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At least 1,045 records · Page 58Linked to original sources

USGS2009

Gas hydrate drilling transect across northern Cascadia margin - IODP Expedition 311

A transect of four sites (U1325, U1326, U1327 and U1329) across the northern Cascadia margin was established during Integrated Ocean Drilling Program Expedition 311 to study the occurrence and formation of gas hydrate in accretionary complexes. In addition to the transect sites, a fifth site (U1328) was established at a cold vent with active fluid flow. The four transect sites represent different typical geological environments of gas hydrate occurrence across the northern Cascadia margin from the earliest occurrence on the westernmost first accreted ridge (Site U1326) to the eastward limit of the gas hydrate occurrence in shallower water (Site U1329). Expedition 311 complements previous gas hydrate studies along the Cascadia accretionary complex, especially ODP Leg 146 and Leg 204 by extending the aperture of the transect sampled and introducing new tools to systematically quantify the gas hydrate content of the sediments. Among the most significant findings of the expedition was the occurrence of up to 20 m thick sand-rich turbidite intervals with gas hydrate concentrations locally exceeding 50% of the pore space at Sites U1326 and U1327. Moreover, these anomalous gas hydrate intervals occur at unexpectedly shallow depths of 50-120 metres below seafloor, which is the opposite of what was expected from previous models of gas hydrate formation in accretionary complexes, where gas hydrate was predicted to be more concentrated near the base of the gas hydrate stability zone just above the bottom-simulating reflector. Gas hydrate appears to be mainly concentrated in turbidite sand layers. During Expedition 311, the visual correlation of gas hydrate with sand layers was clearly and repeatedly documented, strongly supporting the importance of grain size in controlling gas hydrate occurrence. The results from the transect sites provide evidence for a structurally complex, lithology-controlled gas hydrate environment on the northern Cascadia margin. Local shallow occurrences of high gas hydrate concentrations contradict the previous model of gas hydrate formation at an accretionary prism. However, long-lived fluid flow (part of the old model) is still required to explain the shallow high gas hydrate concentrations, although it is most likely not pervasive throughout the entire accretionary prism, but rather localized and focused by the tectonic processes. Differences in the fluid flow regime across all of the transect drill sites indicate site-specific and probably disconnected (compartmented) deeper fluid sources in the various parts of the accretionary prism. The data and future analyses will yield a better understanding of the geologic controls, evolution and ultimate fate of gas hydrate in an accretionary prism as an important contribution to the role of gas hydrate methane gas in slope stability and possibly in climate change. ?? The Geological Society of London 2009.

Geological Society Special Publication↗

USGS2002

Hydrostructural maps of the Death Valley regional flow system, Nevada and California

The locations of principal faults and structural zones that may influence ground-water flow were compiled in support of a three-dimensional ground-water model for the Death Valley regional flow system (DVRFS), which covers 80,000 square km in southwestern Nevada and southeastern California. Faults include Neogene extensional and strike-slip faults and pre-Tertiary thrust faults. Emphasis was given to characteristics of faults and deformed zones that may have a high potential for influencing hydraulic conductivity. These include: (1) faulting that results in the juxtaposition of stratigraphic units with contrasting hydrologic properties, which may cause ground-water discharge and other perturbations in the flow system; (2) special physical characteristics of the fault zones, such as brecciation and fracturing, that may cause specific parts of the zone to act either as conduits or as barriers to fluid flow; (3) the presence of a variety of lithologies whose physical and deformational characteristics may serve to impede or enhance flow in fault zones; (4) orientation of a fault with respect to the present-day stress field, possibly influencing hydraulic conductivity along the fault zone; and (5) faults that have been active in late Pleistocene or Holocene time and areas of contemporary seismicity, which may be associated with enhanced permeabilities. The faults shown on maps A and B are largely from Workman and others (in press), and fit one or more of the following criteria: (1) faults that are more than 10 km in map length; (2) faults with more than 500 m of displacement; and (3) faults in sets that define a significant structural fabric that characterizes a particular domain of the DVRFS. The following fault types are shown: Neogene normal, Neogene strike-slip, Neogene low-angle normal, pre-Tertiary thrust, and structural boundaries of Miocene calderas. We have highlighted faults that have late Pleistocene to Holocene displacement (Piety, 1996). Areas of thick Neogene basin-fill deposits (thicknesses 1-2 km, 2-3 km, and >3 km) are shown on map A, based on gravity anomalies and depth-to-basement modeling by Blakely and others (1999). We have interpreted the positions of faults in the subsurface, generally following the interpretations of Blakely and others (1999). Where geophysical constraints are not present, the faults beneath late Tertiary and Quaternary cover have been extended based on geologic reasoning. Nearly all of these concealed faults are shown with continuous solid lines on maps A and B, in order to provide continuous structures for incorporation into the hydrogeologic framework model (HFM). Map A also shows the potentiometric surface, regional springs (25-35 degrees Celsius, D'Agnese and others, 1997), and cold springs (Turner and others, 1996).

Nevada;California↗

USGS2000

Recognition of primary and diagenetic magnetizations to determine the magnetic polarity record and timing of deposition of the moat-fill rocks of the Oligocene Creede Caldera, Colorado

Sedimentary and volcaniclastic rocks of the Oligocene Creede Formation fill the moat of the Creede caldera, which formed at about 26.9 Ma during the eruption of the Snowshoe Mountain Tuff. Paleomagnetic and rock magnetic studies of two cores (418 and 703 m long) that penetrated the lower half of the Creede Formation, in addition to paleomagnetic and isotopic dating studies of stratigraphically bracketing volcanic units, provide information on the age and the time span of sedimentation of the caldera fill. Normal polarity magnetization are found in Snowshoe Mountain Tuff beneath the moat sediments; in detrital-magnetite-bearing graded tuffs near the bottom of the moat fill; in an ash-fall deposit about 200 m stratigraphically about the top of core 2; and in postcaldera lava flows of the Fisher Dacite that overlie the Creede Formation. Normal polarity also characterizes detrital-magnetite-bearing tuff and sandstone unites within the caldera moat rocks that did not undergo severe sulfidic alteration. The combination of initially low magnitude of remanent magnetization and the destructive effects of subsequent diagenetic sulfidization on detrital iron oxides results in a poor paleomagnetic record for the fine-grained sedimentary rocks of the Creede Formation. these fine-grained rocks have either normal or revered polarity magnetizations that are carried by magnetite and/or maghemite. Many more apparent reversals are found that can be accommodated by any geomagnetic polarity time scale over the interval spanned by the ages of the bracketing extrusive rocks. Moreover, opposite polarity magnetization are found in specimens separated by only a few centimeters, without intervening hiatuses, and by specimens in several tuff beds, each of which represents a single depositional event. These polarity changes cannot, therefore, be attributed to detrital remanent magnetization. Many polarity changes are apparently related to chemical remanent magnetizations carried by postdepositional magnetite and maghemite that formed in rocks in which most or all detrital megnetic iron oxide was destroyed. Incipient oxidation of early diagenetic pyrite may have normal polarity Snowshoe Mountain Tuff (26.89 ± 0.0 Ma, 1 δ) and on the normal polarity postcaldera Fisher lava flows (as young as 26.23 ± 0.05 Ma, 1 δ) indicate that deposition of the Creede Formation spanned about 340-660 k.y. The intermittently defined normal polarity magnetization for the caldera-fill sequence, compared with different versions of the geomagnetic polarity time scale, is consistent with the shorter time span.

