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At least 253 records · Page 14Linked to original sources
USGS2003

Beaufortian stratigraphic plays in the National Petroleum Reserve - Alaska (NPRA)

The Beaufortian megasequence in the National Petroleum Reserve in Alaska (NPRA) includes Jurassic through lower Cretaceous (Neocomian) strata of the Kingak Shale and the overlying pebble shale unit. These strata are part of a composite total petroleum system involving hydrocarbons expelled from source rocks in three stratigraphic intervals, the Lower Jurassic part of the Kingak Shale, the Triassic Shublik Formation, and the Lower Cretaceous gamma-ray zone (GRZ) and associated strata. The potential for undiscovered oil and gas resources in the Beaufortian megasequence in NPRA was assessed by defining eight plays (assessment units), two in lower Cretaceous (Neocomian) topset seismic facies, four in Upper Jurassic topset seismic facies, one in Lower Jurassic topset seismic facies, and one in Jurassic through lower Cretaceous (Neocomian) clinoform seismic facies. The Beaufortian Cretaceous Topset North Play is estimated to contain between 0 (95-percent probability) and 239 (5-percent probability) million barrels of technically recoverable oil, with a mean (expected value) of 103 million barrels. The Beaufortian Cretaceous Topset North Play is estimated to contain between 0 (95-percent probability) and 1,162 (5-percent probability) billion cubic feet of technically recoverable, nonassociated natural gas, with a mean (expected value) of 405 billion cubic feet. The Beaufortian Cretaceous Topset South Play is estimated to contain between 635 (95-percent probability) and 4,004 (5-percent probability) billion cubic feet of technically recoverable, nonassociated natural gas, with a mean (expected value) of 2,130 billion cubic feet. No technically recoverable oil is assessed in the Beaufortian Cretaceous Topset South Play, as it lies at depths that are entirely in the gas window. The Beaufortian Upper Jurassic Topset Northeast Play is estimated to contain between 2,744 (95-percent probability) and 8,086 (5-percent probability) million barrels of technically recoverable oil, with a mean (expected value) of 5,176 million barrels. No technically recoverable gas is assessed in the Beaufortian Upper Jurassic Topset Northeast Play. The Beaufortian Upper Jurassic Topset Northwest Play is estimated to contain between 733 (95-percent probability) and 3,312 (5-percent probability) million barrels of technically recoverable oil, with a mean (expected value) of 1,859 million barrels. No technically recoverable gas is assessed in the Beaufortian Upper Jurassic Topset Northwest Play. The Beaufortian Upper Jurassic Topset Southeast Play is estimated to contain between 2,053 (95-percent probability) and 9,030 (5-percent probability) billion cubic feet of technically recoverable, nonassociated natural gas, with a mean (expected value) of 5,137 billion cubic feet. No technically recoverable oil is assessed in the Beaufortian Upper Jurassic Topset Southeast Play, as it lies at depths that are entirely in the gas window. The Beaufortian Upper Jurassic Topset Southwest Play is estimated to contain between 2,008 (95-percent probability) and 9,265 (5-percent probability) billion cubic feet of technically recoverable, nonassociated natural gas, with a mean (expected value) of 5,220 billion cubic feet. No technically recoverable oil is assessed in the Beaufortian Upper Jurassic Topset Southwest Play, as it lies at depths that are entirely in the gas window. The Beaufortian Lower Jurassic Topset Play is estimated to contain between 0 (95-percent probability) and 210 (5-percent probability) million barrels of technically recoverable oil, with a mean (expected value) of 83 million barrels. The Beaufortian Lower Jurassic Topset Play is estimated to contain between 0 (95-percent probability) and 1,915 (5-percent probability) billion cubic feet of technically recoverable, nonassociated natural gas, with a mean (expected value) of 793 billion cubic feet. The Beaufortian Clinoform Play is estimated to contain between 0 (95-percent probability)

Open-File Report↗
USGS2009

Geologic and Geochronologic Studies of the Early Proterozoic Kanektok Metamorphic Complex of Southwestern Alaska

The Kanektok complex of southwestern Alaska appears to be a rootless terrane of early Proterozoic sedimentary, volcanic, and intrusive rocks which were metamorphosed to amphibolite and granulite facies and later underwent a pervasive late Mesozoic thermal event accompanied by granitic plutonism and greenschist facies metamorphism of overlying sediments. The terrane is structurally complex and exhibits characteristics generally attributed to mantled gneiss domes. U-Th-Pb analyses of zircon and sphene from a core zone granitic orthogneiss indicate that the orthogneiss protolith crystallized about 2.05 b.y. ago and that the protolithic sedimentary, volcanic and granitic intrusive rocks of the core zone were metamorphosed to granulite and amphibolite facies about 1.77 b.y. ago. A Rb-Sr study of 13 whole-rock samples also suggests metamorphism of an early Proterozoic [Paleoproterozoic] protolith at 1.77 Ga, although the data are scattered and difficult to interpret. Seventy-seven conventional 40K/40Ar mineral ages were determined for 58 rocks distributed throughout the outcrop area of the complex. Analysis of the K-Ar data indicate that nearly all of these ages have been totally or partially reset by a pervasive late Mesozoic thermal event accompanied by granitic plutonism and greenschist facies metamorphism. Several biotites gave apparent K-Ar ages over 2 Ga. These ages appear to be controlled by excess radiogenic 40Ar produced by the degassing protolith during the 1.77 Ga metamorphism and incorporated by the biotites when they were at temperatures at which Ar could diffuse through the lattice. Five amphibolites yielded apparent Precambrian 40K/40Ar hornblende ages. There is no evidence that these hornblende ages have been increased by excess argon. The oldest 40K/40Ar hornblende age of 1.77 Ga is identical to the sphene 207Pb/206Pb orthogneiss age and to the Rb-Sr 'isochron' age for six of the 13 whole-rock samples. The younger hornblende ages are interpreted as having been partially reset during the late Mesozoic thermal event. 40Ar/39Ar incremental heating experiments suggest metamorphism occurred at least 1.2 b.y. ago but do not exhibit high temperature plateau ages significantly older than the 40Ar/39Ar total fusion ages of these samples. The age spectra are much more uniform than expected from a terrane with such a complex thermal history, perhaps caused by the small grain size of the samples which may possibly be less than the effective Ar diffusion radii of the analyzed hornblendes.

