TWOPETE FAULT: A MULTIPLY REACTIVATED STRUCTURE THAT LOCALLY INFLUENCED THE DEVELOPMENT OF THE SOUTHERN SELWYN BASIN IN CENTRAL YUKON

In other parts of the world, previous workers have shown that sparry dolomite in carbonate rocks may be produced by the generation and movement of hot basinal brines in response to arid paleoclimates and tectonism, and that some of these brines served as the transport medium for metals fixed in Mississippi Valley-type (MVT) and sedimentary exhalative (Sedex) deposits of Zn, Pb, Ag, Au, or barite. Numerous occurrences of hydrothermal zebra dolomite (HZD), comprised of alternating layers of dark replacement and light void-filling sparry or saddle dolomite, are present in Paleozoic platform and slope carbonate rocks on the eastern side of the Great Basin physiographic province. Locally, it is associated with mineral deposits of barite, Ag-Pb-Zn, and Au. In this paper the spatial distribution of HZD occurrences, their stratigraphic position, morphological characteristics, textures and zoning, and chemical and stable isotopic compositions were determined to improve understanding of their age, origin, and relation to dolostone, ore deposits, and the tectonic evolution of the Great Basin. In northern and central Nevada, HZD is coeval and cogenetic with Late Devonian and Early Mississippian Sedex Au, Zn, and barite deposits and may be related to Late Ordovician Sedex barite deposits. In southern Nevada and southwest California, it is cogenetic with small MVT Ag-Pb-Zn deposits in rocks as young as Early Mississippian. Over Paleozoic time, the Great Basin was at equatorial paleolatitudes with episodes of arid paleoclimates. Several occurrences of HZD are crosscut by Mesozoic or Cenozoic intrusions, and some host younger pluton-related polymetallic replacement and Carlin-type gold deposits. The distribution of HZD in space (carbonate platform, margin, and slope) and stratigraphy (Late Neoproterozoic Ediacaran–Mississippian) roughly parallels that of dolostone and both are prevalent in Devonian strata. Stratabound HZD is best developed in Ediacaran and Cambrian units, whereas discordant HZD is proximal to high-angle structures at the carbonate platform margin, such as strike-slip and growth faults and dilational jogs. Fabric-selective replacement and dissolution features (e.g., collapse breccias, voids with geopetal textures) are common, with remaining void space lined with light-colored dolomite crystals that exhibit zoning under cathodoluminescence. Zoned crystals usually contain tiny ( ∼70 °C. The oxygen isotopic compositions of HZD are consistent with formation temperatures of 50–150 °C requiring brine circulation to depths of 2–5 km, or more. The few HZD occurrences with the highest concentrations of metals (especially Fe, Mn, and Zn) and the largest isotopic shifts are closely associated with Sedex or MVT deposits known to have formed from hotter brines (e.g., Th > 150–250 °C). These relationships permit that HZD formed at about the same time as dolostone, from brines produced by the evaporation of seawater during arid paleoclimates at equatorial paleolatitudes. Both dolostone and HZD may have formed as basinal brines, which migrated seaward from evaporative pans on the platform, with dolostone forming at low temperatures along shallow migration pathways through permeable limestones, and HZD forming at high temperatures along deeper migration pathways through basal aquifers and dilatant high-angle faults. The small MVT deposits were chemical traps where hot brines encountered rocks or fluids containing reduced sulfur. The abundant Sedex deposits mark sites where hot brine discharged at the seafloor in adjacent basins. Thus the distribution of HZD may map deep migration pathways and upflow zones between eastern shallow marine facies, where evaporative brine could have been generated, and western Sedex deposits, where heated brines discharged along faults into platform margin, slope, and basin facies. The small size and scarcity of Pb-Zn deposits and the abundance of barite deposits in the Great Basin suggests the brines were generally reduced, possibly due to reactions with carbonaceous rocks along deep migration pathways. While this scenario may have occurred at several times, the age and abundance of Sedex deposits suggest that such a hydrology was best developed in the Late Ordovician, Late Devonian, and Early Mississippian, possibly in response to episodes of extension and forebulge faults associated with the Antler orogeny. The improved understanding of HZD may aid future exploration for ore deposits in the Great Basin.

