Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “The Sedimentary Record”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 613 records · Page 34Linked to original sources
USGS1990

Hydrogeology of the Point Lookout Sandstone in the San Juan structural basin, New Mexico, Colorado, Arizona, and Utah

This report is one in a series resulting from the U.S. Geological Survey's Regional Aquifer-System Analysis (RASA) study of the San Juan structural basin that began in October 1984. Previous reports in the series describe the hydrogeology of the Dakota Sandstone (Craigg and others, 1989), Morrison Formation (Dam and others, 1990), Gallup Sandstone (Kernodle and others, 1989), Menefee Formation (Levings and others, 1990), and Cliff House Sandstone (Thorn and others, 1990), in the San Juan structural basin. The purposes of the RASA (Welder, 1986) are to: (1) Define and evaluate the aquifer system; (2) assess the effects of past, present, and potential ground-water use on aquifers and streams; and (3) determine the availability and quality of ground water. This report summarizes information on the geology and the occurrence and quality of water in the Point Lookout Sandstone, one of the primary water-bearing units in the regional aquifer system. Data used in this report were collected during the study or were derived from existing records in the U.S. Geological Survey's computerized National Water Information System (NWIS) data base, the Petroleum Information Corporation's database, and the Dwight's ENERGYDATA Inc. BRIN data base. Although all data available for the Point Lookout Sandstone were considered in formulating the discussions in the text, not all those data could be plotted on the illustrations. The San Juan structural basin is in New Mexico, Colorado, Arizona, and Utah and has an area of about 21,600 square miles (fig. 1). The structural basin is about 140 miles wide and about 200 miles long. The study area is that part of the structural basin that contains rocks of Triassic or younger age and, therefore, is less areally extensive than the structural basin. Triassic through Tertiary sedimentary rocks are emphasized in this study because the major aquifers in the basin are present in these rocks. The study area is about 140 miles wide (about the same as the structural basin), 180 miles long, and has an area of about 19,400 square miles. Altitudes in the study area range from about 4,500 feet in San Juan County, Utah, to about 11,000 feet in Cibola County, New Mexico. Annual precipitation in the high mountainous areas along the north and east margins of the basin is as much as 45 inches, whereas annual precipitation in the lower altitude, central basin is generally less than 8 inches. Mean annual precipitation in the study area is about 12 inches. Data obtained from documents published by the U.S. Bureau of the Census, 1980 and 1985, were used to estimate the population of the study area. The population of the study area in 1970 was estimated to be about 134,000. The population rose to about 194,000 in 1980, 212,000 in 1982, 221,000 in 1984, and then fell to about 210,000 in 1985. The economy of the basin is supported by exploration and development of petroleum, natural gas, coal, and uranium resources; urban enterprise, farming "and ranching; tourism; and recreation. The rise and fall in population were related to changes in the economic strength of the mining, petroleum, and natural-gas industries, and support services. Uranium mining and milling activities grew rapidly until the late 1970's when most uranium-mining activity ended in the study area. Likewise, the oil and gas industry prospered until about 1983 and then declined rapidly, also affecting many jobs in support industries.

Arizona, Colorado, New Mexico, Utah↗
USGS1977

Paleomagnetic investigation of late Quaternary sediments of south San Francisco Bay, California

Paleomagnetic inclinations of the Late Quaternary sediments of South San Francisco Bay were determined from bore hole samples collected near Dumbarton Bridge. The sediments consist of estuarine muds and nonmarine sand deposits, floored by bedrock of the Mesozoic Franciscan Formation. - Beneath Dumbarton Bridge the entire sedimentary fill is normally polarized; therefore, the fill postdates the Brunhes-Matayama polarity reversal (700,000 y. B.P.). Magnetic time lines such as the Mono Lake excursion (24,000 y. B.P.) and the reversed Blake event (110,000 y B.P.) were not found in this bore hole. In addition to Holocene and modern deposits of San Francisco Bay, an older estuarine unit occurs in the stratigraphic section. The older unit was deposited during a period of high sea level, tentatively correlated with the Sangamon interglacial period. Because evidence of the Blake event is not present in the older estuarine unit, the proposed age of this unit could not be confirmed. Although the Holocene estuarine deposits of South San Francisco Bay carry stable remanent magnetization, a reliable record of geomagnetic secular variation could not be recovered because the water-saturated sdiment was deformed by drilling.

