Abstract The Paleogene sedimentary deposits of the Colorado Headwaters Basin provide a detailed proxy record of regional deformation and basin subsidence during the Laramide orogeny in north-central Colorado and southern Wyoming. This field trip presents extensive evidence from sedimentology, stratigraphy, structure, palynology, and isotope geochronology that shows a complex history that is markedly different from other Laramide synorogenic basins in the vicinity. We show that the basin area was deformed by faulting and folding before, during, and after deposition of the Paleogene rocks. Internal unconformities have been identified that further reflect the interaction of deformation, subsidence, and sedimentation. Uplift of Proterozoic basement blocks that make up the surrounding mountain ranges today occurred late in basin history. Evidence is given to reinterpret the Independence Mountain uplift as the result of significant normal faulting (not thrusting), probably in middle Tertiary time. While the Denver and Cheyenne Basins to the east were subsiding and accumulating sediment during Late Cretaceous time, the Colorado Headwaters Basin region was experiencing vertical uplift and erosion. At least 1200 m of the upper part of the marine Upper Cretaceous Pierre Shale was regionally removed, along with Fox Hills Sandstone shoreline deposits of the receding Interior Seaway as well as any Laramie Formation–type continental deposits. Subsidence did not begin in the Colorado Headwaters Basin until after 60.5 Ma, when coarse, chaotic, debris-flow deposits of the Paleocene Windy Gap Volcanic Member of the Middle Park Formation began to accumulate along the southern basin margin. These volcaniclastic conglomerate deposits were derived from local, mafic-alkalic volcanic sources (and transitory deposits in the drainage basin), and were rapidly transported into a deep lake system by sediment gravity currents. The southern part of the basin subsided rapidly (roughly 750–1000 m/m.y.) and the drainage system delivered increasing proportions of arkosic debris from uplifted Proterozoic basement and more intermediate-composition volcanic-porphyry materials from central Colorado sources. Other margins of the Colorado Headwaters Basin subsided at slightly different times. Subsidence was preceded by variable amounts of gentle tilting and localized block-fault uplifts. The north-central part of the basin that was least-eroded in early Paleocene time was structurally inverted and became the locus of greatest subsidence during later Paleocene-Eocene time. Middle Paleocene coal-mires formed in the topographically lowest eastern part of the basin, but the basin center migrated to the western side by Eocene time when coal was deposited in the Coalmont district. In between, persistent lakes of variable depths characterized the central basin area, as evidenced by well-preserved deltaic facies. Fault-fold deformation within the Colorado Headwaters Basin strongly affected the Paleocene fluvial-lacustrine deposits, as reflected in the steep limbs of anticline-syncline pairs within the McCallum fold belt and the steep margins of the Breccia Spoon syncline. Slivers of Proterozoic basement rock were also elevated on steep reverse faults in late Paleocene time along the Delaney Butte–Sheep Mountain–Boettcher Ridge structure. Eocene deposits, by and large, are only gently folded within the Colorado Headwaters Basin and thus reflect a change in deformation history. The Paleogene deposits of the Colorado Headwaters Basin today represent only a fragment of the original extent of the depositional basin. Basal, coarse conglomerate deposits that suggest proximity to an active basin margin are relatively rare and are limited to the southern and northwestern margins of the relict basin. The northeastern margin of the preserved Paleogene section is conspicuously fine-grained, which indicates that any contemporaneous marginal uplift was far removed from the current extent of preserved fluvial-lacustrine sediments. The conspicuous basement uplifts of Proterozoic rock that flank the current relict Paleogene basin deposits are largely post-middle Eocene in age and are not associated with any Laramide synuplift fluvial deposits. The east-west–trending Independence Mountain fault system that truncates the Colorado Headwaters Basin on the north with an uplifted Proterozoic basement block is reinterpreted in this report. Numerous prior analyses had concluded that the fault was a low-angle, south-directed Laramide thrust that overlapped the northern margin of the basin. We conclude instead that the fault is more likely a Neogene normal fault that truncates all prior structure and belongs to a family of sub-parallel west-northwest–trending normal faults that offset upper Oligocene-Miocene fluvial deposits of the Browns Park–North Park Formations.

