Volcanic clasts incorporated in the lower portion of the Tertiary Santa Fe Group sedimentary rocks of the Culebra graben, San Luis Basin, Colorado, provide constraints on the timing of regional tectonic events by provenance determination. Based on currently exposed volcanic terrains, possible clast sources include Spanish Peaks and Mount Mestas to the east, the San Juan volcanic field to the west, and the Thirtynine Mile volcanic field, a remnant of the Central Colorado volcanic field, to the north and east of the San Luis Basin. Provenance was determined by a variety of geochemical, mineral chemical, and geochronologic data. Large porphyritic Santa Fe Group volcanic clasts are potassic with a wide compositional range from potassic trachybasalt to rhyolite. The whole-rock chemistry of the Culebra graben clasts is similar to that of the Thirtynine Mile and San Juan volcanic fields. Culebra graben amphibole and biotite chemistry is generally consistent with that of rocks of the San Juan volcanic field, but not with Spanish Peaks samples. Trace-element data of Culebra graben volcanic clasts overlap with those of the San Juan and Thirtynine Mile volcanic fields, but differ from those of the Mount Mestas. Thermobarometric calculations using mineral chemistry suggest that many Culebra graben rocks underwent a three-stage crystallization history: ~1120 °C at 7–10 kbar, ~1100 °C at 2.3–4.6 kbar, and hornblende formation ~800 °C at 3 kbar. Within the Culebra graben clasts, zircon rim U-Pb geochronologic systematics as well as amphibole and biotite 40 Ar/ 39 Ar plateau data yield ages ranging from 36 to 29 Ma. These ages are consistent with ages of the Thirtynine Mile volcanic field (36–27 Ma) and the Conejos Formation of the San Juan volcanic field (35–29 Ma), but predate Spanish Peaks (ca. 27–21 Ma) and Mount Mestas (ca. 25 Ma). Based on these data, Spanish Peaks and Mount Mestas are excluded as potential source areas for the Santa Fe Group volcanic clasts in the Culebra graben. The San Juan volcanic field is also an unlikely source due to the distance from the depositional site, the inconsistent paleo-current directions, and the pressure-temperature conditions of the rocks. The most likely scenario is that the Central Colorado volcanic field originally extended proximal to the current location of the Culebra graben and local delivery of volcanic clasts was from the north and northeast prior to the uplift of the Culebra Range and Sangre de Cristo Mountains.

Abstract The Southern Rocky Mountain volcanic field contains widespread andesite and dacitic lavas erupted from central volcanoes; associated with these are ~26 regional ignimbrites (each 150–5000 km 3 ) emplaced from 37 to 23 Ma, source calderas as much as 75 km across, and subvolcanic plutons. Exposed plutons vary in composition and size from small roof-zone exposures of porphyritic andesite and dacite to batholith-scale granitoids. Calderas and plutons are enclosed by one of the largest-amplitude gravity lows in North America. The gravity low, interpreted as defining the extent of a largely concealed low-density silicic batholith complex, encloses the overall area of ignimbrite calderas, most of which lack individual geophysical expression. Initial ignimbrite eruptions from calderas aligned along the Sawatch Range at 37–34 Ma progressed southwestward, culminating in peak eruptions in the San Juan Mountains at 30–27 Ma. This field guide focuses on diverse features of previously little-studied ignimbrites and caldera sources in the northeastern San Juan region, which record critical temporal and compositional transitions in this distinctive eastern Cordilleran example of Andean-type continental-margin volcanism.

