both enhancing long-term stratification of the lake’s water column.

 


“Thrust faults as a mechanism of attenuation in the steep limbs of forced folds”

Vince Matthews
Director, Colorado Geological Survey

East-dipping thrust faults in the steeply-dipping limb of the basin-bounding monocline along the east flank of the Colorado Front Range have been ascribed to a variety of origins:

• Backthrusting from the upper detachment of a triangle zone (Sterne and Raynolds, 2001; Sterne, 2002))
• Shortening from east-west compression (Siddoway, 2002)
• Rotated growth faults (Weimer, 2004)

An analysis of this large displacement structure (associated with the Golden and Rampart Range faults), as well as several small-displacement folds, shows that the east dipping faults are part of a conjugate thrust system. The conjugate faults are a primary mechanism of attenuation as the strata are folded over the edge of an uplifted basement block. Thinning of 19 percent by this mechanism was measured in a shale unit in one outcrop.

Analyzing the orientation of thrust faults in the steep limbs of a variety of structures along the Front Range and in Central Colorado indicates that this is a common phenomenon in extended steep limbs of forced folds. These thrust faults appear to be part of an orderly system of layer-parallel extension. Thrust faults that dip into the uplifted block will place older strata over younger strata and thrust faults that dip away from the uplifted block place younger strata over older strata.

This system appears to be scale-independent, as it is observed operating on the sub-meter scale and the sub-kilometer scale. The process can create isolated, rhombic boudins or fault panels. These conjugate systems can tectonically isolate a sandstone boudin within a shale unit.

Our results suggest that the conjugate set may initiate when the strata have been tilted about 60 degrees from horizontal. With further rotation of the steep limb (even to overturned) the conjugate set is also rotated. With rotation, one of the sets becomes more favorably oriented to slip and becomes dominant.

 


October 2006

“Evolved” triangle zone along the southeastern flank of the Colorado Front Range”

Edward J. (Ned) Sterne
Petro-Hunt LLC

Faults along the eastern flank of the Colorado Front Range display a diversity of attitudes and juxtaposed age relationships. This paper proposes a modified triangle zone model, one that allows back thrusts within the intercutaneous wedge, to explain the observed fault data. The “evolved” triangle zone model predicts a variety of characteristic fault types including: 1) foreland-dipping roof thrusts, which either show no stratigraphic separation or anomalous younger-over-older relationships; 2) foreland-dipping intercutaneous back thrusts which exhibit both older-over-younger and younger-over-older bedding relationships; and 3) hinterland-dipping floor thrusts that show older-over-younger bedding relationships.

Evolved triangle zones are found at multiple stratigraphic and structural levels along the range. These stacked detachment levels accommodate displacement transfer along the range and give rise to some surprisingly complex but restorable structures. This model may be helpful in understanding apparently anomalous thrust relationships in a variety of tectonic settings.

 


“Lake Alamosa and Middle Pleistocene Integration of the Rio Grande”

Michael Machette
U.S. Geological Survey

In 1910, Claude Siebenthal proposed the existence of a Pleistocene lake in the San Luis Basin of southern Colorado based on fossils and sediments exposed in Hansen Bluff, about 10 km southeast of Alamosa. Extensive studies of the bluff exposures in the 1980s by Karel Rogers and others revealed an early to middle Pleistocene section that has abundant plant and animal fossils, two volcanic ashes, and a paleomagnetic record that includes the Brunhes/Matayama boundary. The depositional environments at Hansen Bluff and in nearby cores include shallow to perennial lakes, playas, and meandering streams. However, in the century since Siebenthal’s first work, no one had found morphological expression of the Lake Alamosa or documented its maximum altitude or lateral extent.

During geologic mapping of the Alamosa 1/2° x 1° sheet, I found several new exposures of shallow near-shore and lacustrine deposits. Beach gravels are preserved as bars, spits, and remnant deposits in saddles between bedrock-cored hills. Associated wave-eroded hills (bedrock) suggest that the ancient lake reached a maximum altitude of 2325-2330 m in the San Luis Basin. The most prominent features are well preserved south and east of Alamosa, along the northern margin of the San Luis Hills. Distinct spits at 2310-2330 m wrap around the southwestern side of Sierro del Ojita and Saddleback Mountain and a kilometer-long spit is well preserved on the eastern side of the Rio Grande, just north of the basin’s outlet. None of these or the dozen other constructional lake features in the area is well exposed, but trenching shows they have well developed, meter thick, stage III calcic soils (i.e., 300-500 ka of soil development).

In the middle Pleistocene, the upper Rio Grande had its headwaters in northern New Mexico. At that same time, the uppermost reach of the modern Rio Grande emptied into Lake Alamosa, which probably formed as a result of blockage by voluminous eruption of Servilleta flood basalt in the southern part of the San Luis Basin (SLB) between 3.7 and 4.8 Ma. Servilleta basalt covers most of the Taos Plateau northward to the Colorado/New Mexico border. On the western side of the SLB, basalt is at the surface north to Antonito, Colorado. New USGS aeromagnetic data suggest the subsurface basalt flows continue at least as far north as La Jara at depths of 30-100 m. Servilleta basalt is also preserved on the eastern side of the SLB beneath the Costilla Plain, on San Pedro Mesa, and in the Culebra graben north to Fort Garland, Colorado. Drill-hole data show that basalt extends northward to Blanca in the shallow subsurface. Thus, by middle Pliocene time (ca. 3.5 Ma), Servilleta basalt probably blocked south-flowing drainages in the basin: the resulting closed basin was occupied (episodically) by Lake Alamosa in which the sediments of the Alamosa Formation were deposited. This formation is well known for thick “blue clays” that form confining layers within fluvial aquifers of the SLB.