Colorado↗

USGS2006

The A.D. 1835 eruption of Volcán Cosigüina, Nicaragua: A guide for assessing local volcanic hazards

The January 1835 eruption of Volcán Cosigüina in northwestern Nicaragua was one of the largest and most explosive in Central America since Spanish colonization. We report on the results of reconnaissance stratigraphic studies and laboratory work aimed at better defining the distribution and character of deposits emplaced by the eruption as a means of developing a preliminary hazards assessment for future eruptions. On the lower flanks of the volcano, a basal tephra-fall deposit comprises either ash and fine lithic lapilli or, locally, dacitic pumice. An overlying tephra-fall deposit forms an extensive blanket of brown to gray andesitic scoria that is 35–60 cm thick at 5–10 km from the summit-caldera rim, except southwest of the volcano, where it is considerably thinner. The scoria fall produced the most voluminous deposit of the eruption and underlies pyroclastic-surge and -flow deposits that chiefly comprise gray andesitic scoria. In northern and southeastern sectors of the volcano, these flowage deposits form broad fans and valley fills that locally reach the Gulf of Fonseca. An arcuate ridge 2 km west of the caldera rim and a low ridge east of the caldera deflected pyroclastic flows northward and southeastward. Pyroclastic flows did not reach the lower west and southwest flanks, which instead received thick, fine-grained, accretionary-lapilli–rich ashfall deposits that probably derived chiefly from ash clouds elutriated from pyroclastic flows. We estimate the total bulk volume of erupted deposits to be ∼6 km 3 . Following the eruption, lahars inundated large portions of the lower flanks, and erosion of deposits and creation of new channels triggered rapid alluviation. Pre-1835 eruptions are poorly dated; however, scoria-fall, pyroclastic-flow, and lahar deposits record a penultimate eruption of smaller magnitude than that of 1835. It occurred a few centuries earlier—perhaps in the fifteenth century. An undated sequence of thick tephra-fall deposits on the west flank of the volcano records tens of eruptions, some of which were greater in magnitude than that of 1835. Weathering evidence suggests this sequence is at least several thousand years old. The wide extent of pyroclastic flows and thick tephra fall during 1835, the greater magnitude of some previous Holocene eruptions, and the location of Cosigüina on a peninsula limit the options to reduce risk during future unrest and eruption.

GSA Special Papers↗

USGS1980

The Rocky Mountain population of the western Canada goose: Its distribution, habitats, and management

The western Canada goose ( Branta canadensis moffitti ) was divided into a Rocky Mountain population (RMP) and a Pacific population (PP) on the basis of band recovery patterns examined in this study and recovery data from other investigators. Habitat information obtained from nine cooperating wildlife agencies within the RMP's range provided a base line for evaluating future changes in nesting, molting, and wintering areas. The habitat inventory indicated that none of the seasonal habitats were currently limiting the size of the RMP. The RMP's range is divided into 15 reference areas and these are briefly described. Past studies of Canada geese in the Intermountain Region are reviewed. Topics covered in the discussion of breeding biology are nesting chronology, spring population composition, breeding age, clutch size, nesting success. artificial nesting structures, and gosling survival. Much of the mortality of Canada geese occurs before the birds are fledged. Man-made nesting structures reduce losses during incubation. but research is needed on the relations between brooding sites and gosling survival. Some western Canada geese, mainly prebreeders and unsuccessful nesters, make molt migrations to and from molting areas during and after the brood-rearing season. More than half of these molt-migrants are yearlings too young to nest; there are indications that even some successful nesters leave nesting areas to molt before the fledging of their offspring. Geese 2 years old or older may serve as guides to traditional molting areas for the first-time migrants (i.e., yearlings). Lack of disturbance appears to influence selection of specific molting areas within the nesting range of moffitti , whereas movements of molters out of the Intermountain Region may be related to the evolution of this subspecies. Apparently. molters of both the PP and RMP that leave the Region go to the Northwest Territories of Canada. Although the taxonomic status of moffitti as related to the giant Canada goose ( B. c. maxima ) is unclear. these two subspecies are closely related. as evidenced by similar molt migrations to subarctic Canada. similar blood serum proteins. and only dinal differences in body size ,!!nd color. Mean annual survival rates for birds banded on nesting areas averaged 53 ± 2% ( X̿ ± SE) for immatures and 64 ± 1 % for adults. Mean annual survival rates of adults captured on molting areas averaged 70 ± 1 %. Sport hunting accounts for more than 86% of the mortality of fledged Rocky Mountain geese. and hunting may limit the population's growth. Because the number of waterfowl hunters in the Rocky Mountain West is increasing, the continued expansion or future maintenance of the RMP may require more restrictive hunting regulations. Other management recommendations include the refinement and standardization of spring and winter aerial surveys, and more accurate age and sex determinations when geese are banded and color-marked.

Alberta, Arizona, California, Colorado, Idaho, Mon↗

USGS1998

Sponges of the Permian Upper Capitan Limestone Guadalupe Mountains, New Mexico and Texas

Demosponge "sphinctozoans" and inozoid calcareous sponges are major constituents of the Upper Permian, Upper Capitan Limestone in the Guadalupe Mountains of New Mexico and Texas. Systematic description, taxonomy, and the stratigraphic distribution of these sponges are documented in collections from exposures of the Upper Capitan Limestone in the vicinity of Carlsbad Caverns in New Mexico. The fauna appears diverse on a local scale, but when compared to diversity of assemblages of similar age in Tunisia and in Southern China, the assemblage is species poor, with 34 species of "calcareous" sponges and demosponges. Whether this is a local time or geographic gradient must wait additional investigations of sponge faunas from older parts of the Guadalupian series in the Guadalupe Mountains, as well as in localities southward in Texas and Mexico. Upper Capitan exposures near Carlsbad Caverns are at the northernmost end of the long Delaware Basin that was restricted by the Hovey channel to the south. As a consequence of either that restricted ecologic limitation or a time stratigraphic factor, Late Capitan assemblages are characterized by abundant individuals of only a few endemic species, and by relatively primitive cosmopolitan genera and species that were able to persist beyond the ranges of more specialized forms. Species in the collections include the ceractinomorphid porate "sphinctozoans" Cystothalamia guadalupensis (Girty, 1908a), Amblysiphonella cf. A. merlai Parona, 1933, Amblysiphonella species A, Amblysiphonella species B, Discosiphonella mammilosa (King, 1943), Tristratocoelia rhythmica Senowbari-Daryan and Rigby, 1988, Exaulipora permica (Senowbari-Daryan, 1990), type species of the new genus Exaulipora, Parauvanella minima Senowbari-Daryan, 1990, and Platythalamiella(?) sp., all from the families Sebargasiidae Steinmann, 1882, and Colospongiidae Senowbari-Daryan, 1990. The family Solenolmiidae Engeser, 1986 is represented by the new species Preverticillites parva. Among the ceractinomorphid Aporata, the family Thaumastocoeliidae Ott, 1967 is represented by Sollasia ostiolata Steinmann, 1882, and Girtyocoelia beedei (Girty, 1908b). The sclerospongiid Guadalupiidae Termier and Termier, in Termier, et al., 1977a, which includes the family Guadalupiidae Girty, 1908a, is represented by the species Guadalupia zitteliana Girty, 1908a, and Guadalupia explanata (King, 1943), Lemonea cylindrica (Girty, 1908a), Lemonea conica Senowbari-Daryan, 1990, Lemonea polysiphonata Senowbari-Daryan, 1990, and the new species Lemonea exaulifera and Lemonea micra. The Calcarea are represented within the subclass Aspiculata Rigby and Senowbari-Daryan, 1996a, and order Inozoida Rigby and Senowbari-Daryan, 1996a, by the Auriculospongiidae Termier and Termier, 1977a, which includes the large Gigantospongia discoforma Rigby and Senowbari-Daryan, 1996b. and Cavusonella caverna Rigby, Fan, and Zhang, 1989b. Also included are the Peronidellidae Wu, 1991, represented by the species Peronidella cf. P. rigbyi Senowbari-Daryan, 1991, Peronidella(?) delicata new species, and Minispongia constricta (Girty, 1908a), and the new genus and species Bicoelia guadalupensis. The family Virgulidae Termier and Termier, 1977a, is redefined to include the genus Virgola and the species Virgola neptunia (Girty, 1908a), and Virgola rigida (Girty, 1908a,). The family Polysiphonellidae Wu, 1991, (not Polysiphonellidae Belyaeva in Boiko, et al., 1991) is interpreted to include most of the subfamilies originally included by Rigby and Senowbari-Daryan (1996a) in the Virgulispongiidae. The sponge Grossotubinella parallela Rigby, Fan, and Zhang, 1989b is included there in the Preeudinae, with Pseudovirgula tenuis Girty, 1908a. Heliospongid demosponges are represented in the Upper Capitan by Heliospongia ramosa Girty, 1908b, Heliospongia vokesi King, 1943, and Neoheliospongia(?) cf. N. typica Deng, 1981. Fossils of unknown taxonomy, possibly sponges, hydrozoans or algae, are incl

