Northern Gondwana Research Articles

Stratification and productivity in the Western Tethys (NW Algeria) during early Toarcian

Profound environmental changes punctuated the Early Jurassic period, as recorded by marked carbon and oxygen isotope anomalies and major biotic crises. The response of low-latitude regions of Northern Gondwana to such intense changes is not documented as well as that of other Tethys areas. We present new calcareous nannofossil assemblages from three sections located in NW Algeria, in the Sahara and Tlemcen Basins, respectively. New stable carbon and nitrogen isotope data are provided from the Tlemcen Basin to reconstruct local environmental conditions in the wider context of a Toarcian greenhouse climate. We first established a solid chemo- and biostratigraphic framework by integrating stable carbon isotope data and calcareous nannofossil events. Calcareous nannofossil assemblages show common trends in the two basins, such as the occurrence in high proportions of the deep-dweller species Mitrolithus jansae , likely indicating stratification of the water column with a deep nutricline. This taxon dominated the assemblage during the negative carbon isotope excursion (CIE) interval, often used to delineate the base of the Toarcian anoxic event (T-OAE). Such a nannofossil record is unique in the Western Tethys domain, as M. jansae is known to drastically decrease in abundance during the T-OAE until its disappearance in the aftermath of the event. The NW Algeria nannofossil record indicates prolonged thermal stratification of water-masses, finally triggering hyper-oligotrophy and low productivity in shallow waters during the Toarcian CIE. Such peculiar conditions are likely related to the combined effects of a warm and arid climate dominating along the northern Gondwana margin and the presence of a strong clockwise gyre over the epicontinental shelf, which brought warm equatorial waters from the Tethys Ocean to the NW Algeria shelf. • The NW Algeria nannofossil record suggests prolonged thermal stratification of water-masses. • Hyper-oligotrophy and low productivity in shallow-waters occurred during the Toarcian Event. • Such conditions are likely related to the effects of a warm and arid climate of the Northern Gondwana.

Early Paleozoic granodiorites and gabbros related to asthenosphere upwelling in the eastern Wuyi-Yunkai Orogen, South China

The early Paleozoic Wuyi-Yunkai Orogen (WYO) developed in the inner part of the South China Block (SCB) and underwent large scale felsic magmatism. Different tectonic models have been proposed to explain the evolution of the WYO, including intracontinental and collisional orogenesis. The debates focus on whether the asthenospheric materials were involved during evolution of the WYO. We conducted detailed petrographic and geochemical analyses on gabbros from the Qiaotou pluton and granodiorites and the hosted MMEs from the Xinsi pluton in the eastern WYO, and present the first evidence of asthenosphere upwelling that induced significant lithosphere remobilization in the SCB during early Paleozoic. Granodiorites from the Xinsi pluton (441 ± 2 Ma) contain abundant amphibole and are metaluminous, which originated from the igneous basement in the Cathaysia Block. The hosted MMEs (440 ± 2 Ma) have variable SiO2 and enriched Sr-Nd isotopic compositions with high zircon δ18O values, which were formed through hybridization of felsic melts and mafic magmas from the enriched lithosphere mantle. Gabbros from the Qiaotou pluton (448 ± 8 Ma) have low K2O with insignificant Nb-Ta-Ti anomalies and show variable Sr-Nd-O isotopic compositions. They were derived from the depleted asthenosphere and underwent crustal assimilation. The asthenosphere upwelling may be induced by slab breakoff during the collision between the western Cathaysia Block and an unknown block near northern Gondwana. The continental collision possibly transferred compressional stress and caused crustal uplifting along the weak zone in the inner SCB. The asthenosphere upwelling provided tremendous heat for the lithosphere remobilization and voluminous magmatism in the WYO.

