Rates of Quaternary Landscape Change in the Eastern Grand Canyon
Lon Abbott, Senior Instructor and Associate Chair, University of Colorado—Boulder
Abstract: The Grand Canyon is a type example of arid region geomorphic processes, making the constraint of process rates, such as river incision and scarp retreat, desirable. Such constraints are particularly important here for two additional reasons. First, the Colorado River is the master stream that drains most of the American Southwest, thereby exerting first-order control on rates of other geomorphic processes throughout the region. Second, the Colorado River’s incision history provides fundamental constraints on models of the surface uplift history of the Colorado Plateau. When and how the plateau was raised to its present average elevation of about 2 km remains controversial.
Quantification of such process rates requires the dating of suitable Quaternary datums. Travertine is the prime datum candidate in eastern Grand Canyon due to its abundance and its amenability to U-series geochronology. Here I present process rate determinations for the last ~1 m.y. based on U-series ages and stratigraphic positions of travertine outcrops in the vicinity of Hermit Creek.
Our primary conclusions are: 1) The minimum incision rate of the Inner Gorge near Hermit Creek was 313 +/- 77 m/m.y. and the maximum rate was 436 +/- 104 mm/m.y. from ~1 Ma to ~400 ka. The rate then slowed to a maximum of 183 +/- 30 m/m.y. thereafter. This slowing of incision is consistent with the passage of an upstream-migrating knick point sometime between 1 Ma and 400 ka; 2) The average incision rate over the last ~1 m.y. has been 254 +/- 31 m/m.y., a rate sufficient to carve the entire Grand Canyon since 6 Ma, as many researchers have proposed. Conversely, Wernicke (2011) argues that the current Canyon was carved to within 380 m of its current depth prior to 16 Ma (mostly by an ancestral ‘California River’). Our data admit such a hypothesis but demand vastly slower pre-Quaternary incision rates, as hypothesized by Wernicke (2011); 3) The Redwall escarpment has been retreating from the river at a rate of 465 +/- 51 m/m.y. during the last ~1 m.y.; 4) Travertine Creek, an ephemeral Colorado River tributary, aggraded its bed between 53–11 ka (to a depth of 21.6 m in its middle reaches), then incised that accumulated fill and an additional 1 m of bedrock toward the present. Today, its bed lies at a nearly identical elevation to the one it occupied at 53 ka.
Biography: Lon Abbott grew up in Boulder and fell in love with the mountains thanks to all of the time he spent in them. That passion for mountains fueled his desire to major in geology when he went to college. He completed bachelor’s degrees in geology and geophysics at the University of Utah and then worked for two years after graduation as a field geophysicist for a small aeromagnetics firm based in Salt Lake City. He has done two internships in seismic processing in Denver, one at Marathon and the other at Amoco. After the aeromag. work, he went to University of California, Santa Cruz to do graduate work focused on mountain-building processes, sponsored by an NSF Graduate Fellowship. His study site in northern Papua New Guinea constitutes the world’s youngest mountain range. For his Ph.D. he studied the evolution of the arc-continent collision, using both field mapping and marine geophysics. He stayed on to do a post-doc during which he quantified the rate of surface uplift for the range.
After his post-doc, he joined the faculty at Prescott College, a small liberal arts college in Arizona. His research interest in the Grand Canyon began there, thanks in part to several undergraduate senior theses he sponsored on local geology. He stayed at Prescott College for ten years, serving as the Chair of the Faculty Senate for four years and Associate Dean for Academic Affairs for one. He then moved to the faculty of Red Rocks Community College for a year before joining the Geological Sciences faculty at CU, where he serves as a Senior Instructor and the Associate Chair for Undergraduate Studies.
With his geologist wife, Terri, he has co-authored several recent geology articles in Earth magazine and two books on Arizona geology aimed at a popular audience: Hiking The Grand Canyon’s Geology and Geology Underfoot in Northern Arizona. Their third popular geology title, Geology Underfoot Along Colorado’s Front Ranges, is scheduled for release this fall.
