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At least 1,207 records · Page 67Linked to original sources
USGS1957

Geology of the Basin Quadrangle, Montana

The Basin quadrangle, in the northern part of the Boulder Mountains between Butte and Helena, Montana, is underlain principally by igneous rocks that include Late Cretaceous quartz latitic and andesitic Elkhorn Mountains volcanics, quartz monzonite and related rocks of the Boulder batholith, Oligocene(?) quartz latitic volcanic rocks, and late Miocene(?)-early Pliocene(?) rhyolitic volcanic rocks. Sedimentary rocks of the Late Jurassic Morrison formation, the Early Cretaceous Kootenai formation, and the Early Cretaceous lower part of the Colorado formation, crop out in the northwest part of the quadrangle. The batholithic rocks include early stage quartz monzonite, intermediate or main stage quartz monzonite and granodiorite, and late stage aplite and alaskite. The rocks of the main stage are in discontinuous layers approximately conformable to the folded Elkhorn Mountains volcanics that form the roof of the batholith, and may be part of a sill-like body rather than of a batholith in the classic sense. Metamorphic changes in the roof rocks are not conspicuous except in one stratigraphic unit that probably was especially susceptible to thermal reorganization. The batholithic rocks and Elkhorn Mountains volcanics are jointed, and are cut by faults that trend about east, north, N. 20° E., northeast, and northwest. The east-trending faults are most abundant, especially in the eastern part of the quadrangle, and cut only the batholithic and pre-batholithic rocks, whereas many of the faults of other trends cut Tertiary volcanic rocks and a few cut Pleistocene glacial deposits. A surface of moderate relief was cut before eruption of the Oligocene(?) volcanic rocks, and the late Miocene(?)-early Pliocene(?) volcanic rocks covered a deeply weathered surface of low relief. By the Pleistocene a landscape essentially like that of today had been formed, and during the one period of Pleistocene glaciation, valley glaciers and a mountain ice sheet modified the earlier landforms and left extensive deposits of till and outwash. These deposits have been modified in many places by mass-wasting processes that have dominated post-glacial erosion. Mineral deposits in the quadrangle include deposits of disseminated auriferous pyrite, base- and precious-metal bearing quartz veins that occupy the east-trending fault zones, placer deposits of gold and tin, and a few non-metallic deposits, chiefly stone, gravel, and dumortierite. Nearly all of the metallic minerals mined in the quadrangle have come from the east-trending quartz veins and from the placer deposits.

Montana↗
USGS1967

Geology of the Windsor quadrangle, Massachusetts

The Windsor quadrangle lies on the boundary between the eugeosynclinal and miogeosynclinal rocks of the Appalachian geosyncline on the western flank of the metamorphic high in western New England. Precambrian rocks crop out in a north-trending belt in the central part of the quadrangle. They have been classified into 2 formations. The Stamford Granite Gneiss crops out in the eastern half of the Precambrian terrane. It is a microcline-quartz-biotite augen gneiss. Stratified Precambrian rocks (the Hinsdale Gneiss) crop out entirely the west of the Stamford Granite Gneiss. They are predominantly highly metamorphosed felsic gneisses and .quartzites with minor calc-silicate rock, amphibolite, and graphitic gneiss. Eugeosynclinal rocks (the Hoosac Formation and the Rowe Schist), .ranging in age from Lower Cambrian to Lower Ordovician, crop out in a north-trending belt east of the Precambrian terrane. They are composed predominantly of albite schist and muscovite-chlorite schist with minor garnet schist, quartz-muscovite-calcite schist, felsic granulite and gneiss, quartzite, greenschist, and carbonaceous phyllite and schist. West of the Precambrian rocks, the Hoosac Formation is overlain by a miogeosynclinal sequence (the Dalton Formation, Cheshire Quartzite, Kitchen Brook Dolomite, Clarendon Springs Dolomite, Shelburne Marble, and the Bascom Formation) ranging in age from Lower Cambrian to Lower Ordovician. These rocks are unconformably overlain by the Berkshire Schist of Middle Ordovician age that is composed of carbonaceous schist, phyllite, and quartzite. The relationships in the zone of transition between the miogeosynclinal and eugeosynclinal rocks are unknown because the rocks of this zone are no longer present. The contact between the eugeosynclinal Hoosac Formation and the Dalton Format ion is conformable and deposition. The dominant structure is a large recumbent, northwest-facing anticline (the Hoosac nappe) with a Precambrian co re. The miogeosynclinal rocks are inverted in the northwestern part of the quadrangle and upright in the southwestern part of the quadrangle. A later generation of open, post-metamorphic folds has folded the recumbent folds in the miogeosynclinal rocks. The eugeosynclinal rocks show 3 phases of folding. The earliest folds are isoclinal, have steep plunges, were synmetamorphic, and have a strong axial plane schistosity. Two post-metamorphic generations of folds are more open and have axial plane cleavage. The development of the Hoosac nappe and the isoclinal folds was accompanied by regional metamorphism of the garnet zone. The pressure exceeded the pressure for the triple point of the Al2SiO 5 polymorphs. The composition of the paragonite coexisting with muscovite suggests a period of retrograde metamorphism for the Paleozoic rocks as well as the Cambrian rocks that were originally of higher grade (sillimanite? ). Later events include high-angle faulting (Triassic?), erosion, and Pleistocene glaciation.

