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At least 1,207 records · Page 67Linked to original sources
USGS1978

lowaphyllum (rugose coral) from the Upper Devonian of Arizona

The rugose coral genus lowaphyllum is uncommon but widely distributed in rocks of Devonian age. It is here reported for the first time from western North America (Late Devonian). lowaphyllum is also known from the late Middle and Late Devonian of Eastern North America, but the lack of Early and early Middle Devonian species anywhere in North America leaves open the question of derivation of these forms and their relationship to Early Devonian species in Europe and Australia.

Arizona↗
USGS2017

Datasheet: Pseudogymnoascus destructans (white-nose syndrome fungus)

Pseudogymnoascus destructans is a psychrophilic (cold-loving) fungus that causes white-nose syndrome (WNS), an emerging disease of North American bats that has caused unprecedented population declines. The fungus is believed to have been introduced to North America from Europe or Asia (where it is present but does not cause significant mortality), but the full extent of its native range is unknown. The route of introduction is also unknown. In North America, hibernating bats become infected with P. destructans when body temperature decreases during winter torpor into the range permissive for growth of this fungus. Infected bats may develop visible fungal growth on the nose or wings, awaken more frequently from torpor, and experience a cascade of physiologic changes that result in weight loss, dehydration, electrolyte imbalances, and death. P. destructans persists in the environments of underground bat hibernation sites (hibernacula) and is believed to spread primarily by natural movements of infected bats. The first evidence of WNS in North America is from a photograph of a hibernating bat taken during winter of 2005-2006 in a hibernaculum near Albany, New York. P. destructans subsequently spread rapidly from the northeastern United States throughout much of the eastern portions of the United States and Canada, and most recently (as of May 2017) was detected in Washington State. It has killed millions of bats, threatening some species with regional extirpation and putting at risk the valuable environmental services that bats provide by eating harmful insects.

Book chapter↗
USGS2019

Conservation status of the world’s swan populations, Cygnus sp. and Coscoroba sp.: a review of current trends and gaps in knowledge

Recent estimates of the world’s swan Cygnus sp. populations indicate that there are currently between 1.5–1.6 million birds in 8 species, including the Coscoroba Swan Coscoroba coscoroba as an honorary swan. Monitoring programmes in Europe and North America indicate that most populations increased following the introduction of national and international legislation to protect the species during the early- to mid-20th century. A switch from feeding primarily on aquatic vegetation to foraging on farmland (especially high-energy arable crops) in winter during the second half of the 20th century, is also considered a contributing factor. Trumpeter Swans Cygnus buccinator famously increased from just 69 individuals known to exist in 1935 (although small numbers were missed) to c. 76,000 at the present time, and most of the northern hemisphere swan populations have continued to show increasing/stable trends over the last 20 years. The exception to this pattern is a decline since 1995 in the Northwest European Bewick’s Swan population, following an increase in its population size during the 1970s–1980s, which is now being addressed through implementation of an International Single Species Action Plan. A proposal to change enforcement regulations of the Migratory Bird Treaty Act in the United States is also of concern, as potentially undermining protection for Trumpeter Swans in North America, illustrating the importance of politics and legislation as well as on-the-ground measures for species conservation. Elsewhere, less is known about the trends and conservation status for swans in central and eastern Asia, though count and research programmes introduced in China, added to those underway in Japan and Korea, have recently greatly enhanced our knowledge of swan populations on the East Asian flyway. Trends for the Black Swan Cygnus atratus in Australia and for the Black-necked Swan Cygnus melancoryphus in South America are also poorly known, because of the large numbers involved for the former and a lack of coordinated counts across difficult terrain for the latter. These southern hemisphere species are considered vulnerable to water resource developments ( i.e . where diversion of water is shrinking wetlands), and to droughts associated with El Nino events and climate change. More extensive monitoring is therefore required to determine whether swan populations and species are stable, fluctuating or in decline.

