ABSTRACT The Morrison-Golden Fossil Areas National Natural Landmark, Colorado, USA, including Dinosaur Ridge, is rich in geological and paleontological history, ranking historically as the premier location and type area for Late Jurassic dinosaurs like Stegosaurus and Diplodocus . As the type area for the Morrison Formation, it famously became central in the “Bone Wars” underway in the 1870s. After a brief historical introduction at Stop 1, the trip will explore the ‘mid’-Cretaceous Dakota Sandstone (Stop 2), which yields the top-ranked dinosaur tracksite in the United States, with two type ichnospecies and the most accessible “nest scrape display trace” evidence of dinosaurian sexual display found anywhere. It also an important location for the study of microbial mat in association with dinosaur tracks, now photogrammetrically surveyed in detail. Dinosaur Ridge serves as the type area for the “Dinosaur Freeway” and the transgression of the Western Interior Seaway. Its diverse invertebrate traces have also been described in detail. After lunch at the Dinosaur Ridge Visitor’s Center exhibit (Stop 3), the trip will move a short distance to Golden to view the type localities for the first bird and crocodilian tracks ever reported from the Mesozoic (Stop 4). We will also visit the younger (Late Cretaceous) Laramie Formation, exposed in the Golden clay pits, cut by the Golden fault, and the source of historically famous paleofloras, the first known Ceratopsian tracks, and other type traces now developed as Triceratops Trail (Stop 5), constituting part of the well- documented Fossil Trace and School of Mines Geological Trail complex. The field excursion will involve easy walks and no strenuous climbs.

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.

The three field guides in this volume, associated with GSA Connects 2022 held in Denver, Colorado, USA, tackle some interesting aspects of Colorado geology and paleontology. Learn about dinosaur tracks, microbial mat, and applied photogrammetry at Dinosaur Ridge; explore the nature and extent of the Mesoproterozoic Picuris orogeny in Colorado; and learn more about Paleoproterozoic tectonics of the northern Colorado Rocky Mountains Front Range in the context of the authors’ proposed tectonic models.

Abstract Paleoproterozoic supracrustal rocks in the region near Big Thompson Canyon, northern Colorado, have long been recognized as a spectacularly exposed example of regionally zoned metamorphism, preserving an apparently complete sequence from biotite- to migmatite-zones. Due to its location and relatively easy access, the Big Thompson Metamorphic Suite has also provided a valuable field-based educational experience for universities and colleges all along the Front Range and from elsewhere. In addition to a number of other studies, the pioneering work of William Braddock and graduate students from the University of Colorado resulted in more than a dozen M.Sc. and Ph.D. theses from the 1960s to the 1990s. Despite the volume of ground-breaking science conducted on these rocks in the past, there remain a number of fundamental questions regarding the metamorphic history and overall tectonic significance of many of the observable features. Several lines of evidence suggest there is potential for a complex tectonometamorphic history that likely spans from ~1.8 to 1.4 Ga. These include: thermochronologic and geochronologic data supporting multiple thermal and magmatic episodes, structural evidence for multiple deformation events, multiple generations of typical Barrovian minerals (e.g., staurolite), and the widespread occurrence of minerals not commonly associated with a classic Barrovian sequence (e.g., andalusite, cordierite). One purpose of this fieldtrip is to foster new ideas and stimulate new research directions that will utilize the Big Thompson Metamorphic Suite, and the Colorado Rockies in general, as field laboratories for better understanding fundamental orogenic processes.

Depth-dependent variations in the structure and composition of continental crust can be studied via integrated investigations of isobaric terranes. In this contribution, we summarize three isobaric terranes in Archean to Proterozoic crust. In western Canada, 35–45-km-deep lower crust is exposed over an area of more than 20,000 km 2 . The Upper Granite Gorge of Grand Canyon, Arizona, provides a transect of 20–25-km-deep middle crust. The Proterozoic basement of central Arizona represents an isobaric exposure of 10–15-km-deep middle crust. Isobaric terranes yield a conceptual image of continental crust that can be compared to seismic images, xenolith data, and drill core data to clarify rheology, coupling/decoupling of crustal levels, and the interplay between deformation, metamorphism, and plutonism. General observations include: (1) The crust is heterogeneous at all levels and cannot be accurately modeled as a simple progression from quartz-rich to feldspar-rich lithologies or from felsic to mafic bulk compositions. (2) The crust is segmented into foliation domains that alternate between steeply dipping and shallowly dipping. (3) Magmatism is expressed differently at different depths due to different background temperatures and a general upward distillation from mafic to felsic composition, and may be the most important control on crustal architecture and rheology. The strength of continental crust (and its potential for low-viscosity flow) is not simply a function of temperature, depth, and compositional layering, but is controlled by the size and distribution of rheological domains. The rheological character of a particular layer can vary in space and time at any crustal level.