California↗
USGS2021

Geologic map of the Aeolis Dorsa Region, Mars

The Aeolis Dorsa region of Mars, located just north of the global dichotomy boundary, includes the Aeolis and Zephyria Plana, and a depositional basin between them. This interplana region consists of extensive networks of ridges—the eponymous Aeolis Dorsa—and is interpreted as having formed by topographic inversion of fluvial and alluvial deposits. To the south is a nearly 1-km-deep trough (Aeolis Chaos) and the southern highlands. These elements of the map area compose a landscape of extensive erosional and depositional sedimentary processes. The plana are pervasively abraded into yardangs, and the interplana area shows scattered yardangs superposed on the underlying terrain. The Aeolis Dorsa fluvial deposits are concentrated within and around the margins of the interplana region, exposed and (or) inverted by this pervasive aeolian abrasion. During this period of extensive erosion and deposition, impacts have also redistributed material. The geologic mapping of this region, conducted at 1:500,000 scale to enable depiction of the fine-scale fluvial features, divides the landscape into six unit groups. The highlands units group comprises three units, located in the southwestern map area. These units, having the highest elevation in the map area, consist of mesas surrounded by more gently sloping terrain. The transitional units group, located northeast of the highlands units, includes the Aeolis Chaos, denoted as a chaos terrain unit, and two transitional units, differentiated on the basis of surface texture and relative elevation. The plana units group comprises five units. The oldest unit consists of mesas similar to those of the highlands mesas unit but located about 200 kilometers north of the highlands in Aeolis Planum. Three other plana units, located on the Aeolis and Zephyria Plana, are differentiated on the basis of yardang texture, crosscutting relations, and relative elevations. They are interpreted as abraded sedimentary and (or) volcaniclastic deposits. The fifth plana unit, which crops out in the north corners of the map area, is at low elevation, has numerous small craters, and is interpreted as cratered lava plains. The interplana units group hosts a hummocky unit and a mounds unit, differentiated on the basis of texture and relief. In the Aeolis Dorsa units group, four units are mapped on the basis of dorsa morphology and adjacent textures. Stratigraphic relations indicate a decrease in discharge over time. The crater units group includes a crater unit, found throughout the map area although concentrated within the interplana region, and a crater fill unit that is found within several craters in this interplana region. Based on this mapping, the interpretation of the regional geologic record begins with emplacement of the highlands terrain during the Noachian Period. Emplacement was followed during the Early Hesperian by erosion and redistribution of this high-standing terrain to form the transitional units, and Aeolis Chaos formed after deposition of the other transitional units. The high-standing plana were emplaced and eroded repeatedly throughout the Hesperian and Early Amazonian time, and impact cratering occurred at decreasing crater sizes. The fluvial and alluvial activity that gave rise to the Aeolis Dorsa also extended throughout this time, leaving their diagnostic signature on this region.

Scientific Investigations Map↗
USGS1992

Interoceanic variation in the rare earth, major, and trace element depositional chemistry of chert: Perspectives gained from the DSDP and ODP record

Rare earth element (REE), major, and trace element abundances and relative fractionations in forty nodular cherts sampled by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) indicate that the REE composition of chert records the interplay between terrigenous sources and scavenging from the local seawater. Major and (non-REE) trace element ratios indicate that the aluminosilicate fraction within the chert is similar to NASC (North American Shale Composite), with average Pacific chert including ~7% NASC-like particles, Indian chert ~ 11% NASC, Atlantic chert ~ 17% NASC, and southern high latitude (SHL) chert 53% NASC. Using La as a proxy for ∑REE, approximations of La ex (the amount of La in excess of that supplied by the detrital aluminosilicate fraction) indicate that Pacific chert contains the greatest La ex (85% of La total ) and SHL chert the least (38% of La total ). As shown by interelement associations, this La ex is most likely an adsorbed component onto aluminosilicate and phosphatic phases. Accordingly, chert from the large Pacific Ocean, where deposition occurs relatively removed from significant terrigenous input, records a depositional REE signal dominated by adsorption of dissolved REEs from seawater. Pacific chert CeCe * ⪡ 1 and La n Yb n ~ 0.8-1, resulting from adsorption of local Ce-depleted seawater and preferential adsorption of LREEs from seawater (e.g., La n Yb n ~ 0.4), which increases the La n Yb n ratio recorded in chert. Chert from the Atlantic basin, a moderately sized ocean basin lined by passive margins and with more terrigenous input than the Pacific, records a mix of adsorptive and terrigenous REE signals, with moderately negative Ce anomalies and La n Yb n "> La n Yb n ratios intermediate to those of the Pacific and those of terrigenous input. Chert from the SHL region is dominated by the large terrigenous input on the Antarctic passive margin, with inherited Ce Ce * ~1 "> CeCe * ~1 and inherited La n Yb n "> La n Yb n values of ~1.2–1.4. Ce Ce * "> ~1.2–1.4.CeCe * does not vary with age, either throughout the entire data base or within a particular basin. Overall, Ce Ce * "> CeCe * does not correlate with P 2 O 5 concentrations, even though phosphatic phases may be an important REE carrier. This and previous studies of the large-scale controlling parameters of sedimentary REEs across ocean basins collectively indicate that REE indices of depositional regime (e.g., Ce Ce * "> CeCe * , La n Yb n "> La n Yb n , La ex ) are reproducible in a variety of sediment and rock lithologies, ages, and ocean basins, and present a coherent tool for paleoceanographic and tectonic basin reconstructions.

Geochimica et Cosmochimica Acta↗
USGS2023

Mapping ancient sedimentary organic matter molecular structure at nanoscales using optical photothermal infrared spectroscopy

Elucidating the molecular structure of sedimentary organic matter (SOM) is key to understanding petroleum generation processes, as well as ancient sedimentary environments. SOM structure is primarily controlled by biogenic source material (e.g., marine vs. terrigenous), depositional conditions, and subsurface thermal history. Additional factors, e.g., strain, may also impact the molecular structure of SOM. Multiple spatially resolved approaches exist for in situ evaluation of SOM, including Raman and infrared spectroscopies, as well as mass spectrometric methods. While these methods have enabled increased understanding of the occurrence and distribution of SOM functional groups, they suffer from disadvantages including low spatial resolution (infrared spectroscopy), limited molecular information (Raman spectroscopy), and sample destruction (mass spectrometric methods). Recent technological advances have resulted in infrared spectrometers capable of breaking the Abbe diffraction limit, greatly increasing the spatial resolutions accessible for an infrared measurement. Here we utilize optical photothermal infrared spectroscopy (O-PTIR) to record maps of functional group distributions at 500 nm spatial resolution in Tasmanites (algal microfossils) from the Upper Devonian Ohio Shale. These data allow for discrimination between Tasmanites , adjacent SOM, and fine-grained minerals. Additionally, functional group distributions within Tasmanites were found to be generally homogenous, although slight variations exist between the body and fold apices (zones of greatest deformation) which may indicate strain-induced chemical reactions. The data presented here represent the first application of O-PTIR to study SOM, highlighting the promise of this analytical approach for future studies evaluating the molecular composition of geologic materials at sub-micron scales.

