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At least 1,549 records · Page 86Linked to original sources
USGS1997

Regional landslide-hazard evaluation using landslide slopes, Western Wasatch County, Utah

Landsliding has historically been one of the most damaging geologic hazards in western Wasatch County, Utah. Accordingly, we mapped and analyzed landslides (slumps and debris slides) in the area to provide an empirical basis for regional landslide-hazard evaluation. The 336 landslides in the 250-sq-mi (650-km 2 ) area involve 20 geologic units, including Mississippian- to Quaternary-aged rock and unconsolidated deposits. Landsliding in western Wasatch County is characterized by a strong correlation between geologic material and landslide-slope inclination. From a simple statistical analysis of overall slope inclinations of late Holocene landslides, we determined “critical” slope inclinations above which late Holocene landsliding has typically occurred and used these as the primary basis for defining relative landslide hazard. The critical slopes vary for individual geologic units and range from 15 to 50 percent (9°–27°). The critical slope values and landslide locations were used in conjunction with geologic and slope maps to construct qualitative landslide-susceptibility maps for use by county planners. The maps delineate areas of low, moderate, and high relative hazard and indicate where studies should be completed prior to development to evaluate site-specific slope-stability conditions. Critical slopes as determined in this study provide a consistent empirical reference that is useful for evaluating relative landslide hazard and guiding land-use-planning decisions in large, geologically complex areas.

Utah↗
USGS1989

A close-range photogrammetric technique for mapping neotectonic features in trenches

Close-range photogrammetric techniques and newly available computerized plotting equipment were used to map exploratory trench walls that expose Quaternary faults in the vicinity of Yucca Mountain, Nevada. Small-scale structural, lithologic, and stratigraphic features can be rapidly mapped by the photogrammetric method. This method is more accurate and significantly more rapid than conventional trench-mapping methods, and the analytical plotter is capable of producing cartographic definition of high resolution when detailed trench maps are necessary.

Bulletin of the Association of Engineering Geologi↗
USGS2017

Oxygen isotope geochemistry of mafic phenocrysts in primitive mafic lavas from the southernmost Cascade Range, California

Previously reported whole-rock δ 18 O values (5.6–7.8‰) for primitive quaternary mafic lavas from the southernmost Cascades (SMC) are often elevated (up to 1‰) relative to δ 18 O values expected for mafic magmas in equilibrium with mantle peridotite. Olivine, clinopyroxene, and plagioclase crystals were separated from 29 geochemically well-characterized mafic lavas for δ 18 O measurements by laser fluorination to assess modification of the mantle sources by ancient and modern subducted components. Oxygen isotope values of olivine phenocrysts in calc-alkaline lavas and contemporaneous high alumina olivine tholeiitic (HAOT) lavas generally exceed depleted mantle olivine values (~4.9–5.3‰). Modern addition of up to 6 wt% slab-derived fluid from Gorda serpentinized peridotite dehydration (~15‰) or chlorite dehydration (~10‰) within the serpentinized peridotite can provide the 18 O enrichment detected in olivine phenocrysts (δ 18 O olivine = 5.3–6.3‰) in calc-alkaline mafic lavas, and elevate 18 O in overlying mantle lithosphere, as well. Specifically, although HAOT δ 18 O olivine values (5.5–5.7‰) may reflect partial melting in heterogeneous 18 O enriched mantle source domains that developed during multiple subduction events associated with terrane accretion (e.g., <1 wt% of ~15‰ materials), an additional 18 O enrichment of up to 2 wt% of 10–15‰ slab-derived hydrous fluids might be accommodated. The calc-alkaline primitive magmas appear to have experienced a continuous range of open system processes, which operate in the mantle and during rapid magma ascent to eruption, and occasionally post quench. Textural relationships and geochemistry of these lava samples are consistent with blends of mafic phenocrysts and degassed melts in varying states of 18 O disequilibrium. In lenses of accumulated melt within peridotite near the base of the crust, coexisting olivine and clinopyroxene δ 18 O values probably are not at isotopic equilibrium because fluids introduced into the system perturbed the δ 18 O melt values. A “sudden” melt extraction event interrupts 18 O equilibration in phenocrysts and poorly mixed melt(s). Rapid ascent of volatile oversaturated primitive mafic magma through the crust appears to be accompanied by devolatilization and crystallization of anorthite-rich plagioclase with elevated δ 18 O plag values. The (Sr/P) N values for the whole rock geochemistry are consistent with a 87 Sr/ 86 Sr ~0.7027 slab-derived fluid addition into the infertile peridotite source of magmas, and melt devolatilization is recorded in the mixture of disequilibrium δ 18 O values for the constituent phases of lavas. Morbidity of the Gorda Plate as it undergoes intense deformation from the spreading ridge to the trench is likely a key factor to developing the carrying capacity of hydrous fluids and mineral phases in the slab subducting into the SMC mantle.

