Nonmarine lower Miocene rocks widely exposed in nearly continuous outcrop over approximately 3100 km 2 (1,200 mi 2 ) of the Hartville Table in southeastern Wyoming and western Nebraska indicate a semiarid continental interior, with seasonal climate characterized by sandy ephemeral or intermittent braided streams, interchannel plains mantled by fine-grained volcaniclastic loess, and shallow ephemeral holomictic lakes. These paleoenvironments are recognized on the basis of distinctive sedimentologic, faunal, and taphonomic characteristics. Stream sediments (10 percent or less of total outcrop) are primarily tuffaceous silty sandstones, deposited as reworked pyroclastic debris in wide shallow valleys. These valleys first filled with fluvial fine-grained volcaniclastics, but with the cessation of streamflow in the region, filling was completed by air-fall volcaniclastic loess that blanketed both valleys and interchannel reaches. Fluvial sediments within the valleys include much spatially dispersed mammal bone that had been scavenged and subaerially weathered prior to burial. Waterholes, situated in or adjacent to the valleys, filled with tuff and carbonate mud containing freshwater ostracods, pulmonate gastropods, diatoms, and charophyte algae. These tuffaceous waterhole muds intertongue with fluvial volcaniclastic sediments and are the locus of major mammalian bone beds, the best known preserved at Agate Fossil Beds National Monument. Bones of chalicothere, rhinoceros, and entelodont are common in waterhole bone beds and in fluvial sediments in the region. Massive tuffaceous air-fall silty sandstones (87 percent of outcrop) punctuated by silcrete paleosols were deposited in the interchannel reaches; mammal remains are commonly represented by widely scattered, isolated bones and partial skeletons of young and aged ungulates, chiefly oreodonts and camels, indicative of attritional deaths over time. No bone beds occur. Thin silicified carbonate mudstones (about 2 percent of outcrop) with ostracods, plant debris, and aquatic pulmonate gastropods (but without fish or other aquatic vertebrates) indicate shallow, holomictic, ephemeral lakes that filled with homogeneous micrite mud. These lakes were isolated sheet-like bodies of water unassociated with stream sediments. Following desiccation, lacustrine sediments were commonly overprinted by pedogenic features. Eolian transport of fine pyroclastic detritus into the North American midcontinent was essential to preservation of these sedimentary environments and their rich fossil record. In the Americas and in Africa during the Cenozoic, fine-grained volcaniclastic sediments blanketed large geographic areas within the continental interiors, preserving significant temporal intervals of the vertebrate fossil record. If volcanism had not occurred, these intervals would exist as major hiatuses in our knowledge of vertebrate, particularly mammalian, evolution. The important role of fine-grained volcaniclastics in preservation of mammalian faunas and their associated depositional environments in the Americas and in Africa during the Cenozoic deserves greater emphasis.

Abstract The Lagerstätte at Ashfall Fossil Beds—the result of supervolcanic eruption—preserves a mass-death assemblage of articulated skeletons of reptiles, birds, and mammals in a 3-m-thick pure volcanic ash near the base of the Cap Rock Member of the Ash Hollow Formation in Antelope County, Nebraska. The ash originated from the Bruneau-Jarbidge caldera in southwest Idaho, some 1600 km away, and it is geochemically matched with the Ibex Hollow tuff (11.93 Ma). Ashfall is a critical Clarendonian North American Land Mammal Age locality. More than 20 taxa—predominantly medium- and large-sized ungulates preserved in three dimensions—are buried in a late Miocene paleodepression (waterhole) filled with tephra reworked from the landscape by wind and water. Smaller taxa, such as birds, turtles, and moschids, died shortly after the pyroclastic airfall event and their remains are preserved in the basal ash. Remains from the medium-sized ungulates (equids and camelids) are separated from the underlying smaller skeletons by several centimeters of ash, indicating that these animals died at a slightly later time. In turn, more than 100 mostly intact skeletons of the barrel-bodied rhinoceros, Teleoceras major , overlie the remains of the medium-sized taxa. Pathologic bone on the limbs and skulls of the horses, camels, and rhinos suggests short-term survival and slow death several weeks or months after the pyroclastic airfall event. Exquisite preservation in an information-rich context allows aspects of the behavior, social structure, intraspecific variability, and pathology of extinct species to be reconstructed.

Abstract The John Day Basin of central Oregon contains a remarkably detailed and well-dated Early Eocene–Late Miocene sedimentary sequence, known for its superb fossils. This field trip examines plant fossil assemblages from throughout the sequence in the context of their geological and taphonomic setting and regional and global significance. The Early to Late Eocene (>54–39.7 Ma) Clarno Formation contains fossil plants and animals that occupied an active volcanic landscape near sea level, interspersed with meandering rivers and lakes. Clarno assemblages, including the ca. 44 Ma Nut Beds flora, record near-tropical “Boreotropical” rainforest, which was replaced during late Clarno time by more open and seasonal subtropical forest. The overlying John Day Formation (39.7–18.2 Ma) was deposited in a backarc landscape of low hills dotted with lakes and showered by ashfalls from the Western Cascades. Fossils and paleosols record the advent of the “Icehouse” Earth during the earliest Oligocene, with decreasing winter temperature and more seasonal rainfall that supported open deciduous and coniferous forest, much like that of the southern Chinese highlands today. Sixteen and a half million years ago the Picture Gorge flood basalt covered the region. Animals and plants fossilized in the overlying (ca. 16 to >12 Ma) Mascall Formation occupied a relatively flat landscape during a warm and moist period known as the Middle Miocene Climatic Optimum. In total this sequence preserves a detailed series of time slices illustrating regional biotic and landscape evolution during the Cenozoic that is highly relevant for deciphering regional and global biotic, climatic, and geological trends.

