January 1997 – Emmons Lecture

TECTONIC EVOLUTION OF THE EASTERN TIBETAN PLATEAU: A DIFFERENT KIND OF MOUNTAIN BUILDING

C. Clark Burchfiel
Massachusetts Institute of Technology

Our studies of active tectonism in the eastern part of the Tibetan Plateau indicate that processes of crustal thickening and elevation of the plateau are related to the complex structures that transfer deformation between strike-slip, shortening, and extension at scales of 1,000 to 100 km. We divide active tectonism of the eastern Tibetan Plateau into three areas with different structural activity. A northern region, the Qilian Shan range and Qaidam basin, bounded on the south by an east-trending belt, the Qinling-Kunlun deformational belt, rotates clockwise and moves NE, transferring motion on the Altyn Tagh fault zone into shortening. A central region, the Longmen Shan-Sichuan basin, bounded on the south by the Xianshuihe-Xiaojiang fault system, deforms at a slow rate (<5 mm/yr) and shortening within this region is too small to form the high plateau to its west. A southern region, south and west of the Xianshuihe-Xiaojiang fault system and extending into Indonesia and Burma, rotates clockwise around the eastern Himalayan syntaxis where velocities relative to South China reach 20 mm/yr. The southern region does not behave as a rigid fragment, but contains smaller fragments which transfer strike-slip motion into shortening and extension.

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.

 

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February 1997

THE DEVONIAN ALAMO IMPACT BRECCIA: UPDATE

John E. Warme (Presenter), Hans-Christian Kuehner, Department of Geology and Geological Engineering, Colorado School of Mines, Golden; Charles A. Sandberg, Geologist Emeritus, USGS, Denver; Jared R. Morrow, Department of Geological Sciences, University of Colorado, Boulder

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).

REFERENCES

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

This talk will describe the experiences over a year and a half of a scientist in the White House. Most scientific matters there are handled by the Office of Science and Technology Policy (OSTP) in the Executive Office of the President which provides an overview of Federal science and technology (S&T) and its importance for public policy. The office, which has a staff of approximately 50, most of whom hold science Ph.D.s, is divided into four divisions: science, technology, environment, and national security and international affairs. OSTP has undertaken the task of trying to coordinate the hundreds of Federal S&T programs which account for $40 billion in Federal S&T spending (less than 10% of the discretionary budget). This has been accomplished through the creation of the National Science and Technology Council (NSTC), a cabinet-level group chaired by the President. Its mission is to coordinate the interagency S&T policy-making process, and to implement and integrate the President’s S&T agenda across the federal government. The NSTC has established nine R&D coordinating committees and a plethora of subcommittees and working groups.

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.

 

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March 1997

ICE AGE MAMMALS OF COLORADO

Russ Graham
Curator and Head of Earth Sciences, Denver Museum of Natural History, Denver, Colorado 

The Ice Age mammal fauna of Colorado was much more diverse than the modern fauna. Large animals like giant ground sloth, camel, mammoth, horse, bison, and flatheaded pecarry were relatively common elements of Pleistocene mammal communities. Most of these species were adapted to grassland, parkland, or savanna environments. Many species went extinct at the end of the Pleistocene (ca. 11,000 years ago). The cause of this extinction is debatable, but both climate change and overhunting by prehistoric humans have been suggested. In any case, the composition of our modern mammal fauna is a direct result of these terminal Pleistocene changes.