California↗
USGS1986

Geology of the platanares geothermal area, Departamento de Copan, Honduras

Platanares is located 16 km west of Santa Rosa de Copan, Honduras, along the Quebrada del Agua Caliente. The thermal manifestations are along faults in tuffs, tuffaceous sedimentary rocks, and lavas of the Padre Miguel Group. These tuffs are silicified near the faults, are fractured, and may provide the fracture permeability necessary for the hydrothermal system. Tuffs are overlain by a wedge of terrace gravels up to 60 m thick. Quaternary conglomerates of the Quebrada del Agua Caliente are cemented by silica sinter. The Platanares area contains numerous faults, all of which appear to be extensional. There are four groups of faults (N80/sup 0/E to N70/sup 0/W, N30/sup 0/ to 60/sup 0/W, N40/sup 0/ to 65/sup 0/E, and N00/sup 0/ to 05/sup 0/W). All hot springs at this site are located along faults that trend mostly northwest and north. Twenty-eight spring groups were described over an area of 0.2 km/sup 2/; half were boiling. Based on surface temperatures and flow rates, between 0.7 and 1.0 MW thermal energy is estimated for the area. The increased temperature of the stream flowing through the thermal area indicates that several megawatts of thermal energy are being added to the stream. We recommend that a dipole-dipole resistivity line be run along the Quebrada del Agua Caliente to identify zones of fracture permeability associated with buried faults and hot water reservoirs within those fault zones. A thermal gradient corehole should be drilled at Platanares to test temperatures, lithologies, and permeability of the hydrothermal system.

Journal of Volcanology and Geothermal Research↗
USGS2009

High-resolution hydro- and geo-stratigraphy at Atlantic Coastal Plain drillhole CR-622 (Strat 8)

We interpret borehole geophysical logs in conjunction with lithology developed from continuous core to produce high-resolution hydro- and geo-stratigraphic profiles for the drillhole CR-622 (Strat 8) in the Atlantic Coastal Plain of North Carolina. The resulting hydrologic and stratigraphic columns show a generalized relation between hydrologic and geologic units. Fresh-water aquifers encountered are the surficial, Yorktown, Pungo River and Castle Hayne. Geologic units present are of the middle and upper Tertiary and Quaternary periods, these are the Castle Hayne (Eocene), Pungo River (Miocene), Yorktown (Pliocene), James City and Flanner Beach (Pleistocene), and the topsoil (Holocene). The River Bend Formation (Oligocene) is missing as a distinct unit between the Pungo River Formation and the Castle Hayne Formation. The confining unit underlying the Yorktown Aquifer corresponds to the Yorktown Geologic Unit. The remaining hydrologic units and geologic units are hydrologically transitional and non-coincident. The lower Pungo River Formation serves as the confining unit for the Castle Hayne Aquifer, rather than the River Bend Aquifer, and separates the Pungo River Aquifer from the upper Castle Hayne Aquifer. All geologic formations were bound by unconformities. All aquifers were confined by the anticipated hydrologic units. We conclude that CR-622 (Strat 8) represents a normal sequence in the Atlantic Coastal Plain.

