Devonian Time Research Articles

New data on the Silurian–Devonian palaeontology and biostratigraphy of Bolivia

In order to precisely establish the bio- and chronostratigraphic position of several levels of the Silurian-Devonian succession of Bolivia, with respect to the International Time Scale, additional Silurian and Devonian localities belonging to the Eastern Cordillera and the Interandean Zone areas have been recently revised and sampled, both for macrofossils and palynomorphs. Specifically, the localities are Nunumayani, and Muruhuta from the late Silurian-early Devonian time interval. Correlations with other fossiliferous localities are discussed, namely Huacallani and Rumicorral. A new outcrop yielding Middle Devonian tabulate corals at Pisacavina is described; its faunal content is described, and compared with the historical coral collection. As a result, it appears clearly that the Silurian-Devonian Bolivian fauna needs a thorough revision, as well as the selection of stratigraphically significant macrofossils. Besides macrofossils, the analysis of palynomorph assemblages is significant in establishment of both intra-Bolivian and international correlations before any reliable biogeographic reconstruction is done. This paper records the first discovery of a plant of the genus Protolepidodendron from the Devonian of Bolivia. An ostracod assemblage from Pisacavina is described, illustrated, and discussed for the first time; it suggests an Early Devonian or an Eifelian age. Newly described taxa include the coral Parastriatopora boliviana sp. nov., and the chonetoidean brachiopods Sanjuanetes glemareci sp. nov., and Kentronetes giae sp. nov.

Onshore–offshore potential field analysis of the Møre–Trøndelag Fault Complex and adjacent structures of Mid Norway

The Møre–Trøndelag Fault Complex (MTFC) is one of the most prominent fault complexes of Scandinavia. It exerted a strong control on the development of offshore basins of Mid Norway and onshore topography. However, the relationships between the faults observed in the field onshore and those identified by means of seismic profiling offshore remain obscure. Regional gravity and aeromagnetic data are used to map regional-scale faults and, in particular, to delineate the main geophysical features related to the MTFC. The advantage of potential field data is to provide an almost continuous coverage and to tie bedrock mapping onshore to seismic interpretation offshore. In this paper, we apply potential field transformations to focus on the regional and deep-seated structures in order to extract new geological information. Also the tilt derivative technique (TDR) is applied to gravity and magnetic data with the aim of enhancing linear trends. The results indicate the possible onshore–offshore links of large scale structural elements like the MTFC and late-Caledonian detachments (e.g. Kollstraumen Detachment). The locations of different segments of the MTFC are detected and possible new faults/lineaments are depicted. Correlating petrophysical data with gravity and magnetic maps explains the influence of the MTFC on the deformation and mineralisation of bedrock along its strike. In addition, the structural pattern seen in the enhanced lineaments is diagnostic for the sinistral strike–slip movements that are known to have occurred in Devonian time along the MTFC. This confirms the important role of the MTFC in the tectonic setting and geological evolution of Mid Norway.

Geodynamic interpretation of the 40Ar/39Ar dating of ophiolitic and arc-related mafics and metamafics of the northern part of the Anadyr-Koryak region

Isotope datings of amphibole-bearing mafics and metamafics in the northern part of the Anadyr-Koryak region allow clarification of the time of magmatic and metamorphic processes, which are synchronous with certain stages of the geodynamic development of the northwest segment of the Pacific mobile belt in the Phanerozoic. To define the 40Ar/39Ar age of amphiboles, eight samples of amphibole gabbroids and metamafics were selected during field work from five massifs representing ophiolites and mafic plutons of the island arc. Rocks from terranes of three foldbelts: 1) Pekulnei (Chukotka region), 2) Ust-Belaya (West Koryak region), and 3) the Tamvatnei and El’gevayam subterranes of the Mainits terrane (Koryak-Kamchatka region), were studied. The isotope investigations enabled us to divide the studied amphiboles into two groups varying in rock petrographic features. The first was represented by gabbroids of the Svetlorechensk massif of the Pekulnei Range and by ophiolites of the Tamvatnei Mts.; their magmatic amphiboles show the distribution of argon isotopes in the form of clearly distinguished plateau with an age ranging within 120–129 Ma. The second group includes metamorphic amphiboles of metagabbroids and apogabbro amphibolites of the Ust-Belaya Mts., Pekulnei and Kenkeren ranges (El’gevayam subterranes). Their age spectra show loss of argon and do not provide well defined plateaus the datings obtained for them are interpreted as minimum ages. Dates of amphiboles from the metagabbro of the upper tectonic plate of the Ust-Belaya allochthon points to metamorphism in the suprasubduction environment in the fragment of Late Neoproterozoic oceanic lithosphere in Middle-Late Devonian time, long before the Uda-Murgal island arc system was formed. The amphibolite metamorphism in the dunite-clinopyroxenite-metagabbro Pekulnei sequence was dated to occur at the Permian-Triassic boundary. The age of amphiboles from gabbrodiorites of the Kenkeren Range was dated to be Early Jurassic that confirmed their assignment to the El’gevayam volcanic-plutonic assemblage. These data are consistent with geological concepts and make more precise the available age dates. Neocomian-Aptian 40Ar/39Ar age of amphibolites from the Pekulnei and Tamvatnei gabbroids make evident that mafics of these terranes (varying in geodynamic formation settings and in petrogenesis) were generated in later stages of the development of the West Pekulnei and Mainits-Algan Middle-Late Jurassic-Early Cretaceous island arc systems, presumably due to breakup of island arcs in the Neocomian.