Organic Geochemistry↗
USGS2023

Spatial distribution and variability of lobe facies in a large sand-rich submarine fan system: Neoproterozoic Zerrissene Group, Namibia

The deposits of the upper Neoproterozoic Zerrissene Group of central-western Namibia represent a large siliciclastic deep-water depositional system that showcases the intricacies of facies and architectural relationships from bed-scale to fan-system-scale. The lack of vegetation in the Namib Desert and regular east–west repetition of folded stratigraphy (reflecting ca 50% tectonic shortening) provides quasi-three-dimensional exposure over a current area of approximately 2700 square kilometres. The Brak River Formation, the middle sand-rich unit of the Zerrissene Group, consists of nearly 600 m of strata exposed in multiple parallel continuous outcrops up to ca 10 km in length and oriented obliquely to depositional dip. Ten stratigraphic sections are correlated ca 32 km ( ca 64 km restored) across the basin and offer exposure comparable in scale to modern submarine fans. Six sedimentary facies are identified and grouped into four facies associations that represent axial-to-marginal portions of deep-water lobes in an unconfined submarine fan system. Spatial facies patterns, regional thickness variations, and palaeocurrents indicate that Brak River Formation sediments were transported primarily from the north to south–south-west through a trough-like basin, and deposited within an unconfined basin plain at the junction of the Adamastor and Khomas oceans. The unique outcrop exposure and extent permits the documentation of system-scale architecture and basin configuration of the Brak River submarine fan system. A transition from the sand-rich lower Brak River Formation to more intercalated mudstone-dominated intervals in the middle and upper Brak River Formation is interpreted to record a change from aggradational to compensational stacking of lobe deposits. This records the evolution of a large submarine fan as it filled the subtle seafloor topography and became less confined at the system-scale. The documentation of these deep-water deposits from centimetre-scale to basin-scale provides a new model for a system with extensive long-distance transport of sand-rich sediment gravity flows to submarine lobes without apparent channelization.

Sedimentology↗
USGS2008

Hydrodynamic implications of textural trends in sand deposits of the 2004 tsunami in Sri Lanka

Field observations and sediment samples at a coastal-plain setting in southeastern Sri Lanka were used to document the erosional and depositional impacts of the 2004 Indian Ocean tsunami and to interpret the hydrodynamic processes that produced an extensive sand-sheet deposit. Tsunami deposit thicknesses ranged from 6 to 22??cm with thickness being controlled partly by antecedent topography. The deposit was composed of coarse to medium sand organized into plane-parallel laminae and a few laminasets. Vertical textural trends showed an overall but non-systematic upward fining and upward thinning of depositional units with an upward increase in heavy-mineral laminations at some locations. Repeated patterns in the vertical textural trends (upward fining, upward coarsening, uniform) were used to subdivide and correlate the deposit into five hydro-textural stratigraphic units. The depositional units were linked to hydrodynamic processes and upcurrent conditions, such as rates of sediment supply and composition of the sediment sources. Vertical changes in grain-size distributions recorded the depositional phases associated with flow acceleration, initial unsteady pulsating flow, relatively stable and uniform flow, flow deceleration, slack water, and return flow or flow redirection. Study results suggest that vertical textural trends from multiple cross-shore sections can be used to interpret complex tsunami flow histories, but at the location examined, interpretation of the lateral textural trends did not provide a basis for identifying the correct sediment transport pathways because flow near the landward boundary was multidirectional.