Stratigraphy↗
USGS2024

Chronology and Paleoenvironment of the Tunga Formation, a new lowermost Miocene sequence in the East Pisco Basin of southern Peru

The East Pisco Basin, occupying the coastal plain of Peru between 13°S and 16°S, is widely known for its extensive Eocene to Quaternary biosiliceous deposits and excellent preservation of fossil marine vertebrates. Biochronologic studies published over the past 35 years record a hiatus of about 13 million years (*32–19 Ma) separating the youngest Paleogene deposits (Otuma Formation) from the oldest Neogene deposits (Chilcatay Formation). We describe a newly identified Lower Miocene depositional sequence that lies below documented Chilcatay strata, rich in diatoms and benthic foraminifers, which we name the Tunga Formation. A low-latitude diatom assemblage from the Tunga Formation indicates an age of 21.6 to 20.5 Ma age, whereas an ash within basal Tunga strata yields an 206Pb/238U weighted mean age of 20.58 ± 0.13 Ma. Benthic foraminifer paleodepth analysis and the diatom assemblage of the Tunga Formation indicate deposition took place along the continental margin at upper bathyal depths under hypoxic conditions beneath highly productive waters, a scenario also supported by the presence of abundant clupeoid fish scales and dolomitized horizons. The Tunga Formation is characterized by a profound scarcity of cetacean fossils, unlike the cetacean-rich neritic sediments of the overlying Chilcatay Formation.

East Pisco Basin↗
USGS1985

Mid-Atlantic Ridge coccolith and silicoflagellate biostratigraphy, Deep Sea Drilling Project Sites 558 and 563.

Low-latitude coccolith zonation can be used for biostratigraphy at Mid-Atlantic Ridge sites DSDP 558 (lat. 38°N) and DSDP 563 (lat. 34°N). The low-latitude zonal sequence from lower Oligocene to Holocene is interrupted by coolwater assemblages in upper middle Miocene and by hiatuses that removed the lower Pliocene and part of the upper Pliocene. A gap in the range of zonal guide fossil Discoaster druggii in the lower Miocene, also identified in other ocean basins, occurs at both DSDP 558 and 563. Coccoliths are abundant and moderately overgrown at both sites. Pentaliths occur in the Oligocene at DSDP 563 but are missing at DSDP 558, probably the result of diagenesis. New taxa of coccoliths identified include Cyclolithellai neoaprica Bukry, n. sp., and Sphenolithus calyculus Bukry, n. sp. Silicoflagellates are limited to the upper Quaternary at DSDP 558 with warm-water assemblages of the Dictyocha aculeata Zone and possibly the upper Mesocena quadrangula Zone, as indicated by the presence of Dictyocha lingii. A new silicoflagellate species, Distephanus floridus Bukry, n. sp., is described.

Initial Reports of the D.S.D.P.↗
USGS1985

Tropical Pacific silicoflagellate zonation and paleotemperature trends of the late Cenozoic

Quantitative study of late Cenozoic silicoflagellates at tropical Pacific DSDP Sites 572 and 575 shows that the greatest amplitude of fluctuation in relative paleotemperature values occurred in the late Miocene. The coolest minimum paleotemperature values (near 75 = 30) also occurred in the late Miocene. The warmest intervals (Ts = 80 to 100) occurred in the middle Miocene and late Pliocene to Quaternary. In detail, the silicoflagellate relative paleotemperature curve correlates fairly well with the eustatic sea-level curve and deep-sea hiatus sequence. The only upper Cenozoic low-latitude biostratigraphic units not identified are the Distephanus speculum haliomma Subzone and Naviculopsis quadrata Zone, owing to the absence of the nominative species, which probably had nontropical ecologic preference. Several Naviculopsis occurrence events within the Naviculopsis ponticula Zone correlate between DSDP Hole 575A and DSDP Hole 495 off Guatemala. Many local and regional biostratigraphic events are recognized. New taxa identified from DSDP Leg 85 include Dictyocha nola Bukry, n. sp., Distephanus stradneri var. grandis Bukry, n. var., Mesocena elliptica var. rhomboidea Bukry, n. van, and Naviculopsis obtusarca var. acicula Bukry, n. var.

Initial Reports of the D.S.D.P.↗
USGS1989

Geochemical expression of early diagenesis in middle Eocene-lower Oligocene pelagic sediments in the southern Labrador Sea, Site 647, ODP Leg 105

