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40 Ar/ 39 Ar dating results of 133 samples from 84 late Cenozoic volcanic rocks provide emplacement ages that constrain the timing of evaporite collapse and the incision rates of the Colorado River. Our samples are from areas in west-central Colorado, both within and outside of the Carbondale and Eagle collapse centers. Significant pulses of volcanic activity occurred in the intervals from 24 to 22, 16 to 13, 11 to 9, and 8 to 7 Ma. In addition, small flows, widely spaced in time and space were emplaced during the last 4 m.y. Although individual basaltic flows appear to be chemically and isotopically homogeneous, there are significant geochemical and isotopic differences between flows, even between some flows that apparently have the same age within the limits of analytical precision.A low-relief early to middle Miocene erosional surface has been postulated in west-central Colorado. Our studies are consistent with the existence of a low-relief paleotopographic surface that is now at a minimum elevation range of ϳ2.9-3.4 km outside areas of collapse. Elevation departures from this range suggest that Ͼ1000 m of subsidence due to evaporite removal has locally occurred in the Carbondale and Eagle collapse centers. 40 Ar/ 39 Ar ages from downdropped and disrupted basaltic flows in the Carbondale center constrain initial collapse to Ͼ13 Ma, the timing of much of Downloaded from M.J. Kunk et al. 214 the evaporite-related collapse to the past 10-8 m.y., and an increase in the rate of collapse during the last 3 m.y.Ages and elevations of basaltic rocks above the Colorado River in Glenwood Canyon are used to calculate average apparent incision rates for the Colorado River in Glenwood Canyon of 24 mm/k.y. from 7.8 to 3.0 Ma. The average apparent incision rate increased by an order of magnitude to 242 mm/k.y. during the last 3 m.y. C o lo r a d o R iv e r E agle R iv e r F o r k Flank of Mt. Sopris E a gl e R iv e r W h it e R iv e r C o lo r a d o R iv er

A paleoseismological investigation of fl exural-slip faults related to interstratal evaporite dissolution suggests that such gravitational structures might have the potential to generate earthquakes with damaging magnitude. The Carbondale collapse center, in the southern Rocky Mountains of Colorado, is a morpho-structural depression of ~1200 km 2 where Miocene volcanic rocks are downdropped as much as 1200 m due to interstratal dissolution of halite-bearing evaporites. On the western margin of the collapse center, the debuttressing effect related to active evaporite dissolution drives unfolding of the steeply dipping late Laramide Grand Hogback monocline, accompanied by displacement on bedding-parallel faults. These fl exural-slip faults rupture unconformable Miocene basalts and Quaternary mantled pediments, generating conspicuous half-graben depressions bounded by antislope fault scarps parallel to the underlying strata of the monocline. Two trenches dug across fl exural-slip fault scarps developed in each stratigraphic marker (basalt cap, mantled pediment) revealed unexpected evidence of multiple late Quaternary faulting events (e.g., faulted colluvial wedge, sharp unconformities), with displacement-perevent values of ≥1 m. Three faulting events were inferred from the trench dug in the pediment (<32 ka, 32-28 ka, 5.6-1.5 ka), and four events from the trench sited in the basalts, all probably older than 20 ka. The probable length (~25 km) and downdip width (~7.5 km) of the fl exural-slip faults associated with the Carbondale collapse center suggests that they might have the potential to generate damaging "unfolding earthquakes" with moment magnitude (M w ) around 6.

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This study focused on characterizing the phylotypic composition of acid mine drainage (AMD) communities associated with the Solomon Mine near Creede, Colorado, and its relative diversity compared to microbial communities found in the East Willow Creek (EWC) watershed. AMD from the Solomon Mine adit flows into an existing passive bioremediation wetland system located next to the Solomon Mine adit that currently is ineffective and is under consideration for renovation. We are interested in gaining an understanding of the baseline microbial communities present in AMD/EWC and to monitor them during future wetland renovation. Prokaryotic community profiling was approached using SSU 16S rRNA marker gene amplification coupled with next generation sequencing. Bioinformatics analysis included raw read preprocessing, data visualization, and statistical testing using a combination of USEARCH and QIIME-based scripts. A pH and conductivity gradient were observed for water moving through the currently inefficient wetland system at the Solomon Mine. The EWC microbiomes had statistically significant higher alpha diversity compared to the AMD microbiomes. Beta diversity analysis parsed the sample locations into statistically significant groups including core AMD microbiomes, the wetland Cell 3 microbiome, and EWC microbiomes using multidimensional scaling. Taxa driving beta diversity included the phylum Proteobacteria for the core AMD microbiomes, the phyla Firmicutes and Chloroflexi for the constructed wetland Cell 3, and the phyla Bacteroidetes and Verrucomicrobia for EWC. Our data suggests that the microbial community in constructed wetland Cell 3 is likely where limited sulfate reduction activity is operating at low capacity, which will be further investigated via shotgun metagenomic analysis.