Open-File Report↗
USGS2022

Preliminary geologic map of the Cherry Hill quadrangle, Dinwiddie, Sussex, and Greensville Counties, Virginia

The Cherry Hill 7.5-minute quadrangle straddles the Coastal Plain and Piedmont Provinces along the Tidewater Fall Line. Rocks of the eastern Piedmont Roanoke Rapids terrane crop out in the western part of the quadrangle and consist of greenschist- to amphibolite-facies Neoproterozoic felsic to intermediate metavolcanic rocks, some of which contain flattened quartz phenocrysts and are locally isoclinally folded; greenstone that locally preserves primary layering; and intrusive metadiorite and metagabbro, much of which has been altered to amphibolite. Most of these rocks are strongly foliated and jointed. Greenschist-facies metasiltstone that preserves primary bedding also occurs locally in the Roanoke Rapids terrane. Neoproterozoic mica schist, middle Paleozoic foliated metagranite, and late Paleozoic massive and porphyritic granite crop out in the eastern part of the quadrangle and are part of the Dinwiddie terrane and the late Paleozoic De Witt pluton. Upper greenschist- to lower amphibolite-facies mica schist consists of stringers and boudins of vein quartz and contains porphyroclasts of staurolite that preserve an earlier foliation as inclusion trails. Porphyroblasts of garnet, staurolite, and kyanite also occur locally. Foliation in granites of the De Witt pluton may be magmatic. Separating the Dinwiddie terrane from the Roanoke Rapids terrane are greenschist-facies, highly strained granitic mylonite and bodies of less deformed granite within the Nottoway River fault zone, which is a strand of the eastern Piedmont fault system. Paleozoic pegmatite dikes and quartz veins cross-cut rocks of the Dinwiddie terrane, and quartz veins and Jurassic diabase dikes cross-cut rocks of the Roanoke Rapids terrane. Sand and gravel deposits of the Atlantic Coastal Plain overlie Piedmont rocks. Two units assigned to the upper part of the Neogene Chesapeake Group occur at elevations up to 295 feet (90 meters) above sea level atop the Richmond plain in the central part of the quadrangle. Two units of the Quaternary Bacons Castle Formation occupy the Essex plain and Norge uplands at elevations up to 180 feet (55 meters) above sea level in the eastern part of the quadrangle. In the western part of the quadrangle, multiple levels of terrace deposits are the fluvial equivalent of estuarine to marine units of the Atlantic Coastal Plain to the east. Holocene alluvium occurs along creeks and the Nottoway River. Quaternary colluvial deposits occur locally. Numerous Carolina bays pock the landscape of the Richmond and Essex plains, and three abandoned channelways represent former locations of Sappony Creek, one of the major drainages of the quadrangle. Brittle faults juxtapose Piedmont basement rocks against Neogene sediments of the upper part of the Chesapeake Group. These Cenozoic faults were first uncovered in mine excavations in the late 1990s; new mapping indicates that many of these faults are reactivated silicified cataclasite zones that occur throughout the Piedmont basement rocks. Silicified cataclasites and associated quartz veins are typically mineralized with iron and iron sulfide minerals. The quadrangle was the focus of extensive mining for heavy minerals, including ilmenite and zircon, in upland Atlantic Coastal Plain deposits beginning in the mid-1990s. Other mineral resources, including precious metals, clay for structural brick, crushed stone, and building stone for millstones, have also been prospected or quarried in the quadrangle.

