Journal articles on the topic 'Middle Ordovician metapelitic rocks'
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1
VAIDA, M., H. P. HANN, G. SAWATZKI, and W. FRISCH. "Ordovician and Silurian protolith ages of metamorphosed clastic sedimentary rocks from the southern Schwarzwald, SW Germany: a palynological study and its bearing on the Early Palaeozoic geotectonic evolution." Geological Magazine 141, no. 5 (September 2004): 629–43. http://dx.doi.org/10.1017/s0016756804009641.
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Sedimentation ages of metamorphosed clastic sedimentary rocks in the southern Schwarzwald were determined by associations of palynomorphs. In the northern subunit of the Badenweiler–Lenzkirch Zone, two lithostratigraphic assemblages could be discerned in low-grade metamorphic units by their facies and age, thus revealing a more complex internal structure of this zone than previously assumed. Lower Ordovician metagreywackes and metapelites were discerned from Silurian metasiltstones. In the cataclastically overprinted metasiltstones and phyllites of the southern subunit of the Badenweiler–Lenzkirch Zone, only poorly preserved microfossil remains could be detected. These show that the sedimentation ages must be Ordovician or younger, but still probably Early Palaeozoic. High-grade metapelitic rocks of the South Schwarzwald Gneiss Complex contain chitinozoans in lenses and layers of schists, that are rich in biotite and graphite. They yielded mid-Silurian ages and show that this crystalline complex does not represent an older basement unit but was the result of marine sedimentation at that time. The new age determinations have a bearing on geodynamic reconstructions of the internal Variscides in Early Palaeozoic time. They show that sedimentation in the oceanic realm of the Badenweiler–Lenzkirch Zone or its margins did not occur before the Ordovician. After transformation of the northern passive into an active continental margin, younger greywackes not older than Middle Devonian received detritus from a volcanic arc, forming above the subduction zone.
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ŽÁK, JIŘÍ, JIŘÍ SLÁMA, and MIROSLAV BURJAK. "Rapid extensional unroofing of a granite–migmatite dome with relics of high-pressure rocks, the Podolsko complex, Bohemian Massif." Geological Magazine 154, no. 2 (February 11, 2016): 354–80. http://dx.doi.org/10.1017/s0016756816000030.
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AbstractThe Podolsko complex, Bohemian Massif, is a high-grade dome that is exposed along the suprastructure–infrastructure boundary of the Variscan orogen and records snapshots of its protracted evolution. The dome is cored by leucocratic migmatites and anatectic granites that enclose relics of high- to ultrahigh-pressure rocks and is mantled by biotite migmatites and paragneisses whose degree of anatexis decreases outwards. Our new U–Pb zircon ages indicate that the leucocratic migmatites were derived from Early Ordovician (c. 480 Ma) felsic igneous crust; the same age is inferred for melting the proto-source of the metapelitic migmatites. The relics of high- to ultrahigh-pressure rocks suggest that at least some portions of the complex witnessed an early Variscan subduction to mantle depths, followed by high-temperature anatexis and syntectonic growth of the Podolsko dome in the middle crust at c. 340–339 Ma. Subsequently, the dome exhumation was accommodated by crustal-scale extensional detachments. Similar c. 340 Ma ages have also been reported from other segments of the Variscan belt, yet the significance of this tectonothermal event remains uncertain. Here we conclude that the 340 Ma age post-dates the high-pressure metamorphism; the high temperatures required to cause the observed isotopic resetting and new growth of zircon were probably caused by heat input from the underlying mantle and, finally, the extensional unroofing of the complex requires a minimum throw of about 8–10 km. We use this as an argument for significant early Carboniferous palaeotopography in the interior of the Variscan orogen.
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Jirásek, Jakub, Dalibor Matýsek, and Martin Sivek. "Minerály „ottrélitových“ břidlic u Vápenného Podola v Železných horách (Česká republika)." Bulletin Mineralogie Petrologie 28, no. 2 (2020): 339–46. http://dx.doi.org/10.46861/bmp.28.339.
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From the belt of Ordovician metapelites in the Železné hory Mountains, ottrélite was described in 1882. Although the original paper stated the virtual absence of manganese, many papers and books from the 20th century copied just the original name of the mineral, without respect to its chemistry. Since the quantitative analysis was not available, we decided to revise this occurrence. Material newly collected in the vicinity of the Bučina Hill (606 m a.s.l.) SW from the Vápenný Podol village fits the original description, i.e. felsic rocks rich in quartz and illite-muscovite, with significant schistosity and abundant porphyroblasts of dark green mineral of the chloritoid group up to 3 mm large. Rietveld refinement of powder X-ray diffraction using different input structural models gave the best fit (the lowest RBragg) for the triclinic chloritoid of P-1 space group. Unit cell parameters are as follow: a = 5.483(1), b = 5.479(1), c = 9.1476(5) Å, α = 83.452(10)°, β = 76.639(11)°, γ = 59.993(15)°. Its average formula from seven WDS spots is (Fe0.83Mg0.17Mn0.01)Σ1.01 Al1.97(SiO4)Σ1.02O0.92(OH)2.00, and therefore must be classified as a chloritoid. As accessory minerals in the schist, we found rutile crystals and aggregates, prismatic zircons, a mineral from the chlorite group, and paragonite. Attention was paid to the unexpected occurrence of possibly primary rare grains of xenotime-(Y) up to 10 μm with average formula (Y0.71Sm0.01Gd0.03Tb0.01Dy0.07Ho0.01Er0.05Tm0.01Yb0.04Lu0.01)Σ0.96(P1.02Si0.01)Σ1.03O4.00 and more common rhabdophane-(Ce), which forms acicular, partly skeletal crystals in cavities, possibly after leached apatite. Its average formula is Y0.01La0.18 Ce0.40Pr0.04Nd0.15Sm0.03Eu0.01Gd0.04Al0.02Ca0.18Fe0.04Th0.02)Σ1.12(P0.95Si0.01S0.01)Σ0.97O4.00·0.97 H2O. We suggest using the term “chloritoid schist” for these metapelites formed at the contact of Middle to Late Ordovician graphite shales with the intrusion of the Variscan biotite granite of the Železné Hory Mts. Plutonic Complex.
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Bock, B., S. M. McLennan, and G. N. Hanson. "The Taconian orogeny in southern New England: Nd-isotope evidence against addition of juvenile components." Canadian Journal of Earth Sciences 33, no. 12 (December 1, 1996): 1612–27. http://dx.doi.org/10.1139/e96-122.
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Nd-isotope data for pre-Taconian (meta)sedimentary and igneous rocks, syn-Taconian (meta)sedimentary rocks, and Late Ordovician–Silurian plutonic rocks indicate that the Ordovician Taconian orogeny did not add significant amounts of juvenile crust to the Laurentian margin in southern New England. Nd-isotope compositions of Grenvillian crust and Late Proterozoic to Early Cambrian rift sediments range from εNd of −3.1 to −6.6 at 450 Ma. Sedimentary rocks deposited during the Cambrian and the early Middle Ordovician, which represent the drift stage of Laurentia, and earliest Taconian sedimentary rocks show more negative εNd(450 Ma), with a range from −11.7 to −13.3. Sedimentary rocks deposited in response to the Taconian orogeny have uniform εNd(450 Ma) values of about −8. Middle to Late Ordovician and Permian plutonic rocks from southwestern Connecticut have εNd(450 Ma) values of −2 to −5, which indicates that these rocks contain older crustal components. Rocks with juvenile Nd characteristics are the early Paleozoic Maltby Lake Volcanics (εNd(450 Ma) +8) from southwestern Connecticut, and Middle Ordovician igneous samples from the Hawley Formation (εNd(450 Ma) +6 to −0.6) in Massachusetts.
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Norford, B. S., and M. P. Cecile. "Ordovician emplacement of the Mount Dingley Diatreme, Western Ranges of the Rocky Mountains, southeastern British Columbia." Canadian Journal of Earth Sciences 31, no. 10 (October 1, 1994): 1491–500. http://dx.doi.org/10.1139/e94-132.
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External and internal morphologies are well shown by a newly discovered diatreme that is exceptionally well exposed in a cirque within the north face of Mount Dingley. The diatreme contains abundant brecciated host rocks mixed with highly altered, fine-grained, light-green igneous fragments (minerals include muscovite, chlorite, quartz, carbonate, and some remnant K-feldspar). The diatreme cuts Lower Ordovician rocks of the McKay Group. Olistostromes and other volcaniclastic rocks that are directly associated with the diatreme are bevelled beneath a regional unconformity below the Upper Ordovician Beaverfoot Formation. Lower Ordovician gastropods are present just below the volcaniclastic rocks and within what appears to be a lens of sediment within one of the olistostrome beds. These occurrences indicate a mid-Early Ordovician time of intrusion, but there is the possibility that the pipe was emplaced later within the interval mid-Early to early Late Ordovician. In the Western Ranges, three other episodes of emplacement of diatremes have been documented previously as within the intervals early Middle to early Late Ordovician, latest Early Silurian to early Middle Devonian, and Late Permian. Many of the diatremes are broadly contemporaneous with widespread, but volumetrically small, Ordovician and Lower Paleozoic volcanic and intrusive rocks found throughout the Canadian Cordillera. These volcanic and intrusive rocks have been interpreted as evidence of continued Lower Paleozoic extensional tectonism and some are associated with large base-metal deposits.
