Abstract Lithosphere-scale seismic experiments, structural geometries, and minor structures in the Bighorn region of the northern Rockies show that Laramide deformation was controlled by horizontal shortening driven by detachment in the lower crust. This field trip visits three exposures in the Colorado Front Range that helped generate the detachment hypothesis, and uses observations from both areas to generate a thrust belt model for basement-involved foreland orogens. Starting north of Boulder, a traverse following the trace of the classic Six Mile fold reveals minor structures showing early layer-parallel shortening overprinted by a progressively tightening, non-self-similar fault-propagation fold. The overlying petroliferous Niobrara marls show how sequential deformation predicts the unit’s natural fractures, which are critical to the success of individual wells in this major resource play. South of Boulder in Eldorado Canyon (known worldwide for near-vertical climbing), the trip traverses excellent exposures of the Paleozoic strata in the hanging-wall above the Golden thrust fault. Out-of-the-basin and into-the-basin thrusts allow the restoration of the Rocky Flats 2D seismic line just to the south. From a revealing overlook, Front Range and Bighorn (using the seismic and structural results from the newly completed Bighorn Project) arch geometries can be generalized into a 4D (3D space + time) model for basement-involved foreland orogens. The final stop visits the enigmatic Boulder-Weld County fault system in the Denver Basin to the east to discuss the relative importance of regional tectonic setting (e.g., low-angle subduction), stress, lithospheric rheology, gravity, and fluids to the formation of basement-involved foreland thrust belts.

Contractional, basement-involved foreland deformation in the Rocky Mountains of the conterminous United States occurred during the latest Cretaceous to Paleogene Laramide orogeny. Current kinematic hypotheses for the Laramide orogeny include single-stage NE- to E-directed shortening, sequential multidirectional shortening, and transpressive deformation partitioned between NW-striking thrust and N-striking strike-slip faults. In part due to this kinematic uncertainty, the links between Laramide deformation and plate-margin processes are unresolved, and proposed driving forces range from external stresses paralleling plate convergence to internal stresses due to gravitational collapse of the Cordilleran thrust belt. To determine the tectonic controls on Laramide deformation, kinematic data from minor faults ( n = 21,129) were combined with a geographic information system (GIS) database quantifying Rocky Mountain structural trends. Minor fault data were collected from a variety of pre-Laramide units to calculate average Laramide slip (N67E-01) and maximum compressive stress (N67E-02) directions for the Rocky Mountains. These largely unimodal, subhorizontal slip and compression directions vary slightly in space; more E-W directions occur in the southern and eastern Rockies, and more NE-SW directions are found near the Colorado Plateau. This kinematic framework was extended to the entire orogen using map data for faults, folds, arches, and Precambrian fabrics from Wyoming, Colorado, northern New Mexico, southeastern Utah, and northeastern Arizona. Vector mean calculations and length-weighted rose diagrams show that Precambrian fabrics are at a high angle to most larger Phanerozoic structures but are commonly reactivated by smaller structures. Ancestral Rocky Mountain structures are subparallel to Laramide structures, suggesting similar tectonic mechanisms. Laramide faults, defined by their involvement of Mesozoic and Paleogene strata but not Neogene strata, are complex, and preexisting weaknesses and minor strain components commonly predominate. In contrast, Laramide fold (avg. N24W) and arch (avg. N23W) axis trends are oriented perpendicular to minor fault slip and compression directions due to their generation by thrust-related folding. Laramide deformation shows the primary external influence of ENE-directed shortening paralleling published convergence vectors between the North American and Farallon plates. Slightly radial shortening directions, from more NE-directed to the north to more E-directed to the south, suggest focused contraction originating near current-day southern California. A slight clockwise rotation of shortening directions going from west to east is consistent with proposed changes in Farallon–North American plate trajectories as the orogen expanded eastward. Additional complexities caused by localized preexisting weaknesses and impingement by the adjoining Cordilleran thrust belt have provided structural diversity within the Laramide province. Obliquities between convergence directions and the northern and southeastern boundaries of the Laramide province have resulted in transpressive arrays of en echelon folds and arches, not major through-going strike-slip faults.

