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At least 991 records · Page 55Linked to original sources
USGS2019

Sediment classification and the characterization, identification, and mapping of geologic substrates for the glaciated Gulf of Maine seabed and other terrains, providing a physical framework for ecological research and seabed management

A geologic substrate is a surface (or volume) of sediment or rock where physical, chemical, and biological processes occur, such as the movement and deposition of sediment, the formation of bedforms, and the attachment, burrowing, feeding, reproduction, and sheltering of organisms. Seabed mapping surveys in the Stellwagen Bank region off Boston, Massachusetts, from 1993 to 2004 have led to the development of a methodology for characterizing, identifying, and mapping geologic substrates. The resulting high-resolution interpretive maps (1:25,000) show the distribution of substrates in a glaciated terrain of banks and basins in water depths of 30 to 185 meters. Data sources used to characterize substrates are multibeam sonar bathymetric and backscatter imagery to document seabed topography and patterns of sediment and rock distribution, grain-size analyses of sediment samples to determine substrate composition, and video and photographic imagery of the seabed to aid in the interpretation of multibeam sonar imagery and to provide information on substrate layering and mobility, seabed structures, and sediments and nonsediment materials that cannot be physically sampled. Sediment composition is a major property of many seabed substrates. Sediment grains belong to a continuum of grain-diameter sizes previously classified into grades (for example, fine sand, medium sand) and into aggregates (mud, sand, gravel). The definition of grade and aggregate boundaries in a classification is arbitrary, and a useful classification is limited to as few classes as are needed to effectively organize and apply information. For the purpose of mapping substrates, sediment grades and aggregates were simplified and re-classified into eight composite grades based on grain-size content, mode of transport, and ecological role. Five composite grades are identified using grain-size analysis and three are identified using video and photographic imagery of the seabed. Naturally occurring sediments contain various amounts of the aggregates mud, sand, and gravel. The separation of naturally occurring sediments into sediment classes, based on grain-size analysis, requires that limits be set on the amount of mud, sand, and gravel each class contains. Fifteen previously identified basic sediment classes provided interpretive information on sediment transport by emphasizing gravel content (a low 0.01-weight-percent threshold) and on winnowing processes based on the sand-to-mud ratio. The present study recognizes 20 basic sediment classes that are combinations of aggregates in which the lower limits for recognition of mud and sand are 10 weight percent and of gravel, 25 weight percent. These sediment classes can be made more specific by listing their content of the composite grades fine-grained sand (3 and 4 phi), which is transported in suspension, and coarse-grained sand (0, 1, and 2 phi), which is transported as bedload. Additional sediment classes and nonsediment classes that cannot be sampled are recognized on the basis of visual analysis of seabed video and photographic imagery and include pebble, cobble, and boulder gravel, rock outcrops, and shell beds, among others. Substrates are not classified because their properties are too varied for a classification to be concise and useful. Rather, substrates are characterized and identified by sediment grain-size composition (the sediment class); the distribution, in millimeters, of grain diameters in the sediment; the presence of nonsediments (for example, rock outcrops); substrate mobility based on the presence of sediment ripples; substrate layering (for example, a partial veneer of sand on gravel); and seabed structures. These properties have interpretive value by providing information about sedimentary processes acting on a substrate and about its ecological function. A geologic substrate, when it is associated with one or more species, is an important element of a habitat. This methodology was developed to map a glaciated terrain characterized by geologic substrates that typify a wide range of erosional and depositional sedimentary environments, and it likely will be useful for mapping substrates in other terrains. Substrate maps provide the physical framework required for identifying sediment transport processes, validating sediment transport models, studying the ecology of species and communities, and managing marine resources and seabed usage.

Massachusetts↗
USGS2004

Water flow and nutrient flux from five estuarine rivers along the southwest coast of the Everglades National Park, Florida, 1997-2001

Discharge and nutrient fluxes for five tidally affected streams were monitored and evaluated as a part of the U.S. Geological Survey Place-Based Studies Initiative and the U.S. Department of the Interior Critical Ecosystem Studies Initiative. Locations on Lostmans Creek, and Broad, Harney, Shark, and North Rivers were selected using the criterion that a large amount of the water that flows through Shark River Slough must pass these sites. Discharge and nutrient-concentration data collection started at the Broad, Harney, and Shark River stations in January 1997 and ended in early 2001. Discharge and nutrient-concentration data collection started at the Lostmans Creek and North River stations in April 1999 and ended in early 2001. Each station was equipped with a vertically oriented acoustic-velocity sensor, water-level pressure transducer, bottom water-temperature thermistor, and specific conductance four-electrode sensor. Data collected using a vessel-mounted acoustic discharge measurement system were used to calibrate regression models of the mean river velocities and the in-situ index velocities. Information from these stations, in conjunction with data from other ongoing studies, will help to determine environmental effects on the southwest coast estuaries as changes in water management of the Everglades National Park continue. Discharges from the Lostmans Creek, and Broad, Harney, Shark, and North River stations are influenced by semidiurnal tides, meteorological events, and surface- and ground-water inflow. Each of the five rivers is usually well mixed, having no greater than 500 microSiemens per centimeter at 25? Celsius difference in specific conductance from top to bottom during flood and ebb tides. Instantaneous flood discharges (water moving upstream) are typically of greater magnitude and shorter duration than instantaneous ebb discharges (water moving downstream). Instantaneous discharge data were filtered using a low-pass filter to remove predominant tidal frequencies, and the filtered data were used to compute daily mean and monthly mean residual discharges. Lostmans Creek, and Broad, Harney and Shark Rivers each contributed from 20 to 27 percent of the total measured discharge to the Gulf of Mexico, whereas North River contributed approximately 4 percent. The main discharge region of the Shark River Slough extends from as far north as Lostmans Creek to as far south as North River. North River discharge has similar response characteristics to the other four rivers measured, but with a lesser magnitude of discharge. Comparisons of monthly mean discharges from the Tamiami Canal flow control structures S-12-A, B, C, and D located on U.S. Highway 41 (Tamiami Trail) to the five station total monthly mean discharges indicate that the discharges from the five rivers are approximately 2 to 3 times the S-12-A, B, C, D discharges, and that the measured southwest coast discharge peaks lead the S-12-A, B, C, D discharge peaks by approximately 1 month. Residual total nitrogen and total phosphorus fluxes were estimated using linear regression models of discharge and flux. Monthly mean total nitrogen residual fluxes for the five southwest coast rivers ranged from approximately 0 to 390 short tons, whereas monthly mean total phosphorus residual fluxes ranged from approximately 0 to 6 short tons. Total nitrogen and total phosphorus residual fluxes at Lostmans Creek, and Broad, Harney, and Shark Rivers were similar in magnitude, each accounting for between 20 to 29 percent of the total measured residual flux. North River contributed between 3 to 4 percent of the total nitrogen and total phosphorus residual flux from the five rivers.

