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At least 19 recordsLinked to original sources
USGS2014

Stitching the western Piedmont of Virginia: Early Paleozoic tectonic history of the Ellisville Pluton and the Potomac and Chopawamsic Terranes

The theme of the 2014 Virginia Geological Field Conference is the tectonic development, economic geology, and seismicity of the western Piedmont of Louisa County, Virginia. It is timely for the conference to turn its attention here, for during the past decade these aspects of western Piedmont geology have garnered the renewed attention of researchers. In terms of regional tectonics, it has been hypothesized that the major structure in the region, the Chopawamsic fault system, represents the most significant boundary in the Appalachian orogen, the main Iapetan suture (Hibbard et al., 2014). Economically, recent elevated market values of metals— particularly that of gold—has spurred reconsideration of the economic geology of the western Piedmont. Finally, the August 23, 2011, M5.8 earthquake, with its epicenter in our field area, startled the North American east coast and has revived awareness of the seismic potential of the region. This renewed interest in the geology of the western Piedmont of north-central Virginia has led to new detailed bedrock mapping, detailed surficial mapping, high-resolution UPb TIMS zircon geochronology, U-Pb LA-ICPMS detrital zircon geochronology, radiogenic isotope geochemistry, major/minor/REE geochemistry, and geophysical studies (e.g. Bailey et al., 2005, 2008; Bailey and Owens, 2012: Berti et al., 2012; Burton et al., 2014; Burton, in progress; Harrison, 2012; Horton et al., 2010, in press; Hughes, 2010, 2014; Hughes et al., 2013a, 2013b, 2014, in press a, in press b; Malenda, in progress; Owens et al., 2013; Spears and Gilmer 2012; Spears et al. 2013, Terblanche, 2013; Terblanche and Nance, 2012). A host of institutions have taken part in the research, including North Carolina State University, the Virginia Department of Mines, Minerals, and Energy, the U.S. Geological Survey, Virginia Tech, Lehigh University, and the College of William and Mary. Many of these investigations remain active. The majority of the data presented herein is the product of research conducted from 2010 to 2014 by geologists at North Carolina State University. This field trip guide is intended to complement a Geological Society of America field guide (Hughes et al., 2014) that covers the western Piedmont geology along strike to the northeast in the vicinity of Fredericksburg. Geologic mapping and geochronologic and geochemical sampling were coordinated between these two areas as part of a study funded in part by the National Science Foundation and the USGS EDMAP program. Some of the stops in this guide have previously been written up in past field guides (Hughes, 2010; Burton et al., 2014) and are reused here because of their ease of access for large groups and because of new data that update the context and our understanding of the outcrops.

Virginia↗
USGS2005

Birth of the lower Colorado River–Stratigraphic and geomorphic evidence for its inception near the conjunction of Nevada, Arizona, and California

A detailed record of the late Cenozoic history of the lower Colorado River can be inferred from alluvial and (likely) lacustrine stratigraphy exposed in dissected alluvial basins below the mouth of the Grand Canyon. Numerous sites in Mohave, Cottonwood, and Detrital valleys contain stratigraphic records that directly bear on the mode, timing, and consequences of the river’s inception and integration in the latest Miocene–early Pliocene and its subsequent evolution through the Pleistocene. This field trip guide describes and illustrates many of these key stratigraphic relationships and, in particular, highlights evidence that supports the hypothesis of cascading lake-overflow as the principal formative mechanism of the river’s course downstream from the Grand Canyon.

