ABSTRACT This field trip focuses on accessible and inclusive design in field-based teaching and learning through a broad investigation of the geology of Arizona, followed by more detailed exercises that focus on the Upper Triassic stratigraphic sequences in Petrified Forest National Park (PEFO). The first day of the field trip will traverse the three physiographic provinces of Arizona, from fault-bounded, basement-cored uplifts and valleys of the Basin and Range in the greater Phoenix area, through the Transition Zone to the Mogollon Rim, and ending in Upper Triassic sedimentary rocks of the Colorado Plateau at Holbrook. The second day of the field trip will encompass more detailed, collaborative exercises in PEFO that utilize the expertise of both student and faculty participants in mixed-ability groups. The main priority of this accessible field experience is the development of an inclusive community of learning driven by paired student-faculty interactions, facilitated as needed by technology integration to mitigate barriers and foster engagement, communication, and collaboration across a spectrum of ability and content knowledge. Please note that the collection of archaeological artifacts, fossils, rocks, or other natural history objects without an active research and collection permit is illegal at Petrified Forest National Park. Please refrain from collecting samples or specimens of any kind from anywhere in the park.

Four different sand types (termed FSP1, FSP2, FSP3, and FSP4) have been recognized in the Paleocene succession of the Faroe-Shetland Basin, NE Atlantic, on the basis of conventional heavy mineral analysis, major element geochemistry of garnet, trace element geochemistry of rutile, U-Pb dating of detrital zircon, and palynofloral analysis. Sand types FSP1, FSP2, and FSP4 were all sourced from the eastern margin of the basin, whereas FSP3 was supplied from the west. No single technique discriminates all four sand types. Conventional heavy mineral analysis discriminates FSP3 from the other three sand types but does not discriminate FSP1, FSP2, and FSP4. Garnet geochemistry distinguishes FSP1, FSP2 and FSP4, but FSP3 garnet populations overlap those of FSP1 and FSP2. Rutile geochemistry distinguishes FSP2 from FSP1 and FSP4 but cannot be easily applied to FSP3 owing to the scarcity of rutile in this sand type. Zircon age spectra in FSP1, FSP2, and FSP4 are similar to one another, but FSP4 can be recognized on the basis of a higher proportion of Archean zircons. Some of the individual techniques have certain limitations: e.g., one of the key conventional heavy mineral parameters is the presence of clinopyroxene, but this is not always reliable owing to the instability of this mineral during burial diagenesis. Likewise, garnet geochemistry cannot be applied to the most deeply buried sandstones in the Faroe-Shetland Basin owing to complete garnet dissolution. Furthermore, care is required when interpreting garnet data from sandstones that have undergone partial garnet dissolution, as there may have been modification of the range of garnet compositions as a result of the greater instability of Ca-rich garnets compared with Ca-poor types. Finally, the “Greenland flora,” which occurs in association with sand type FSP3, has been found in some wells that lack FSP3 sandstones. This discrepancy is attributed to the difference in hydrodynamic behavior of palynomorphs compared with sand particles. This chapter illustrates the importance of adopting an integrated approach, as significant detail would have been lost if only one technique had been applied, and integration of a number of different techniques overcomes limitations associated with individual approaches. An integrated approach also builds a more comprehensive picture of source area characteristics.

The source of volcanic material in the Upper Triassic Chinle Formation on the Colorado Plateau has long been speculated upon, largely owing to the absence of similar-age volcanic or plutonic material cropping out closer than several hundred kilometers distant. These strata, however, together with Upper Triassic formations within El Antimonio and Barranca Group sedimentary rocks in northern Sonora, Mexico, yield important clues about the inception of Cordilleran magmatism in Triassic time. Volcanic clasts in the Sonsela Member of the Chinle Formation range in age from ca. 235 to ca. 218 Ma. Geochemistry of the volcanic clasts documents a hydrothermally altered source region for these clasts. Detrital zircons in the Sonsela Member sandstone are of similar age to the clasts, as are detrital zircons from the El Antimonio and Barranca Groups in Sonora. Most noteworthy about the Colorado Plateau Triassic zircons, however, are their Th/U ratios, which range from ~1 to 3.5 in both clast and detrital zircons. Thorium/uranium ratios in the Sonoran zircons, in contrast, range from ~0.4 to ~1. These data, together with rare-earth-element geochemistry of the zircons, shed light on likely provenance. Geochemical comparisons support correlation of clasts in the Sonsela Member with Triassic plutons in the Mojave Desert in California that are of the same age. Zircons from these Triassic plutons have relatively low Th/U ratios, which correspond well with values from El Antimonio and Barranca Group sedimentary rocks, and support derivation of the strata, at least in part, from northern sources. The Sonsela Member zircons, in contrast, match Th/U values obtained from Proterozoic through Miocene volcanic, volcaniclastic, and plutonic rocks in the eastern and central Mojave Desert. Similarly, rare-earth-element compositions of zircons from Jurassic ignimbrites in the Mojave Desert, though overlapping those of zircons from Mojave Desert plutons, also closely resemble those from Sonsela Member zircons. We use these data to speculate that erosion of Triassic volcanic fields in the central to eastern Mojave Desert shed detritus that became incorporated into the Chinle Formation on the Colorado Plateau.