Wildfowl↗
USGS2022

Data resources for NGA-subduction project

A relational database was developed over a five-year period to support ground motion model (GMM) development for the Next Generation Attenuation-Subduction (NGA-Sub) project. The relational database has components that interact according to a database schema, including a source and path component used to describe attributes of seismic sources in global subduction regions and to compute source-to-site distances, a site component that describes attributes of sites where recordings have been made, and a ground motion component. The source component of the database has information for 1880 earthquakes, mainly from the following regions: the Pacific Northwest region of North America, Alaska and the Aleutian Islands, Japan, Taiwan, New Zealand, South America, Central America, and Mexico. Of the 1880 earthquakes, 88 have finite fault models (FFMs) from the literature that were systematically reviewed, distilled to one more rectangular shapes, and trimmed according to procedures based on percentage of total slip. For earthquakes without FFMs, a simulation routine is used to represent finite fault effects required for distance calculations. This simulation routine was adjusted and made more uniform in its application than in prior NGA projects. All earthquakes are classified as interface, intraslab, shallow crustal, or outer rise, using uniform protocols developed for this project. All earthquakes are also assigned class designations adapted from a prior NGA project for active regions, that allows foreshock, mainshock, and aftershock events to be distinguished. The site component of the database is described in a companion paper (Ahdi et al. 2020 [1]). The ground motion component of the database consists of median – and maximum – horizontal component peak parameters (peak ground acceleration, PGA and peak ground velocity, PGV) and pseudo-spectral accelerations (PSa) at 111 oscillator periods and 11 damping ratios. Response spectra were also computed for the vertical component. Fourier amplitude spectra (FAS) and duration metrics were also computed. The ground motion recordings were obtained from collaborating organizations world-wide as uncorrected (Vol 1) digital recordings, that were corrected (componentspecific low – and high – pass filters and baseline correction, as needed) following Pacific Earthquake Engineering Research Center (PEER)/NGA protocols. The relational database operates on each of these (and other) database components to dynamically draw relevant parameters into a single file, known as a flatfile, that is used by researchers engaged in GMM development. The flatfiles used in model development are being published with the NGA-Sub GMMs as products of the NGA-Sub project.

Extramural-Authored Publication Paper↗
USGS2024

The impact of future changes in climate on breeding waterfowl pairs in the US Prairie Pothole Region

Millions of small (< 10 ha) waterbodies embedded in grassland and agroecosystems in midcontinental North America provide breeding habitat to an estimated 50–80% of North America’s migratory ducks. Tens of millions of dollars are invested annually to conserve and enhance upland and wetland habitats for breeding ducks by prioritizing locations predicted to have high densities of breeding pairs under average precipitation conditions. An implicit assumption of this approach is that the distribution of breeding habitat remains relatively static. Climate change is an identified risk to this strategy. To assess this assumption and plan for potential forthcoming conditions, we estimated changes in potential breeding duck pairs under different climate scenarios by combining results of 1) a mechanistic hydrology model that simulates ecosystem processes for a subset of wetlands distributed across the U.S. Prairie Pothole Region (USPPR); 2) four downscaled climate model projections at mid- and late-century time horizons; and 3) U.S. Fish and Wildlife Service multi-decadal datasets and predictive breeding waterfowl pair statistical models. We conducted virtual and in-person informational sessions with partners to inform them on the best practices of using downscaled global circulation models and approaches for climate scenario planning. This close coordination led to a joint presentation at a monthly North Central Climate Adaptation Science Center seminar. We are also co-developing simulated wetland- waterfowl responses under different climate futures for wetlands. Information from these robust predictions of waterfowl habitat and settling patterns in this region provides land-management agencies insights in prioritizing current conservation actions given uncertainty. In addition, understanding how many breeding pairs the USPPR might support in coming decades will likely influence overall breeding population sizes and sustainable harvest objectives across North America.

Iowa, Minnesota, Nebraska, North Dakota, South Dak↗
USGS1971

Liberian age province (about 2,700 m.y.)and adjacent provinces in Liberia and Sierra Leone

Whole-rock Rb-Sr dating of rocks from the crystalline basement has disclosed an age province in Liberia and Sierra Leone of about 2,700 m.y. The approximate eastern boundary of this province with the adjoining Eburnean age province of about 2,000 m.y. has been found in eastern Liberia. Much younger rocks of PanAfrican age (about 550 m.y.) bound the ancient province in Sierra Leone and western Liberia in a belt adjacent to the coast. Groups of infolded metasedimentary and metavolcanic rocks have been tentatively identified as associated with two of these ages. The Kambui Schists of Sierra Leone show whole-rock Rb-Sr ages of about 2,700 m.y. The pelitic and iron-bearing metamorphic rocks in the Marampa Formation of Sierra Leone, and similar sections in the Nimba Range, Liberia, appear to be about 2,200 m.y. old, and may fall within the typical Eburnean age range. The age values are scattered, however, and the Nimba rocks may be equivalent t o the Kambui. The Kasila Group of Sierra Leone, and a coastal belt in Liberia believed to be a continuation of the Kasila, yield typical PanAfrican ages of about 550 m.y. © 1971, The Geological Society of America, Inc.