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CASAS, J. M. "Ordovician deformations in the Pyrenees: new insights into the significance of pre-Variscan (‘sardic’) tectonics." Geological Magazine 147, no. 5 (January 25, 2010): 674–89. http://dx.doi.org/10.1017/s0016756809990756.
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AbstractTwo deformational events which developed prior to the Variscan structures can be characterized in the Palaeozoic rocks of the Pyrenees: a Middle (?) Ordovician folding event and a Late Ordovician fracture episode. The Middle (?) Ordovician folding event gives rise to NW–SE- to N–S-oriented, metric- to hectometric-sized folds, without cleavage formation or related metamorphism. These folds can account for the deformation and uplift of the pre-Upper Ordovician (Cambro-Ordovician) sequence and for the formation of the Upper Ordovician unconformity. Ordovician folds control the orientation of the Variscan main-folding-phase minor structures, fold axes and intersection lineation in the Cambro-Ordovician sediments. The Late Ordovician fracture episode gave rise to normal faults affecting the lower part of the Upper Ordovician series, the basal unconformity and the underlying Cambro-Ordovician metasediments. Displacement of some of these faults diminishes progressively upwards of the series and tapers off in the upper part of the Upper Ordovician rocks, indicating that the faults became inactive during Late Ordovician times before deposition of the Ashgillian metasediments. Normal faults can be linked to the Upper Ordovician volcanic activity, which has been extensively described in the Pyrenees. The aforementioned deformation episodes took place after the Early Ordovician magmatic event, which gave rise to a large volume of plutonic rocks in the Pyrenees as in other segments of the European Variscides. This Middle Ordovician contractional event separated two extensional events in the Pyrenees from Early Ordovician to Silurian times. This event prevents us from assuming the existence of a continuous extensional regime through Ordovician and Silurian times, and suggests a more complex evolution of this segment of the northern Gondwana margin during the Ordovician.
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Dixon, O. A. "The heliolitid coral Acidolites in Ordovician–Silurian rocks of eastern Canada." Journal of Paleontology 60, no. 1 (January 1986): 26–52. http://dx.doi.org/10.1017/s002233600002148x.
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Acidolites Lang, Smith and Thomas occurs in upper Middle and Upper Ordovician, Lower and lower Middle Silurian rocks of Ontario and Quebec. On Anticosti Island, Quebec, the genus is represented by A. tenuis (Billings) in the Upper Ordovician (Gamachian) Ellis Bay Formation; the new species A. arctatus, A. compactus and A. helianthus in the Ordovician–Silurian boundary beds at the top of the Ellis Bay Formation; the new species A. arctatus, A. compactus and A. lindströmi in the lower Llandoverian Becscie Formation; A. arctatus in the mid-Llandoverian Gun River Formation; and an unnamed species in the upper Llandoverian Jupiter Formation. The lower Llandoverian Clemville Formation of the Gaspé Peninsula, Quebec, contains Protaraea clemvillensis Parks, now considered to be Acidolites. The upper Middle to lower Upper Ordovician Cobourg Formation near Ottawa, Ontario, contains A. cf. arctatus, formerly included in Protaraea vetusta (Hall). The lower Wenlockian Amabel Formation in southern Ontario contains a species of Acidolites as yet unnamed.
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Botting, Joseph P. "Llanvirn (Middle Ordovician) echinoderms from Llandegley Rocks, central Wales." Palaeontology 46, no. 4 (July 2003): 685–708. http://dx.doi.org/10.1111/1475-4983.00316.
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Gehrels, George E., Jason B. Saleeby, and Henry C. Berg. "Geology of Annette, Gravina, and Duke islands, southeastern Alaska." Canadian Journal of Earth Sciences 24, no. 5 (May 1, 1987): 866–81. http://dx.doi.org/10.1139/e87-086.
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Geologic mapping, U–Pb (zircon) geochronometry, and conodont studies indicate that the major pre-Jurassic assemblages on Annette, Gravina, Duke, and adjacent smaller islands include pre-Middle Ordovician metavolcanic and metasedimentary rocks (Wales metamorphic suite); Cambrian metaplutonic rocks; Ordovician – Early Silurian volcanic (Descon Formation), dioritic, and gabbroic rocks; Silurian trondhjemitic plutons; Early Devonian sedimentary (Karheen Formation) and volcanic rocks; Late Triassic sedimentary and volcanic rocks (Hyd Group); and a large body of Late Triassic pyroxene gabbro.Stratigraphic, structural, and intrusive relations record episodes of regional deformation, metamorphism, and uplift during Middle Cambrian – Early Ordovician time (Wales orogeny) and during middle Silurian – earliest Devonian time (Klakas orogeny). Upper Triassic strata were apparently deposited during a latest Paleozoic(?) – Triassic rifting event.Comparison with the geology of Prince of Wales Island indicates that the Annette and Craig subterranes of the Alexander terrane belong to the same tectonic fragment and that the Clarence Strait fault has ~15 km of right-lateral displacement at this latitude. Our geochronologic data indicate that the pyroxene gabbro on Duke Island is Triassic in age and therefore probably unrelated to nearby Cretaceous(?) zoned ultramafic bodies.
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Johnston, J. D., and W. E. A. Phillips. "Terrane amalgamation in the Clew Bay region, west of Ireland." Geological Magazine 132, no. 5 (September 1995): 485–501. http://dx.doi.org/10.1017/s0016756800021154.
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AbstractThe Caledonides of the west of Ireland provide a well-exposed and well-mapped example of an oblique collision zone. The east-northeast trending Deer Park and Achill Beg Fault system is a crustal scale ductile sinistral strike-slip duplex of late Ordovician age, imbricating late Precambrian granulite facies lower crustal rocks, near eclogite facies supracrustal rocks, up to amphibolite facies Dalradian metasedimentary rocks and greenschist facies Cambro-Ordovician rocks. This fault system is correlated with a pre-Devonian component of the Highland Boundary Fault system in southern Scotland. In the Clew Bay area, the high pressure-low temperature facies metamorphic rocks, in tectonic contact with greenschist facies Cambro-Ordovician rocks, are together interpreted as an accretionary prism complex related to northwestward directed subduction. Both of these are allocthonous terrains with respect to the Dalradian terrane to the north (North West Mayo). To the south, the Cambro-Ordovician rocks docked with a probable Dalradian block containing ultramafic intrusives (Deer Park Complex) during the late Ordovician. The Deer Park Complex and South Mayo Trough linked earlier, during the Arenig.Silurian and Lower-Middle Devonian redbed successions sit unconformably on the metamorphic rocks. Deposition and deformation of these cover rocks was controlled by oblique strike-slip movements on the Leek Fault whose strike swings from west-northwest to north-northeast, following earlier basement trends, as it is traced eastwards from Clew Bay. The Leek Fault System may be correlated with the Leannan Fault of northwest Donegal, a splay of the Great Glen Fault system of central Scotland. East of Clew Bay, this sinistral shear generated local dilation on the more northerly trending bend of the Leek Fault. Lower and Middle Old Red Sandstone redbeds were developed here. The west-northwest trend of the Leek Fault in Clew Bay acted as a compressional bend during these sinistral movements and transpressional southwest directed thrusting developed in Silurian rocks. Post-Middle Old Red Sandstone pre-late Tournaisian dextral displacement on the Leek Fault reversed this pattern with transtension in Clew Bay allowing intrusion of small carbonated peridotite bodies into Silurian rocks and easterly directed thrusting of Middle Old Red Sandstone rocks east of the Bay on the transpressional north-south bend.A tectonic model for the region is presented here. This model involves a northwestward directed subduction system, 150 to 750 km of Arenig sinistral strike slip movement, and eastwards insertion of the Connemara block with formation of the Ordovician South Mayo Trough as a pull-apart basin. Subsequently, a further 130 to 650 km eastward displacement of rocks took place south of the Deer Park Fault in later Ordovician times. The magnitudes of these estimates are directly proportional to an assumed maximum wavelength of 1500 km for promontories on the original Laurentian margin, and using the current juxtaposition of terranes, a minimum wavelength of 300 km is inferred.
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Winchester, J. A., C. R. van Staal, and J. P. Langton. "The Ordovician volcanics of the Elmtree–Belledune inlier and their relationship to volcanics of the northern Miramichi Highlands, New Brunswick." Canadian Journal of Earth Sciences 29, no. 7 (July 1, 1992): 1430–47. http://dx.doi.org/10.1139/e92-115.
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An investigation of the geology and chemistry of the basic igneous rocks in the Elmtree and Belledune inliers in northern New Brunswick shows that the bulk of the Middle Ordovician rocks of the ophiolitic Fournier Group are best interpreted as the products of volcanism and sedimentation in an extensive ensimatic back-arc basin southeast of a volcanic arc. The oceanic back-arc-basin igneous rocks form the basement to renewed arc-related basaltic volcanism in late Middle to Late Ordovician time. The Fournier Group is separated from the structurally-underlying, shale-dominated Elmtree Formation of the Tetagouche Group by an extensive tectonic melange, which incorporates lenses of serpentinite, mafic volcanic rocks, and sedimentary rocks of both the Tetagouche and Fournier groups. The mafic volcanic rocks in the Elmtree Formation correlate best with those intercalated with the lithologically similar sediments of the Llandeilian–Caradocian Boucher Brook Formation in the northern Miramichi Highlands. The melange and the present structural amalgamation of the Tetagouche and Fournier groups result from closure of the marginal basin by northward-directed subduction at the end of the Ordovician. Most mafic suites in the Elmtree and Belledune inliers can be chemically correlated with similar suites in the northern Miramichi Highlands, showing that the two areas are not separated by a terrane boundary.