Understanding along-strike variability in controlling mechanisms of paleoenvironmental conditions and stratigraphic architecture: Ordovician successions in the Alborz Mountains of Iran at the northern Gondwana margin

Changes in stacking patterns of facies, stratal architecture and shoreline trajectory along depositional strike are mainly dependent on along-strike differences in factors such as physical processes (rivers, waves, tides), tectonics, eustatic changes, sediment supply, and local physiography. The lower-upper Ordovician successions (Lashkerak Formation) deposited in a Peri-Gondwana shallow continental shelf have been interpreted using the sequence stratigraphic concepts. This approach allowed to highlight main differences along-strike in the co-existing depositional systems (coastal plains, estuaries, deltas, and shoreline to shelf systems) in the Alborz basin along the northern edge of Gondwana. On the basis of sedimentary and ichnological characteristics, the siliciclastic successions are divided into four facies associations: (i) Wave-dominated estuarine, (ii) Wave-dominated shoreface-offshore complex, (iii) Mixed fluvial (F)- and wave (W)-influenced delta (Fw and Wf), and (iv) Tide-dominated, fluvial-influenced delta (Tf). Six third-order depositional sequences (DS1-DS6) were recognized, each of them are bounded by unconformities and pronounced shifts in the facies. Along depositional strike (NE/SW- a distance of c. 150 km), significant intrabasinal changes in sediment supply produced evident lateral changes in the shoreline trajectory and stratal architecture, of the DS1-DS3 (Tremadocian-Floian), may be attributed to differences in physical processes (rivers, waves, tides activity) and topographic irregularities derive probably by different rates of subsidence in the basin. During the deposition of the DS1-DS3, a significant along-strike differences in the composition of ichnological suites and the lifestyle of benthonic organisms, dependent on the relative supremacy of wave, storm, river, and tidal processes. Sedimentary supply and processes are the main controlling factors for the stratigraphic architecture rather than the tectonic activity between Tremadocian and Floian. While during the Darriwilian-Katian interval (DS4-DS6), the tectonic activity predominates with the beginning of extensional phases related to the rifting in the Alborz basin causing the activation of syn-depositional normal faults and the development of block-faulted craton-ward margin. Typically, differential subsidence due to extensional tectonic activity causes the uplift of tectonic blocks west of the Alborz Basin into the emerging areas, on one hand, and simultaneous deepening-upward stacking patterns (i.e., increased subsidence rates) in the hanging-wall depocenters of east of the Alborz Basin, on the other hand. This outcrop-based study suggests that the identification of genetically related depositional units in a larger paleogeographic context is fundamental for understanding along-strike variability in controlling mechanisms of the spatial distribution and stratigraphic architecture of coexisting and linked depositional systems.

Ordovician-Silurian volcanism in northern Iran: Implications for a new Large Igneous Province (LIP) and a robust candidate for the Late Ordovician mass extinction

Proxies such as mercury and Hg/TOC anomalies document signatures for temporal correlations between major pulses of the Late Ordovician mass extinction (LOME) event and a postulated Large Igneous Province (LIP) as the main cause of extinction. Herein, for the first time, we report a series of voluminous intraplate volcanic events of the Middle Ordovician-Silurian from northern Iran, representing the erosional and deformed remnants of a LIP as well as a robust candidate as the cause of the LOME. These volcanic rocks, distributed over a length of ca. 1700 km and more than 1000 m thickness in some cases, were erupted during initial rifting of the Paleotethys Ocean in northern Gondwana. Based on fieldwork, relative ages, and high precision U-Pb ID-TIMS dating results we have identified one plutonic (granitic) and six volcanic (mainly basaltic) phases of mostly short duration for this chain of magmatism. The onset of volcanic events at 468.70 ± 0.30 Ma (2σ) coincided with the earliest Darriwilian, followed by a huge bimodal volcanic event between the Sandbian-Katian boundary and 450.61 ± 0.27 million years ago, and the climax of volcanism occurred during the late Katian-Hirnantian. An approximate coincidence between the onset of basaltic eruption and weathering in northern Iran and the beginning of significant global decline in seawater 87Sr/86Sr during the Darriwilian Stage may be a sign of the initial volcanic activities of our newly identified LIP. We suggest that the Middle Ordovician-Silurian volcanic rocks from northern Iran, and other related places, are remnants of this newly proposed LIP (herein labeled the Alborz LIP), with high potential to be the main cause of environmental and climatic changes that led to the LOME.