Brigham Young University Geology Studies↗

USGS2010

Erosion of tilted fault blocks and deposition of coarse sediments in half-graben basins during late stages of extension: Gold Butte area, Basin and Range Province

The provenance and stratigraphic architecture of basin-filling Miocene sediments around the Gold Butte area, southern Nevada, and adjacent highlands record the erosion of fault blocks that progressively tilted during extension. This study focuses especially on upper Miocene correlatives of the red sandstone unit and the Muddy Creek Formation that were deposited during waning stages of extension. Upper parts of the underlying middle Miocene Horse Spring Formation are also addressed. The large east-tilted South Virgin–White Hills block, including the Gold Butte block, was the primary source of coarse detritus into the adjacent half-graben basins on both sides. Voluminous, very coarse-grained sediments were shed eastward down the back slope of this tilt block into the Grand Wash Trough. This suggests that there were large middle and late Miocene catchments on that side of the block, possibly inherited from a gentler dip slope early in the tilting history. The block uplifted and tilted during slip on the west-dipping South Virgin–White Hills normal fault that bounds the west side of the block. Its exposed footwall shed coarse-grained debris to the west. While the fault was active, this debris included rock-avalanche megabreccias. Longitudinal transport of coarse-grained sediment also occurred along the axes of basins on both sides of the block. In the late Miocene, fault death at ca. 10 Ma followed rotation of the South Virgin–White Hills fault, and the along-strike Quail Spring fault, from initial dips >55° to dips <30°. This cessation of faulting coincided with and likely caused an eastward shift in locus of faulting to the steeper Wheeler fault system. Coarse sediment shed from the South Virgin–White Hills tilt block gradually declined as deformation waned and limestone-rich sedimentation expanded onto the basin margins against the block. Where the rising sedimentary fills eventually bridged across the block and connected basins on either side, these bridge sites served to focus later integrated regional drainage—the Pliocene Colorado River. Progressive Miocene tilting of the highland block would have broadened its structural footwall on the west and narrowed its east-dipping back slope. Migration of the drainage divide by erosion and piracy, influenced by changing tilt slopes, can explain the modern position of the divide in the Gold Butte block as one that separates drainage roughly equally down the two sides.

Nevada↗

USGS2018

Magma supply to Kīlauea Volcano, Hawai‘i, from inception to now: Historical perspective, current state of knowledge, and future challenges

Meticulous field observations are a common underpinning of two landmark studies conducted by Don Swanson dealing with the rate at which magma is supplied to Kīlauea Volcano, Hawai‘i. The first combined effusion rate and ground deformation observations to show that the supply rate to Kīlauea was constant at ~0.11 km 3 /yr during three sustained eruptions from 1952 to 1971, a quiescent period at neighboring Mauna Loa volcano. This rate was also interpreted as the steady supply rate from the mantle to both volcanoes combined throughout historical time. The second breakthrough involved field evidence that activity at Kīlauea alternates between dominantly effusive and explosive styles over time scales of several centuries, and that the magma supply rate during explosive periods is only 1%-2% of the rate during effusive periods. For the historical period, several later studies concluded that the supply rate to Kīlauea has varied by as much as an order of magnitude, contrary to Swanson’s earlier suggestion. All such estimates are fraught with uncertainty, given the poorly known amount of magma stored within the volcano’s rift zones as a function of time—an enduring problem and active research topic. Nonetheless, Swanson’s original work remains an important touchstone that spurred many subsequent investigations and refinements. For example, there is strong evidence that Kīlauea experienced a surge in magma supply during 2003–2007 that exceeded the historical average by as much as a factor of two, and that the surge was followed by a comparable lull before the supply rate returned to “normal” by 2016. There is also evidence for supply-rate variations of similar magnitude during the latter part of the twentieth century and possibly earlier, subject to the aforementioned uncertainty in rift-zone storage. The extent to which variations in the magma supply to Kīlauea can be attributed to partitioning between Kīlauea and Mauna Loa, a long-debated topic, remains uncertain. Since Kilauea’s inception, the net magma supply to the volcano (and also to Lō‘ihi Seamount, since it began growing) has increased, while Mauna Loa’s growth rate has slowed, suggesting that the volcanoes compete for the same magma supply. However, geochemical differences between lavas erupted at Kīlauea and Mauna Loa indicate that they do not share a homogeneous mantle source or common lithospheric magma plumbing system. Both ideas might be correct; i.e., Kīlauea and Mauna Loa magmas may be sourced in differing portions of the same melt accumulation zone and ascend through different crustal pathways, but those pathways interact through stress or pressure changes that modulate the supply to each volcano. Currently, magma supply-rate estimates are facilitated by comprehensive imaging of surface deformation and topographic change coupled with measurements of gas emissions. Physics-based models are being developed within a probabilistic framework to provide rigorous estimates of model parameters, including magma supply rate, and their uncertainties. Further refinement will require intensive multiparameter observations of the entire magmatic system—from source to surface and above, and from the volcanoes’ summits to their submerged lower flanks—in order to account fully for a complex magma budget.