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Staal, C. R. Van, J. A. Winchester, and J. H. Bédard. "Geochemical variations in Middle Ordovician volcanic rocks of the northern Miramichi Highlands and their tectonic significance." Canadian Journal of Earth Sciences 28, no. 7 (July 1, 1991): 1031–49. http://dx.doi.org/10.1139/e91-094.
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A detailed geochemical study of Middle Ordovician volcanic rocks, undertaken in the northern Miramichi Highlands of New Brunswick, shows that 10 basaltic suites can be distinguished. These suites are assigned to the Tetagouche and Fournier groups. The contact between these two groups is a major thrust zone, marked for over 70 km by a prominent blueschist zone. All the Tetagouche Group volcanic rocks have chemistries consistent with extrusion in a continental rift, but most Fournier Group basalts in the Miramichi Highlands have chemistries suggestive of an oceanic back-arc setting. The chemical signatures, stratigraphic variations, and structural data indicate that the northern Miramichi Highlands preserve a section across a telescoped Middle Ordovician back-arc basin that initially opened as a result of asthenospheric injection near the rear part of a Lower Ordovician ensialic arc.
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Sanford, Bruce V. "Stratigraphic and Structural Framework of Upper Middle Ordovician Rocks in the Head Lake-Burleigh Falls Area of South-Central Ontario." Géographie physique et Quaternaire 47, no. 3 (November 23, 2007): 253–68. http://dx.doi.org/10.7202/032956ar.
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ABSTRACT Field investigations in the Head Lake-Burleigh Falls area of south-central Ontario, that focused mainly on the Covey Hill(?), Shadow Lake, Gull River and Coboconk formations along the Paleozoic escarpment, provided clear evidence of faulting. Observed and inferred structural deformation, coupled with faciès changes within the Shadow Lake and lower Gull River, point to a succession of basement movements during the Phanerozoic. These range from Hadrynian-Early Cambrian, through Middle Ordovician to post-late Middle Ordovician times. Some of the earlier movements (Hadrynian-Early Cambrian to late Middle Ordovician) appear to be coincident with, and probably related to, plate tectonic events and the associated Taconian orogeny that were in progress along the southeastern margins of the North American continent. Post-Middle Ordovician block faulting, on the other hand, may have been triggered by any number of epeirogenic events related to late stage Taconian, Acadian or Alleghanian orogenies in Late Ordovician to Carboniferous times, or possibly to rifting associated with continental break-up and initiation of seafloor spreading processes in the early Mesozoic. Manuscrit révisé accepté le 5 août 1993
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Yanev, Slavcho, and Yavor Khrischev. "Comparative lithological and geochemical studies of the Palaeozoic rocks from western Bulgaria." Geologica Balcanica 29, no. 1-2 (June 30, 1999): 61–87. http://dx.doi.org/10.52321/geolbalc.29.1-2.61.
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First attempt to link the lithological data for Ordovician, Silurian, Devonian and Upper Carboniferous rocks from the Svoge Anticlinorium and the Sofiyska Stara Planina Mountain to the results from the multivariate mathematics analysis of data for macro- and microchemical composition of the rocks (correspondence and cluster analysis) was made in the present study. Four groups with enhanced content of the certain macroelements were divided: I - with SiO2; II - with CaO; III - with Na2О; III - with Al, Fe, Mn, Mg, K, volatile and Ti; and 9 subgroups (Ia, Ib, Ic, IIIa, IIIb, IIIc, IVa, IVb, IVc). The group Ia includes Ordovician quartzites, Upper Devonian sandstones, Devonian silicites and Upper Carboniferous sandstones; Ib is composed of mainly Upper Carboniferous polymictic sandstones; Ic consists of argillites of Ordovician, Silurian and Devonian ages. Group II contains sandstones abundant in Ca. Group III comprises sandstones of various ages and with diverse degree of transformation (catagenetic and low metamorphic) or with assistance of volcanic rocks in the source province. Group IV contains argillites: IVa - from the Ordovician Grohoten Formation, tightly linked to Al and K; IVb - from the Tzeretzel Formation, from Upper Silurian, Lower and Middle Devonian, and a part of the Upper Carboniferous argillites; IVc - argillites from the transitional interval between Silurian and Devonian as well as chert argillites from the Lower Silurian. Four groups were determined on the dendrogram obtained after cluster analysis with respect to the microcomposition. The first group (I) contains samples with high amount of Mn (Tzeretzel Formation, Upper Silurian, Middle and Upper Devonian silicites and argillites, Carboniferous sandstones and argillites). The second group (II) is abundant of V, Cr, Rb (Silurian graptolite argillites, less Devonian, Ordovician and Upper Carboniferous argillites). The group III is abundant of Zr (clastic rocks from the Grohoten Formation and from Upper Carboniferous) and group IV is composed of samples with high Sr content (mainly Upper Devonian). The detached groups regarding to macro- and microcomposition are resulting from mineralogical and petrographical features of the rocks and thus they correlate with each other in a certain degree. A certain tendency of variations in the geochemical features of the rocks were established regarding to their evolution from mature sediments (Ordovician and Silurian) to immature sediments (Upper Devonian, Upper Carboniferous), to he change of marine sedimentation (Ordovician - Devonian) to continental one (Upper Carboniferous); to the change in the provenance (sedimentary rocks in the Ordovician and the Silurian, the same plus metamorphic in the Devonian, previous plus intrusivc rocks in the Upper Carboniferous); to the influence of the climate (weaker in the marine deposits and stronger in the continental Upper Carboniferous) etc.
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Robson, Sean P., and Brian R. Pratt. "Cambrian and Ordovician linguliform brachiopods from the Shallow Bay Formation (Cow Head Group), western Newfoundland." Journal of Paleontology 75, no. 2 (March 2001): 241–60. http://dx.doi.org/10.1017/s0022336000018060.
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Linguliform brachiopods were recovered from the Upper Cambrian Downes Point Member (lower Sunwaptan) and from the Middle Ordovician Factory Cove Member (Arenig) of the Shallow Bay Formation, Cow Head Group, of western Newfoundland. These rocks are a series of Middle Cambrian to Middle Ordovician conglomerates, lime mudstones, and shales that formed a sediment apron at the base of the lower Paleozoic continental slope of Laurentia. The linguliform brachiopod fauna consists of sixteen species assigned to twelve genera. Three new species are described: Picnotreta lophocracenta, Neotreta humberensis, and Siphonotretella parvaducta.
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KEPPIE, J. D., J. DOSTAL, J. B. MURPHY, and B. L. COUSENS. "Palaeozoic within-plate volcanic rocks in Nova Scotia (Canada) reinterpreted: isotopic constraints on magmatic source and palaeocontinental reconstructions." Geological Magazine 134, no. 4 (July 1997): 425–47. http://dx.doi.org/10.1017/s001675689700719x.
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Palaeozoic volcanism in the Avalon Terrane of northern Nova Scotia occurred during three time intervals: Cambrian–early Ordovician, late Ordovician–early Silurian and middle–late Devonian. In the Meguma Terrane of southern Nova Scotia, Palaeozoic volcanism is limited to the middle Ordovician. Geochemical data show that most of these volcanic rocks are bimodal, within-plate suites. Initial εNd signatures range from +5.4 to −1.9 in the rhyolites and +6.8 to +2.7 in the basalts, a difference attributable to the absence or presence, respectively, of a significant crustal component. The data and regional tectonic settings of the Avalon and Meguma terranes suggest that the volcanism was generated in three different within-plate settings: (1) Cambrian–early Ordovician volcanism related to thermal decay of late Proterozoic arc magmatism during transtensional deformation; (2) middle Ordovician–early Silurian volcanism during sinistral telescoping between Laurentia and Gondwana where extensional bends in the Appalachians produced rifting; and (3) Devonian volcanism resulting from lithospheric delamination during dextral transpression and telescoping. In each setting, active faults served as conduits for the magmas. Nd isotopic data indicate that the source of the Palaeozoic felsic volcanic rocks is isotopically indistinguishable beneath southern and northern Nova Scotia and did not substantially change with time. This crustal source appears to have separated from the mantle during the Proterozoic, a conclusion consistent with the hypothesis that the Palaeozoic rocks in Nova Scotia were deposited upon a late Proterozoic oceanic–cratonic volcanic arc terrane. The Nd data, when combined with published faunal, palaeomagnetic and U–Pb isotopic data, suggest that the Avalon Terrane was peripheral to Gondwana off northwestern South America during Neoproterozoic and early Palaeozoic times.
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Jutras, Pierre, J. Brendan Murphy, Dennis Quick, and Jaroslav Dostal. "Evolution of Subduction Dynamics beneath West Avalonia in Middle to Late Ordovician Times." Lithosphere 2020, no. 1 (September 23, 2020): 1–22. http://dx.doi.org/10.2113/2020/8837633.