Abstract Studies of Quaternary dune fields during pluvial intervals have shown that, given appropriate shifts in climate, eolian sand seas can become verdant landscapes with widespread surface water. The Early Jurassic Navajo Sandstone of the western interior United States, although it appears superficially to have been an immense, arid, active dune sea during the entirety of its depositional history, contains subtle evidence of pluvial episodes. In situ fossil tree stumps, abundant dinosaur tracks, bioturbated strata up to 20 m thick, and large stromatolite-bearing, mass flow-capped interdune deposits bear witness to long-lived pluvial episodes. The Navajo accumulated within the tropics; dunes were swept by cross-equatorial winds in winter and watered by monsoonal rains in summer. Most of the evidence for pluvial conditions occurs around the southern and eastern margins of the Navajo dune sea, in the lower half of the formation.

Abstract The history of fault initiation and reactivation in the southern Rocky Mountains remains highly debated, as does the region’s exhumation history. Nowhere has the evidence been more contested than in the southern Sangre de Cristo Mountains, where major, 30+ km dextral separations of basement rocks and their aeromagnetic anomalies have been attributed to Proterozoic, Ancestral Rocky Mountain and Laramide orogenies. Since the sum of these dextral separations is in the range of 100 km, unambiguous determination of the age(s) of faulting would have major implications to Rocky Mountain tectonics. Likewise, the history of exhumation and stabilization of the western North American craton provides an important example of continental lithospheric evolution. This field trip will start by visiting excellent exposures of spectacularly brecciated yet indurated basement rocks and flanking Paleozoic sedimentary rocks along the Picuris-Pecos fault system, which has 37 km of dextral separation of Proterozoic contacts. Hypotheses for the age(s) of slip will be examined in light of stratigraphic and fault relationships, thin section petrography and isotopic analyses. The region’s history of fault reactivation and associated K-metasomatism will be discussed by combining thermochronology, largely based on new 40Ar/39Ar K-feldspar analyses, with recent seismic data across the Laramide front of the Sangre de Cristo Mountains. The regional tectonic implications of new geologic mapping, fault analyses, 40Ar/39Ar thermochronology and seismic studies will be discussed on the outcrop, with a full examination of all hypotheses.

Abstract Paleogene and Neogene faults and fractures on the eastern edge of the Colorado Plateau are present in Mesaverde Group coal and sandstone beds. Recent observations of coal cleat orientation in relation to faults in coal mines have significant impacts for mine planning in the area. Faults, coal cleats, and natural fractures are interpreted to show a structural evolution of the Mesaverde Group through time. This field trip will include a visit to two active underground coal mines, the Bowie Resources’ Bowie No. 2 Mine, and Mountain Coal’s West Elk Mine. Mine geologists will discuss structural styles including fault orientations and timing, cleat development, and rotation. Geologic encounters ranging from fault flooding, subsidence, mine fires, methane gas problems, and land use restrictions will also be discussed. Coal cleat development and open-mode fractures in adjacent sandstones will be observed on outcrops and compared to underground measurements in coal mines in the Somerset Coal Field, Colorado’s most productive. Coal cleat orientations along a reverse fault in one mine will show rotation in relation to possible Neogene age displacement. This two-day trip begins at the Convention Center in downtown Denver, Colorado. Participants will be transported in vans westbound on Interstate 70 to Glenwood Springs, then south on State Highway 82 to Carbondale, then southwest on State Highway 133 to Somerset, with a lunch stop in Redstone to observe 100-year-old coking coal beehive-shaped ovens. The first afternoon will include a stop at Paonia Reservoir Dam for introductory remarks on the regional fracture development of the Somerset Coal Field and a mine tour at the West Elk Mine, which has encountered warm water flooding in large-scale faults associated with the West Elk Mountain uplift. We will head to Paonia for dinner and overnight. The second day will include a stop at the reclaimed Bowie No. 1 portal to observe fracture patterns on the west side of the coal field and an underground tour of the Bowie No. 2 Mine and possibly the new Bowie No. 3 Mine, where the mine geologist will discuss observations of coal cleat orientation changes in relation to faulting. Recent coalbed methane exploration in the southern Piceance Basin will also be addressed. Then we will drive a four-hour route back to Denver with a quick stop at the Muddy Creek Landslide, a 2.5 mi2 (6.5 km2) active earth flow complex.

