January 1998 – Emmons Lecture
Evidence for Life in a Martian Meteorite (! or ?)
Harry Y. McSween, Jr.
Department of Geological Sciences
University of Tennessee, Knoxville, TN 37996
The controversial hypothesis that the ALH84001 meteorite contains relics of ancient Martian life has spurred new findings, but the question has not yet been resolved. Organic matter probably results, at least in part, from terrestrial contamination by Antarctic ice meltwater. The origin of nanophase magnetites and sulfides, suggested, on the basis of their sizes and morphologies, to be biogenic, remains contested, as does the formation temperature of the carbonates that contain all of the cited evidence for life. The reported nanofossils may be magnetite whiskers and platelets, probably grown from a vapor. New observations, such as the possible presence of biofilms and shock metamorphic effects in the carbonates, have not yet been evaluated. Regardless of the ultimate conclusion, this controversy continues to help define strategies and sharpen tools that will be required for a Mars exploration program focused on the search for life.
February 1998
SEAFLOOR HYDROTHERMAL SYSTEMS:
20 YEARS OF REMARKABLE DISCOVERIES
W.C. Pat Shanks III
U.S. Geological Survey, MS 973, Denver Federal Center, Denver, CO 80225
Widespread hydrothermal activity related to mid-ocean ridges (MORs) was first indicated by anomalous heat flow patterns and by the discovery of Fe- and Mn-enriched metalliferous sediments on the ridge flanks. Oxygen isotope analyses of greenstones dredged from the seafloor implicated basalt alteration by hydrothermal seawater circulating at temperatures of 200-300 °C. Discovery of “black smoker” vents using the submersible Alvin at 21°N on the East Pacific Rise proved the role of seawater in MOR hydrothermal systems. NSF-RIDGE supported studies of vent fluids and metal sulfide deposits from a variety of different mid-ocean ridge sites have confirmed the importance of circulating, heated seawater and have revealed remarkable variations in salinity due to phase separation (sub-critical or supercritical boiling) processes. In fact, roughly 25% of the global heat loss is due to hydrothermal circulation at the MORs and basalt-seawater reactions substantially influence global chemical balances of the elements. Recent explorations using the drillship Joides Resolution under the auspices of the Ocean Drilling program have led to the discovery and delineation of major massive sulfide deposits on the Mid-Atlantic Ridge (TAG deposit) and on the northern Juan de Fuca Ridge (Bent Hill Massive Sulfide deposit). These discoveries prove the analogy between some massive sulfide deposits that are mined on the continents for copper, zinc, silver and gold AND modern deposits that are actively forming on the mid-ocean ridges. Some recent studies have suggested the presence of magmatic water in vent fluids immediately following ridge-crest volcanic eruptions. The importance of magmatic volatiles, the role of hyperthermophilic bacteria, and the environmental problems of future seafloor mining activities are fertile topics for additional research.
CONTINENTAL GROWTH AND OROGENIC LODE GOLD FORMATION
Richard J. Goldfarb
U.S. Geological Survey, MS 973, Denver Federal Center, Denver, CO 80225
Orogenic gold deposits are hosted within deformed metamorphic terranes of all ages. Observations from throughout the world’s preserved Archean greenstone belts and more recently active Phanerozic orogens indicate a strong gold-greenschist association. Ores were formed during deformation near plate margins over 10’s-100 million years of collision. Subduction-related thermal events initiate long-distance hydrothermal fluid migration. Resulting gold-bearing quartz veins are emplaced over a unique 15-20 km depth range for hydrothermal ore deposits. The spatial association between gold deposits and subduction-related thermal processes in peripheral orogens is well recognized. Models for the Mesozoic-Cenozoic goldfields of westernmost North America indicate a variety of plate-margin tectonic processes can initiate ore formation. Relationships within Paleozoic internal orogens, such as the gold-rich Variscan belt extending from NE China to southern Europe, indicate continent-continent collisional sutures are also favorable ore sites. A key point in all examples is that hydrated marine rocks were added to craton margins and, during this growth, the accreted rocks experienced high geothermal gradients.
Understanding of gold-forming processes in oldest Paleozoic to Precambrian orogens is typically complicated by hundreds of millions of years of additional geologic time, but ore genesis is still associated with continental growth. Gold veins of the Tasman Orogen in SE Australia make it clear that the ore-forming process need not require a “Cordilleran-style” of deformation; compression-related deformation may be solely intraplate rather than concentrated along terrane sutures. As with the Phanerozoic lodes, many significant Archean deposits are now also viewed as products of deep crustal, continental margin thermotectonic events.