Arizona, California, Nevada↗
USGS2022

Developing landslide chronologies using landslide-dammed lakes in the Oregon Coast Range

The Oregon Coast Range is a dynamic landscape that is continually shaped by shallow and deep-seated landslides that can have disastrous consequences to infrastructure and human lives. Searching for evidence of potentially coseismic mass wasting is incredibly difficult, particularly when historical observations are limited. Landslide-dammed lakes with submerged “ghost forests” in the Oregon Coast Range present the unique opportunity to establish landslide chronologies with subannual accuracy when dendrochronology is applied. This field guide will visit the unique landslide-dammed Klickitat Lake and explore a drowned ‘ghost forest’ to discuss methods used to establish a prehistoric landslide chronology in western Oregon, USA. After exploring the lake and exposing its geomorphic secrets, the guide will end with a stop on Marys Peak, a mafic volcanic intrusion composed of gabbroic dikes and pillow basalt that forms the highest point in the Oregon Coast Range. With the landscape of western Oregon laid out before us, we will discuss short- and long-term geomorphic evolution of the Oregon Coast Range and Willamette Valley.

Oregon↗
USGS2023

Geology and paleontology of Cretaceous and Paleocene sediments of the Cabin Branch, Cabin Creek (Cappy Avenue), and Tinkers Creek outcrops, Prince George’s County, Maryland

This field guide presents a one-day excursion in Prince George’s County, Maryland, USA, and documents the transition across the Cretaceous-Paleogene boundary by examining sediments from the upper Maastrichtian of the Severn Formation to the Paleocene sediments of the Brightseat and Aquia formations. Emphasis is placed on understanding how differences in depositional character and lithostratigraphy are related to changes in both microfossil and macrofossil assemblages. Particular attention is given to the difficulty in distinguishing Upper Cretaceous sediments from lower Paleocene sediments in the field, a problem that has traditionally led to misrepresentation of the distribution and thickness of these units and their correlation on a regional scale. Regarding the Late Cretaceous geology, the guide presents information on the lithology and microfossil biostratigraphy of the Severn Formation, which consists predominantly of silty quartz sand, with less than 5% clay. These sediments are placed in calcareous nannofossil Zone CC25a, suggesting an early late Maastrichtian age. Low abundances of planktic foraminifera combined with sedimentological evidence suggest deposition most likely occurred in a middle neritic environment. Macrofossils in the outcrops along the field trip consist primarily of fragmented bivalve mollusk and cephalopod shell material. A hiatus of ~5 m.y. separates the Cretaceous sediments from the overlying Paleocene deposits. As for the Paleocene geology, the guide presents information on the Brightseat and Aquia formations. The Brightseat represents early Danian age deposition and consists of clayey, silty sand at the base that grades upward into a silty sand. Glauconite is present at <5% throughout the formation in outcrop. Sediments of the Brightseat Formation are placed in calcareous nannofossil Zone NP3. Macrofossils are limited to small bivalve fragments that are scattered throughout. A hiatus representing ~3 m.y. separates the Brightseat from the overlying Aquia Formation, which is Selandian to Thanetian in age and consists of a glauconite-rich (~10%–20%), silty sand with common to abundant macrofossils, including both fragmented and complete gastropods and bivalves.

Maryland↗
USGS2016

Pinedale glacial history of the upper Arkansas River valley: New moraine chronologies, modeling results, and geologic mapping

This field-trip guide outlines the glacial history of the upper Arkansas River valley, Colorado, and builds on a previous GSA field trip to the area in 2010. The following will be presented: (1) new cosmogenic 10 Be exposure ages of moraine boulders from the Pinedale and Bull Lake glaciations (Marine Isotope Stages 2 and 6, respectively) located adjacent to the Twin Lakes Reservoir, (2) numerical modeling of glaciers during the Pinedale glaciation in major tributaries draining into the upper Arkansas River, (3) discharge estimates for glacial-lake outburst floods in the upper Arkansas River valley, and (4) 10 Be ages on flood boulders deposited downvalley from the moraine sequences. This research was stimulated by a new geologic map of the Granite 7.5′ quadrangle, in which the mapping of surficial deposits was revised based in part on the interpretation of newly acquired LiDAR data and field investigations. The new 10 Be ages of the Pinedale terminal moraine at Twin Lakes average 21.8 ± 0.7 ka ( n = 14), which adds to nearby Pinedale terminal moraine ages of 23.6 ± 1.4 ka ( n = 5), 20.5 ± 0.2 ka ( n = 3), and 16.6 ± 1.0 ka ( n = 7), and downvalley outburst flood terraces that date to 20.9 ± 0.9 ka ( n = 4) and 19.0 ± 0.6 ka ( n = 4). This growing chronology leads to improved understanding of the controls and timing of glaciation in the western United States, the modeling of glacial-lake outburst flooding, and the reconstruction of paleotemperature through glacier modeling.