Abstract Geomorphic, thermochronologic, geochemical, structural, and geophysical data all lend support to the hypothesis that the Colorado Rocky Mountains are an example of dynamic topography that has responded variably to broad epeirogenic uplift since the late Miocene. Our view is that this epeirogenic uplift is primarily related to mantle buoyancy and to a lesser extent, on isostatic adjustments caused by regional denudation. Neogene uplift components were superimposed on earlier (Laramide and mid-Tertiary) uplift events such that the present-day high topography of the Colorado Rocky Mountains reflects a composite uplift history. Newly recognized gravels of the ancestral Colorado River located beneath lava flows of Grand Mesa suggest that the Colorado River flowed west from the Rocky Mountains out onto the Colorado Plateau by 11 Ma. Radiometric dating of late Miocene basalt flows and thermochronologic data for western Colorado indicate that regional denudation and river incision became much more rapid ca. 9.5–6.0 Ma, prior to integration of the Colorado River through Grand Canyon. Post-10 Ma river incision rates and magnitudes are variable, and we interpret these variations to reflect differential uplift of discrete areas of the Rocky Mountain region during the late Cenozoic. For instance, in areas such as Grand Mesa, the Flat Tops, and the Park Range, incision rates and magnitudes are generally ~100–150 m/m.y. and 1.0–1.5 km, respectively. For comparison, these values are 2–3 times larger than incision rates and magnitudes over the past 5–10 Ma in the upper Green River basin. Differential incision across major structural boundaries indicates that post-10 Ma river incision has been driven by Neogene rock uplift. Additional evidence of post-Laramide deformation includes warping of the Oligocene to early Miocene pre-Browns Park Formation erosion surface. This surface has up to 1.8 km of structural relief that has been produced since the Miocene. For basalt-capped regions such as Grand Mesa and the Flat Tops, Neogene rock uplift of 1.0–1.5 km has probably been accompanied by a commensurate amount of surface uplift, resulting in increased local relief ( England and Molnar, 1990 ). Flexural isostatic modeling suggests that isostatic adjustments account for only 10%–40% of the post-10 Ma rock uplift recorded in western Colorado, and that 850–1500 m of probable post-10 Ma rock uplift cannot be accounted for by the isostatic response to denudation. Areas such as Grand Mesa and the Flat Tops are associated with the largest magnitudes of post-10 Ma rock uplift, and generally overlie areas of anomalously low P-wave velocities (the largest of which is the Aspen Anomaly), which suggests that mantle buoyancy could have driven Neogene uplift of the Colorado Rocky Mountains. 3 He/ 4 He ratios strongly suggest that there are direct mantle-to-surface interactions of neotectonics and surface waters. In summary, we conclude that the Rocky Mountains are continuing to evolve in response to neotectonic events that accelerated in the late Miocene, and which are continuing to drive base-level changes and drainage integration of the modern upper Colorado River system. The combined data are best explained by broad epeirogenic Neogene uplift of the Colorado Rocky Mountains driven by mantle flow and buoyancy.

Abstract Recent debris flow studies in Colorado indicate that the state is most susceptible to debris flows that initiate from surface-water runoff that erodes and entrains hillslope and channel sediment. These runoff-initiated debris flows grow in size by entraining sediment along travel paths, thereby increasing their destructive potential. Yet, the mechanics of initiation, erosion, and entrainment processes for runoff-initiated debris flows are poorly understood. The steep, bedrock-dominated flanks of the formerly glaciated Chalk Creek Valley near Nathrop, Colorado, generate an average of two runoff-initiated debris flows per year, making the valley an ideal natural laboratory for debris-flow research. This two-day field trip to the Chalk Creek Valley will examine debris-flow initiation areas, transport zones, deposits, and the impact of large pulses of debris-flow sediment on the morphology of Chalk Creek. On the first day, participants will hike into a particularly active basin at Chalk Cliffs where debris flows are being monitored by the U.S. Geological Survey, the University of Colorado, and East Carolina University. The second day will focus on debris-flow deposits in Chalk Creek and on recent debris flows in and near the community of Alpine in the central part of the valley.

Eocene and Oligocene floras of the western United States show a climatic deterioration from warmer conditions to much cooler and drier conditions. Recent 40 Ar/ 39 Ar dates and magnetic stratigraphy have greatly improved their correlation. In this study, the uppermost Eocene Antero Formation, Colorado, is entirely reversed in polarity, and is correlated with late Chron C13r, based on 40 Ar/ 39 Ar dates of 33.77–33.89 Ma. The early Oligocene Pitch-Pinnacle flora of Colorado is within rocks of normal polarity, and best correlated with Chron C12n (30.5–31.0 Ma), based on 40 Ar/ 39 Ar dates of 32.9–29.0 Ma (although correlation with Chron C11n is also possible). The late Oligocene ( 40 Ar/ 39 Ar dated 26.26–26.92 Ma) Creede flora of southwestern Colorado is correlated with Chron C8r. The early Oligocene ( 40 Ar/ 39 Ar dated at 31.5 Ma) Granger Canyon flora in the Warner Mountains, near Cedarville, northeastern California, is correlated with Chron C12r. These results are compiled with previously published dates and magnetic stratigraphy of the Eugene-Fisher floral sequence in western Oregon, the Bridge Creek floras in central Oregon, other floras in the Warner Mountains of northeast California, and the Florissant flora of central Colorado. In Colorado, the climatic change seems to have occurred between the Florissant and Antero floras, and is dated between 33.89 and 34.07 Ma, or latest Eocene in age, although the Pitch-Pinnacle flora suggests that the deterioration was less severe and took place in the early Oligocene. In northeast California, the dating is not as precise, so the climatic change could have occurred between 31.5 and 34.0 Ma (probably early Oligocene). In western Oregon (Eugene and Fisher Formations), the change occurs between the early Oligocene Goshen flora (33.4 Ma) and the early Oligocene Rujada flora (31.5 Ma). In the John Day region of Oregon, it occurs before the oldest Bridge Creek flora, dated at 33.62 Ma (right after the Eocene-Oligocene boundary). Thus, only two of these four floral sequences (Eugene, Oregon, and Cedarville, California) clearly show the early Oligocene climatic change consistent with that documented in the global marine record, whereas the climatic change was seemingly abrupt in the late Eocene in Colorado between 33.89 and 34.07 Ma, and also sometime during the late Eocene (before 33.62 Ma) in central Oregon.