Abstract This field trip highlights recent research into the Laramide uplift, erosion, and sedimentation on the western side of the northern Colorado Front Range. The Laramide history of the North Park-Middle Park basin (designated the Colorado Headwaters Basin in this paper) is distinctly different from that of the Denver basin on the eastern flank of the range. The Denver basin stratigraphy records the transition from Late Cretaceous marine shale to recessional shoreline sandstones to continental, fluvial, marsh, and coal mires environments, followed by orogenic sediments that span the K-T boundary. Upper Cretaceous and Paleogene strata in the Denver basin consist of two mega-fan complexes that are separated by a 9 million-year interval of erosion/non-deposition between about 63 and 54 Ma. In contrast, the marine shale unit on the western flank of the Front Range was deeply eroded over most of the area of the Colorado Headwaters Basin (approximately one km removed) prior to any orogenic sediment accumulation. New 40 Ar- 39 Ar ages indicate the oldest sediments on the western flank of the Front Range were as young as about 61 Ma. They comprise the Windy Gap Volcanic Member of the Middle Park Formation, which consists of coarse, immature volcanic conglomerates derived from nearby alkalic-mafic volcanic edifices that were forming at about 6561 Ma. Clasts of Proterozoic granite, pegmatite, and gneiss (eroded from the uplifted at Laramide basin subsidence, sedimentation, and deformation in north-central Colorado, in Morgan, L.A., and Quane, S.L., eds., Through the Generations: core of the Front Range) seem to arrive in the Colorado Headwaters Basin at different times in different places, but they become dominant in arkosic sandstones and conglomerates about one km above the base of the Colorado Headwaters Basin section. Paleocurrent trends suggest the southern end of the Colorado Headwaters Basin was structurally closed because all fluvial deposits show a northward component of transport. Lacustrine depositional environments are indicated by various sedimentological features in several sections within the >3 km of sediment preserved in the Colorado Headwaters Basin, suggesting this basin may have remained closed throughout the Paleocene and early Eocene. The field trip also addresses middle Eocene(?) folding of the late Laramide basin-fill strata, related to steep reverse faults that offset the Proterozoic crystalline basement. Late Oligocene magmatic activity is indicated by dikes, plugs, and eruptive volcanic rocks in the Rabbit Ears Range and the Never Summer Mountains that span and flank the Colorado Headwaters Basin. These intrusions and eruptions were accompanied by extensional faulting along predominantly northwesterly trends. Erosion accompanied the late Oligocene igneous activity and faulting, leading to deposition of boulder conglomerates and sandstones of the North Park Formation and high-level conglomerates across the landscape that preserve evidence of a paleo-drainage network that drained the volcanic landscape.

An evaluation of the poorly understood Cenozoic hydrologic history of the American Southwest using combined geological and biological data yields new insights with implications for tectonic evolution. The Mesozoic Cordilleran orogen next to the continental margin of southwestern North America probably formed the continental divide. Mountain building migrated eastward to cause uplift of the Rocky Mountains during the Late Cretaceous to early Tertiary Laramide orogeny. Closed drainage basins that developed between the two mountain belts trapped lake waters containing fish of Atlantic affinity. Oligocene-Miocene tectonic extension fragmented the western mountain belt and created abundant closed basins that gradually filled with sediments and became conduits for dispersal of fishes of both Pacific and Atlantic affinity. Abrupt arrival of the modern Colorado River to the Mojave-Sonora Desert region at ca. 5 Ma provided a new conduit for fish dispersal. Great dissimilarities in modern fish fauna, including differences in their mitochondrial deoxyribonucleic acid (DNA), indicate that late Miocene runoff from the Colorado Plateau did not flow down the Platte or Rio Grande, or through the Lake Bonneville Basin. Fossil fishes from the upper Miocene part of the Bidahochi Formation on the Colorado Plateau have characteristics that reflect a habitat of large, swift-moving waters, and they are closely related to fossil fishes associated with the Snake and Sacramento Rivers. This evidence suggests that influx of fishes from the ancestral Snake River involved a major drainage, not merely small headwater transfers.