Florida↗
USGS2002

Water quality in the Little Sac River basin near Springfield, Missouri, 1999-2001

The Little Sac River, north of Springfield, Missouri, flows through mainly agricultural and forest land. However, the quality of the river water is a concern because the river flows into Stockton Lake, which is a supplemental drinking water source for Springfield. Large bacterial densities and nutrient concentrations are primary concerns to the water quality of the river. A 29-river mile reach of the Little Sac River is on the 1998 list of waters of Missouri designated under section 303(d) of the Federal Clean Water Act because of fecal coliform densities larger than the Missouri Department of Natural Resources standard (hereinafter referred to as Missouri standard) of 200 colonies per 100 milliliters for whole-body contact recreation. During an investigation of the water quality in the Little Sac River by the U.S. Geological Survey, in cooperation with the Watershed Committee of the Ozarks, fecal coliform bacteria densities exceeded the Missouri standard (the standard applies from April 1 through October 31) in one sample from a site near Walnut Grove. At other sites on the Little Sac River, the Missouri standard was exceeded in two samples and equalled in one sample upstream from the Northwest Wastewater Treatment Plant (NW WTP) and in one sample immediately downstream from the NW WTP. Effluent from the NW WTP flows into the Little Sac River. Annually from April 1 through October 31, the effluent is disinfected to meet the Missouri standard for whole-body contact recreation. Fecal coliform bacteria densities in samples collected during this period generally were less than 100 colonies per 100 milliliters. For the rest of the year when the effluent was not disinfected, the bacteria densities in samples ranged from 50 (sample collected on November 1, 2000) to 10,100 colonies per 100 milliliters (both counts were non-ideal). When the effluent was disinfected and the fecal coliform bacteria density was small, samples from sites upstream and downstream from the NW WTP had a bacteria density larger than the density in the effluent. Other sources of bacteria are likely to be present in the study area in addition to the NW WTP. These potential sources include effluent from domestic septic systems and animal wastes. Nutrient concentrations in the Little Sac River immediately downstream from the NW WTP were affected by effluent from the NW WTP and possibly other sources. At two sites upstream from the NW WTP, median nitrite plus nitrate concentrations were 1.1 and 1.4 milligrams per liter. The median nitrite plus nitrate concentration for the effluent from the NW WTP was 6.4 milligrams per liter, and the median concentration decreased downstream in the Little Sac River to 2.2, 1.2, and 0.56 milligrams per liter. The effects of the effluent from the NW WTP on the water quality of the Little Sac River downstream from the NW WTP were reflected in an increase in discharge (effluent from the NW WTP can be as much as 50 percent of the flow at the site about 1.5 river miles downstream from the NW WTP), an increase in specific conductance values, an increase in several inorganic constituent concentrations, including calcium, magnesium, and sulfate, and a large increase in sodium and chloride concentrations. The effluent from the NW WTP seemed to have no effect on the pH value, temperature, and dissolved oxygen concentrations in the Little Sac River. Results of repetitive element polymerase chain reaction (rep-PCR) pattern analysis indicated that most Escherichia coli ( E. coli ) bacteria in water samples probably were from nonhuman sources, such as horses and cattle. The rep-PCR pattern analysis indicated that horses were an important source of E. coli downstream from the NW WTP, which was consistent with horses pastured adjacent to the sampling site. Fecal coliform bacteria loads increased upstream from the NW WTP from the most upstream site to the site immediately upstream from the NW WTP. Loads in the effluent from the NW WTP and also those in the Little Sac River downstream from the NW WTP were dependent on the treatment of the effluent. When the effluent was not disinfected, the loads in the effluent increased from those upstream. Downstream in the Little Sac River, the loads decreased, but then increased at the most downstream site. The increase may be a result of increased loads from tributaries and other sources not sampled during the study. When the effluent was disinfected, fecal coliform bacteria loads were less than loads in samples from the Little Sac River downstream from the NW WTP.

Water-Resources Investigations Report↗
USGS2004

Migration stopover ecology of western avian populations: A southwestern migration workshop

The importance of migration stopover sites in ensuring that migratory birds successfully accomplish their journeys between breeding and non-breeding ranges has come to the forefront of avian research. Migratory birds that breed in western United States (US) and Canada and overwinter primarily in western Mexico migrate across the arid region of northern Mexico and southwestern US. Many of these migrants use lowland riparian stopover habitats, which comprise less than 0.1% of the western U.S. landscape. These habitats represent a significant conservation priority. Recognizing the importance of migration stopover habitats in the arid southwest, the U.S. Fish and Wildlife Service (USFWS) Region 6 partnered with the U.S. Geological Survey (USGS) to support a project---“Migration stopover ecology of western avian populations: patterns of geographic and habitat distribution.” A primary objective of the project was to convene a workshop for avian researchers, conservation professionals, and land managers involved in stopover needs of migratory birds that breed in western North America. The workshop included presentations on our current state of knowledge regarding passerine migration in western North America, techniques and technologies potentially useful in researching migration, and efforts that agencies and other partners are conducting within the realm of migration. Workshop presentations provided a backdrop for subsequent discussions, the goals of which were to identify research needs and initiate a coordinated approach to research of western migration stopover ecology. Workshop presentations spanned a wide range of concerns and interests. Highlights included indications that mid- and high-elevation riparian and montane shrubland habitats may be as crucial to western migrants in fall migration as lowland riparian habitats are in spring migration. Comparisons of eastern versus western migration systems elucidated large differences in stopover habitats used and the intensity with which certain types are used, underscoring the potential need to develop separate management approaches for eastern and western stopover sites. Presentations on techniques and technology for migration research revealed that rate of lipid deposition can serve as an indicator of habitat quality; that genetics and stable isotope analyses of feathers can be valuable tools to elucidate linkages between breeding and wintering areas; that radar imagery can be used to track large-scale movement patterns and habitat use; and that there are analytical options for combining multiple sources of information. Other presentations focused on partnership perspectives (USFWS and Sonoran Joint Venture), the genesis of a western migration monitoring network, premises of Coordinated Bird Monitoring, and how collaborative efforts could benefit migration research (e.g., combined bird and bat migration studies; linking avian researchers with fluvial geomorphologists; linking research throughout western North America; linking surveys and banding). Priority research needs and questions identified during the open discussions fell into three main categories: (1) habitat/landscape/climate relationships, (2) en route bird distribution patterns, and (3) general migration ecology. Tasks within these categories included: define the relative importance of various habitat types to migrants in spring and fall, determine what distinguishes high- from poor-quality stopover habitat; determine geographic patterns of loss in stopover habitats; model landscape attributes associated with species richness and abundance; identify effects of climate change and current climate anomalies on plant phonologies, associated insect flushes, and timing of migration; and determine effects of hydrologic changes on riparian vegetation, food availability, and stopover habitat quality. Workshop participants discussed a coordinated approach for addressing immediate research needs regarding migration patterns and crucial stopover sites and types. They envisioned a three­-tiered, coordinated approach: (1) long-term research to address effects of climate change and other large-scale patterns, (2) intensive, short-term survey and monitoring efforts using a stratified random design within habitats of interest to elucidate regional patterns of distribution and habitat use, and (3) research conducted at existing survey and banding sites to address more in-depth questions (e.g., rates of lipid deposition, microhabitat use, isotope analyses). There was considerable interest in developing common research proposals to blend the broad expertise represented at this workshop. A second meeting is recommended to build on the momentum of these discussions, to facilitate collaborations, and further the goals of integrated approaches to broadscale research on migration stopover ecology.