Detrital zircon U-Pb geochronology can make an extremely valuable contribution to provenance studies and paleogeographic reconstructions, but the technique cannot distinguish grains with similar ages derived from different sources. Hafnium isotope analysis of zircon crystals combined with U-Pb dating can help make such distinctions. Five Paleogene formations in West Java have U-Pb age populations of 80–50 Ma (Late Cretaceous–Paleogene), 145–74 Ma (Cretaceous), 298–202 Ma (Permian–Triassic), 653–480 Ma (mid-Neoproterozoic–latest Cambrian), and 1290–723 Ma (late Mesoproterozoic–early Neoproterozoic). Hf-isotopes have been analyzed for 311 zircons from these formations. Differences in zircon U-Pb age and Hf-isotope populations reflect changing sources with time. Late Cretaceous and Paleogene zircons are interpreted as having been derived from two temporally discrete volcanic arcs in Java and West Sulawesi, respectively. The Java arc was active before micro-continent collision, and the W Sulawesi arc developed later, on newly accreted crust at the SE Sundaland margin. The collision age is estimated to be ca. 80 Ma. U-Pb age and 176 Hf/ 177 Hf i characteristics allow a distinction to be made between Cretaceous granitic and volcanic arc sources. Zircons that are older than ca. 80 Ma have a continental Sundaland provenance. Mid-Cretaceous zircons in all upper Eocene and lower Oligocene formations were derived from granites of the Schwaner Mountains of SW Borneo. Permian–Triassic zircons were derived predominantly from granites in the SE Asian Tin Belt. 176 Hf/ 177 Hf i ratios permit distinction between Tin Belt granites in the Main Range and Eastern Provinces, and indicate that only the lower Oligocene Cijengkol Formation contains significant input from the Main Range Province, suggesting a partial change in drainage pattern. Older zircon ages are more difficult to interpret but probably record contributions from allochthonous basement and sedimentary rocks that were deposited prior to rifting of continental blocks from Gondwana in the early Mesozoic.