Virginia↗
USGS1965

Geology of the Romanzof Mountains, Brooks Range, northeastern Alaska

This remote 700 square mile area in the Brooks Range is topographically rugged and geologically diverse; it contains a granitic pluton, low-grade metamorphic rocks, sedimentary rocks, and mafic igneous rocks, as well as glacial features. Rocks of sedimentary origin include from oldest to youngest: 1.Neruokpuk Formation Middle and Upper Devonian(?), more than 4000 feet thick, a variety of units which represent the greenschist facies, including quartzitic- and schistose-feldspathic graywacke; phyllite, argillite, and slate, as well as dark limestone, sandy limestone, and silicified carbonate rocks. The succession of units in parts of the area is uncertain. Correlations between these units and with others in the eastern Brooks Range are provisional. 2.Kekiktuk Conglomerate and Kayak(?) Shale (Upper Devonian(?) to Upper Mississippian), a single map unit, from absent(?) to 400+ feet thick, containing dark shale Kayak(?) in its uppermost part and quartzite, interbedded dark shale, and some pebble- to boulder-conglomerate in the locally absent lower part (Kekiktuk). The unit overlies the Neruokpuk with angular unconformity, which may reflect either a pre-Kayak(?) or pre-Kekiktuk hiatus or both. 3.Lisburne Group, almost entirely carbonate rocks, and relatively thin in this area, 600 to 800 feet thick. Alapah Limestone (Upper Mississippian), to 560 feet thick, includes gray sandy, crystalline, and cherty limestone; minor dark shale; and dark cherty carbonate rocks in the upper part. The lower contact is gradational with the Kayak(?). Wahoo(?) Limestone (Pennsylvanian(?) to Permian) conformably overlies the Alapah, is absent to 200+ feet thick, and is characterized by light-gray crinoidal limestones in its upper part. 4. Sadlerochit Formation, consisting of three intraconformable units: ferruginous sandstone member (Permian) of ironstained orthoquartzite and dark slate, 175 to 240 feet thick which unconformably overlies the Wahoo(?) and Alapah Limestones; shale member of dark shale, slate, and minor quartzite averaging 400 feet in thickness; and quartzite member (Lower(?) Triassic), 700 feet thick, mostly orthoquartzite with minor shale and conglomerate. The basal clastics were probably shed from the north. 5.Shublik Formation (Middle(?) and Upper Triassic), 600 to 700 feet thick, with the thin phosphatic sandstone member overlain by dark phosphatic limestones and limy shales of the limestone member. 6.Kingak Formation (Jurassic), more than 1000 feet thick. The siltstone member, resistant sandstone and siltstone 75 to 150 feet thick, is overlain by an undetermined thickness of black shale. The basal part contrasts sharply with the underlying Shublik, indicating possible disconformity. 7.Ignek(?) Formation (Cretaceous), represented in the foothills where lithic graywacke, shale, and coaly shale constitute the few exposures examined. 8.Glacial and glaciofluvial materials of five advances recognized on the basis of morphology and position, which are tentatively correlated with five glaciations 15 miles west of the area. 9. Recent alluvial and colluvial deposits including fans which appear to represent at least three stages of encroachment. The Ramanzof granite, exposed in the Okpilak batholith and Jago stock, is mostly light-gray quartz monzonite to granite, and contains essential quartz, perthitic microcline, albite-oligoclase, and partly chloritized biotite. Limited modal and chemical data are presented. Three textural facies are: 1) porphyritic (marginal), with abundant large microcline megacrysts; 2) variable (middle to marginal), which exhibits textural and mineralogical banding; and 3) coarse (inner to marginal), which is gneissoid to equigranular. Facies relationships appear to be mostly gradational but may be locally intrusive. Some schistose metasedimentary(?) rock occurs in the granite. Aplite dikes, inclusions, tourmaline veins and replacements, and chlorite and quartz veins are locally common, as well as quartz monzonite and mafic igneous dikes. Contacts with Neruokpuk Formation rocks are mostly abrupt, concordant to cross-cutting, and locally adjoin tactite and hornfels of the albite-epidote-hornfels and hornblende-hornfels facies. Contacts with Kekiktuk Conglomerate are apparently gradational through a schistoze zone. Both primary and secondary structural elements are present in the Romanzof in granite. Textural and mineralogical banding and, in general, feldspar foliation are considered to be primary in origin; biotite foliation, gneissic and schistose foliation, and schistose zones are considered secondary. Lead-alpha age of zircons appears to be Late Devonian, K-Ar age of biotite is Cretaceous, possibly indicating updating by later reheating. Field age relationships are inconclusive but suggest pre-Kayak(?) (Upper Devonian) granite emplacement. The pluton is interpreted to be essentially the product of melt crystallization, synorogenically emplaced by forceful injection with minor stoping, and may include marginally granitized rock. Mafic igneous rocks of altered basaltic composition (greenstones) include dikes in granitic and Neruokpuk Formation rocks, and volcanics(?). A late Paleozoic age is suggested for them. Structural grain strikes east-northeast; south-dipping elements are common. Structures include the major positive nature of the area (first order), relatively broad folds (second order) which contain small tight folds (third order). Related south-dipping cleavage, schistosity, and biotite foliation in granite in the northern part of the area are cut by prominent sets of transverse joints and faults. Other features are longitudinal normal and reverse faults, at least one large-scale overthrust fault, and sheared zones in granite with possible attendant retrograde metamorphism. Although Mesozoic and Tertiary deformational features are dominant in northern Alaska, the Romanzof area may have been part of a Late Devonian orogenic belt continuous with one in northern Canada. Three alternate trends of such a belt in northern Alaska are discussed, but evidence is inconclusive. The mineral potential of the area is largely unknown. Minor amounts of metallic sulfides and oxides are present in granite and Neruokpuk Formation rocks. Analyses of stream silt samples suggest the possibility of tin and beryllium potential. The Shublik Formation contains rock phosphate.

Alaska↗
USGS1972

Reconnaissance geology of the Jabal Bitran quadrangle, Kingdom of Saudi Arabia

The Jabal Bitten quadrangle covers an area of 2833 sq km in the eastern part of the Precambrian Shield in Saudi Arabia. The rocks in the quadrangle are divided geographically alone arcuate north-trending lines into an eastern area of granite intruded by a swarm of dikes of rhyolite and andesite, and a western area of dominantly pelitic chlorite-sericite schist, separated by the narrow central complex of the Idsas Range. This complex is composed of pyroclastic rocks, lava, conglomerate, marble, and plutonic mafic rocks that have been intricately modified by episodes of metamorphism, igneous intrusion, and faulting. The Idsas Range contains ancient gold and copper mines, and deposits of magnetite, copper, asbestos, and chromite. The rocks in the Jabal Bitten quadrangle are here interpreted to consist of three major sedimentary and volcanic groups, the lowermost of which was deposited unconformably on hornblende-biotite granite gneiss, and all of which are intruded by granite dikes and plutons. From oldest to youngest the layered rocks are called Halaban Group, Bi'r Khountina Group, and Murdama Group, A biotite-hornblende granite is older than uppermost Bi'r Khountina, and peralkalic granite is younger than Murdama. The layered rocks of these groups are generally metamorphosed to the greenschist facies. The metamorphic grade rises abruptly at the Idsas Range to the albite-epidote-amphibolite facies and lower subfacies of the amphibolite facies in parts of the Halaban Group; some skarn east of the range may be in the upper part of the amphibolite facies. Characteristically, the Halaban Group has the highest grade and the greatest range in metamorphic grade, and the Murdama Group has the lowest but most uniformly developed metamorphic grade. The metamorphism of the rocks was caused by three successive pulses of regional dynamothermal metamorphism plus contact metamorphism around the younger bodies of plutonic igneous rocks. Four major structural elements of the quadrangle are reflected in the geography and geologic units. These are a mantled gneiss dome on the east separated from a north-plunging synclinorium in rocks of the Murdama and Bi?r Khountina Groups on the west by a narrow dejective zone of the Halaban and lower Bi?r Khountina. The dejective zone is much modified by impricate overthrusts and accompanying tear faults. These major faults have pushed elements of the Halaban and Bi?r Khountina westward over Bi?r Khountina and Murdama, with the result that very complex fault patterns have evolved. Open geochemical reconnaissance of the area disclosed one positive anomaly for nickel and 40 threshold indications of several elements, principally nickel, chromium, copper, and tungsten. Heavy-mineral and radiometric reconnaissance showed 18 areas containing scheelite and/or powellite and four areas of anomalous radioactivity. Most of these features are in the dejective zone, as are five of the nine ancient workings, the massive and disseminated magnetite, most of the secondary copper minerals, and the traces of asbestos, magnesite, and chromite known in the quadrangle. The mantled gneiss dome and a complex of gabbro and amphibolite on its southwestern flank are the next most mineralized areas. Scant evidence of mineralization is present in the Murdama Group west of the dejective zone. Magnetite deposits at Jabal Idsas have the greatest potential of the mineral deposits in the Jabal Bitran quadrangle. Further study of gold at Fawara and Selib mines is recommended, as is investigation of a positive nickel anomaly that shows threshold cobalt and above background radioactivity. The garnetiferous skarn in the east-central part of the quadrangle should be examined for composition and abrasive character of the garnet and for the remote possibility of tungsten in scheelite and beryllium in helvite.