Arkansas River valley↗
USGS2005

Evolution of a late Cenozoic supradetachment basin above a flat-on-flat detachment with a folded lateral ramp, SE Idaho

Uplift and exposure of the Bannock detachment system and the synextensional basin fill deposits of the Salt Lake Formation provide a unique exposure of the three-dimensional geometries of a low-angle normal fault system and the stratal architecture of the overlying supradetachment basin. Within this system, structural and stratigraphic analyses, outcrop patterns, tephra geochronology, and geological cross sections document several important relationships: (1) the Bannock detachment system developed above the Sevierage Cache-Pocatello culmination and resembles the Sevier Desert detachment in its geometry, structural setting, and kinematic evolution; (2) the Bannock detachment system initiated and slipped at low angles; (3) flat-on-flat, ramp-flat, and lateral ramp geometries, as well as excision, can significantly affect the hanging wall deformation style due to the shallow depth (~2–4 km) of the Bannock detachment fault during late stages of slip; (4) late Miocene–Pliocene tuffaceous synrift deposits of the Salt Lake Formation record deposition in a supradetachment basin, display an unroofing sequence, and a three-stage evolution that includes pre-translation, translation, and breakup phases. Recycled pre-translation and translation phase deposits are diagnostic of this evolution; and (5) beginning in mid- to late Pliocene time, high-angle, north-striking Basin and Range faults disrupted and dismembered the Bannock detachment system.

Idaho↗
USGS2013

Making the case for the Picuris orogeny: Evidence for a 1500 to 1400 Ma orogenic event in the southwestern United States

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.

Geological Society of America Field Guides↗
USGS2017

Geology of Seattle, a field trip

Seattle’s geologic record begins with Eocene deposition of fluvial arkosic sandstone and associated volcanic rocks of the Puget Group, perhaps during a time of regional strike-slip faulting, followed by late Eocene and Oligocene marine deposition of the Blakeley Formation in the Cascadia forearc. Older Quaternary deposits are locally exposed. Most of the city is underlain by up to 100 m of glacial drift deposited during the Vashon stade of Fraser glaciation, 18–15 cal k.y. B.P. Vashon Drift includes lacustrine clay and silt of the Lawton Clay, lacustrine and fluvial sand of the Esperance Sand, and concrete-like Vashon till. Mappable till is absent over much of the area of the Vashon Drift. Peak local ice thickness was 900 m. Isostatic response to this brief ice loading was significant. Upon deglaciation, global ice-equivalent sea level was about −100 m and local RSL (relative sea level) was 15–20 m, suggesting a total isostatic depression of ~115–120 m at Seattle. Subsequent rapid rebound outstripped global sea-level rise to result in a newly recognized marine low-stand shoreline at −50 m. The Seattle fault is a north-verging thrust or reverse fault with ~7.5 km of throw. Conglomeratic Miocene strata may record initiation of shortening. Field relations indicate that fault geometry has evolved through three phases. At present, the north-verging master fault is blind, whereas several surface-rupturing faults above the master fault are south verging. The 900–930 A.D. Restoration Point earthquake raised a 5 km × 35 km (or larger) area as much as 7 m. The marine low-stand shoreline is offset by a similar amount, thus there has been only one such earthquake in the last ~11 k.y. Geomorphology is largely glacial: an outwash plain decorated with ice-molded flutes and large, anastomosing tunnel valleys carved by water flowing beneath the ice sheet. Euro-Americans initially settled here because of landscape features formed by uplift in the Restoration Point earthquake. But steep slopes and tide flats were not conducive to commerce: starting in the 1890s and ending in the 1920s, extensive regrading removed hills, decreased slopes, and filled low areas. In steep slopes the glacial stratigraphy is prone to landslides when saturated by unusually wet winters. Seismic hazards comprise moderately large (M 7) earthquakes in the Benioff zone 50 km and more beneath the city, demi-millennial M 9 events on the subduction zone to the west, and infrequent local crustal earthquakes (M 7) that are likely to be devastating because of their proximity. Seismic shaking and consequent liquefaction are of particular concern in Pioneer Square, SoDo, and lower Duwamish neighborhoods, which are largely built on unengineered fill that was placed over estuarine mud. Debris from past Mount Rainier lahars has reached the lower Duwamish valley and a future large lahar could pose a sedimentation hazard.