Sedimentary Geology↗
USGS1989

Geology and water resources of Owens Valley, California

Owens Valley, a long, narrow valley located along the east flank of the Sierra Nevada in east-central California, is the main source of water for the city of Los Angeles. The city diverts most of the surface water in the valley into the Owens River-Los Angeles Aqueduct system, which transports the water more than 200 miles south to areas of distribution and use. Additionally, ground water is pumped or flows from wells to supplement the surface-water diversions to the river-aqueduct system. Pumpage from wells needed to supplement water export has increased since 1970, when a second aqueduct was put into service, and local concerns have been expressed that the increased pumpage may have had a detrimental effect on the environment and the indigenous alkaline scrub and meadow plant communities in the valley. The scrub and meadow communities depend on soil moisture derived from precipitation and the unconfined part of a multilayered aquifer system. This report, which describes the hydrogeology of the aquifer system and the water resources of the valley, is one in a series designed to (1) evaluate the effects that ground-water pumping has on scrub and meadow communities and (2) appraise alternative strategies to mitigate any adverse effects caused by pumping. Two principal topographic features are the surface expression of the geologic framework--the high, prominent mountains on the east and west sides of the valley and the long, narrow intermountain valley floor. The mountains are composed of sedimentary, granitic, and metamorphic rocks, mantled in part by volcanic rocks as well as by glacial, talus, and fluvial deposits. The valley floor is underlain by valley fill that consists of unconsolidated to moderately consolidated alluvial fan, glacial and talus, and fluvial and lacustrine deposits. The valley fill also includes interlayered recent volcanic flows and pyroclastic rocks. The bedrock surface beneath the valley fill is a narrow, steep-sided graben that is structurally separated into the Bishop Basin to the north and the Owens Lake Basin to the south. These two structural basins are separated by (1) a bedrock high that is the upper bedrock block of an east-west normal fault, (2) a horst block of bedrock (the Poverty Hills), and (3) Quaternary basalt flows and cinder cones that intercalate and intrude the sedimentary deposits of the valley fill. The resulting structural separation of the basins allowed separate development of fluvial and lacustrine depositional systems in each basin. Nearly all the ground water in Owens Valley flows through and is stored in the saturated valley fill. The bedrock, which surrounds and underlies the valley fill, is virtually impermeable. Three hydrogeologic units compose the valley-fill aquifer system, a defined subdivision of the ground-water system, and a fourth represents the valley fill below the aquifer system and above the bedrock. The aquifer system is divided into horizontal hydrogeologic units on the basis of either (1) uniform hydrologic characteristics of a specific lithologic layer or (2) distribution of the vertical hydraulic head. Hydrogeologic unit 1 is the upper unit and represents the unconfined part of the system, hydrogeologic unit 2 represents the confining unit (or units), and hydrogeologic unit 3 represents the confined part of the aquifer system. Hydrogeologic unit 4 represents the deep part of the ground-water system and lies below the aquifer system. Hydrogeologic unit 4 transmits or stores much less water than hydrogeologic unit 3 and represents either a moderately consolidated valley fill or a geologic unit in the valley fill defined on the basis of geophysical data. Nearly all the recharge to the aquifer system is from infiltration of runoff from snowmelt and rainfall on the Sierra Nevada. In contrast, little recharge occurs to the system by runoff from the White and Inyo Mountains or from direct precipitation on the valley floor. Ground water flows from the margins of the valley towards the center of the valley; the ground water then flows south to the terminus of the system at Owens (dry) Lake. Ground water flows south from Bishop Basin to Owens Lake Basin through the narrows that constrict the flow opposite Poverty Hills. The aquifer system in the northern half of Owens Lake Basin is divided into east and west halves by the barrier effect caused by the Owens Valley fault. Discharge from the aquifer system is primarily by pumpage and evapotranspiration, and to a lesser extent by flowing wells, springs, underflow, and leakage to the Owens River-Los Angeles aqueduct system. Withdrawals from pumped or flowing wells is the largest component of discharge and accounts for about 50 percent of the outflow from the system. Transpiration by scrub and meadow plant communities, and to a lesser extent by irrigated alfalfa pasture, accounts for about 40 percent of the system's discharge. Natural hydraulic conductivity ranges from less than 400 to about 12,000 feet per day in the basalt flows, the more permeable material in the aquifer system. Where the basalts are fractured by explosives and drilling techniques, actual transmissivities can be greater than 1,000,000 feet squared per day. Hydraulic conductivities in sedimentary deposits of the aquifer system range from less than a few feet per day in lacustrine clays to more than 300 feet per day in gravel stringers and beach deposits in the transition zone between alluvial fan deposits and fluvial and lacustrine deposits. Degree of confinement in the aquifer system generally increases to the south and east in both the Bishop and Owens Lake Basins. The vertical hydraulic gradient across hydrogeologic unit 2 and confining beds in hydrogeologic units 1 and 3 is a function of (1) the asymmetric recharge and hydraulic head created by the dominant recharge from Sierra Nevada runoff and (2) the areal extent and thickness of the confining beds. Although most of the pumpage is from hydrogeologic unit 3, some coincident drawdown has been recorded in nonpumped wells that tap unit 1. Drawdown in hydrogeologic unit 1 is a function of changes in (1) lateral flow through hydrogeologic unit 1, (2) upward flow of ground water through the confining beds, (3) downward leakage of water from hydrogeologic unit 1 to unit 3 through wells, (4) direct withdrawal from well intervals open to hydrogeologic unit 1, and (5) increased evapotranspiration. The water in the aquifer system is generally of excellent quality for public supply and irrigation, with the exception of water stored in thick sequences of lacustrine silts and clays near Owens Lake. The water is principally a calcium bicarbonate type and dissolved-solids concentrations range from approximately 104 to 325 milligrams per liter. Water in the lacustrine sediments of Owens (dry) Lake is a sodium bicarbonate type and dissolved-solids concentrations are about 5,400 milligrams per liter.

California↗
USGS2021

Preliminary geohydrologic assessment of Buenos Aires National Wildlife Refuge, Altar Valley, southeastern Arizona