First Perigondwanan record of actinolepids (Vertebrata: Placodermi: Arthrodira) from the Lochkovian (Early Devonian) of Spain and its palaeobiogeographic significance

Different palaeogeographic models have been proposed for the position of Laurussia (including Baltica) and Gondwana-derived microcontinents (including Ibero-Armorica) during Ordovician to Late Carboniferous times. Principal differences concern the presence and duration of a large ocean, the Rheic Ocean, acting as a faunal barrier between these areas. The timing of the collision of Laurussia with Gondwana and/or Gondwana-derived terranes continues to be debated. Here we present new faunal data revealing close biogeographical relations between Ibero-Armorica (“Perigondwanan” or Gondwanan derivate terranes) and Podolia (SE margin of Baltica, in Laurussia). The placoderm assemblage found in the mid-late Lochkovian (Lower Devonian) of Celtiberia (north-central Spain), consisting of the ‘actinolepid’ species Kujdanowiaspis podolica, Erikaspis zychi and the acanthothoracid Palaeacanthaspis aff. P. vasta, is similar, both in terms of taxonomy and stratigraphic record, to that encountered in the Lochkovian of Podolia (Ukraine; Laurussia) and until now considered as endemic to this region. Moreover, vertebrate faunal links between Podolia and Celtiberia are also extended to the chondrichthyan scale-based species Seretolepis elegans and Altholepis composita previously documented exclusively from Laurussian localities (Podolia and Mackenzie Mountains in Canada), which occur together with the placoderm remains described herein. These evidences support the hypothesis that intermittent shallow neritic migration paths between Podolia (as well as “Avalonia”) and Iberia existed in the late Lochkovian, agreeing with a palaeogeographic reconstruction showing close proximity between peri-Gondwanan or Gondwana-derived terranes and Laurussia. It supports the palaeogeographic model of the non-oceanic Variscan Mobile Crustal Field and it corroborates the arguments against wide oceans, acting as biogeographical relevant barriers, between Baltica and Gondwana in early Devonian times. The distribution patterns of heavy-shelled ostracods, turbidicolous brachiopods, and Rhenish trilobites also support these conclusions.

The Gramscatho Basin, south Cornwall, UK: Devonian active margin successions

Deep marine deposits of the Gramscatho Basin of south Cornwall reflect two tectonic regimes; Early to Middle Devonian rifting of continental lithosphere with formation of oceanic lithosphere to the south, and Middle Devonian to earliest Carboniferous convergence along its southern margin. Sediments on thinned continental crust to the north and oceanic lithosphere to the south were juxtaposed in the Late Devonian when nappes of deep water flysch and olistostrome were thrust up on to the northern continental margin of the basin. Basin closure was accommodated by forward propagating thrust nappes, accompanied by penecontemporaneous sedimentation. The stratigraphical sequences of major nappes illustrate the progradation of flysch with climactic sedimentation of olistostrome in late Mid- to Late Devonian times. The Lizard Complex, including the Lizard ophiolite, within that nappe stack, constitutes part of one of the GCR sites which are largely in the allochthonous rocks. Many of those sites feature the olistostrome, Roseland Breccia Formation, with its great variety of sedimentary, igneous and metamorphic clasts (up to 1.5 km), and the association of ocean floor basalt and penecontemporaneous acidic volcanics indicative of the coming together of oceanic and continental plates. A site at the top of the parautochthonous continental margin succession displays the erosion products of the youngest nappe as it emerged and advanced across the sediment surface, marking closure of the oceanised Gramscatho Basin and continental collision.