Open-File Report↗
USGS2008

Description of Existing Data for Integrated Landscape Monitoring in the Puget Sound Basin, Washington

This report summarizes existing geospatial data and monitoring programs for the Puget Sound Basin in northwestern Washington. This information was assembled as a preliminary data-development task for the U.S. Geological Survey (USGS) Puget Sound Integrated Landscape Monitoring (PSILM) pilot project. The PSILM project seeks to support natural resource decision-making by developing a 'whole system' approach that links ecological processes at the landscape level to the local level (Benjamin and others, 2008). Part of this effort will include building the capacity to provide cumulative information about impacts that cross jurisdictional and regulatory boundaries, such as cumulative effects of land-cover change and shoreline modification, or region-wide responses to climate change. The PSILM project study area is defined as the 23 HUC-8 (hydrologic unit code) catchments that comprise the watersheds that drain into Puget Sound and their near-shore environments. The study area includes 13 counties and more than four million people. One goal of the PSILM geospatial database is to integrate spatial data collected at multiple scales across the Puget Sound Basin marine and terrestrial landscape. The PSILM work plan specifies an iterative process that alternates between tasks associated with data development and tasks associated with research or strategy development. For example, an initial work-plan goal was to delineate the study area boundary. Geospatial data required to address this task included data from ecological regions, watersheds, jurisdictions, and other boundaries. This assemblage of data provided the basis for identifying larger research issues and delineating the study-area boundary based on these research needs. Once the study-area boundary was agreed upon, the next iteration between data development and research activities was guided by questions about data availability, data extent, data abundance, and data types. This report is not intended as an exhaustive compilation of all available geospatial data, rather, it is a collection of information about geospatial data that can be used to help answer the suite of questions posed after the study-area boundary was defined. This information will also be useful to the PSILM team for future project tasks, such as assessing monitoring gaps, exploring monitoring-design strategies, identifying and deriving landscape indicators and metrics, and visual geographic communication. The two main geospatial data types referenced in this report - base-reference layers and monitoring data - originated from numerous and varied sources. In addition to collecting information and metadata about the base-reference layers, the data also were collected for project needs, such as developing maps for visual communication among team members and with outside groups. In contrast, only information about the data was typically required for the monitoring data. The information on base-reference layers and monitoring data included in this report is only as detailed as what was readily available from the sources themselves. Although this report may appear to lack consistency between data records, the varying degree of details contained in this report are merely a reflection of varying source detail. This compilation is just a beginning. All data listed also are being catalogued in spreadsheets and knowledge-management systems. Our efforts are continual as we develop a geospatial catalog for the PSILM pilot project.

Open-File Report↗
USGS2016

Environmental conditions in the Namskaket Marsh Area, Orleans, Massachusetts: A summary of studies by the U.S. Geological Survey, 1989–2011

Namskaket Marsh and its tidal creek system are potential receptors for a treated wastewater plume originating from a septage treatment facility in the northwest part of Orleans, Massachusetts, on Cape Cod. From 1989 to 2011, the U.S. Geological Survey, in cooperation with State and local partners, conducted a series of studies in the Namskaket Marsh area to characterize the potential effects of the plume on the marsh and its tidal creek system. Studies included characterizing the baseline vegetation and salinity distribution in the marsh, monitoring the movement of the wastewater plume downgradient of the septage treatment facility, and sampling nutrient concentrations in the tidal creek system during a baseline period prior to the arrival of the plume at the marsh boundary. The Inner Namskaket Marsh baseline vegetation survey in 1995 found it to be dominated by Phragmites australis (common reed, 44 percent of vegetative cover), Spartina patens (salt marsh hay, 17 percent), and Spartina alterniflora (cordgrass, 9 percent). Phragmites occurrence was correlated with shallow pore-water salinity in the marsh peat and was largely confined to areas with salinities less than 4 parts per thousand. Baseline, ebb-tide nutrient concentrations at the tidal creek sampling stations during 1994–96 showed strong seasonal variations for ammonium, likely associated with the seasonal cycle of growth and senescence for the dominant salt marsh grasses ( S. alterniflora and S. patens ). The seasonal cycle for nitrate was generally less pronounced. The movement of the wastewater plume has been monitored from its source at the septage treatment facility to areas immediately adjacent to and beneath the most inland part of the marsh. In late 1994, the plume was first detected by borehole geophysical logging in observation wells along the Cape Cod Rail Trail (rail trail), 600 feet northwest of the infiltration beds, at an elevation of 47 to 53 feet below the National Geodetic Vertical Datum of 1929 (NGVD 29). At the rail trail, the plume was largely confined below a 3- to 8-foot-thick silt/clay layer detected by borehole geophysical logging and confirmed by lithologic samples. By early 1998, a second plume segment was detected above this silt/clay layer at the rail trail, near the plume’s southwest boundary. Groundwater sampling in 2003–4 at additional stations southwest of the main plume, as well as beneath Namskaket Marsh, defined the extent of this shallow plume segment in glacial sands underlying the marsh. The tidal creek sampling stations established in the 1990s were resampled in 2003–4 and 2010–11 to evaluate potential effects of the treated wastewater plume on creek water quality. The annual medians of the 2011 biweekly nitrate and total dissolved nitrogen concentrations were determined for each station and compared to the annual medians of biweekly samples for the baseline years 1994, 1995, and 1996. At all stations, the 2011 median nitrate concentrations were within the range of medians for the 3 baseline years. A similar result was obtained for total dissolved nitrogen. We conclude that the 2011 creek samples, collected approximately 8 years after the shallow plume segment was first detected beneath the marsh, do not show evidence of elevated nitrate or total dissolved nitrogen concentrations attributable to discharge of either the shallow or deep segments of the treated wastewater plume.

