ABSTRACT Detrital zircon U-Pb and (U-Th)/He ages from latest Cretaceous–Eocene strata of the Denver Basin provide novel insights into evolving sediment sourcing, recycling, and dispersal patterns during deposition in an intracontinental foreland basin. In total, 2464 U-Pb and 78 (U-Th)/He analyses of detrital zircons from 21 sandstone samples are presented from outcrop and drill core in the proximal and distal portions of the Denver Basin. Upper Cretaceous samples that predate uplift of the southern Front Range during the Laramide orogeny (Pierre Shale, Fox Hills Sandstone, and Laramie Formation) contain prominent Late Cretaceous (84–77 Ma), Jurassic (169–163 Ma), and Proterozoic (1.69–1.68 Ga) U-Pb ages, along with less abundant Paleozoic through Archean zircon grain ages. These grain ages are consistent with sources in the western U.S. Cordillera, including the Mesozoic Cordilleran magmatic arc and Yavapai-Mazatzal basement, with lesser contributions of Grenville and Appalachian zircon recycled from older sedimentary sequences. Mesozoic zircon (U-Th)/He ages confirm Cordilleran sources and/or recycling from the Sevier orogenic hinterland. Five of the 11 samples from syn-Laramide basin fill (latest Cretaceous–Paleocene D1 Sequence) and all five samples from the overlying Eocene D2 Sequence are dominated by 1.1–1.05 Ga zircon ages that are interpreted to reflect local derivation from the ca. 1.1 Ga Pikes Peak batholith. Corresponding late Mesoproterozoic to early Neoproterozoic zircon (U-Th)/He ages are consistent with local sourcing from the southern Front Range that underwent limited Mesozoic–Cenozoic unroofing. The other six samples from the D1 Sequence yielded detrital zircon U-Pb ages similar to pre-Laramide units, with major U-Pb age peaks at ca. 1.7 and 1.4 Ga but lacking the 1.1 Ga age peak found in the other syn-Laramide samples. One of these samples yielded abundant Mesozoic and Paleozoic (U-Th)/He ages, including prominent Early and Late Cretaceous peaks. We propose that fill of the Denver Basin represents the interplay between locally derived sediment delivered by transverse drainages that emanated from the southern Front Range and a previously unrecognized, possibly extraregional, axial-fluvial system. Transverse alluvial-fluvial fans, preserved in proximal basin fill, record progressive unroofing of southern Front Range basement during D1 and D2 Sequence deposition. Deposits of the upper and lower D1 Sequence across the basin were derived from these fans that emanated from the southern Front Range. However, the finer-grained, middle portion of the D1 Sequence that spans the Cretaceous-Paleogene boundary was deposited by both transverse (proximal basin fill) and axial (distal basin fill) fluvial systems that exhibit contrasting provenance signatures. Although both tectonic and climatic controls likely influenced the stratigraphic development of the Denver Basin, the migration of locally derived fans toward and then away from the thrust front suggests that uplift of the southern Front Range may have peaked at approximately the Cretaceous-Paleogene boundary.

ABSTRACT Analysis of detrital zircon U-Pb ages from the Phanerozoic sedimentary record of central Colorado reveals variability in sediment transport pathways across the middle of the North American continent during the last 500 m.y. that reflects the tectonic and paleogeographic evolution of the region. In total, we present 2222 detrital zircon U-Pb ages from 18 samples collected from a vertical transect in the vicinity of Colorado’s southern Front Range. Of these, 1792 analyses from 13 samples are published herein for the first time. Detrital zircon U-Pb age distributions display a considerable degree of variability that we interpret to reflect derivation from (1) local sediment sources along the southern Front Range or other areas within the Yavapai-Mazatzal Provinces, or (2) distant sediment sources (hundreds to thousands of kilometers), including northern, eastern, or southwestern Laurentia. Local sediment sources dominated during the Cambrian marine transgression onto the North American craton and during local mountain building associated with the formation of the Ancestral and modern Rocky Mountains. Distant sediment sources characterize the remaining ~75% of geologic time and reflect transcontinental sediment transport from the Appalachian or western Cordilleran orogenies. Sediment transport mechanisms to central Colorado are variable and include alluvial, fluvial, marine, and eolian processes, the latter including windblown volcanic ash from the distant mid-Cretaceous Cordilleran arc. Our results highlight the importance of active mountain building and developing topography in controlling sediment dispersal patterns. For example, locally derived sediment is predominantly associated with generation of topography during uplift of the Ancestral and modern Rocky Mountains, whereas sediment derived from distant sources reflects the migrating locus of orogenesis from the Appalachian orogen in the east to western Cordilleran orogenic belts in the west. Alternating episodes of local and distant sediment sources are suggestive of local-to-distant provenance cyclicity, with cycle boundaries occurring at fundamental transitions in sediment transport patterns. Thus, identifying provenance cycles in sedimentary successions can provide insight into variability in drainage networks, which in turn reflects tectonic or other exogenic forcing mechanisms in sediment routing systems.