Palynology of the Lower and Middle Devonian deposits in southern and central Poland

A palynological investigation has been carried out on samples obtained from the Upper Silesian Block, Małopolska Block and the Łysogóry Block from southern and central Poland. Lower and Middle Devonian deposits in this area have never been investigated palynologically at such a refined scale. Samples were obtained from eleven boreholes, a trench, and a quarry representing the Upper Pragian to Eifelian interval which is recognized as a switching point from siliciclastic to carbonate deposition. In many cases, the investigated samples were obtained from transitional facies where faunal information is insufficient and only palynostratigraphical data is reliable. The oldest samples were dated as being part of the polygonalis–emsiensis Zone (Su Interval Zone in Streel et al., 1987), while the younger ones are tentatively dated to the lower part of the velata–langii spore Zone. Palynofacies observations suggest terrestrial to shallow-marine conditions during the transitional period between Early and Mid Devonian times. The upper Pragian to Emsian sedimentary rocks from the Upper Silesian Block and the Małopolska Block were deposited in marginal marine and/or alluvial conditions. Fully marine conditions prevailed only in the upper Emsian in the Łysogóry Block. During the early Eifelian similar marine conditions were established across the investigated region. Spore color analyses have been carried out revealing different palaeotemperatures from over several dozen degrees to 200 °C. One new miospore ( Dibolisporites capillaris sp. nov.) and two new acritarcha ( Lophosphaeridium densum sp. nov. and Lophosphaeridium fiberum sp. nov.) species are formally erected.

Jared R. Morrow (October 8, 1959-October 7, 2010)

Jared R. Morrow (October 8, 1959-October 7, 2010)

Syn- and post-collisional granitoids in the Central Tianshan orogen: Geochemistry, geochronology and implications for tectonic evolution

The eastern part of the Central Tianshan Belt in China is characterized by abundant granitoids, which are not only indicators of the interaction between crust and mantle but also can be used for tracking the tectonic evolutionary history of the orogen. Four granitic plutons, muscovite granites (MG), biotite monzonitic granites (BMG), biotite granites (BG), and alkali granites (AG), have been recognized to have intruded in the Mishigou area, Central Tianshan Belt. The MG has pronounced a S-type affinity, which is strongly peraluminous with a high aluminum saturation index, and displays positive Rb, Th, U and LREE anomalies with a strong negative Ba, Nb, Sr, P, Ti and Eu anomalies. Therefore, the magma of the MG is considered to have been derived from melting of thickened continental crust, which is composed mainly of the sediments eroded from a pre-Silurian continental island-arc. The U–Pb zircon age of 424.5 ± 2.6 Ma constrains the age of MG and crust thickening. The BG and BMG granitoids display I-type geochemical features and have affinities to subduction arcs. Both of them are characterized by calc-alkaline peraluminous granites with a significant LREE enrichment and a negative Eu anomaly, as well as a depletion in Ba, Nb, Ta, Sr, P and Ti and an enrichment in Rb, Th, U, Nd, Zr and Sm. These characteristics indicate that they were derived from a mixed magma source between lower continental crust and the input of components derived from the mantle wedge above the subduction zone. The LA-ICPMS zircon U–Pb ages of 411 ± 4.7 Ma and 402 ± 3.4 Ma represent the formation ages of the BG and BMG granitoids, respectively, which also constrain that the subduction of the South Tianshan oceanic crust also occurred during Early Devonian time. The AG shows a typical A-type granite affinity with depletion of Nb, Ta, Sr, P, Ti and Y. It is interpreted that AG was formed in a within-plate tectonic setting, and related to continental up-doming and rifting zones as a consequence of the extensional collapse after the collision of the Tianshan orogen. The U–Pb zircon age of 290 ± 5.1 Ma represents the formation age of the AG, and the time of the extensional collapse and rifting.

Detrital zircon geochronology and provenance of Devono-Mississippian strata in the northern Canadian Cordilleran miogeoclineThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.Northwest Territories Geoscience Office Contribution 0047. Geological Survey of Canada Contribution 20100432.