Open-File Report↗
USGS1970

Economic geology of the Zipaquira quadrangle and adjoining area, Department of Cundinamarca, Colombia

At least four evaporite sequences are interbedded with Cretaceous strata in the Bogotga area of the Cordillera Oriental of Colombia. The easternmost and oldest evaporite interval is of probable Berriasian-Valanglnian age; the next oldest is of probable late Barremian-early Aptian age, and is followed by a possible late Aptian sequence. The westernmost and best known sequence is Turonian-early Coniacian in age, in the Sabana de Bogota. This youngest sequence contains the thickest known salt deposits and is probably the most widespread geographically. Three gypsum deposits of probable Barremian-Valanginian age are in the eastern part of the area under investigation. These deposits may have been leached from former salt accumulations. No other evaporites are exposed, but numerous brine springs are known, That the sources of these brines are neither deep not distant is suggested by the generally high concentrations, of the brines, the local presence of rute (leached salt residue), and the commonly significant amounts of H2S gas emitted at these springs. The rock salt exposed in three accessible mines commonly has a characteristic lamination caused by alternating layers of relatively pure halite and very argillaceous halite. Ubiquitously scattered throughout all salt deposits are small clasts of black, commonly pyritic, marly claystone. This lithology is also present as large claystone bodies conformably interbedded in the salt strata. Anhydrite is rare and is apparently more abundant at the Zipaquira mine that at the Nemocon and Upin mine. Paleontologic evidence in the Sabana de Bogota demonstrates that the salt-claystone series, hematite impregnated strata, and carbonaceous to locally coaly claystone are coeval. The salt-claystone facies may have been deposited in shallow evaporite pans that were separated within the overall evaporite interval by barriers on which the locally hematitic strata were deposited. The carbonaceous facies may also have formed in barrier areas or on the edges of the evaporite basins. Whether or not this facies relationship prevails in the older evaporite intervals is not known; meager evidence suggests that it does. Nonmetallic mineral resources other than the evaporite minerals are phosphate rock, limestone, kaolinite, and emeralds. Metallic mineral deposits present in the Zone include hematite at Pericos, La Caldera, Tibirita, Nueva Vizcaya, and Cerro de Montecristo; chalcopyrite at Cerro do Cobre and at Farallones de Medina; galena in several places along the Rio Farallones and Rio Gacheta; and spahlerite in the Junin district.