Colorado Meteorites and the 2004 Berthoud Meteorite
Abstract: Some 85 meteorites are known from Colorado, of which some of the most notable are the Guffey iron meteorite (309 kg), the Bear Creek iron meteorite (227 kg) and the Rifle iron meteorite (103 kg). Of the 5 witnessed and recovered meteorite falls in Colorado, the most recent was the Oct. 5, 2004, Berthoud, Weld County, meteorite fall. (1 kg = 2.2 lbs.)
The Berthoud meteorite is a eucrite, a stony meteorite of basaltic composition. It belongs to the HED (howardite-eucrite-diogenite) group of meteorites that are believed to have been derived from the asteroid, Vesta. NASA’s Dawn spacecraft mission to study two asteroids was launched in 2007; it is currently studying and photographing Vesta, and in Feb. 2015, it will arrive in proximity to the asteroid Ceres.
Thanks to a donor who contributed the funds, the Colorado Meteorite Society (COMETS) purchased a 50 gram slice of the meteorite for $12,000, which they donated to the Colorado School of Mines Geology Museum, where it is currently on display.
The Berthoud meteorite originally had a mass of 960 grams; after various pieces were sawn off and removed and sold, the largest remaining piece (main mass) weighs 247 grams, and is 70% covered by a fusion crust. It has been owned since 2010 by Oregon meteorite dealer Edwin Thompson, who would like to see it end up in some Colorado museum, but because he needs to raise money to help pay for medical expenses, he is planning to sell the meteorite by the end of this month. Thompson is asking $45,000 for the 247-gram meteorite, an amount which, at $182/gram, is in line with the prices commonly commanded by the more unusual types of meteorites; for example, specimens of the Johnston, Colorado meteorite, a shower which fell in 1924, with 40.3 kg total known weight of meteorite fragments, sell for over $200/gram. Interestingly, the Johnstown meteorite fall was a diogenite, another of the HED group; Colorado is the only state in the U.S. known to have two different meteorite falls of Vestian origin.
Biography: Fred Olsen has studied and collected meteorites from Colorado and elsewhere, and has traveled extensively worldwide to visit meteorites and localities where they have been found. Fred has helped assemble the meteorite display at the Colorado School of Mines Geology Museum.
Fred Olsen is hoping to find a donor or donors who might be able to purchase and preserve the Berthoud meteorite for a Colorado public institution, during the very short window of time remaining before it is sold; the owner will be taking it to Tucson sometime shortly after Jan. 20 for probable sale to another buyer or institution if no Colorado purchaser is forthcoming. Fred (303–748–7400 or firstname.lastname@example.org) would be happy to speak with anyone who has ideas or suggestions about raising funds for this purpose.
Unroofing and uplift history of the Grand Canyon region of the Colorado Plateau from apatite (U-Th)/He thermochronometry and future directions in the new (U-Th)/He lab at CU-Boulder
Rebecca Flowers, University of Colorado Boulder
Abstract: The source of buoyancy for the ~1.9 km elevation gain of the Colorado Plateau since Late Cretaceous time, and its relationship to carving of the ~1.5-km-deep Grand Canyon by the Colorado River, are longstanding and controversial problems. We used apatite (U-Th)/He thermochronometry throughout the southwestern quadrant of the Colorado Plateau to address these issues. The data document overall southwest to northeast unroofing from plateau margin to plateau interior during denudation phases in Late Cretaceous/Early Tertiary (80 to 55 Ma), mid Tertiary (28 to 16 Ma), and Late Tertiary (<6 Ma) times. Distributions of apatite dates suggest that eastern Grand Canyon samples from the basement and the Kaibab surface nearby had similar Early to mid-Tertiary thermal histories, despite their ~1,500 m of stratigraphic separation. If these models are correct, they indicate that a significant (≥ 1,000-m deep) paleo-Grand Canyon was carved in post-Paleozoic sediments in this region during Early Tertiary time. Kilometer-scale topographic relief would require substantial uplift during the Sevier/Laramide, pointing toward hydration of the lithospheric mantle, partial lithospheric removal, or dynamic topography associated with evolution of the Farallon slab as causative mechanisms for the initial rise of the Colorado Plateau. The evidence for an Early Tertiary paleo-Grand Canyon also has intriguing implications for the paleodrainage evolution of the Colorado River, and runs counter to the widespread view of a Late Tertiary origin for the canyon.