Geological Society of America Bulletin↗
USGS2012

Accumulation of impact markers in desert wetlands and implications for the Younger Dryas impact hypothesis

The Younger Dryas impact hypothesis contends that an extraterrestrial object exploded over North America at 12.9 ka, initiating the Younger Dryas cold event, the extinction of many North American megafauna, and the demise of the Clovis archeological culture. Although the exact nature and location of the proposed impact or explosion remain unclear, alleged evidence for the fallout comes from multiple sites across North America and a site in Belgium. At 6 of the 10 original sites (excluding the Carolina Bays), elevated concentrations of various "impact markers" were found in association with black mats that date to the onset of the Younger Dryas. Black mats are common features in paleowetland deposits and typically represent shallow marsh environments. In this study, we investigated black mats ranging in age from approximately 6 to more than 40 ka in the southwestern United States and the Atacama Desert of northern Chile. At 10 of 13 sites, we found elevated concentrations of iridium in bulk and magnetic sediments, magnetic spherules, and/or titanomagnetite grains within or at the base of black mats, regardless of their age or location, suggesting that elevated concentrations of these markers arise from processes common to wetland systems, and not a catastrophic extraterrestrial impact event.

Proceedings of the National Academy of Sciences of↗
USGS2000

Detrital zircon geochronology of the Shoo Fly Complex, northern Sierra terrane, northeastern California

U-Pb analyses have been conducted on 92 individual detrital zircon grains from 4 of the main thrust sheets of the Shoo Fly Complex . Samples from the Culbertson Lake allochthon, Duncan Peak allochthon, and Lang sequence yield mainly 1.80-2.10, 2.20-2.45, and 2.55-2.70 Ga ages, which suggests that sediments in these units originated in a cratonal region containing Paleoproterozoic and Archean igneous rocks. These ages match those of basement provinces from the northwestern Canadian shield, suggesting a provenance link with northwestern North America during early Paleozoic time. The Sierra City melange, however, has significantly different zircon ages of 551-635 and 1170-1319 Ma, with only a subordinate population of >1.8 Ga grains. These grains apparently were derived originally from an outboard Neoproterozoic-Cambrian(?) volcanic arc and from 1.0-1.7 Ga basement rocks of south- western North America . The occurrence of all three sets of ages in a sandstone that accumulated outboard of the Lang, Culbertson Lake, and Duncan Peak thrust sheets indicates that most rocks of the Shoo Fly Complex formed inboard of a volcanic arc located in proximity to the southern portion of the Cordilleran margin.

California↗
USGS2005

Paleoseismic study of the South Lajas fault: First documentation of an onshore Holocene fault in Puerto Rico

The island of Puerto Rico is located within the complex boundary between the North America and Caribbean plates. The relative motion along this boundary is dominantly left-lateral strike slip, but compression and extension are locally significant. Although tectonic models proposed for the region suggest the presence of onshore active faults in Puerto Rico, no faults with Holocene displacement have been documented on the island before this study. Current seismic hazard assessments primarily consider only the impact of distant, offshore seismic sources because onshore fault hazard is unknown. Our mapping and trenching studies demonstrate Holocene surface rupture on a previously undocumented fault in southwestern Puerto Rico. We excavated a trench across a scarp near the southern edge of the Lajas Valley that exposed a narrow fault zone disrupting alluvial deposits. Structural relations indicate valley-side-down fault slip, with a component of strike-slip motion. Radiocarbon analyses of organic material collected from the sediments suggest that the most recent surface rupture occurred during the past 5000 yr, but no minimum age has yet been established. This fault may be part of a larger fault zone that extends from the western end of the Lajas Valley toward Ponce, the second largest city in Puerto Rico. ?? 2005 Geological Society of America.

