ABSTRACT The Mesoproterozoic is a controversial time within the Earth’s history, and is characterized by high temperature/pressure ratios in metamorphic rocks, a large volume of extensional plutons, very few economic mineral deposits, and possibly a slowdown in plate tectonic processes. In Laurentia, ca. 1.48–1.35 Ga is well known as a time of voluminous ferroan magmatism, which led to conflicting tectonic interpretations that range from continental extension to convergent margin settings. Recently, a ca. 1.50–1.35 Ga orogenic belt was proposed that spanned Laurentia from present-day eastern Canada to the southwestern United States. Unlike the preceding Paleoproterozoic Yavapai/Mazatzal orogenies and the subsequent late Mesoproterozoic Grenville orogeny, the early–mid-Mesoproterozoic Picuris orogeny in the southwestern United States was relatively unrecognized until about two decades ago, when geochronology data and depositional age constraints became more abundant. In multiple study areas of Arizona and New Mexico, deposition, metamorphism, and deformation previously ascribed to the Yavapai/Mazatzal orogenies proved to be part of the ca. 1.4 Ga Picuris orogeny. In Colorado, the nature and extent of the Picuris orogeny is poorly understood. On this trip, we discuss new evidence for the Picuris orogeny in the central Colorado Front Range, from Black Hawk in the central Colorado Front Range to the Wet Mountains, Colorado. We will discuss how the Picuris orogeny reactivated or overprinted earlier structures, and perhaps controlled the location of structures associated with Cambrian rifting, the Cretaceous–Paleogene Laramide orogeny, and the Rio Grande rift, and associated mineralization. We will also discuss whether and how the Picuris orogeny, and the Mesoproterozoic in general, were unique within the Earth’s history.

ABSTRACT Two models have been proposed to explain continental crust generation in accretionary orogens. One model suggests that accretionary orogens are formed by the successive collision of juvenile arcs. The second model invokes tectonic switching, which is the repeated cycles of slab rollback and extensional backarc basin formation followed by basin collapse caused by collision, shallow subduction, and/or increased convergence rate. The northern Colorado Front Range, specifically in and around the Big Thompson, Rist, and Poudre Canyons, offers excellent exposures of Paleoproterozoic rocks to test which accretionary model best explains crust generation for a portion of the Yavapai Province. In this contribution we have two goals: The first is to provide a field-trip guide that augments Mahan et al.’s (2013) field guide, which uses many stops that have become inaccessible or have changed because of catastrophic flooding that occurred in September 2013. This more current guide focuses on a variety of mostly Paleoproterozoic rocks within what some call the Poudre Basin. These rocks include clastic metasedimentary rocks, amphibolite, the Big Thompson Canyon tonalite suite, the northern Front Range granodiorite, granitic pegmatites, and Mesoproterozoic Silver Plume granite. The second goal is to present and synthesize new and existing geochemistry, geochronology, and isotopic data, and then discuss the origins, age, deformation, and metamorphism of these rocks in the context of the proposed tectonic models. These data were synthesized into the following tectonic model for the Poudre Basin. At ca. 1780 Ma, the juvenile Green Mountain arc, located today along the Colorado-Wyoming border, formed and extended shortly thereafter during slab rollback, resulting in the extensional backarc Poudre basin between the diverging arc fragments. Sedimentation within the basin began at inception and continued to ca. 1735 Ma when basin rocks were intruded by the Big Thompson Canyon tonalite suite and the northern Front Range granodiorite, all of which were subsequently metamorphosed and deformed at ca. 1725 Ma. Felsic magmatism and deformation within the basin were perhaps driven by the northward shallow subduction of an oceanic plateau or seamount. This suggests that following accretion of the Green Mountain Arc, tectonic switching explains formation and collapse of the Poudre Basin and creation of some of northern Colorado’s crust.