<p>This contribution reports LA-ICP-MS zircon ages and Rb-Sr biotite ages from the Troiseck-Floning Nappe, forming the northeasternmost extension of the Silvretta-Seckau Nappe System in the Eastern Alps. The Troiseck-Floning Nappe comprises a basement formed by the Troiseck Complex and a Permo-Triassic cover sequence. The basement consists of paragneiss with intercalations of micaschist, amphibolite and different types of orthogneiss, which was affected by a Variscan (Late Devonian) amphibolite facies metamorphic overprint. The cover sequence includes Permian clastic metasediments and metavolcanics, as well as Triassic quartzite, rauhwacke, calcitic marble and dolomite. During the Eoalpine (Cretaceous) tectonothermal event the nappe experienced deformation at lower greenschist facies conditions.</p> <p>Detrital zircon grains from paragneiss are in the range of 530-590 Ma, indicating an Ediacarian to earliest Cambrian source and a Cambrian to Ordovizian deposition age of the protolith. Late Cambrian to Ordovician crystallization ages from leucogranitic intrusions represent the earliest magmatic event of the Troiseck Complex. The amphibolite bodies derived from basalt with a calc-alkaline to island arc tholeiitic signature.</p> <p>Leucocratic orthogneiss with K-feldspar porphyroclasts and a calc-alkaline granitic composition plots in the field of volcanic arc granite. The youngest zircon grains indicate a Late Devonian crystallization. Two pegmatite gneisses with a calc-alkaline composition are early Mississippian in age.</p> <p>Mylonitic orthogneiss with a pronounced stretching lineation appears as irregularly shaped layers. It is leucocratic, very fine grained and contains scattered feldspar porphyroclasts with a round shape and a diameter of about 1 mm. Its chemical composition is granitic/rhyolitic with an alkali-calcic signature. In classification diagrams it plots in the field of syn-collision granite. Zircon ages of about 270 Ma indicate a Permian crystallization. Similar rocks interpreted as Permian rhyolitic metavolcanics appear in the cover sequence. They share a similar chemical composition and crystallization age of 270 Ma. Associated intermediate metavolcanics developed from calc-alkaline basaltic andesite.</p> <p>According to Rb-Sr biotite ages cooling of the Troiseck-Floning Nappe below c. 300°C occurred at about 85 Ma in the west and 75 Ma in the east.</p> <p>In summary, the Troiseck Complex developed from Cambrian to Ordovizian clastic metasediments and granitic and basaltic magmatic rocks emplaced in the same time range. During the Late Devonian, it was affected by the Variscan collisional event, causing deformation at amphibolite facies conditions and intrusion of calc-alkaline granites. In early Mississippian time pegmatite dikes intruded, maybe induced by decompression and exhumation. The deposition of clastic sediments and (sub)volcanic rocks (rhyolite and basaltic andesite) constrains a surface position of the Troiseck Complex during the Permian. Based on regional considerations an extensional environment is assumed. In Triassic times carbonate platform sediments were deposited. During the Eo-Alpine collision in the Cretaceous the unit was part of the tectonic lower plate and subducted to shallow crustal levels, indicated by a lower greenschist facies metamorphic overprint. The Troiseck-Floning Nappe was formed and exhumed since about 85 Ma. Rb-Sr as well as apatite fission track data from the literature indicate tilting with more pronounced exhumation and erosion in the eastern part during Miocene lateral extrusion of the Eastern Alps.</p>

SEDEX (SEDimentary EXhalative) deposits are important resources of Zn and Pb. In addition to Zn and Pb, other potentially economic commodities are: Ag, Au, Cu, Cd, Sb, Sn, and barite. Major metallogenic districts in the Canadian Cordillera that host SEDEX deposits are: - Mesoproterozoic Sullivan district in southeastern British Columbia, which hosts the world-class Sullivan deposit and other smaller deposits such as North Star and Kootenay King. - Late Cambrian Anvil district in the Selwyn Basin of central Yukon, which hosts the Faro, Grum, Vangorda, DY, and Swim deposits. - Early Silurian Howard's Pass district in the Selwyn Basin of the northeastern Yukon, which hosts the world-class Howard's Pass deposits (XY, Brodel, HC, Don, Anniv, OP, Pelly North). - Late Devonian Gataga district of the Kechika Trough (southern extension of Selwyn Basin) in northeastern British Columbia, which hosts the Cirque, Driftpile, and Akie deposits. - Late Devonian MacMillan's Pass district in the Selwyn Basin of northeastern Yukon, which hosts the Tom and Jason deposits. SEDEX deposits are defined as being predominantly composed of Zn and Pb hosted in sphalerite and galena that were deposited at or near the seafloor from basinal metalliferous fluids discharged into riftcontrolled anoxic sedimentary basins. They consist of vent-distal and vent-proximal facies. The former is composed of interbedded sphalerite, galena, iron sulphides and clastic sediments, and the latter of variably veined, infilled and replaced bedded sulphides. Cordilleran SEDEX deposits formed in settings with dynamic redox fronts controlling sulphide and sulphate precipitation within carbonaceous sediments. Zinc, Pb, Cu and other metals are leached from deeply buried clastic sediments during metamorphic mineralogical transformations driven by increasing temperature and pressure during burial. At the Howard's Pass (HP) deposits (Yukon), sulphides precipitated from dense bottom-hugging metalliferous brines that accumulated in a bathymetric low, distal to vent complex(es), and percolated into porous unconsolidated sulphidic carbonaceous muds. At the MacMillan Pass (MP) deposits (Yukon), hydrothermal sulphides precipitated sub-seafloor, proximal to vent complex(es), due to interaction of hot (>250°C), acidic (pH ? 