The late Eocene to early Miocene Renova Formation records initial post-Laramide sediment accumulation in the intermontane basin province of southwest Montana. Recent studies that postulate deposition of the Renova Formation were restricted to a broad, low-relief, tectonically quiescent basin on the eastern shoulder of an active rift zone vastly differ from traditional models in which the Renova Formation was deposited in individual intermontane basins separated by basin-bounding uplands. This study utilizes detrital zircon geochronology to resolve the paleogeography of the Renova Formation. Detrital zircon was selected as a detrital tracer that can be used to differentiate between multiple potential sources of similar mineralogy but with distinctly different U-Pb ages. Laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) U-Pb detrital zircon ages were determined for 11 sandstones from the Eocene-Oligocene Renova Formation exposed in the Sage Creek, Beaverhead, Frying Pan, Upper Jefferson, Melrose, and Divide basins. Detrital zircon ages, lithofacies, paleoflow, and petrography indicate that provenance of the Renova Formation includes Paleogene volcanics (Dillon volcanics and Lowland Creek volcanics), Late Cretaceous igneous intrusions (Boulder batholith, Pioneer batholith, McCartney Mountain pluton), Mesozoic strata (Blackleaf Formation, Beaverhead Group), Belt Supergroup strata, and Archean basement. The oldest deposits of the Renova are assigned Bridgerian to Uintan North American Land Mammal (NALM) ages and contain detrital zircons derived from volcanic, sedimentary, and metamorphic rocks constituting the “cover strata” to uplift-cored Late Cretaceous plutonic bodies. Regional unroofing trends are manifested by a decreased percentage of cover strata–sourced zircon and an increased percentage of pluton-sourced zircon as Renova deposits became younger. Zircon derived from Late Cretaceous plutonic bodies indicate that initial unroofing of the McCartney Mountain pluton, Pioneer batholith, and Boulder batholith occurred during Duchesnean time. Facies assemblages, including alluvial fan, trunk fluvial, and paludal-lacustrine lithofacies, are integrated with detrital zircon populations to reveal a complex Paleogene paleotopography in the study area. The “Renova basin” was dissected by paleo-uplands that shed detritus into individual intervening basins. Areas of paleo-relief include ancestral expressions of the Pioneer Range, McCartney Mountain, Boulder batholith–Highland Range, and Tobacco Root Range. First-order alluvial distributary systems fed sediment to two noncontiguous regional-trunk fluvial systems during the Chadronian. A “Western fluvial system” drained the area west of the Boulder batholith, and an “Eastern fluvial system” drained the area east of the Boulder batholith. Chadronian paleodrainages parallel the regional Sevier-Laramide structural grain and may exhibit possible inheritance from Late Cretaceous fluvial systems. Detrital zircons of the Renova Formation can be confidently attributed to local sources exposed in highlands that bound the Divide, Melrose, Beaverhead, Frying Pan, Upper Jefferson, and Sage Creek basins. The data presented in this study do not require an Idaho batholith provenance for the Renova Formation.

The particle size and provenance signature of glacial till from the Lonewolf Nunataks at the head of Byrd Glacier, Antarctica, show evidence of subglacial origin and therefore provide new information about ice-covered bedrock of East Antarctica. Particle-size data from ice-cored moraines at Lonewolf Nunataks show more abundant silt and clay (>50% fines) than active lateral moraines along downstream sites (<10% fines), and 25% of pebbles are faceted and/or striated. Sand and pebbles from moraines at Lonewolf Nunataks are a mix of locally derived Beacon Supergroup rocks and exotic felsic igneous and metamorphic rocks. The U/Pb detrital zircon data from the Lonewolf Nunataks till show significant populations of zircon ages, including early Ross and/or Pan-African ages of ca. 565–610 Ma, Grenville ages (ca. 950–1270 Ma), several Proterozoic peaks, and one prominent late Archean peak at ca. 2700 Ma. 40 Ar/ 39 Ar analyses of detrital hornblende and mica also show Ross and/or Pan-African ages from ca. 500 to 580 Ma, with a population of Grenville-age hornblende grains of ca. 1150–1250 Ma. This combination of geochronological tools can be used to identify recycled versus primary age populations eroded by the ice sheet, and so provide constraints on the age and distribution of unmapped, ice-covered bedrock. Our data show that petrologic and geochronologic signatures in East Antarctic till can be used to address geologic problems ranging from Cenozoic ice sheet history to Precambrian bedrock geology.

Single-step, laser-fusion 40 Ar/ 39 Ar ages of single muscovite grains with an automated micro-extraction system is a precise and relatively rapid way of analyzing large numbers of grains. This study used >500 muscovite grains from a late Wisconsinan sandy loess from eastern Long Island, New York, in order to evaluate the potential of Ar-Ar ages of single grain muscovite as a provenance tool for loess. The samples for dating were from a 2.7 m core of sediments from a small kettle hole in Wildwood State Park on the north shore of Long Island. These eolian deposits consist of a bimodal distribution of poorly sorted medium silt and medium sand that are buff colored, homogeneous, and unstratified. Long Island is a good place to test this approach, because the 40 Ar/ 39 Ar and K/Ar ages for muscovite in the potential bedrock sources to the north in New England vary systematically from ca. 450 Ma in the west to ca. 200 Ma in the east. The majority of muscovite ages in the loess range from 250 to 400 Ma, and muscovite age populations along the core show a change in proportion of muscovite input from the different provenances in New England. The results of this study confirm that using 40 Ar/ 39 Ar ages of a large number of single muscovite grains is a good method for examining the provenance of muscovite in loess, and thus understanding processes that produce loess.

