January 1999 – Emmons Lecture
“A THREAT FROM THE BIPOLAR SEESAW”
Wallace Broecker
Newberry Professor, Lamont-Doherty Earth Observatory, Columbia University
Summary: During the last glacial period, Earth’s climate underwent frequent large and abrupt global changes. This behavior appears to reflect the ability of the ocean’s thermohaline circulation to assume more than one mode of operation. The record in ancient sedimentary rocks suggests that similar abrupt changes plagued the Earth at other times. The trigger mechanism for these reorganizations may have been the antiphasing of polar insolation associated with orbital cycles. Were the ongoing increase in atmospheric CO2 levels to trigger another such reorganization, it would be bad news for a world striving to feed 10 billion people. Major climate changes can occur abruptly in a few years. The oceans likely hold the secrets to these abrupt changes. For the stability of the earth’s climate system, the oceans play a critical role, as illustrated by the recent El Niño events.
February 1999
ARITHMETIC, POPULATION, AND ENERGY
Albert A. Bartlett
Department of Physics, University of Colorado at Boulder
albert.bartlett@colorado.edu
“The greatest shortcoming of the human race is our inability to understand the exponential function.”
With these words, Professor Bartlett starts his one-hour talk. First he gives a very elementary introduction to the arithmetic of steady growth, showing what steady growth of population means in Boulder, in Colorado, and in the world. Then he examines the situation where one has steady growth in a finite environment and the results of this are applied to fossil fuels, particularly to petroleum and coal. Data from the U.S. Department of Energy are used to show that the realistic lifetimes of U.S. coal, U.S. petroleum and world petroleum are much shorter than the optimistic figures that are so often quoted. Reassuring statements from experts, the press, scientists, political leaders, and others, that are wildly at odds with the facts, are examined. The widespread worship of economic growth and population growth throughout the western world is discussed. These facts give the listener a better understanding of the real meaning of “sustainability,” which Prof. Bartlett gives as the First Law of Sustainability:
“You cannot sustain population growth and/or growth in the rates of consumption of resources.”
This allows the listener to appreciate fully the implications of the growth path of western society and in particular, of the United States. The talk closes with a plea for the widespread education of people on the arithmetic and consequences of growth.
March 1999
ITALY, “TERRAEMOTUS” COUNTRY: A BIRD’S-EYE VIEW ON THE 1997 ASSISI EARTHQUAKES
Paolo Marco De Martini
Istituto Nazionale di Geofisica, Rome, Italy
In 1997, a series of devastating earthquakes struck the central Apennines, which are the structural backbone of the boot of Italy. The September 26th earthquake (M6), caught the attention of the world because it caused 12 deaths, 4 from within the Basilica of St. Francis de Assisi. Italian seismologists, geologists, and geophysicists working together have been able to capture 3-dimensional data on the faults that caused these historic earthquakes. Satellite-based laser interferometry shows a striking pattern of general subsidence and localized uplift associated with the faults and geodetic releveling of pre-earthquake surveying stations confirm the same general pattern. These data, plus geologic studies of the surface ruptures, give us a bird’s-eye view on the 1997 Assisi earthquakes.
HYDROTHERMAL SYSTEMS UNDER YELLOWSTONE LAKE
Pat Shanks
U.S. Geological Survey
No abstract available
April 1999
UPHEAVAL IN THE SOUTHERN ROCKIES: CLIMATE VERSUS TECTONICS FOR STREAM INCISION IN THE LATE CENOZOIC
Emmett Evanoff
University of Colorado Museum, Boulder
The deep canyons cut into the Southern Rocky Mountains have been explained by two scenarios. The first, traditional explanation, is that the region has undergone late Cenozoic uplift. By this model, elevations in the middle Cenozoic were lower than at present and the present elevations result from regional uplift that steepened the gradients of the rivers resulting in the rivers cutting their canyons. The second scenario is that the average elevations of the region were as high as today since the middle Cenozoic, and that climatic change provided the streams with more power to cut their canyons. The apparent uplift of the region resulted from the isostatic adjustment of the mountains to the removal of materials by the rivers and that the mean regional elevation has not changed throughout the Cenozoic.
A wide variety of evidence has been presented for and against both interpretations. The traditional view has been largely supported from evidence of geomorphology, while the second model has been supported by paleobotanical interpretations. Supporters of each explanation have regarded the two scenarios to be mutually exclusive, but both models may be acting in the region. Climatic change has occurred in the late Cenozoic and did affect the drainage systems, primarily by integration of the large river systems. Erosion has been a large factor, but primarily in the basins adjacent to the mountains. Isostatic rebound may have occurred from erosion of the basins. Finally, river drainages have had major readjustments from the middle Cenozoic to the late Cenozoic, that could not have been from climatic change alone. Thus, the models may be the end members of a complex interaction of events that produced the modern topography in the Southern Rockies.