Geochemical analyses of the middle Eocene through lower Oligocene lithologic Unit IIIC (260-518 meters below sea floor [mbsf]) indicate a relatively constant geochemical composition of the detrital fraction throughout this depositional interval at Ocean Drilling Program (ODP) Site 647 in the southern Labrador Sea. The main variability occurs in redox-sensitive elements (e.g., iron, manganese, and phosphorus), which may be related to early diagenetic inability in anaerobic pore waters during bacterial decomposition of organic matter. Initial preservation of organic matter was mediated by high sedimentation rates (36 m/m.Y.). High iron (Fe) and manganese (Mn) contents are associated with carbonate concretions of siderite, manganosiderite, and rhodochrosite. These concretions probably formed in response to elevated pore-water alkalinity and total dissolved carbon dioxide (CO2) concentrations resulting from bacterial sulfate reduction, as indicated by nodule stable-isotope compositions and pore-water geochemistry. These nodules differ from those found in upper Cenozoic hemipelagic sequences in that they are not associated with methanogenesis. Phosphate minerals (carbonate-fluorapatite) precipitated in some intervals, probably as the result of desorption of phosphorus from iron and manganese during reduction. The bulk chemical composition of the sediments differs little from that of North Atlantic Quaternary abyssal red clays, but may contain a minor hydrothermal component. The silicon/ aluminum (Si/Al) ratio, however, is high and variable and probably reflects original variations in biagenic opal, much of which is now altered to smectite and/or opal CT. An increase in the sodium/potassium (Na/K) ratio in the upper Eocene corresponds to the beginning of coarser-grained feldspar flux to the site, possibly marking the onset of more vigorous deep currents. Although the Site 647 cores provide a nearly complete high-resolution, high-latitude Eocene-Oligocene record, the high sedimentation rate and somewhat unusual diagenetic conditions have led to variable alteration of benthic foraminifers and fine-fraction carbonate and have overprinted the original stable-isotope records. Planktonic foraminifers are less altered, but on the whole, there is little chance of sorting out the nature and timing of environmental change on the basis of our stable-isotope analyses.

Proceedings of the Ocean Drilling Program: Scienti↗
USGS2000

Data Report: Intra-annual variability of the diatom assemblages at Hole 1034B (Saanich Inlet) near 9 ka

Ocean Drilling Program (ODP) Site 1034 (48°38.000´N, 123°30.000´W) was drilled at a water depth of 200 m in the Saanich Inlet, an anoxic fjord on the southeastern coast of Vancouver Island, British Columbia, to a depth of 118.2 meters below seafloor (mbsf). The uppermost 50 m consists of very well-laminated (triplet varves) diatomaceous muds deposited over the past 7000 yr. Below, sediments become progressively less distinctly laminated and reflect better oxygenated bottom-water conditions. The oldest sediments recovered at Site 1034 were dated as 14 to 15 ka (see Shipboard Scientific Party, 1998). Varved sediments recovered during Leg 169S in the Saanich Inlet offer an excellent opportunity to compare the interannual variability of recent climatic elements (e.g., sea-surface temperature, salinity, precipitation) with those of late Quaternary periods that were characterized by climates appreciably different from those of today. A large amount of recent diatom data from Saanich Inlet exists, which can serve as a baseline for such comparisons: Sancetta (1989a, 1989b, 1990) investigated modern processes controlling the accumulation of diatoms and spacial and temporal trends of diatom flux in the Inlet; Sancetta and Calvert (1988) documented the annual cycle of sedimentation in the fjord. McQuoid (1995) and McQuoid and Hobson (1997) studied the modern pattern of diatom succession in the Saanich Inlet and analyzed the diatoms in laminae couplets in frozen sediment cores for the years 1900 to 1991 A.D. During the hypsithermal warming of the early Holocene (~10-6 ka), climatic conditions throughout much of northern North America were warmer and drier than those of the present (Pielou, 1991; Hebda and Whitlock, 1997), largely as a result of increased solar insolation, which peaked between 10 and 9 ka at 65°N (Berger and Loutre, 1991). Temperatures are estimated to have been 2° to 4°C warmer than today for most of this interval, reaching a maximum between ~9 and 7 ka (Hebda and Whitlock, 1997). According to Heusser (1983) and Heusser (1985), rapid warming occurred at ~10 ka in southwestern British Columbia with summer conditions that were drier and as warm or warmer than today lasting until ~6 ka. Clague and Mathewes (1989) report that treeline elevation in the southeastern coast mountains of British Columbia reached elevations that were between 60 and 130 m higher than today between 9.1 and 8.2 ka. Thompson et al. (1993) argue that the driest conditions (period of maximum summer drought) of the Holocene were reached in western North America at 9 ka. The warmer and drier conditions of this Holocene thermal maximum were gradually replaced by cooler and wetter conditions (Hebda, 1995; Hebda and Whitlock, 1997). This report describes the intra-annual variability of the diatom assemblages at Site 1034, during a 8-yr interval near 9 ka. Pollen and dinoflagellates are being studied from the same samples by R. Hebda and P. Mudie (unpubl. data).