The late glacial and deglacial history of the Southeastern Laurentide Ice Sheet involves the southward advance and subsequent northward retreat from southeastern Canada and the northeastern United States. Superposed on this advance and retreat are three major ice-rafting events associated with Heinrich events 2 and 1 (H2 and H1) and the Younger Dryas. Nd, Sr, and Pb isotopes were measured on the 63–150 μm, de-carbonated marine sediment for the period 24–10.5 14 C ka, from marine sediment core EW9303-GGC31, collected from the top of Orphan Knoll, a topographic high 550 km northeast of Newfoundland, Canada. In general, one of the problems with understanding ice-rafting records is the disparate provenance strategies that have been used in different studies. Nd and Sr isotopes have been widely used in the study of North Atlantic sediment provenance, and Pb isotopes and 40 Ar/ 39 Ar hornblende ages have also been used for provenance assessment in a number of studies. The new Nd, Sr, and Pb isotope data presented here are complementary to the published hornblende data from the same samples, and provide a data set that allows more confident comparison of this record with other published provenance studies. The results are consistent with reconstructions based on a combination of marine and land-based geomorphic observations.

Detrital geochronological analyses, combined with information on river sediment load, are widely employed to constrain erosion patterns in orogenic belts. Major assumptions in most detrital studies are that detrital samples are fully representative of eroding bedrock, and variation in original mineral concentration, often referred to as fertility, is negligible. Nevertheless, hydraulic sorting effects during transport may strongly affect sediment composition, and mineral fertility strongly depends on bedrock lithology. In this detrital geochronology study, we illustrate how hydraulic sorting effects can be properly evaluated, and how mineral fertility in bedrock can be determined from detrital samples, in order to infer reliable erosion patterns on short-term time scales. Fission-track, bulk-petrography, and geochemical analyses were carried out on modern sands of Rivers Dora Baltea and Arc in the Western Alps. These rivers drain in opposite directions two major fault-bounded blocks (Eastern and Western Blocks) that have undergone contrasting exhumation paths since the Miocene. Samples were collected from different sites along the river trunk, in order to investigate how the detrital signal evolves when detritus from different sub-basins is progressively added to the system. In the Dora Baltea catchment, petrographic data indicate that 29% of the total river load was derived from the Western Block, whereas the Eastern Block contributes the remaining 71%. Petrographic signatures in the modern Arc sands are more homogeneous, thus preventing a precise discrimination of the sources. Apatite fission-track data from the Dora Baltea River show that the Western Block yields 43% of the total apatite load, and the Eastern Block the remaining 57%. In the Arc catchment, apatite contribution is 29% from the Eastern Block, 14% from the Houiller-Subbriançonnais units, and 57% from the Belledonne-Dauphinois units. We assessed apatite fertility in source rocks by measuring apatite content in processed sediments, after checking for anomalous hydraulic concentrations by geochemical analyses. Apatite flux from each sub-basin was converted into a specific sediment yield to infer the short-term erosion pattern in the drainage. The annual sediment load measured along the trunk was then partitioned between sub-basins, in order to calculate erosion rates during the late- to post-glacial time interval. Results document focused erosion in the External Massifs, at rates of 0.4–0.5 mm/a, irrespective of their position inside the drainage, and a westward migration of erosional foci through time along the Western Alps transect.