DISSOLUTION OF EVAPORITES CAUSES WIDESPREAD ACTIVE COLLAPSE IN WEST-CENTRAL COLORADO
Robert M. Kirkham
Colorado Geological Survey
Geologic mapping of 7.5-minute quadrangles in west-central Colorado in the vicinity of Glenwood Springs has led to the discovery of widespread active collapse that is a result of dissolution and flowage of underlying Pennsylvanian evaporitic rocks. Neogene extrusive rocks preserved in the region provide an excellent datum by which the extent of the collapse areas and style and timing of deformation can be evaluated. A cooperative investigation of the Neogene igneous rocks by the Colorado Geological Survey and U. S. Geological Survey involving large numbers of Ar40/Ar39 dates, major and trace element geochemical analyses, paleomagnetic determinations, and petrographic studies allows for individual lava flows to be correlated across the collapse areas.
The Neogene igneous rocks are downdropped as much as 3,000 to 4,000 feet in the Carbondale collapse center. Laramide structures bound the Carbondale collapse center, but the direction of collapse is generally opposite to movement during the Laramide. For example, the Laramide Grand Hogback Monocline has “unfolded” or collapsed during the Neogene as evaporitic rocks dissolved and/or flowed from beneath it. A variety of unusual geologic features related to salt tectonism occur within the collapse area. They include sets of parallel bedding-plane faults, orthogonal fault patterns, sag structures, sharp monoclines, intrusive contacts between sedimentary formations, valley anticlines, complexly deformed collapse debris, and locally thick accumulations of sediments. Faulted and folded surficial deposits indicate the collapse continued into the Quaternary, while historic sinkholes and the high salinity loads of streams in the area suggest dissolution is ongoing today.
May 1999
Growth of Gold Nuggets During Active Mountain Building
Dave Craw
University of Otago, New Zealand
A series of broad antiformal ranges and intervening synformal basins is forming to the east of the Australian-Pacific plate boundary in southern New Zealand. Ranges are rising at ca. 1 mm/year, principally by folding above thrust faults at depth, in an arid rain shadow with low erosion rates. The ranges are asymmetrical, with steep southeastern slopes and gentle northwestern backslopes. Miocene gold-bearing quartz-pebble conglomerates are preserved on the backslopes and in the synformal valleys, but are eroded from the steep southeastern slopes. Debris flows consisting of recycled Miocene sediments and fresh basement detritus are shed sporadically from the rising ranges, especially on the steep southeastern slopes. Groundwater percolating through debris flow material has low oxygen content, is locally acidic, and authigenic pyrite or marcasite is forming with authigenic clay minerals. Miocene gold flakes (ca. 300 microns) are locally dissolved and gold is reprecipitated onto neighbouring grains, enhancing the gold grain size. Secondary gold has distinctly lower Ag content (ca. 1 wt %) than primary gold (ca. 5 wt %). Continued uplift results in further erosion and recycling of gold particles with continued grain size enhancement up to 2 cm nuggets. Chemical processes in the alkaline soil also remobilise gold during erosion and recycling, contributing to the grain size enhancement and nugget formation, but Ag is conserved. The grain size enhancement has been important for the economics of placer mining, by facilitating higher recovery than would be possible with finer gold. Similar processes might be expected to occur, at slower rates, along the eastern margin of the Colorado Front Range as topography, climate, and erosion processes are similar.
The history of El Niño and its impacts: Lessons from the paleoclimate record
Julie Cole
Department of Geological Sciences, University of Colorado Boulder, Colorado
How unusual was last year’s El Niño event? This question can’t be answered with the existing instrumental climate data, which span only the past several decades over most of the tropical Pacific. This lack of data presents a challenge to paleoclimatologists, who are learning to decipher the climate record preserved in ancient corals that grow throughout the islands of the Pacific. New coral data from the central Pacific show a strong warming trend over the past 150 years, as well as changes in the strength and the recurrence interval of El Niño events. Other paleoclimatic records, notably tree-rings, offer information on the impact of El Niño on North American climate, particularly regional drought. Climate reconstructions from corals and tree rings indicate that the past century has witnessed long-term changes in both the El Niño system and its extratropical impacts.