A new (U-Th)/He thermochronology lab is being established at the University of Colorado, Boulder. Over the last fifteen years, (U-Th)/He thermochronology has evolved from a newly emergent technique to a broadly used tool for addressing problems in earth science. Like in the example above, apatite (U-Th)/He dating has been widely applied to decipher the thermal histories of rocks in the upper few kilometers of the Earth’s crust and thereby gain insight into burial and unroofing, topographic development and elevation change, and the underlying controls on these processes. However, innovative applications of the method continue to emerge, including diverse hydrocarbon exploration studies to predict hydrocarbon maturation through reconstruction of basinal thermal histories, new work to decipher the origin and evolution of hydrothermal ore deposits, and sediment tracer investigations to quantify the spatial distribution of relief change and catchment erosion. The explosion of new studies on minerals such as goethite, magnetite, zircon, monazite, calcite, and clinker have enabled new applications ranging from dating soil development and determining coal fire frequency, to providing new angles for constraining the influence of climate change on the rates and patterns of landscape evolution. We plan to pursue a variety of new research avenues in the new CU-Boulder lab, and are interested in exploring collaborative opportunities with others in the region.
My adventure to the Driest, Coldest, and Windiest place on Earth
Laura Wolton, NOAA
Abstract: South Pole Station in Antarctica is located at 9,300’ (2900m), however the equivalent pressure elevation, based on polar atmospheric conditions, will vary from 10,800’ (3300m) to 13,120’ (4,000m). On average, it is the driest place on earth and actually considered a desert because of its minimal precipitation, which is on average less that 1 inch per year. The remote, elevated and dry conditions make the South Pole an excellent place for research. However, the challenges to research in Antarctica and particularly at the South Pole can be many, including wind, months of darkness, isolation and extreme cold.
Biography:Laura Wolton runs the Tropospheric Surface Ozone program at NOAA. She recently visited McMurdo Station and South Pole Sation, Antarctica, to calibrate ozone monitors that reside at the stations, a visit that was timed with the 100th anniversary of Scott’s arrival at the South Pole. Her talk about travel, atmospheric research, work and fun at the two stations is accompanied by a photo show.
Complex Uplift History of the Front Range deduced from the Synorogenic sediments in the Denver, Cheyenne, North-Middle and South Park Basins, Colorado
Marieke Dechesne, Denver Museum of Nature and Science (DMNS) and USGS,
Jim Cole, USGS,
and Bob Raynolds, DMNS
Abstract: The basins surrounding the Front Range reveal information about the timing and location of uplift of this basement block. Each basin tells part of the story and combining clues from all basins allows interpretation of the regional paleogeographic evolution for this region. This study combines data collected by the Denver Museum of Nature & Science, USGS, Colorado Geological Survey and University of Washington. This talk will be given in three parts:
Bob will discuss the basin-fill succession and characteristics of the Denver Basin. The stratigraphy of the Denver Basin serves as a temporal and spatial framework for comparison with the other basins. The Western Interior Seaway covered this region completely before the Front Range showed any signs of uplift. The character of the seaway sediments follows the linear pattern of its coastlines. The first hint of uplift in the hinterland is recorded by sparse pebbles in the regressive shorelines of the Fox Hills Sandstone. Several hundred feet higher in the section, the Arapahoe Conglomerate marks the onset of hinterland uplift. Not much later, the Front Range basement started to emerge and fan systems developed off its flanks in distributary patterns that thin eastward and northward. These depositional patterns are an important factor in understanding aquifer distribution in the basin.