The forearc of Central Chile (33°–34°S) is formed by three N-S–trending morphostructural units, including, from west to east, the Coastal Cordillera, the Central Depression, and the Principal Cordillera. The Cenozoic sedimentary rocks that could represent the erosional material generated throughout the morphotectonic evolution of these units accumulated in the marine Navidad Basin. The age of the marine deposits is controversial, as foraminifer biostratigraphy indicates that marine deposition started during the late Miocene, whereas 87 Sr/ 86 Sr data indicate that deposition started during the early Miocene. We carried out single heavy mineral microprobe analysis and standard heavy mineral analysis of these deposits in order to qualitatively identify the geological units subjected to erosion in the central Chilean forearc during Cenozoic times. Our analysis focused mainly on unweathered and unaltered detrital garnet, pyroxene, and amphibole. The textural characteristics and geochemical signature of these minerals were used to determine their original rock type; their magmatic affinity, in the case of pyroxenes of volcanic origin; and their metamorphic grade, in the case of amphiboles of metamorphic origin. We have also compared the composition of detrital garnet, pyroxene, and amphibole with preexisting chemical data of these minerals in the possible source rocks, which, along with the analysis of the detrital heavy mineral suite in each sample, allows us to determine the specific geological unit from which they were generated. Three erosional-depositional stages are recorded by our analysis. Whereas the chemistry of pyroxene and amphibole characterized volcanic-subvolcanic sources within the present-day Central Depression for the first stage, the Central Depression and the Principal Cordillera for the second stage, and the Principal Cordillera for the third stage; the composition of garnet is indicative of metamorphic and plutonic sources within the Coastal Cordillera during all three stages. If marine deposition inside the Navidad Basin started during the early Miocene, the provenance results would record erosion and deposition contemporary with volcanic activity. On the other hand, if marine deposition started during the late Miocene, the provenance results show a retrograde erosive response to landscape for a regional uplift event proposed for that period in the study area. Also, assuming that provenance results are directly related to the action of faults, our data indicate that the main relief-generating fault during the early stages of Andean uplift corresponds to the Los Ángeles–Infiernillo Fault, rather than the San Ramón Fault, as stated by the proposed morphotectonic models for the study area. In addition, the ubiquitous provenance from the Coastal Cordillera is more likely to represent the erosion of nearshore basement rocks affected by faulting along the eastern border of the Navidad Basin, rather than uplift and erosion of the Coastal Cordillera, as previously considered. Single-mineral geochemical analysis of detrital pyroxene and amphibole can be used in other sedimentary basins related to arc-magmatic systems with short transport distances, like the ones in the western Andean border, where these minerals tend to be largely unweathered. In particular, our work represents an advance in this field, as the chemistry of detrital amphibole has not been used before to discriminate source rocks presenting different geochemical signatures.

Upper Jurassic sandstones deposited in a shallow-marine deltaic setting in the Piper Field of the Outer Moray Firth area, North Sea, show high-frequency fluctuations in apatite:tourmaline ratios that appear to be related to sea-level change. Because apatite and tourmaline are both stable during burial diagenesis and have similar hydraulic behavior, variations in the apatite:tourmaline ratio indicate either differences in sediment provenance or in the extent of floodplain weathering, apatite being unstable during weathering. Other provenance-sensitive heavy mineral ratios (rutile:zircon, monazite:zircon, chrome spinel:zircon) and mineral-chemical data from detrital garnet assemblages show that sandstones with high apatite:tourmaline have the same provenance as sandstones with low apatite:tourmaline. Fluctuations in apatite:tourmaline ratios are therefore attributed to the extent of weathering during floodplain residence prior to the sediment entering the marine system. Sedimentological data indicate that sandstones with high apatite:tourmaline were deposited during sea-level highstands, whereas sandstones with low apatite:tourmaline were deposited during lowstands. The implication of this observation is that during sea-level lowstands, sediment undergoes more prolonged floodplain residence than during highstands, apparently the direct result of the increase in areal extent of the floodplain. The fluctuations in apatite:tourmaline offer an opportunity for high-resolution correlation in the Piper Field. If similar patterns become apparent in other areas, variations in apatite:tourmaline ratios could also provide a basis for identifying highstand and lowstand events, and help establish whether deep-water submarine fan sandstones were deposited during highstands or lowstands.

Sandstone provenance is commonly characterized by point counting thin sections using a petrographic microscope. An analytical tool (QEMScan™: Quantitative Evaluation of Materials by Scanning Electron Microscopy) newly applied to provenance analyses provides complementary data and alternatives for quantifying modal compositions of sandstones. QEMScan combines scanning electron microscope (SEM) imaging and elemental analyses to create mineralogical maps of solid materials. Three different applications of QEMScan (mineralogic maps, bulk mineralogy calculations, and automated disaggregate counts) were compared to traditional (petrographic) point-count results using a test data set of 12 samples from Utah and Mongolia. Results indicate that QEMScan can provide semi-automated and rapid analyses of sandstone provenance. In the case of the manual QEMScan point-count method, the new technique largely removes operator error in grain identification. However, direct comparison to petrographic data currently requires time-consuming image processing, and adjusting QEMScan processors to recognize grain boundaries and complex grain-mineral types. Moreover, comparison of these methods provided a means to assess the operator error associated with point counting. The results of the petrographic and QEMScan methods generated comparable results, indicating that operator error does not significantly affect modal compositions through traditional techniques.