Special Paper of the Geological Society of America↗
USGS1954

Caribbean land and sea through the ages

The oldest part of the Caribbean region proper is in northern Central America, where Permian (?) and Lower Permian marine deposits rest on metamorphic rocks of unknown, possibly middle Paleozoic, age. According to present dating, geosynclinal deposition spread eastward in Late Jurassic time to include Cuba, farther eastward and southward in Early Cretaceous time to include Hispaniola and probably Jamaica, and still farther eastward in Late Cretaceous time to include Puerto Rico, the Virgin Islands, and St. Croix. Throughout the Caribbean region, the Cretaceous is characterized by volcanics of great thickness, pyroclastics being more widespread and thicker than flows. These volcanics evidently were derived from lands of unknown size that are now under the waters of the Caribbean Sea. Land still persisted south of eastern Cuba during Eocene time. Thereafter no geological evidence is now available pointing to land in the Caribbean Sea. © 1954, The Geological Society of America, Inc.

Puerto Rico, the U.S. Virgin Islands, British Virg↗
USGS1969

Mesozoic California and the underflow of Pacific mantle

The Mesozoic evolution of California is interpreted as dominated by the underflow of oceanic mantle beneath the continental margin. Underflow during part of Late Cretaceous time of more than 2000 km of the eastern Pacific plate seems required by the marine magnetic data. Correspondingly, varied oceanic environments—abyssal hill, island arc, trench, oceanic crust, and upper mantle, perhaps also continental rise and abyssal plain—appear to be represented in the eugeosynclinal terranes of California. The rock juxtapositions accord with the concept that these materials were scraped off against the continent as the oceanic plate slid beneath it along Mesozoic Benioff seismic zones, which are now seen as serpentine belts separating profoundly different rock assemblages. The chaotic Franciscan Formation of coastal California consists of deep-ocean Late Jurassic to Late Cretaceous sedimentary, volcanic, crustal, and mantle materials. As open-ocean abyssal oozes and the oceanic crust beneath them were swept into the Benioff-zone trench at the continental margin, they were covered by terrigenous clastic sediments, and the entire complex was carried beneath the correlative continental-shelf and continental-slope deposits (Great Valley sequence) and the older Mesozoic complexes. The other eugeosynclinal terranes of California can be interpreted, albeit with less confidence, in similar terms of underflow of Pacific mantle. In the Klamath Mountains and northern Sierra Nevada, for example, Ordovician and Silurian ocean-floor materials, overlain by or juxtaposed against an Upper Silurian to Permian island arc, were swept in first to the continent, along with a large fragment of oceanic crust and mantle and another fragment of an old orogenic belt. This debris was followed by Permian and Triassic ocean-floor deposits. Late Triassic and Jurassic volcanic products from stocks and batholiths forming in the welded complexes lapped across both landward and oceanward sides of the region. Reversal of Cenozoic extension, strike-slip faulting, and volcanic crustal growth in the western United States reveals a Cretaceous tectonic pattern strikingly like the modern pattern of the Andes, so the paleotectonic setting of North America can be inferred from the South American present. The Mesozoic batholiths of North America, like the late Cenozoic volcanic belt of the central Andes, are products of the same rapid motion of oceanic plates that carried oceanic sediments against the continent to form eugeosynclinal terranes. Magmas generated in the Benioff zones formed the batholiths and the volcanic fields which initially capped them.

California↗
USGS1971

Sierra Nevada plutonic cycle: Part I, origin of composite granitic batholiths

Intrusion of Mesozoic batholiths in California and the western North America Cordillera began in the Late Triassic 210 m.y. ago and ended in the Late Cretaceous 80 m.y. ago. Emplacement of granitic rocks was apparently not continuous but was accomplished during five major epochs of intrusion at approximately 30 m.y. intervals, each epoch taking 10 to 20 m.y. to complete. A progressive transgression of epicontinental seas onto the midcontinent occurred during the same interval of time as the batholithic emplacement to the west. A penecontemporaneous deformation near the loci of granitic emplacement and a temporary regression during the major progressive transgression of seas onto the midcontinent are correlated with each intrusive epoch. The locus of Mesozoic granitic rocks was a source of sediments during most of the period of time required to emplace the batholiths; the origin of the batholithic magmas cannot be related only to localized down-warping of geosynclines. The source of the major proportion of the mobile granodioritic magmas of the Sierra Nevada was within the mantle, as is indicated by Sr isotope data. All plutons now exposed in the Sierra Nevada, whether of Cretaceous age or older, were emplaced at depths of a very few kilometers, the shallowest having been emplaced at depths of 4 km or less. The spatial relationships among these synchronous geologic phenomena and the geochemical and geophysical data from the same region are accounted for by a northwestward drift of North America in the region of the western Cordillera of the United States onto and across a Mesozoic feature that had characteristics like present-day oceanic rises.