Massachusetts↗
USGS2004

Pesticides in the Lower Clackamas River Basin, Oregon, 2000-01

In 2000-01, the U. S. Geological Survey sampled the Clackamas River and its major lower-basin tributaries during storm runoff conditions for 86 dissolved pesticides and selected breakdown products. Twenty-seven compounds, including 18 herbicides, 7 insecticides, and 2 pesticide breakdown products, were detected in 18 stream samples. The most commonly detected pesticides, in decreasing frequency, included atrazine, simazine, diazinon, metolachlor, and diuron, which variously occurred in 46-92% of samples collected from the tributaries. Of these, atrazine, simazine, and metolachlor, plus six other compounds, also were detected in the main-stem Clackamas River. Pesticides were detected more frequently and at higher concentrations in the four lowermost tributaries (Deep, Richardson, Rock, and Sieben Creeks). In these streams, 12 to 18 pesticides were detected per stream in samples collected during spring and fall. Pesticides always occurred with at least one other pesticide, and about half of the samples, including one sample from the Clackamas River in October 2000, contained six or more pesticides. Nine pesticides, including the insecticide diazinon and the herbicides 2,4-D, atrazine, dichlobenil, diuron, imazaquin, metolachlor, simazine, and trifluralin, were detected in five water samples of Clackamas River water. No pesticides were detected in three samples of treated Clackamas River water used for drinking-water supply. Concentrations of six compounds--carbaryl, chlorpyrifos, diazinon, dieldrin, malathion, and the breakdown product of DDT (p,p'-DDE)--exceeded established or recommended criteria for the protection of aquatic life in some of the tributaries, sometimes for multiple pesticides in one sample. Identifying the sources of pesticides detected in the Clackamas River Basin is difficult because of the diverse land use in the basin and the multiple-use nature of many of the pesticides detected. Of the 25 parent compounds detected, 22 have agricultural uses, 23 have urban uses, 16 are applied to golf courses, 11 are applied along roads and other right-of-ways, and 5 have or had forestry applications. Because only a small fraction of the thousands of pesticide products registered for use in Oregon were tested for in this study, future monitoring could benefit from knowledge of what pesticides are applied so that potential problems can be identified and managed.

Water-Resources Investigations Report↗
USGS2015

California State Waters map series — Offshore of Pigeon Point, California

Introduction In 2007, the California Ocean Protection Council initiated the California Seafloor Mapping Program (CSMP), designed to create a comprehensive seafloor map of high-resolution bathymetry, marine benthic habitats, and geology within the 3-nautical-mile limit of California’s State Waters. The CSMP approach is to create highly detailed seafloor maps through collection, integration, interpretation, and visualization of swath sonar data, acoustic backscatter, seafloor video, seafloor photography, high-resolution seismic-reflection profiles, and bottom-sediment sampling data. The map products display seafloor morphology and character, identify potential marine benthic habitats, and illustrate both the surficial seafloor geology and shallow subsurface geology. The Offshore of Pigeon Point map area is located in central California, on the Pacific Coast about 50 km south of San Francisco and 25 km northwest of Santa Cruz. The onshore part of the map area is sparsely populated. The nearest significant onshore cultural center is Pescadero, an unincorporated community with a population of well under 1,000. The hilly coastal area is virtually undeveloped, used primarily for agricultural or as grazing land for sheep and cattle. Agriculture is limited to the coastal uplifted Pleistocene marine terraces and upper Pleistocene alluvial fan deposits, which lie between the shoreline and the northwest-trending Santa Cruz Mountains. The map area is cut by the San Gregorio Fault Zone, and is located a few kilometers southwest of the San Andreas Fault Zone. Coastal uplift and folding in the map area has been attributed to a westward bend in the San Andreas Fault Zone and also to right-lateral movement along the San Gregorio Fault Zone. The irregular coastal geomorphology of this area, which consists of low, rocky cliffs and sparse, small pocket beaches backed by low, terraced hills, is partly attributable to this ongoing deformation. The shelf in the map area is underlain by variable amounts (0 to 20 m) of upper Quaternary nearshore and shelf sediments deposited as sea level fluctuated in the late Pleistocene. The southern part of the map is characterized by the presence of uplifted bedrock that has been linked to a local zone of transpression in the San Gregorio Fault Zone. This uplift, coupled with high wave energy, has resulted in little or no sediment cover in this area where exposures of bedrock are present at water depths of as much as 45 m. The thickest deposits of sediment are located in the northern part of the map area. Coastal sediment transport in the map area is characterized by north-to-south littoral transport of sediment that is derived mainly from streams in the Santa Cruz Mountains and also from local coastal erosion. Shoreline-change studies indicate long-term erosion; within the region between San Francisco and Davenport, the highest long- and short-term coastal-erosion rates occur in the map area, just north of Point Año Nuevo. During the last approximately 300 years, as much as 18 million cubic yards (14 million cubic meters) of sand-sized sediment has been eroded from the area between Año Nuevo Island and Point Año Nuevo and transported south. Once widened by this pulse of eroded sediment, beaches south of Point Año Nuevo are now narrowing as the tail end of this mass of sand progresses farther south. The Offshore of Pigeon Point map area lies within the cold-temperate biogeographic zone that is called either the “Oregonian province” or the “northern California ecoregion.” This biogeographic province is maintained by the long-term stability of the southward-flowing California Current, the eastern limb of the North Pacific subtropical gyre that flows from southern British Columbia to Baja California. At its midpoint off central California, the California Current transports subarctic surface (0–500 m deep) waters southward, about 150 to 1,300 km from shore. Seasonal northwesterly winds that are, in part, responsible for the California Current, generate coastal upwelling. The south end of the Oregonian province is at Point Conception (about 335 km south of the map area), although its associated phylogeographic group of marine fauna may extend beyond to the area offshore of Los Angeles in southern California. The ocean off of central California has experienced a warming over the last 50 years that is driving an ecosystem shift away from the productive subarctic regime towards a depopulated subtropical environment. Seafloor habitats in the Offshore of Pigeon Point map area lie within the Shelf (continental shelf) megahabitat. Significant rocky outcrops, which support kelp-forest communities in the nearshore and rocky-reef communities in deeper water, dominate the inner shelf waters. Biological productivity resulting from coastal upwelling supports populations of Sooty Shearwater, Western Gull, Common Murre, Cassin’s Auklet, and many other less populous bird species. In addition, an observable recovery of Humpback and Blue Whales has occurred in the area; both species are dependent on coastal upwelling to provide nutrients. The large extent of exposed inner shelf bedrock supports large forests of “bull kelp,” which is well adapted for high-wave-energy environments. Common fish species found in the kelp beds and rocky reefs include lingcod and various species of rockfish and greenling.