Among supracrustal sequences of the Jurassic magmatic arc of the southwestern Cordillera, the Middle Jurassic Topawa Group, Baboquivari Mountains, south-central Arizona, is remarkable for its lithologic diversity and substantial stratigraphic thickness, ≈8 km. The Topawa Group comprises four units (in order of decreasing age): (1) Ali Molina Formation—largely pyroclastic rhyolite with interlayered eolian and fluvial arenite, and overlying conglomerate and sandstone; (2) Pitoikam Formation—conglomerate, sedimentary breccia, and sandstone overlain by interbedded silt-stone and sandstone; (3) Mulberry Wash Formation—rhyolite lava flows, flow breccias, and mass-flow breccias, with intercalated intraformational conglomerate, sedimentary breccia, and sandstone, plus sparse within-plate alkali basalt and comendite in the upper part; and (4) Tinaja Spring Porphyry—intrusive rhyolite. The Mulberry Wash alkali basalt and comendite are genetically unrelated to the dominant calcalkaline rhyolite. U-Pb isotopic analyses of zircon from volcanic and intrusive rocks indicate the Topawa Group, despite its considerable thickness, represents only several million years of Middle Jurassic time, between approximately 170 and 165 Ma. Sedimentary rocks of the Topawa Group record mixing of detritus from a minimum of three sources: a dominant local source of porphyritic silicic volcanic and subvolcanic rocks, identical or similar to those of the Topawa Group itself; Meso-proterozoic or Cambrian conglomerates in central or southeast Arizona, which contributed well-rounded, highly durable, polycyclic quartzite pebbles; and eolian sand fields, related to Middle Jurassic ergs that lay to the north of the magmatic arc and are now preserved on the Colorado Plateau. As the Topawa Group evidently represents only a relatively short interval of time, it does not record long-term evolution of the Jurassic magmatic arc, but rather represents a Middle Jurassic “stratigraphic snapshot” of the arc. This particular view of the arc has been preserved primarily because the Topawa Group accumulated in deep intra-arc basins. These nonmarine basins were fundamentally tectonic and extensional, rather than volcano-tectonic, in origin. Evidence from the Topawa Group supports two previous paleogeographic inferences: the Middle Jurassic magmatic arc in southern Arizona was relatively low standing, and externally derived sediment was introduced into the arc from the continent (northeast) side, without appreciable travel along the arc. We speculate that because the Topawa Group intra-arc basins were deep and rapidly subsiding, they became the locus of a major (though probably intermittent) fluvial system, which flowed into the low-standing magmatic arc from its northeast flank.

Upper Jurassic strike-slip intra-arc basins formed along the axis of earlier Lower to Middle Jurassic extensional intra-arc basins in Arizona. These strike-slip basins developed along the Sawmill Canyon fault zone, which may represent an inboard strand of the Mojave-Sonora megashear system that did not necessarily produce large-scale translations. Subsidence in the Lower to Middle Jurassic extensional arc was uniformly fast and continuous, whereas at least parts of the Upper Jurassic arc experienced rapidly alternating uplift and subsidence, producing numerous large-scale intrabasinal unconformities. Volcanism occurred only at releasing bends or stepovers in the Upper Jurassic arc, producing more episodic and localized eruptions than in the earlier extensional arc. Sediment sources in the Upper Jurassic strike-slip arc were also more localized, with restraining bends shedding sediment into nearby releasing bends. Normal fault scarps were rapidly buried by voluminous pyroclastic debris in the Lower to Middle Jurassic extensional arc, so epiclastic sedimentary deposits are rare, whereas pop-up structures in the Upper Jurassic strike-slip arc shed abundant epiclastic sediment into the basins. Three Upper Jurassic calderas formed along the Sawmill Canyon fault zone where strands of the fault progressively stepped westward in a releasing geometry relative to paleo-Pacific–North America plate motion. We hypothesize that strike-slip basins in the Upper Jurassic arc formed in response to changing plate motions that induced northward drift of North America, causing sinistral deformation of the paleo-Pacific margin. Drift out of the northern horse latitudes into northern temperate latitudes brought about wetter climatic conditions, with eolianites replaced by fluvial, debris-flow, and lacustrine sediments. “Dry” eruptions of welded ignimbrite were replaced by “wet” eruptions of nonwelded, easily reworked ignimbrite and phreatoplinian fall. This Late Jurassic transition from hyperarid to more temperate climatic conditions may thus form a superregional “time line” that ties the Cordilleran plate margin to events in the interior of the continent.