California↗
USGS1990

Remagnetization and northward translation of Mesozoic red chert from Cedros Island and the San Benito Islands, Baja California, Mexico

Samples for paleomagnetic analysis were collected from red radiolarian ribbon chert in a Franciscan-like subduction complex (Western Baja terrane) exposed along the western margin of the Baja California Peninsula. A 40-m-thick section of chert was sampled on Cedros Island, and a 30-m-thick and two smaller sections of chert were sampled on the San Benito Islands to the west. A site in pillow basalt underlying the chert also was sampled on Cedros Island. The sections of chert on Cedros Island and the San Benito Islands contain radiolaria ranging in age from Late Triassic to Early Cretaceous. All of these rocks have similar normal-polarity direction of magnetization, indicating that they were remagnetized long after deposition. Results from a chert site, including an intrastratal fold, also indicate that the magnetization is postdepositional. A significantly improved grouping (95% confidence level) of directions after correction for the structural attitudes of the Cedros and San Benito chert sections, however, implies that the remagnetization predates breaking up and incorporation of the chert into the subduction complex. Studies of modern subduction zones imply that the attitude of the ancient subducting slab, including the chert sections, was near horizontal (dip angles <10° at the time of remagnetization. Thus, assuming paleohorizontality, a paleolatitude of 12° ± 5° N can be estimated for the Western Baja terrane in mid-Cretaceous time. The paleomagnetic directions also indicate a subsequent poleward translation of 25° ± 7° in latitude (2,800 km) and a clockwise rotation of 56° ± 7° of the Western Baja terrane with respect to North America. The paleolatitude error limits at the 95% confidence level represent minimum values, because deviations of the bedding planes from horizontal during remagnetization would contribute to the error. A 10° NE dip of the subducting slab, however, would account for only 4° of the paleolatitude anomaly, and a ≃60° NE dip would be needed to account for the entire anomaly. Modern analogs, in combination with plate-reconstruction models, imply that the northward translation of the Western Baja terrane along the ancient convergent margin of North America could have been accomplished by arc-parallel strike-slip faulting that was driven by oblique subduction.

Baja California↗
USGS2001

Detrital zircon provenance of Mesoproterozoic to Cambrian arenites in the western United States and northwestern Mexico

U-Pb isotopic dating of detrital zircon from supracrustal Proterozoic and Cambrian arenites from the western United States and northern Mexico reveal three main age groups, 1.90 to 1.62 Ga, 1.45 to 1.40 Ga, and 1.2 to 1.0 Ga. Small amounts of zircons with ages of 3.1 to 2.5 Ga, 1.57 Ga, 1.32 Ga, 1.26 Ga, 0.7 Ga, and 0.5 Ga are also present. Detrital zircons ranging in age from 1.90 to 1.62 Ga and from 1.45 to 1.40 Ga are considered to have been derived from Proterozoic crystalline basement rocks of these known ages, and probably in part from reworked Proterozoic supracrustal sedimentary rocks, of the western United States. The 1.2 to 1.0 Ga detrital zircon ages from California, Arizona, and Sonora are characterized by distinct spikes (1.11 Ga, in particular) in the age-probability plots. These spikes are interpreted to indicate the influx of zircon from major silicic volcanic fields. Igneous rocks such as the Pikes Peak Granite (1.093 Ga) of Colorado, and the Aibo Granite (1.110 Ga) of Sonora, Mexico, may represent the deeply eroded roots of such volcanic fields. Samples from farther north along the Cordilleran margin that contain abundant 1.2–1.0 Ga detrital zircons do not show spikes in the age distribution, but rather ages spread out across the entire 1.2–1.0 Ga range. These age spectra resemble those for detrital zircons from the Grenville province, which is considered their source. Less common detrital zircons had a variety of sources. Zircons ranging in age from 3.36 to 2.31 Ga were apparently derived from inland parts of the North American continent from Wyoming to Canada. Zircons of about 1.577 Ga are highly unusual and may have had an exotic source; they may have come from Australia and been deposited in North America when Australia and North America were juxtaposed as part of the hypothetical Rodinian supercontinent. Detrital zircon of ∼1.320 Ga apparently had the same source as that for tuff (1.320 Ga) in the Pioneer Shale of the Apache Group in Arizona. Detrital zircons of about 1.26 Ga in the Apache Group and Troy Quartzite appear to be related to local, approximately coeval volcanic fields. Zircons of about 0.7 Ga may have had a source in igneous rocks related to rifting of the Proterozoic supercontinent of Rodinia, and 0.5 Ga zircons a source in relatively small areas of granitic rocks of this known, or inferred, age in Oklahoma, Texas, New Mexico, and Colorado.