Open-File Report↗
USGS1979

Metals in Devonian kerogenous marine strata at Gibellini and Bisoni properties in southern Fish Creek Range, Eureka County, Nevada

A kerogen-rich sequence of siliceous mudstone, siltstone, and chert as much as 60 m thick on ridge 7129 in the southern Fish Creek Range, referred to as Gibellini facies of the Woodruff Formation, has been evaluated on the surface and in drill holes principally for its potential resources of vanadium, zinc, selenium, molybdenum, and syncrude oil content. The strata are part of a strongly deformed allochthonous mass of eugeosynclinal Devonian marine rocks that overlie deformed allochthonous Mississippian siliceous rocks and relatively undeformed autochthonous Mississippian Antler flysch at this locality. The vanadium in fresh black rocks obtained from drill holes and fresh exposures in trenches and roadcuts occurs chiefly in organic matter. Concentrations of vanadium oxide (V2O5) in unoxidized samples range from 3,000 to 7,000 ppm. In oxidized and bleached rock that is prevalent at the surface, concentrations of vanadium oxide range from 6,000 to 8,000 ppm, suggesting a tendency toward enrichment due to surficial weathering and ground-water movement. Zinc occurs in sphalerite, and selenium occurs in organic matter; molybdenum appears to occur both in molybdenite and in organic matter. Concentrations of zinc in unoxidized rock range from 4,000 to 18,000 ppm, whereas in oxidized rock they range from 30 to 100 ppm, showing strong depletion due to weathering. Concentrations of selenium in unoxidized rock range from 30 to 200 ppm, whereas in oxidized rock they range from 200 to 400 ppm, indicating some enrichment upon weathering. Concentrations of molybdenum in unoxidized rock range from 70 to 960 ppm, whereas in oxidized rock they range from 30 to 80 ppm, indicating strong depletion upon weathering. Most fresh black rock is low-grade oil shale, and yields as much as 12 gallons/short ton of syncrude oil. Metahewettite is the principal vanadium mineral in the oxidized zone, but it also occurs sparsely as small nodules and fillings of microfractures in unweathered strata. In fresh rock, bluish-white opaline-like silica (chalcedonic quartz) fills microfractures, and is believed to have originated by diagenetic mobilization of opaline silica from radiolarian tests and sponge spicules. As revealed by microscopic study, the Gibellini facies originally consisted of siliceous muds, slimes, and oozes high in organic constituents. The organic matter is amorphous flaky and stringy sapropel, and probably includes remains of bacteria, phytoplankton, zooplankton, and minor higher plants. Recognizable organic remnants include radiolarian tests, sponge spicules, conodonts, brachiopod shells, algae, and humic debris. Diagnostic radiolarians indicate a Late Devonian age for the Gibellini facies of the Woodruff Formation. Some pyrite is disseminated through the rock and may be primary (syngenetic) but significant pyrite and marcasite occur in chalcedonic quartz veinlets and appear to be diagenetic. In fresh rock, black solid bitumen and liquid oil fill voids and microfractures. These early phase hydrocarbons probably were released during diagenesis from complex nonhydrocarbon molecular structures originating from living organisms, and formed without any major thermal degradation of the kerogen. Gas chromatographic analysis of the saturated hydrocarbon fraction indicates a very complex mixture dominated by branched and cyclic compounds. Conodont and palynomorph color alteration, vitrinite reflectance, and other organic geochemical data suggest that the organic matter in the rock is thermally immature and has not been subjected to temperatures greater than 60?C since deposition in Devonian time. All of these characteristics are consistent with the interpretation of a relatively low temperature and a shallow-burial history for the Gibellini facies on ridge 7129.

Open-File Report↗
USGS1980

Ordovician and Silurian Phi Kappa and Trail Creek formations, Pioneer Mountains, central Idaho: Stratigraphic and structural revisions, and new data on graptolite faunas