Abstract The Cretaceous-Tertiary (K-T) boundary section at the West Bijou Site is remarkable because many of the methods used to constrain the position of a terrestrial K-T boundary have been successfully applied to a local section. These include palynology, magnetostratigraphy, shocked quartz and iridium analysis, vertebrate paleontology, geochronology, and paleobotany. The West Bijou Site K-T boundary records the extinction of the Wodehouseia spinata Assemblage Zone palynoflora (21%), followed immediately by the presence of a fern-spore abundance anomaly (74%) and the subsequent appearance of the P1 palynoflora. This palynological extinction is coincident with the presence of shock-metamorphosed quartz grains (5+ planes of parallel lamellae) and an iridium spike of 619 ± 32 parts per trillion within the 3-cm-thick boundary claystone. The boundary lies within a reversely magnetized interval, recognized as subchron C29r, substantiated by a radiometrically dated tuff 4.5 m below the boundary with an age of 65.73 ± 0.13 Ma. Dinosaur remains attributable to the late Maastrichtian Triceratops Zone were discovered 4 m below the boundary clay, and a partial jaw of a diagnostic Pu1 mammal was discovered 12 m above. Fossil plants are most abundant in the Paleocene and document a low diversity ecosystem recognizable as the southernmost extension of the FUI disaster recovery flora that radiated in North America following the K-T boundary cataclysm.

Abstract Recent geoarchaeological research on the High Plains of northwestern Kansas has yielded new information regarding the location of buried landscapes that may harbor the material record of the earliest humans in the region. Soil-stratigraphic and geomorphic investigations in playas and in the valleys of small, intermittent streams (draws) located high in drainage networks indicate these geomorphic setting were zones of slow sedimentation and episodic soil development during the terminal Pleistocene and early Holocene. Deeply buried paleosols in the draws have yielded Clovis and possibly pre-Clovis cultural deposits, and a landscape/soil-stratigraphic model has been developed to systematically search for additional early sites in buried contexts. This field guide presents results of geoarchaeological investigations at five Paleo-Indian sites in the upper Beaver Creek drainage basin. Also, locations with buried Paleo-Indian landscapes but no recorded sites are described.

Abstract Along a transect across the Front Range from Denver to the Blue River valley near Dillon, the trip explores the geologic framework and Laramide (Late Cretaceous to early Eocene) uplift history of this basement-cored mountain range. Specific items for discussion at various stops are (1) the sedimentary and structural record along the upturned eastern margin of the range, which contains several discontinuous, east-directed reverse faults; (2) the western structural margin of the range, which contains a minimum of 9 km of thrust overhang and is significantly different in structural style from the eastern margin; (3) mid- to late-Tertiary modifications to the western margin of the range from extensional faulting along the northern Rio Grande rift trend; (4) the thermal and uplift history of the range as revealed by apatite fission track analysis; (5) the Proterozoic basement of the range, including the significance of northeast-trending shear zones; and (6) the geologic setting of the Colorado mineral belt, formed during Laramide and mid-Tertiary igneous activity.

Abstract The Central Front Range of the Colorado Rockies is dominated by an early Proterozoic (ca. 1.8–1.7 Ga) metamorphosed volcanic and sedimentary sequence. In terms of plate tectonics, these rocks are interpreted as island arc, backarc, and sedimentary basin-fill units formed during the accretion of Colorado onto the North American craton. Despite good exposures, which we will be able to observe throughout the field trip, and their proximity to a large metropolitan area, these rocks are still not well understood. New research is under way to better understand accretionary processes in this region. The boundaries of the Central Front Range arc sequence are currently undefined. On the east and west, the sequence is terminated by Laramide-age faulting. The Pike’s Peak Batholith obscures the southern boundary, and the northern boundary is problematic. The main units present in the Central Front Range arc sequence are amphibolites, felsic gneisses, calc-silicate gneisses, mica schists and gneisses, iron formations, metagraywackes, quartzites, and metaconglomerates. These units as a whole are often called the “Idaho Springs Formation,” which is no longer considered a valid formation name. The degree of metamorphism is generally upper amphibolite grade, high temperature–low pressure. Anatectic conditions were reached in the felsic gneisses and mica schists over much of the area. This field trip examines these units in an area of slightly lower grade, where the character of the rocks is not masked by complications of anatectic melting.