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Abstract Middle to Upper Ordovician volcanic rocks in the Arisaig area of Nova Scotia, Canada, constitute the only known record of volcanism in West Avalonia during that interval. Hence, they have been extensively studied to test paleocontinental reconstructions that consistently show Avalonia as a drifting microcontinent during that period. Identification of volcanic rocks with an intermediate composition (the new Seaspray Cove Formation) between upper Darriwilian bimodal volcanic rocks of the Dunn Point Formation and Sandbian felsic pyroclastic rocks of the McGillivray Brook Formation has led to a reevaluation of magmatic relationships in the Ordovician volcanic suite at Arisaig. Although part of the same volcanic construction, the three formations are separated by significant time-gaps and are shown to belong to three distinct magmatic subsystems. The tectonostratigraphic context and trace element contents of the Dunn Point Formation basalts suggest that they were produced by the high-degree partial melting of an E-MORB type source in a back-arc extensional setting, whereas trace element contents in intermediate rocks of the Seaspray Cove Formation suggest that they were produced by the low-degree partial melting of a subduction-enriched source in an arc setting. The two formations are separated by a long interval of volcanic quiescence and deep weathering, during which time the back-arc region evolved from extension to shortening and was eventually onlapped by arc volcanic rocks. Based on limited field constraints, paleomagnetic and paleontological data, this progradation of arc onto back-arc volcanic rocks occurred from the north, where an increasingly young Iapetan oceanic plate was being subducted at an increasingly shallow angle. Partial subduction of the Iapetan oceanic ridge is thought to have subsequently generated slab window magmatism, thus marking the last pulse of subduction-related volcanism in both East and West Avalonia.
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Plado, J., L. Ainsaar, M. Dmitrijeva, K. Põldsaar, S. Ots, L. J. Pesonen, and U. Preeden. "Magnetic susceptibility of Middle Ordovician sedimentary rocks, Pakri Peninsula, NW Estonia." Estonian Journal of Earth Sciences 65, no. 3 (2016): 125. http://dx.doi.org/10.3176/earth.2016.11.
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Bauer, Jeffrey A. "Conodont biostratigraphy and paleoecology of Middle Ordovician rocks in eastern Oklahoma." Journal of Paleontology 63, no. 1 (January 1989): 92–107. http://dx.doi.org/10.1017/s0022336000041019.
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Conodonts from the upper Burgen, Tyner, and Fite Formations (Middle Ordovician) of eastern Oklahoma include two new species, Phragmodus harrisi and Plectodina tynerensis. The conodont fauna indicates that the upper Burgen through middle Tyner is Whiterockian (pre- to earliest Chazyan) and that the upper Tyner–Fite is probably Kirkfieldian in age.The Whiterockian Burgen–Tyner preserves a regressive succession of shoreface, lagoonal, and intertidal deposits. That change is reflected by the conodont succession, which shows replacement of a fauna dominated by species of Neomultioistodus, Scandodus?, and Paraprioniodus by one dominated by species of Phragmodus, Plectodina, and Erismodus.Carbonate deposits of the Kirkfieldian(?) upper Tyner and Fite Formations follow a major hiatus and reflect shallow, subtidal to intertidal conditions. The conodont fauna is composed of species of Aphelognathus, Plectodina, Curtognathus, Erismodus, and Oulodus, among others.
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Tucker, R. D., S. J. O'Brien, and B. H. O'Brien. "Age and implications of Early Ordovician (Arenig) plutonism in the type area of the Bay du Nord Group, Dunnage Zone, southern Newfoundland Appalachians." Canadian Journal of Earth Sciences 31, no. 2 (February 1, 1994): 351–57. http://dx.doi.org/10.1139/e94-032.
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In south Newfoundland, an extensive tract of metamorphosed Ordovician metavolcanic, metasedimentary, and granitoid rocks (Bay du Nord Group) lies north of a late Precambrian basement inlier of peri-Gondwanan affinity, separated from the latter by Silurian rocks. In the Bay du Nord Group type area, the metavolcanic and metasedimentary rocks were ductilely sheared and locally fault imbricated with metagabbro prior to emplacement of the Baggs Hill Granite, herein dated at 477.6 ± 1.8 Ma. Some of the volcano-sedimentary strata within this succession, however, contain foliated clasts of Baggs Hill Granite, and these strata must comprise a younger stratigraphic sequence which, in this area, is thrust northwestward over the older rocks.The pre-477.6 ± 1.8 Ma tectonic interleaving of the Dunnage Zone gabbro and stratified rocks is significantly earlier than the Llandovery (early Salinic) recumbent folding and thrusting of Middle Ordovician Exploits Subzone rocks in the east-central Hermitage Flexure. In both areas, the Dunnage Zone rocks were inhomogeneously thickened and tectonically telescoped north of a rigid block of late Precambrian peri-Gondwanan basement. The emplacement of the Baggs Hill Granite is coeval with intrusion of similar Ordovician granite into ophiolites obducted southeastward onto the Gondwanan margin during Arenigian (Penobscot) orogenesis.
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Garzione, Carmala N., P. Jonathan Patchett, Gerald M. Ross, and JoAnne Nelson. "Provenance of Paleozoic sedimentary rocks in the Canadian Cordilleran miogeocline: a Nd isotopic study." Canadian Journal of Earth Sciences 34, no. 12 (December 1, 1997): 1603–18. http://dx.doi.org/10.1139/e17-129.
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Nd isotopes and trace elements in sedimentary rocks of the Yukon, the Northwest Territories, and northern British Columbia are used to examine the source of sediments in the Canadian Cordilleran miogeocline. Previous Nd isotope studies in southern Alberta demonstrated that strata of Neoproterozoic to Late Ordovician age were derived from Archean and Proterozoic Canadian Shield sources, whereas by the Late Devonian, a shift of 6 εNd units to younger crustal sources (εNd (T) = −6 to −9) had occurred. In this study, we found that the shift to younger crustal Nd isotopic signatures in the Yukon and Northwest Territories occurred much earlier than in southern Alberta. Cambrian and older strata have εNd(T) values of −10.0 to −21.1, consistent with derivation from Canadian Shield sources. Lower Ordovician through Permian strata in the Yukon and Northwest Territories, including the Innuitian-derived Imperial Assemblage, have εNd(T) values of −5 to −11.4. In northern British Columbia, the shift to a younger source reflects a wider range of εNd(T) values, from -−8.7 to −14.6 in Middle Ordovician through Middle Devonian strata, suggesting continued input from Canadian Shield sources. By the Middle Devonian, a complete shift to younger crustal signatures (εNd(T) = −5.9 to −10.5) had occurred in northern British Columbia. Several sources for the more juvenile sediments include (1) a mixture of locally erupted volcanic rocks with Canadian Shield sources, (2) a Grenville source, and (3) an Innuitian source. We propose that Ordovician to Lower Devonian strata were derived from a mixture of locally erupted, juvenile volcanics and pre-Cambrian Canadian Shield sources, and post-Middle Devonian strata were sourced from the Innuitian orogen in the Canadian Arctic.
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Dean, W. T., O. Monod, and Yilmaz Günay. "Lower Palaeozoic stratigraphy in the southern and central Amanos Mountains, south central Turkey." Geological Magazine 123, no. 3 (May 1986): 215–26. http://dx.doi.org/10.1017/s0016756800034713.
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AbstractFormations of Cambrian and Ordovician age originally identified by the letters A to E in the southern Amanos Mountains are reviewed using lithostratigraphic units known from the western Taurus Mountains and south-eastern Turkey. The youngest Cambrian rocks in the southern Amanos belong to the Sosink Formation and their age, based on trilobite evidence, ranges from the Badulesia Zone to the Solenopleuropsis Zone of the Middle Cambrian; they are overlain unconformably by conglomerates dated as Triassic? by means of plant fossils. Ordovician rocks – Seydişehir Formation (approx. Arenig) and Bedinan Formation (Caradoc in part) – are exposed further north, in the central Amanos. The unusual position of the allochthonous ophiolites, which rest in part directly upon Lower Palaeozoic rocks, is discussed.
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Harper, D. A. T. "The brachiopod Ptychopleurella lapworthi (Davidson) from the Ordovician of Girvan, S.W. Scotland." Journal of Paleontology 60, no. 4 (July 1986): 845–50. http://dx.doi.org/10.1017/s0022336000042980.
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The small, distinctive, glyptorthinine brachiopod Ptychopleurella Schuchert and Cooper is widely distributed in rocks of early Ordovician to late Silurian age. Several species are known from the Barr and Ardmillan successions (middle-upper Ordovician) of the Girvan district, S.W. Scotland, one of which, ‘Orthis Lapworthi’ Davidson, has not been described in modern terms as there has been some confusion concerning its true identity. Recognition of this species of Ptychopleurella permits comparison with congeners elsewhere, strengthens the correlation of this part of the Girvan Succession with the middle Ordovician of North America, and provides a more complete record of this genus in the slope sedimentary facies of the northwestern margin of the Iapetus Ocean.
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Brower, James C. "Carabocrinid crinoids from the Ordovician of northern Iowa and southern Minnesota." Journal of Paleontology 70, no. 4 (July 1996): 614–31. http://dx.doi.org/10.1017/s0022336000023593.