March 1998
Owens (Dry) Lake, California: A Human-Induced Dust Problem
Marith C. Reheis
U.S. Geological Survey, MS 980, Denver Federal Center, Denver, CO 80225
The dry bed of Owens Lake, on the east side of the Sierra Nevada, has produced enormous amounts of windblown dust since the desiccation of the lake due to water diversions by Los Angeles in the early 1900’s. The lake bed is the largest single source in the United States of PM10 dust, which is regulated by the EPA because these small dust particles can be inhaled deeply into the human respiratory tract. Aerosols in Owens dust storms commonly contain significant amounts of arsenic. Owens dust storms also degrade visibility in three national parks and several national forests and wilderness areas, and cause suspension of operations at China Lake Naval Weapons Station resulting in millions of dollars in economic losses.
Salt-rich dust derived from the Owens Lake playa is deposited in significant quantities to distances of at least 40 km from the playa. Semi-annual measurements of dust 2 meters above the ground show that: (1) dust in Owens Valley is mainly derived from the playa, but is mixed with local sources, (2) dust flux is much higher in the winter than in the summer, and (3) Owens dust has soluble-salt content as high as 35 percent, much higher than that of dust elsewhere in the region. Significant quantities of salt-rich dust are probably being added to the soils in the region around Owens Lake playa, which may affect soil pH and vegetation. In addition, preliminary trace-element data suggest that dust sampled 100-300 km away from the playa may contain a significant component of Owens dust.
The Central Importance Of Geologic Ideas To The Issue Of Sustainability
Peter Palmer
President, Institute for Cambrian Studies
Can Earth handle the population of 10 BILLION people anticipated by 2050? At what level of consumption? Can it handle more if population is not brought under control by then? For how long? These are the central questions behind increasingly serious concerns about a sustainable future for humanity. Geological understandings that bring this problem into focus, and which we as geologists have long ago internalized, are clearly lacking in a majority of the public and many of our political and religious leaders.
The theme of “What should my neighbor know?” that underlies changing attitudes about science education is particularly apt with regard to the issue of sustainability. Our time and space context needs to be understood by all educated citizens. The ideas aren’t difficult to grasp: We live on Spaceship Earth; there is no place else to go. Earth systems that involve issues such as climate and geographic change, groundwater recharge, and soil replenishment, operate on geologic time scales, not human ones. Non-renewable resources have limits; the oil window will begin to close by mid-century. Renewable resources can become unavailable through over use. The geological record of life on earth shows us that too-rapid environmental changes have potentially bad side effects on the biosphere. Each human being has a more or less measurable ecological footprint and the ecological area of Earth is finite.
Most of these issues do not have a technological fix. We, the geological community, should use every opportunity to broaden public understanding of our geological context. It may make a difference in the quality of life available for our grandchildren.
April 1998
The Use of Imaging Spectroscopy Data for Geological and Environmental Studies
Roger N. Clark
U.S. Geological Survey, Denver, CO
The U.S. Geological Survey is using imaging spectroscopy data to access the geological and environmental character of public and private lands in the United States. Imaging spectroscopy is a departure from traditional remote sensing concepts in that the data represent continuous spectral coverage over a selected portion of the electromagnetic spectrum. Spectroscopic processing delineates absorption features due to individual chemical bonds in surface materials and in the atmosphere and when used with image analysis, maps their occurrence and distribution. The narrow spectral channels of an imaging spectrometer form a continuous sampling of the reflectance spectrum of the Earth’s surface, in contrast with the 4 to 7 discontinuous broad channels of the older imaging instruments (Landsat Thematic Mapper (TM) and Multispectral Scanner (MSS)). Analysis of imaging spectroscopy data allows minerals, vegetation types, man-made materials, water, snow, and many other mediums to be mapped if they display unique and identifiable absorption features in the 0.4 to 2.45 mm spectral wavelength region.