The Buenos Aires National Wildlife Refuge is located in the southern part of Altar Valley, southwest of Tucson in southeastern Arizona. The primary water-supply well at the Buenos Aires National Wildlife Refuge has experienced a two-decade decrease in groundwater levels in the well, as have other wells in the southern part of Altar Valley. In part to understand this trend, a study was undertaken by the U.S. Geological Survey, in cooperation with the U.S. Fish and Wildlife Service, to summarize what is known about the geohydrologic system on the refuge and analyze groundwater-level trends and precipitation-groundwater correlations. In addition, available data were compiled where possible on the climate, land cover, soils, geology, and hydrology to provide a foundation for future modeling of the system. Altar Valley is a sedimentary basin bounded by a mixture of Paleozoic to Tertiary sedimentary, volcanic, granitic, and metamorphic rocks. The valley fill is undifferentiated Tertiary to Quaternary sediments underlain by middle Miocene to Pliocene rocks that consist of moderately to strongly consolidated conglomerate and sandstone. Surface water, when present in the predominantly ephemeral streams of the valley, flows from south to north. Arivaca Creek has a cienega (or wetland) where groundwater surfaces before it flows as a short perennial reach out of Arivaca Basin. Groundwater maps compiled between 1934 and 2016 showed groundwater flowing from south to north. Before the 1980s, temporal patterns of groundwater levels in wells in Altar Valley varied substantially from one well to another. In the mid-1980s, comparatively high levels of precipitation occurred: the 1980s median value was 15.3 inches, whereas the median for the period of record was 13.2 inches. In addition, apparently corresponding groundwater level increases were seen in nearly all wells studied. After this initial increase, two different groundwater-level trends began to be observed in two spatially distinct sets of wells: in the northern part, groundwater levels were relatively steady, whereas in the southern part, groundwater levels declined from 10 to 20 feet between 1990 and 2019. Annual groundwater pumpage declined substantially in the northern part of the valley beginning in the early 1980s, but it began to increase again in the 1990s. Pumpage in the southern part has remained low and relatively steady compared to the northern part. Although the precise reasons for the declining groundwater levels in the southern part remain unclear, groundwater levels may be affected by factors such as climate cycles, long-term drought, and temperature-induced declines in recharge, resulting in increased evapotranspiration. Preliminary analyses of two wells, one selected from each part of the valley, using linear regression and lag correlation to investigate correlation between annual precipitation and groundwater levels, showed a maximum correlation at a lag of about 17 years in the southern part of the valley and about 25 years in the northern part, indicating that, although variable sources and traveltimes of recharged water may be needed to propagate to each location, the strongest correlation at each well is with precipitation that was recharged 17 and 25 years prior to the groundwater response in that well. Assuming a constant flow of groundwater from the southern to the northern part of the valley, a decrease in recharge is expected to lead to a decrease in aquifer storage. As to the comparatively stable groundwater levels in the northern part, pumpage is still only about one-half what it was in the early 1980s, even though pumpage has increased there since the 1990s. Water levels in most wells in the northern part were drawn down prior to the decrease in pumping in the early 1980s, possibly owing to a combination of pumping and the nearly 20-year midcentury drought that occurred between 1940 and 1960. Water levels were in the process of recovering when the increase in pumping occurred in the 1990s. Because the water levels were recovering (increasing) instead of remaining static, the increased pumping may have only limited the recovery rather than causing a decrease in water levels, as a new quasi-equilibrium state may have been reached. Additional possible causes for the stable groundwater levels include (1) upgradient aquifer transmissivity that was high enough to offset pumping, (2) a low-permeability barrier, such as bedrock or clay, at the north end of the valley that caused groundwater pooling, (3) higher lateral inflow of groundwater in the northern part of the valley, (4) a delay in the effect of storage declines propagating from the south, or (5) some combination thereof.

Arizona↗
USGS2003

Age constraints on Tarkwaian palaeoplacer and lode-gold formation in the Tarkwa-Damang district, SW Ghana

Two major epigenetic gold-forming events are recorded in the world-class gold province of southwest Ghana. A pre-Tarkwaian event was the source of the world-class Tarkwa palaeoplacers whereas post-Birimian and Tarkwaian deformation, which was related to the Eburnean orogeny, gave rise to the world-class (e.g. Prestea) to giant (e.g. Obuasi) orogenic gold deposits which have made the region famous for more than 2,500 years. A maximum age of 2133 ?? 4 Ma for Tarkwaian sedimentation is provided by 71 of 111 concordant SHRIMP II U Pb dates from detrital zircons in Tarkwaian clastic rocks from Damang and Bippo Bin, northeast of Tarkwa. The overall data distribution broadly overlaps the relatively poorly constrained ages of Birimian volcanism and associated Dixcove-type granitoid emplacement, indicating syntectonic development of the Tarkwaian sedimentary basin. These zircon ages argue against derivation of the palaeoplacer gold from an orogenic gold source related to the compressional phase of an orogeny significantly older than the Eburnean orogeny. Instead, they suggest that the gold source was either orogenic gold lodes related to an earlier compressional phase of a diachronous Eburnean orogeny or ca. 2200-2100 Ma intrusion-related gold lode. The CO2-rich fluid inclusions in associated vein-quartz pebbles are permissive of either source. At the Damang deposit, an epigenetic, orogenic lode-gold system clearly overprinted, and sulphidised low-grade palaeoplacer hematite magnetite gold occurrences in the Banket Series conglomerate within the Tarkwaian sedimentary sequence. Gold mineralisation is demonstrably post-peak metamorphism, as gold-related alteration assemblages overprint metamorphic assemblages in host rocks. In alteration zones surrounding the dominant, subhorizontal auriferous quartz veins, there are rare occurrences of hydrothermal xenotime which give a SHRIMP U Pb age of 2063 ?? 9 Ma for gold mineralisation. The similar structural timing of epigenetic gold mineralisation in Tarkwaian host rocks at Damang to that in mainly Birimian host rocks elsewhere in southwest Ghana, particularly at Obuasi, suggests that 2063 ?? 9 Ma is the best available age estimate for widespread orogenic gold mineralisation in the region. Argon-argon ages of 2029 ?? 4 and 2034 ?? 4 Ma for hydrothermal biotite from auriferous quartz veins appear to represent uplift and cooling of the region below about 300??C, as estimates of the temperature of gold mineralisation are higher, at around 400??C. If peak metamorphism, with temperatures of about 550??C, is assumed to have occurred at about 2100 Ma, the biotite ages, in combination with the xenotime age, suggest a broadly constant uplift rate for the region of about 1 km per 10 million years from about 2100 to 2025 Ma.