U–Pb ages have been determined on detrital zircons from the Upper Devonian Imperial Formation and Upper Devonian – Lower Carboniferous Tuttle Formation of the northern Canadian Cordilleran miogeocline using laser ablation – multicollector – inductively coupled plasma – mass spectrometry. The results provide insights into mid-Paleozoic sediment dispersal in, and paleogeography of, the northern Canadian Cordillera. The Imperial Formation yielded a wide range of detrital zircon dates; one sample yielded dominant peaks at 1130, 1660, and 1860 Ma, with smaller mid-Paleozoic (∼430 Ma), Neoproterozoic, and Archean populations. The easternmost Imperial Formation sample yielded predominantly late Neoproterozoic – Cambrian zircons between 500 and 700 Ma, with lesser Mesoproterozoic and older populations. The age spectra suggest that the samples were largely derived from an extensive region of northwestern Laurentia, including the Canadian Shield, igneous and sedimentary provinces of Canada’s Arctic Islands, and possibly the northern Yukon. The presence of late Neoproterozoic – Cambrian zircon, absent from the Laurentian magmatic record, indicate that a number of grains were likely derived from an exotic source region, possibly including Baltica, Siberia, or Arctic Alaska – Chukotka. In contrast, zircon grains from the Tuttle Formation show a well-defined middle Paleoproterozoic population with dominant relative probability peaks between 1850 and 1950 Ma. Additional populations in the Tuttle Formation are mid-Paleozoic (∼430 Ma), Mesoproterozoic (1000–1600 Ma), and earlier Paleoproterozoic and Archean ages (>2000 Ma). These data lend support to the hypothesis that the influx of sediments of northerly derivation that supplied the northern miogeocline in Late Devonian time underwent an abrupt shift to a source of predominantly Laurentian affinity by the Mississippian.

Thickness trends and sequence stratigraphy of the Middle Devonian Marcellus Formation, Appalachian Basin: Implications for Acadian foreland basin evolution

Analysis of more than 900 wireline logs indicates that the Middle Devonian Marcellus Formation encompasses two third-order transgressive-regressive (T-R) sequences, MSS1 and MSS2, in ascending order. Compositional elements of the Marcellus Formation crucial to the successful development of this emerging shale gas play, including quartz, clay, carbonate, pyrite, and organic carbon, vary predictably within the proposed sequence-stratigraphic framework. Thickness trends of Marcellus T-R sequences and lithostratigraphic units reflect the interplay of Acadian thrust-load-induced subsidence, short-term base-level fluctuations, and recurrent basement structures. Rapid thickening of both T-R sequences, especially MSS2, toward the northeastern region of the basin preserves a record of greater accommodation space and proximity to clastic sources early in the Acadian orogeny. However, local variations in T-R sequence thickness in the western, more distal, area of the basin may reflect the reactivation of inherited Eocambrian basement structures, including the Rome trough and northwest-striking cross-structural discontinuities, induced by Acadian plate convergence. Episodes of block displacement locally warped the basin into northeast-southwest–trending regions of starved sedimentation and/or erosion adjacent to depocenters in which regressive systems tract deposits were ponded. Block movement appears to have initiated in late Early Devonian time, resulting first in thinning and local erosion of the Oriskany sandstone in northwest Pennsylvania. This study, in addition to providing the basis for a predictive sequence-stratigraphic model that can be used to further Marcellus exploration, tells of a foreland basin more tectonically complex than accounted for by simple flexural models.

The Neoproterozoic-early Paleozoic metamorphic and magmatic evolution of the Eastern Sierras Pampeanas: an overview