California↗
USGS1992

Aquifer-test results, direction of ground-water flow, and 1984-90 annual ground-water pumpage for irrigation, lower Big Lost River Valley, Idaho

The study area (see index map of Idaho), part of the Big Lost River drainage basin, is at the northern side of the eastern Snake River Plain. The lower Big Lost River Valley extends from the confluence of Antelope Creek and the Big Lost River to about 4 mi south of Arco and encompasses about 145 mi 2 (see map showing water-level contours). The study area is about 18 mi long and, at its narrowest, 4 mi wide. Arco, Butte City, and Moore, with populations of 1,016, 59, and 190, respectively, in 1990, are the only incorporated towns. The entire study area, except the extreme northwestern part, is in Butte City. The study area boundary is where alluvium and colluvium pinch out and abut against the White Knob Mountains (chiefly undifferentiated sedimentary rock with lesser amounts of volcanic rock) on the west and the Lost River Range (chiefly sedimentary rock) on the east. Gravel and sand in the valley fill compose the main aquifer. The southern boundary is approximately where Big Lost River valley fill intercalates with or abuts against basalt of the Snake River Group. Spring ground-water levels and flow in the Big Lost River depend primarily on temperature and the amount and timing of precipitation within the entire drainage basin. Periods of abundant water supply and water shortages are, therefore, related to the amount of annual precipitation. Surface reservoir capacity in the valley (Mackay Reservoir, about 20 mi northwest of Moore) is only 20 percent of the average annual flow of the Big Lost River (Crosthwaite and others, 1970, p. 3). Stored surface water is generally unavailable for carryover from years of abundant water supply to help relieve drought conditions in subsequent years. Many farmers have drilled irrigation wells to supplement surface-water supplies and to increase irrigated acreage. Average annual flow of the Big Lost River below Mackay Reservoir near Mackay (gaging station 13127000, not shown) in water years 1905, 1913-14, and 1920-90 was about 224,600 acre-ft; average annual flow of the Big Lost River near Arco (gaging station 13132500; see map showing water-level contours) in water years 1947-61, 1967-80, and 1983-90 was about 79,000 acre-ft (Harenberg and others, 1991, p. 254-255). Moore Canal and East Side Ditch divert water from the Big Lost River at the Moore Diversion, 3 mi north of Moore (see map showing water-level contours) and supply water for irrigation near the margins of the valley. When water supply is average or greater, water in the Big Lost River flows through the study area and onto the Snake River Plain, where it evaporates or infiltrates into the Snake River Plain aquifer. When water supply is below average, water in the Big Lost River commonly does not reach Arco; rather, it is diverted for irrigation in the interior of the valley, evaporates, or infiltrates to the valley-fill aquifer. This report describes the results of a study by the U.S. Geological Survey, in cooperation with the Idaho Department of Water Resources, to collect hydrologic data needed to help address water-supply problems in the Big Lost River Valley. Work involved (1) field inventory of 81 wells, including 46 irrigation wells; (2) measurement of water levels in 154 wells in March 1991; (3) estimation of annual ground-water pumpage for irrigation from 1984 through 1990; and (4) analysis of results of an aquifer test conducted southwest of Moore. All data obtained during this study may be inspected at the U.S. Geological Survey, Idaho District office, Boise.

Idaho↗
USGS1946

Lower Middle Ordovician stratigraphy of the Shenandoah Valley, Virginia

In classifying the lower Middle Ordovician of the Shenandoah Valley, the formation names Stones River, Mosheim, Lenoir, Holston, Whitesburg, and Athens have been used without adequate evidence. Detailed study shows that the so-called Athens and Whitesburg, as developed near Harrisonburg, are laterally continuous with the greater part of the Chambersburg limestone, which is supposed to be younger than the Athens. The newly discovered relations of these formations affect the classification of the Middle Ordovician in much of the northern Appalachian region. The present study has been high-lighted by the discovery that Cryptophragmus antiquatus , widely regarded as a valid guide to the lower Black River, ranges through several hundred feet of beds, possibly as high as lower Trenton. In the Shenandoah Valley, this fossil is most abundant near the top of the Chambersburg, which is supposed to be late Black River or early Trenton. In the proposed reclassification, the lower Middle Ordovician is divided into six time-stratigraphic units, in ascending order: the New Market limestone, Whistle Creek limestone, Lincolnshire limestone, Edinburg formation, Oranda formation, and Collierstown limestone. The Edinburg embraces two equivalent facies: one of cobbly limestone (Lantz Mills facies) which is mainly developed in the northern and western parts of the Shenandoah Valley; and a relatively thicker body of black limestone and shale (Liberty Hall facies) which is typically developed in the Harrison-burg-Staunton area. In the western part of Shenandoah County, the topmost division of the Edinburg formation is composed of light-gray calcilutite and calcarenite, named the St. Luke limestone member. The rusty-brown granular limestones just below Butts' Athens in the Harrisonburg-Staunton-Lexington area are here named the Botetourt limestone member of the Edinburg formation. At least part of the New Market limestone is linked with a part of the New York Chazy and type Lenoir, but the Lincolnshire seems to be post-Chazy. All the succeeding beds, comprising the greater part of the lower Middle Ordovician succession, are Black River or Trenton.

Virginia↗
USGS2025

Streamflow extents and hydraulic characteristics of Meadow Valley Wash at Stuart Ranch, near Rox, Nevada