Open-File Report↗
USGS1971

Geology of the Stroudsburg quadrangle and Adjacent areas, Pennsylvania--New Jersey

The Stroudsburg area is within the Valley and Ridge and Great Valley physiographic provinces, Northampton and Monroe Counties, Pennsylvania, and Warren County, New Jersey. The northeast-trending subparallel valleys and ridges resulted from erosion of folded heterogeneous sedimentary rocks. These are Middle Ordovician to Middle Devonian in age and are more than 17,000 feet thick. Deposition of a thick flysch sequence (Martinsburg Formation of Ordovician age) accompanied onset of Taconic orogenesis. It was followed by deposition of a thick molasse sequence of Silurian and Early Devonian age (continental and marginal-marine clastics--Shawangunk Formation and Bloomsburg Red Beds--overlain by predominantly marginal-marine and subtidal limestone, dolomite, shale, and sandstone--Poxono Island Formation through Oriskany Group). Basin deepening and gradual shallowing occurred during Esopus through Mahantango deposition, heralding the Acadian clastic wedge exposed north of the Stroudsburg area. Interpretation of sedimentary structures and regional stratigraphic relations suggest that the Silurian and Devonian rocks were deposited in the following environments: A1luviated coastal plain (meandering and braided streams), tidal flats (supratidal and intertidal), barrier zone, and neritic zone (upper and lower). The rock stratigraphic units have been grouped into four lithotectonic units, each having a different style of deformation. Folds produced in these rocks are disharmonic, and it is believed that each rock sequence is set off from units above and below by decollements, or zones of detachment. Movement was northwest into the Appalachian basin, primarily by gravitational sliding. The contact between the Shawangunk Formation of Silurian age and Martinsburg Formation of Ordovician age, is one zone of detachment as well as an angular unconformity. Deformational effects of the Middle to Late Ordovician Taconic orogeny are elusive, but it appears that the folds and most minor structures, including the prominent regional cleavage, were produced during the late Paleozoic Appalachian orogeny and are superimposed upon larger Taconic folds and faults. Field relations and microscopic study suggest that the regional cleavage in the Stroudsburg area is due to laminar flow of pelitic material along cleavage folia accompanied by mechanical reorientation of platy and elongate minerals and neocrystallization of mica, quartz, chlorite, and probably albite. Numerous lines of evidence point to the conclusion that cleavage developed after the rock was indurated and formed at, and Just below, conditions of low-grade metamorphism. Intensity of cleavage development increases to the southeast across the area. Second-generation slip cleavage, also believed to be Appalachian in age, formed by mechanical reorientation of minerals as well as by limited new mineral growth. The topography had a profound effect on the direction of movement of the Wisconsin glacier, as well as the manner of its retreat and the deposits that were formed. Till and stratified drift of Wisconsin age and till of Illinoian(?) age are common in the area. Wisconsin deglaciation occurred by northeastward retreat and by stagnation. A conspicuous terminal moraine marks the limit of Wisconsin ice movement. Lake Sciota was dammed between the retreating ice, the moraine, and the surrounding ridges north of Godfrey Ridge. Several deltas mark ice stand positions during the retreat of the ice. Lake-bottom and kame deposits are locally common in Cherry Valley. South of Kittatinny Mountain, on the other hand, melt water was freely discharged to the south. The wind and water gaps in the Stroudsburg area (including Delaware Water Gap and Wind Gap) are structurally controlled; specifically they are located where folds die out in short distances, where folding is locally more intense, or where resistant rocks dip steeply and have a narrow width of outcrop. This conclusion is contrary to

Open-File Report↗
USGS1983

Reconnaissance geology of the Ghazzalah Quadrangle, sheet 26/41 A, Kingdom of Saudi Arabia