Alluvial and lacustrine sediment nearly filled the upper SLB prior to the lake’s overflow, sometime around 450 ka as estimated from a preliminary 3He exposure date of ~439±6 ka on a boulder or the spit at Saddleback Mountain. When the lake rose during a middle Pleistocene glacial cycle (perhaps marine OIS 12), it overtopped a hydrologic sill on Oligocene Tertiary rock of the Fairy Hills and cut a deep gorge. The integration of the upper SLB into the upper Rio Grande drainage led to downstream incision of the river, especially north of the Red River and added about 18,000 km2 to the river’s drainage area.

 

 


November 2006

Regional zoning of alteration and mineralization of Espino Iron-oxide Copper Gold (IOCG) district, Coastal Cordillera of Northern Chile


by Gloria Lopez
Department of Geology and Geological Engineering, Colorado School of Mines

 

The Espino district is located in the Chilean Coastal Range to the south of other known IOCG deposits.  The district contains a number of copper and gold veins that have been exploited since colonial times.

The regional setting of Espino is characterized by an Early Cretaceous volcanic arc formed at a continental margin with recurrent marine transgressions.  The district contains a series of intermediate stocks of granodioritic to dioritic composition that intruded the volcanic sequence, and a relatively small marine to transitional sedimentary basin.  Both the intrusive rocks and the volcanic-sedimentary rocks have been hydrothermally altered.

Sodic alteration characterized by albite is extensively developed in the district at low to intermediate structural levels.  Sodic alteration is overprinted by a spatially more restricted sodic-calcic alteration (88.4±0.6 Ma) at intermediate and low levels.  Potassic alteration has been recognized in limited exposures at the structurally lowest portions of the hydrothermal system.  Hydrolytic alteration (87.9±0.6 Ma) occurs at higher levels of the system, and, locally, at intermediate levels.  Hydrolytic alteration is focused along N- to NE-striking veins composed of chlorite-sericite or quartz-sericite accompanied by iron oxide and sulfide with anomalous gold and copper.  Quartz, with minor hematite, is abundant at the highest portion of the system, whereas at intermediate levels hematite is abundant and quartz is minor.  At deeper levels, quartz is rare and magnetite is dominant.


Shatter Cone Occurrences Indicate A Possible Impact Structure Near Santa Fe, New Mexico


by Siobhan Fackelman, University of Northern Colorado

 

Shatter cone-like features have been documented ~5km northeast of Santa Fe, New Mexico, where they are exposed along, and laterally adjacent to, Hyde Park Road.  Several continuous exposures of nested, sub-conical, planar to slightly curved multiply striated fracture surfaces occur within Proterozoic rocks including, granitic gneiss, amphibolite, mica schist, and quartzite.  We interpret these features to be shatter cones, indicating the remnant of an old, eroded impact structure.  The striated surfaces are best developed within equigranular, potassium feldspar-rich granitic gneiss.  Mapping to date has identified the cone structures only within Proterozoic metamorphic and crystalline rocks; the structures apparently are not present in nearby outcrops of the unconformably overlying sedimentary rocks of the Carboniferous Madera Group.  If impact evidence is proven to be lacking within the Paleozoic sequence, this would constrain initially the age of the event to post-Mesoproterozoic and pre-Early Carboniferous.  The shatter cones, which are well exposed for more than 1 km along and north of Hyde Park Road, are individually up to 1 m long, and display a general NE trend of their cone axes, which plunge to the SW.  The trend of the cone axes cuts the foliation of the host rock, which strikes S to SE.  An average apical angle of approximately 65 degrees was measured for the master cones and the minor cones, conjunctively.  Ongoing research is further documenting the structure, orientation, texture, and petrography of the shatter cones and the detailed structural fabric of their host rocks.

 


Processes of Magma Evolution: A Trace Element Study of Magmatic Epidote


by Ian Merkel, Colorado State University

Two weakly deformed Cretaceous pegmatitic tonalite dikes, representing part of the high-pressure magmatic plumbing of the North Western U.S. paleo-volcanic arc system, occur in the North Cascade Mountains, Washington.  These dikes contain quartz, plagioclase, muscovite, epidote and rare garnet, and apatite.  Significantly, the epidote occurs in more than seven texturally and chemically distinct populations, with individual crystals in excess of 13 cm.  Trace element (TE) analyses have been carried out on the epidote populations to test hypotheses of magmatic processes associated with an evolving tonalite magma system (magma recharge, country rock assimilation, differentiation, phase-reaction evolution), as well as to constrain the relative chronology of these populations.

Geochemical interpretations indicate that two of the hypothesized magmatic processes were dominant in the early evolution of this system.  First, magmatic differentiation, resulting from fractionated high temperature/pressure magmatic phases, led to decreasing TE concentrations; second, the breakdown of a heavy rare earth element (HREE) phase significantly adjusted to HREE geochemical patterns of subsequent epidote populations.  It has also been determined that TE variations of these epidotes can be used to construct the relative chronology of the epidote populations.  These results are important because they demonstrate the ability to use distinct populations of trace minerals to acquire insight into early magmatic processes and geochemical evolution.  Early magmatic processes, which often lack constraints due to evolutionary late crystallization processes that dominate the geochemical signature of the final rocks, are important in reconstructions of tectonic regimes, in particular, the deep plumbing systems of continental volcanic arcs.

 


December 2006

A new look at old friends—The paleogeography of the ancestral Rocky Mountains of Colorado


by Chuck Kluth
Director, Center of Research Excellence,  Colorado School of Mines

 

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.