Mineralium Deposita↗
USGS1989

Application of the Graphic Correlation method to Pliocene marine sequences

Biostratigraphy — the use of paleontological evidence to establish relative chronologies, forms the cornerstone of many sedimentary geological investigations. Several different approaches to biochronology are available. Traditional interval zones, defined on lowest and/or highest occurrences of selected taxa, are used to place bodies of rock in a relative chronological framework. Fossil datum levels, which are more numerous than zones, are often used as chronohorizons for correlation purposs. The Graphic Correlation method, like interval zonations, synthesizes information from a number of different taxa but does not assume synchrony of any one taxon. A magnetobiostratigraphic model for deep-sea Pliocene sequences has been constructed by graphic correlation of Deep Sea Drilling project cores from the North Atlantic (606), Caribbean Sea (502), South Atlantic (516), Tasman Sea (590), Equatorial Pacific (573) and North Pacific (577). All cores are hydraulic piston cores which contain abundant planktonic foraminifers, calcareous nannofossils and which record many of the magnetic reversals expected in the Pliocene. The model is based on internally consistent paleontologic data gathered by the author. This study demonstrates the advantages of graphic correlation over conventional biostratigraphic procedures. Accurate inter-regional correlations can be made between core sites without resorting to multiple microfossil zonations and without invoking synchrony of fossil events. Important results of this study are: (1) many Pliocene planktonic foraminifer and calcareous nannofossil events are diachronous by more than 0.20 m.y., (2) Globorotalia truncatulinoides first occurs in the Southwest Pacific Ocean, approximately 0.50 m.y. earlier than previously reported, (3) a previously undetected hiatus of short duration (0.38 m.y.) exists just above the Cochiti subchron at DSDP 577A.

Marine Micropaleontology↗
USGS2021

Onshore flow characteristics of the 1755 CE Lisbon tsunami: Linking forward and inverse numerical modeling

The 1755 CE Lisbon earthquake triggered the largest historical tsunami ever impacting the Atlantic coasts of Europe. Despite recent efforts to better understand this event, there are still unanswered questions about the location of its epicenter and whether physical and historical evidences are in agreement. Inverse modeling using tsunami sediments can be applied to quantify onshore flow characteristics. Forward numerical modeling is also a powerful tool capable of simulating tsunami hydrodynamics and the induced sediment transport. This work presents novel results from a combination of inverse and forward modeling to assess tsunami characteristics onshore. The study site is located on the Portuguese southern coast, at the Salgados lowland where inverse modeling was performed using TsuSedMod ( Jaffe and Gelfenbaum, 2007 ) based on data retrieved from sediment samples. Forward modeling, including tsunami generation and propagation, was performed using the FLOW module of Delft3D suite model. Onshore topography was corrected for the 1755 CE scenario based on extensive tsunami sedimentary deposit thickness data. The tsunami source was chosen based on recent results from the authors that pointed to a good correlation between modeled and field tsunami data for the Marques de Pombal Fault (MPF), Horseshoe Fault (HSF) and a hypothetical scenario represented by a simple combination between Gorringe Bank and Horseshoe Fault (Scenario 1 - SC1). Results from inverse model show tsunami onshore average speed varying from 7.3 up to 9.3 m/s and shear velocities from 0.52 up to 0.66 m/s. Forward modeling results show a wide variation according to the seismic source and tsunami onshore velocities can range from around 7 m/s when considering MPF to even an absence of inundation (SC1). The good agreement between both modeling approaches estimating tsunami velocity confirms the potential of numerical modeling coupled with geological records to improve the understanding of historical events.

Marine Geology↗
USGS2015

Update of the Graizer-Kalkan ground-motion prediction equations for shallow crustal continental earthquakes

A ground-motion prediction equation (GMPE) for computing medians and standard deviations of peak ground acceleration and 5-percent damped pseudo spectral acceleration response ordinates of maximum horizontal component of randomly oriented ground motions was developed by Graizer and Kalkan (2007, 2009) to be used for seismic hazard analyses and engineering applications. This GMPE was derived from the greatly expanded Next Generation of Attenuation (NGA)-West1 database. In this study, Graizer and Kalkan’s GMPE is revised to include (1) an anelastic attenuation term as a function of quality factor (Q0) in order to capture regional differences in large-distance attenuation and (2) a new frequency-dependent sedimentary-basin scaling term as a function of depth to the 1.5-km/s shear-wave velocity isosurface to improve ground-motion predictions for sites on deep sedimentary basins. The new model (GK15), developed to be simple, is applicable to the western United States and other regions with shallow continental crust in active tectonic environments and may be used for earthquakes with moment magnitudes 5.0–8.0, distances 0–250 km, average shear-wave velocities 200–1,300 m/s, and spectral periods 0.01–5 s. Directivity effects are not explicitly modeled but are included through the variability of the data. Our aleatory variability model captures inter-event variability, which decreases with magnitude and increases with distance. The mixed-effects residuals analysis shows that the GK15 reveals no trend with respect to the independent parameters. The GK15 is a significant improvement over Graizer and Kalkan (2007, 2009), and provides a demonstrable, reliable description of ground-motion amplitudes recorded from shallow crustal earthquakes in active tectonic regions over a wide range of magnitudes, distances, and site conditions.

Open-File Report↗
USGS2002

Stratigraphy and paleontology of Lower Permian rocks north of Cananea, northern Sonora, Mexico