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Four species of carabocrinids from the Ordovician of northern Iowa and southern Minnesota are described, namelyCarabocrinus radiatusE. Billings,C. vancortlandtiE. Billings,C. magnificusSardeson from the Middle Ordovician Dunleith Formation andC. slocomiFoerste from the Upper Ordovician Maquoketa Formation.Carabocrinus radiatusandC. vancortlandtiare also known from the Middle Ordovician of Ontario and Quebec. In addition,C. magnificusandC. vancortlandtiare recorded from the Decorah of the Twin Cities area and the Curdsville Limestone of Kentucky, respectively. Biogeographically, the Middle Ordovician carabocrinids from Iowa and Minnesota are most similar to those from rocks of similar age in the northern Appalachians.Development of the cup and its component plates inC. slocomiis almost entirely isometric so that the its shape is largely constant regardless of size. This species exhibits ridge canals on the shoulders of the radial plates in the cup. The ridge canals probably served for respiration. As expected, the number of ridge canals and their length increase with positive allometry compared to the size and volume of the cup. Growth of the ridge canals restricts the width of the radial facets.
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Rohr, David M. "Ordovician (Whiterockian) gastropods of Nevada: Bellerophontoidea, Macluritoidea, and Euomphaloidea." Journal of Paleontology 68, no. 3 (May 1994): 473–86. http://dx.doi.org/10.1017/s0022336000025865.
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Gastropods are abundant in the Middle Ordovician (Whiterockian) Antelope Valley Formation of Nevada. Because Whiterockian rocks are absent in much of central and eastern North America, these Nevada gastropods play a significant role in understanding the Early to Middle Ordovician transition of the class. The shell and operculum of a new genus and species of macluritoid, Monitorella auricula, is described. New euomphaloid taxa include Walcottoma frydai n. gen. and sp., Rossospira harrisae n. gen. and sp., Barnesella measuresae n. sp., Helicotoma gubanovi n. sp., Lytospira yochelsoni n. sp., and Malayaspira hintzei n. sp.
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Pang, Bo, Junqing Chen, Xiongqi Pang, Tuo Liu, Haijun Yang, Hui Li, Yingxun Wang, and Tao Hu. "Possible new method to discriminate effective source rocks in petroliferous basins: A case study in the Tazhong area, Tarim Basin." Energy Exploration & Exploitation 38, no. 2 (September 11, 2019): 417–33. http://dx.doi.org/10.1177/0144598719871414.
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Sediments with organic matter content (total organic carbon) TOC ≤ 0.5% which can act as effective source rocks are critical and challenging in the field of petroleum geology. A new method is proposed through a case study to identify and evaluate the effective source rocks, which is applied to study the changing characteristic of hydrocarbon-generation potential index with depth. The burial depth corresponding to the beginning of hydrocarbon-generation potential index reduction represents the hydrocarbon expulsion threshold in source rocks. Then, new identification standards are established for discrimination of effective source rocks of Middle–Upper Ordovician in Tarim Basin. The critical value of TOC for effective source rocks change with their burial depth: the TOC > 0.5% with source rock depth > 4000 m, TOC > 0.4% with depth >4500 m, TOC > 0.3% with depth > 5000 m, TOC > 0.2% with depth >5500 m. Based on the new criteria, effective source rocks in the Middle–Upper Ordovician are identified and their total potential hydrocarbon resources is evaluated, reaches 0.68 × 109 t in the Tazhong area, which is 65.4% higher than that of previous studies and consistent with the exploration result. Thus, this new method is of significance to resource evaluation and can be applied in the carbonate source rocks and mudstone source rocks with high degree of exploration.
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Dean, W. T., and O. Monod. "Revised stratigraphy and relationships of Lower Palaeozoic rocks, eastern Taurus Mountains, south central Turkey." Geological Magazine 127, no. 4 (July 1990): 333–47. http://dx.doi.org/10.1017/s0016756800014898.
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AbstractLithostratigraphic terminology is revised for rocks of Ordovician age in the eastern Taurus. The Seydişehir Formation, of late middle Cambrian to early Ordovician age, is shown to be applicable in the eastern and western Taurus as well as in south-eastern Turkey. Near Degˇirmentaş, northeast of Adana, the Seydişehir Formation is overlain, with inferred unconformity, by clastic strata of Ashgill age referred to the Şort Tepe Formation, first defined south of Hakkâri, in the Border Folds of southeastern Turkey. Palaeontological evidence is cited for the early Silurian (Llandovery) age of the unconformably overlying Halityayla Formation farther south, on the coast near Ovacik, where the Şort Tepe Formation is as yet unrecorded. A general model is proposed showing relationships of Cambrian and Ordovician rocks and faunas in southern and southeastern Turkey.
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Ryazantsev, А. V. "Ordovician gabbro-tonalite-trondhjemite complex and associated volcanic rocks in the paleozoic suture between Urals and Kazakhstan." Доклады Академии наук 485, no. 2 (May 20, 2019): 202–6. http://dx.doi.org/10.31857/s0869-56524852202-206.
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This paper reports new data on the Early Ordovician age established for granitoids of the gabbro-tonalite-trond- hjemite complex in the Denisovka ophiolite zone (suture) on the boundary between the Paleozoic structures (Paleozoides) of the Urals and Kazakhstan. The ages of 482.6 ± 3.6 Ma and 486.2 ± 6.7 Ma were established by the U/Pb method (SHRIMP II) on the age data for zircon extracted from granitoids. The plutonic complex associates with the Early-Middle Ordovician effusive complex, composed of a series of rocks differentiated from basalts to rhyolites. The volcano-plutonic association was formed in the supra-subduction settings, as is evidenced by the composition of the rocks.
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Key, Marcus M. "The halloporid trepostome bryozoans from the Ordovician Simpson Group of Oklahoma." Journal of Paleontology 65, no. 2 (March 1991): 200–212. http://dx.doi.org/10.1017/s0022336000020436.
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The Bromide Formation of the Middle Ordovician Simpson Group of Oklahoma contains one of the oldest diverse bryozoan faunas in North America. The early divergence of many trepostome clades is revealed in these rocks. Three trepostome bryozoan species belonging to family Halloporidae are described from this fauna. Discriminant analysis is used to define the following halloporid species: Diplotrypa schindeli n. sp., Tarphophragma karklinsi n. sp., and Tarphophragma macrostoma (Loeblich). Preliminary cladistic analysis indicates that the family Halloporidae was already a distinct lineage by the Middle Ordovician. This suggests that by this time, many of the major trepostome clades were already established.
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Zhemchugova, T. A. "CHARACTERISTICS OF THE OIL DEPOSITS FORMATION ON THE TERRITORY OF THE VARANDEY-ADZVINSKAYA OIL AND GAS REGION." Вестник Пермского университета. Геология 21, no. 2 (2022): 167–72. http://dx.doi.org/10.17072/psu.geol.21.2.167.
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The structure, oil and gas potential of the Varandey-Adzvinskaya oil and gas region is considered in the article. Physicochemical properties and composition of oil and source rocks of the structural zone are analysed. Based on the determined features of the oil deposits formation of study area, 3 oil systems have been identified: Middle Ordovician-Lower Devonian, Middle Devonian-Lower Fransian-Timanian and Middle Frasnian-Lower Triassic, which are combined in the General geological section of the Varandey-Adzvinskaya zone. In each system, a connection between oils of various types with oil source rocks was established. The connection of different type oil with source rocks was established that made it possible to predict the migration paths of hydrocarbons for new oil and gas deposits exploration.
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BOTTING, JOSEPH P. "EXCEPTIONALLY WELL-PRESERVED MIDDLE ORDOVICIAN SPONGES FROM THE LLANDEGLEY ROCKS LAGERSTATTE, WALES." Palaeontology 48, no. 3 (May 2005): 577–617. http://dx.doi.org/10.1111/j.1475-4983.2005.00470.x.
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West Jr., David P., Heather M. Beal, and Timothy W. Grover. "Silurian deformation and metamorphism of Ordovician arc rocks of the Casco Bay Group, south-central Maine." Canadian Journal of Earth Sciences 40, no. 6 (June 1, 2003): 887–905. http://dx.doi.org/10.1139/e03-021.
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The Casco Bay Group in south-central Maine consists of a sequence of Late Cambrian to Early Ordovician interlayered quartzofeldspathic granofels and pelite (Cape Elizabeth Formation) overlain by Early to Late Ordovician back-arc volcanic (Spring Point Formation) and volcanogenic sedimentary rocks (Diamond Island and Scarboro formations). These rocks were tightly folded and subjected to low-pressure amphibolite-facies metamorphism in the Late Silurian. This phase of deformation and metamorphism was followed by the development of a variety of structures consistent with a period of dextral transpression in Middle Devonian Early Carboniferous time. Previously dated plutons within the sequence range in age from 422389 Ma and record a period of prolonged intrusive activity in the region. Similarities in age, volcanic rock geochemistry, and lithologic characteristics argue strongly for a correlation between rocks of the Casco Bay Group and those in the Miramichi belt of eastern Maine and northern New Brunswick. The Cape Elizabeth Formation correlates with Late Cambrian to Early Ordovician sediments of the Miramichi Group (Gander Zone) and the Spring Point through Scarboro formations correlate with Early to Late Ordovician back-arc basin volcanics and volcanogenic sediments of the Bathurst Supergroup. The folding and low-pressure metamorphism of the Casco Bay Group is attributed to Late Silurian to Early Devonian terrane convergence and possible lithospheric delamination that would have resulted in a prolonged period of intrusive activity and elevated temperatures at low pressures. Continued convergence and likely plate reconfigurations in the Middle Devonian to Carboniferous led to widespread dextral transpression in the region.