Hawai‘i↗

USGS2015

Organic sedimentation in modern lacustrine systems: A case study from Lake Malawi, East Africa

This study examines the relationship between depositional environment and sedimentary organic geochemistry in Lake Malawi, East Africa, and evaluates the relative significance of the various processes that control sedimentary organic matter (OM) in lacustrine systems. Total organic carbon (TOC) concentrations in recent sediments from Lake Malawi range from 0.01 to 8.80 wt% and average 2.83 wt% for surface sediments and 2.35 wt% for shallow core sediments. Hydrogen index (HI) values as determined by Rock-Eval pyrolysis range from 0 to 756 mg HC g−1 TOC and average 205 mg HC g−1 TOC for surface sediments and 228 mg HC g−1 TOC for shallow core samples. On average, variations in primary productivity throughout the lake may account for ~33% of the TOC content in Lake Malawi sediments (as much as 1 wt% TOC), and have little or no impact on sedimentary HI values. Similarly, ~33% to 66% of the variation in TOC content in Lake Malawi sediments appears to be controlled by anoxic preservation of OM (~1–2 wt% TOC), although some component of the water depth–TOC relationship may be due to physical sediment transport processes. Furthermore, anoxic preservation has a minimal effect on HI values in Lake Malawi sediments. Dilution of OM by inorganic sediment may account for ~16% of variability in TOC content in Lake Malawi sediments (~0.5 wt% TOC). The effect of inputs of terrestrial sediment on the organic character of surface sediments in these lakes is highly variable, and appears to be more closely related to the local depositional environment than the regional flux of terrestrial OM. Total nitrogen and TOC content in surface sediments collected throughout the lake are found to be highly correlated (r2 = 0.95), indicating a well-homogenized source of OM to the lake bottom. The recurring suspension and deposition of terrestrial sediment may account for significant amounts of OM deposited in offshore regions of the lake. This process effectively separates denser inorganic sediment from less dense OM and allows terrestrial OM to preferentially be transported farther offshore. The conclusion is that for the organic carbon content in these regions to be elevated a mixed terrestrial-lacustrine origin is required. The hydrodynamic separation of mineral and organic constituents is most pronounced in regions with shallow bathymetric gradients, consistent with previous findings from Lake Tanganyika.

Lake Malawi↗

USGS1916

Guidebook of the western United States: Part C - The Santa Fe route, with a side trip to Grand Canyon of the Colorado

The United States of America comprise an area so vast in extent and so diverse in natural features as well as in characters due to human agency that the American citizen who knows thoroughly his own country must have traveled widely and observed wisely. To 'know America first' is a patriotic obligation, but to meet this obligation the railroad traveler needs to have his eyes directed toward the more important or essential things within his field of vision and then to have much that he sees explained by what is unseen in the swift passage of the train. Indeed, many things that attract his attention are inexplicable except as the story of the past is available to enable him to interpret the present. Herein lie the value and the charm of history, whether human or geologic. The present stimulus given to travel in the home country will encourage many thousands of Americans to study geography at first hand. To make this study most profitable the traveler needs a handbook that will answer the questions that come to his mind so readily along the way. Furthermore, the aim of such a guide should be to stimulate the eye in the selection of the essentials in the scene that so rapidly unfolds itself in the crossing of the continent. In recognition of the opportunity afforded in 1915 to render service of this kind to an unusually large number of American citizens, as well as to visitors from other countries, the United States Geological Survey has prepared a series of guidebooks covering four of the older railroad routes west of the Mississippi. These books are educational in purpose, but the method adopted is to entertain the traveler by making more interesting what he sees from the car window. The plan of the series is to present authoritative information that may enable the reader to realize adequately the scenic and material resources of the region he is traversing, to comprehend correctly the basis of its development, and above all to appreciate keenly the real value of the country he looks out upon, not as so many square miles of territory represented on the map in a railroad folder by meaningless spaces, but rather as land - real estate, if you please - varying widely in present appearance because differing largely in its history and characterized by even greater variation in values because possessing diversified natural resources. One region may be such as to afford a livelihood for only a pastoral people; another may present opportunity for intensive agriculture; still another may contain hidden stores of mineral wealth that may attract large industrial development; and taken together these varied resources afford the promise of long-continued prosperity for this or that State. Items of interest in civic development or references to significant epochs in the record of discovery and settlement may be interspersed with explanations of mountain and valley or statements of geologic history. In a broad way, the story of the West is a unit, and every chapter should be told in order to meet fully the needs of the tourist who aims to understand all that he sees. To such a traveler-reader this series of guidebooks is addressed. To this interpretation of our own country the United States Geological Survey brings the accumulated data of decades of pioneering investigation, and the present contribution is only one type of return to the public which has supported this scientific work under the Federal Government. In preparing the description of the country traversed by the Santa Fe Route the geographic and geologic information already published as well as unpublished material in the possession of the Geological Survey has been utilized, but to supplement this material Mr. Darton made a field examination of the entire route in 1914. Information has been furnished by Erasmus Haworth, J. E. Todd, and R. T. Hill, as well as by others whose writings are listed in the bibliography at the end of the text. Cooperation has been rendered by the United States Reclamation Service and by bureaus of the Department of Agriculture. Railroad officials and other citizens have also generously given their aid, and other members of the Survey have freely cooperated in the work. For the purpose of furnishing the traveler with a graphic presentation of each part of his route, the accompanying maps, 25 sheets in all, have been prepared, with a degree of accuracy probably never bsfore attained in a guidebook, aiid their arrangement has been planned to meet the convenience of the reader. The special topographic surveys necessary to complete these maps of the route were made by W. O. Tufts.

Bulletin↗

USGS2013

Oblique transfer of extensional strain between basins of the middle Rio Grande rift, New Mexico: Fault kinematic and paleostress constraints

The structural geometry of transfer and accommodation zones that relay strain between extensional domains in rifted crust has been addressed in many studies over the past 30 years. However, details of the kinematics of deformation and related stress changes within these zones have received relatively little attention. In this study we conduct the first-ever systematic, multi-basin fault-slip measurement campaign within the late Cenozoic Rio Grande rift of northern New Mexico to address the mechanisms and causes of extensional strain transfer associated with a broad accommodation zone. Numerous (562) kinematic measurements were collected at fault exposures within and adjacent to the NE-trending Santo Domingo Basin accommodation zone, or relay, which structurally links the N-trending, right-stepping en echelon Albuquerque and Española rift basins. The following observations are made based on these fault measurements and paleostresses computed from them. (1) Compared to the typical northerly striking normal to normal-oblique faults in the rift basins to the north and south, normal-oblique faults are broadly distributed within two merging, NE-trending zones on the northwest and southeast sides of the Santo Domingo Basin. (2) Faults in these zones have greater dispersion of rake values and fault strikes, greater dextral strike-slip components over a wide northerly strike range, and small to moderate clockwise deflections of their tips. (3) Relative-age relations among fault surfaces and slickenlines used to compute reduced stress tensors suggest that far-field, ~E-W–trending σ 3 stress trajectories were perturbed 45° to 90° clockwise into NW to N trends within the Santo Domingo zones. (4) Fault-stratigraphic age relations constrain the stress perturbations to the later stages of rifting, possibly as late as 2.7–1.1 Ma. Our fault observations and previous paleomagnetic evidence of post–2.7 Ma counterclockwise vertical-axis rotations are consistent with increased bulk sinistral-normal oblique shear along the Santo Domingo rift segment in Pliocene and later time. Regional geologic evidence suggests that the width of active rift faulting became increasingly confined to the Santo Domingo Basin and axial parts of the adjoining basins beginning in the late Miocene. We infer that the Santo Domingo clockwise stress perturbations developed coevally with the oblique rift segment mainly due to mechanical interactions of large faults propagating toward each other from the adjoining basins as the rift narrowed. Our results suggest that negligible bulk strike-slip displacement has been accommodated along the north-trending rift during much of its development, but uncertainties in the maximum ages of fault slip do not allow us to fully evaluate and discriminate between earlier models that invoked northward or southward rotation and translation of the Colorado Plateau during early (Miocene) rifting.