The former Stuart Ranch, now managed by the Bureau of Land Management, is transected by Meadow Valley Wash, where 4,600 feet of perennial stream and adjacent riparian vegetation provide critical habitat for several wildlife and aquatic species protected under the Endangered Species Act. The stream has been altered by prior construction of irrigation diversions, gravel mining, and removal of riparian vegetation, resulting in the loss of instream and riparian vegetation and disconnected floodplains. The stream alteration has also resulted in the loss of native species and increased non-native invasive species and changes in ecological cycles. With the goal of improving habitat extent and quality for native threatened and endangered species, the Bureau of Land Management (BLM) is considering establishing perennial streams through braided side channels by constructing beaver dam analogs, excavating side channel connectors, and grading an irrigation reservoir berm on the floodplain. The U.S. Geological Survey (USGS) provided hydraulic modeling to assist the BLM in evaluating how possible restoration modifications could affect the extent of aquatic, riparian, and other habitat types. Three two-dimensional (2-D) hydraulic models were developed to simulate 2021 conditions (when most of the topographic data were collected), minor restoration modifications (one excavated side channel and a beaver dam analog), and major restoration modifications (three excavated side channels, a beaver dam analog, and an excavated and graded area to remove the irrigation reservoir) to determine streamflow-inundation extents and hydraulic characteristics (depth and velocity) for base flow and various flood (50-, 20-, 10-, 4-, 2-, and 1-percent annual exceedance probability [AEP]) scenarios. An average summer base flow of 0.92 cubic feet per second was estimated based on data from a USGS streamgage in the study area. The 50-, 20-, 10-, 4-, 2-, and 1-percent AEP streamflows were estimated based on a flood-frequency analysis of data from the streamgage. The base flow and AEP floods were combined with surveyed topographic data to create a 2-D unsteady hydraulic model. The hydraulic model was used to simulate the base flow and flood-inundation extents and hydraulic characteristics under 2021 conditions and with two possible restoration modification scenarios. Under 2021 conditions, flow remains in a single channel until the most downstream end of the modeled reach, where flow then expands into slower velocity pools. During floods, streamflow begins to enter the side channels at the 50-percent flood, expands into the east floodplain at 20-percent flood, and flows in the irrigation reservoir at 4-percent flood. Compared to 2021 conditions with no terrain modification, base flow under the possible restoration modifications enters and remains in the side channels, thus increasing the likelihood of expanding riparian habitat. Additionally, during floods under the major restoration modifications, streamflow expands into the modified terrain surrounding the irrigation reservoir at 10-percent AEP, as opposed to 4-percent AEP under 2021 conditions. For all modeled streamflow scenarios, streamflow is deepest in the center of the main and side channels, as well as the downstream pooled areas. Streamflow is fastest in the narrow sections of the channels, especially in the upper 1,220 feet of the modeled reach.

Nevada↗
USGS2021

Characterization of groundwater quality and discharge with emphasis on selenium in an irrigated agricultural drainage near Delta, Colorado, 2017–19

Selenium is a water-quality constituent of concern for aquatic ecosystems in the lower Gunnison River Basin. Selenium is derived from bedrock of the Mancos Shale and is mobilized and transported to groundwater and surface water by application of irrigation water. Although it is recognized that groundwater contributes an appreciable amount of selenium to surface water, few studies have addressed interactions between the two. The U.S. Geological Survey in cooperation with the Colorado Water Conservation Board conducted a study during 2017–19 to characterize the quality and quantity of groundwater discharging to an agricultural drainage near Delta, Colorado, locally known as Sunflower Drain. Water quality in the study area is characterized by high dissolved solids with elevated concentrations of selenium and nitrate resulting from dissolution of soluble salts in the Mancos Shale. Selenium concentrations have decreased by 50 percent since the early 2000s, possibly in response to irrigation system improvements. Stable water isotopes indicate streamflow is dominated by canal water during the irrigation season (April to October) and, during the nonirrigation season (November to March), is dominated by groundwater that has undergone some degree of evaporation. Pesticide and pharmaceutical compounds were infrequently detected, and results indicate they were derived from sources outside the study area such that they do not appear to be useful as tracers of groundwater sources. Stable isotopes of nitrate indicate that nitrate originates from the Mancos Shale, and the isotopic composition is enriched by denitrification in the groundwater system. Using a mass-balance approach, estimated groundwater discharge rates to Sunflower Drain ranged from 0.15 to 0.27 cubic feet per second per mile with one losing reach identified. Selenium, sulfate, and nitrate concentrations in groundwater estimated by mass-balance calculations were similar to concentrations measured in the Poly 17 observation well, located in a largely irrigated area in east tributary. One tributary reach had higher concentrations of selenium, sulfate, and nitrate likely reflecting localized inputs of more concentrated groundwater, similar to the concentrations in the Poly 7 observation well, which is downgradient from a residential area in the west tributary. Three pilot studies were conducted, including fiber optic distributed temperature sensing to detect groundwater discharge zones in the stream channel, a passive seismic technique to estimate depth to bedrock, and use of radon-222 as a geochemical tracer of groundwater discharge. All three techniques show promise as additional approaches for investigating groundwater discharge surface-water systems in irrigated drainage areas on Mancos Shale. The factors that affect groundwater movement mainly include when and where irrigation water is transported and applied, and the distribution of bedrock of the Mancos Shale and overlying alluvial deposits. The average groundwater recharge rate for the study area was estimated at 8.1 inches per year, based on mass balance calculations from synoptic survey data. Along the western tributary of Sunflower Drain, there was evidence that spills from the East Canal may recharge the groundwater aquifer adjacent to the stream channel. Groundwater movement to the stream channel may be controlled by the topography of the alluvial/bedrock interface or focused along human-made features, such as tile drains and ditches constructed around irrigated fields. On larger scales, the top of bedrock was also important, creating a topographic constriction that caused a zone of groundwater discharge. The groundwater system is complex, and further study could better define the system, possibly through application of a groundwater flow model and more extensive studies using some of the exploratory methods evaluated in this study.

Colorado↗
USGS2016

Aquatic Trophic Productivity model: A decision support model for river restoration planning in the Methow River, Washington

Introduction The U.S. Geological Survey (USGS) has developed a dynamic food-web simulation model to provide decision support for Bureau of Reclamation (Reclamation) river restoration projects in the Methow River, Washington. This modeling effort was done to contribute to Reasonable and Prudent Alternative actions 56 and 57of the 2014 Federal Columbia River Power System Biological Opinion (FCRPS BO), which calls for exploration of modeling as a means to help evaluate Endangered Species Act (ESA)-listed fish response to river restoration efforts. In the Methow River, these species of concern include Upper Columbia River (UCR) spring Chinook salmon ( Oncorhynchus tshawytscha ) and UCR summer steelhead ( Oncorhynchus mykiss ). Additionally, the Independent Scientific Advisory Board (ISAB) for the Columbia River has identified the need for modeling (Independent Scientific Advisory Board, 2011a)—including models that incorporate food-web dynamics (Independent Scientific Advisory Board, 2011b)—to better understand how restoration and management strategies might enhance salmon and steelhead populations. Dynamic food-web models, even relatively simple ones, can be valuable tools for exploring responses to river restoration. Although these models have rarely been applied to rivers and streams (but see Mcintire and Colby, 1978; Power and others, 1995), they are commonly used for management decisions in terrestrial and ocean ecosystems (Christensen and Pauly, 1993; Evans and others, 2013). One of the main strengths of these models is that they are rooted in the fundamental laws of thermodynamics (that is, mass balance). Moreover, these models can be easily adapted to different contexts by adding or subtracting different species from the web and by mechanistically linking the dynamics of web members to local environmental conditions, such as water temperature, stream discharge, and channel hydraulics (Power and others, 1995; Doyle, 2006). Alternative management actions can then be evaluated by changing these environmental conditions to simulate potential outcomes following restoration. In this report, we outline the structure of a stream food-web model constructed to explore how alternative river restoration strategies may affect stream fish populations. We have termed this model the “Aquatic Trophic Productivity model” (ATP). We present the model structure, followed by three case study applications of the model to segments of the Methow River watershed in northern Washington. For two case studies (middle Methow River and lower Twisp River floodplain), we ran a series of simulations to explore how food-web dynamics respond to four distinctly different, but applied, strategies in the Methow River watershed: (1) reconnection of floodplain aquatic habitats, (2) riparian vegetation planting, (3) nutrient augmentation (that is, salmon carcass addition), and (4) enhancement of habitat suitability for fish. For the third case study, we conducted simulations to explore the potential fish and food-web response to habitat improvements conducted in 2012 at the Whitefish Island Side Channel, located in the middle Methow River.