Lower Permian carbonate and overlying red bed clastic rocks are present in a 2 km2 stratigraphic window in the vicinity of Rancho La Cueva, Santa Cruz sheet (scale 1:50,000), northern Sonora, Mexico. This exposure lies unconformably beneath predominantly intermediate Upper Cretaceous volcanics yielding 40Ar/39Ar ages of 73.4?? 0.18 and 71.1 ?? 0.35 Ma. The lower part of the Permian succession consists of light- to medium-gray colored limestones of the Colina Limestone, with a minimum thickness of 235 m. Sedimentary features suggest shallow water, slightly restricted depositional environments. Although lacking observable fossils for the most part, two intervals of richly fossiliferous, silicified shell beds are present near the base and top of the Colina Limestone. The lower fauna consist mostly of gastropods and bivalves. The presence of a new microdomatid gastropod species. Glyptospira sonorensis n. sp., close to Glytospira arelela Plas, suggests a late Wolfcampian age for this horizon. The upper fauna are predominantly molluscan dominated (gastropods and bivalves), but some brachiopods (productids and the rhynchonellid genus Pontisia) are also present. Gastropod genera include Bellerophon, Warthia, Euomphalus (represented by the species, Euomphalus kaibabensis Chronic), Baylea, Worthenia, Naticopsis, Goniasma, Kinishbia, Cibecuia, and Glyptospira. The gastropods suggest a Leonardian (late Early Permian) age for this horizon, and many of the species have previously been recorded from the Supai Group and Kaibab Formation of northern and central Arizona. The Colina Limestone is conformably overlain by 11.2 m of light-gray lime mudstone and dolostone, assigned here to the Epitaph Dolomite, which in turn is succeeded by 58.8 m of red-colored sandstone and gray lime mudstone, assigned here to the Scherrer Formation. This Lower Permian succession is significant because it further strengthens the stratigraphic ties of southeastern Arizona rocks with those of northern Sonora and confirms the presence of North American cratonal stratigraphy in the northern part of the state of Sonora, Mexico. Published by Elsevier Science Ltd.

Journal of South American Earth Sciences↗
USGS1964

Chemical quality of surface waters and sedimentation in the Saline River basin, Kansas

This report gives the results of an investigation of the sediment and dissolved minerals that are transported by the Saline River and its tributaries. The Saline River basin is in western and central Kansas; it is long and narrow and covers 3,420 square miles of rolling plains, which is broken in some places by escarpments and small areas of badlands. In the western part the uppermost bedrock consists predominantly of calcareous elastic sedimentary rocks of continental origin of Pliocene age and in most places is covered by eolian deposits of Pleistocene and Recent age. In the central part the ex posed bedrock consists predominantly of calcareous marine sedimentary rocks of Late Cretaceous age. In the eastern part the exposed bedrock consists mainly of noncalcareous continental and littoral elastic sedimentary rocks of Early Cretaceous and Permian age. Fluvial deposits are in the valleys, and eolian materials are present over much of the uplands. Average precipitation increases rather uniformly from about 18 inches per year in the west to almost 28 inches per year in the east. Runoff is not affected by irrigation nor regulated by large structures, but it is closely related to precipitation. Average runoff increases from less than 0.2 inch per year in the west to more than 1.5 inches per year in the east. Aquifers of the flood-plain and terrace deposits and of the Cretaceous Dakota Sandstone are the major sources of ground-water accretion to the streams. In the upper reaches of the Saline River, the water is only slightly mineralized; during the period of record the specific conductance near Wakeeney never exceeded 750 micromhos per centimeter. In the lower reaches, however, the water is slightly mineralized during periods of high flow and is highly mineralized during periods of low flow; the specific conductance near Russell exceeded 1,500 micromhos per centimeter more than 80 percent of the time. Near Russell, near Wilson, and at Tescott the water is of the calcium bicarbonate type when the specific conductance is less than about 1,000 micromhos per centimeter, but it is of the sodium chloride type when the specific conductance is more than about 1,500 micromhos per centimeter. The water is off the calcium bicarbonate, sodium bicarbonate, or sodium chloride type when the conductance is between 1,000 and 1,500 micromhos per centimeter. Most of the increase in mineralization of the water is caused by inflow of highly mineralized ground water. The ground-water inflow was estimated to be 22 percent of the total streamflow at Tescott in 1948 and 60 percent in 1952. Mineralization increases and water quality deteriorates progressively downstream along nearly the entire Saline River, especially in the part of the area directly underlain by the Dakota Sandstone between the vicinities of Fairport and Wilson: sodium and chloride are the principal constituents of water contributed by the Dakota. The total percentage of the salt in the Saline River that comes from oil-field brines is considered to be small. The water in the upper Saline River is of good quality for domestic use except that it is hard; the water in the lower Saline River is of poor quality for domestic use because most of the time it is highly mineralized, is hard, and contains high concentrations of chloride and sulfate. In the upper reaches of the river, the water is of good quality for irrigation. In the lower reaches, if the water were impounded in a reservoir, it would be of good quality for irrigation during years of high flow and of very poor quality during years of low flow. The water in the lower reaches is of poor quality for industrial use because it is highly mineralized most of the tinge. Relations of suspended-sediment discharge to water discharge were used with the long-term streamflow duration curves to compute the long-term aver age suspended-sediment discharges and concentrations at five indications. Sediment discharge is closely related to runoff. S

Water Supply Paper↗
USGS2022

The role of organic matter diversity on the Re-Os systematics of organic-rich sedimentary units: Insights into the controls of isochron age determinations from the lacustrine Green River Formation