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Williams, Harold, and M. A. J. Piasecki. "The Cold Spring Melange and a possible model for Dunnage–Gander zone interaction in central Newfoundland." Canadian Journal of Earth Sciences 27, no. 8 (August 1, 1990): 1126–34. http://dx.doi.org/10.1139/e90-117.
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Structural relationships at Cold Spring Pond and the recognition of ophiolitic melange bear on the important questions of timing and style of structural superpositioning of Dunnage Zone rocks above Gander Zone rocks in central Newfoundland. The latest models emphasize ductile shear boundaries and orogen-parallel movements. Previous models proposed west-to-east or head-on obduction of Dunnage ophiolitic rocks across the Gander Zone.At the Dunnage (Exploits Subzone) – Gander (Meelpaeg Subzone) boundary at Cold Spring Pond, discrete, outcrop-size ultramafic blocks and smaller quartzite blocks are randomly distributed, and they are surrounded by, or are embedded in, homogeneous black graphitic shale or phyllite. The ultramafic blocks are typical of nearby Early Ordovician Dunnage ophiolite suites, the quartzite blocks are typical of adjacent Early Ordovician or earlier Gander clastic rocks, and the matrix black shales are similar to those of Middle or Early Ordovician age that occur throughout central Newfoundland. This chaotic mixture of almost coeval lithologies at Cold Spring Pond is interpreted as an olistostromal melange; the Cold Spring Melange. It resembles melanges that are dated as Ordovician elsewhere in Newfoundland.The Cold Spring Melange is overprinted by the full range of structures and metamorphic effects evident in adjacent rocks of the Exploits (Dunnage) and Meelpaeg (Gander) subzones. These include the development of lineations, cleavages, schistosities, zones of ductile shearing, regional metamorphism, and contact metamorphism. The oldest of these effects are interpreted as Silurian, based on isotopic dating in southern Newfoundland.The formation of olistostromal, ophiolitic melange implies disruption of the oceanic tract (Exploits Subzone of the Dunnage Zone), and in the case of the Cold Spring example, juxtapositioning or transport of Exploits Subzone ophiolite suites against or across the supracrustal rocks of the Meelpaeg Subzone (Gander Zone). The age and provenance of Cold Spring components, lack of post-Ordovician components, overprinting structural relationships, and comparison with other Newfoundland melanges all support an Ordovician age of formation. Overprinting relationships indicate that major ductile shears at other Dunnage–Gander zone boundaries postdate initial Dunnage–Gander superpositioning.
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Mao, Qigui, Jingbin Wang, Wenjiao Xiao, Brian F. Windley, Karel Schulmann, Songjian Ao, Mingjing Yu, Ji’en Zhang, and Tonghui Fang. "From Ordovician nascent to early Permian mature arc in the southern Altaids: Insights from the Kalatage inlier in the Eastern Tianshan, NW China." Geosphere 17, no. 2 (February 5, 2021): 647–83. http://dx.doi.org/10.1130/ges02232.1.
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Abstract The Kalatage inlier in the Dananhu-Haerlik arc is one of the most important arcs in the Eastern Tianshan, southern Altaids (or Central Asian orogenic belt). Based on outcrop maps and core logs, we report 16 new U-Pb dates in order to reconstruct the stratigraphic framework of the Dananhu-Haerlik arc. The new U-Pb ages reveal that the volcanic and intrusive rocks formed in the interval from the Ordovician to early Permian (445–299 Ma), with the oldest diorite dike at 445 ± 3 Ma and the youngest rhyolite at 299 ± 2 Ma. These results constrain the ages of the oldest basaltic and volcaniclastic rocks of the Ordovician Huangchaopo Group, which were intruded by granite-granodiorite-diorite plutons in the Late Ordovician to middle Silurian (445–426 Ma). The second oldest components are intermediate volcanic and volcaniclastic rocks of the early Silurian Hongliuxia Formation (S1h), which lies unconformably on the Huangchaopo Group and is unconformably overlain by Early Devonian volcanic rocks (416 Ma). From the mid- to late Silurian (S2-3), all the rocks were exhumed, eroded, and overlain by polymictic pyroclastic deposits. Following subaerial to shallow subaqueous burial at 416–300 Ma by intermediate to felsic volcanic and volcaniclastics rocks, the succession was intruded by diorites, granodiorites, and granites (390–314 Ma). The arc volcanic and intrusive rocks are characterized by potassium enrichment, when they evolved from mafic to felsic and from tholeiitic via transitional and calc-alkaline to final high-K calc-alkaline compositions with relatively low initial Sr values, (87Sr/86Sr)i = 0.70391–0.70567, and positive εNd(t) values, +4.1 to +9.2. These new data suggest that the Dananhu-Haerlik arc is a long-lived arc that consequently requires a new evolutionary model. It began as a nascent (immature) intra-oceanic arc in the Ordovician to early Silurian, and it evolved into a mature island arc in the middle Silurian to early Permian. The results suggest that the construction of a juvenile-to-mature arc, in combination with its lateral attachment to an incoming arc or continent, was an important crustal growth mechanism in the southern Altaids.
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Rigby, J. Keith, and A. W. Potter. "Ordovician Sphinctozoan Sponges from the Eastern Klamath Mountains, Northern California." Journal of Paleontology 60, S20 (July 1986): 1–47. http://dx.doi.org/10.1017/s0022336000060820.
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Extensive silicified faunules of Middle and Late Ordovician sphinctozoan sponges have been assembled from the northern part of the eastern Klamath Mountains in northern California. The sponges are from eugeosynclinal rocks that are the westernmost Middle Ordovician to Late Devonian rocks at that latitude in North America. Seventeen new species occur in the assemblages, including 10 porate and 7 aporate forms. New genera of porate forms areAmblysiphonelloidesandCorymbospongia, and new porate species include:Amblysiphonella grossa, Amblysiphonelloides tubula, A. reticulata, Imperatoria mega, I. media, I. minima, I. irregularis, Corymbospongia adnata, C. mica, andC.(?)perforata.These are the first reported occurrences ofAmblysiphonellaandImperatoriain the Ordovician. New aporate genera areCystothalamiellaandPorefieldia, and new aporate species include:Cystothalamiella ducta, C. craticula, C. tuboides, Porefieldia robusta, Girtyocoelia epiporata, andG. canna.This is the oldest known occurrence ofGirtyocoelia, which is generally considered a Late Paleozoic form.Cliefdenella obconican. sp. is characterized by an obconical growth form in contrast to other species of the genus that are more massive or explanate.Cliefdenellais considered here as an imperforate sphinctozoan.Minor isolated hexactines and hexactine-derived spicules of Hexactinellida were associated with the sphinctozoans. No particular taxa within the class can be distinguished from these individual elements.
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Trettin, H. P. "Pearya: a composite terrane with Caledonian affinities in northern Ellesmere Island." Canadian Journal of Earth Sciences 24, no. 2 (February 1, 1987): 224–45. http://dx.doi.org/10.1139/e87-025.
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In Ellesmere Island, the Canadian Shield and Arctic Platform are flanked on the northwest by the lower Paleozoic Franklinian mobile belt, which comprises an unstable shelf (miogeocline) and a deep-water basin, divisible into an inner sedimentary belt and an outer sedimentary–volcanic belt. Both are tied to the shelf by interlocking facies changes, but additional exotic units may be present in the outer belt.Pearya, bordering the deep-water basin on the northwest, is divisible into four successions. Succession I comprises sedimentary and(?) volcanic rocks, deformed, metamorphosed to amphibolite grade, and intruded by granitic plutons at 1.0–1.1 Ga. Succession II consists mainly of platformal sediments (carbonates, quartzite, mudrock), with smaller proportions of mafic and siliceous volcanics, diamictite, and chert ranging in age from Late Proterozoic (Hadrynian) to latest Cambrian or Early Ordovician. Its concealed contact with succession I is tentatively interpreted as an angular unconformity. Succession III (Lower to Middle Ordovician?) includes arc-type and ocean-floor volcanics, chert, mudrock, and carbonates and is associated with fault slices of Lower Ordovician (Arenig) ultramafic–mafic complexes–possibly dismembered ophiolites. The faulted contact of succession III and the ultramafics with succession II is unconformably overlapped by succession IV, 7–8 km of volcanic and sedimentary rocks ranging in age from late Middle Ordovician (Blackriverian = early Caradoc) to Late Silurian (late Ludlow?). The angular unconformity at the base of succession IV represents the early Middle Ordovician (Llandeilo–Llanvirn) M'Clintock Orogeny, which was accompanied by metamorphism up to amphibolite grade and granitic plutonism. Pearya is related to the Appalachian–Caledonian mobile belt by the Grenville age of its basement, the age of its ultramafic–mafic complexes, and evidence for a Middle Ordovician orogeny, comparable in age and character to the Taconic. By contrast, the Franklinian mobile belt has a Lower Proterozoic (Aphebian) – Archean basement and was not deformed in the Ordovician. Stratigraphic–structural evidence suggests that Pearya was transported by sinistral strike slip as three or more slices and accreted to the Franklinian deep-water basin in the Late Silurian under intense deformation. The inferred sinistral motion is compatible with derivation from the northern Caledonides.