Obsidian Artifacts in Patagonia, Southern Chile
Charles Stern
Department of Geology, University of Colorado, Boulder
Patagonia is covered by large plateaus of Cenozoic basalts. Outcrops of rhyolites are unknown, but obsidian artifacts used by the prehistoric inhabitants are found in many of the archaeological sites of Patagonia, some dating back as far as 12,000 yrs. BP. Chemical finger-printing of one distinctive jet black type of obsidian suggests a single source for this material, which is widespread over a region stretching for more than 1000 miles along the Andean foothills of southern Patagonia. During field work last year, I found cobbles of this obsidian in dry stream beds crossing a fluvial-glacial plateau near the Cave of the Hands, one of the archaeological sites where obsidian artifacts are abundant. Apparently this single locality was the source of all the black obsidian in southernmost Patagonia, implying a much greater amount of cultural and material interchange than archaeologists working in this region had suspected.
May 1998
The Geology of Denver’s Building Stones
Jack Murphy
Denver Museum of Natural History
Building stones in Denver’s Landmark buildings reflect Colorado’s geologic history and can be used as an educational resource. A survey of historic landmark structures shows that native granite, sandstone, rhyolite, marble, and travertine were mainly quarried for dimension stone until the Silver Crash of 1893. Pikes Peak Granite was widely used as foundation stones. South Beaver Creek Granite from south of Gunnison makes up the exterior walls of the State Capitol. Many mansions and churches dating from the late 1800’s is made up of blocks of Lyons Sandstone from the Red Rock Canyon quarries south of Garden of the Gods, El Paso County. Marble from Yule, Colorado is probably the State’s best known stone because of its use nationally.
The Paleohydrology of Machu Picchu
Ken Wright
Wright Water Engineers, Inc.
Abstract not available.
September 1998
Stratigraphic Cycles and Controls Within a Continental Basin-Fill Succession in the Hornelen Basin (Middle Devonian), Western Norway
Donna S. Anderson
Consulting Geologist, Golden, CO
The Hornelen Basin (Middle Devonian) of western Norway is the most completely preserved of a series of structurally-linked, rapidly subsiding extensional basins that formed after the Silurian, continent-to-continent collision of the North American and Eurasian plates (Caledonian orogeny). The tectonic style of extension following compression is reminiscent of post-Laramide extension in the Great Basin of the western U.S.A. The syntectonic continental basin-fill succession is also analogous to Tertiary successions of the Great Basin, U.S.A.
In a study area along the north-central margin of the Hornelen Basin, sedimentary rocks consisting of interfingering deposits of the alluvial-fan, lake, and braidplain facies tracts are spectacularly exposed from the faulted basin margin to >4 km basinward. Mapping the organized relationships of coeval and interfingering alluvial-fan, lake, and braidplain facies in the study area shows three, nested (hierarchical) scales of stratigraphic cycles ranging from 4 to 120 m thick. Mapping also shows that the retreat or basinward advance of the alluvial-fan-facies tract is fundamentally linked to the expansion and contraction of the lake and braidplain facies tracts at all cycle scales.
Previous studies of Hornelen Basin strata are the sources of the “classic” and oft-cited tectonically-induced coarsening-upward alluvial-fan cycle. However, this study suggests that climate and an autocyclic mechanism, in addition to tectonics, controlled cyclicity of strata across all facies tracts. High sediment-accumulation rates in the alluvial-fan-facies tract correspond to high tectonic subsidence rates immediately adjacent to the faulted northern basin margin. But within this high subsidence background, climatically controlled lakes formed an important component of accommodation space. The mostly in-phase contraction and expansion of the alluvial-fan and braidplain facies tracts, that are sourced from entirely different geographic areas, also suggests a climatic control on cyclicity. An autocyclic mechanism produced by a huge difference in depositional topography between the alluvial-fan and adjacent braidplain facies tracts forms an important control at all cycle scales. Stratigraphic cycles in the continental strata of the Hornelen Basin are not simply the products of one forcing function. Rather they are products of independent and simultaneously operating controls.
Paradise Gained and Lost: A Geologic Tour of Tenerife, the Jewel of the Canary Islands
W. Ian Ridley
U.S. Geological Survey
Welcome to Tenerife. The majestic volcano Pico del Teide rises to 3718 meters through the cloud banks that shroud the lower flanks of the island of Tenerife, one of the seven major volcanic islands of the Canary Archipelago. Teide sits within a large, semi-arid caldera whose origin has been debated for half a decade, but which, nonetheless, reveals the complex geologic history of the past 3 million years. In contrast, deeply incised, sub-tropical valleys within the western (Teno) and eastern (Anaga) peninsulas illuminate their earliest sub-aerial geologic history. Large landslide depressions on the northern and southern flanks of the island, and about 1800 cubic kilometers of debris on the adjacent seafloor, represent major sector zone collapse that is the dominant mechanism of destruction of the island. This geologic/travelog talk will illustrate the growth and destruction of the island.