Washington↗
USGS2008

Cenozoic evolution of the abrupt Colorado Plateau–Basin and Range boundary, northwest Arizona: A tale of three basins, immense lacustrine-evaporite deposits, and the nascent Colorado River

In northwest Arizona, the relatively unextended Colorado Plateau gives way abruptly to the highly extended Colorado River extensional corridor within the Basin and Range province along a system of major west-dipping normal faults, including the Grand Wash fault zone and South Virgin–White Hills detachment fault. Large growth-fault basins developed in the hanging walls of these faults. Lowering of base level in the corridor facilitated development of the Colorado River and Grand Canyon. This trip explores stratigraphic constraints on the timing of deformation and paleogeographic evolution of the region. Highlights include growth-fault relations that constrain the timing of structural demarcation between the Colorado Plateau and Basin and Range, major fault zones, synextensional megabreccia deposits, nonmarine carbonate and halite deposits that immediately predate arrival of the Colorado River, and a basalt flow interbedded with Colorado River sediments. Structural and stratigraphic relations indicate that the current physiography of the Colorado Plateau–Basin and Range boundary in northwest Arizona began developing ca. 16 Ma, was essentially established by 13 Ma, and has changed little since ca. 8 Ma. The antiquity and abruptness of this boundary, as well as the stratigraphic record, suggest significant headward erosion into the high-standing plateau in middle Miocene time. Thick late Miocene evaporite and lacustrine deposits indicate that a long period of internal drainage followed the onset of extension. The widespread distribution of such deposits may signify, however, a large influx of surface waters and/or groundwater from the Colorado Plateau possibly from a precursor to the Colorado River. Stratigraphic relations bracket arrival of a through-flowing Colorado River between 5.6 and 4.4 Ma.

Arizona, California, Nevada↗
USGS2002

Field guide to geologic excursions in southwestern Utah and adjacent areas of Arizona and Nevada

This field guide contains road logs for field trips planned in conjunction with the 2002 Rocky Mountain Section meeting of the Geological Society of America held at Southern Utah University in Cedar City, Utah. There are a total of eight field trips, covering various locations and topics in southwestern Utah and adjacent areas of Arizona and Nevada. In addition, the field guide contains a road log for a set of Geological Engineering Field Camp Exercises run annually by the University of Missouri at Rolla in and around Cedar City. Two of the field trips address structural aspects of the geology in southwestern Utah and northwestern Arizona; two trips deal with ground water in the region; and along with the Field Camp Exercises, one trip, to the Grand Staircase, is designed specifically for educators. The remaining trips examine the volcanology and mineral resources of a large area in and around the Tusher Mountains in Utah; marine and brackish water strata in the Grand Staircase-Escalante National Monument; and the Pine Valley Mountains, which are cored by what may be the largest known laccolith in the world. The "Three Corners" area of Utah, Arizona, and Nevada is home to truly world-class geology, and I am confident that all of the 2002 Rocky Mountain Section meeting attendees will find a field trip suited to their interests.