The Eastern Sierras Pampeanas were structured by three main events: the Ediacaran to early Cambrian (580–510 Ma) Pampean, the late Cambrian–Ordovician (500–440 Ma) Famatinian and the Devonian-Carboniferous (400–350 Ma) Achalian orogenies. Geochronological and Sm–Nd isotopic evidence combined with petrological and structural features allow to speculate for a major rift event (Ediacaran) dividing into two Mesoproterozoic major crustal blocks (source of the Grenvillian age peaks in the metaclastic rocks).This event would be coeval with the development of arc magmatism along the eastern margin of the eastern block. Closure of this eastern margin led to a Cambrian active margin (Sierra Norte arc) along the western margin of the eastern block in which magmatism reworked the same crustal block. Consumption of a ridge segment (input of OIB signature mafic magmas) which controlled granulite-facies metamorphism led to a final collision (Pampean orogeny) with the western Mesoprotrozoic block. Sm–Nd results for the metamorphic basement suggest that the T DM age interval of 1.8–1.7 Ga, which is associated with the less radiogenic values of εNd(540) (−6 to −8), can be considered as the mean average crustal composition for the Eastern Sierras Pampeanas. Increasing metamorphic grade in rocks with similar detrital sources and metamorphic ages like in the Sierras de Córdoba is associated with a younger T DM age and a more positive εNd(540) value. Pampean pre-540 Ma granitoids form two clusters, one with T DM ages between 2.0 and 1.75 Ga and another between 1.6 and 1.5 Ga. Pampean post-540 Ma granitoids exhibit more homogenous T DM ages ranging from 2.0 to 1.75 Ga. Ordovician re-activation of active margin along the western part of the block that collided in the Cambrian led to arc magmatism (Famatinian orogeny) and related ensialic back-arc basin in which high-grade metamorphism is related to mid-crustal felsic plutonism and mafic magmatism with significant contamination of continental crust. T DM values for the Ordovician Famatinian granitoids define a main interval of 1.8–1.6, except for the Ordovician TTG suites of the Sierras de Córdoba, which show younger T DM ages ranging from 1.3 to 1.0 Ga. In Devonian times (Achalian orogeny), a new subduction regime installed west of the Eastern Sierras Pampeanas. Devonian magmatism in the Sierras exhibit process of mixing/assimilation of depleted mantle signature melts and continental crust. Achalian magmatism exhibits more radiogenic εNd(540) values that range between 0.5 and −4 and T DM ages younger than 1.3 Ga. In pre-Devonian times, crustal reworking is dominant, whereas processes during Devonian times involved different geochemical and isotopic signatures that reflect a major input of juvenile magmatism.

The geodynamics of collision of a microplate (Chilenia) in Devonian times deduced by the pressure–temperature–time evolution within part of a collisional belt (Guarguaraz Complex, W-Argentina)

The Guarguaraz Complex in West Argentina formed during collision between the microplate Chilenia and South America. It is composed of neritic clastic metasediments with intercalations of metabasic and ultrabasic rocks of oceanic origin. Prograde garnet growth in metapelite and metabasite occurred between 1.2 GPa, 470°C and 1.4 GPa, 530°C, when the penetrative s2-foliation was formed. The average age of garnet crystallization of 390 ± 2 Ma (2σ) was determined from three four-point Lu–Hf mineral isochrones from metapelite and metabasite samples and represents the time of collision. Peak pressure conditions are followed by a decompression path with slight heating at 0.5 GPa, 560°C. Fluid release during decompression caused equilibration of mineral compositions at the rims and also aided Ar diffusion. An 40Ar/39Ar plateau age of white mica at 353 ± 1 Ma (1σ) indicates the time of cooling below 350–400°C. These temperatures were attained at pressures of 0.2–0.3 GPa, indicative of an average exhumation rate of ≥1 mm/a for the period 390–353 Ma. Late hydrous influx at 0.1–0.3 GPa caused pervasive growth of sericite and chlorite and reset the Ar/Ar ages of earlier coarse-grained white mica. At 284–295 Ma, the entire basement cooled below 280°C (fission track ages of zircon) after abundant post-collisional granitoid intrusion. The deeply buried epicontinental sedimentary rocks, the high peak pressure referring to a low metamorphic geotherm of 10–12°C/km, and the decompression/heating path are characteristics of material buried and exhumed within a (micro) continent–continent collisional setting.

Solar variance recorded in lacustrine deposits from the Devonian and Proterozoic of Scotland

Abstract: Continental lacustrine laminites deposited in low latitudes have been examined from the Devonian and Torridonian (Mesoproterozoic) strata of northern Scotland to investigate the consistency of sunspot periodicity through geological time. The thicknesses of 2448 annual laminae were measured from the Middle Devonian ( c . 388 Ma) Achanarras Limestone Member, Lower Caithness Flagstone Group, and of 2821 laminae from the Mesoproterozoic (1.2 Ga) Poll a'Mhuilt Member of the Stoer Group. Time-series analysis resolved periodicities of 13.8 and 13.9 years in the complete Devonian and Mesoproterozoic datasets respectively. When sedimentological variance was considered and the data were examined as evolutive spectra those sections consisting of the most robust data consistently yielded periodicities between 8.6 and 13.7 years for the Devonian section and 9.2 and 10.6 years for the Mesoproterozoic section. These correspond well to Schwabe cycles. Evidence for the presence of the longer Hale cycles (20 years) was also recognized in the Devonian time series. The sunspot periodicities identified within the Devonian and Mesoproterozoic show little variance from published examples from the Quaternary and Eocene and therefore provide evidence for the consistency of solar processes and solar–atmospheric interactions for the past 1.2 Ga.