 

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EVIDENCE FOR THE COLLAPSE OF LARGE ICE SHEETS: PAST AND FUTURE

John Andrews
INSTAAR, University of Colorado at Boulder 

A scientific paradigm for much of the 1970s-1980s was that the fluctuations of the world’s great ice sheets were driven mainly by forcing associated with the Milankovitch orbital variations. Thus, ice sheets waxed and waned slowly and predictably on time-scales of 90,000 to 20,000 years. However, in the late 1970s there were bits and pieces of evidence that suggested that large ice sheets, particularly the North American (Laurentide) Ice Sheet (LIS), had a much more unstable behavior. In 1988, Heinrich, and then in 1992, Broecker et al., Bond et al., and Andrews and Tedesco, presented evidence for dramatic collapses of the LIS approximately every 7,000 years (e.g. 14.5, 20.5, 27, 35 ka BP). These events spread debris over a broad swath of the North Atlantic from the Canadian Arctic to off Spain/Portugal, probably associated with iceberg rafting. These events call into question the future stability of the West Antarctic Ice Sheet, especially in view of the evidence for global sea level to have been 5 m above present at the height of the last interglacial, ca. 125 ka.

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.

 

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April 1997

OLD, VERY HIGH LEVELS OF PLUVIAL LAKE LAHONTAN, NEVADA

Marith Reheis
U.S. Geological Survey

During the early and middle Pleistocene, Lake Lahontan in western Nevada repeatedly rose to levels that substantially exceeded its late Pleistocene high stands. Reconstructing these lake levels has important implications for paleoclimate, local tectonic effects, and migration of fish and other aquatic species in the Great Basin. Evidence for the extreme high stands include sequences of tephra- and fossil-bearing lacustrine and deltaic deposits grading up into beach and shoreface gravel, remnants of V-bars and arcuate shoreline berms, and benches cut on bedrock that bear remnants of rounded, polished beach gravel well above the late Pleistocene level. The ages of these lake cycles are constrained by tephra correlations, fossils, and paleomagnetic data. Lake Columbus-Rennie and Lake Russell, south and west of Lake Lahontan, also exhibit evidence of very high, old shorelines. Together, these findings suggest that western Nevada was much wetter during pluvial periods of the early and middle Pleistocene than during those of the late middle to late Pleistocene.

CONSEQUENCES AND KINEMATICS OF THE HIMALAYAN COLLISION

Roger Bilham
University of Colorado at Boulder

In the past few years we have determined India’s approach velocity and Tibet’s escape velocity to an accuracy of approximately 10% using space geodesy. The net collision rate between Tibet and INdia is 20.5 mm/year and this results in localized uplift of the mountains at 8 mm/year. The uplift has been detected in spirit-leveling data along the road connecting Lhasa and Kathmandu and is confined to a region less than 20 km wide characterized by steep river gorges and high relief on the surface, and by high microseismicity rates at depth. We propose that north of this region the Indian plate slides aseismically beneath Tibet, whereas to its south great earthquakes occur, permitting the bulk of the Himalaya to slide incrementally over the Indian plate. When it does so, in infrequent M>8 earthquakes, the slip is radially outward along the arc. The radius of the Himalaya is determined by the ratio of the long-term extrusion velocity of Tibet over India and the evident east-west expansion rate of Tibet.

 

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May 1997

SANDSTONE-HOSTED COPPER DEPOSITS, LISBON VALLEY, UTAH

John Thorson
Consulting Geologist

Copper deposits occur within several sandstone units of the Mesozoic sedimentary section adjacent to the structural margins of salt dome anticlines in the Paradox Basin. Three of these deposits are undergoing active exploration and pre-development by Summo USA Corp.

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

Gary Heckman
Chief, Space Environment Services Center, NOAA, Boulder, Colorado

We have recently reached the end of an eleven-year solar cycle. A new cycle is beginning. By the end of the millennium, solar activity will reach another peak. Astronauts on the International Space Station will be interrupting their experiments in a zero-gravity environment to dodge space debris whose orbit has been changed as a result of rising solar activity. At the same time, scientists at Houston will be monitoring the crew’s dosimiters to avoid the chance of exposure to energetic solar particles that are a radiation hazard for the crews. Power companies will be struggling to stabilize power-distribution grids while the aurora borealis surges overhead. Flight attendants finishing their flights on the Concorde will be calling the Space Environment Center in Boulder, Colorado to inquire whether they have been exposed to excess radiation doses. GPS systems will face their first extensive testing under adverse conditions as pilots use them to descend to the end of a runway in limited visual conditions. Communication satellites will experience sudden shutdowns and crews of defense-monitoring systems will have to face false alarms in their warning systems. Police on the Golden Gate Bridge will find themselves talking to a Minneapolis dispatcher when they call for backup. Garage doors on the California coast will be opening in response to Navy radio signals. The rise in solar activity will produce these and other unexpected effects. Forecasters at the Space Environment Center in Boulder will be issuing daily forecasts and alerts of the activity. This talk will describe solar activity, illustrated with pictures, and the terrestrial effects that result. It will offer a glimpse into the working day of a space weather forecaster, one of the smallest occupational groups in the country.