Arizona, Nevada, Utah↗
USGS2019

Appalachian Basin stratigraphy, tectonics, and eustasy from the Blue Ridge to the Allegheny Front, Virginia and West Virginia

This guide is from a two-day field trip in western Virginia and eastern West Virginia held before the 2015 Geological Society of America annual meeting in Baltimore, Maryland. The field trip examines exposures of Paleozoic sedimentary strata in the Appalachian Basin starting in the Blue Ridge physiographic province, going through the Valley and Ridge physiographic province, and ending in the Appalachian Plateau physiographic province. Most of the field-trip stops are along US 48 (Corridor H) in West Virginia.

Virginia, West Virginia↗
USGS2006

Metallogeny of the Great Basin: Crustal evolution, fluid flow, and ore deposits

The Great Basin physiographic province in the Western United States contains a diverse assortment of world-class ore deposits. It currently (2006) is the world’s second leading producer of gold, contains large silver and base metal (Cu, Zn, Pb, Mo, W) deposits, a variety of other important metallic (Fe, Ni, Be, REE’s, Hg, PGE) and industrial mineral (diatomite, barite, perlite, kaolinite, gallium) resources, as well as petroleum and geothermal energy resources. Ore deposits are most numerous and largest in size in linear mineral belts with complex geology. U.S. Geological Survey (USGS) scientists are in the final year of a research project initiated in the fall of 2001 to increase understanding of relations between crustal evolution, fluid flow, and ore deposits in the Great Basin. Because of its substantial past and current mineral production, this region has been the focus of numerous investigations over the past century and is the site of ongoing research by industry, academia, and state agencies. A variety of geoinformatic tools was used to organize, reinterpret, and display, in space and time, the large amounts of geologic, geophysical, geochemical, and hydrologic information deemed pertinent to this problem. This information, in combination with concentrated research on (1) critical aspects of the geologic history, (2) an area in northern Nevada that encompasses the major mineral belts, and (3) important mining districts and deposits, is producing new insights about the interplay between key tectonic events, hydrothermal fluid flow, and ore genesis in mineral belts. The results suggest that the Archean to Holocene history of the Great Basin was punctuated by several tectonic events that caused fluids of different origins (sea water, basinal brine, meteoric water, metamorphic water, magmatic water) to move through the crust. Basement faults reactivated during these events localized deformation, sedimentation, magmatism, and hydrothermal fluid flow in overlying rocks to form mineral belts that contain ore deposits of different types and ages that are locally superimposed (demonstrating inheritance). Fluid flow in these systems also was influenced by the distribution of permeable lithologies and paleotopographic highs and lows. Hydrothermal fluids evolved from their initial chemistries towards compositions that reflect the ƒ O 2 and ƒ S 2 buffering capacity of, and the ligands and metals present in, the rocks (±older mineralization) through which they moved. In northern Nevada, where gold deposits are relatively common, carbonaceous, pyritic strata buffered fluids of diverse origins to H 2 S-rich compositions so they could transport gold repeatedly over Paleozoic-Cenozoic time (convergent evolution). Ore formed where metal-laden fluids encountered effective physicochemical traps. Maps of Neogene basin fill and erosion surfaces identify areas where preexisting ore deposits have been progressively exposed or concealed. Comparisons with analogous terrains and deposit types in other parts of the world provide global context. The initial findings and some of the databases, geologic maps, sections, reconstructions, hydrogeologic models, topical syntheses, regional overviews, short courses, field guides, and deposit comparisons produced by project staff and associated managers, contractors, and collaborators have been presented in numerous abstracts, symposia, USGS publications, and professional journals over the last 5 years (see the extensive bibliography). Notable among these was the 2005 Geological Society of Nevada symposium in Reno, Nevada, and the 2005 Geological Society of America annual meeting in Salt Lake City, Utah, where project results were presented to audiences from around the nation and world. The final results of the project will be submitted for publication in 2007 to appropriate USGS and professional journals. A special issue of GEOSPHERE, scheduled for publication in 2007, will be devoted to the results of this project and related work. This special issue will reach an international audience and be available worldwide on the internet. Much of the research for this project has concentrated on areas that will receive the focused attention of the mining industry in the future. As such, the data and interpretations generated by this project have direct use for land-use managers in Federal, State, and local agencies. Improved hydrogeologic models developed by this project will considerably enhance ongoing and future water resource investigations in the region. The increased understanding of when, where, and how hydrothermal systems produce significant economic deposits has direct uses for mineral exploration and for future USGS mineral resource assessments in the Great Basin and other parts of the world.