Abstract The Powder River Basin of northeast Wyoming and southeast Montana contains coal resources larger than those of any other basin of comparable size in the United States. These coal resources are developed in 16 strip mines for coal-fired power plants and for coalbed methane in more than 20,000 wells for distribution throughout the country. During this field trip, strip mining methodology and technology at the Wyodak coal mine, and operations at the nearby Wyodak power plant and in coalbed methane fields will be observed and discussed. The power plant utilizes feed coal from the mine and supplies electricity for the City of Gillette and surrounding areas. Coalbed methane development in the basin has coexisted with coal mining since the early 1980s. The same coal beds that are being mined yield gas in the subsurface, which is produced, collected, and moved to pipelines to access the interstate pipeline grid to serve the Rocky Mountains, Midwest, and California customers. Keywords: Powder River Basin, Wyoming, coal, coalbed methane, gas.

Abstract The 300-km-long Books Cliffs are a world-class field laboratory for studying clastic sedimentology, sedimentary architecture, and sequence stratigraphy, and serve as an outcrop analog for fluvio-deltaic and shoreface-to-shelf hydrocarbon reservoirs worldwide. These famous rocks have been used to develop, test and refine sedimentological and stratigraphic ideas and models over the years, including the principles and concepts of sequence stratigraphy. This field guide focuses on the following themes: (a) sedimentology, sedimentary architecture, and sequence stratigraphy of fluvial, coastal plain, river- and wave-dominated deltas, and shoreface-to-shelf deposits, (b) stacking patterns in high versus low accommodation settings, (c) distribution of reservoir and non-reservoir facies in a predictive sequence stratigraphic framework, (d) relationship between relative sea level, shoreline position, and stratigraphic architecture, and (e) alternative sequence stratigraphic models for the interpretation of channel-shoreface packages. Keywords: accommodation space, Book Cliffs, Cretaceous, deltaic, fluvial, sedimentaryarchitecture, sedimentology, sequence stratigraphy, shelf, shoreface, Western InteriorSeaway

Abstract The Grizzly Creek shear zone is a newly discovered Proterozoic structure coincident with the southern margin of the Laramide White River uplift in west-central Colorado. The shear zone includes a 10-15 m basal mylonite oriented 255/44°NW that truncates foliation and magmatic fabrics in the footwall. The mylonite is overlain by >400 m of moderately north-dipping tectonites dominated by protomylonitic to mylonitic gneiss intercalated with mainly concordant pseudotachylyte veins and cmscale ultramylonite bands. At the upper boundary of the shear zone, the high-strain tectonic fabrics grade into relatively undeformed rocks of the hanging wall. The shear zone separates supracrustal gneisses and coarse-to-megacrystic granitoids in the footwall (south block) from fine-grained, foliated granite and supracrustal gneisses in the hanging wall (north block). Mutually overprinting brittle and plastic fault rocks, including cataclasites, pseudotachylyte fault veins, mylonitized pseudotachylyte, and ultramylonite bands, record cyclic deformation by both seismogenic rupture and plastic creep within the shear zone. Mineral lineations on mylonitic foliation surfaces plunge N-NE, and a variety of microscopic and mesoscopic shear-sense indicators indicate top-to-south reverse displacement. The Grizzly Creek shear zone is truncated by the Cambrian-Precambrian nonconformity, which is underlain by a 1-2-m-thick paleoregolith containing altered pseudotachylyte. We interpret the shear zone to record cyclic seismogenic faulting and aseismic plastic flow at the brittle-plastic transition during Proterozoic ~N-S shortening. The age, kinematics, and tectonic style of the Grizzly Creek shear zone are consistent with the 1.4 Ga Colorado mineral belt shear zone system; a NE-trending intracontinental deformation zone that formed under a regional strain field involving N-S shortening, E-W extension, and complex transpression on subvertical shear zones 55 km to the southeast. The Grizzly Creek shear zone was brittley reactivated as a Laramide, south-vergent reverse fault with a displacement of > 200 m. We suggest that the Proterozoic Grizzly Creek shear zone represents a persistent zone of weakness in the lithosphere that significantly impacted the Cenozoic structural evolution and present geomorphology of the region. Keywords: pseudotachylyte, ultramylonite, brittle-plastic deformation, tectonic inheritance, White River uplift, Glenwood Canyon.