Jim will describe the North and Middle Park Basins on the west side of the Front Range. The basin-fill sequence here reveals a complex history of broad uplift, widespread erosion and subtle deformation before sediment started to accumulate. Fossil leaf margin analysis shows that at about 58 Ma this area was elevated significantly higher than the nearby Denver Basin. Within the synorogenic strata of the basin, angular unconformities are present and (combined with the other observations) indicate multiple phases of deformation and fault activity. Depositional facies indicate that coarse, proximal, fluvial-alluvial deposits are only present in a few locations at the base of the synorogenic strata. Fine-grained basal deposits elsewhere around the basin margins indicate the uplift source areas were more distant. The synclinal basin observed today is a smaller, folded relic of the original depositional basin.
Marieke will highlight the thick package of latest Cretaceous sediments in the Cheyenne Basin and the sediment sequence in South Park. Evidence from all discussed basins will be integrated together in a series of maps showing the paleogeographic evolution of the region between 70–55 Ma.
Marieke Dechesne has been working with Bob Raynolds on the Laramide synorogenic sediments of the Denver Basin since 2002, and more recently also with the CGS. Expanding this work to the north into the Cheyenne Basin with the DMNS and the University of Washington sparked her curiosity to find out what was happening on the other side of the mountains. Since 2010 she is also working with Jim Cole on the North Park – Medicine Bow Mountains project to learn more about the history of the Front Range. She has a degree from the University of Utrecht in the Netherlands, been a researcher at the Colorado School of Mines, and taught at CSM and CU.
Jim Cole earned a Ph.D. in Geology from the University of Colorado-Boulder, and has been a mapping geologist with the USGS for over 35 years. His career work has covered Precambrian basement rocks in Colorado, island-arc terranes in Saudi Arabia, Paleozoic stratigraphy and Mesozoic thrusting in southern Nevada, Tertiary stratigraphy and groundwater resources in New Mexico, and Pliocene drainage integration of the Rio Grande. For the last 7 years, he’s returned to the northern Front Range to take another look at the Laramide history, and late Tertiary uplift and drainage evolution.
Bob Raynolds is a Research Associate at the Denver Museum of Nature & Science. His dissertation research focused on sedimentary rocks that accumulated at the foot of the Himalayas. This experience led him to study rocks in the Denver Basin that record the uplift of the Front Range. Bob has taught at the Center for Excellence in Geology at Peshawar University in Pakistan, at Dartmouth College, and at the Colorado School of Mines where he is an adjunct faculty member. His recent lectures focus on the impact of climate change on Colorado’s ecology and water resources.
What’s New in the Cambrian?
Two presentations by James “Whitey” Hagadorn, Denver Museum of Nature and Science
(1) Death of a Megapredator
Abstract: Anomalocaridids were the first large predators, and are thought to have been ferocious consumers of trilobites and other prey that inhabited early oceans. Like Tyrannosaurus rex, sabre-toothed cats, and similar apex predators, they first became known from a famous deposit— the Cambrian-age (510-million-year-old) Burgess Shale. Aside from their iconic status, anomalocaridids also play an important role in our understanding of Earth history and evolution because they are keystone organisms. They are keystones because our interpretations of their biology and ecology, based on the morphology of fossils, anchor our understanding of how entire marine ecosystems functioned. Yet new finite element modeling, taphonomic, and mineralogic analyses of these extinct creatures seems to contradict the longstanding interpretation of the biology and ecology of these animals, suggesting rather that they may have been small, that their mouths were soft, that they could not close their mouth, and that they could not have possibly eaten trilobites. Similarly, analyses of malformed or “bitten” trilobites interpreted to be prey suggests that they could not have been deformed by the mouth or appendages of any known anomalocaridid. Analysis of putative anomalocaridid coprolites (i.e., fossilized feces) from the same deposits suggests that coprolites were produced by other animals or inorganic processes. Based on this new work, it is unclear if these iconic predators were really just small worm-suckers or whether they were early plankton combers; regardless, this new data substantially changes the way we interpret the complexity of ancient ecosystems, the evolution of large arthropods and their kin, and biogeochemical feedback loops in organic-rich muddy settings.