British Columbia↗
USGS2011

Groundwater quality in the San Diego Drainages Hydrogeologic Province, California

More than 40 percent of California's drinking water is from groundwater. To protect this vital resource, the State of California created the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The Priority Basin Project of the GAMA Program provides a comprehensive assessment of the State's groundwater quality and increases public access to groundwater-quality information. The San Diego Drainages Hydrogeologic Province (hereinafter referred to as San Diego) is one of the study units being evaluated. The San Diego study unit is approximately 3,900 square miles and consists of the Temecula Valley, Warner Valley, and 12 other alluvial basins (California Department of Water Resources, 2003). The study unit also consists of all areas outside defined groundwater basins that are within 3 kilometers of a public-supply well. The study unit was separated, based primarily on hydrogeologic settings, into four study areas: Temecula Valley, Warner Valley, Alluvial Basins, and Hard Rock (Wright and others, 2005). The sampling density for the Hard Rock study area, which consists of areas outside of groundwater basins, was much lower than for the other study areas. Consequently, aquifer proportions for the Hard Rock study area are not used to calculate the aquifer proportions shown by the pie charts. An assessment of groundwater quality for the Hard Rock study area can be found in Wright and Belitz, 2011. The temperatures in the coastal part of the study unit are mild with dry summers, moist winters, and an average annual rainfall of about 10 inches. The temperatures in the mountainous eastern part of the study unit are cooler than in the coastal part, with an annual precipitation of about 45 inches that occurs mostly in the winter. The primary aquifers consist of Quaternary-age alluvium and weathered bedrock in the Temecula Valley, Warner Valley, and Alluvial Basins study areas, whereas in the Hard Rock study area the primary aquifers consist mainly of fractured and decomposed granite of Mesozoic age. The primary aquifers are defined as those parts of the aquifers corresponding to the perforated intervals of wells listed in the California Department of Public Health (CDPH) database. Public-supply wells typically are drilled to depths between 200 and 700 feet, consist of solid casing from the land surface to a depth of about 60 to 170 feet, and are perforated, or consist of an open hole, below the solid casing. Water quality in the shallow and deep parts of the aquifer system may differ from water quality in the primary aquifers. Municipal water use accounts for approximately 70 percent of water used in the study unit; the majority of the remainder is used for agriculture, industry, and commerce. Groundwater accounts for approximately 8 percent of the municipal supply, and surface water, the majority of which is imported, accounts for the rest. Recharge to groundwater occurs through stream-channel infiltration from rivers and their tributaries, infiltration in engineered recharge basins, and infiltration of water from precipitation and irrigation. The primary source of discharge is water pumped from wells.

California↗
USGS1969

Ground-water data as of 1967, Central Coastal Subregion, California

Most usable ground water in the predominantly mountainous Central Coastal Subregion occurs in alluvium-filled valleys and coastal plains and in deeper aquifers of Quaternary and Tertiary age. The intervening mountainous areas are underlain by consolidated sedimentary, igneous, and metamorphic rocks, mainly of Mesozoic age. These older rocks contain only small quantities of recoverable ground water and, therefore, are not considered a major source of ground water. In the Central Coastal Subregion, 24 basins have been identified as significant sources of ground water. The total area of the 24 basins is about 3,500 square miles. The water-bearing deposits range in thickness from about 200 to 4,000 feet. Depending on local conditions, recharge infiltrates at rates of less than 1½ feet per day to more than 10 feet per day in the upper part of alluvial fans and stream channels and at the outcrops of the deeper aquifers. The maximum measured depth to water in the water-bearing deposits is 568 ft. In several valleys there are flowing wells. Total storage capacity of 16 of the basins is more than 20,000,000 acre-feet . The usable storage capacity of 18 of the basins is more than 7,600,000 acre-feet; the limiting factors are sea-water intrusion and high pumping lift. Ground-water temperature ranges from about 55° to about 75°F . The dissolved-solids content of the water is generally less than 800 parts per million, but locally is more than 11,000 parts per million. The predominant water type is calcium bicarbonate, but sodium, magnesium, sulfate, and chloride are present locally in significant quantities. Properly constructed wells in some areas can yield 425 gallons per minute.