Abstract The Cripple Creek district is renowned for epithermal gold telluride veins which have produced over 22 million ounces of gold from an intensely altered diatreme complex (total production + economic resources of >1000 tons). The district is also renowned for its association with a rare class of alkaline igneous rocks. The volcanism at Cripple Creek was part of a regionally extensive episode of Oligocene magmatism, including large volumes of calc-alkaline rocks and smaller, but widely distributed alkaline centers. Amongst the mid-Tertiary alkaline intrusive complexes, only Cripple Creek is associated with a giant (>500 ton) gold deposit. Further study of the magmatic and hydrothermal evolution of these systems will be necessary to explain this apparent disparity in gold enrichment. Cripple Creek’s gold mineralization principally occurs as telluride minerals hosted by swarms of narrow veins. Most geological studies over the last century have focused on the high-grade veins and to a lesser degree, adjacent hydrothermal alteration, but metasomatism is now shown to be broadly developed and demonstrably accompanied many events throughout the evolution of the igneous complex. Alteration types ranged from minor early pyroxenestable varieties through various biotite-bearing assemblages into voluminous K-feldspar stable types. Hydrolytic (acid) styles of alteration are present but minor. Economic gold mineralization is intimately associated only with late, voluminous K-feldspar-pyrite alteration which affected >5 km 3 of the explored portion (upper 1 km) of the complex. Although similar to other gold deposits related to alkaline magmatism, Cripple Creek differs markedly from other epithermal systems in terms of its large volume of K-feldspar added and paucity of quartz and acid alteration. Keywords : alkaline, epithermal, Cripple-Creek-Colorado, phonolite, metasomatism, hydrothermal-alteration, tellurides, diatreme

Abstract Middle Park, a high-altitude basin in the Southern Rocky Mountains of north central Colorado, contains abundant evidence of Paleoindian occupation. At Barger Gulch Locality B, an extensive Folsom assemblage (ca. 10,900-10,200 14C yr B.P.) occurs within a buried soil informally referred to as the Barger soil, a composite of a truncated latest Pleistocene soil and a younger soil forming during the early to middle Holocene. Erosion documented in valley fill adjacent to the excavation area between ca. 10,200 and 9700 14C yr B.P. is likely related to truncation of the latest Pleistocene soil. Relatively rapid valley filling until sometime after ca. 9350 14C yr B.P. was followed by stability and Barger soil formation until ca. 6000 14C yr B.P. Soil dates in the excavation area indicate the Barger soil was forming as late as ca. 5200 14C yr B.P. The highest of three Holocene terraces along the main axis of Barger Gulch records erosion occurring ca. 10,000-9700 14C yr B.P. followed by rapid aggradation ca. 9700-9500 14C yr B.P. Charcoal layers and lenses are common in earliest Holocene valley fills, suggesting relatively frequent and widespread burning in the Barger Gulch drainage. Aggradation rates slowed and cumulic soil formation occurred between ca. 9500 and 7500 14C yr B.P., at least partially overlapping with Barger soil formation at Locality B. Chipped stone in association with charcoal dated to ca. 8500 14C yr B.P. occurs in the cumulic soil. Morphological characteristics of upland soils in Barger Gulch and elsewhere in Middle Park suggest expansion of forest and grass cover during the early and middle Holocene in areas currently characterized by sagebrush steppe. Geoarchaeological research in Barger Gulch has thus yielded information important to understanding late Quaternary landscape evolution, characteristics of Paleoindian landscapes, and Holocene environmental change. Keywords: geoarchaeology, Southern Rocky Mountains, Paleoindian, Folsom, soil stratigraphy

