January 1997 – Emmons Lecture
TECTONIC EVOLUTION OF THE EASTERN TIBETAN PLATEAU: A DIFFERENT KIND OF MOUNTAIN BUILDING
C. Clark Burchfiel
Massachusetts Institute of Technology
Present differences in structural styles demonstrate contemporaneous deformation within the eastern part of the Tibetan Plateau are not uniform within the India-Eurasia collisional system. The present morphology of the plateau gives the appearance of uniform crustal processes, but this may be related to Tibetan crustal rheology and decoupled flow in the mid or lower crust. Differences in topography on the plateau may be partly compensated by changes in thickness and flow in the lower crust.
Despite the high elevation and thickened crust present in eastern Tibet, geological field observation and satellite geodesy indicate that little crustal shortening has occurred along the central and southern portion of the eastern Tibetan Plateau since about 4 Ma. Instead, we observe rapid clockwise rotation of southeastern Tibet, little or no motion of central eastern Tibet relative to southeastern China, extension (rather than shortening) along the edge of the high plateau, and the absence of young foredeep-basin subsidence east of the plateau. Geodynamic modeling suggests that these phenomena share a common dynamic origin and are the natural result of continental convergence where the lower crust evolves to become very weak, so that upper crustal motions are decoupled from the behavior of the lower crust and mantle, except along the edges of the plateau. Thickening and consequent high topography of the eastern part of the Tibetan Plateau results more from lateral flow of the lower crust than from crustal shortening. Neither our observations nor our model results suggest major extrusion of Tibetan crust beyond the margins of the plateau, nor do they suggest convective removal of Tibetan mantle lithosphere, as has been widely accepted in much of the recent literature.
THE DEVONIAN ALAMO IMPACT BRECCIA: UPDATE
The Alamo breccia was discovered in 1990, and by 1992 was documented in the Upper Devonian stratigraphic section in many of the mountain ranges around the southern Nevada town of Alamo, 150 km north of Las Vegas. Because the breccia is clearly a stratigraphic anomaly–being a thick, rapidly deposited, graded unit of catastrophic derivation interbedded with normal thin-bedded cyclic Devonian platform carbonates–it was suspected to be the product of the impact of an extraterrestrial object with Earth.
Since 1992 much of the breccia has been mapped. It occupies a minimum of 4,000 sq. km, and perhaps 15,000 sq km or more, across southern Nevada. It is distributed within an eastward-directed semicircle, being 135 m in thickness in the center and thinning to a feather edge around the periphery. Biostratigraphic dating by conodonts precisely places the breccia within the early Frasnian punctata zone, at ~367 Ma. In our early work we found shocked quartz and a weak iridium anomaly within the breccia, supporting our impact theory (Warme and Sandberg, 1996). New findings include breccia-in-breccia clasts of accretionary carbonate lapilli, interpreted to have formed in the ejecta cloud and embedded in the breccia as it flowed. Detailed internal stratigraphy shows that the breccia is composed of as many as five separate thinning- and fining-upward graded beds, each separated by erosional surfaces that were commonly deformed as the breccia accumulated.
The Alamo breccia is perhaps the best-exposed impact deposit yet discovered. It serves as an accessible field example for comparison with other impact sites, and to test impact models. The physical attributes of the breccia match those produced by a theoretical sequence of shock- and tsunami-processes that should occur upon shallow-water impact (Kuehner and others, 1996; Warme and others, 1996).
Kuehner, H-C., Warme, J. E., and Oberbeck, V. R., 1996, Impact stratigraphy: comparison of synthetic model with the Late Devonian Alamo breccia: 1996 GSA Annual Meeting Abstracts, Denver, p.A-181.
Warme, J. E., and Sandberg, C. A., 1996, Alamo megabreccia: record of a Late Devonian impact in southern Nevada: GSA Today, v. 6, p. 1-7.
Warme, J. E., Sandberg, C. A., and Morrow, J. R., 1996, Late Devonian Alamo impact megabreccia, southern Nevada: 1996 GSA Annual Meeting Abstracts, Denver, p. A-181.
SCIENCE AND PUBLIC POLICY: A VIEW FROM THE WHITE HOUSE
Murray W. Hitzman
Department of Geology and Geological Engineering, Colorado School of Mines, Golden
For the earth sciences, the committee of most importance is the Committee on Environment and Natural Resources (CENR). The CENR contains subcommittee teams which examine specific activities, such as resource use and management, water, global change, and natural hazard reduction. The teams are composed of natural and social scientists, economists, engineers, and policy makers from throughout the Federal government. These teams have inventoried Federal programs, compiled program budgets, established clear national goals, and developed strategic plans including specific benchmarks.