Miospores from the Upper Devonian and lowermost Carboniferous strata of the Khoshyeilagh area, northeastern Alborz, Iran

Diverse and well-preserved terrestrial palynomorphs occur in the Padeha, Khoshyeilagh, and Mobarak formations in the Khoshyeilagh area, northeastern Alborz, northern Iran. The spore assemblages consist of 36 species belonging to 24 genera. The vertical distribution of index spores allows the erection within these strata of five local biozones (BT, FF, LE, LN, and VI). The data show that assemblages recorded from the Khoshyeilagh section can be correlated with the VCo, LE, LN, and VI Miospore biozones of Europe, Canada, and USA. Many of the palynomorph species, such as Archaeoperisaccus ovalis, Ancyrospora ampulla, Diducites mucronatus, D. versabilis, Grandispora cornuta, G. echinata, Indotriradites explanatus, Retispora lepidophyta, Retusotriletes incohatus, R. phillipsii, Rugospora flexuosa, Teichertospora torquata, Tumulispora malevkensis, Vallatisporites pusillites, Verrucosisporites bulliferus, and Verrucosisporites nitidus, are closely comparable with coeval assemblages recorded from Belgium, Portugal, Canada, North Africa, South America, and North America. This indicates the close relationship of the Iranian Platform to other parts of the northern Gondwana and southern Laurentian domain during this time interval. Moreover, parent plants of the Late Devonian miospores in the Khoshyeilagh area generally belong to herbaceous Class Rhyniopsida (orders: Rhyniales, Trimerophytales), Zosterophyllopsida, and various classes such as, Lycopsida (herbaceous order: Isoetales, Selaginellales, Protolepidodendrales), Progymnospermopsida (Order Archaeopteridales and Aneuropthytales), and Filicopsida. The presence of marine shelly macrofauna (brachiopods and corals) suggest a nearshore depositional environment comprising rivers, fluvial deltas, tidal flats, and lagoons for the Upper Devonian and lowermost Carboniferous deposits studied herein.

On Traces of Non-Equilibrium States in the Evolution of Terrestrial Vertebrate Communities across the Paleozoic–Mesozoic Boundary

Evolutionary change in a living system starts with a disturbance to its equilibrium and an increase in its internal disorderliness. Typical manifestations of this process are exemplified by the transformation of the tetrapod faunas across the Permian–Triassic boundary. These are seen both at the end of the transition to a new equilibrium (in the pioneer Triassic faunas) and at its beginning (in the terminal Permian communities). In the first case, the most distinct manifestations are: unfinished state of structural changes within some new groups; a high degree of endemism of regional pioneer communities, and the heterogeneity of their change with time. In the second case, as can be seen from analysis of the early-crisis (Vyaznikovian) biota of Eastern Europe, the observed events primarily include the “precocious” appearance of some future dominants; the “return” of a number of relics to the fossil record; and examples of explosive group variability in some taxa close to extinction. Actual or probable traces of non-equilibrium changes, similar to those noted in the Vyaznikovian biota, can also be partly traced in other Late Permian faunas of Eurasia and in coeval communities of southern and northern Gondwana.