The Ghazzalah quadrangle is located in the northern Precambrian shield of Saudi Arabia between lat 26?30' and 27?00' N. and long 41?00' and 41?30' E. The area is underlain by two lithologically distinct, Precambrian volcanosedimentary units and a wide range of dioritoid and granitoid plutonic intrusive rocks. The only Phanerozoic rocks consist of one outcrop of Tertiary(?) basalt and widespread but thin deposits of Quaternary detritus. The Banana greenstone, the oldest rock in the quadrangle, consists of intermediate volcanic and subvolcanic rocks and minor interbedded marble, which have been metamorphosed to greenschist-facies assemblages. Volcanic rocks mainly range in composition from basalt to andesite, and subvolcanic rocks consist of diorite and diabase. The Banana greenstone is unconformably overlain by silicic volcanic rocks and minor arkosic sandstone and breccia of the Hadn formation. Preservation of delicate volcanic textures suggests that the rocks have been only incipiently metamorphosed. Unpublished rubidium/strontium isotopic data for the Hadn formation suggest an age of 620 to 610 Ma. Intrusive rocks are separable according to their ages relative to the Hadn formation. Those that are unconformably overlain by the Hadn formation consist of hornblende quartz diorite and gabbro, which may be consanguineous with the Banana greenstone, and younger tonalite, biotite-hornblende granodiorite, syenogranite, and monzogranite. Plutons of monzogranite, alkali-feldspar g,ranite, syenbgranite, peralkaline granite, and hypabyssal intrusions of granophyre were probably emplaced during a period coincident with and (or) following Hadn volcanism. Uranium-lead and rubidium/strontium isotopic data for two plutons in the adjacent Al Qasr quadrangle suggest that plutonic activity persisted in the region until about 580 to 570 Ma. Faulting appears to postdate all of the plutonic rocks. The dominant faults belong to a northeast-trending system of right-lateral shears; a subordinant system consists of mainly north- to northwest-trending faults. The peralkaline-granite plutons underlying Jibal Ba'gham and Jibal ar Rumman have the most economic potential. Wadi samples from these areas show an anomalous concentrations of tin, lead, niobium, and yttrium. Localized, intense radiometric anomalies in the Ba'gham intrusive complex are associated with high concentrations )f thorium, uranium, andrare-earth elements.

Open-File Report↗
USGS1983

Reconnaissance geology of the Thaniyah Quadrangle, sheet 20/42 C, Kingdom of Saudi Arabia

The Thaniyah quadrangle, sheet 20/42 C, is located in the transition zone between the Hijaz Mountains and the Najd Plateau of southwestern Saudi Arabia between lat 20?00' and 20?30' N., long 42?00' to 42?30' E. The quadrangle is underlain by Precambrian metavolcanic, metasedimentary, plutonic, and dike rocks. Metavolcanic rocks consist of metamorphosed basalt and andesite with minor dacite and rhyolite and underlie three discontinuous northwest-trending belts. Metasedimentary rocks are confined to small areas underlain by quartzite, metasandstone, marble, and calc-silicate rock. Plutonic rocks include an extensive unit of tonalite and quartz diorite and a smaller unit of diorite and quartz diorite, which occupy much of the central part of the quadrangle. A small body of diorite and gabbro and a two-part zone of tonalite gneiss are also present. All of these plutonic rocks are assigned to the An Nimas batholith. Younger plutonic rocks include extensive graphic granite and rhyolite in the northeastern part of the quadrangle and several smaller bodies of granitic rocks and of gabbro. The metavolcanic rocks commonly have strong foliation with northwest strike and steep to vertical dip. Diorite and quartz diorite are sheared and brecciated and apparently syntectonic. Tonalite and quartz diorite are both foliate and nonfoliate and were intruded in episodes both preceding and following shearing. The granitic rocks and gabbro are post-tectonic. Trends of faults and dikes are mostly related to the Najd faulting episode. Radiometric ages, mostly from adjacent quadrangles, suggest that the An Nimas batholith is 835 to 800 Ma, gabbro and granite, except the graphic granite and rhyolite unit, are about 640 to 615 Ma, and the graphic granite and rhyolite 575 to 565 Ma old. Metavolcanic rocks similar to those hosting copper and gold mineralization in the Wadi Shuwas mining district adjacent to the southwestern part of the quadrangle are abundant. An ancient copper mine was discovered at the edge of the tonalite gneiss belt east of Wadi Ranyah. Granite and gabbro have economic potential as building stone.

Open-File Report↗