Washington↗
USGS2011

Geochemistry of environmentally sensitive trace elements in Permian coals from the Huainan coalfield, Anhui, China

To study the geochemical characteristics of 11 environmentally sensitive trace elements in the coals of the Permian Period from the Huainan coalfield, Anhui province, China, borehole samples of 336 coals, two partings, and four roof and floor mudstones were collected from mineable coal seams. Major elements and selected trace elements were determined by inductively coupled plasma optical emission spectrometry (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), and hydride generation atomic absorption spectrometry (HAAS). The depositional environment, abundances, distribution, and modes of occurrence of trace elements were investigated. Results show that clay and carbonate minerals are the principal inorganic constituents in the coals. A lower deltaic plain, where fluvial channel systems developed successively, was the likely depositional environment of the Permian coals in the Huainan coalfield. All major elements have wider variation ranges than those of Chinese coals except for Mg and Fe. The contents of Cr, Co, Ni, and Se are higher than their averages for Chinese coals and world coals. Vertical variations of trace elements in different formations are not significant except for B and Ba. Certain roof and partings are distinctly higher in trace elements than underlying coal bench samples. The modes of occurrence of trace elements vary in different coal seams as a result of different coal-forming environments. Vanadium, Cr, and Th are associated with aluminosilicate minerals, Ba with carbonate minerals, and Cu, Zn, As, Se, and Pb mainly with sulfide minerals.

Huainan coalfield↗
USGS1966

Isotopic study of galenas from the upper Mississippi Valley, the Illinois-Kentucky, and some Appalachian Valley mineral districts

Analyses of lead isotopes in galena in carbonate rock ore deposits collected from several mineralized districts in the Central and Eastern Interior of the United States support previous studies in establishing that the galena is anomalously radiogenic ("J"type")- This study , using care-fully selected samples to test some géologie relationships, shows region-wide and distrietwide systematic variations in isotope ratios. The genetic implications of thèse variations are discussed. Two mechanisms of origin are considered to explain thèse variations in isotopic ratios. One involves mixing of leads from multiple sources, the other calls for variable lead-isotope ratios originating from a single source. Several source materials for the lead ores have been suggested; whether a single source has predominated, or two or more main sources have predominated is still unresolved. Slopes from Pb206/Pb204 versus Pb207/Pb204 plot are compatible with a contribution of lead by the 1300 ± 300 m.y. basement rocks that underlie the deposits. The spatial relationships of alkaline igneous rocks to several of the minerai districts in the Mississippi Valley région suggest a possible mag-matic source for the lead. Isotopic analyses of galenas closely associated with several of the intrusive rocks are similar in isotopic composition to adjacent ore leads, but genetic implications are ambiguous. Galena oc-curring in shales in the Mississippi Valley région outside mineralized districts contains ordinary lead. Théories that call upon such shales to be the major source of the lead in the depoaits of the Mississippi Valley type must explain this distinct différence in isotopic compositions. Galena collected from districts within the Appalachian Ridge and Valley Province contains notably less radiogenic lead than that from the Mississippi Valley minerai districts , and the lead is of the ordinary type.

Illinois, Kentucky↗
USGS1973

Geology and quality of water in the Modesto-Merced area, San Joaquin Valley, California, with a brief section on hydrology

The Modesto-Merced area includes about 1,800 square miles on the northeast side of the San Joaquin Valley. The physiographic units in the area are (1) Sierra Nevada, (2) dissected uplands, (3) low alluvial plains and fans, (4) river flood plains and channels, and (5) overflow lands. Geologic units consist of consolidated rocks and unconsolidated deposits. The dominant structure of the geologic units is that of a homocline, which reflects the southwestward-tilted fault block of the Sierra Nevada. The consolidated rocks include: (1) basement complex (pre-Tertiary), (2) marine sandstone and shale (Cretaceous), (3) Ione Formation and other sedimentary rocks (Eocene), (4) Valley Springs Formation (late? Miocene), and (5) the Mehrten Formation (Miocene and Pliocene). In the eastern part of the area, the consolidated rocks generally yield small quantities of water to wells except for the Mehrten Formation which is an important aquifer. The unconsolidated deposits include: (1) continental deposits (Pliocene and Pleistocene?), (2) lacustrine and marsh deposits (Pleistocene), (3) older alluvium (Pleistocene and Holocene?), (4) younger alluvium (Holocene), and (5) flood-basin deposits (Holocene). The continental deposits and older alluvium are the main water-yielding units in the unconsolidated deposits. The lacustrine and marsh deposits (E-clay) and the flood-basin deposits yield little water to wells, and the younger alluvium in most places probably yields only moderate quantities of water to wells. There are three ground-water bodies in the Modesto-Merced area: (1) the unconfined water body, (2) the confined water body, and (3) the water body in consolidated rocks. The unconfined water body occurs in the unconsolidated deposits above and east of the E-clay, except in the western and southern parts of the area where clay lenses occur and semiconfined conditions exist. The confined water body occurs in the unconsolidated deposits below the E-clay and extends downward to the base of fresh water. The water body in consolidated rocks occurs under both perched and confined conditions. Ground-water movement in the unconfined and the confined water bodies is generally westward toward the valley trough. In the unconfined water body, the water also moves toward the major rivers, and toward pumping depressions at Modesto and near El Nido. Because of the higher head in the unconfined water body, water slowly moves from it through the E-clay to the underlying confined water body. Surface water is used extensively for irrigation in most of the study area; consequently, shallow water in the unconfined water body has to be controlled by pumping. Nevertheless, water levels near Modesto declined about 6 feet between 1958 and 1962, for the most part during the dry years 1959-61. Near El Nido, water levels declined about 70 feet between 1942 and 1967. In the confined water body, water levels were high in the winter and spring and low in the summer and fall, reflecting irrigation practices. In the southwestern part of the area, water levels in the confined water body declined about 14 feet from 1962 to 1968 and rose slightly after 1968, whereas in the northwestern part water levels remained fairly constant. Water from the upper reaches of the Stanislaus, Tuolumne, Merced, and Chowchilla Rivers is calcium bicarbonate in chemical type and is of excellent quality. As those rivers cross the valley floor, their water quality is generally degraded by return flows from irrigated land, and in the Tuolumne River by saline water discharged from abandoned gas wells. Average dissolvedsolids content in the major rivers, as indicated by chemical analyses, do not exceed 400 mg/1 (milligrams per liter) except in the San Joaquin River where average dissolved-solids content has not exceeded 1,050 mg/l. Water from minor streams in the area is bicarbonate in chemical type with dissolved-solids content ranging from 56 mg/1 to about 350 mg/l. Although chloride-type fresh ground water occurs in the unconfined and confined water bodies and in the water body in consolidated rocks, most of the fresh ground water is a bicarbonate type that has a dissolved-solids content of less than 500 mg/l. Water having dissolved solids in excess of about 2,000 mg/1 is considered to be saline. Saline water extends below the base of fresh water to the basement complex and, except in the extreme eastern part, underlies most of the study area. Saline water also occurs as lenses above the base of fresh water in the unconfined and confined water bodies.