Middle Proterozoic and Lower Paleozoic of the Sierra de Umango, Andean Foreland (29°S), Argentina: U-Pb ages and isotopic characteristics.

Igneous and metamorphic units formed mainly during the Middle Proterozoic and Early Paleozoic times have been distinguished in the Sierra de Umango crystalline basement. These include granitic orthogneisses, a sequence of schists and marbles, amphibolites, and three small granitic plutons. Geochemical analyses of granitoids and amphibolites, as well as age determinations using U-Pb and K-Ar methods were carried out. The U-Pb age in zircon of the Juchi Orthogneiss (1108±13 Ma) confirms the previous Rb-Sr Mesoproterozoic dating (1030±30 Ma). The U-Pb in zircon age of El Penon Granite (473±17 Ma) is slighty older than the Rb-Sr age (469±9 Ma). The K-Ar mouscovite dates from El Penon Granite (409±14 and 381±11 Ma) are related to a Devonian deformation and metamorphism. On the basis of Nd and Sr isotopic characteristics, two different sets of units can be identified, one related to the Grenvillian cycle and the second to the Pampean-Famatinian cycles. According to the data obtained for the Sierra de Umango, we interpret that in this part of the Western Sierras Pampeanas there are relics of a Mesoproterozoic basement (Grenville cycle), together with more important exposures of a schists and marble succession, with amphibolites and small granitic plutons, that were formed during Late Proterozoic to Devonian times (Pampean and Famatinian cycles). The major basement structure, formed during Ordovician to Devonian, shows a clear tectonic vergence toward the west, and consists of thrust sheets (nappes) typical of collisional tectonics. These sheets are transported from the inner parts of the orogen (Sierras Pampeanas Orientales) toward the contiguous foreland of the interacting terrane. The Upper Ordovician to Devonian sedimentary rocks of the Precordillera (foreland basin) could complete the geodynamic environment.

New data on the composition and age of basement rocks of the Tagil paleo-island arc system

The Tagil paleo-island arc terrane is composed of Late Ordovician-Devonian intrusive, volcanic, and volcano-sedimentary complexes. The western margin of the terrane is comprised of dunite-clinopyroxenite-gabbro massifs of the Ural platinum-bearing belt, which are fringed by rock strata of widely different metamorphic grades. Work on isotope systematics of olivine gabbros enabled us to infer a Vendian age (550–540 Ma) of homogenization of the Sm-Nd isotopic system of the Kytlym and Knyaspa massifs within the Ural platinum-bearing belt. The Sm-Nd ages for metamorphic rocks of the Belaya Gora complex surrounding the studied massifs also agree with a Vendian age (573–574 Ma). Our results suggest that metamorphites of the Belaya Gora complex (amphibolites, plagiogneisses, two-mica and biotite gneisses, schists containing garnet, cordierite, staurolite, gedrite, and sillimanite) and dunite-clinopyroxenite-gabbro intrusions of the proto-platinum-bearing belt may have been formed in a convergence setting above a mantle plume, most likely in a back-arc (?) extension region. Reactivation of this plume during the late Precambrian resulted in the opening of the Ural paleo-ocean. The Late Ordovician-Early Devonian times were marked by metamorphic reworking and tectonic transport of pre-Paleozoic complexes into an accretionary prism setting of the Tagil paleo-arc that was accompanied by generation of gabbroid and granitoid magmas. Based on the obtained results, the Tagil terrane can be now considered as part of the Paleozoic paleo-island arc system developed on a heterogeneous Proterozoic basement.

A structural study of western Anticosti Island, St. Lawrence platform, Quebec: a fracture analysis that integrates surface and subsurface structural data