4.5) metal-bearing hydrothermal fluids with H S generated during a number of processes (bacterial 2 and thermochemical sulphate reduction, barite dissolution, and sulphate reduction coupled with anaerobic methane oxidation) in the carbonaceous mudstones. At HP and MP, most of the Zn-Pb mineralisation was precipitated below the seafloor as replacement of early barite (at MP) and finegrained sediments during early diagenesis. Close proximity to a carbonate platform may have been an important factor for the mineralising systems, enhancing access to sources for saline, metal complexing brines. Factors that have potential application in the search for SEDEX include the presence of: 1. Deep-seated synsedimentary faults expressed as abrupt changes in facies and isopachs, intraformational breccias, slumps, debris flows, and fault scarp talus. 2. Sedimentary basins hosting organic-rich sediments with >1% C ; adjacent to carbonate platform. org 3. Ore-stage diagenetic pyrite that are texturally sooty (i.e. inclusion-rich) and anomaleous in Tl, As, Sb and possibly Mn. 4. Anomalous concentrations of redox-sensitive trace elements (e.g. V, Tl, Cd, U, V/Mo, Re/Mo) Mo, Re/Mo, and Ce/Ce* in the host rocks. 5. Widespread hydrothermal alteration (muscovite, carbonates, and silicates). 6. Laterally and vertically extensive distal sediments that are mineralogically and chemically zoned around seafloor vents. 7. Regional euxinic condition is not a prerequisite for the formation of SEDEX deposits. 8. Basin has high biological productivity; should include outer shelf and slope settings and not focus exclusively on anoxic or sulphidic basins.

The Northern Sierra terrane is one of a series of Paleozoic terranes outboard of the western Laurentian margin that contain lithotectonic elements generally considered to have originated in settings far removed from their present relative locations. The Lower to Middle Paleozoic Shoo Fly Complex makes up the oldest rocks in the terrane and consists partly of thrust-imbricated deep-marine sedimentary strata having detrital zircon age signatures consistent with derivation from the northwestern Laurentian margin. The thrust package is structurally overlain by the Sierra City mélange, which formed within a mid-Paleozoic subduction zone and contains tectonic blocks of Ediacaran tonalite and sandstone with Proterozoic to early Paleozoic detrital zircon populations having age spectra pointing to a non–western Laurentian source. Island-arc volcanic rocks of the Upper Devonian Sierra Buttes Formation unconformably overlie the Shoo Fly Complex and are spatially associated with the Bowman Lake batholith, Wolf Creek granite stock, and smaller hypabyssal felsic bodies that intrude the Shoo Fly Complex. Here, we report new results from U-Pb sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) dating of 15 samples of the volcanic and intrusive rocks, along with geochemical studies of the dated units. In addition, we report U-Pb laser ablation–inductively coupled plasma–mass spectrometry ages for 50 detrital zircons from a feldspathic sandstone block in the Sierra City mélange, which yielded abundant Ordovician to Early Devonian (ca. 480–390 Ma) ages. Ten samples from the composite Bowman Lake batholith, which cuts some of the main thrusts in the Shoo Fly Complex, yielded an age range of 371 ± 9 Ma to 353 ± 3 Ma; felsic tuff in the Sierra Buttes Formation yielded an age of 363 ± 7 Ma; and three felsic hypabyssal bodies intruded into the Sierra City mélange yielded ages of 369 ± 4 Ma to 358 ± 3 Ma. These data provide a younger age limit for assembly of the Shoo Fly Complex and indicate that arc magmatism in the Northern Sierra terrane began with a major pulse of Late Devonian (Famennian) igneous activity. The Wolf Creek stock yielded an age of 352 ± 3 Ma, showing that the felsic magmatism extended into the early Mississippian. All of these rocks have similar geochemical features with arc-type trace-element signatures, consistent with the interpretation that they constitute a petrogenetically linked volcano-plutonic system. Field evidence shows that the felsic hypabyssal intrusions in the Sierra City mélange were intruded while parts of it were still unlithified, indicating that a relatively narrow time span separated subduction-related deformation in the Shoo Fly Complex and onset of Late Devonian arc magmatism. Following recent models for Paleozoic terrane assembly in the western Cordillera, we infer that the Shoo Fly Complex together with strata in the Roberts Mountains allochthon in Nevada migrated south along a sinistral transform boundary prior to the onset of arc magmatism in the Northern Sierra terrane. We suggest that the Shoo Fly Complex arrived close to the western Laurentian margin at the same time as the Roberts Mountains allochthon was thrust over the passive margin during the Late Devonian–early Mississippian Antler orogeny. This led to a change in plate kinematics that caused development of a west-facing Late Devonian island arc on the Shoo Fly Complex. Due to slab rollback, the arc front migrated onto parts of the Sierra City mélange that had only recently been incorporated into the accretionary complex. In the mélange, blocks of Ediacaran tonalite, as well as sandstones having detrital zircon populations with non–western Laurentian sources, may have been derived from the Yreka and Trinity terranes in the eastern Klamath Mountains, where similar rock types occur. If so, this suggests that these Klamath terranes were in close proximity to the developing accretionary complex in the Northern Sierra terrane in the Late Devonian.