New Mexico↗

USGS1995

Jurassic tectonics of northeastern Nevada and northwestern Utah from the perspective of barometric studies

Jurassic tectonism in the northeastern Great Basin produced varied structures, many closely associated with widespread magmatism at ca. 155–165 Ma and with local metamorphism. Many of the plutons are of suitable mineralogy for Al-in-hornblende barometry, providing the potential for depth data. We have studied conditions of metamorphism in the Pilot Range and barometry for six Jurassic plutons across the northeastern Great Basin. All barometry results are in harmony with pressures estimated from stratigraphic data, requiring little or no tectonic thickening. On the basis of structural styles and barometric data, we divide the northeastern Great Basin into three Jurassic tectonic provinces. An eastern extensional province, largely in western Utah, is characterized by Paleozoic strata that were thrust faulted and then intruded by shallow plutons shortly after or during normal and strike-slip faulting. Extension was probably a short-lived event associated with magmatism, but its west trend indicates a total reorientation of stress at this time, perhaps within transtensional strike-slip zones. A central province of modest, and possibly locally extreme, Jurassic shortening in eastern Nevada is characterized by metamorphosed Paleozoic rocks and by thrusts and kilometer-scale southeast-vergent folds. Upper amphibolite facies, but low pressure (3–4 kbar) metamorphism is present near Jurassic plutons in the Pilot Range and Ruby Mountains, probably indicating metamorphism induced by heat from magmas. In contrast, metamorphism in other ranges, which is known only to be pre–Late Cretaceous, indicates thickening of 10–20 km. This thickening may have entirely postdated the Jurassic. A western province in north-central Nevada is characterized by preserved Jurassic volcanic rocks and shallow plutons, indicating that little erosion, and probably surface uplift, occurred during the late Mesozoic. Folds and thrust faults indicate minor Jurassic shortening but many structures are undated. The low-pressure upper-crustal conditions for demonstrably Jurassic events suggest that higher-pressure metamorphism recorded in the central province is younger (Cretaceous) in age. We suggest that Jurassic structures were caused by distributed minor crustal shortening, manifested mainly as small-scale thrust faults. Local thermal highs created by plutonism produced metamorphic zones in relatively shallow crust. Shortening in the east was manifested by zones of strike-slip, within which plutons were emplaced in tensile niches. Lack of a deep foreland basin and lack of evidence for massive erosion argue against high-relief mountain belts caused by significant crustal shortening. Paleozoic rocks metamorphosed at pressures far in excess of stratigraphic burial are restricted to narrow lenses exhumed during Late Cretaceous and Tertiary extension and are bordered by rocks that always have been part of the shallow crust. The abundant shallow-crustal rocks preserved across the region indicate that a conventional hypothesis of large-scale, regional crustal thickening causing many kilometers of surface uplift and consequent erosion is unlikely to have taken place in the Mesozoic.

Nevada, Utah↗

USGS2014

Sulfur and oxygen isotopic study of Paleozoic sediment-hosted Zn-Pb(-Ag-Au-Ba-F) deposits and associated hydrothermal alteration zones in the Nome Complex, Seward Peninsula, Alaska

Results of sulfur and oxygen isotope studies of sedimentary exhalative (SEDEX) Zn-Pb(-Ag-Au-Ba-F) deposits hosted in metamorphosed Paleozoic clastic and carbonate rocks of the Nome Complex, Seward Peninsula, Alaska, are consistent with data for similar deposits worldwide. Stable isotopic studies of the Nome Complex are challenging because the rocks have undergone Mesozoic blueschist- and greenschist-facies metamorphism and deformation at temperatures estimated from 390–490 °C. Studies of sulfur and oxygen isotopes in other areas suggest that, in the absence of chemical and mineralogical evidence for metasomatism, the principal effect of metamorphism is re-equilibration between individual minerals at the temperature of metamorphism, which commonly leads to a narrowing of the overall range of isotope values for a suite of rocks, but generally does not significantly modify the average whole rock value for that suite. Sulfur isotope studies of the stratabound and locally stratiform sulfide lenses at the Aurora Creek-Christophosen deposit, which is of possible Late Devonian-early Carboniferous age, show a large range of δ34Ssulfide values from -9.7 to 39.4‰, suggesting multiple sulfur sources and possibly complex processes of sulfide formation that may include bacterial sulfate reduction, thermochemical sulfate reduction, and Rayleigh distillation. Low δ34S values probably represent bacterial sulfide minerals remobilized from the host metasedimentary rocks either during the original seafloor mineralization or are related to a Cretaceous mineralizing event that produced Au-vein and base-metal replacement deposits; the latter process is supported by Pb isotope data. The Wheeler North deposit is similar to Aurora Creek-Christophosen but does not have negative δ34S values. It also probably formed in an euxinic sub-basin. The stratabound Nelson deposit, and the deformed veins at the Galena and Quarry deposits, may be older than the Aurora Creek-Christophosen and Wheeler North deposits. The Nelson deposit has a lower and narrower range of δ34S values (1.9 to 10.4‰), averaging about 8‰. The Galena and Quarry veins display δ34S values that are similar to those of the stratabound Nelson deposit. Barite samples from the Aurora Creek-Christophosen, Wheeler North, and Quarry deposits have 34S-enriched δ34S values between 25 and 30‰ that are consistent with derivation of the sulfur from coeval (Paleozoic) seawater sulfate. Given their δ34S values, it is likely that the Aurora Creek-Christophosen and Wheeler North deposits formed in closed sub-basins with euxinic conditions that led to extreme Rayleigh distillation to produce the very large range and very high δ34S values. The Nelson deposit probably formed within an anoxic but not euxinic sub-basin. At Nelson, sulfide was likely derived by a subsurface thermochemical sulfate reduction (TSR) reaction, similar to reactions that are inferred to have produced the sulfides in the Galena and Quarry deposits, which are interpreted as feeder veins for the stratabound deposits. Calculations of oxygen isotope temperatures are based on the assumption that evolved seawater with δ18O of 3‰ was the mineralizing and altering fluid related to the formation of the sulfide deposits. Temperatures of aluminous alteration and sulfide mineralization were between 109 and 209 °C, determined on the basis of oxygen isotope fractionations between the mineralizing fluid and proportionate amounts of quartz and muscovite in the rocks. These temperature estimates agree well with known temperatures of SEDEX mineralization worldwide. Sulfur isotope values also are generally consistent with the known ranges in SEDEX deposits worldwide (δ34S ≈ -5 to 25‰).