September 1999
Why is Mount Sopris So Big?: A Middle Tertiary Pluton Caught With Its Pants Down
Randall K. Streufert
Summit Geology & Consulting, LLC, Silverthorne, Colorado
At 12,953 feet, the twin summits of Mount Sopris loom nearly 7000 feet above the confluence of the modern Crystal River with the Roaring Fork River near Carbondale, Colorado. Exposure of this magnitude rivals or exceeds that of some of the 14,000-ft peaks throughout Colorado, and is interpreted to result, in part, from Neogene deformation in the lower Crystal River valley that involves widespread collapse due to dissolution of Pennsylvanian evaporitic rocks. Evidence of collapse occurs 1 mile north of the Mount Sopris stock near Potato Bill Creek where thick Tertiary sediments are in contact with Mesozoic rocks across a normal fault. The collapse area contains thick surficial deposits that cover bedrock and conceal the structure of the collapse margin.
Rock Glacier History of Mount Sopris, Colorado
Peter Birkeland
University of Colorado, Boulder
Rock glaciers are prominent in the large valleys facing northwest through east on Mount Sopris. Some are nearly 2 km long, and at least parts of all of the rock glaciers are still moving. Aspect plays a major role in both the length of the rock glaciers and the lower altitude of their moving fronts. Several ages of rock glacier formation can be distinguished on the basis of post-depositional alteration of surfaces of the rock glaciers. Such alteration includes the progressive development of lichen growth on and weathering features of the clasts, and loess thickness and soil development of the rock glacier mantle. It is difficult to date the mantles of the rock glacier deposits, but they seem best correlated with the latter part of the Pinedale Glaciation and several cool episodes within the Holocene. These criteria can be used to suggest ages for rock glaciers in other parts of the Rocky Mountains.
October 1999 – Family Night
Dig a Hole in your Backyard: New Discoveries in Denver’s Geology
Kirk Johnson
Denver Museum of Natural History
Denver is built on bedrock that contains dinosaurs and the remains of ancient rain forests. As construction has followed the economic boom time of the 1990s, excavations have created amazing opportunities to find fossils literally in our own backyards. Join Kirk Johnson, Denver Museum of Natural History’s curator of paleontology, as he describes the museum’s effort to save these fossils and to understand the amazing geological history of our home town.
November 1999 – Student Night
The Importance and Characterization of Pseudotachylytes from the Outer Hebrides Fault Zone
Trista Thornberry
Department of Earth Resources, Colorado State University
Pseudotachylytes from the Outer Hebrides Isles, Scotland have been characterized on the basis of mineralogy, texture and chemical compositions. This was done using a petrographic microscope, back-scattered electron imagine and microprobe analysis. Every method indicates an entirely crystalline matrix, with no glass yet seen. Intermediate (sodic oligoclase to labradorite) plagioclase microlites are arranged in a crude spherulitic fashion and can reach lengths as long as 300 microns. Intergrown with the plagioclase lathes are slightly aluminous amphibole/clinopyroxene microlites and magnetite dendrites. Partially melted quartz clasts are the most common host rock remnants of these pseudotachylytes. Sparse apatite, zircon, and rutile clasts also persist in the matrix. Altered veins are scarce and easy to identify by their different textural and mineral components. Commonly altered veins show replacement minerals such as epidote, adularia, and chlorite, and a lack of microlites. Chemically, the pseudotachylyte groundmass shows SiO2 percentages as low as 48.06. This is in marked contrast with the rather felsic Lewisian gneiss host rocks and could indicate incomplete melting and a low viscosity melt. Also important for dating, is the fact that volatiles are low and the K2O percentage is moderately high. In conclusion, the microlitic matrix indicates only a moderately high cooling rate. This, in conjunction with high temperature microlites, could point to a mid-crustal, high ambient temperature source. The large microlites could be useful in applying a quartz-plagioclase-hornblende thermobarometer. Lastly, the unaltered pseudotachylyte with ample K2O concentrations provides a likely candidate for 40Ar/39Ar dating.
Fault Slip Rates, Structural Style, Seismic Moment and Magnitudes of the Last 2.3 Ka – Lake County Uplift, New Madrid Region
Jocasta Champion
Department of Geological Sciences, University of Colorado
Trenching, geomorphology and structural analysis of subtle, fault-related folds in the Lake County uplift (LCU) suggest uplift is accommodated mostly by fault-bend folding above the blind Reelfoot thrust. Trench exposures indicate the Reelfoot scarp is comprised of 2-3 east-facing kink bands that dip 5-16°. These form above bends in the underlying blind thrust, collectively accommodating ~20-25° flattening of the thrust at the top of a ramp dipping 55° west. Seismic profiles image the scarp as a monocline comprised of overlapping kink-bands (multibend fold). Additional shortening is defined in trench excavations and seismic data as a ~4 km wide fold that forms above a fault tip (i.e. a fault-propagation fold). Fault-related fold theory and radiocarbon dates on folded sediments allow slip rates on the thrust to be determined. The average width of kink bands in the trenches is ~14.0 m. Limb width is equivalent to fault slip; this and the age of folded sediments (~2.3 ± 0.1 ka) yields a slip rate of 5.8 ± 0.7 mm/yr. Another method uses the 9.1 m of total structural relief across the Reelfoot scarp, the age of folded sediments and uplift on a 55° thrust to yield a slip rate of 4.8 ± 0.2 mm/yr. Vector transformation of these rates onto the strike slip Cottonwood Grove fault (CGF) indicates slip on the CGF of 1.8 ± 2.2 mm/yr. This is in conflict with recent assertions based on GPS that New Madrid is now inactive. Seismic moment and moment magnitude calculations for faults in New Madrid used fault geometry from our model, slip inferred by fold geometry and published recurrence intervals. For a period of 500 years, seismic moment for the thrust is 8.37 X 1026 dyne-cm (Mw = 7.25); for the strike slip CGF, moment is 4.93 X 1026 (Mw = 7.10). This is consistent with historical records from the 1811/12 sequence where greater shaking was felt over the LCU.