OSTP is working to assure continuance of Federal S&T accomplishments by providing policymakers with the realistic, coordinated analysis necessary for the tough choices. The budget and policy decisions made by the little-known OSTP directly affect you if you are interested in science or have Federal funding for research.
ICE AGE MAMMALS OF COLORADO
Curator and Head of Earth Sciences, Denver Museum of Natural History, Denver, Colorado
EVIDENCE FOR THE COLLAPSE OF LARGE ICE SHEETS: PAST AND FUTURE
INSTAAR, University of Colorado at Boulder
The talk will review the evidence for rapid collapses of large ice sheets based on current research in the Labrador Sea, Baffin Bay, the eastern margin of Greenland, and West Antarctica.
OLD, VERY HIGH LEVELS OF PLUVIAL LAKE LAHONTAN, NEVADA
U.S. Geological Survey
CONSEQUENCES AND KINEMATICS OF THE HIMALAYAN COLLISION
University of Colorado at Boulder
SANDSTONE-HOSTED COPPER DEPOSITS, LISBON VALLEY, UTAH
The Lisbon Valley copper project is located at the southeastern end of the faulted Lisbon salt anticline in southeastern Utah. Host rocks are Cretaceous Dakota Fm. and Burro Canyon Fm., both dominated by braided fluvial sandstones. Copper occurs adjacent to the Lisbon Valley Fault, a major axial structure with up to 4000 feet of displacement, and outward from the fault as much as 1500 feet along favorable sandstone beds. Ore grades are continuous for >200 feet at the Centennial deposit. Two other significant deposits are included within the project.
The Lisbon Valley project has announced reserves of 42.6 million tons of 0.45% Cu, and Summo is confident that this reserve can be expanded. The depoists are amenable to open-pit mining, heap leaching, and SX-EW recovery. The Lisbon Valley project is currently in the permitting phase of preparation for a 12,000 ton per day open-pit mine that would recover 34 million pounds of copper per year over a 10 year mine life.
SPACE WEATHER AND THE COMING SOLAR CYCLE
Chief, Space Environment Services Center, NOAA, Boulder, Colorado
Quantifying the Sources and Sinks of Atmospheric Methyl Bromide
James H. Butler
NOAA Climate Monitoring and Diagnostics Laboratory Boulder, Colorado
Water-sediment Interaction in Holocene Carbonate Islands, NE Panama
Colorado School of Mines
October 1997 – Family Night
THE EXPLORATION OF MARS
Department of Geological Sciences and Laboratory for Atmospheric and Space Physics
University of Colorado at Boulder
Although there is some evidence that life might have existed on Mars, found within meteorites that came from the martian surface, that evidence is at best ambiguous and uncertain. The missions over the next decade will culminate in the return to Earth of rocks from the martian surface, and their analysis in laboratories here for evidence pertaining to life.
The question of whether there is life on Mars, and the broader question of whether there is life elsewhere in the universe, is one that is of immense interest to everybody. It touches the core of how we view ourselves as individuals and as a society. As with the issue four hundred years ago of whether the Earth goes around the Sun or vice versa, answering this question will help us to understand our place in the universe; this is the case even though the answer may not change our day-to-day behavior.
Should we continue this exploration? Our ongoing exploration of ourselves, the solar system, and the universe, is one of the hallmarks of our existence as a society. As a society, we all remember the last time in history that we did not search out the world around us to understand it better–that was the middle ages, also known as the dark ages, a period whose ending we celebrate as the Renaissance!
I will discuss the role of exploration in our society, the scientific issues surrounding the search for life elsewhere, and the recent and ongoing exploration of Mars.
November 1997 – Student Night
Sm-Nd ISOTOPE SYSTEMATICS IN METAMORPHIC MONAZITE FROM NORTHERN NEW MEXICO, IMPLICATIONS FOR THE ORIGIN OF 1.4 GA HIGH TEMPERATURE-LOW PRESSURE METAMORPHISM
Colorado College, Colorado Springs
In northern New Mexico, the Taos and Cimmaron Mountains are cored by a ca 1.76-1.6 Ga complex of plutonic and volcanic rocks, which have had a complex history of deformation and metamorphism. The timing of metamorphism and deformation in these rocks is contentious. However it is clear that ca 1.4 Ga these rocks were overprinted by a high temperature (500-700 °C) low to moderate pressure (3-5 kb) metamorphism. Sillimanite-bearing paragneisses contain abundant clear grains of monazite which are 1.42 Ga. Amphibolitic rocks contain 1.42-1.4 Ga metamorphic sphene and zircon. No 1.4 Ga plutons have been recognized anywhere in the range although volumetrically insignificant pegmatitic pods occur locally. A major question is, therefore, what is the source of the heat for this pervasive metamorphism.
