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.

Paleontological studies at Florissant have been ongoing for more than 13 decades. As the focus of these studies has shifted through this time, the site has provided important insights into the evolution of paleontology as a science from its beginnings in the nineteenth century through its subsequent development. Early studies focused on the description of new taxa from collections that were being made by the early scientific surveys of the American West, particularly the Hayden Survey during the early 1870s and an expedition from Princeton in 1877. The first studies and descriptions of these fossils were by Leo Lesquereux on the fossil plants, S.H. Scudder on the fossil insects, and E.D. Cope on the fossil vertebrates. At the beginning of the twentieth century, T.D.A. Cockerell conducted field expeditions in 1906–1908, and subsequently published ∼130 papers on fossil plants, insects, and mollusks. Work by these early researchers was the first to consider the implications of the Florissant fossils for evolution, extinction, biogeography, and paleoclimate. Even greater emphasis on these broader implications began when H.D. MacGinitie made excavations during 1936–1937 and published a comprehensive monograph on the fossil flora in 1953, including numerous taxonomic revisions and detailed interpretations of stratigraphic context, paleoecology, paleoclimate, paleoelevation, biogeography, and taphonomy. Other workers during the late 1900s initiated the first studies on pollen, dicotyledonous woods, and multiple organ reconstructions of extinct plant genera, and developed more quantified methods for determining paleoelevation and paleoclimate. Current work emphasizes plant-insect interactions, the use of diatoms as fresh-water paleoen-vironmental indicators and as agents in macrofossil taphonomy, and the use of insects as terrestrial environmental indicators.

Abstract The central Colorado landscape bears a strong imprint of post-Laramide (late Eocene to Quaternary) tectonics, volcanism, climate change, and drainage rearrangement. This field trip will examine the post-Laramide evolution of central Colorado, traversing the Front Range, from the Colorado Piedmont on the east to the upper Arkansas valley segment of the Rio Grande Rift on the west (Fig. 1 ). The first day of the trip will involve a transect from the Denver-Colorado Springs section of the Piedmont across the southern Front Range, South Park, and Mosquito Range to the upper Arkansas valley. On this day we will focus on questions concerning the roles of tectonics and climate in driving post-Laramide landscape changes, examining structural, sedimentological, paleontological, geomorphic, and fission track evidence that has been used to reconstruct post-Laramide history. We will end the day with an initial overview of rift-related structures, sediments, and geomorphology as we enter the upper Arkansas valley. We will spend the second day in the southern portion of the upper Arkansas valley and the adjacent Poncha Pass transfer zone, examining structural and sedimentological evidence for the nature and timing of Neogene and Quaternary faulting and graben formation, and the character of the transfer zone. On our final day we will traverse back to the Piedmont, this time traveling down the canyons of the Arkansas River. We will examine rift-related structures and sediments in the Pleasant Valley graben and at the northern end of the Wet Mountain Valley, and will discuss the history of Cenozoic and earlier faulting in the area, the evolution of the Arkansas River drainage, and its recent downcutting history. We will end the trip with a discussion of the Neogene and Quaternary erosional history of the High Plains and Piedmont, and possible implications of this history for the driving mechanisms of landscape change.

Abstract The early Mesoproterozoic (ca. 1400 Ma) is an enigmatic time in the tectonic evolution of southern Laurentia. Circa 1400 Ma granites within Laurentia and multiple other continents have distinctive geochemistry consistent with crustal extension or mantle upwelling. In the southwestern United States, these granites are commonly foliated and are often spatially associated with km-scale ductile shear zones. Deformation is attributed to intracontinental tectonism driven by active convergence along the distal southern margin of Laurentia. The recent discovery of deformed and metamorphosed, ca. 1450 Ma sedimentary rocks in northern New Mexico has strengthened the case for regional deformation and orogenesis. However, important questions remain about the tectonic significance of these events and how to reconcile tectonic models with granite petrology at the regional to global scale. This trip focuses on the protolith age of Proterozoic metasedimentary rocks and the kinematics, timing, and tectonic significance of deformation, magmatism, and metamorphism for the Mesoproterozoic across different crustal levels in the southern Rocky Mountains to highlight the ongoing questions and controversies regarding the Mesoproterozoic tectonic setting of Laurentia. This field trip will examine some of the diverse and most recently discovered evidence for ca. 1400 Ma orogenesis in the southern Rocky Mountains. We hope this trip will promote new interest and discussion about the Mesoproterozoic tectonic evolution of Laurentia. We will visit multiple outcrops in the Wet Mountains of southern Colorado and the Picuris Mountains of northern New Mexico. Stops in the Wet Mountains are arranged from north to south to examine contrasting styles of ca. 1400 Ma deformation with increasing paleodepth across the tilted Proterozoic crustal section. In the Picuris Mountains, we focus on detrital zircon geochronology and revisions to the lithostratigraphy of Paleoproterozoic and recently documented Mesoproterozoic metasedimentary rocks, the nature of regional metamorphism, and the style of deformation, ca. 1450–1400 Ma.

Abstract In 1901, Charles Van Hise asked Samuel Emmons and Whitman Cross to organize a grand excursion across Colorado as part of the combined meeting of the American Association for the Advancement of Science, GSA, and the Colorado Scientific Society (CSS). This trip replays part of that 10-day excursion across Colorado. Shortened to three days, this trip takes in some of the same sites as the 1901 trip, plus adds others of interest along the route where CSS members are reinventing geological interpretations. The trip will follow the precedent set in 1901; CSS members will serve as “site or stop hosts” in addition to the trip leader and drivers. While walking in the steps of the most famous of our profession we will also see some of the most magnificent scenery of Colorado.