GSA Special Papers↗

USGS1936

Supplementary report on the ground-water supplies of the Atlantic City region

This report is the second progress report on the ground-water investigations in the Atlantic City region. Many important problems still remain to be solved, however, and it is in no sense a final report. The report covers the area immediately surrounding Atlantic City, extending from Brigantine to Sea Isle City along the coast and from Absecon to Somers Point on the mainland. In addition to this, a few data are presented bearing on the area along the coast as far south as Wildwood. The area lies in the southern part of the New Jersey Coastal Plain, and the water-bearing formations considered are all unconsolidated and of Miocene or more recent age. The major formations in the region dip gently toward the ocean and possibly extend out under the ocean to the edge of the Continental Shelf, about 100 miles from Atlantic City. The principal ground-water supplies in the area are derived from the so-called "800-foot sand," a member of the Kirkwood formation, and from the overlying Cohansey sands. The 800-foot sand is of wide extent and apparently fairly uniform. The Cohansey sands, on the other hand, cover a wide area but are by no means uniform. At the Atlantic City Water Works two Cohansey sands are recognized-the so-called "100-foot" and "200-foot" sands. Neither of these sands can be differentiated from the other sands of the Cohansey formation over a distance greater than 4 or 5 miles in any direction. In addition to the supplies derived from ground water, some surface water is used at present by two of the public water supplies. The quality of the water from all the sources of supply now used is satisfactory. The total consumption of water in the region has increased gradually over the entire period of record, except for a moderate decline from 1929 to 1934. Additional water supplies can be obtained from either of two fairly large streams near the region, and possibly also from a more widespread development of the Cohansey sands. The 800-foot sand should not be counted upon as a source of additional water supply, in view of its liability to salt-water contamination. The same danger also exists in the Cohansey sands on the mainland near the shore, but farther inland it is not a serious menace to the supply from these sands The 100-foot sand at the Atlantic City Water Work has been overdeveloped since 1930, with the result that the head of the water in it has been lowered materially and salt water has been drawn into it through holes in the overlying clay beneath the nearby salt marshes. Three of the five new large-capacity wells drilled to this sand in 1930 have been temporarily or permanently abandoned on account of salt-water intrusion, and the two others will probably have to be abandoned also, unless suitable remedial measures are promptly adopted. It is recommended that the wells to the 100-foot sand be used only when needed to supply the seasonal peak demand and that consideration be given to a project to transform the tidal marshes into a fresh-water pond by means of a suitable dam in order to protect the formation from further contamination. If detailed study proves that the dam and fresh-water pond would not be economically justified, a smaller pond and an embankment and tide gates on the main stream to keep the salt water from flooding the marshes are recommended as less effective but less expensive remedial measures. In view of the experience with the 100-foot sand at the Atlantic City Water Works, it is further recommended that any additional development of the Cohansey sands be preceded by a comprehensive test-well program that will indicate not only the capacity of the sands, but the location of salt water in them and the possibility of its being drawn into existing wells or the proposed new wells. The salt-water intrusion into the 100-foot sand was effectively studied by means of driven-well points, which, it was found, could easily be driven to a depth of about 100 feet. The fact that the screen of these wells was driven with the casing and that no water was used in the drilling process made it possible to collect true samples of water from every sand encountered in them. This, in turn, made it possible to study the vertical distribution of salt water in each well. The interpretation of the vertical distribution of salt water in these wells was very helpful in arriving at a final decision as to the source of the salt water. The 200-foot sand at the Atlantic City Water Works has also been subjected to a considerably increased draft since 1930. Although there is no evidence at present that this sand has been overdeveloped, a study of its characteristics suggests that it may not be capable of yielding permanently the capacity of the present wells that tap it. Three test wells have been installed between the well field and the source from which this sand might derive salt water, and they should be sampled regularly to determine the danger of salt-water intrusion into the sand. In a landward direction this sand merges into the other Cohansey sands. It is therefore advisable that any additional development of the Cohansey sands should be undertaken so far inland that the pumping from it will not affect the present wells to the 200-foot sand and thereby increase the danger of salt-water contamination in them. At present more potable water is taken from the Atlantic City 800-foot sand than from any other source of supply for the region. This sand is the sole source for some of the smaller communities on the barrier beaches. The original static head of the water in it at Atlantic City was between 20 and 25 feet above sea level. The head has been lowered more than 50 feet over much of the region, and in parts of Atlantic City it has been lowered considerably more than 100 feet. A consideration of the principles governing the relation between salt water and fresh water in water-bearing sands indicates that the 800-foot sand probably contained salt water at a distance of 5 or 10 miles out from Atlantic City before any water was pumped from it. The evidence collected in this investigation indicates that the cone of depression created by the pumping from this sand in the Atlantic City region has probably extended inland to the intake area of the sand, the nearest part of which is probably about 40 miles from Atlantic City. If this is so, the conclusion is almost inescapable that it has also extended oceanward for a distance considerably greater than the 5 or 10 miles to the original zone of contact between the fresh and salt waters, and that salt water is probably being drawn toward the Atlantic City region through this sand. The time of its arrival will depend primarily upon the rate of pumping in the region and upon how much of the fresh water that originally lay between the region and the zone of contact must be removed before the salt water can reach the region. It may arrive in the near future if it advances in the form of a narrow tongue. On the other hand, if it advances along a broader front; so that more of the intervening fresh water must be pumped out of the formation, its arrival may be delayed for some time.

New Jersey↗

USGS2005

Reconnaissance study of late quaternary faulting along Cerro Goden fault zone, western Puerto Rico

The Cerro Goden fault zone is associated with a curvilinear, continuous, and prominent topographic lineament in western Puerto Rico. The fault varies in strike from northwest to west. In its westernmost section, the fault is ∼500 m south of an abrupt, curvilinear mountain front separating the 270- to 361-m-high La Cadena de San Francisco range from the Rio Añasco alluvial valley. The Quaternary fault of the Añasco Valley is in alignment with the bedrock fault mapped by D. McIntyre (1971) in the Central La Plata quadrangle sheet east of Añasco Valley. Previous workers have postulated that the Cerro Goden fault zone continues southeast from the Añasco Valley and merges with the Great Southern Puerto Rico fault zone of south-central Puerto Rico. West of the Añasco Valley, the fault continues offshore into the Mona Passage (Caribbean Sea) where it is characterized by offsets of seafloor sediments estimated to be of late Quaternary age. Using both 1:18,500 scale air photographs taken in 1936 and 1:40,000 scale photographs taken by the U.S. Department of Agriculture in 1986, we identified geomorphic features suggestive of Quaternary fault movement in the Añasco Valley, including aligned and deflected drainages, apparently offset terrace risers, and mountain-facing scarps. Many of these features suggest right-lateral displacement. Mapping of Paleogene bedrock units in the uplifted La Cadena range adjacent to the Cerro Goden fault zone reveals the main tectonic events that have culminated in late Quaternary normal-oblique displacement across the Cerro Goden fault. Cretaceous to Eocene rocks of the La Cadena range exhibit large folds with wavelengths of several kms. The orientation of folds and analysis of fault striations within the folds indicate that the folds formed by northeast-southwest shortening in present-day geographic coordinates. The age of deformation is well constrained as late Eocene–early Oligocene by an angular unconformity separating folded, deep-marine middle Eocene rocks from transgressive, shallow-marine rocks of middle-upper Oligocene age. Rocks of middle Oligocene–early Pliocene age above unconformity are gently folded about the roughly east-west–trending Puerto Rico–Virgin Islands arch, which is well expressed in the geomorphology of western Puerto Rico. Arching appears ongoing because onshore and offshore late Quaternary oblique-slip faults closely parallel the complexly deformed crest of the arch and appear to be related to extensional strains focused in the crest of the arch. We estimate ∼4 km of vertical throw on the Cerro Goden fault based on the position of the carbonate cap north of the fault in the La Cadena de San Francisco and its position south of the fault inferred from seismic reflection data in Mayaguez Bay. Based on these observations, our interpretation of the kinematics and history of the Cerro Goden fault zone includes two major phases of motion: (1) Eocene northeast-southwest shortening possibly accompanied by left-lateral shearing as determined by previous workers on the Great Southern Puerto Rico fault zone; and (2) post–early Pliocene regional arching of Puerto Rico accompanied by normal offset and right-lateral shear along faults flanking the crest of the arch. The second phase of deformation accompanied east-west opening of the Mona rift and is inferred to continue to the present day.