We have analyzed the Sm and Nd isotopic composition of single grains of 1.42 Ga metamorphic monazite from the Taos Range and Cimmaron Mountains. The monazites have uniform BSE images with no evidence of growth zoning. We have obtained a range in initial (1.4 Ga) ENd from +2 to +6. The 147Sm/144Nd values vary from .08 to 0.1. The observation that the TDM ages of the monazites are, within uncertainties, the same as the crystallization age suggests that they were not formed from an average 1.7 to 1.6 Ga crustal reservoir of Nd. Instead, the data suggest that the Nd was derived from a long-term Sm depleted reservoir and implicate a major transfer of mantle-derived fluids and/or magma coincident with the 1.4 Ga metamorphic event.
INSTAAR, Colorado University, Colorado Springs
The two dominant geomorphic landscapes on southeastern Cumberland Peninsula are (1) glacially modified low-lying areas and (2) weathered high plateaus lacking evidence of recent glacial activity. The orientation of striations indicates that ice flowed SW and SE into the low-lying areas of Cumberland Peninsula before entering the Sound. In addition, mapping of the limits and elevations of postglacial marine submergence (raised marine features such as wave-washing and beaches above present sea level) along the coast of Cumberland Peninsula indicate that the Sound was isostatically depressed by the LIS. Dates based on 26Al and 10Be (cosmogenic) isotope concentrations in glacially-modified bedrock range from 12,000 to 20,000 years, constraining the timing of both glacial activity on the Peninsula and the presence of the LIS. Cosmogenic isotope dates from the higher plateaus are older and, as expected, show more scatter: dates range from 29,000 to 61,000 years, indicating complex exposure histories. Together, these lines of evidence suggest that recently much of Cumberland Peninsula was affected by erosive (warm-based) local glaciers while the adjacent Sound was inundated by a low-surface slope ice stream. During this time the high plateaus must have been covered by thin cold-based ice and/or have remained unglaciated. The latter explanation is more compatible with recent lake sediment studies on some of the plateaus. Numerical modeling allows simulation of this hypothesized reconstruction in addition to providing insights into the necessary boundary conditions for such glaciologic behavior. This study provides the first data for the glacial history of southeastern Cumberland Peninsula. The results highlight the glaciologic role Cumberland Sound may have played during the Late Quaternary Period, given its influence on the dynamics and configuration of the northeastern LIS.
Colorado School of Mines, Golden
Outcrops of the Permian Brushy Canyon Fm. in the Delaware Mountains of West Texas provide an excellent analog to many large, deep-water sandstone reservoirs. The Brushy Canyon Fm. is interpreted to be a slope and basin low-stand sequence of fine-grained sandstones and siltstones deposited in deep-water channels and fans. Migration of carbonate- and hydrocarbon-bearing fluids accompanied Basin and Range normal faulting.
Sedimentological architecture within the Brushy Canyon Fm. contributes significantly to fracture development. Scan line surveys along cliffs demonstrate that fracture spacing is related, in part, to lithology and bed thickness. Fracture spacing and bed thickness generally show a positive correlation. Spacing and variance of fractures are smallest in thin fine-grained sandstone beds, and largest in organic-rich siltstone beds.
Also, faults and associated fractures can act as fluid conduits, fluid barriers, or both, depending on the history of diagenesis related to fluid migration. Fracture spacing in sandstone beds increases parabolically away from faults. Carbonate-filled veins are concentrated near faults. Outcrop seismic velocity measurements quantify a diagenetic alteration halo across faults resulting from multiple fluid migration and precipitation events. Furthermore, fracture orientation and spacing are dissimilar in the hanging-wall vs. foot-wall of some faults, suggesting that cementation-related fault sealing and reservoir damage may be asymmetric around these faults.
Colorado School of Mines, Golden
The reservoir is the Permian San Andres Formation, which is composed of stacked cyclic shallow marine carbonates deposited in a distally steepened ramp setting. Pervasive dolomitization, fracturing and subsequent plugging with anhydrite cement, in addition to moldic, interparticle and intercrystalline porosities are the result of extensive diagenesis.
Time-lapse logging using resistivity logs and injectivity profiles, and the use of porosity-resistivity overlays show that there are significant zones of unswept pay. Borehole images provide information to interpret bedding and fracture orientation, and allows recognition of breakouts that define the in-situ stress orientation. Combining these elements with the sequence stratigraphic interpretation has allowed the creation of a 3-D geological model using Stratamodel. This software package provides input into the Eclipse reservoir simulator using scaling up approaches contained in Geolink/Gridgenr software packages. Stratamodel is also used to distribute the attributes needed for the 3-D synthetic seismogram.