Bedrock and surficial geology of the Maritime Provinces

Na tIe zroznicowania litologicznego utworow czerwonego spągowca w Wielkopolsce zaproponowano, w myśl Zasad polskiej klasyfikacji ... (1975), wyrozniac jednostki formalne: formacje, ogniwa, podgrupy i grupe. Ponadto omowiono niektore wyroznione nieformalnie alloformacje (P.H. Karnkowski, 1987). LITHOSTRATIGRAPHY OF THE ROTLIEGENDES IN WIELKOPOLSKA (WESTERN POLAND) The author divides the Rotliegendes formations in Wielkopolska into the following formal lithostratigraphical units (Tab. 1): the Dolsk Formation - between the folded, Lower Carboniferous basement and the volcanites; the Wyrzeka Volcanite Formation - where the formation borders are determined by the bottom of the first and the top of the last volcanite occurrence; the Ksiąz Wlkp. Conglomerate Formation - the main coarse·clastic rocks originated from destruction of the Wolsztyn Ridge: and the Siekierki Sandstone Formation - with a dominated sandstone member. Within the Siekierki Sandstone Formation the Polwica Conglomerate Member and the Solec Conglomerate Member are distinguished. Twenty three borehole profiles (Tab. 2, Figs. 1-5) have been chosen to present the formal litho· stratigraphical units. The Dolsk Formation (Figs. 2, 6) consist of the grey, black and red·brown clastic rock complex, fine· and medium.grained sandstones dominate in its lower part, and mudstones and claystones with accessorial, sandstones and conglomerates in its upper part. This formation can be divided, as an auxiliary into two parts regarding to the colour of sediments (in a cat he gory of allostratigraphical units). In this respect the lower part of grey and black sediments (the Upper Carboniferous) may be distinguished as the Kaczawa Alloformation, and the upper red· brown part as the Kwisa Alloformation. Changes in sediment colour in this case result in mainly palaeoclimatic changes. The Wyrzeka Volcanite Formation is composed of various volcanic rocks: in the west of the Poznan - Oleśnica Dislocation Zone - mainly acid volcanic rocks and their tuffs, in the east - neutral volcanic rocks and their tuffs (Fig. 7). The volcanites do not form a continuous cover, they occur in separately preserved lobes. It is presumed however that originally they covered much larger area (Fig. 8). Rocks occurring in the Wyrzeka region are mainly trachybasalts. Quartz porphyris have been documented in the west from the Poznan - Oleśnica Dislocation Zone (Figs. 4, 5, 7). All the volcanic rocks in the area show a highrated alteration (e.g. silification). Based on chemical composition the acid volcanic rocks can be classified as rhyolites and rhyodacites. The second rock formation is composed of quartz porphyries, that may be distinguished macroscopicaly from actual volcanic rocks due to the sedimentary rock debris presence. Mineral composition of tuffs does not differ radically from the extrusive rocks. The sedimentary rock debris found frequently in tuffs originated most commonly from the Lower Carboniferous basement. Besides it has been stated that flowing lavas formed the lava flows what has been emphasized by parallel structure. The Ksiąz Wlkp. Conglomerate Formation form mainly conglomerates and breccias (Figs. 3 - 5). The Wolszyn Ridge peripheries (Fig. 9) is a typical area of the occurrence. In the northern side of the ridge these sediments have a wider range and greater thickness than in the south (Fig. 10). The breccias and conglomerates 'are chiefly composed of debris of the volcanic rocks and Palaeozoic basement rocks. Selection of the coarse·grained material has not been stated in the KsiClZ Wlkp. conglomerate profile. Lithological composition of the coarse·grained material is also variable. The Siekierki Sandstone Formation is mainly composed of sandstones (Figs. 2, 4, 11). Quartz with a variable degrees of gradation and roundness is a chief lithologic component of the sandstones. Besides, the potash feldspars, plagioclases, sedimentary and volcanic rock chippings occur. The variability of granulation is another factor characterizing the sandstones. The medium- and fine-grained sandstones are most frequent, the coarse- and various-grained are also found. Most sandstones are oblique bedded. The Polwica Conglomerate Member is separated from the lower Siekierki Sandstone Formation (Figs. 2. 3, 5). It is mainly composed of volcanic rock debris. The Poznan - Kalisz Dislocation Zone and marginal zones of lava covers are typical area of the occurrence (Figs. 12, 13). Four detailed profiles are presented to draw nearer the rock characteristics (Fig. 14). Based on sedimentological and lithological characteristics it can be stated that sedimentary conditions of the Polwica Conglomerate Member and the Ksiąz Wlkp. Conglomerate Formation were similar. The Solec Conglomerate Member bas been distinguished in the middle Siekierki Sandstone Formation (Figs. 2. 3). This member occupies relatively small area in Wielkopolska (Figs. 15, 16). Based on the lithological differentiation of conglomerates two following regions can be discriminated: the Kaleje-Solec region (mainly trachybasalt debris) and the Kleka region (white quartzitic sandstone debris). To characterize the Solec Conglomerate Member it can be stated that identical features and phenomena occur here and in the other coarse silastic complexes. The Lower Silesia Subgroup has been distinguished to link the volcanic and subvolcanic sedimentary rocks in one unit (Tab. 1). Such a link has had longlasted tradition in science research. The Wielkopolska Subgroup links supra volcanic sedimentary rocks of Rotliegendes in one lithostratigraphical unit (Tab. 1). To present the geological structure of the said subgroup the map of the base. the quantitative-lithofacies map (Fig. 17) and the isopachous map (Fig. 10) have been made. The Rotliegendes Group consist of the Lower Silesia Subgroup and Wielkopolska Subgroup Formations (Tab. 1, Fig. 18). The synthetic geological map presenting all the formations in Wielkopolska is a recapitulation of the Rotliegendes complexity. Mosaic occurrence of particular formations and members has caused the discrimination of 17 various mutual combinations of lithostratigraphical units (Fig. 19).