Abstract The origin of the subsurface fire burning since 1910 in the South Cañon Number 1 Coal Mine west of Glenwood Springs, Colorado, is unknown. Surface manifestations of the fire include gas vents (some encrusted with minerals), burnt vegetation, subsidence features, ash, sulfur, and red-oxidized shale. The minerals tschermigite, mascagnite, and gypsum formed in association with coal-fire gas exhaled from a gas vent on the western slope of South Canyon. Tschermigite and gypsum are reaction products of the gas with feldspar grains in the Williams Fork Formation. Gas collected from a vent and from soil above a burn zone in former underground workings on the eastern slope of South Canyon was found to contain numerous hydrocarbons, including n-alkanes, iso-alkanes, cyclo-alkanes, alkyl aromatics, alkenes, ketones, ethers, and a number of other volatile organic com-pounds, as well as sulfur compounds. Drill casings currently present in voids in the D coal seam on the western slope trail are useful for collecting gas samples, monitoring the temperature of subsur-face burning, and measuring the concentration of gases, such as carbon monoxide and carbon dioxide, in the field. The likely success of conventional fire-containment methodologies in South Canyon is questionable, although additional studies including drilling data may eventually suggest a useful control procedure. The 2002 “Coal Seam Fire” that burned over 12,000 acres and destroyed numer-ous buildings in and around Glenwood Springs exemplifies the potential danger an underground coal fire poses for igniting a surface fire. Coal-fire gas and the solid by-products of combustion contribute to the destruction of floral and faunal habitats and may be responsible for a variety of human diseases; hence, the study of coal gas and its by-products may prove useful in understanding environmental pollution created by coal-mine fires. Keywords: South Cañon Number 1 Coal Mine, coal-mine subsidence, coal fires, coal-fire gas, gas-vent minerals, coal-fire pollution.

Abstract Ernie Anderson, Lauren Wright, and Bennie Troxel were among the first to rec-ognize the coeval nature of magmatism and crustal extension in the Colorado River and Death Valley extensional terrains, respectively. The earliest magmatic rocks in the Colorado River extensional terrain include the 15.7 Ma Aztec Wash and 15.8-16.9 Ma Searchlight plutons as well as the 15.3-17.4 Ma Spirit Mountain batholith. The Aztec Wash and Searchlight plutons are well exposed, stratified plutons that show a similar range in composition from 45 to 75 wt% SiO 2 . Homogeneous granites that compose about one-third of each intrusion are nearly identical in texture, major- and trace-element geochemistry, and isotopic composition; mafic rocks that are present in both plutons document basaltic input into felsic magma chambers. Isotopic data suggest that mafic magmas were derived from enriched lithospheric mantle with minor crust-al contamination; the more felsic rocks crystallized from hybrid melts that represent mixtures of juvenile basaltic magma + crustal melt, or products of anatexis of ancient crust + young mafic magmas that underplate the crust during extension. Although emplacement generally proceeded peak east-west extension, the Aztec Wash pluton is in part coeval with extension in the Colorado River extensional terrain, as indicated by the uniform north-south orientation of late dikes and mafic injections into the magma chamber. The Spirit Mountain batholith is more homogeneous and felsic than the Aztec Wash and Searchlight plutons; granite composition varies from 66.6 to 76.4 wt% SiO 2 across the batholith. Geochemical and isotopic data suggest that granites within the Spirit Mountain batholith may be derived by the melting of K-feldspar-dominated crust that includes both ancient and juvenile rocks, or by other processes involving fresh mantle input and Proterozoic rocks deep in the Miocene crust. The oldest and youngest plutonic rocks coeval with crustal extension in the Death Valley extensional terrain are the 12.4 Ma granite of Kingston Peak and the 9.8 Ma Shoshone pluton, respectively. Both plutons are texturally zoned and are charac-terized by rapakivi textures. Chemical and isotopic data suggest that the granite of Kingston Peak represents a partial melt of Mesozoic plutonic rocks at mid-crustal levels; the juvenile isotopic composition of this granite as well as the Shoshone plu-ton probably reflects post-subduction hybridization of mantle-derived mafic magmas with the crustal melt. Keywords: magmatism, crustal extension, Colorado River, Death Valley.