Great Basin↗
USGS1978

Biostratigraphic guide to Upper Devonian and Mississippian rocks along the Wasatch Front and Cordilleran hingeline, Utah

This paper provides a guide to measured sections of Upper Devonian (Famennian) and Mississippian (Kinderhookian, Osagean, and Meramecian) rocks visited on a three-day field trip between Provo, Utah, on the Wasatch Front and the Crawford Mountains in the Overthrust Belt along the Utah-Wyoming State line. The field trip, sponsored by the Pander Society, was held on April 30-May 2, 1978 in conjunction with the meeting of the Rocky Mountain section of the Geological Society of America at Provo, Utah. The rocks that may be sampled for conodonts are represented by the Middle to Upper Scaphignathus velifer (middle Famennian) through Cavusgnathus (middle Meramecian) conodont Zones. The measured sections and field trip provide a west to east traverse of the Sevier thrust system. The sections demonstrate the biostratigraphic, paleogeographic, and sedimentational differences among equivalent Famennian rocks as well as the transition from the Osagean and early Meramecian starved phosphatic basin on the west to time-equivalent carbonate rocks of the shelf margin on the east.

Utah↗
USGS2006

Field-trip guide to volcanic and volcaniclastic deposits of the lower Jurassic Talkeetna formation, Sheep Mountain, south-central Alaska

This guide provides information for a one-day field trip in the vicinity of Sheep Mountain, just north of the Glenn Highway in south-central Alaska. The Lower Jurassic Talkeetna Formation, consisting of extrusive volcanic and volcaniclastic sedimentary rocks of the Talkeetna arc complex, is exposed on and near Sheep Mountain. Field-trip stops within short walking distance of the Glenn Highway (approximately two hours’ drive from Anchorage) are described, which will be visited during the Geological Society of America Penrose meeting entitled Crustal Genesis and Evolution: Focus on Arc Lower Crust and Shallow Mantle , held in Valdez, Alaska, in July 2006. Several additional exposures of the Talkeetna Formation on other parts of Sheep Mountain that would need to be accessed with longer and more strenuous walking or by helicopter are also mentioned.

Alaska↗
USGS2025

A scientific vision and roadmap for earthquake rupture forecast developments, a USGS perspective

We articulate a scientific vision and roadmap for the development of improved Earthquake Rupture Forecast models, which are one of the two main modeling components used in modern seismic hazard and risk analysis. One primary future objective is to provide fully time-dependent models that include both elastic rebound and spatiotemporal clustering nationwide, which is particularly important for shorter-term hazard and risk considerations (e.g., earthquake insurance products). We also discuss the importance and perennial challenges associated with quantifying epistemic uncertainties, including those associated with deformation-model slip rates, un-quantified sampling errors with respect to off-fault seismicity, and any spatial covariances. The need for more physics-based approaches is also emphasized, as is the benefit of adding model valuation (quantifying usefulness) to our verification and validation protocols. Given the multidisciplinary and system-level nature of this activity, modular design is critical. Future updates will also draw from best-available science by both the United States Geological Survey and the external community. The primary goal of this paper is to highlight plans that guide research and facilitate community engagement with model development, especially with respect to lowering the entry barrier for early career scientists and engineers. The paper is written so readers can focus on the sections that interest them most (see table of contents), with the Introduction and Discussion providing a stand-alone overview and summary.