Research Article| September 01, 1984 Stratigraphy and structure of the Schoonover sequence, northeastern Nevada: Implications for Paleozoic plate-margin tectonics E. L. MILLER; E. L. MILLER 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar B. K. HOLDSWORTH; B. K. HOLDSWORTH 2Department of Geology, University of Keele, Keele, Staffordshire ST5 5BG, England Search for other works by this author on: GSW Google Scholar W. B. WHITEFORD; W. B. WHITEFORD 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar D. RODGERS D. RODGERS 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1984) 95 (9): 1063–1076. https://doi.org/10.1130/0016-7606(1984)95<1063:SASOTS>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation E. L. MILLER, B. K. HOLDSWORTH, W. B. WHITEFORD, D. RODGERS; Stratigraphy and structure of the Schoonover sequence, northeastern Nevada: Implications for Paleozoic plate-margin tectonics. GSA Bulletin 1984;; 95 (9): 1063–1076. doi: https://doi.org/10.1130/0016-7606(1984)95<1063:SASOTS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Radiolarian biostratigraphy and detailed geologic mapping have been used to resolve the complex structure and stratigraphy of part of the Golconda allochthon in the Independence Mountains, Nevada. Here, the Schoonover sequence is latest Devonian to Early Permian in age, spanning the time interval from the emplacement of the Roberts Mountains allochthon onto the shelf in the earliest Mississippian Antler orogeny to the inception of the Sonoma orogeny. The history of the Schoonover basin is tied to that of the adjacent shelf in Nevada and provides important insights into the upper Paleozoic paleogeographic framework of the continental margin.In the Schoonover sequence, latest Devonian (Famennian) chert overlies basaltic and andesitic greenstone and is in turn overlain by Early Mississippian (Kinderhookian) chert interbedded with tuff and volcaniclastic rocks derived from an island arc. The Kinderhookian volcaniclastic rocks grade upward, with no obvious depositional break, into siliciclastic turbidites and pebbly mudstones that contain debris derived primarily from erosion of the Antler orogenic belt, although in some beds volcanic rock fragments and greenschist-grade metasedimentary clasts are also common. The age of underlying (Kinderhookian) and interbedded and overlying (Osagian to Meramecian) chert sequences indicates that the siliciclastic turbidites are synchronous with the early deposits of the autochthonous foredeep basin of the Antler orogenic belt. The dual or composite source terranes represented by the Schoonover siliciclastic rocks place the basin between an arc and the Antler orogen on the edge of the shelf.Meramecian-age basalt flows in the Schoonover are coeval with subsidence and basaltic volcanism in autochthonous shelf sequences in northern Nevada, suggesting that a rifting event may have occasioned the end of Antler-age compression in Nevada. The onset of limestone turbidite deposition in the Schoonover corresponds to the reestablishment of carbonate shelf conditions on the continental margin in latest Mississippian–earliest Pennsylvanian time.The Late Devonian–earliest Mississippian part of the Schoonover depositional history has not yet been documented elsewhere in the Golconda allochthon, but Late Mississippian to Permian rocks in the Schoonover sequence are analogous to those of the Havallah sequence, suggesting deposition in the same basin.Thrust faults in the Schoonover repeat the Late Devonian to Permian section, detaching the basinal sequence from its depositional basement. Thrust faults extend along strike for at least 10 km, but thrust plates are <1 km thick, reflecting the originally thin sequence involved in thrusting. Fold and fault data indicate that thrust plates formed and were emplaced due to northwest-southeast shortening and southeast-directed thrusting. Deformation within the allochthon postdates deposition of Early Permian strata, and the emplacement of the allochthon postdates deposition of latest Permian autochthonous strata and predates intrusion of Jurassic plutons.The stratigraphic relations documented in the Schoonover sequence are compatible with a back-arc thrusting model for the formation of both the Roberts Mountains and Golconda allochthons, but they are more difficult to reconcile with models that interpret the allochthons as accretionary prisms developed in front of farther-traveled arcs that collided with a passive margin.In a broader context, the stratigraphy of upper Paleozoic allochthonous rocks in Nevada records short-lived episodes of crustal shortening along the continental margin, separated by longer episodes of extensional and/or transcurrent tectonics. This data suggest that the western United States was a southwest Pacific–style active margin at least as far back as the Devonian. This content is PDF only. 