(2) Surfing Cambrian Coasts: First Animals on Land
Abstract: Why did animals crawl out of the ocean and onto land? How did they do it, and who first succeeded? What was “land” like? An understanding of early animal-influenced coastal systems is emerging from Cambrian strata of North America and Arabia, where it is possible to tie animal activities directly to environments by linking fossils to facies. For example, eolian foresets of the Furongian (“Late” Cambrian) Gunter and Lamotte Sandstones of Missouri, and the Series 2 (“Middle” Cambrian) Keeseville Member of the Potsdam Group of New York have arthropod trackways that were produced subaerially, by euthycarcinoid-like animals crawling on coastal dune slip faces. These fossils represent some of the first evidence of successful land-going animals. However, in order to get to these dunes, animals must first have crawled across other intermittently wet coastal landforms, such as tide- and sand-flats and beaches. It is in these settings where they first experienced the vicissitudes of crossing the subaqueous-subaerial divide—learning to withstand loss of buoyancy and aqueous respiration, and the hazards of increased desiccation, ultraviolet radiation and temperature variation. Evidence of these pioneering forays comes from Series 2 (“Early” Cambrian) to Series 3 intertidal-supratidal lithofacies of the Elk Mound Group of Wisconsin. There, a low diversity community of large euthycarcinoid-like arthropods, soft-bodied or weakly sclerotized molluscs, and ?annelids? crawled across exposed sand flats during and between episodes of subaerial exposure. Although the arthropods are represented by both body and trace fossils, most of this community is only known from its trackways or body impressions. The trackways link the timing of animal activity to specific depositional events, including individual episodes of subaerial exposure. Coeval strata of the Arabian Peninsula, which effectively represent a Gondwanan tectonic regime, contain similar information about the colonization of land—but Terreneuvian (i.e., “Early” Cambrian) strata also contain evidence for arthropods crawling up actively flowing rivers. Given that freshwater environments were not colonized by animals until tens of millions of years later, it is possible that such fossils represent ?unsuccessful? exploratory ventures or other behaviors.
Biography: James “Whitey” Hagadorn is currently the Tim and Kathryn Ryan Curator of Geology at the Denver Museum of Nature & Science. There, he curates the mineral, rock, meteorite, and invertebrate fossil collections. Although originally from San Diego, California he has been fortunate to have also lived in Pennsylvania, Montana, Massachusetts, and Texas. Everything about “deep time” fascinates him, and he has been lucky to have spent the last 20 years studying modern and ancient environments all over the world. Much of his research has focused on the latest part of the Precambrian (700–542 million years ago) and the early parts of the Paleozoic (542–450 million years ago), intervals of time that witnessed some of the most profound changes in environments and biota in all of Earth history. Through fieldwork, labwork, and collaboration with academic and citizen scientists, he has published papers focused on interpreting ancient sedimentary environments, tectonics, ocean chemistry, fossils, microbial structures, and a variety of enigmatic ‘whatsits’. Although all of this work contributes to improving our understanding of ancient Earth systems, Hagadorn is cognizant of the need to leverage our understanding of ancient Earth systems to better understand how our Earth will change in the future—as a result of human activities.
The Ancient Surface of Venus is Saturated with Impact Structures, and its Lowlands are Covered with Marine Sediments
Warren Hamilton, Distinguished Senior Scientist, Department of Geophysics, Colorado School of Mines
Abstract: Radar imagery of Venus shows thousands of rimmed circular depressions, up to 2,000 km in inside-rim diameter, that saturate both highlands and lowlands, have apparent impact morphology, and must be mostly older than 3.9 Ga if indeed impact structures. Conventional wisdom force-fits interpretations to the incompatible assumption that the planet is too active internally to preserve an ancient landscape, ignores most of the large circular structures, and assigns those that are considered to young and diverse, up and down “plumes” of “unique to Venus” types. Imagery and geodesy falsify these popular assumptions. There is broad overlap in degree of modification between the 1,000 variably-modified small craters accepted by all as “young” impacts, and the far more numerous and mostly larger and older circular structures. Purported evidence for terrestrial plumes has all been disproved, so there is no earthly basis for export of plume fantasies to another planet. Venusian plains display abundant sedimentary features, but no igneous ones despite their popular designation as flood basalts. The sediments variably bury terrain-saturating and water-reworked impact structures, and are compacted into them.