Variations in the Au-Bearing L1 and L2 Liese Quartz Zones, Pogo Deposit, East Central Alaska
Keri Moore
Department of Geology and Geological Engineering, Colorado School of Mines
Gold at the Pogo deposit (estimated 10 million tons, average grade 0.52 opt) is hosted in the subhorizontal, subparallel L1 and L2 Liese quartz zones. These zones crosscut Proterozoic (?) to Paleozoic amphibolite-grade gneisses and Cretaceous granitoids of the Yukon-Tanana terrane. The quartz can be divided into three types based on color, texture, and mineralogy. The quartz types are correlatable across the deposit and appear to be closely related in time. Type 1 is massive, milky white quartz with sulfides dominated by pyrrhotite and pyrite; biotite alteration occurs locally along wallrock contacts. Type 2 is massive, gray, and commonly mottled. Locally, however, this type is dark gray and highly strained. Sulfides are dominated by arsenopyrite and pyrite, and feldspars and micas in wallrocks have been altered to ferroan dolomite and sericite. Type 3 is pale gray to white with a distinctive “granular” texture consisting of subrounded to subangular quartz grains with intergranular potassium feldspar. It is not clear whether this texture is primary or secondary. Pyrrhotite, arsenopyrite, and pyrite are all present in type 4. Gold, with or without various associated Au-Bi-Pb-Te±S-Ag minerals, is found in all quartz types, though visible gold is preferentially concentrated in type 2.
December 1999 – President’s Address
The Yellowstone Hotspot, Greater Yellowstone Ecosystem, and Human Geography
Ken Pierce
U.S. Geological Survey
The landscapes of the Greater Yellowstone ecosystem (GYE) are shaped by geologic processes of volcanism, faulting, and uplift, all of which we associate with the Yellowstone hotspot. As the North American Plate moved SW, hotspot volcanism progressed NE and arrived at Yellowstone 2 Ma. Thousands of feet of recent uplift of the GYE have resulted in ongoing erosion of deep, steep-walled valleys in readily erodible rock.
Modern and Pleistocene weather and resultant vegetation patterns strongly relate to hotspot topography and its Snake-River-Plain track. Moist Pacific airmasses traverse the Snake River Plain and rise onto the Yellowstone Plateau and adjacent mountains to produce deep snows, and east of the mountains, a precipitation shadow. Such deep orographic snows produced extensive Pleistocene glaciers that covered the core GYE and produced many of the landscape features on which modern soils have formed, as well as outwash gravels (commonly covered with sagebrush-grassland) and silty lake sediments (commonly covered by lush grassland such as Hayden Valley).
Rhyolitic hotspot volcanism constructed the Pleistocene Yellowstone Plateau. Streams eroding the steep edges of this plateau form scenic canyons and waterfalls. Rhyolite is poor in nutrients and forms sandy, well-drained soils that support the monotonous, fire-prone, lodgepole pine forest of the Yellowstone Plateau. Older andesite and other rocks surround this plateau and support more varied vegetation, including spruce-fir and whitebark pine forests broken by grassy meadows. Upwelling waters heated by hotspot magmas drive Yellowstone’s famed geysers, hotsprings, and mudpots which provide habitat for specialized, primitive ecosystems of algae and bacteria.
Human settlement and use of the GYE reflects the hotspot processes of uplift, volcanism, and faulting. Uplift formed a remote highland from which streams drain radially outward like spokes from a hub. Humans have settled around Yellowstone along these drainages and established roads, irrigation systems, and political associations along them. Decision-making involving the GYE is complicated by multiple jurisdictions athwart this hotspot highland, including 18 counties, seven National Forests, three states, and two National Parks.
This talk is based on a manuscript written with co-authors Don Despain, Lisa Morgan, and John Good.