 

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September 1997

Quantifying the Sources and Sinks of Atmospheric Methyl Bromide

James H. Butler
NOAA Climate Monitoring and Diagnostics Laboratory Boulder, Colorado

Methyl bromide (CH₃Br) is a trace atmospheric gas that has been implicated in the depletion of stratospheric ozone. It is present in the atmosphere at about 10 parts per trillion, which is 10-50 times lower than chlorofluorocarbon (CFC) mixing ratios, yet its contribution to ozone depletion is considerable because it contains bromine. Free, stratospheric bromine, in combination with current levels of stratospheric chlorine, is 40-100 times more effective at removing ozone on a per-atom basis than chlorine alone. Thus, CH₃Br is considered to be on par with some of the CFC’s in contributing to stratospheric ozone depletion. Because of this, CH₃Br was recently included by international agreement as a controlled ozone depleting substance, along with CFC’s and halons. Methyl bromide differs from these other gases, however, in that its sources are not entirely anthropogenic. Also, its sinks result not only from reactions in the atmosphere, but also from interaction with the oceans and land. Thus, estimating the contribution of industrially produced CH₃Br to the depletion of stratospheric ozone and calculating the atmospheric lifetime of CH₃Br are more difficult than for the other regulated halocarbons. This does not mean that obtaining an adequate description of the behavior of atmospheric CH₃Br is impossible. Considerable research has been done and is being done to understand the budget of atmospheric CH₃Br. This talk will address how the new research has changed our view of the cyclic nature and atmospheric lifetime of CH₃Br, what constraints are imposed upon its budget, and what gaps remain in our understanding of the behavior of this atmospheric gas.

Water-sediment Interaction in Holocene Carbonate Islands, NE Panama

John Humphrey
Colorado School of Mines

The San Blas Archipelago of islands extends along the northeastern coast of Panama, from Punta San Blas to nearly the border with Colombia. Over 340 Holocene islands occur on this shallow and extensive carbonate platform. Although relatively narrow, shallow-water carbonate environments are spread along 200 km of Caribbean coastline. Onshore waves and currents confine siliciclastic sediment derived from mainland Panama to the near-coastal environment. Clean reef-derived carbonate sands dominate the platform, although no true barrier reef is developed along the margin. The islands occur on topographic highs where pre-existing late Pleistocene reefs developed. Islands are composed predominantly of Holocene skeletal carbonate sand, are generally less than one km across, and are commonly less than two meters in height. High annual rainfall in San Blas has led to the development and maintenance of year-round meteoric phreatic lenses. Holocene sediments, composed of metastable biogenic aragonite and high-Mg calcite, are undergoing diagenesis through interaction with these ground waters. Hydrochemical and petrological data indicate the occurrence of dissolution of metastable mineralogies and concomitant precipitation of low-Mg calcite. Only a minor proportion of low-Mg calcite is precipitated as cement; the vast majority occurs as neomorphosed skeletal carbonate. The San Blas islands provide an excellent natural laboratory for investigating early, near-surface mineralogical transformations in carbonate sediments.

 

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October 1997 – Family Night

THE EXPLORATION OF MARS

Bruce Jakosky
Department of Geological Sciences and Laboratory for Atmospheric and Space Physics
University of Colorado at Boulder

Our nation has recently embarked on a program of exploring the planet Mars with spacecraft during the coming decade. The beginning of this exploration took place in 1997 with the successful operation of the Pathfinder lander and rover and the Mars Global Surveyor orbiting spacecraft. The goal of these missions is to search for evidence that would tell us whether life ever existed on Mars or might exist today.