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The Antler orogenic belt of central Nevada is a zone of tectonic activity which profoundly affected sedimentation patterns during the late Paleozoic. Lower Paleozoic through Middle Devonian strata indicate deposition in miogeosynclinal to eugeosynclinal environments with deposition of carbonate rocks on the east and shale, chert, and volcanic rocks on the west. In Late Devonian time, the depositional sequence was interrupted by eastward thrusting of the Roberts Mountains system which carried siliceous eugeosynclinal sediments onto miogeosynclinal carbonate rocks in the Antler belt. Debris eroded and transported eastward from this thrust system overwhelmed carbonate deposition during latest Devonian and earliest Mississippian time. Early Mississippian through Early Pennsyl anian tectonism with a strong vertical component provided a source terrane which again shed coarse to fine detritus eastward. These synorogenic sediments are herein interpreted as having been deposited in a shallow-marine and marginal-marine environment although carbonate rocks were at times deposited. This interpretation differs significantly from others that interpret the rocks as deepwater flysch deposits. As tectonism slowed, carbonate deposition recurred in central Nevada and the Antler belt was overlapped by Lower Permian sediments. With the overlap, shelf-to-basin sedimentation similar to that of the early Paleozoic was resumed. Antler tectonism is thus interpreted as having interrupted a well-established depositional system which persisted in a fragmentary manner throughout the t me of deformation, and which finally was reestablished across the deformed zone. End_of_Article - Last_Page 738------------

The Jonestown Volcanic Field is a five kilometer by fifteen kilometer area of volcanic and hypabyssal rocks, basalt and diabase, located in southeastern Pennsylvania. These igneous rocks presently occur within an allochthonous belt of Ordovician deep water sedimentary rocks and Taconic flysch rocks known collectively as the Hamburg Klippe. Detailed field mapping during this project has revealed that the contacts between the igneous rocks and the flysch are not conformable. The volcanic rocks are associated with the Ordovician limestone that is adjacent to these volcanics. This limestone also is not conformable to the Hamburg Klippe sediments, rather it shares outcrop characteristics of some Laurentian platform carbonates in the region. The association between the igneous rocks and the limestone suggests that the igneous rocks were emplaced on a carbonate platform. Trace element data gathered from whole rock geochemical analysis suggests that the volcanic and hypabyssal rocks formed from different melts. They could, however, have formed in the same magmatic province. The volcanic rocks share geochemical characteristics with rocks emplaced on continental forelands, while the hypabyssal rocks show evidence of continental lithospheric influence on the magma, suggesting the Jonestown igneous rocks were emplaced on a carbonate platform on the Laurentian foreland, not a seamount. Their origin may be related to the approach of the Taconic arc. The structural geology indicates the flysch rocks and the igneous rocks were originally deformed and juxtaposed during the Taconic orogeny. Detailed mapping has also shown that a sandstone unit in the Bunker Hills region previously mapped as part of the Hamburg Klippe sequence is more likely an outlier of the Silurian Tuscarora Formation, and it is probably not conformable to any of the other rocks in the field area. The geometry of the deformation of the sandstone in the Bunker Hills region suggests there was again thrusting in the region during the Alleghanian orogeny.

The Great American Carbonate Bank (GACB) comprises the carbonates (and related siliciclastics) of the Sauk megasequence, which were deposited on and around the Laurentian continent during Cambrian through earliest Middle Ordovician, forming one of the largest carbonate-dominated platforms of the Phanerozoic. The Sauk megasequence, which ranges upwards of several thousand meters thick along the Bank's margin, consists of distinctive Lithofacies and fauna that are widely recognized throughout Laurentia. A refined biostratigraphic zonation forms the chronostratigraphic framework for correlating disparate outcrops and subsurface data, providing the basis for interpreting depositional patterns and the evolution of the Bank. GACB hydrocarbon fields have produced 4 BBO and 21 TCFG, mostly from reservoirs near the Sauk-Tippecanoe unconformity. The GACB is also a source of economic minerals and construction material and, locally, serves as either an aquifer or repository for injection of waste material. This Memoir comprises works on biostratigraphy, ichnology, stratigraphy, depositional facies, diagenesis, and petroleum and mineral resources of the GACB. It is dedicated to James Lee Wilson who first conceived of this publication and who worked on many aspects of the GACB during his long and illustrious career.