Middle Permian Mixed Siliciclastic‐Carbonate System on the Northwestern Margin of the Indian Plate, Pakistan: Implications for Paleoclimate and Carbonate Platform Evolution

AbstractA mixed siliciclastic‐carbonate system that responds to changes in Permian climate and subsequent carbonate platform evolution is investigated using microscopic details of the Middle Permian Amb Formation (Fm.), in Saiyiduwali section, Khisor Range, northern Pakistan. Thin sections were made from rocks throughout the stratigraphic section of the Amb Fm. and analyzed with an emphasis on carbonate and clastic microfacies, and the latter interpreted within the existing chronostratigraphic framework. Outcrop observations reveal that the units comprise coarse‐grained, channelized, ripple‐marked, and burrowed sandstone and sandy, fossiliferous limestone with minor marls and shale intercalations, suggesting deposition in a subaqueous tide‐dominated delta to beach barrier. Based on the determined seven microfacies coupled with outcrop observation, the Amb Fm. was deposited in a tide‐influenced subaqueous delta to middle shelf environment under fluctuating sea level. The deposition of compositionally mature sandstone in the lower part of the formation suggests reworking of detritus from the rift shoulders and an adjacent source area with an ambient warm and humid climate. The stratal mixing of carbonates and compositionally mature siliciclastic units in the middle part suggest deposition under tectonic and climate‐induced terrigenous and carbonate fluxes to the basin. Thus the deposition shows a perfect transition from clastic‐dominated deltaic to pure carbonate platform settings as a result of warm climate and tectonics. This Middle Permian warming is confirmed by sea‐level rise and the presence of a temperature‐sensitive fusulinid fauna in association with photozoan‐based ooids. Deposition of the Amb Fm. and establishment of a carbonate platform are envisaged to be associated with major rifting of northern Gondwana, which subsequently resulted in the development of a rift basin at the passive margin of the NW Indian Plate then in northern Pakistan.

Running across the Silurian/Devonian Boundary along Northern Gondwana: A Conodont Perspective

The Global Stratotype Section and Point (GSSP) of the Silurian/Devonian boundary, Lower Devonian Series and Lochkovian Stage was formally placed in 1977 at Klonk, in the Czech Republic, at the first appearance of the graptolite Uncinatograptus uniformis uniformis (Přibyl). However, since then, correlation of this limit has been often hampered in carbonate facies where graptolites are uncommon or totally absent. A large calcareous deposition occurred at the Silurian/Devonian boundary along the northern and peri-Gondwana margin, thus representing an ideal location to select and test a possible additional biostratigraphic marker of the limit among conodonts. The first appearance of Caudicriodus hesperius almost simultaneously at the base of the Devonian in Bohemia, the Carnic Alps, Sardinia, Morocco and elsewhere indicates that this taxon is the conodont that best approximates the beginning of the Period. The first or last appearance of other species (e.g., Ozarkodina confluens, Zieglerodina klonkensis, Z. remscheidensis and Caudicriodus woschmidti) may help to recognise the boundary as well.

Cambro–Ordovician ferrosilicic magmatism along the northern Gondwana margin: constraints from the Cézarenque–Joyeuse gneiss complex (French Massif Central)

It is well-acknowledged that the northern margin of the Gondwana supercontinent was affected by a major magmatic event at late Cambrian (Furongian) to early Ordovician (Tremadocian–Floian) times. However, an accurate assessment of its extent, origin, and significance is partly hampered by the incomplete characterization of the numerous gneiss massifs exposed in the inner part of the Variscan belt, as some of them possibly represent dismembered and deformed Furongian–Lower Ordovician igneous bodies. In this study, we document the case of the “Cézarenque–Joyeuse” gneiss complex in the Cévennes parautochthon domain of the French Massif Central. The gneisses form decametre- to kilometre-thick concordant massifs interlayered within a pluri-kilometric sequence of mica- and quartz schists. They encompass two main petrological types: augen gneisses and albite gneisses, both typified by their blue and engulfed quartz grains with the augen facies differing by the presence of centimetre-sized pseudomorphs after K-feldspar and the local preservation of igneous textures. Whole-rock geochemistry highlights that many gneisses have magmatic ferrosilicic (acidic with anomalously high FeOt and low CaO) compositions while others are akin to greywackes. Collectively, it is inferred that the bulk of the Cézarenque–Joyeuse gneisses represents former rhyodacite lava flows or ignimbrites and associated epiclastic tuffs. Volumetrically subordinate, finer grained, and strongly silicic leucogneisses are interpreted as microgranite dykes originally intrusive within the volcanic edifices. LA–ICP–MS U–Pb dating of magmatic zircon grains extracted from an augen gneiss and a leucogneiss brackets the crystallization age of the silicic magmas between 486.1 ± 5.5 Ma and 483.0 ± 5.5 Ma which unambiguously ties the Cézarenque–Joyeuse gneisses to the Furongian–Lower Ordovician volcanic belt of SW Europe. Inherited zircon date distributions, Ti-in-zircon and zircon saturation thermometry demonstrate that they formed by melting at 750–820 °C of Ediacaran sediments. Zircon Eu/Eu* and Ce/Ce* systematics indicate that the melts were strongly reduced (fO2 probably close to the values expected for the iron–wüstite buffer), possibly because they interacted during ascent with Lower Cambrian black shales. This would have enhanced Fe solubility in the melt phase and may explain the peculiar ferrosilicic signature displayed by many Furongian–Lower Ordovician igneous rocks in the northern Gondwana realm. We infer that crustal melting resulted from a combination of mantle-derived magma underplating in an intracontinental rift setting and anomalously elevated radiogenic heat production within the Ediacaran sedimentary sequences.