Abstract This field trip highlights various stages in the evolution of the Snake River Plain- Yellowstone Plateau bimodal volcanic province, and associated faulting and uplift, also known as the track of the Yellowstone hotspot. The 16 Ma Yellowstone hotspot track is one of the few places on Earth where time-transgressive processes on continental crust can be observed in the volcanic and tectonic (faulting and uplift) record at the rate and direction predicted by plate motion. Recent interest in young and possible renewed volcanism at Yellowstone along with new discoveries and synthesis of previous studies, i.e., tomographic, deformation, bathymetric, and seismic surveys, provide a framework of evidence of plate motion over a mantle plume. This 3-day trip is organized to present an overview into volcanism and tectonism in this dynamically active region. Field trip stops will include the young basaltic Craters of the Moon, exposures of 12–4 Ma rhyolites and edges of their associated collapsed calderas on the Snake River Plain, and exposures of faults which show an age progression similar to the volcanic fields. An essential stop is Yellowstone National Park, where the last major caldera-forming event occurred 640,000 years ago and now is host to the world’s largest concentration of hydrothermal features (>10,000 hot springs and geysers). This trip presents a quick, intensive overview into volcanism and tectonism in this dynamically active region. Field stops are directly linked to conceptual models related to hotspot passage through this volcano-tectonic province. Features that may reflect a tilted thermal mantle plume suggested in recent tomographic studies will be examined. The drive home will pass through Grand Teton National Park, where the Teton Range is currently rising in response to the passage of the North American plate over the Yellowstone hotspot. Keywords Yellowstone, hotspot, track, volcanism, faulting, uplift.

Abstract The Black Canyon of the Gunnison and Unaweep Canyon in western Colorado have long been viewed as classic examples of post-Laramide Plio-Pleistocene uplift, which in the case of Unaweep, is thought to have forced the Gunnison River to abandon the canyon. Ongoing field studies of the incision histories of these canyons and their surrounding regions, however, suggest that post-Laramide rock uplift has been regional, rather than local in nature. River incision rates calculated using ca. 10 Ma basaltic lava flows as a late Miocene datum suggest that long-term incision rates range from 61 to 142 m/m.y. with rates decreasing eastward towards the central Rocky Mountains. Incision rates calculated using the ca. 640 ka Lava Creek B ash range from 95 to 162 m/m.y., decrease eastward towards the mountains, and are broadly similar in magnitude to the longer-term incision rates. Locally, incision rates are as high as 500–600 m/m.y. along the lower reaches of the Black Canyon of the Gunnison, and these anomalously high values reflect transient knickpoint migration upvalley. Knickpoint migration was controlled, in part, by downvalley base-level changes related to stream piracy. For example, abandonment of Unaweep Canyon by the Gunnison River could have led to rapid incision through erodible Mancos Shale as the Gunnison River joined the Colorado River on its course around the northern end of the Uncompahgre Plateau. Geophysical data show that abandonment of Unaweep Canyon was not caused by differential uplift of the crest of Unaweep Canyon relative to the surrounding basins. Instead, the ancestral (Plio-Pleistocene?) Gunnison River flowed through Cactus Park, a major paleovalley that feeds into Unaweep Canyon, and continued downvalley to its juncture with the Dolores River near present-day Gateway, Colorado. The average gradient of the ancestral Gunnison River through the canyon prior to abandonment was ~7.5–7.6 m/km. Lithological and mineralogical considerations suggest that the Colorado River also flowed through and helped to carve Unaweep Canyon, although the Colorado River probably exited Unaweep Canyon prior to abandonment by the Gunnison River. The ancestral Gunnison River remained in its course and incised through bedrock for a long enough period of time to produce terrace remnants in the Cactus Park region that range in elevation from 2000 to 1880 m. Abandonment of the canyon by the Gunnison River was followed by formation of a natural dam that probably led to deposition upvalley of ~50 m of lacustrine sediments in Cactus Park. Recent mapping in the lower reaches of Unaweep Canyon indicate that a landslide could have led to damming of Unaweep Canyon, perhaps while it was occupied by underfit streams.