ABSTRACT Sedimentary rocks, ranging in age from Pennsylvanian to Cretaceous, crop out in the vicinity of the Dakota hogback. The rocks overlie Precambrian igneous and metamorphic rocks. The strata were deformed in late Cretaceous and early Tertiary time and dip to the east. Downcutting occurred in the Tertiary and Quaternary, formed the major canyons, and was interrupted by intervals of alluvial deposition east of the range front. Landslides mantle the east front of the hogback.

Abstract The South Cañon Number 1 Coal Mine fire, in South Canyon west of Glenwood Springs, Colorado, is a subsurface fire of unknown origin, burning since 1910. Subsidence features, gas vents, ash, condensates, and red oxidized shales are surface manifestations of the fire. The likely success of conventional fire-containment methodologies in South Canyon is questionable, although drilling data may eventually suggest a useful control procedure. Drill casings in voids in the D coal seam on the western slope trail are useful for collecting gas samples, monitoring the temperature of subsurface burning, and measuring the concentration of gases such as carbon monoxide and carbon dioxide in the field. Coal fire gas and mineral condensates may contribute to the destruction of floral and faunal habitats and be responsible for a variety of human diseases; hence, the study of coal gas and its condensation products may prove useful in understanding environmental pollution created by coal mine fires. The 2002 Coal Seam Fire, which burned over 12,000 acres and destroyed numerous buildings in and around Glenwood Springs, exemplifies the potential danger an underground coal fire poses for igniting a surface fire.

Abstract This field trip in the vicinity of the Florissant fossil beds includes five stops that examine the Precambrian Cripple Creek Granite and Pikes Peak Granite, and the late Eocene Wall Mountain Tuff, Thirtynine Mile Andesite lahars, and Florissant Formation. The Cripple Creek Granite and Pikes Peak Granite formed in balholilhs ca. 1.46 and 1.08 Ga, respectively. Uplifted during the Laramide Orogeny of the Late Cretaceous and early Tertiary, the Precambrian rocks were exposed along a widespread erosion surface of moderate relief by the late Eocene. The late Eocene volcanic history of the Florissant area is dominated by two separate events: (1) a caldera eruption of a pyroclastic flow that resulted in the emplacement of the Wall Mountain Tuff, a welded tuff dated at 36.73 Ma; and (2) stratovolcanic eruptions of tephra and associated lahars from the Guffey volcanic center of the Thirtynine Mile volcanic field. This volcanic activity from the Guffey volcanic center had a major influence on the development of local landforms and on sedimentation in the Florissant Formation, which was deposited in a fluvial and lacustrine setting and is dated as 34.07 Ma. The Florissant Formation contains a diverse flora and insect fauna consisting of more than 1700 described species. Most of these fossils are preserved as impressions and compressions in a diatomaceous tuffaceous paper shale and as huge petrified trees that were entombed in a lahar deposit.

Abstract A revised lithostratigraphy for Lower Paleozoic strata in New Mexico and west Texas was developed through detailed sedimentological study of the Bliss and Hitt Canyon Formations within a refined temporal framework assembled from precise biostratigraphic (trilobite and conodont) and chemostratigraphic (carbon isotope) data. Member boundaries within the Hitt Canyon now correspond with mappable and essentially isochronous horizons that represent major depositional events that affected sedimentation in basins throughout Laurentian North America. This trip is designed to examine these and other important intervals, such as the extinction horizons at the base and top of the Skullrockian Stage, and to demonstrate the utility of associated faunas and isotopic excursions for correlation within and beyond the region.

Abstract This field trip focuses on the impacts of geologic hazards, natural resources development, and other geologic features on human activity along the mountain front west of the Denver metropolitan area. The trip serves both as a trip for those in the Denver area and as a model of how common, technically oriented field trips can be converted into trips to educate the general public about such impacts. The many consequences of living with geology present questions about how geologic characteristics and processes should be recognized and mitigated. The questions’ answers involve complex economic and political issues that must be answered on an individual or regional basis. The primary job of the geologic community is to educate the public about the impacts and their consequences so that informed public policy decisions can be made.