Eocene and Oligocene floras of the western United States show a climatic deterioration from warmer conditions to much cooler and drier conditions. Recent 40 Ar/ 39 Ar dates and magnetic stratigraphy have greatly improved their correlation. In this study, the uppermost Eocene Antero Formation, Colorado, is entirely reversed in polarity, and is correlated with late Chron C13r, based on 40 Ar/ 39 Ar dates of 33.77–33.89 Ma. The early Oligocene Pitch-Pinnacle flora of Colorado is within rocks of normal polarity, and best correlated with Chron C12n (30.5–31.0 Ma), based on 40 Ar/ 39 Ar dates of 32.9–29.0 Ma (although correlation with Chron C11n is also possible). The late Oligocene ( 40 Ar/ 39 Ar dated 26.26–26.92 Ma) Creede flora of southwestern Colorado is correlated with Chron C8r. The early Oligocene ( 40 Ar/ 39 Ar dated at 31.5 Ma) Granger Canyon flora in the Warner Mountains, near Cedarville, northeastern California, is correlated with Chron C12r. These results are compiled with previously published dates and magnetic stratigraphy of the Eugene-Fisher floral sequence in western Oregon, the Bridge Creek floras in central Oregon, other floras in the Warner Mountains of northeast California, and the Florissant flora of central Colorado. In Colorado, the climatic change seems to have occurred between the Florissant and Antero floras, and is dated between 33.89 and 34.07 Ma, or latest Eocene in age, although the Pitch-Pinnacle flora suggests that the deterioration was less severe and took place in the early Oligocene. In northeast California, the dating is not as precise, so the climatic change could have occurred between 31.5 and 34.0 Ma (probably early Oligocene). In western Oregon (Eugene and Fisher Formations), the change occurs between the early Oligocene Goshen flora (33.4 Ma) and the early Oligocene Rujada flora (31.5 Ma). In the John Day region of Oregon, it occurs before the oldest Bridge Creek flora, dated at 33.62 Ma (right after the Eocene-Oligocene boundary). Thus, only two of these four floral sequences (Eugene, Oregon, and Cedarville, California) clearly show the early Oligocene climatic change consistent with that documented in the global marine record, whereas the climatic change was seemingly abrupt in the late Eocene in Colorado between 33.89 and 34.07 Ma, and also sometime during the late Eocene (before 33.62 Ma) in central Oregon.

During the past five years, renewed prospecting and collecting of mammalian fossils in the Florissant Formation within Florissant Fossil Beds National Monument in central Colorado has nearly tripled the known diversity of fossil mammals from this rock unit. Taxa first recorded here from the Florissant Formation include the eomyid rodent Paradjidaumo trilophus , the lagomorph Palaeolagus , and the rare artiodactyl Pseudoprotoceras longinaris . We also describe an isolated deciduous premolar of a protoceratid. We update the mammalian faunal list of the Florissant Formation, which includes some 16 species in 13 families and 6 orders. The mammalian fauna corroborates the Chadronian (latest Eocene) age determined by others. Geographic ranges of Pelycomys , Palaeolagus , and Paradjidaumo trilophus are extended slightly southwest from northeastern Colorado, and the range of Pseudoprotoceras longinaris is extended southwest from Wyoming and Nebraska. Based upon comparison with nearest living relatives and plausible analogs, the mammalian taxa represented in the Florissant Formation seem to be consistent with the moist, warm temperate, relatively high elevation wetland and woodland habitats that have been inferred by others for the area in and around late Eocene Lake Florissant.

ABSTRACT A growing body of evidence suggests that continental arc lower crust and underlying mantle wedge assemblages native to the Mojave Desert (i.e., the southern California batholith) were displaced eastward during Laramide shallow-angle subduction, and reattached to the base of the Colorado Plateau Transition Zone (central Arizona) and farther inboard. On this field trip, we highlight two xenolith localities from the Transition Zone (Camp Creek and Chino Valley) that likely contain remnants of the missing Mojave lithosphere. At these localities, nodules of garnet clinopyroxenite, the dominant xenolith type at both studied localities, yield low jadeite components in clinopyroxene, chemically homogeneous “type-B” garnet, and peak conditions of equilibration at 600–900 °C and 9–28 kbar. These relations strongly suggest a continental arc residue (“arclogite”), rather than a lower-plate subduction (“eclogite”), origin. Zircon grains extracted from these nodules yield a bimodal age distribution with peaks at ca. 75 and 150 Ma, overlapping southern California batholith pluton ages, and suggesting a consanguineous relationship. In contrast, Mesozoic and early Cenozoic igneous rocks native to SW Arizona, with age peaks at ca. 60 and 170 Ma, do not provide as close a match. In light of these results, we suggest that Transition Zone xenoliths: (1) began forming in Late Jurassic time as a mafic keel to continental arc magmas emplaced into the Mojave Desert and associated with eastward subduction of the Farallon plate; (2) experienced a second ca. 80–70 Ma pulse of growth associated with increased magmatism in the southern California batholith; (3) were transported ~500 km eastward along the leading edge of the shallowly subducting Farallon plate; and (4) were reaffixed to the base of the crust at the new location, in central Arizona. Cenozoic zircon U-Pb, garnet-whole rock Sm-Nd, and titanite U-Pb ages suggest that displaced arclogite remained at elevated temperature (>700 °C) for 10s of m.y., following its dispersal, and until late Oligocene entrainment in host latite. The lack of arclogite and abundance of spinel peridotite xenoliths in Miocene and younger mafic volcanic host rocks (such as those at the San Carlos xenolith locality), and the presence of seismically fast and vertically dipping features beneath the western Colorado Plateau, suggest that arclogite has been foundering into the mantle and being replaced by upwelling asthenosphere since Miocene time.