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David, Martin, and Petr Budil. "Complementary Description of the Middle Ordovician Trilobite Associations at Praha-Vokovice." Folia Musei rerum naturalium Bohemiae occidentalis. Geologica et Paleobiologica 49, no. 1-2 (December 1, 2015): 1–7. http://dx.doi.org/10.1515/fbgp-2015-0001.
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AbstractThe temporarily exposed rocks of the Dobrotivá Formation (Middle Ordovician, upper Darriwilian) accessible, due to lower water levels, in Džbán water reservoir (Praha-Vokovice area) have produced common fauna. The newly obtained trilobite-dominated fossil associations enable a more detailed discussion of faunal changes in the lower and lower-middle portions of the Dobrotivá Formation - a transition from the Placoparia Association to the newly recognized Degamella-Zeliszkella Association, dominated by mesopelagic forms.
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Pinet, Nicolas, Sébastien Castonguay, and Alain Tremblay. "Thrusting and back thrusting in the Taconian internal zone, southern Quebec Appalachians." Canadian Journal of Earth Sciences 33, no. 9 (September 1, 1996): 1283–93. http://dx.doi.org/10.1139/e96-097.
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Cambro-Ordovician continental-margin rocks of the Humber zone of the Quebec Appalachians were mainly deformed during the Taconian orogeny (Middle Ordovician to Early Silurian). Two Taconian deformational events are recorded west of the Sutton–Notre-Dame mountains anticlinorium axis. They are characterized, respectively, by northwest-directed faulting and synmetamorphic folding (D1−2) and by southeastward back-thrusting motion (D3); the latter deformation has previously been poorly documented in the Quebec Appalachians. This duality of structural vergence is probably induced by the progressive tectonic wedging of basement rocks during a nearly constant northwest–southeast Taconian shortening. In correlative higher grade metamorphic rocks of New England, back-thrusting structures (D3) have not been described and are most probably absent because their root zone is located well above the present-day erosional surface of that part of the Appalachian belt.
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Chen, Jun-Qing, Xiong-Qi Pang, Song Wu, Zhuo-Heng Chen, Mei-Ling Hu, Luo-Fu Liu, Kui-You Ma, Bo Pang, and Zhi-Peng Huo. "Method for identifying effective carbonate source rocks: a case study from Middle–Upper Ordovician in Tarim Basin, China." Petroleum Science 17, no. 6 (September 19, 2020): 1491–511. http://dx.doi.org/10.1007/s12182-020-00489-z.
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AbstractHydrocarbon expulsion occurs only when pore fluid pressure due to hydrocarbon generation in source rock exceeds the force against migration in the adjacent carrier beds. Taking the Middle–Upper Ordovician carbonate source rock of Tarim Basin in China as an example, this paper proposes a method that identifies effective carbonate source rock based on the principles of mass balance. Data from the Well YW2 indicate that the Middle Ordovician Yijianfang Formation contains effective carbonate source rocks with low present-day TOC. Geological and geochemical analysis suggests that the hydrocarbons in the carbonate interval are likely self-generated and retained. Regular steranes from GC–MS analysis of oil extracts in this interval display similar features to those of the crude oil samples in Tabei area, indicating that the crude oil probably was migrated from the effective source rocks. By applying to other wells in the basin, the identified effective carbonate source rocks and non-source rock carbonates can be effectively identified and consistent with the actual exploration results, validating the method. Considering the contribution from the identified effective source rocks with low present-day TOC (TOCpd) is considered, the long-standing puzzle between the proved 3P oil reserves and estimated resources in the basin can be reasonably explained.
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Sobolev, I. D., A. N. Shadrin, V. A. Rastorguev, and D. A. Kozyreva. "Primitive islandarc granitoids of the Schuchinskaya zone, the Polar Urals (SIMS U-Pb zircon dating)." Moscow University Bulletin. Series 4. Geology, no. 1 (February 28, 2017): 22–32. http://dx.doi.org/10.33623/0579-9406-2017-1-22-32.
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In the Schuchinskaya Zone of the Polar Urals granitoids of the Rechnoy and Yalya-Pe paleovolcanoes have been studied. They were mapped as Khoimpeysky Complex of Silurian age. In addition, granitoids of the Nganotsky-1 and Nganotsky-2 plutons mapped as Yunyaginsky Complex of Early Devonian age have been investigated. It was found that based on the mineral and chemical composition the rocks of all plutons studied correspond to island arc I-type granitoids. U-Pb (SIMS) concordant ages of zircons from granitoids of the Rechnoy and Yalya-Pe paleovolcanoes, and of the Nganotsky-1 pluton are 456±6, 454±4 and 463±3 Ma, respectively, which implies the existence of an island arc in the Schuchinskaya Zone as early as the Middle-Late Ordovician. Establishing the age of granitoids allows to refer volcanic rocks cut by plutons to Syadayskaya Formation, and to clarify the upper stratigraphic limit of its sedimentation as Middle-Upper Ordovician.
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Williams, S. Henry, Elliott T. Burden, and P. K. Mukhopadhyay. "Thermal maturity and burial history of Paleozoic rocks in western Newfoundland." Canadian Journal of Earth Sciences 35, no. 11 (November 1, 1998): 1307–22. http://dx.doi.org/10.1139/e98-045.
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Palynomorphs and graptolites from Paleozoic strata in western Newfoundland are examined and correlated with previously published data to identify fossils which are characteristic of proven and suspected source rocks. Measurements of colour alteration of acritarchs and spores (acritarch alteration index and thermal alteration index), random graptolite reflectance, and vitrinite reflectance are applied to determine regional thermal maturation and burial history. General trends of increasing maturity from south to north along the Northen Peninsula and from west to east across the Port au Port Peninsula are observed. Within these general trends, a more detailed distribution of thermal maturities can be recognized. In the south, Upper Ordovician rocks of the Long Point Group, western Port au Port Peninsula, exhibit the lowest maturity values found in western Newfoundland and are considered immature or marginally mature. Middle Ordovician rocks of the Goose Tickle and Table Head groups and the Lower Ordovician St. George Group are marginally mature. Cambrian strata on the Port au Port Peninsula are mature. Maturation levels increase to the east; Goose Tickle Group black shales in the vicinity of Black Cove, east of Port au Port, are mature. Equivalent sediments extending for another 15-20 km to the east lie within the oil window. Beyond that area, the equivalent rocks are overmature. The best potential source rocks belonging to the allochthonous Cow Head Group contain abundant acritarchs and Gloeocapsamorpha sp. These rocks are marginally mature to mature within Gros Morne National Park; maturation levels increase farther north (e.g., Parsons Pond), becoming overmature somewhere south of Port au Choix. It is concluded that neither the allochthonous Ordovician rocks presently exposed in Gros Morne nor the autochthonous strata exposed on the Port au Port Peninsula have ever been covered by significant thicknesses of overburden (probably 3 km or less), either in the form of structural slices or other sedimentary units since their original deposition.
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Draper, J. "GEORGINA BASIN—AN EARLY PALAEOZOIC CARBONATE PETROLEUM SYSTEM IN QUEENSLAND." APPEA Journal 47, no. 1 (2007): 107. http://dx.doi.org/10.1071/aj06006.
Full textAbstract:
Queensland contains a number of carbonate-bearing basins which are under-explored for petroleum, but contain the elements of potentially economic petroleum systems. The oldest such basin is the Neoproterozoic to Ordovician Georgina Basin which straddles the Queensland-Northern Territory border and is traversed by the Ballera to Mount Isa gas pipeline.The basin developed across several major crustal blocks resulting in regional variations in deposition and deformation. Thick Neoproterozoic rocks of the Centralian Superbasin form the base of the sequence in apparently fault-bounded, extensional sub-basins. These rocks are generally tight and source rocks are unknown. The Cambrian to Ordovician rocks have the best petroleum potential with the most prospective part of the basin being the Toko Syncline. The Burke River Structural Belt is less prospective, but is worthy of further exploration. Basin fill consists of Cambrian and Early Ordovician rocks which are dominantly carbonates, with both limestones and dolostones present. In the Early to Middle Ordovician, the rocks became predominantly siliciclastic.The main phase of deformation affecting the Georgina Basin occurred in the Devonian as part of the Alice Springs Orogeny. The Toomba Fault, which forms the western boundary of the asymmetric Toko Syncline, is a thrust fault with up to 6.5 km of uplift. The angle of thrusting is between less than 40 degrees and up to 70 degrees. Rich, marine source rocks of Middle Cambrian age in the Toko Syncline are mature for oil except in the deepest part of the syncline where they are mature for dry gas. The deeper part of the Toko Syncline may be gas saturated.Potential hydrocarbon targets include large folds associated with fault rollovers, stratigraphic traps and faultbounded traps. Vugular, secondary porosity in dolostones offers the best chance for commercial reservoirs within the Ninmaroo and Kelly Creek formations and Thorntonia Limestone. There are also oolitic carbonates which may have good primary porosity, as well as interbedded sandstones in the carbonates with preserved porosity. Structurally controlled hydrothermal dolomite facies represent potential reservoirs. The dominantly siliciclastic Ordovician sequence is water flushed. Fracture porosity is another possibility (cf. the Palm Valley gas field in the Amadeus Basin). As the deeper part of the Toko Syncline appears to be gas saturated, there may be potential for basin-centred gas. Fine-grained carbonates and shales provide excellent seals. There has not been a valid structural test; although AOD Ethabuka–1 flowed 7,000 m3/d of dry gas, the well was abandoned short of the target depth.