Anthropogenic Global Climate Change
Strong evidence supports the scientific consensus that greenhouse gases and other human-generated pollutants are
affecting global climate, most conspicuously by warming. Diverse and interlocking evidence from atmosphere, cryosphere, and hydrosphere will be summarized. Interactions have been complex, non-linear, and include cascading feedbacks. The large amount of heat going to high latitudes, and the large temporal variations in partitioning between air and water, have been major surprises. Extrapolations and predictions are highly uncertain, and whether or not threshold and runaway effects will be severely damaging is unclear. Ideological and economic campaigns of anti-science disinformation and defamation have contributed much to the gridlock that now precludes rational political discussion of the issues.
Biography: Warren Hamilton moved to the Colorado School of Mines after a long USGS research career. Membership in the National Academy of Sciences, the Penrose Medal of the Geological Society of America, and the Distinguished Service Medal of the U.S. Department of the Interior are among his many research honors. His current work focuses on
changing terrestrial tectonics and geodynamics through time, the mechanism of plate tectonics, and Venus.
Colorado Plateau Molecules: Chemical Stories from the High Desert
Abstract: This photo-illustrated talk poses essential questions about observable natural phenomena common to the Four Corners region. Questions such as:
* If God is not a Broncos fan then why are sunsets orange?
* Breaking Bad: Were the early settlers of the Colorado Plateau drug addicts?
* Why are the rocks of canyon country little more than huge chunks of oxygen?
* Rattlesnake venom: What is it and why are there two kinds?
* What is desert varnish, how does it form and why so much manganese?
Most essays on natural history interpret the traditional fields such as geology, wildlife, wild flowers, insects, weather, birds, forests or ecology. Colorado Plateau Molecules interprets chemistry. That is to say, it interprets all of those disciplines because chemistry defines and refines each of the other fields.
Colorado Plateau Molecules invites the audience members to look beyond what they see into the largely hidden domain of molecules. By providing a molecular level context, common observations become imminently comprehensible and audiences gain a deeper understanding of the natural environment that embraces this amazing world of canyons and mesas.
Colorado Plateau Molecules offers information about more than molecules. It contains stories about the plants and animals, the rocks and minerals, the history and culture found in and around the national parks of the southwest, stories inspired by their unique and interesting chemistry. It explores connections between chemistry, nature, poetry, history, classic literature, popular culture, music and much more.
Biography: Ed Waterman is the author or co-author of five high school chemistry textbooks. He presents photo-essay lectures about the natural history of molecules, engaging the general public in an appreciation for and an understanding of chemistry. He also conducts workshops for teachers on inquiry, differentiation, small-scale chemistry, AP Chemistry, and virtual chemistry laboratory.
Mr. Waterman’s publications include Pearson Chemistry, a popular text for first-year high school chemistry and Small-Scale Chemistry Laboratory, also published by Pearson. In addition, he has published numerous professional papers in peer-reviewed journals including the Journal of the American Chemical Society, the Journal of Organic Chemistry, the Journal of Chemical Education and The Science Teacher.
Mr. Waterman holds a Bachelor of Science degree in chemistry from Montana State University and a Master of Science degree in chemistry from Colorado State University. In his free time he enjoys exploring wild places by hiking, kayaking and cross-country skiing in the Rocky Mountains and on the Colorado Plateau.