This study provides a bridge between geologic, engineering, and seismic data. Such combined data can be used as a tool for helping predict reservoir behavior and improving exploitation in this and other similar carbonate reservoirs. A 3-D model becomes a powerful visualization tool that can help make strategic reservoir-management decisions.
Colorado School of Mines, Golden
My research has incorporated both traditional 2D-GIS and 3D-visualization methods inherent in geoscientific information systems (GSIS) to produce a database with a graphical user interface. Several GIS and GSIS products were integrated to construct these products.
Data from the Harappa excavations include thousands of entities or items, such as site maps, area maps, structure plans and sections, trench plans and sections, artifact information records and collections, and archaeologic interpretations. A subset of the data was used to develop methodologies to manage, synthesize, and visualize; and to examine interrelationships of site-wide topographic and stratigraphic models, paleotopography, archaeologic structures, reconstructions, archaeologic trenches, and artifact distributions.
The prototype includes a multi-scaling user interface to visualize data at appropriate scales (site, structure, and trench) to present topographic/stratigraphic data, to manage and display excavation data, and to present archaeologic reconstructions of structures. The accuracy and appropriateness of the 3D-visualization methodology were evaluated by qualitative and quantitative analyses.
An integrated data management and visualization approach can serve to archive and preserve Harappa data and interpretations for future generations of archaeologists. The results of this work can also be applied to other geologic work such as geotechnical site investigations, seismological work, and environmental engineering work, due to analogies between geology and archaeology.
December 1997 – Presidential Address
Rift Basins of the Central and Northern Rocky Mountains: Inheritance from Laramide Structures
Karl S. Kellogg
U.S. Geological Survey
The Hilgard thrust system is a major east-directed structure in southwestern Montana that strikes north along the western side of the Madison Range and forms the eastern structural margin of the Laramide Madison-Gravelly arch, a large east-directed basement uplift. In most places along the system, basement is thrust over a tight footwall syncline in rocks as young as Late Cretaceous. Basement blocks have demonstrably rotated by as much as that of the basement-cover contact, which in some places is overturned. This relationship underscores a major paradox in basement balancing of basement uplifts: stated simply, why don’t large open spaces form? A possible solution, which fits the empirical evidence, is domino-style rotation of basement blocks and the inevitable formation of bounding breccia zones.
The Hilgard thrust system is approximately parallel to a zone of Neogene valley-bounding normal faults (Madison fault system) along the eastern side of the Madison Valley, which contains a thick basin-fill sequence that dips eastward into the normal faults. In some places, normal faults exploit older thrusts, down-dropping basement blocks into the Madison Valley, leaving only the footwall synclines exposed. This paired thrust-and-normal-fault relationship is strikingly similar to other paired systems across south-western Montana and may be due to the collapse of the crestal zones of the basement uplifts (arches) during Tertiary extension.
In Colorado, three basins of the northern Rio Grande rift: the San Luis basin, the Arkansas River Valley, and Middle Park basin, are complex half grabens that in most places contain thick Miocene and early Pliocene basin-fill deposits that dip into large flanking normal faults. In the first two cases, the basins lie astride asymmetric Laramide uplifts (San Luis and Sawatch, respectively), whose steep, thrust-faulted sides are on the same side as the deep parts of the Neogene basins. An accommodation zone, across which the asymmetry of both the Laramide uplift and the Neogene basin reverses, separates the San Luis basin-San Luis uplift from the Arkansas River Valley-Sawatch uplift. These relations suggest not only that the locations of the basins are inherited from the Laramide uplift (as Ogden Tweto and others have noted), but also that the asymmetry of the basins is inherited from the vergence of the Laramide uplifts. A more complicated case is Middle Park, where the valley of the Blue River is faulted down to the west, the same side as the steep, thrust-faulted west margins of the Front and Gore Ranges.
A model for the inheritance of basins from Laramide uplifts works equally well to explain both the features observed in southwestern Montana and the northern Rio Grande rift. The model proposes that during regional Laramide contraction, localized regions of extension formed in the axial zones of uplifts. These extensional zones developed in response to sagging of the leading, thrust-bounded edges of the uplift into adjacent synorogenic basins. During subsequent crustal extension, beginning in late Oligocene or early Miocene (perhaps slightly older in Montana), the axial zones collapsed. Normal faults exploited the older, listric thrusts and tilted the axial basin toward the thrust-bounded side of the Laramide uplift.