Recent geologic mapping in the northern Pioneer Mountains combined with the identification of graptolites from 116 new collections indicate that the Ordovician and Silurian Phi Kappa and Trail Creek Formations occur in a series of thrust-bounded slices within a broad zone of imbricate thrust faulting. Though confirming a deformational style first reported in a 1963 study by Michael Churkin, our data suggest that the complexity and regional extent of the thrust zone were not previously recognized. Most previously published sections of the Phi Kappa and Trail Creek Formations were measured across unrecognized thrust faults and therefore include not only structural repetitions of graptolitic Ordovician and Silurian rocks but also other tectonically juxtaposed lithostratigraphic units of diverse ages as well. Because of this discovery, the need to reconsider the stratigraphic validity of these formations and their lithology, nomenclature, structural distribution, facies relations, and graptolite faunas has arisen. The Phi Kappa Formation in most thrust slices has internal stratigraphic continuity despite the intensity of deformation to which it was subjected. As revised herein, the Phi Kappa Formation is restricted to a structurally repeated succession of predominantly black, carbonaceous, graptolitic argillite and shale. Some limy, light-gray-weathering shale occurs in the middle part of the section, and fine-grained locally pebbly quartzite is present at the base. The basal quartzite is here named the Basin Gulch Quartzite Member of the Phi Kappa. The Phi Kappa redefined on a lithologic basis represents the span of Ordovician time from W. B. N. Berry's graptolite zones 2-4 through 15 and also includes approximately 17 m of lithologically identical shale of Early and Middle Silurian age at the top. The lower contact of the formation as revised is tectonic. The Phi Kappa is gradationally overlain by the Trail Creek Formation as restricted herein. Most of the coarser clastic rocks reported in previously measured sections of the Phi Kappa, as well as the sequence along Phi Kappa Creek from which the name originates, are excluded from the Phi Kappa as revised and are reassigned to two structural plates of Mississippian Copper Basin Formation; other strata now excluded from the formation are reassigned to the Trail Creek Formation and to an unnamed Silurian and Devonian unit. As redefined, the Phi Kappa Formation is only about 240 m thick, compared with the 3,860 m originally estimated, and it occupies only about 25 percent of the outcrop area previously mapped in 1930 by H. G. Westgate and C. P. Ross. Despite this drastic reduction in thickness and the exclusion of the rocks along Phi Kappa Creek, the name Phi Kappa is retained because of widely accepted prior usage to denote the Ordovician graptolitic shale facies of central Idaho, and because the Phi Kappa Formation as revised is present in thrust slices on Phi Kappa Mountain, at the head of Phi Kappa Creek. The lithic and faunal consistency of this unit throughout the area precludes the necessity for major facies telescoping along individual faults within the outcrop belt. However, tens of kilometers of tectonic shortening seems required to juxtapose the imbricated Phi Kappa shale facies with the Middle Ordovician part of the carbonate and quartzite shale sequence of east central Idaho. The shelf rocks are exposed in the Wildhorse structural window of the northeastern Pioneer Mountains, and attain a thickness of at least 1,500 m throughout the region north and east of the Pioneer Mountains. The Phi Kappa is in direct thrust contact on intensely deformed medium- to high-grade metamorphic equivalents of the same shelf sequence in the Pioneer window at the south end of the Phi Kappa-Trail Creek outcrop belt. Along East Pass, Big Lake, and Pine Creeks, north of the Pioneer Mountains, some rocks previously mapped as Ramshorn Slate are lithologically and faunally equivalent to the P

Idaho↗
USGS2007

Sedimentology and sequence stratigraphy of the Lower Cretaceous Fortress Mountain and Torok Formations exposed along the Siksikpuk River, North-Central Alaska

An exposure of the Lower Cretaceous Fortress Mountain and Torok Formations along the Siksikpuk River in north-central Alaska provides a rare opportunity to observe the stratigraphic contact between these two formations and to interpret the depositional facies and sequence stratigraphy of the exposed strata. The Fortress Mountain Formation at the base of the measured section includes braided-fluvial and coastal-plain facies deposited in a lowstand-systems tract, and an overlying succession of mostly shallow marine facies deposited in the basal part of a transgressive-systems tract. The overlying Torok Formation includes a thick, upward-deepening succession of marine-shelf to marine-slope facies deposited in the upper part of the transgressive-systems tract. The upper part of the section includes marine-slope and incised-slope-channel turbidite deposits of the Torok Formation, interpreted as a highstand-systems tract. Consideration of the balance between accommodation and sediment flux inferred from the sequence-stratigraphic analysis suggests that both tectonics and eustasy may have influenced deposition of the lowstand-systems and transgressive-systems tracts. In contrast, the highstand-systems tract may have been primarily influenced by progradation of a regional sediment-dispersal system and by subsidence induced by sediment loading.

Alaska↗
USGS2013

Stratigraphy of lower to middle Paleozoic rocks of northern Nevada and the Antler orogeny

Commonly accepted concepts concerning the lower Paleozoic stratigraphy of northern Nevada are based on the assumption that the deep-water aspects of Ordovician to Devonian siliceous strata are due to their origin in a distant oceanic environment, and their presence where we find them is due to tectonic emplacement by the Roberts Mountains thrust. The concept adopted here is based on the assumption that their deep-water aspects are the result of sea-level rise in the Cambrian, and all of the Paleozoic strata in northern Nevada are indigenous to that area. The lower part of the Cambrian consists mainly of shallow-water cross-bedded sands derived from the craton. The upper part of the Cambrian, and part of the Ordovician, consists mainly of deep-water carbonate clastics carried by turbidity currents from the carbonate shelf in eastern Nevada, newly constructed as a result of sea-level rise. Ordovician to mid-Devonian strata are relatively deep-water siliceous deposits, which are the western facies assemblage. The basal contact of this assemblage on autochthonous Cambrian rocks is exposed in three mountain ranges and is clearly depositional in all three. The western facies assemblage can be divided into distinct stratigraphic units of regional extent. Many stratigraphic details can be explained simply by known changes in sea level. Upper Devonian to Mississippian strata are locally and westerly derived orogenic clastic beds deposited disconformably on the western facies assemblage. This disconformity, clearly exposed in 10 mountain ranges, indicates regional uplift and erosion of the western facies assemblage and absence of local deformation. The disconformity represents the Antler orogeny.