California↗
USGS1989

Seismic methods for determining earthquake source parameters and lithospheric structure

The seismologic methods most commonly used in studies of earthquakes and the structure of the continental lithosphere are reviewed in three main sections: earthquake source parameter determinations, the determination of earth structure using natural sources, and controlled-source seismology. The emphasis in each section is on a description of data, the principles behind the analysis techniques, and the assumptions and uncertainties in interpretation. Rather than focusing on future directions in seismology, the goal here is to summarize past and current practice as a companion to the review papers in this volume. Reliable earthquake hypocenters and focal mechanisms require seismograph locations with a broad distribution in azimuth and distance from the earthquakes; a recording within one focal depth of the epicenter provides excellent hypocentral depth control. For earthquakes of magnitude greater than 4.5, waveform modeling methods may be used to determine source parameters. The seismic moment tensor provides the most complete and accurate measure of earthquake source parameters, and offers a dynamic picture of the faulting process. Methods for determining the Earth’s structure from natural sources exist for local, regional, and teleseismic sources. One-dimensional models of structure are obtained from body and surface waves using both forward and inverse modeling. Forward-modeling methods include consideration of seismic amplitudes and waveforms, but lack the formal resolution estimates obtained with inverse methods. Two- and three-dimensional lithospheric models are derived using various inverse methods, but at present most of these methods consider only traveltimes of body waves. Controlled-source studies of the Earth’s structure are generally divided by method into seismic refraction/wide-angle reflection and seismic reflection studies. Seismic refraction profiles are usually interpreted in terms of two-dimensional structure by forward modeling of traveltimes and amplitudes. The refraction method gives excellent estimates of seismic velocities, but relatively low resolution of structure. Formal resolution estimates are not possible for models derived from forward modeling, but informal estimates can be obtained by perturbing the best-fitting model. Inversion methods for seismic refraction data for one-dimensional models are well established, and two- and three-dimensional methods, including tomography, have recently been developed. Seismic reflection data provide the highest resolution of crustal structure, and have provided many important geological insights in the past decade. The acquisition and processing of these data have been greatly advanced by the hydrocarbon exploration industry. However, reliable crustal velocity control is generally lacking, and the origin of deep crustal reflections remains unclear, resulting in nonunique interpretations. A new form of lithospheric seismology has recently emerged that combines the advantages of seismic refraction and seismic reflection profiles, and the distinction between the two methods is steadily diminishing. Major challenges for future work will be the collection of data that are more densely sampled in space, and the development of interpretation methods that provide quantitative estimates of the uncertainties in the calculated models.

GSA Memoirs↗
USGS1979

Genesis of ore deposits in the San Juan Volcanic Field, Southwestern Colorado-lead isotope evidence

In the San Juan volcanic area of southwestern Colorado, the isotopic composition of lead in ores and ore prospects of Cenozoic age ranges widely: 17.72 to 21.13 for 206 Pb/ 204 Pb; 15.50 to 15.81 for 207 Pb/ 204 Pb; and 37.21 to 38 for 208 Pb/ 204 Pb. Examination of the lead isotope data indicates that once deposition of lead minerals begins, further exchange of lead between fluid and wall rock is insignificant. This conclusion is supported by the relatively constant isotopic composition of the lead in these ores, which is not affected by the grade of ore mineralization or type of wall rock. The values of 206 Pb/ 204 Pb in some vein-type deposits exceed the maximum value known for all Mesozoic and Cenozoic igneous rocks of the Rocky Mountain region. These isotopic relations show that if ore-forming solutions are composed of meteoric water, as indicated in studies of light stable isotopes, they must have penetrated deep enough to acquire lead from Pre-cambrian rocks or sediments derived from them. The fact that some of these vein ores are now in Cenozoic igneous rocks indicates the ore fluid had an upward vertical component to its movement. The lead isotope data therefore support a circulating cell hypothesis for these kinds of ores, as suggested by many recent studies of light stable isotopes as related to mineralization. Some other deposits (Summitville, Jasper, Red Mountain district) have values of 206 Pb/ 204 Pb similar to those of the large volumes of altered rock that enclose them ( 206 Pb/ 204 Pb ∼ 18.5), suggesting that in places the lead may have been locally derived by leaching of the adjacent rocks or from magmatogenic fluids. When the lead isotope data are treated in detail, the rocks and galenas of the Platoro caldera complex, of the central San Juan caklera complex, and of the Baughman Creek center appear to contain significant components derived from 1,400- to l,500-m.y.-old and 1,700- to 1,800 m.y.-old source materials. These also are the ages of the two main groups of rocks that comprise the Precambrian basement under the San Juan volcanic area. Although the data from the western San Juan caldera complex scatter considerably, the only obvious source for the lead seems to be the 1,700- to l,800-m.y.-old rocks or detritus of such age in Phanerozoic sediments. Where the involvement of 1,400- to l,500-m.y.-old sources is greatest, the Th/U ratio in the source material is small (calculated to be about 0.7), whereas the Th/U ratio of the 1,700- to l,800-m.y.-old source material appears to be 2.3 to 3.3. The largest Th/U values are from the western San Juan caldera complex. © 1979 Society of Economic Geologists, Inc.

Colorado↗