Abstract Field data from fracture sets, folds and faults affecting generally flat lying Lower Ordovician to Lower Silurian strata on western Anticosti Island, Quebec, are integrated with remote sensing and seismic reflection data. Spatial and temporal relationships among different structural features observed on the Island and in the Gulf of St. Lawrence are assessed with the aim of understanding the role of these structural features for petroleum migration through the Anticosti Platform. The tectonic impact of Appalachian orogenic events on Anticosti Platform is considered, together with Jurassic and Quaternary tectonics. Four fracture sets are identified in Lower Silurian rocks: 1) a NS-EW orthogonal fracture system (Sets I and II); 2) a N40–70 set (Set III); 3) and a N135–165 set (Set IV). Some irregular, randomly oriented fractures are also observed. The orthogonal system may be related to Early Acadian events, which may have induced extensional stresses within the Platform during Late Silurian-Early Devonian time. Normal faults observed in the Lower Silurian succession may be related to this extensional event. The N135–165 and N40–70 sets may have developed either during the Middle Devonian Acadian orogeny or later in the Jurassic. The irregular fractures could have formed during the final stages of fracture development in response to erosion of the Lower Silurian and younger succession, or in response to isostatic rebound following glaciation. Shuttle Radar Digital Elevation Model lineaments are consistent either with observed fracture set orientations, or subsurface Taconian fault orientations. We infer some Taconian faults and Silurian fracture zones were reactivated, probably during the opening of the Atlantic Ocean, as indicated by the intrusion of two Jurassic dikes. In the subsurface of the Island, major SW-dipping normal faults and extensional fractures affect the Lower to Upper Ordovician succession most. These probably formed in response to an extensional stress field in front of the Taconian deformation front during Late Ordovician time. WSW-trending typically open folds occur on southern Anticosti Island in Chicotte Formation and in the Gulf of St. Lawrence. They involve Lower Silurian to Devonian rocks, and may have formed in a compressive regional setting during the Middle Devonian Acadian orogeny. Thermal maturation and petroleum generation on Anticosti Platform are controlled mainly by the burial history. However, the structures described in this study, in particular Taconian and Early Acadian fractures and faults systems, may have played an important role in petroleum migration and in potential reservoir development.

Log Jams and Flood Sediment Buildup Caused Channel Abandonment and Avulsion in the Pennsylvanian of Atlantic Canada

Abstract Accumulations of logs and flood sediment frequently block modern channels and may trigger avulsion, but these effects are difficult to demonstrate for the ancient record. Braided-fluvial channels in the Pennsylvanian South Bar Formation of Atlantic Canada contain sandstone successions up to 6 m thick of sigmoidal cross-beds, plane beds, and antidunes, deposited rapidly at high-flow-stage. These strata are commonly capped by accumulations up to 2.5 m thick of flattened, coalified logs and coal intraclasts (originally peat fragments), many of which are overlain by mudstone laid down in abandoned channels. The logs include lycopsids, calamiteans, tree ferns, pteridosperms and cordaitaleans, inferred to have grown on inactive braided tracts near the channels. A compaction estimate suggests that one log accumulation was originally more than four times its present thickness. Most accumulations are interpreted as stable “transport log jams” formed during floods, although some may have been “unstable jams” stranded on bars during peak-flow recession. Associated with the logs are extrabasinal gravel and intraclasts of mudstone and coal, which suggest that floods in sediment-choked channels undercut banks of gravelly sand capped by mud and forested peat, widened the channels, and toppled riparian vegetation. An estimated blockage ratio of 8% for one accumulation (ratio of the cross-sectional areas of the log jam and host channel) is close to the 10% value considered to cause substantial blockage in some modern rivers. In two instances, a radical change in paleoflow between pre- and post-abandonment channels is consistent with an interpretation that log jams and flood sediment buildup promoted channel-belt avulsion. Although large trees had evolved by Middle to Late Devonian times, it is unlikely that riparian plants occurred in stands that were sufficiently dense to exert a major influence on river dynamics until the Pennsylvanian. Thus, we report some of the earliest evidence for the effects of woody debris on ancient fluvial systems.

Air-breathing adaptation in a marine Devonian lungfish

Recent discoveries of tetrapod trackways in 395 Myr old tidal zone deposits of Poland (Niedźwiedzki et al. 2010 Nature 463, 43-48 (doi:10.1038/nature.08623)) indicate that vertebrates had already ventured out of the water and might already have developed some air-breathing capacity by the Middle Devonian. Air-breathing in lungfishes is not considered to be a shared specialization with tetrapods, but evolved independently. Air-breathing in lungfishes has been postulated as starting in Middle Devonian times (ca 385 Ma) in freshwater habitats, based on a set of skeletal characters involved in air-breathing in extant lungfishes. New discoveries described herein of the lungfish Rhinodipterus from marine limestones of Australia identifies the node in dipnoan phylogeny where air-breathing begins, and confirms that lungfishes living in marine habitats had also developed specializations to breathe air by the start of the Late Devonian (ca 375 Ma). While invasion of freshwater habitats from the marine realm was previously suggested to be the prime cause of aerial respiration developing in lungfishes, we believe that global decline in oxygen levels during the Middle Devonian combined with higher metabolic costs is a more likely driver of air-breathing ability, which developed in both marine and freshwater lungfishes and tetrapodomorph fishes such as Gogonasus.