Puerto Rico↗

USGS2022

Downhill from Austin and Ely to Las Vegas: U-Pb detrital zircon suites from the Eocene–Oligocene Titus Canyon Formation and associated strata, Death Valley, California

In a reconnaissance investigation aimed at interrogating the changing topography and paleogeography of the western United States prior to Basin and Range faulting, a preliminary study made use of U-Pb ages of detrital zircon suites from 16 samples from the Eocene–Oligocene Titus Canyon Formation, its overlying units, and correlatives near Death Valley. The Titus Canyon Formation unconformably overlies Neoproterozoic to Devonian strata in the Funeral and Grapevine Mountains of California and Nevada. Samples were collected from (1) the type area in Titus Canyon, (2) the headwaters of Monarch Canyon, and (3) unnamed Cenozoic strata exposed in a klippe of the Boundary Canyon fault in the central Funeral Mountains. Red beds and conglomerates at the base of the Titus Canyon Formation at locations 1 and 2, which contain previously reported 38–37 Ma fossils, yielded mostly Sierran batholith–age detrital zircons (defined by Triassic, Jurassic, and Cretaceous peaks). Overlying channelized fluvial sandstones, conglomerates, and minor lacustrine shale, marl, and limestone record an abrupt change in source region around 38–36 Ma or slightly later, from more local, Sierran arc–derived sediment to extraregional sources to the north. Clasts of red radiolarian-bearing chert, dark radiolarian chert, and quartzite indicate sources in the region of the Golconda and Roberts Mountains allochthons of northern Nevada. Sandstones intercalated with conglomerate contain increasing proportions of Cenozoic zircon sourced from south-migrating, caldera-forming eruptions at the latitude of Austin and Ely in Nevada with maximum depositional ages (MDAs) ranging from 36 to 24 Ma at the top of the Titus Canyon Formation. Carbonate clasts and ash-rich horizons become more prevalent in the overlying conglomeratic Panuga Formation (which contains a previously dated 15.7 Ma ash-flow tuff). The base of the higher, ash-dominated Wahguyhe Formation yielded a MDA of 14.4 Ma. The central Funeral Mountains section exposes a different sequence of units that, based on new data, are correlative to the Titus Canyon, Panuga, and Wahguyhe Formations at locations 1 and 2. An ash-flow tuff above its (unexposed) base provided a MDA of 34 Ma, and the youngest sample yielded a MDA of 12.7 Ma. The striking differences between age-correlative sections, together with map-based evidence for channelization, indicate that the Titus Canyon Formation and overlying units likely represent fluvial channel, floodplain, and lacustrine deposits as sediments mostly bypassed the region, moving south toward the Paleogene shoreline in the Mojave Desert. The profound changes in source regions and sedimentary facies documented in the Titus Canyon Formation took place during ignimbrite flareup magmatism and a proposed eastward shift of the continental divide from the axis of the Cretaceous arc to a new divide in central Nevada in response to thermal uplift and addition of magma to the crust. This uplift initiated south-flowing fluvial systems that supplied sediments to the Titus Canyon Formation and higher units.

Arizona, California, Nevada, Utah↗

USGS2010

Pesticides in groundwater in the Anacostia River and Rock Creek watersheds in Washington, D.C., 2005 and 2008