Abstract Mesas and buttes of the central Colorado Piedmont are composed of at least twodistinct rock types, which differ in their cohesiveness and resistance to erosion. Thelower parts of the exposed stratigraphic section are poorly cemented, Upper Cretaceousto Middle Eocene sandstones of the Dawson Formation. The caprocks arecomposed of one or more resistant formations of Late Eocene age: the Castle RockConglomerate, Wall Mountain Tuff, and the conglomerate of Larkspur Butte. Theseformations were originally deposited in topographic lows, but due to their resistance,they now cap prominent buttes and mesas of the Colorado Piedmont. Erosion of thecaprock through progressive retreat of the butte scarp produces colluvium that has ahigher resistance to erosion than the poorly cemented underlying sandstone. Once the caprock of a butte has been removed by erosion, the underlying weaklycemented Dawson Formation is readily eroded. Ultimately, the armored lower slopesof the former butte remain as a circular ridge standing as much as 100 m above thesurrounding topography. This process produces a topographic low surrounded byrelict faceted slopes where the flat top of the butte once stood. Prominent alluvial fans are associated with some of these annular features, andthey record the main phases of butte removal and excavation of the central part of thearmored slopes. Multiple generations of alluvial fans contain coarse- and fine-grainedfacies that represent changes in effective stream power and record alternating phasesof aggradation and erosion. The degree of soil development in the fan alluvium andheight of the fan surfaces above streams indicates the oldest preserved gravel fandeposit is of late-middle Pleistocene age. The youngest luminescence (optically stimulatedluminescence) dated alluvial fans were deposited during the late Pleistoceneabout the time of the Pinedale glacial maximum in Colorado, ca. 21,000 yr B.P. Keywords: Colorado Piedmont, talus flatiron, talus flatiron ring, inverted topography.

Abstract The Boulder Creek Watershed, within the Front Range region of Colorado, istypical of many western watersheds because it is composed of a high-gradient upperreach mostly fed by snowmelt, a substantial change in gradient at the range front, andan urban corridor within the lower gradient section. A stream ecosystem within anurban landscape not only can provide water for municipal, industrial, and agriculturalneeds, but also can be utilized for recreation, esthetic enjoyment, and wastewaterdisposal. The purpose of this 26 km bicycle field trip is to explore the hydrology andgeochemistry of Boulder and South Boulder Creeks and to discuss topics includingflood frequency and hazards, aqueous geochemistry of the watershed, and potentialimpacts of invasive species and emerging contaminants on stream ecology. Keywords: Colorado Front Range, flood hazard, water quality, invasive species

Abstract Badlands are common arid and semiarid landscapes long recognized in slope development and erosion rate studies by preeminent geomorphologists including Gilbert, Davis, and Schumm. The trip described here will examine in detail Quaternarystrata and landscape evolution in arguably the most famous badlands, the White River Badlands of South Dakota, which were pivotal during development of vertebrate paleontology in North America. Geologists have collected fossils from the White River Group there nearly every field season since the mid-1800s; however, until recently, little work was reported on the extensively exposed Quaternary strata. The White River Badlands are also a proposed dust source for the widespread PeoriaLoess of the Central Great Plains. The research highlighted on this trip includes (1) luminescence and radiocarbon ages from late Pleistocene through Holocene eolian sand, (2) radiocarbon ages from Holocene eolian cliff-top deposits, (3) luminescenceages from late Pleistocene fluvial silts, (4) radiocarbon ages of late Holocene fluvial silts, and (5) cosmogenic ages on ventifacts from the adjoining upper prairie. These new studies will facilitate discussions, including (1) late Quaternary paleoenvironments,(2) late Quaternary fluvial incision rates and episodes, (3) up-wind sediment supply of late Quaternary nonglaciogenic loess, (4) landscape evolution spanning late Pleistocene tableland through late Holocene sod table development, and (5) modern erosion-pedimentation rates. Keywords: Badlands, fluvial, eolian, geochronology, geomorphology.

Abstract The Paleocene Tongue River Member of the Fort Union Formation in the northern Bighorn Basin of south-central Montana and Wyoming, and the Cretaceous Eagle Formation at Elk Basin, Wyoming, contain abundant clastic dikes, sills, and other soft-sediment deformation structures. Clastic sills and tabular clastic dikes, which show evidence of forceful, upward injection of liquefied sediments, have been cited as evidence for past seismic shaking. Sills and dikes with such characteristics are common in the field trip area and we interpret them to be paleoseismites that formed during the late Cretaceous-Eocene Laramide orogeny. Other structures indicating liquefaction and flow of unconsolidated sediments are present in the same geographic areas and stratigraphic horizons as the dikes and sills. We speculate that these features may also be a result of seismic shaking. Paleoseismites in this region can be classified into two groups, clastic dikes and sills, and convolute bedding, which includes ball-and-pillow structures, slumps, and diapirs. Clastic dikes and sills are present in the proximal, conglomerate-bearing, alluvial-fan facies of the Tongue River Member and in interbedded Cretaceous sandstone and shale at Elk Basin. Clastic dikes are rare or absent in distal fluvial and lacustrine deposits, where the seismites are most commonly convolute bedding. Field mapping and stratigraphic measurements show that source-bed thickness and depositional environment are the major controls on the type of seismite that formed. Keywords : paleoseismite, seismite, Laramide, Bighorn Basin, Elk Basin.