Nevada↗
USGS2015

Revised hydrogeologic framework of the Floridan aquifer system in Florida and parts of Georgia, Alabama, and South Carolina

The hydrogeologic framework for the Floridan aquifer system has been revised throughout its extent in Florida and parts of Georgia, Alabama, and South Carolina. The updated framework generally conforms to the original framework established by the U.S. Geological Survey in the 1980s, except for adjustments made to the internal boundaries of the Upper and Lower Floridan aquifers and the individual higher and contrasting lower permeability zones within these aquifers. The system behaves as one aquifer over much of its extent; although subdivided vertically into two aquifer units, the Upper and Lower Floridan aquifers. In the previous framework, discontinuous numbered middle confining units (MCUI–VII) were used to subdivide the system. In areas where less-permeable rocks do not occur within the middle part of the system, the system was previously considered one aquifer and named the Upper Floridan aquifer. In intervening years, more detailed data have been collected in local areas, resulting in some of the same lithostratigraphic units in the Floridan aquifer system being assigned to the Upper or Lower Floridan aquifer in different parts of the State of Florida. Additionally, some of the numbered middle confining units are found to have hydraulic properties within the same order of magnitude as the aquifers. A new term “composite unit” is introduced for lithostratigraphic units that cannot be defined as either a confining or aquifer unit over their entire extent. This naming convention is a departure from the previous framework, in that stratigraphy is used to consistently subdivide the aquifer system into upper and lower aquifers across the State of Florida. This lithostratigraphic mapping approach does not change the concept of flow within the system. The revised boundaries of the Floridan aquifer system were mapped by considering results from local studies and regional correlations of lithostratigraphic and hydrogeologic units or zones. Additional zones within the aquifers have been incorporated into the framework to allow finer delineation of permeability variations within the aquifer system. These additional zones can be used to progressively divide the system for assessing groundwater and surface-water interaction, saltwater intrusion, and offshore movement of groundwater at greater detail if necessary. The lateral extent of the updip boundary of the Floridan aquifer system is modified from previous work based on newer data and inclusion of parts of the updip clastic facies. The carbonate and clastic facies form a gradational sequence, generally characterized by limestone of successively younger units that extend progressively farther updip. Because of the gradational nature of the carbonate-clastic sequence, some of the updip clastic aquifers have been included in the Floridan aquifer system, the Southeastern Coastal Plain aquifer system, or both. Thus, the revised updip limit includes some of these clastic facies. Additionally, the updip limit of the most productive part of the Floridan aquifer system was revised and indicates the approximate updip limit of the carbonate facies. The extent and altitude of the freshwater-saltwater interface in the aquifer system has been mapped to define the freshwater part of the flow system.

Alabama, Florida, Georgia, South Carolina↗
USGS2011

Sedimentology and reservoir heterogeneity of a valley-fill deposit– A field guide to the Dakota Sandstone of the San Rafael Swell, Utah

Valley-fill deposits form a significant class of hydrocarbon reservoirs in many basins of the world. Maximizing recovery of fluids from these reservoirs requires an understanding of the scales of fluid-flow heterogeneity present within the valley-fill system. The Upper Cretaceous Dakota Sandstone in the San Rafael Swell, Utah contains well exposed, relatively accessible outcrops that allow a unique view of the external geometry and internal complexity of a set of rocks interpreted to be deposits of an incised valley fill. These units can be traced on outcrop for tens of miles, and individual sandstone bodies are exposed in three dimensions because of modern erosion in side canyons in a semiarid setting and by exhumation of the overlying, easily erodible Mancos Shale. The Dakota consists of two major units: (1) a lower amalgamated sandstone facies dominated by large-scale cross stratification with several individual sandstone bodies ranging in thickness from 8 to 28 feet, ranging in width from 115 to 150 feet, and having lengths as much as 5,000 feet, and (2) an upper facies composed of numerous mud-encased lenticular sandstones, dominated by ripple-scale lamination, in bedsets ranging in thickness from 5 to 12 feet. The lower facies is interpreted to be fluvial, probably of mainly braided stream origin that exhibits multiple incisions amalgamated into a complex sandstone body. The upper facies has lower energy, probably anastomosed channels encased within alluvial and coastal-plain floodplain sediments. The Dakota valley-fill complex has multiple scales of heterogeneity that could affect fluid flow in similar oil and gas subsurface reservoirs. The largest scale heterogeneity is at the formation level, where the valley-fill complex is sealed within overlying and underlying units. Within the valley-fill complex, there are heterogeneities between individual sandstone bodies, and at the smallest scale, internal heterogeneities within the bodies themselves. These different scales of fluid-flow compartmentalization present a challenge to hydrocarbon exploration targeting paleovalley deposits, and producing fields containing these types of reservoirs may have significant bypassed pay, especially where well spacing is large.

Utah↗
USGS2014

Three-dimensional geologic mapping of the Cenozoic basin fill, Amargosa Desert basin, Nevada and California

Understanding the subsurface geologic framework of the Cenozoic basin fill that underlies the Amargosa Desert in southern Nevada and southeastern California has been improved by using borehole data to construct three-dimensional lithologic and interpreted facies models. Lithologic data from 210 boreholes from a 20-kilometer (km) by 90-km area were reduced to a limited suite of descriptors based on geologic knowledge of the basin and distributed in three-dimensional space using interpolation methods. The resulting lithologic model of the Amargosa Desert basin portrays a complex system of interfingered coarse- to fine-grained alluvium, playa and palustrine deposits, eolian sands, and interbedded volcanic units. Lithologic units could not be represented in the model as a stacked stratigraphic sequence due to the complex interfingering of lithologic units and the absence of available time-stratigraphic markers. Instead, lithologic units were grouped into interpreted genetic classes, such as playa or alluvial fan, to create a three-dimensional model of the interpreted facies data. Three-dimensional facies models computed from these data portray the alluvial infilling of a tectonically formed basin with intermittent internal drainage and localized regional groundwater discharge. The lithologic and interpreted facies models compare favorably to resistivity, aeromagnetic, and geologic map data, lending confidence to the interpretation.