The U.S. Geological Survey (USGS), in cooperation with the District Department of the Environment, conducted a groundwater-quality investigation to (a) determine the presence, concentrations, and distribution of selected pesticides in groundwater, and (b) assess the presence of pesticides in groundwater in relation to selected landscape, hydrogeologic, and groundwater-quality characteristics in the shallow groundwater underlying the Anacostia River and Rock Creek watersheds in Washington, D.C. With one exception, well depths were 100 feet or less below land surface. The USGS obtained or compiled ancillary data and information on land use (2001), subsurface sediments, and groundwater samples from 17 wells in the lower Anacostia River watershed from September through December 2005, and from 14 wells in the lower Anacostia River and lower Rock Creek watersheds from August through September 2008. Twenty-seven pesticide compounds, reflecting at least 19 different types of pesticides, were detected in the groundwater samples obtained in 2005 and 2008. No fungicides were detected. In relation to the pesticides detected, degradate compounds were as or more likely to be detected than applied (parent) compounds. The detected pesticides chiefly reflected herbicides commonly used in urban settings for non-specific weed control or insecticides used for nonspecific haustellate insects (insects with specialized mouthparts for sucking liquid) or termite-specific control. Detected pesticides included a combination of pesticides currently (2008) in use, banned or under highly restricted use, and some that had replaced the banned or restricted-use pesticides. The presence of banned and restricted-use pesticides illustrates their continued persistence and resistance to complete degradation in the environment. The presence of the replacement pesticides indicates the susceptibility of the surficial aquifer to contamination irrespective of the changes in the pesticides used. A preliminary review of the data collected in 2005 and 2008 indicated that differences in the surficial geology, land use (as a surrogate for pesticide use), and above-average precipitation for most of 2004 through 2008, as well as differences in the number and performance of USGS laboratory methods used, could have led to more pesticides detected in groundwater samples collected in 2008 than in groundwater samples collected in 2005. Thus, although data from both years of collection were used for interpretive analysis, emphasis was placed on the analysis of the data obtained in 2008. The presence of pesticides in shallow groundwater (less than approximately 100 ft (feet), or 30 m (meters), below land surface) indicated at least the upper surficial aquifer in Washington, D.C. was susceptible to contamination. One or more herbicides or insecticides were detected in groundwater samples collected from 50 percent of the shallow wells sampled in 2005, and from 62 percent of the shallow wells sampled in 2008. Differences among types of pesticides in shallow groundwater were apparent. The most frequently detected class of herbicides was the s-triazine compounds—atrazine, simazine, or prometon, or the atrazine-degradate compounds—2-chloro-4-ethylamino-6-amino-s-triazine (desethylatrazine or CIAT) and 2-chloro-4-isopropylamino-6-amino-s-triazine (hydroxyatrazine or OIET). The next most frequently detected classes of herbicides were the chloroacetanilides, including metolachlor and acetochlor, and the ureic herbicides, including diuron (and degradate, 3,4-dichloroaniline), fluometuron, metsulfuron methyl, sulfameturon, bromacil, and tebuthiuron. Insecticides also were detected, but less frequently than herbicides, with one or more insecticides present in groundwater samples from 38 percent of shallow wells sampled in 2008. Detected insecticides included parent or degradate compounds commonly used for either nonspecific or haustellate (sucking) insects, including chlorpyrifos and dichlorodiphenyldichloroethane (p,p’-DDD; a degradate of dichlorodiphenyltrichloroethane, DDT), and for termite control, including dieldrin, chlordane, heptachlor epoxide, (a degradate of heptachlor), fipronil, and the sulfone and sulfide degradates of fipronil. The concentrations of individual pesticides in shallow groundwater in both years were low. Maximum concentrations were no greater than a few tenths of a microgram per liter (μg/L); typical concentrations often were less than 0.1 μg/L. Multiple pesticides, however, commonly were present in groundwater. For example, in 2008, approximately 88 percent (7 of 8) of the wells that yielded a sample with at least one detectable pesticide contained five or more pesticides. The highest number of detections occurred in a groundwater sample from well WE Ca 32, which is located in a highly developed urban area; this sample contained 15 different pesticide residues. In relation to human and aquatic health, no pesticide concentration in either 2005 or 2008 exceeded Federal drinking-water standards. Groundwater samples from a few sites, however, contained levels of chiefly banned or restricted-use pesticides that exceeded other human-health and (or) aquatic-health guidelines. For example, concentrations of dieldrin in 2008 groundwater samples from three wells—WE Ca 32 (0.028 μg/L), WE Ba 11 (0.016 μg/L), and WW Ac 8 (0.014 μg/L)—fell within the range of concern for 2004 Federally approved non-regulatory USGS Health-Based Assessment benchmarks (0.002 to 0.2 μg/L), and exceeded earlier (1999) Federal criteria for drinking water (0.000052 μg/L). Other individual compounds whose concentrations exceeded 1999 Federal guidelines for samples from one or more of these three sites, or another site, included p,p’-DDD, dichlorodiphenyldichloroethylene (p,p’-DDE; another degradate of DDT), chlordane, and heptachlor epoxide. Pesticide concentrations in groundwater also were compared to three aquatic-health guidelines for freshwater (United States, Great Lakes, or Canada). One or more of these guidelines were exceeded in groundwater samples obtained in 2005 or 2008 for one or more of the compounds chlordane, dieldrin, heptachlor epoxide, p,p’-DDE, p,p’-DDD, and chlorpyrifos. The spatial distribution of pesticides in the shallow groundwater appeared to be related, in part, to land use, a surrogate for pesticide use. Although most of the wells sampled in this study are in parklands or other relatively open and accessible space, multiple pesticides most often were detected in 2008 groundwater samples collected from wells where a considerable percentage (in excess of 60 percent) of the land within a 500-m radius is developed space (residential, commercial, or other urban infrastructure). Insecticides were detected in wells surrounded by at least 50 percent, and most commonly by more than 80 percent, development. Well WE Ca 32, the site associated with the highest number of pesticide residues in groundwater (8 herbicides and 7 insecticides), is in a small residential park, where 99 percent of the surrounding land is well-maintained residential and commercial development. The vertical distribution of detected pesticides in shallow groundwater appeared to be related, in part, to depth below land surface, surficial-bedrock type, and differences in the chemistry of shallow groundwater. Pesticides were detected at relatively shallow depths in wells that may not have fully penetrated the shallow aquifer. For wells in which at least one pesticide was detected, the median depth below land surface to the top of the well screen was 5.8 m, and the maximum depth was 8.5 m. Among the types of surficial materials in which wells were completed—alluvium, terrace deposits, or Potomac Formation sub- or outcrops in the Coastal Plain Province, and saprolite or fractured bedrock (Laurel and Sykesville Formations) underlying saprolite in the Piedmont Province—no pesticides were detected in groundwater associated with wells completed in the alluvium or fractured bedrock. Detections occurred in some but not all wells completed in the other surficial materials. Overall, the pattern in occurrence appeared related to the local permeability of these sediments and groundwater chemistry. Groundwater with multiple pesticide detections tended to occur in permeable sediments (absent any appreciable overlying clay, silt, or clay-silt layers), in conjunction with other common urban contaminants (elevated chloride in excess of tens to hundreds of milligrams per liter (mg/L), and oxic, rather than reduced, groundwater as evidenced by elevated (in excess of 5 mg/L) concentrations of nitrate). The results of this investigation were compared to results from two other similar and recent studies on pesticide occurrence in the shallow aquifer. These included a study in the nearby Maryland and Delaware Coastal Plain Physiographic Province and one in the Maryland and Virginia Piedmont Physiographic Province. Results from these studies were similar to the current study in relation to (a) the types, frequencies, concentrations, and mixtures of pesticides detected; (b) compounds that exceeded human-and aquatic-health criteria; and (c) the occurrence and distribution of pesticides within the surficial aquifer in relation to depths, sediment types, and groundwater chemistries.

Washington D.C.↗

USGS1993

Entrenchment and widening of the upper San Pedro River, Arizona

The San Pedro River of southeast Arizona is a north-flowing tributary of the Gila River. The area of the drainage basin upstream of the 40-km-long study reach is about 3,200 km 2 . This study traces the historical evolution of the San Pedro River channel—specifically, the deepening, widening, and sediment deposition that have occurred since 1900—and it aims to evaluate the causes of channel widening and deepening, the rate of widening, and the present stability of the channel. Alluvium of the river valley consists of upper Holocene pre- and postentrenchment deposits. The pre-entrenchment alluvium, which forms the principal terrace of the inner valley, accumulated between about A.D. 1450 and 1900 in a relatively sluggish, low-energy fluvial system with extensive marshy reaches and high water table. In contrast, postentrenchment alluvium, which forms the terrace, floodplain, and channel of the San Pedro River, was deposited in a relatively high-energy, entrenched, and meandering fluvial system. The river flowed in a shallow, narrow channel on the surface of the unentrenched valley before 1890. A series of large floods, perhaps beginning as early as 1881, eventually led to entrenchment of the channel between 1890 and 1908. This deepening placed the channel 1 to 10 m below the former floodplain. The channel has widened substantially since entrenchment through lateral migration and expansion of entrenched meanders; its present size is 5.7 times greater than before entrenchment. The rate of channel expansion, however, has decreased since about 1955, coincident with a decrease of peak-flood discharge. Channel area increased at 0.1 km 2 yr −1 from entrenchment until 1955; since then the area increased at only 0.02 km 2 yr −1 , suggesting that the channel has stabilized and that further widening will probably be minor under present conditions of land use, discharge, and climate. The reduction of peak-flow rates was related partly to increased channel sinuosity and to development of floodplains and riparian woodlands. The increased sinuosity produced a reservoir effect that attenuated flood waves, and the development of flood-plains enabled flood waters to spread laterally, thereby increasing transmission losses. In addition, flow rates were probably affected by improved land use and changes of rainfall intensity and short-term rainfall patterns, which reduced runoff and decreased the time necessary for channel stabilization. Livestock grazing decreased steadily after the turn of the century, and numerous stock ponds and small water-retention structures were constructed in tributaries. The cumulative effect of these structures probably reduced peak-flow rates. Short-term rainfall patterns of the wet season (June 15–October 15) have probably changed from annual alternation of above- and below-average rainfall to a biennial or longer pattern. Moreover, frequency of low-intensity rainfall (daily rainfall less than about 1.27 cm) was consistently above average for the decade 1957–1967. These factors probably improved conditions for growth and establishment of vegetation both in and outside of the channel. The causes of the large floods that resulted in entrenchment are poorly understood, although climate and land use were key factors. Floods followed closely the rapid settlement of the area brought about by mining activity in the late 1870s; population rose from a few hundred to 6,000 in less than 5 yr. Extensive wood cutting for mine timber and fuel, suppression of wildfire, and reintroduction of large cattle herds undoubtedly exacerbated entrenchment. Flood-producing wet-season rainfall in the Southwest, however, was unusually heavy before, during, and shortly after entrenchment.

Arizona↗