October 1998 – Family Night
The Incredible Climate of Colorado
Nolan Doesken
Assistant State Climatologist , Colorado Climate Center, and Director of the Fort Collins Weather Station
Thunderstorms, flash floods, wind storms, hail storms, dust storms, blizzards, heat waves, arctic blasts separated by periods of glorious sunshine and occasionally dull and boring weather are all a part of Colorado’s incredible climate. Is the climate changing? How do we track climate variations? What makes Colorado’s climate so interesting? What’s going to happen this winter? These are some of the topics that will be discussed. Come prepared to learn about Colorado’s remarkable climate. There will even be chances to win some interesting prizes.
November 1998 – Student Night
Secondary Hematite: Evidence for Molybdenum Transport under Oxidizing Conditions, Bergslagen District, Sweden
Nick K. Starkey
Colorado State University
Petrographic evidence shows that widespread oxidation of early pyrite to hematite may record the introduction of oxidizing molybdenum-rich fluids in granite systems at Bispbergs klack in the Bergslagen district, central Sweden.
Two ore hosts, distinguished by their deep red and pale gray colors, were investigated. Both are homogeneous, hypidiomorphic, slightly porphyritic, A-type granites. The red ore host differs from the gray ore host by aggrandized alteration of early pyrite to hematite, a higher degree of sericitization and chloritization, and dramatically higher molybdenite grades (locally up to 5% MoS2). Chemical analyses of the two ore hosts exhibit notable differences. The red ore host is enriched in Fe2O3, K2O, Mo, Ba, and Zr, and depleted in SiO2 and Na2O compared to the gray ore host. Most of the red ore has been removed by centuries of mining; some lower grade gray ore remains in outcrop.
A model for the Bispbergs klack deposit is as follows: (1) crystallization of abundant magmatic pyrite (plus minor chalcopyrite and sphalerite) in an early granite phase; (2a) oxidation of early pyrite (related to intrusion of gray granite?) producing widespread hematization and imparting a pervasive red color to the early granite; and (2b) simultaneous precipitation of molybdenite resulting from reduction of oxidized Mo-bearing fluids by reaction with early pyrite.
The bulk mineralogy and chemistry of the Proterozoic Bispbergs klack granites are similar in many respects to Tertiary Climax-type granites (e.g., Colorado Mineral Belt) where the significance of an oxidizing system for the transport of molybdenum has been demonstrated (e.g., d34S >4.0 permil). We propose that Bispbergs klack may be a deeper crustal analog to shallow Climax-type granite porphyries. High confining pressure at Bispbergs klack may have precluded exsolution of sufficient quantities of magmatic fluid to produce the stockwork veining characteristic of shallow Climax-type Mo-porphyry systems. As a result, molybdenite at Bispbergs klack occurs principally as disseminated blebs, and rarely in discontinuous quartz-calcite veins.
Integrated Approach to Three-phase Fluid Flow Modeling: Application to Northern Walker Ridge/Southern Green Canyon, Northern Gulf of Mexico Basin
S. Chereé Stover
University of Colorado
Recent cross-section reconstructions of the northern Gulf of Mexico Basin have allowed for new insights to be gained concerning the complex geologic processes that governed historic submarine structural development. The reconstructions encompass a time span of 66.4 Ma to present day and indicate a dynamic and interconnected evolution of fault growth, sedimentation, and allochthonous salt deformation. Parallel with enhancing interpretation of the historical geology of the basin, the reconstructions have also reinforced a relatively unknown variable that is of substantial interest to both geohydrology and structural academic fields: historic fluid flow patterns. As the Gulf of Mexico Basin developed over time, the evolution and magnitude of basin heterogeneities are likely to have had pronounced influences on historic fluid migration, but the flow patterns and scale of heterogeneity influence remain poorly understood. This research focuses on modeling the effects of heterogeneities on historic fluid flow patterns in the northern Walker Ridge/southern Green Canyon region. Through implementing replicas of each reconstruction stage into a 2-dimensional numerical model and linking a series of fluid flow models throughout the reconstructed period, we are able to predict the resultant fluid flow patterns and comprehensively examine the magnitude of heterogeneity influence on the system.