California;Nevada↗
USGS2001

Hydrogeology of Picacho Basin, south-central Arizona

The hydrogeology of Picacho Basin was studied to define the stratigraphy, basin structure, physical and hydraulic properties of the basin sediments, and predevelopment and postdevelopment conditions of ground-water flow as of 1985. The study area includes about 900 square miles and contains a sedimentfilled asymmetric graben. The greatest sediment thickness occurs along the east margin of the graben. Basin sediments contain the principal water-bearing units and are separated into lower, middle, and upper units. The lower unit is several thousand feet thick and contains a conglomerate facies and a playa facies that contains a thick evaporite sequence. The middle and upper units contain alluvial and playa facies that are as much as 1,500 feet thick. Ground water occurs in lower and upper aquifer systems separated by a middle confining unit that comprises the playa facies of the three units. Hydraulic properties and compressibility of the middle and upper units are much greater than those of the lower unit. Vertical-head gradients exist, and vertical flow occurs within and between the aquifer systems. Early development of surface-water supplies resulted in increased recharge through deep percolation of irrigation water. Later development of the ground-water supplies resulted in extensive water-level declines, changes in the direction of ground-water flow, removal of water from storage, aquifer compaction, land subsidence, and earth fissures. Dewatering of pore spaces in the upper unit has been the primary source of water; however, as much as 80 percent of the water derived from storage in the Eloy area has resulted from compaction of pore spaces.

Arizona↗
USGS1998

Spatial variation in saturated hydraulic conductivity of sediments at a crude-oil spill site near Bemidji, Minnesota

Saturated hydraulic conductivity of aquifer sediments at a crude-oil spill research site near Bemidji, Minnesota were examined using pneumatically-induced head-difference tests and packer/vacuum system tests. Results from slug tests on 58 wells show that hydraulic conductivity varies both horizontally and vertically in the range from about 10 -7 to 10 -4 meters per second (m/s), with a median of 7.28 x 10 -5 m/s. Hydraulic conductivities of the well-sorted medium to fine sand facies, which contains a majority of the oil plume, range from 1.76 x 10 -5 to 9.82 x 10 -5 m/s with a median of 5.42 x 10 -5 m/s. Hydraulic conductivities of the lower sand and gravel unit, which contains a majority of the plume of dissolved petroleum constituents, range from 4.42 x 10 -6 to 5.36 x 10 -4 m/s with a median of 2.32 x 10 -4 m/s. The average linear velocity of ground water near the spill site was calculated to examine the effects of advective flow on migration of the plumes of oil and dissolved petroleum constituents. The average linear velocity in the well-sorted medium to fine sand facies during September 1996 was about 11 meters per year (m/year). If we assume that this was the average velocity during the 17-year period since the spill (1979-96), total advective flow of ground water in this facies was about 187 m. During this 17-year period, oil floating on the water table migrated only about 40 m. By comparison, the lower sand and gravel unit had an average linear velocity of about 29 m/year, or about 3 times greater than the velocity in the well-sorted medium to fine sand facies. Based on a 29 m/year velocity, advective flow of ground water in this unit during the 17-year period since the spill was about 493 m; whereas, the plume of dissolved petroleum constituents migrated only about 200 m. These results indicate that spatial variability of hydraulic conductivity and ground-water velocity at the research site likely is a factor affecting the rate of contaminant migration. Additional research is needed to fully evaluate how the contaminant plumes are affected by changes in hydraulic properties of the various lithologic units.

Minnesota↗
USGS2023

Extrusion tectonism of Indochina reassessed: constraints from 40Ar/39Ar geochronology from the Day Nui Con Voi metamorphic massif, Vietnam

The extrusion tectonic model for the southeastern margin of the Himalayan orogeny links the crustal shear activity along the Red River Shear Zone (RRSZ) to the opening of the South China Sea (SCS). The Day Nui Con Voi (DNCV) metamorphic massif in northern Vietnam strikes NW-SE, is bounded by the RRSZ to the south and continues along the strike where it meets the SCS. The DNCV is thus a critical area to document thermotectonic history in order to advance our understanding of the tectonic evolution of Indochina extrusion and its relationship to the opening of the SCS. Our new 40 Ar/ 39 Ar data combined with microstructural and petrological analyses constrained the timing of the left-lateral shearing of the RRSZ and revealed the thermal evolution of the DNCV metamorphic massif. Three ductile deformation events were observed. D 1 formed NNW-SSE striking upright folds under granulite to upper amphibolite facies conditions. D 2 was a horizontal to sub-horizontal folding event that occurred at amphibolite facies conditions. D 3 was a doming event that formed NW-SE striking up-right folds bounded by left-lateral shearing mylonite belts along the two limbs. The S/C fabrics were defined by muscovite fish, quartz + albite + K-feldspar aggregates, and muscovite folia. The D 3 doming event exhumed the DNCV metamorphic massif from amphibolite facies conditions to the lower greenschist facies conditions. The 40 Ar/ 39 Ar ages obtained from amphibole (∼26 Ma), phlogopite (∼25 Ma), muscovites (∼24-23 Ma), biotite (∼25-23 Ma), and K-feldspars (∼25-22 Ma) from different structural domains of the DNCV metamorphic massif indicated a rapid exhumation ∼26–22 Ma. We interpreted this as the time period for the D 3 event, with the onset of left-lateral shearing occurring around 24 Ma based on ages obtained from syn-kinematic muscovites. This age was much younger than the initiation of sea-floor spreading of the SCS (since 32 Ma) but coincided with the age for the ridge jump event in the SCS. Based on these new data, we proposed that extrusion tectonism cannot be the cause for the initial opening of the SCS. Rather, the extrusion of the Indochina block was temporally correlative with the southward ridge jump event of the already opened SCS.