Bulletin of the Seismological Society of America↗
USGS2016

Continuity of the West Napa–Franklin fault zone inferred from guided waves generated by earthquakes following the 24 August 2014 Mw 6.0 South Napa Earthquake

We measure peak ground velocities from fault‐zone guided waves (FZGWs), generated by on‐fault earthquakes associated with the 24 August 2014 M w 6.0 South Napa earthquake. The data were recorded on three arrays deployed across north and south of the 2014 surface rupture. The observed FZGWs indicate that the West Napa fault zone (WNFZ) and the Franklin fault (FF) are continuous in the subsurface for at least 75 km. Previously published potential‐field data indicate that the WNFZ extends northward to the Maacama fault (MF), and previous geologic mapping indicates that the FF extends southward to the Calaveras fault (CF); this suggests a total length of at least 110 km for the WNFZ–FF. Because the WNFZ–FF appears contiguous with the MF and CF, these faults apparently form a continuous Calaveras–Franklin–WNFZ–Maacama (CFWM) fault that is second only in length (∼300 km) to the San Andreas fault in the San Francisco Bay area. The long distances over which we observe FZGWs, coupled with their high amplitudes (2–10 times the S waves) suggest that strong shaking from large earthquakes on any part of the CFWM fault may cause far‐field amplified fault‐zone shaking. We interpret guided waves and seismicity cross sections to indicate multiple upper crustal splays of the WNFZ–FF, including a northward extension of the Southhampton fault, which may cause strong shaking in the Napa Valley and the Vallejo area. Based on travel times from each earthquake to each recording array, we estimate average P ‐, S ‐, and guided‐wave velocities within the WNFZ–FF (4.8–5.7, 2.2–3.2, and 1.1–2.8 km/s, respectively), with FZGW velocities ranging from 58% to 93% of the average S ‐wave velocities.

California↗
USGS2015

Sixth International Limnogeology Congress: field trip guidebook, Reno, Nevada, June 15-19, 2015

Limnogeology is the study of modern lakes and lake deposits in the geologic record. Limnogeologists have been active since the 1800s, but interest in Limnogeology became prevalent in the early 1990s when it became clear that lake deposits contain continental environmental and climate records. A society that is focused on Limnogeology would allow greater communication and access to research on these important subjects and contribute to providing sound science used to understand rapid global changes in our modern world; thus the International Association of Limnogeology was founded in 1995 at the first International Limnogeology Congress (ILIC) held in Copenhagen, Denmark. The Sixth International Limnogeology Congress (ILIC6) was held in Reno, Nevada, from June 15&ndash;19, 2015. The ILIC meetings have been held every 4 years since the first meeting in1995 and were subsequently convened in Brest, France (1999), Tucson, USA (2003), Barcelona, Spain (2007), and Konstanz, Germany (2011). The Congress in Reno, USA marks the second time the Congress has been held in the United States and more than 150 scientists from every part of the world participated. As part of the Congress, ILIC6 included pre- and post- Congress field trips, the descriptions of which are included as separate trips in this Open-File Report. Trip 1 provides information on the pluvial and post-glacial Lakes of the eastern Great Basin, led by Paul Jewell, University of Utah, Ben Laabs, State University of New York-Geneseo, Jeff Munroe, Middlebury College, and Jack Oviatt, Kansas State University. Trip 2 contains information on the lake sequences of closed-basin lakes in the Eocene Green River Formation in Wyoming, led by Michael Smith, Northern Arizona University and Jennifer Scott, Mount Royal University. Trip 3 provides the background for the field trip to Pleistocene and modern lakes in the Great Basin of North America that was led by Susan Zimmerman, Lawrence Livermore National Laboratory, Ken Adams, Desert Research Institute, and Michael Rosen, U.S. Geological Survey. Trip 4 contains the information for a trip to the modern lakes in Lassen National Park that was led by Paula Noble and Kerry Howard, both from the University of Nevada, Reno. The U.S. Geological Survey has sponsored each ILIC that has been held in the United States because of the importance of understanding paleoclimate and contaminant histories of lakes, two main themes of the Congress. This field trip guide provides a permanent record of some of the wide variety of studies that are being conducted in modern lakes and ancient lake deposits in western North America, and it provides a starting point for any one desiring to visit these exceptional sites or begin work in these areas.

California, Nevada, Utah, Wyoming↗