Late Devonian through Early Mississippian depositional patterns in eastern Nevada and western Utah reflect transition from passive to collisional margin regimes. The Late Devonian (Frasnian) passive margin sequence (Devils Gate Limestone, Guilmette Limestone) was flexurally warped by thrust loading of the Roberts Mountains allochthon during eastward tectonic emplacement onto the continental margin. The foreland basin received minimal clastic input consisting primarily of bedded chert and hemipelagic claystone (Pine Cone Sequence, Woodruff Formation). Paleocurrent data from the northeast-southwest-trending back-bulge basin (Pilot basin) indicate that clastic detritus was derived from the forebulge to the north-northwest. The southern Pilot basin was the site of relatively shallow water carbonate deposition (West Range Limestone). During the Late Devonian (Famennian) and Early Mississippian (early Kinderhook), northern siliciclastic strata prograded over the southern carbonates, and the axis of the Pilot basin migrated eastward in conjunction with migration of the forebulge, foreland basin, and Antler thrust front. During the Early Mississippian (early Kinderhook), the forebulge migrated rapidly eastward through eastern Nevada and western Utah to produce local erosional surfaces of shoaling-upward sequences. The cratonward edge of the foreland basin was the site basin, west of the carbonate bank, shale and siltstone were deposited and grade westward into hemipelagicmore » clay (Webb Formation). During the Early Mississippian (Osage), carbonate turbidites (Tripon Pass Limestone) derived from eroded highlands to the east in Utah and to the southeast in southern Nevada were deposited in the foreland trough.« less

The stratigraphy of the cordillera can be interpreted in terms of stratigraphic assemblages that are unique in distribution, gross lithology, and lateral facies variations. Models of depositional basins in which these assemblages accumulated are essential in exploration for mineral deposits whose distribution is controlled by stratigraphy. End_Page 1437------------------------------ Proterozoic and lower Paleozoic strata in the cordillera comprise an assemblage of clastic and carbonate with minor volcanic rocks that appears to represent a continental-terrace wedge built along the margin of an earlier Precambrian continent. All units show a distinct polarity of facies distribution and thickness relative to the source area. This assemblage contains most of the known stratiform mineral deposits of gypsum, iron, copper, zinc, and lead in the cordillera. The distinctive elements of a Late Devonian and Early Mississippian assemblage suggest, at least in the northern cordillera, the presence of a foredeep and related source areas in the west and northwest. In the southern and eastern parts of the cordillera, however, the rocks reflect a continuing shelf-platform environment linked to the craton. The mineral potential of these rocks has been considered low but needs further study in view of an important zinc-lead deposit in eastern Selwyn basin. Distinctive rocks of oceanic character ranging in age from Mississippian to Middle Triassic underlie parts of the cordilleran intermontane belt. The important mineral deposits in these rocks include asbestos deposits in ultramafic rocks in the northern cordillera. Generally, however, mineral discoveries, other than those in ultramafic rocks have been few. In the eastern cordillera a shelf environment prevailed. The association of copper with volcanic rocks of Late Triassic and Early Jurassic ages is well known. The volcanic rocks, together with spatially and temporally associated plutons, are thought to outline a system of evolving island arcs probably roughly coincident with the mapped distribution of these rocks. Between the arcs and the craton, strata were deposited in a marginal basin with little or no evidence of the volcanism that occurred farther west. The remaining stratigraphic units, ranging in age from Early Jurassic to the Cenozoic, are described as successor-basin and foredeep assemblages whose distribution and lithology reflect a close relation to bounding uplifts of metamorphic and plutonic terrains. Because they are a late-stage phenomenon in the evolution of the cordillera these assemblages have potential for a variety of placer deposits. They also contain all of the known coal reserves of the region. End_of_Article - Last_Page 1438------------

Paleozoic volcanic and intrusive rocks from the Selwyn Basin are being investigated to establish if there is genetic relationship between volcanism and sediment-hosted massive sulphide (SEDEX) mineralization with the aim of improving mineral exploration models. The spatial and temporal distribution of dominantly highly alkalic magmatism and SEDEX deposits in the Selwyn Basin are similar, but traditional models for SEDEX deposit formation have excluded any role of magmatism in their genesis. This study is testing whether magmatic systems supply heat and possibly metals and/or volatiles to the ore system. Herein we report preliminary petrological and geochemical data for samples from the Keno Hill, Anvil and MacMillan Pass districts, and Misty Creek Embayment. Most of the volcanic rocks in the Anvil and MacMillan Pass districts are alkalic and mafic, although Paleozoic volcanic rocks and later dykes in the Keno Hill district are subalkalic. Significantly, most volcanic rocks in all districts are enriched to highly enriched in Ba, Cs, Nb and Th, and show a positive relationship between barium and thallium, similar to the Howards Pass SEDEX deposit, suggesting either the volcanic rocks have been altered by similar hydro-thermal fluids as those that generated SEDEX mineralization or that the volcanic rocks formed from magmas that may have contributed metals and metalloids (e.g. Ba, Tl) to ore-forming magmatic-hydrothermal fluids. Thallium, strontium, carbon and oxygen isotopic analysis, combined with U-Pb dating are planned to constrain the connections between alkalic volcanism and SEDEX formation in the Selwyn Basin.