President’s Address: Colorado Pegmatites
Pete Modreski, USGS
Abstract: Colorado is known worldwide among geologists, mineralogists, and mineral collectors for its granitic pegmatites and the often spectacular minerals they have produced. The best known pegmatites are those within the ~1.08-Ga anorogenic Pikes Peak batholith. Crystal-lined pegmatites occur as miarolitic “pockets,” sometimes localized along dikes or fractures, which may contain crystals of smoky quartz, amazonite (blue-green microcline), and albite, as well as topaz, fluorite, zinnwaldite, goethite, and a host of other accessory minerals. The pegmatites are concentrated around late intrusive plutons within the batholith, at such places as Lake George (Crystal Peak), Crystal Park, Glen Cove, St. Peters Dome, Devils Head, Wigwam Creek, Harris Park, and the Tarryall Mountains. Details of exactly how, and at what temperatures, the euhedral crystals in the open-pocket pegmatites formed have always been a puzzle. Very few fluid-inclusion studies have been done on the crystals, but it seems that crystal growth spanned the continuum from true pegmatite (water-rich silicate melt + coexisting supercritical aqueous fluid) to hydrothermal conditions. Mostly in the northern half of the batholith, a distinctive type of large, chimney-like, zoned pegmatites occur, constituting the South Platte pegmatite district. Most of these were once mined for feldspar and decorative white quartz; some contain significant amounts of fluorite, topaz, and rare-earth and Nb-Th-U minerals. The pegmatites of the batholith are classic NYF (niobium-yttrium-fluorine) type pegmatites, enriched in those elements.
Pegmatites also occur within the older Proterozoic igneous rocks of Colorado, mostly within the ~1.7-Ga metamorphic rocks (“Idaho Springs Formation”) that form the country rock to these plutons. It has always been a challenge to interpret whether the pegmatites are genetically related to the older, synorogenic, ~1.7-Ga Routt Plutonic Suite (“Boulder Creek age”) granitic rocks (granodiorite, quartz monzonite, granite), as most of them probably are, or to the younger, essentially anorogenic, ~1.4-Ga Berthoud Plutonic Suite (“Silver Plume age”) granite to quartz monzonite plutons. The pegmatites occur in a number of swarms in the Front Range and adjacent parts of Colorado: in Larimer County (Crystal Mountain pegmatite district); between Mt. Evans and Golden (“Denver Mountain Parks area”); near the Arkansas River (Guffey-Micanite, Eight Mile Park, and Cotopaxi-Texas Creek districts); near Trout Creek Pass; and further west in the Quartz Creek district, Gunnison Co., and south in the Crestone-Villa Grove area. Many of these pegmatites contain beryl, black tourmaline (schorl) and are rich in radioactive oxide minerals, and a few contain lithium minerals (spodumene, lithium-bearing tourmaline, lepidolite, montebrasite) and would be geochemically classed as LCT (lithium-cesium-tantalum) type pegmatites.
Distinct and unique in Colorado are the ~30-Ma miarolitic cavity pegmatites in the Mount Antero Granite on Mounts Antero and White in the Sawatch Range, famous for their crystals of aquamarine, the State Gemstone.
In the past few years, I’ve had the pleasure of collaborating with Dr. Luis Sánchez-Muñoz (Institute for Ceramics and Glasses, CSIC, Madrid, Spain), in sampling of Colorado pegmatites for his studies of what twinning and exsolution in microcline show about the genesis and cooling history of their host pegmatites.1 This work holds promise toward better correlating the pegmatites to their genetically linked granites.
1Sánchez-Muñoz, Luis, P.J. Modreski, and B.R. Frost, 2011, K-feldspar twin-structures from orogenic and anorogenic granitic pegmatites in central North America: Asociación Geológica Argentina, Serie D, Publicación Especial No. 14, p. 179–183.
Biography: Dr. Peter J. Modreski has been a geochemist since 1979 with the U.S. Geological Survey, Lakewood, Colorado. He has a B.A. (chemistry) from Rutgers College and an M.S. and Ph.D. from Penn State (geochemistry). His research interests include mineralogy, gemstones, Colorado geology, ore deposits, and alkaline igneous rocks, and he is the USGS geological resource specialist for abrasives, gemstones, quartz, beryllium, cesium, and rubidium. He is presently responsible for public and educational outreach for the USGS Office of Communications. Pete is a co-author of Minerals of Colorado (1997), a Consulting Editor of Rocks and Minerals magazine, and a Research Associate with the Earth Sciences Department, Denver Museum of Nature and Science.