Geological Society of America Bulletin↗
USGS2006

A community effort to construct a gravity database for the United States and an associated Web portal

Potential field data (gravity and magnetic measurements) are both useful and costeffective tools for many geologic investigations. Significant amounts of these data are traditionally in the public domain. A new magnetic database for North America was released in 2002, and as a result, a cooperative effort between government agencies, industry, and universities to compile an upgraded digital gravity anomaly database, grid, and map for the conterminous United States was initiated and is the subject of this paper. This database is being crafted into a data system that is accessible through a Web portal. This data system features the database, software tools, and convenient access. The Web portal will enhance the quality and quantity of data contributed to the gravity database that will be a shared community resource. The system's totally digital nature ensures that it will be flexible so that it can grow and evolve as new data, processing procedures, and modeling and visualization tools become available. Another goal of this Web-based data system is facilitation of the efforts of researchers and students who wish to collect data from regions currently not represented adequately in the database. The primary goal of upgrading the United States gravity database and this data system is to provide more reliable data that support societal and scientific investigations of national importance. An additional motivation is the international intent to compile an enhanced North American gravity database, which is critical to understanding regional geologic features, the tectonic evolution of the continent, and other issues that cross national boundaries. ?? 2006 Geological Society of America. All rights reserved.

Special Paper of the Geological Society of America↗
USGS2006

The Hancock County tetrapod locality: A new Mississippian (Chesterian) wetlands fauna from western Kentucky (USA)

The earliest tetrapods are known from a handful of Upper Devonian and Lower Carboniferous localities in Europe, North America, and Australia. All Upper Devonian sites and virtually all Early Carboniferous faunas are regarded as predominantly aquatic and most occur within, or are associated with, wetland habitats. A new mid- Carboniferous (Elvirian, Namurian A) fossil locality in Kentucky preserves the fi rst tetrapod fauna from the eastern portion of the Illinois Basin. Four distinct facies at the locality have yielded vertebrate material. Diverse faunas have been found in an abandoned channel/oxbow facies and a fl oodplain/lake facies. The abandoned channel/oxbow facies contains Colosteidae, Embolomeri, Rhizodontida, Dipnoi, Xenacanthiformes, Palaeonisciformes, and Gyracanthidae remains. This assemblage is similar to known Mississippian freshwater and brackishwater faunas, providing further evidence of a cosmopolitan tetrapod province during the Mississippian. A different fauna, rich in tetrapods but lacking fi sh, is associated with granular carbonate masses, rooting structures, and a paleosol in the fl oodplain/ lake facies. Isolated and associated tetrapod elements from this facies exhibit morphological adaptations that may suggest a fauna of more highly terrestrial vertebrates than previously known from the North American Mississippian. ?? 2006 Geological Society of America.

Special Paper of the Geological Society of America↗
USGS2008

Devonian brachiopods of southwesternmost laurentia: Biogeographic affinities and tectonic significance