The range of 187 Re/ 188 Os values measured from samples of five organic-rich lacustrine mudstones units in the Eocene Green River Formation in the easternmost Uinta Basin covaries with organic matter diversity driven by changing water column conditions. A set of samples from the Douglas Creek Member has the highest pristane/phytane ratio and lowest β-carotane/ n -C 30 ratio compared to overlying units, indicating deposition in an oxic-anoxic environment with low salinity that would have allowed for the accumulation of a diverse assemblage of aquatic organisms. These samples define the broadest 187 Re/ 188 Os range of 1504. In contrast, samples from the R6 and Mahogany zones possess lower pristane/phytane ratios and higher β-carotane/ n -C 30 ratios indicating deposition in a more restricted lacustrine environment with elevated salinities and alkalinities that would have limited aquatic organic matter diversity. The R6 and Mahogany zones have the narrowest range of 187 Re/ 188 Os values measured in this study of 254.9 and 154.6, respectively. As noted by previous workers, these results suggest that organic matter diversity plays a primary role in determining the range of 187 Re/ 188 Os ratios in a sample set, and in turn the uncertainty of Re-Os age determinations from organic-rich sedimentary rocks. The Re-Os data from the R3 zone and R6 zone yield ages of 49.7 ± 3.4 Ma and 42.0 ± 18 Ma, respectively, which are statistically indistinguishable based on 2σ uncertainty from three previously reported Re-Os age determinations and those provided by 40 Ar/ 39 Ar geochronology of interbedded volcanic ash beds. Although the age uncertainty is high, these findings further highlight the importance of Re-Os geochronology in lacustrine basins, particularly those with thick mudstone successions that lack volcanic ash layers, reliable biostratigraphy , or magnetostratigraphic control. In these cases, even ages with large uncertainties can be useful to constrain burial history and thermal history models. Together, the initial 187 Os/ 188 Os ratios of five sets of samples analyzed from the Uinta Basin define the largest Os isotope stratigraphic record from any lacustrine basin compiled to date and record a shift from a value of 1.40 to 1.48 between the R3 and R4 zones in the lower part of the Parachute Creek Member. This small shift may signify a change in the chemical weathering products that entered the lake preserved 20 to 50 m above the contact between the Douglas Creek and the lower Parachute Creek members during a period when the basin transitioned from a shallow lake with mostly open hydrology to an alkaline lake with more frequent basin restrictions.

Colorado, Utah, Wyoming↗
USGS2016

Tomographic Rayleigh-wave group velocities in the Central Valley, California centered on the Sacramento/San Joaquin Delta

If shaking from a local or regional earthquake in the San Francisco Bay region were to rupture levees in the Sacramento/San Joaquin Delta then brackish water from San Francisco Bay would contaminate the water in the Delta: the source of fresh water for about half of California. As a prelude to a full shear-wave velocity model that can be used in computer simulations and further seismic hazard analysis, we report on the use of ambient noise tomography to build a fundamental-mode, Rayleigh-wave group velocity model for the region around the Sacramento/San Joaquin Delta in the western Central Valley, California. Recordings from the vertical component of about 31 stations were processed to compute the spatial distribution of Rayleigh wave group velocities. Complex coherency between pairs of stations were stacked over 8 months to more than a year. Dispersion curves were determined from 4 to about 18 seconds. We calculated average group velocities for each period and inverted for deviations from the average for a matrix of cells that covered the study area. Smoothing using the first difference is applied. Cells of the model were about 5.6 km in either dimension. Checkerboard tests of resolution, which is dependent on station density, suggest that the resolving ability of the array is reasonably good within the middle of the array with resolution between 0.2 and 0.4 degrees. Overall, low velocities in the middle of each image reflect the deeper sedimentary syncline in the Central Valley. In detail, the model shows several centers of low velocity that may be associated with gross geologic features such as faulting along the western margin of the Central Valley, oil and gas reservoirs, and large cross cutting features like the Stockton arch. At shorter periods around 5.5s, the model’s western boundary between low and high velocities closely follows regional fault geometry and the edge of a residual isostatic gravity low. In the eastern part of the valley, the boundaries of the low velocity zone and gravity anomaly are better aligned at longer periods (around 10.5s) suggesting that the eastern edge of the gravity low is associated with deeper structure. There is a strong correspondence between a low in gravity near the Kirby Hills fault and low velocities from the ambient noise tomography. At longer periods, higher velocities creep in from the east and narrow the overall dimension defined by the lower velocities. Overall, there is a strong correspondence between the shape and location of low velocities in the Rayleigh wave velocity images, and geological and geophysical features.

California↗
USGS2016

Continental Shelf Morphology and Stratigraphy Offshore San Onofre, CA: The Interplay Between Rates of Eustatic Change and Sediment Supply

New high-resolution CHIRP seismic data acquired offshore San Onofre, southern California reveal that shelf sediment distribution and thickness are primarily controlled by eustatic sea level rise and sediment supply. Throughout the majority of the study region, a prominent abrasion platform and associated shoreline cutoff are observed in the subsurface from ~ 72 to 53 m below present sea level. These erosional features appear to have formed between Melt Water Pulse 1A and Melt Water Pulse 1B, when the rate of sea-level rise was lower. There are three distinct sedimentary units mapped above a regional angular unconformity interpreted to be the Holocene transgressive surface in the seismic data. Unit I, the deepest unit, is interpreted as a lag deposit that infills a topographic low associated with an abrasion platform. Unit I thins seaward by downlap and pinches out landward against the shoreline cutoff. Unit II is a mid-shelf lag deposit formed from shallower eroded material and thins seaward by downlap and landward by onlap. The youngest, Unit III, is interpreted to represent modern sediment deposition. Faults in the study area do not appear to offset the transgressive surface. The Newport Inglewood/Rose Canyon fault system is active in other regions to the south (e.g., La Jolla) where it offsets the transgressive surface and creates seafloor relief. Several shoals observed along the transgressive surface could record minor deformation due to fault activity in the study area. Nevertheless, our preferred interpretation is that the shoals are regions more resistant to erosion during marine transgression. The Cristianitos fault zone also causes a shoaling of the transgressive surface. This may be from resistant antecedent topography due to an early phase of compression on the fault. The Cristianitos fault zone was previously defined as a down-to-the-north normal fault, but the folding and faulting architecture imaged in the CHIRP data are more consistent with a strike-slip fault with a down-to-the-northwest dip-slip component. A third area of shoaling is observed off of San Mateo and San Onofre creeks. This shoaling has a constructional component and could be a relict delta or beach structure. (C) 2015 Elsevier B.V. All rights reserved.

California↗