Detrital zircon geochronology and its provenance implications: responses to Jurassic through Neogene basin‐range interactions along northern margin of the Tarim Basin, Northwest China

ABSTRACTPreviously published research from the Kuqa Subbasin along northern margin of the Tarim Basin shows five tectonic‐depositional phases from Triassic to Neogene time. In order to reveal additional detailed information on the nature of provenance terrains and tectonic attributes since late Mesozoic time, five typical sandstone samples from Jurassic–Neogene strata were collected for U–Pb dating of detrital zircons. Geochronological constitution of detrital zircons of the Middle Jurassic sample is essentially unimodal and indicates major contributions from the South Tian Shan even Yili–Central Tian Shan, wherein most 370–450 Ma zircons probably resulted from tectonic accretion events between the Yili–Central Tian Shan block and South Tian Shan Ocean during Silurian and Devonian time, with sandstone provenance tectonic attributes of passive continental margin. The Lower Cretaceous sample shows a complicated provenance detrital zircon signature, with new peak ages of 290–330 Ma as well as 370(or 350)–450 Ma showing evident arc orogenic provenance tectonic attribute, probably reflecting a new provenance supply that resulted from denudation process whthin the South Tian Shan and South Tian Shan suture. There are no obvious changes within age probability spectra of detrital zircons between the Cretaceous and early Paleogene samples, which suggests that similar provenance types and basin‐range framework continued from Cretaceous to Early Paleogene time. However, unlike the Cretaceous and early Paleogene samples, an age spectra of the Miocene sample is relatively unimodal and similar to that of the Pliocene sample, with peak ages ranging between ∼392 and ∼458 Ma older than the comparable provenance ages (peak ages about 370–450 Ma) of the Middle Jurassic and Lower Cretaceous samples. Therefore, we conclude that the South Tian Shan was rapidly exhumated and the southern South Tian Shan had become the main source of clastics for the Kuqa Subbasin since the Miocene epoch.

Geochemical and palynological study of the Upper Famennian Dasberg event horizon from the Holy Cross Mountains (central Poland)

AbstractIntegrated palynological, organic and inorganic geochemical and petrographical methods have been used for deciphering the depositional redox conditions and character of organic matter of the Famennian Dasberg event horizon from the deep-shelf Kowala succession of the Holy Cross Mountains. The ages of the investigated samples have been established, using miospore data, as VF (Diducites versabilis–Grandispora famenensis) and LV (Retispora lepidophyta–Apiculiretusispora verrucosa) miospore Zones of the Middle/Upper Famennian. In the standard conodont zonation, this corresponds to the uppermost postera to lowermost praesulcata Zones. The presence of green sulphur bacteria biomarkers and dominance of small-sized framboids together with the presence of large framboids and low values of the U/Th ratio may indicate that during sedimentation of the lower Dasberg shale, intermittent anoxia occurred in the water column, or the anoxic conditions prevailed in the upper part of the water column, while the bottom waters were oxygenated, at least briefly. Deposition of the upper Dasberg shale was characterized by both bottom water and water column anoxia. The lack of acritarcha taxa from these intervals could have been due to anoxia in the photic zone. Moreover, organic content is high in those samples. There is no geochemical evidence for anoxia during sedimentation of the deposits sandwiched between the lower and upper Dasberg shales, or in the deposits which underlie and overlie both Dasberg shale horizons. The two discrete anoxic events are interpreted to be the result of major transgressions and the blooming of primary producers. Above the Dasberg shales, small fragments of charcoal and raised concentrations of polycyclic aromatic hydrocarbons are detected. This supports the presence of wildfires during deposition of shales just above the boundary of VF/LV palynological zones. Temperatures calculated from the fusinite reflectance values suggest that the charcoal was formed in low-temperature ground and/or surface fires. The typical marine character of sedimentation combined with the high proportion of charcoals suggests that wildfires were large-scale, and that there was intensive transport of terrestrial material. The main causes of intensive wildfires were a significant rise of O2 in the atmosphere and important progress in the land plant diversity during Late Devonian times. Palynofacies studies suggest that the transgression corresponds to the part IIf of the Late Devonian sea-level curve.