Three brachiopod faunas discussed herein record different depositional and tectonic settings along the southwestern margin of Laurentia (North America) during Devonian time. Depositional settings include inner continental shelf (Cerros de Los Murcielagos), medial continental shelf (Rancho Placeritos), and offshelf continental rise (Rancho Los Chinos). Ages of Devonian brachiopod faunas include middle Early (Pragian) at Rancho Placeritos in west-central Sonora, late Middle (Givetian) at Cerros de Los Murcielagos in northwestern Sonora, and late Late (Famennian) at Rancho Los Chinos in central Sonora. The brachiopods of these three faunas, as well as the gastropod Orecopia, are easily recognized in outcrop and thus are useful for local and regional correlations. Pragian brachiopods dominated by Acrospirifer and Meristella in the "San Miguel Formation" at Rancho Placeritos represent the widespread Appohimchi Subprovince of eastern and southern Laurentia. Conodonts of the early to middle Pragian sulcatus to kindlei Zones associated with the brachiopods confirm the ages indicated by the brachiopod fauna and provide additional information on the depositional setting of the Devonian strata. Biostratigraphic distribution of the Appohimchi brachiopod fauna indicates continuous Early Devonian shelf deposition along the entire southern margin of Laurentia. The largely emergent southwest-trending Transcontinental arch apparently formed a barrier preventing migration and mixing of many genera and species of brachiopods from the southern shelf of Laurentia in northern Mexico to the western shelf (Cordilleran mio-geocline) in the western United States. Middle Devonian Stringocephalus brachiopods and Late Devonian Orecopia gastropods in the "Los Murcielagos Formation" in northwest Sonora represent the southwest-ernmost occurrence of these genera in North America and date the host rocks as Givetian and Frasnian, respectively. Rhynchonelloid brachiopods (Dzieduszyckia sonora) and associated worm tubes in the Los Pozos Formation of the Sonora allochthon in central Sonora are also found in strati-form-barite facies in the upper Upper Devonian (Famennian) part of the Slaven Chert in the Roberts Mountains allochthon (upper plate) of central and western Nevada. Although these brachiopods and worm tubes occur in similar depositional settings along the margin of Laurentia in Mexico, they occur in allochthons that exhibit different tectonic styles and times of emplacement. Thus, the allochthons containing the brachiopods and worm tubes in Sonora and Nevada are parts of separate orogenic belts and have different geographic settings and tectonic histories. Devonian facies belts and faunas in northern Mexico indicate a continuous continental shelf along the entire southern margin of Laurentia. These data, in addition to the continuity of the late Paleozoic Ouachita-Marathon-Sonora orogen across northern Mexico, contradict the early Late Jurassic Mojave-Sonora megashear as a viable hypothesis for large-magnitude offset (600-1100 km) of Proterozoic through Middle Jurassic rocks from California to Sonora.

Special Paper of the Geological Society of America↗
USGS2009

The glacial/deglacial history of sedimentation in Bear Lake, Utah and Idaho

Bear Lake, in northeastern Utah and southern Idaho, lies in a large valley formed by an active half-graben. Bear River, the largest river in the Great Basin, enters Bear Lake Valley ???15 km north of the lake. Two 4-m-long cores provide a lake sediment record extending back ???26 cal k.y. The penetrated section can be divided into a lower unit composed of quartz-rich clastic sediments and an upper unit composed largely of endogenic carbonate. Data from modern fluvial sediments provide the basis for interpreting changes in provenance of detrital material in the lake cores. Sediments from small streams draining elevated topography on the east and west sides of the lake are characterized by abundant dolomite, high magnetic susceptibility (MS) related to eolian magnetite, and low values of hard isothermal remanent magnetization (HIRM, indicative of hematite content). In contrast, sediments from the headwaters of the Bear River in the Uinta Mountains lack carbonate and have high HIRM and low MS. Sediments from lower reaches of the Bear River contain calcite but little dolomite and have low values of MS and HIRM. These contrasts in catchment properties allow interpretation of the following sequence from variations in properties of the lake sediment: (1) ca. 26 cal ka-onset of glaciation; (2) ca. 26-20 cal ka-quasicyclical, millennial-scale variations in the concentrations of hematite-rich glacial fl our derived from the Uinta Mountains, and dolomite- and magnetite-rich material derived from the local Bear Lake catchment (reflecting variations in glacial extent); (3) ca. 20-19 cal ka-maximum content of glacial fl our; (4) ca. 19-17 cal ka-constant content of Bear River sediment but declining content of glacial fl our from the Uinta Mountains; (5) ca. 17-15.5 cal ka-decline in Bear River sediment and increase in content of sediment from the local catchment; and (6) ca. 15.5-14.5 cal ka-increase in content of endogenic calcite at the expense of detrital material. The onset of glaciation indicated in the Bear Lake record postdates the initial rise of Lake Bonneville and roughly corresponds to the Stansbury shoreline. The lake record indicates that maximum glaciation occurred as Lake Bonneville reached its maximum extent ca. 20 cal ka and that deglaciation was under way while Lake Bonneville remained at its peak. The transition from siliciclastic to carbonate sedimentation probably indicates increasingly evaporative conditions and may coincide with the climatically driven fall of Lake Bonneville from the Provo shoreline. Although lake levels fluctuated during the Younger Dryas, the Bear Lake record for this period is more consistent with drier conditions, rather than cooler, moister conditions interpreted from many studies from western North America. Copyright ?? 2009 The Geological Society of America.

Special Paper of the Geological Society of America↗