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 Continuous sediment, pollen, and charcoal records were developed from an 8.46-m-long sediment core taken from Hermit Lake in the northern Sangre de Cristo mountain range of Colorado. Presently, vegetation around the lake is upper subalpine forest, consisting of Picea engelmannii (Englemann spruce) with some Abies lasiocarpa (subalpine fir), and the lake lies >200 m below present tree line. We used several pollen ratios to reconstruct the relative position of the tree line and the occurrence of clay layers to infer landscape instability through time. Deglaciation of the Hermit Lake drainage began during the Bølling-Allerød interval. Between ca. 13.5 and 12.4 ka, high Artemisia (sagebrush) pollen abundance, low Picea / Pinus (spruce/pine; S/P) ratios, and sporadic occurrence of Picea macrofossils indicate alpine tundra-spruce conditions. Though the pollen record shows no transition to the Younger Dryas, the subsequent absence of Picea needle fragments suggests a lowering of tree line. By ca. 10.2 ka, a subalpine forest of Picea and Pinus grew there. Based on pollen ratios, tree line was higher than today from ca. 9.0 to ca. 3.8 ka, after which the tree line began to lower to its present elevation. Maximum expansion of the Picea-Abies subalpine forest, determined from both pollen and macrofossils, was coincident with the highest influx of charcoal particles and maximum deposition of postfire erosion (clay layers) into the lake. The period ca. 7.8–6.2 ka was the driest period, as shown by aquatic indicators, but pollen ratios suggest that ca. 6.2–3.8 ka was the warmest period of the Holocene, accompanied by high rates of burning, and consequently elevated erosion of clays into the lake. During the late Holocene, declining S/P ratios are interpreted as declining alpine tree line, while decreases in both Picea to Artemisia (S/Art) and Pinus to Artemisia (P/Art) ratios suggest climate cooling. Pollen evidence suggests expansion of the lower-elevation Colorado piñon ( Pinus edulis ), which has been documented as part of a widespread phenomenon noted by other studies.

ABSTRACT Late diagenesis records a common history of fluid flow in sub-Permian strata in the midcontinent, where fluid inclusion Th are higher than burial temperatures and Tm ice show evolving salinity. Most negative δ 18 O dolomite and highest Th are at the top of the Mississippian. Fluid inclusion and geochemical data point to advective fluid flow out of basins utilizing Cambrian–Ordovician–Mississippian strata as an aquifer for hydrothermal fluids. The Pennsylvanian was a leaky confining unit. This system evolved from: Stage 1 Pennsylvanian–early Permian pulsed hydrothermal migration of connate brine and gas; between Stages 1 and 2, low-temperature Permian brine reflux; Stage 2 mixing between high-temperature and low-temperature brines during the Permian; and Stage 3 large-scale migration of hydrothermal brines and oil later during the Permian or after. Stages 1–3 were the most important late processes affecting Mississippian reservoirs, and record an inverted thermal structure with most impact of hot fluids at the top of the Mississippian. Stage 4 shows radiogenic 87 Sr/ 86 Sr in calcite, supporting a transition to localized fault pumping from basement, likely driven by Laramide fault reactivation. Stage 5 is the current system, with Ozark and Front Range uplift-driven fluid flow and potential for small-scale sporadic fault pumping.

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 U-Pb dating of detrital zircons in fluvial sandstones provides a method for reconstruction of drainage basin and sediment routing systems for ancient sedimentary basins. This paper summarizes a detrital-zircon record of Cenomanian paleodrainage and sediment routing for the Gulf of Mexico and U.S. midcontinent. Detrital zircon data from Cenomanian fluvial deposits of the Gulf of Mexico coastal plain (Tuscaloosa and Woodbine formations), the Central Plains (Dakota Group), and the Colorado Front Range (Dakota Formation) show the Appalachian-Ouachita orogen represented a continental divide between south-draining rivers that delivered sediment to the Gulf of Mexico, and west- and north-draining rivers that delivered sediment to the eastern margins of the Western Interior seaway. Moreover, Cenomanian fluvial deposits of the present-day Colorado Front Range were derived from the Western Cordillera, flowed generally west to east, and discharged to the western margin of the seaway. Western Cordillera-derived fluvial systems are distinctive because of the presence of Mesozoic-age zircons from the Cordilleran magmatic arc: the lack of arc zircons in Cenomanian fluvial deposits that dis-charged to the Gulf of Mexico indicates no connection to the Western Cordillera. Detrital zircon data facilitate reconstruction of contributing drainage area and sediment routing. From these data, the dominant system for the Cenomanian Gulf of Mexico was an ancestral Tennessee River (Tuscaloosa Formation), which flowed axially through the Appalachians, had an estimated channel length of 1200-1600 km, and discharged sediment to the east-central Gulf of Mexico. Smaller rivers drained the Ouachita Mountains of Arkansas and Oklahoma (Woodbine Formation), had length scales of <300 km, and entered the Gulf through the East Texas Basin. From empirical scaling relationships between drainage-basin length and the length of basin-floor fans, these results predict significant basin-floor fans related to the paleo-Tennessee River system and very small fans from the east Texas fluvial systems. This predictive model is consistent with mapped deep-water systems, as the largest fan system was derived from rivers that entered the Gulf of Mexico through the southern Mississippi embayment.