Abstract This field trip addresses multidisciplinary field research conducted since 2001 in Ash Hollow basin at both archaeological sites and cut bank exposures. Six lithostratigraphic units (I through VI, from oldest to youngest) are recognized in alluvium near the base of the valley fill below the 18-m-high Terrace 2 in the basin. Unit I is presumably Late Wisconsin, while units II through VI are early Holocene alluvial units comprised of silt loam and distinguished on the basis of color, sedimentary structures, and texture. Unit V contains the cultural material (multiple components at both archaeological sites), and was deposited at a rate of between about 0.1 and 1.5 cm/year, based on calibrated radiocarbon ages of charcoal in cut bank exposures. With such rapid rates of sedimentation, artifacts at both the Clary Ranch site (25GD106) and O.V. Clary site (25GD50) were well preserved. Landscape evolution is reconstructed in the context of the Late-Paleoindian components at the two archaeological sites. Keywords: geoarchaeology, Early Holocene, Nebraska, Paleoindian.

Abstract The Morrison Formation is a laterally extensive terrestrial deposit representingan ecologically diverse assemblage of paleoenvironments from the Late Jurassic ofwestern North America. Although the Morrison Formation has recently been interpretedas a semiarid lowland savannah on the basis of geological and paleobiologicalindicators, many microenvironments within this system are more consistent with newinterpretations of the Morrison as a ground-water dominated “wetland” deposit. Herewe report new fossils from a little-studied exposure of the Morrison Formation in andaround Temple Canyon Park near Cañon City, Colorado. The Temple Canyon sectionshows a relatively thin sequence of conglomerate, sandstone, siltstone, mudstone, andlimestone beds representing alluvial to fluvial and possibly lacustrine deposition. Thesection rests on Precambrian basement and is unconformably overlain by the LowerCretaceous Lytle Formation. The mudstone and limestone beds preserve an abundantfossil flora and fauna distinct from those previously described from the MorrisonFormation. The floral assemblage includes species of algae, bryophytes, ferns, ginkgophytes,horsetails, cycads, bennettites, and conifers; together these plants indicatea warm climate with abundant local water supply. The faunal assemblage containsostracodes, conchostracans, traces of aquatic insect larvae, a terrestrial insect bodyfossil, prosobranch and pulmonate gastropods, many species of fish, a possible frog,and rare turtle remains. The presence of prosobranch gastropods, fish, and aquaticinsect larvae suggests a perennial water body with high oxygen content, while thepresence of conchostracans and pulmonate gastropods may indicate some fluctuationin water quality. Keywords: Morrison Formation, Jurassic, Cañon City, Colorado, paleoecology.

Abstract Prepared following the 2007 GSA Annual Meeting in Denver, Colorado, these 15 guides illustrate the latest geological and archeological thinking on a variety of current research themes. Regional-scale topics include landscape responses to dynamic processes of volcanism and uplift in Yellowstone and western Colorado, geomorphic evolution along the Front Range of Colorado and on the High Plains of South Dakota, and geoarchaeological research in central Colorado and western Nebraska. A series of papers illustrates tectonic and stratigraphic processes through time and space, with discussions of Precambrian structures in western Colorado, Jurassic deposition in south-central Colorado, and near-shore stratigraphic patterns in the Cretaceous strata of the Book Cliffs. One paper reviews potential seismic signatures in Cretaceous and Early Tertiary strata in northern Wyoming and Montana, and another discusses patterns of extension in southern Nevada and adjacent portions of California. Other topics in this well-rounded volume include the history of volcanism and gold mineralization at Cripple Creek, development of coalbed methane resources in the Powder River Basin, and a long-lived subsurface coal fire in western Colorado. Follow in the footsteps of these field trips, and see for yourself the patterns and evidence discussed.