Day Nui Con Voi massif↗
USGS2001

Minimum age of the Neoproterozoic Seven Hundred Acre Island Formation and the tectonic setting of the Islesboro Formation, Islesboro block, Maine

Two plat formal stratigraphic sequences occur on Islesboro, Penobscot Bay, Maine. The older Seven Hundred Acre Island Formation is at least 200 m thick, its base is not exposed, and it makes up fault-bounded blocks of siliceous colour-banded dolomitic marble, muscovite-rich quartzite, coarse-grained splendent muscovite-garnet-staurolite-andalusite schist, and calcareous metapelite, with minor garnet amphibolite and amphibolite. It was initially metamorphosed to lower amphibolite facies and was later retrograded to lower greenschist facies. The lower amphibolite facies metamorphism is Neoproterozoic (670 to 650 Ma) as inferred from the 40 Ar/ 39 Ar high temperature release spectra of hornblende separates. A U-Pb zircon age of 646.7 ± 2.7 Ma obtained for a pegmatite that intruded deformed rocks is taken to be the minimum age of the Formation. The platformal Islesboro Formation probably unconformably overlies the Seven Hundred Acre Island Formation. It is primarily turbiditic pelite with many beds of quartzite, impure dolomitic marble, some conglomerate, and a few feldspar-rich volcaniclastic beds and is thought to be either Neoproterozoic or Cambrian. It was metamorphosed only to lower greenschist facies, possibly in the same event that retrograded the Seven Hundred Acre Island Formation. Geochemical interpretations of minor and trace element analyses of six amphibolite and four schist samples from the Seven Hundred Acre Island Formation show that the protoliths of the amphibolite samples were intermediate between tholeiitic and within-plate type basaltic flows or dikes that intruded attenuated continental crust, or were eroded from these basalts. Four amphibolite and three schist samples analyzed for Pb isotopes were found to be enriched in radiogenic Pb. The Pb isotopic compositions are similar to those in peri-Gondwanan basement rocks from Atlantic Canada. The peri-Gondwanan Islesboro block was placed against the peri-Gondwanan Middle and Late Cambrian Ellsworth terrane on the cast by significant Late Silurian strike-slip faulting. The Late Silurian or Early Devonian Turtle Head dextral strike-slip fault separates the Islesboro block from the peri-Gondwanan St. Croix terrane to the west. The timing and nature of the movements of these faults are given from offsets of the isograds around the zircon-dated Laic Silurian Sedgwick Granite (419.5 ± 1.4 Ma) and South Penobscot Intrusive Suite (419.2 ± 2.2 Ma). These terranes and others like them in Atlantic Canada make up a composite terrane that is different from the composite Avalonian terranes sensu stricto of southeastern New England and Atlantic Canada. The composite peri-Gondwanan terrane that included the Islesboro block was amalgamated with the margin of ancestral North America in the Silurian.

Maine↗
USGS1991

Sedimentology and cyclicity in the Lower Permian De Chelly Sandstone on the Defiance Plateau: Eastern Arizona

The Lower Permian (Leonardian) De Chelly Sandstone crops out along a north-south trend on the Defiance Plateau of eastern Arizona. It is divided into lower and upper members separated by a tongue of the Supai Formation that pinches out to the north. Lithofacies in the De Chelly Sandstone consist of (1) a large-scale trough to tabular- and/or wedge-planar cross-stratified sandstone facies of large-scale eolian dune origin, (2) a small- to medium-scale, trough cross-stratified sandstone also of eolian dune origin, (3) a horizontally stratified, wind-rippled sandstone of sand sheet origin, (4) a wavy, horizontally stratified, wind-rippled sandstone of sabkha origin, and (5) a mud-draped ripple-laminated sandstone of mud-flat origin. The De Chelly Sandstone in the northern Defiance Plateau consists mainly of large-scale dune deposits. Stratigraphic sections in the middle of the plateau are dominated by small- to medium-scale dune and sand sheet deposits whereas those along the southern end of the plateau are composed largely of sabkha and supratidal mud-flat deposits. The lateral distribution of these facies suggests a north-south juxtaposition of central-erg, fore-erg, and mixed sabkha-supratidal depositional environments. Repetitive interbedding of facies in the De Chelly indicates at least twelve depositional cycles in which sabkha and/or supratidal to coastal-plain mud-flats were sequentially overridden by eolian sand sheets and cross-stratified dunes. Lateral and vertical facies relations within the lower and upper members of the De Chelly Sandstone record episodic expansion of the De Chelly erg southward, The comparative abundance of large-scale dune deposits in the upper member suggests that progradation was more extensive during latter stages of deposition. The intervening tongue of Supai Formation and the redbeds that overlie the upper member of the De Chelly at Bonito Canyon document northward transgression of sabkha and supratidal to coastal-plain mud-flat environments. Eolian dune deposition was restricted to the northern Defiance Plateau during deposition of these units.

Arizona↗
USGS2002

Conventional U-Pb dating versus SHRIMP of the Santa Barbara Granite Massif, Rondonia, Brazil

The Santa Ba??rbara Granite Massif is part of the Younger Granites of Rondo??nia (998 - 974 Ma) and is included in the Rondo??nia Tin Province (SW Amazonian Craton). It comprises three highly fractionated metaluminous to peraluminous within-plate A-type granite units emplaced in older medium-grade metamorphic rocks. Sn-mineralization is closely associated with the late-stage unit. U-Pb monazite conventional dating of the early-stage Serra do Cicero facies and late-stage Serra Azul facies yielded ages of 993 ?? 5 Ma and 989 ?? 13 Ma, respectively. Conventional multigrain U-Pb isotope analyses of zircon demonstrate isotopic disturbance (discordance) and the preservation of inherited older zircons of several different ages and thus yield little about the ages of Sn-granite magmatism. SHRIMP U-Pb ages for the Santa Ba??rbara facies association yielded a 207Pb/206Pb weighted-mean age of 978 ?? 13 Ma. The textural complexity of the zircon crystals of the Santa Ba??rbara facies association, the variable concentrations of U, Th and Pb, as well as the mixed inheritance of zircon populations are major obstacles to using conventional multigrain U-Pb isotopic analyses. Sm-Nd model ages and ??Nd (T) values reveal anomalous isotopic data, attesting to the complex isotopic behaviour within these highly fractionated granites. Thus, SHRIMP U-Pb zircon and conventional U-Pb monazite dating methods are the most appropriate to constrain the crystallization age of the Sn-bearing granite systems in the Rondo??nia Tin Province.

Geologia USP - Serie Cientifica↗