Geology, Alteration, and Mineralization of the Cerro San Pedro Gold-Silver Deposit, San Luis Potosi, Mexico
David Winterboume
Colorado School of Mines
The Cerro San Pedro Au-Ag district is located in central Mexico, and has been the site of intermittent mining for over 400 years. District geology is characterized by a complexly folded Cretaceous limestone sequence which has been intruded by a quartz-diorite porphyry. Several episodes of premineral deformation have resulted in north-south fold axes cut by both north-south and younger east-west faults. Intrusion of the porphyry was localized by the north-south structures. Mineralization is characterized by stockwork veins and disseminations in the porphyry and carbonate replacement and manto/chimney bodies in the carbonates. Five hydrothermal alteration styles are observed at Cerro San Pedro, with mineralization associated with both sericitic and acid-sulfate alteration. Sericitic alteration is characterized by fine-grained muscovite, while the acid-sulfate alteration assemblage includes alunite, jarosite, and kaolinite.
The most important structural controls of alteration are the north-south Mendez-type faults and the east-west Princesa fault. Hydrothermal fluids appear to have migrated upward from depth along the Princesa fault, then north into the Mendez-type faults.
Preliminary results from geochemical studies suggest that there are multiple stages of mineralization. Gold and silver were first introduced during an early sericitic alteration. Gold was subsequently enriched and/or remobilized during a later acid-sulfate alteration event.
December 1998 – President’s Address
MULTI-STAGE, MULTI-DIRECTIONAL LARAMIDE(?) SHORTENING AND COMPRESSION IN THE SOUTHERN ROCKY MOUNTAINS
Eric A. Erslev
Department Of Earth Resources, Colorado State University
Despite unequaled geologic control on the upper crust of the Rocky Mountains, the Laramide orogeny remains an enigma. The linkages between plate motions and Laramide deformation are uncertain, with proposed explanations including subcrustal shear during low-angle subduction, crustal delamination and detachment due to plate coupling to the west, and gravitational spreading of the Cordilleran thrust belt.
In the southern Rocky Mountains (south of Wyoming), recent battles between advocates of north-south strike-slip faulting and east-west thrust faulting have centered on attempts to limit strike-slip displacements using sedimentary trends. Unfortunately, the region’s abundant crustal weaknesses make assumptions of linear pre-Laramide sedimentary trends suspect at best.
Our research at C.S.U. has used minor fault patterns to document the complex Laramide-to-Recent evolution of the southern Rocky Mountains. The major Laramide arches formed during an early stage of E-W to NE-SW shortening and compression. This deformation is recorded by roughly equal numbers of thrust, right-lateral and left-lateral faults. In western Colorado, these faults are cut by faults indicating a second stage of NW-SE shortening and compression. In central Colorado and New Mexico, faults from a third stage of N-S shortening overprint the above faults. In New Mexico, these faults are dominantly strike-slip, with few associated thrust and normal faults. Neogene Rio Grande extension is recorded by younger north-striking normal faults.
The timing of these orogenic stages is partially constrained near Galisteo, New Mexico, where all fault sets cut the Paleocene Diamond Tail Formation but only the N-S strike-slip and normal faults cut the Eocene Galisteo Formation. In this area, no strike- slip faults cut the 27 Ma Galisteo mafic dike. Near Raton and Taos, however, north- south strike-slip faults cut the 24 Ma Eagle Rock dike and the 27 Ma Llana Quemado volcanic breccia as well as conglomerates containing clasts of 25 Ma tuff. In Colorado, equivalent late-stage shortening directions are indicated by WNW-trending, Miocene-cored synclines in North Park and near State Bridge as well as north-south strike-slip faults cutting mid-Tertiary plutons. Thus, the latest period of strike-slip shortening may have continued into the early Miocene. It may be the missing link spanning the gap between Laramide shortening and Rio Grande rifting.
Our work has shown the validity of both thrust and strike-slip hypotheses, although neither is adequate in exclusion of the other. Early Laramide deformation is consistent with crustal delamination and detachment due to plate coupling to the west. Subsequent shortening directions do not parallel predicted plate convergence vectors, however, and suggest gravitational spreading of the Cordilleran orogenic welt. Middle Tertiary strike-slip faulting may have localized mid-Tertiary igneous activity and provided the precursor for Rio Grande rifting.