Abstract This field trip will visit a modern, large-scale underground block cave mining operation at the world-class Urad-Henderson porphyry molybdenum deposits on and beneath Red Mountain, in the historic Dailey-Jones Pass mining districts, Clear Creek County, Colorado. The underground tour summarizes the Henderson deposit geology and the current status of mining operations, and offers the opportunity to examine and collect rock specimens. The surface tour summarizes the regional and local geologic and structural setting of the deposits, and surface features that define and characterize the outer, peripheral parts of the intrusive-hydrothermal system. The mine is located in the northern Colorado Mineral Belt, in the Front Range of the Rocky Mountains, ∼75 km west of Denver. The deposits consist of molybdenite-bearing, quartz vein stockworks at the cupola apices of highly evolved, silica-rich, subalkaline, leucorhyolite/leucogranite porphyry stocks. The system formed over ∼3.0 m.y. between ca. 27 and 30 Ma by at least 23 intrusive events. Emplacement of the Red Mountain intrusive center and a second intrusive center at Woods Mountain may have been controlled by the NNE-trending Berthoud Pass–Vasquez Pass structural zone, a major Laramide-reactivated Precambrian shear/fault zone. A peripheral, 7.5 × 12.0 km, NNE-elongated, elliptical, pervasive chlorite alteration zone contains a well-developed system of radial quartz and base-precious metal veins.

Abstract The Front Range of Colorado has been subjected to at least three major mountain-building episodes from Paleozoic time on; this field trip will examine some sedimentary deposits related to the first two of these. The first orogeny uplifted the late Paleozoic Ancestral Rocky Mountains and started a long period of terrestrial sedimentation; the second uplifted the late Mesozoic–early Tertiary Laramide Rocky Mountains. Faults in both cases strike approximately north to northwest; the Laramide Orogeny reactivated Paleozoic faults that may in turn have been reactivated Precambrian faults. The earliest sedimentary rocks (Fountain Formation) deposited on the Precambrian basement in this area were deposited during the Pennsylvanian and Permian Periods under arid climatic conditions as alluvial fans along the eastern side of the Front Range arches, followed by braided rivers and sand seas (Lyons Sandstone). By the Late Jurassic, the climate was more humid and the area was a low-lying meandering river floodplain (Morrison Formation). During the mid-Cretaceous, with the advance of the Western Interior Seaway, the Denver area was beachfront property (Dakota Group) before fully marine conditions returned in the Late Cretaceous. By the end of the Cretaceous, with the initial phase of uplift of the Laramide Rocky Mountains, there was a return to terrestrial conditions (Laramie Formation), which have predominated ever since. Both body and trace fossils of invertebrates and vertebrates have been preserved in several of these formations. In many cases, trace fossils are the only evidence of the animals that inhabited these areas.

Abstract The theme of the 2004 GSA Annual Meeting and Exposition, “Geoscience in a Changing World,” covers both new and traditional areas of the earth sciences. The Front Range of the Rocky Mountains and the High Plains preserve an outstanding record of geological processes from Precambrian through Quaternary times, and thus serve as excellent educational exhibits for the meeting. With energy and mineral resources, geological hazards, water issues, geoarchaeological sites, and famous dinosaur fossil sites, the Front Range and adjacent High Plains region provide ample opportunities for field trips focusing on our changing world. The chapters in this field guide all contain technical content as well as a field trip log describing field trip routes and stops. Of the 25 field trips offered at the Meeting, 14 are described in this guidebook, covering a wide variety of geoscience disciplines, with chapters on tectonics (Precambrian and Laramide), stratigraphy and paleoenvironments (e.g., early Paleozoic environments, Jurassic eolian environments, the K-T boundary, the famous Oligocene Florissant fossil beds), economic deposits (coal and molybdenum), geological hazards, and geoarchaeology.

Abstract The field trip will traverse the highly asymmetrical Front Range to examine the west flank's major Laramide thrusts, subsequent volcanic rocks and normal faults, as well as the east flank's higher-angle thrust and reverse faults and their associated fault-propagation folds. Laramide to Holocene tectonics will be debated on the outcrop and during our evening soak at Hot Sulphur Springs.

Abstract The northeastern Front Range provides an excellent test of Laramide tectonic hypotheses. This trip will feature moderate hikes to excellent exposures of basement-involved structures. We will integrate new fault and balancing data into a model for backlimb deformation in basement-cored foreland arches.