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.

 

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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

 

Brook Holcombe
Colorado College, Colorado SpringsIn the southwestern U.S., the 1.4 Ga “anorogenic perforation” of the continent has been shown to be much more complex than originally conceived. Large areas of 1.7-1.6 Ga rocks have been overprinted by a pervasive high-temperature metamorphism ca 1.4 Ga. Regional geochronological and thermochronological data suggest that following this perturbation, middle crustal rocks cooled (from >500°C to <200°C) very slowly from 1.4- <1.0 Ga.
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) E_Nd from +2 to +6. The ¹⁴⁷Sm/¹⁴⁴Nd values vary from .08 to 0.1. The observation that the T_DM 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.

LATE QUATERNARY GLACIAL HISTORY OF MID-OUTER CUMBERLAND SOUND, EASTERN CANADIAN ARCTIC

 

Michael R. Kaplan
INSTAAR, Colorado University, Colorado SpringsCumberland Sound is a major marine embayment along the southeast coast of Baffin Island, eastern Canadian Arctic. During the Late Quaternary this was an ideal setting for an ice stream connecting the interior of the northeastern Laurentide Ice Sheet (LIS) and the northwestern sector of the North Atlantic Ocean. Therefore, Cumberland Sound and adjacent southeastern Cumberland Peninsula are key areas for studying northeastern LIS dynamics and ice/sheet ocean interactions, despite little current knowledge of this history. Glacial geologic studies along the coastline of Cumberland Peninsula provide evidence for both ice sheet and local ice cap activity whereas numerical modeling tests the physical plausibility of ice sheet reconstructions inferred from the field program in this region.
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.

FRACTURE NETWORK PREDICTABILITY IN RELATION TO BED THICKNESS, LITHOLOGY, AND FAULT PROXIMITY, BRUSHY CANYON FM., WEST TEXAS

 

Aaron John Kullman
Colorado School of Mines, GoldenCharacterization of fracture networks is important for assessing reservoir quality and fault sealing. Knowledge of how lithology, bed thickness, and faulting can control fracture spacing and orientation greatly enhances the predictability of fractures in the subsurface. The effect of fractures on cementation and sealing is also important.
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.

FLOW UNITS AND UPSCALING OF A COMPLEX CARBONATE RESERVOIR USING A 3-D GEOLOGICAL MODEL

 

Max Scuta
Colorado School of Mines, GoldenThe purpose of this study is to define reservoir flow units in a complex carbonate reservoir using cores, logs, outcrop analogs, production data, by applying sequence stratigraphy concepts as well as the interpreted structural evolution for this area. A further objective is to construct a 3-D geological model of this field, which has been upscaled for reservoir simulation purposes and modeled to predict seismic response.
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.

DEVELOPMENT OF A PROTOTYPE GEOSCIENTIFIC INFORMATION SYSTEM OF THE HARAPPA ARCHAEOLOGICAL SITE, PUNJAB PROVINCE, PAKISTAN

 

Wayne R. Belcher
Colorado School of Mines, Golden     The Harappa archaeological site, located in the Punjab Province, Pakistan, consists of several large mounds of cultural material on the Ravi River flood plain. The Harappa site is part of the Bronze Age Indus Valley Civilization, initially discovered at Harappa. Current excavations, which begun in 1986, focus on the evolution and dynamics of this ancient urban center.
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.

 

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December 1997 – Presidential Address

Rift Basins of the Central and Northern Rocky Mountains: Inheritance from Laramide Structures

Karl S. Kellogg
U.S. Geological Survey 

Laramide structures in southwestern Montana profoundly influenced the formation of Neogene extensional basins. A model proposed to explain this association also appears to explain uplifts and basins in the northern reaches of the Rio Grande rift in Colorado.

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.