Neoproterozoic to Paleozoic slope-to-basin facies continental margin strata underlie area 700 x 200 km across central Yukon Territory, Canada, and collectively define the Selwyn Basin. In a Cordilleran framework, Selwyn Basin strata form a strongly deformed and thrust-faulted package located between the Mackenzie foreland fold-and-thrust belt, and accreted terranes and displaced elements of the ancient North American continental margin. Orogeny commenced in the Jurassic as exotic elements of the composite Yukon-Tanana terrane overrode the ancient continental margin. Collision-related deformation had ceased by ca. 100 Ma, and was followed by a Late Cretaceous (post–85 Ma) dextral transcurrent regime, which laterally displaced elements of the newly assembled continental margin along the orogen-parallel Tintina fault. In western Selwyn Basin, more than 100 km of structural overlap was accommodated on two main detachments, the Robert Service and underlying Tombstone thrust faults. Internal deformation within the thrust sheets is intense, characterized by shear-related folds and fabrics. Metamorphic grade reaches lower to middle greenschist facies at the deepest structural levels exposed, and is characterized by chlorite-muscovite schists. The onset of deformation is constrained by the Late Jurassic age of the youngest units deformed during orogeny. The end of ductile deformation is constrained by new 40Ar/39Ar ages for metamorphic muscovite that range from 104 to 100 Ma. Due to the low metamorphic grade, these ages are interpreted to closely follow the waning of deformation. At ca. 93 ± 3 Ma, isolated granitic intrusions of the Tombstone-Tungsten magmatic belt were emplaced across the western Selwyn Basin in a tensional, postcollisional regime. Restoration of displacement on the Tintina fault places the western Selwyn Basin adjacent to the Yukon-Tanana terrane uplands of east-central Alaska in the Early to mid-Cretaceous. Despite their adjacent positioning in cross-orogen section during orogenesis, the two elements feature some significant differences in Jurassic-Cretaceous deformation. Most notably, the Yukon-Tanana terrane uplands record a significant extensional event at 120–105 Ma, which resulted in NW-SE–oriented extension, exhumation of deep structural levels, and voluminous felsic plutonism. In contrast, western Selwyn Basin did not undergo equivalent uplift and extension, and features temporally and spatially restricted plutonism. Within an orogenic framework, the Yukon-Tanana terrane uplands can therefore be considered to represent an exhumed core characterized by high heat flow, whereas the western Selwyn Basin represents an immediate northeastern salient to the exhumed core. These differences have important implications for the geodynamic setting of mid-Cretaceous plutonism across these two major lithologic-tectonic entities of the northern Cordillera.

Late Precambrian to Cretaceous, weakly metamorphosed strata of the ancestral North American margin comprise four sequences: Upper Precambrian to Middle Devonian (more than 3000 m) turbiditic sandstone, deep-water limestone, shale, and chert (Selwyn Basin), flanked southwesterly in the Siluro-Devonian by shallow-water carbonate and clastic sediments (McEvoy Platform). Early Cambrian pelite hosts deposits of stratiform lead-zinc. Upper Devonian and Mississippian turbiditic quartz-chert sandstone and chert-pebble conglomerate (2000 m) shed from elevated fault blocks of Selwyn Basin strata. Stratiform barite occurs within siliceous shale of mid- to Upper Devonian age. Mississippian to Triassic shale, chert, limestone, minor sandstone, and siltstone (1700 m) deposited on a shallow-marine shelf; and Lower Cretaceous clastic sediments (120+ m) derived from Jura-Cretaceous deformation and uplift. Regional Jura-Cretaceous deformation formed décollement style, northwest-trending folds and large, shallow-dipping thrust faults. Incompetent Cambro-Ordovician to Lower Devonian strata are complexly deformed above a regional flat-lying detachment. Deformation was accompanied by obduction of oceanic ultramafite, basalt, chert, and carbonate (Slide Mountain Terrane) as well as siliceous mylonite and schist (Yukon-Tanana Terrane). Granitic intrusions of the mid-Cretaceous (100 Ma) Selwyn Plutonic Suite crosscut regional structure and are responsible for small skarn and base-metal vein deposits. Coeval pyroclastic rocks of the South Fork volcanics are preserved within huge calderas. Cretaceous-Tertiary dextral slip along Tintina Fault has offset geological elements at least 430 km and formed pull-apart basins along and near the fault. Eocene fill of fluvial clastic and bimodal volcanic rocks host epithermal precious-metal veins.