ABSTRACT Wildfire affects ecosystems via the disruption of landforms and biota. Step and pool formations in mountainous streams have important hydraulic and ecological functions, but there is little information on wildfire impacts on step‐pool ecosystems. This chapter describes a study examining the biophysical responses of headwater mountain streams for 2 years following the 2012 Waldo Canyon Fire in Pike National Forest (Front Range of Colorado, USA). We focused on the impacts of this wildfire on stream invertebrate communities, including richness, composition, and traits related to disturbance tolerance and habit types, as well as functional feeding groups. We tested the hypothesis that the postfire responses of invertebrate communities will depend on vegetation burn severity (unburned, low severity, and moderate/high severity), with responses being greater in moderate/high-severity than unburned or low-severity basins. Our results indicated that the relative abundances of sensitive invertebrates decreased and tolerant taxa increased after fire at sites draining catchments burned at moderate/high severity, and few postfire impacts were found on invertebrates at sites draining catchments that burned at low severity, relative to unburned sites. Regarding the relative abundances of trait groups, shredder abundance was higher at low-severity burned sites compared to sites burned at moderate/high severity and unburned sites throughout the study, and sprawler abundance was higher at low-severity burn sites compared to moderate/high-severity burn sites and unburned basins in 2013. However, values of all invertebrate variables in moderately/highly burned reaches generally returned to unburned levels by 2 years postfire. Stream geomorphological variables, specifically changes in the vertical profile of the stream channel over time, were negatively correlated with taxa richness, family biotic index, and percentage of shredders. Fire management strategies that prevent or reduce the severity of catastrophic wildfire, such as forest thinning or prescribed fire, as well as efforts to reestablish natural channel morphology, can ameliorate the impacts of wildfire on stream communities and help to establish stable, healthy, and diverse ecosystems in steep environments in the Front Range and elsewhere.

Thirty to forty m.y. of post-Laramide degradation of the southern Rocky Mountains likely produced relatively low-relief topography within the crystalline cores of the ranges, and capped the adjacent sedimentary basins with easily eroded sediments. We focus on the modern, more dissected topography of these ranges, reflecting late Cenozoic evolution driven by fluvial and glacial exhumation, each of which affects different portions of the landscape in characteristic ways. Ongoing exhumation of the adjacent basins, in places by more than 1 km, is effectively lowering base level of streams draining the crystalline range cores. The streams have incised deep bedrock canyons that now cut the flanks of the range. Over the same time scales, glaciation of the headwaters of the major streams has modified the range crests. We utilize the topography of the northern Front Range of Colorado to explore the response of a Laramide range both to the exhumation of the adjacent basin and to glaciation in the high elevations. We break the problem of whole landscape evolution into three related, one-dimensional problems: evolution of the high smooth summit surfaces; evolution of the longitudinal profiles of adjacent glacial troughs; and evolution of the fluvial profiles downstream of the glacial limit. We review work on the high summit surfaces, showing quantitatively that they are steady-state features lowering at rates on the order of 5 μm/yr, and are entirely decoupled from the adjacent glacial troughs. Glaciers not only truncate these high surfaces, but greatly alter the longitudinal profiles of the major streams: major steps occur at tributary junctions, and profiles above the glacial limit are significantly flattened from their original fluvial slopes. We extend existing models of glacial valley evolution by including processes that allow head-wall retreat. This serves to enhance the headward retreat of east-facing valleys, and explains the asymmetric truncation of the high smooth surfaces that form the spine of the range. Fluvial profiles downstream of the glacial limit commonly display a prominent convexity inboard of the range edge. Stream-power–based numerical models of profile evolution of specific rivers demonstrate that this reflects a transient response of the streams to base-level lowering. This response varies significantly with drainage basin area. We explore the degree to which this differential response controls the location of major remnants of pediments on the edge of the Great Plains, such as the prominent Rocky Flats and adjacent surfaces.