California↗
USGS1982

Preliminary results of potassium-argon age determinations from the Ugashik quadrangle, Alaska Peninsula: A section in The United States Geological Survey in Alaska: Accomplishments during 1980

Early and preliminary results of potassiumargon dating work on samples from 12 sites in the Ugashik quadrangle indicate a continuation of the geologic trends seen in the Chignik and Sutwik Island quadrangles to the south (Wilson, 1980). Tertiary volcanic and hypabyssal rocks apparently fall into two age groups: early Tertiary-late Eocene to earliest Miocene and late Tertiary and Quaternary-late Miocene to Holocene (fig. 53).

Alaska↗
USGS1893

The Glaciation of the Yellowstone Valley North of the Park

The local glaciers of Quaternary times, of which evidences abound throughout the highest portions of the Rocky mountain cordillera, attained an unusually extensive development in that broad elevated region known as the Yellowstone Park. It was indeed the center of a considerable ice sheet whose glaciers spread out and down the valleys leading from this mountain region in all directions. In the northern part of the park two streams of ice found an outlet for their united flow northward down the valley of the Yellowstone, and they have left impressive memorials of the power and size of this stream that at once attract the attention of the observant traveler on the way to the famous geyser basins of the park. The number and size of the erratic bowlders scattered so abundantly over the valley floor and perched high up on the mountain slopes, can not fail to impress the beholder, while the second canyon of the Yellowstone, known as Yankee Jim canyon, through which the river has cut its way to the broad mountain encircled lower valley, is a grand and perfect piece of ice sculpture that affords striking proof of the power and magnitude of the glacier which once filled the valley. While studying and mapping the geology of a portion of the country north of the Yellowstone Park, under the direction of Mr. Arnold Hague, and for the United States Geological Survey, I found a long desired opportunity to study the glaciation of this interesting region.

Bulletin↗
USGS1957

Core logs from two test holes near Kramer, San Bernardino County, California

Between July 1954 and May 1955 two test holes were drilled near Kramer Calif., on the Mojave Desert. Four Corners test hole 1 was drilled in sec. 20 T. 10 N., R. 6 W., San Bernardino base line and meridian, to a depth of 1,561 feet in a basin filled with Quaternary and Tertiary sediments. The core is predominantly sand and conglomerate composed of mostly unweathered quartz monzonite debris. Four Corners test hole 2 was drilled in sec. 5, T. 10 N., R. 8 W., in a separate sediment-filled basin. The core from this hole is less consolidated and more weathered. It is predominantly clay, silt, sand, and gravel composed of quartz monzonitic and volcanic material. Detailed logs of the cores are given in this report.

Bulletin↗
USGS1960

Core logs from five test holes near Kramer, California

In 1957, five test holes were drilled near Kramer, Calif., in the western Mojave Desert, for the U.S. Geological Survey. The drill sites are in topographic basins where gravimetric and geologic surveys indicated the presence, beneath alluvium, of a thick section of Quaternary and Tertiary sedimentary and volcanic rocks. Two holes, which were deeper tests at sites drilled in 1954, cored only silts, sands, and gravels: Four Corners test hole. 1 was drilled in sec. 20, T. 10 N., R. 6 W., to a depth of 3,500 feet. Four Corners test hole 2, in sec. 5, T. 10 N., R. 8 W., was drilled to 2,328 feet. Three holes which were drilled at new sites north of Kramer Junction, the intersection of U.S. Highways 395 and 466 and locally known as Four Corners, penetrated colemanite-bearing sediments. The location and total depth of these holes, are as follows: Four Corners test hole 3, sec. 18, T. 11 N., R. 6 W., depth 2,568. feet; Four Corners test hole 4, near the north edge of sec. 30, T. 11 N., R. 6 W., 1 depth 3,500 feet; Four Corners test hole 5, near the south edge of sec. 30, depth 1,604 feet. The sections of rocks penetrated in these 3 holes are similar. In each, the colemanite is in fine-grained sediments; these lie below sands and gravels, which are about 600 to 800 feet thick, and are underlain by sandstones and conglomerates. Colemanite is most abundant in the cores from Four Corners test hole 5, particularly in the 76 feet of core recovered between depths of 1,051 and 1,131 feet. Chemical analysis indicates that in this section of core the average content of BaOs is above 14 percent. In addition to colemanite, the cores contain sulfides of arsenic, an unusual iron sulfide, and zeolites. This mineral ,content of the colemanite-bearing sediments north of Kramer Junction (Four Corners), together with the general lithology of the lake beds and the occurrence of the sediments as a tilted section of beds below sands and gravels, supports correlation with the upper or marginal parts of the borate-bearing sediments at the Kramer borate mining district, which have similar features. There is, however, no evidence that any beds are exactly equivalent in age.