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Pojeta Jr., John, and Christopher A. Stott. "Nucularcidae: a new family of palaeotaxodont Ordovician pelecypods (Mollusca) from North America and Australia." Canadian Journal of Earth Sciences 44, no. 10 (October 1, 2007): 1479–501. http://dx.doi.org/10.1139/e07-028.
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The new Ordovician palaeotaxodont family Nucularcidae and the new genus Nucularca are described. Included in Nucularca are four previously described species that have taxodont dentition: N. cingulata (Ulrich) (the type species), N. pectunculoides (Hall), N. lorrainensis (Foerste), and N. gorensis (Foerste). All four species are of Late Ordovician (Cincinnatian Katian) age and occur in eastern Canada and the northeastern USA. Ctenodonta borealis Foerste is regarded as a subjective synonym of Nucularca lorrainensis. No new species names are proposed. The Nucularcidae includes the genera Nucularca and Sthenodonta Pojeta and Gilbert-Tomlinson (1977). Sthenodonta occurs in central Australia in rocks of Middle Ordovician (Darriwilian) age. The 12 family group names previously proposed for Ordovician palaeotaxodonts having taxodont dentition are reviewed and evaluated in the Appendix.
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WEST, DAVID P., RAYMOND A. COISH, and PAUL B. TOMASCAK. "Tectonic setting and regional correlation of Ordovician metavolcanic rocks of the Casco Bay Group, Maine: evidence from trace element and isotope geochemistry." Geological Magazine 141, no. 2 (March 2004): 125–40. http://dx.doi.org/10.1017/s0016756803008562.
Full textAbstract:
Ordovician metamorphic rocks of the Casco Bay Group are exposed in an approximately 170 km long NE-trending belt (Liberty-Orrington belt) in southern and south-central Maine. Geochemical analysis of rocks within the Spring Point Formation (469±3 Ma) of the Casco Bay Group indicate that it is an assemblage of metamorphosed bimodal volcanic rocks. The mafic rocks (originally basalts) have trace element and Nd isotopic characteristics consistent with derivation from a mantle source enriched by a crustal and/or subduction component. The felsic rocks (originally rhyolites and dacites) were likely generated through partial melting of continental crust in response to intrusion of the mafic magma. Relatively low initial εNd values for both the mafic (−1.3 to +0.6) and felsic (−4.1 to −3.8) rocks suggest interactions with Gander zone continental crust and support a correlation between the Casco Bay Group and the Bathurst Supergroup in the Miramichi belt of New Brunswick. This correlation suggests that elements of the Early to Middle Ordovician Tetagouche-Exploits back-arc basin can be traced well into southern Maine. A possible tectonic model for the evolution of the Casco Bay Group involves the initiation of arc volcanism in Early Ordovician time along the Gander continental margin on the eastern side of the Iapetus Ocean basin. Slab rollback and trenchward migration of arc magmatism initiated crustal thinning and rifting of the volcanic arc around 470 Ma and resulted in the eruption of the Spring Point volcanic rocks in a back-arc tectonic setting.
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Shang, Zhi, and Yongqing Chen. "Petrogenesis and Tectonic Implications of Early Paleozoic Magmatism in Awen Gold District, South Section of the Truong Son Orogenic Belt, Laos." Minerals 12, no. 8 (July 22, 2022): 923. http://dx.doi.org/10.3390/min12080923.
Full textAbstract:
The Truong Son orogenic belt (TSOB) is one of the most important orogenic belts in the Indochina block. There are numerous mafic to felsic intrusions in the Early Paleozoic caused by the Tethyan orogeny. However, the tectono-magmatic evolution of the TSOB in the Early Paleozoic is still unclear. In this paper, zircon U-Pb dating, whole-rock geochemistry, and the Sr-Nd isotopic data of the Early Paleozoic magmatic rocks have been systematically investigated to explore the petrogenesis and tectonic significance of these rocks in the TSOB. Based on our new results integrated with previous geological data, four major tectono-magmatic episodes are identified. (1) The Middle Cambrian (~507 Ma) is the early stage of northward subduction of the Tamky-Phuoc Son Ocean. (2) The Early Ordovician to Middle Ordovician (483–461 Ma) is the main subduction stage of the Tamky-Phuoc Son Ocean. The intrusive rock associations imply the closure of the Tamky-Phuoc Son Ocean. (3) The Late Ordovician to Early Silurian (461–438 Ma) is the collision stage of the Kontum massif and Truong Son terrane. (4) The Early Silurian to Late Silurian (438–410 Ma) is the late stage of collision accompanied by slab roll-back.
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Shkol’nik, S. I., A. V. Ivanov, L. Z. Reznitsky, E. F. Letnikova, Hyaiyu He, Y. Zhiqlang, Youjueen Li, I. A. Vishnevskaya, and I. G. Barash. "Middle Ordovician effusive rocks of the Khamsara terrane (Tyva) as an indicator complex." Russian Geology and Geophysics 58, no. 9 (September 2017): 1032–44. http://dx.doi.org/10.1016/j.rgg.2017.08.002.
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Costamagna, Luca Giacomo, Franco Marco Elter, Laura Gaggero, and Federico Mantovani. "Contact metamorphism in Middle Ordovician arc rocks (SW Sardinia, Italy): New paleogeographic constraints." Lithos 264 (November 2016): 577–93. http://dx.doi.org/10.1016/j.lithos.2016.09.014.
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Renbin, Zhan, Jin Jisuo, Rong Jiayu, and Liang Yan. "The earliest known strophomenoids (Brachiopoda) from early Middle Ordovician rocks Of South China." Palaeontology 56, no. 5 (September 2013): 1121–48. http://dx.doi.org/10.1111/pala.12039.
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Samygin, S. G. "Formation of the rear slope of the island arc of Chingiz Kaledonid ridge in eastern Kazakhstan." Геотектоника, no. 2 (April 17, 2019): 72–80. http://dx.doi.org/10.31857/s0016-853x2019272-80.
Full textAbstract:
Process of formation of the island-arc rear slope is considered on the example of the Upper Cambrian–Middle Ordovician arc found in the Chingiz ridge in eastern Kazakhstan. Its occurrence is shown at the end of volcanic activity in the island-arc structure, beginning at the end of the early Arenig (from the end of the Flos century of the Early Ordovician) with tephroturbidites appearance. After the cessation of volcanism, two sedimentation cycles were distinguished in the sedimentary section of the slope in the middle Ordovician: (1) transgressive when the island arc submerged, (2) and regressive when the Chingiz arc began to build up at the beginning of the Llanwyrn (Darrivilian) century.
The sedimentation was repeatedly accompanied by landslide processes, which ended in the middle of llanvirna (darrivilia) with the disruption of tectonic-gravity plate composed of Upper Cambrian volcanic rocks with limestone in the sole, resulting in the formation of coarsely fragmented mixtite at the allochthonous mass frontier, the further sedimentation on the rear slope stopped.
Keywords: the island-arc; rear slope; sedimentation cycles; landslide processes
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Fortey, Richard A., and Mary L. Droser. "Trilobites from the base of the type Whiterockian (Middle Ordovician) in Nevada." Journal of Paleontology 73, no. 2 (March 1999): 182–201. http://dx.doi.org/10.1017/s0022336000027712.
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No trilobite species, and very few genera, pass from the Lower (Ibexian) into the Middle Ordovician (Whiterockian), which is a turning point in Laurentian trilobite history. Trilobites from three sections exposing the base of the Whiterock Series (basal Middle Ordovician) in Nevada are described and illustrated. They are attributable to different, and generally more open-shelf, biofacies from the Bathyurid biofacies trilobites described from the Ibex area, Utah, by Fortey and Droser (1996), but include species in common, which allow correlation into the Ibexian type section. At Little Rawhide Mountain the basal Middle Ordovician is developed in Olenid biofacies, described for the first time in western North America from rocks of this age. Correlation based on species-level similarity shows that the “spike” that has been used to define the type base of the Whiterockian (and hence the Middle Ordovician) at Whiterock Canyon is at a level younger than the base of the Whiterockian assumed in recent discussions of its international correlation. The type Whiterock base correlates with the Psephosthenaspis glabrior trilobite Subzone at Ibex, well above the major change in trilobite faunas at the base of the Psephosthenaspis Zone (P. microspinosa Subzone). In all study sections there is an abrupt change of facies after trilobite Zone J, possibly associated with regression. Four new species are described: Cloacaspis tesselata, Harpillaenus rossi, Acidiphorus? lineotuberculatus, and Benthamaspis serus.