Bulletin↗
USGS1997

Clastic rocks associated with the Midcontinent rift system in Iowa

The Middle Proterozoic Midcontinent Rift System (MRS) of North America is a failed rift that formed in response to region-wide stresses about 1,100 Ma. In Iowa, the MRS is buried beneath 2,200?3,500 ft of Paleozoic and Mesozoic sedimentary rocks and Quaternary glaciogenic deposits. An extremely large volume of sediments was deposited within basins associated with the rift at several stages during its development. Although the uplift of a rift-axial horst resulted in the erosional removal of most of these clastic rocks from the central region of the MRS in Iowa, thick sequences are preserved in a series of horst-bounding basins. Recent studies incorporating petrographic analysis, geophysical modeling, and other analytical procedures have led to the establishment of a preliminary stratigraphy for these clastic rocks and interpretations of basin geometries. This information has allowed the refinement of existing theories and history of MRS formation in Iowa. Additionally, drill samples previously interpreted as indicating the existence of early Paleozoic basins overlying the Proterozoic MRS basins were re-examined. Samples previously interpreted as deep-lying Paleozoic rocks are now known to have caved from upper levels of the drillhole and were out of stratigraphic position. No deep Paleozoic basins exist in this area. These investigations led to the development of petrographic parameters useful in differentiating the Proterozoic MRS Red clastics from Paleozoic clastic rocks having similar lithologies.

Bulletin↗
USGS1992

Geologic studies in Alaska by the U.S. Geological Survey, 1990

This collection of papers continues the annual series of U.S. Geological Survey (USGS) reports on geologic investigations in Alaska. From 1975 through 1988, the series was published as USGS circulars. The first of these appeared under the title "The United States Geological Survey in Alaska: Accomplishments during 1975," and the series continued to the last annual circular entitled "Geologic studies in Alaska by the U.S. Geological Survey during 1987," which reflects a title change made in 1986. This 1990 volume continues the bulletin format started in 1988. As in 1989, this volume separates shorter contributions as Geologic Notes from more extensive Articles. This 1990 volume of 18 Articles and 4 Geologic Notes represents the broad range of USGS research activities carried out in Alaska over the past few years. These studies include topics on mineral and other resources such as gold (Y eend), platinum-group elements (Cathrall and Antweiler), coal (Roberts, Stricker, and Affolter), and petroleum (Howell, Bird, and others). Many other investigations provide background geochemical (Kilburn, Box, and others) and geologic data needed for resource evaluation as well as for determining the general geologic framework of Alaska, as in stratigraphic, sedimentologic, and paleontologic and radiometric age studies (Blodgett, Clough, and others; Box and Elder; Dickinson and Skipp; Marincovich and Moriya; McLean and Stanley; Stanley, Flores, and Wiley; Roeske, Pavlis, and others); geophysics (Sampson, Labson, and Long); structure and tectonic evaluations (Bradley and Kosky; Clendenen, Sliter, and Byrne; Karl; Csejtey; Howell, Johnsson, and others); and geomorphic and late Quaternary studies (Carter and Hillhouse; Galloway, Huebner, and others; McGimsey, Richter, and others; Nelson and Carter). These studies span nearly the entire State from the North Slope and Brooks Range to interior, southwestern, and south-central Alaska (fig. 1). Two bibliographies (White) at the end of the volume list (1) reports about Alaska in USGS publications released in 1990 and (2) reports about Alaska by USGS authors in publications outside the USGS in 1990. A bibliography and index of papers in past USGS circulars that are devoted to geologic research and accomplishments in Alaska (1975 to 1986) is published as USGS Open-File Report 87-420.

Alaska↗