Late Devonian (Frasnian–Famennian) palynomorphs from the Padeha and Bahram Formations of Shahzadeh Mohammad section, northwest of Kerman, Iran

A diverse and well-preserved microphytoplankton assemblage is reported from a measured section of Frasnian–Famennian (Upper Devonian) Padeha and Bahram Formations in Shahzadeh Mohammad area, northwestern Kerman, southeast Iran. The palynoflora assemblage contains 17 genera and 23 species of acritarchs, chitinozoa, scolecodont and rich miospore taxa. 57 species (25 genera) of miospores were identified and 5 assemblage biozones were defined from the Shahzadeh Mohammad section. The miospore composition is similar to neighboring localities in Southern and Northern Iran, but it shows significant similarities with palynoflora from more distant localities, e.g. Saudi Arabia, Western Australia or Algeria. This implies a close relationship between the Iranian platform and other areas of the Northern Gondwana and southern Laurentia Domain during the late Devonian. The investigated section was deposited in a shallow marine environment with tropical conditions during the Frasnian-Famennian period.

Organic carbon accumulation at the northern Gondwana paleomargin (Tunisia) during the Toarcian Oceanic Anoxic Event: Sedimentological and geochemical evidence

During the early Toarcian (Early Jurassic; ~183 Ma) global environmental changes promoted the expansion of oxygen-deficient conditions across vast shelf areas around the globe. Ocean margin deoxygenation favored the preservation of organic matter in shelf sediments and is considered to reflect a Toarcian Oceanic Anoxic Event (T-OAE). Higher-level global drivers controlled temperature fluctuations, sea level evolution, oceanic circulation patterns, marine primary productivity, and stable carbon isotope excursions. Lower-level factors, controlled by individual basin bathymetry and paleogeography, operated on local to regional scale and here regulated organic matter accumulation. Accordingly, the T-OAE has been described as global event with a strong regional character. During the Toarcian, preferentially organic matter-lean sediments have been deposited at the northern Gondwana paleomargin. Enhanced connectivity to the Tethys Ocean and low marine primary productivity are considered to have been major factors preventing the formation of oxygen-deficient conditions and organic matter preservation. We here report on early Toarcian sediments from the Châabet El Attaris Section (Tunisia), located at the northern Gondwana paleomargin that episodically became enriched in organic matter. Bio- and chemostratigraphy both confirm that organic matter-rich sediments preferentially formed during the T-OAE but also predate the T-OAE. Sedimentological and micropaleontological characterization in concert with bulk, molecular and stable isotope geochemical investigations reveal that accumulation of marine organic matter occurred under anoxic, but not under euxinic, conditions. Limitation in the supply of reactive iron regulated organic matter sulfurization indicating that during most of the depositional history the levels of free H 2 S in bottom waters were rather low but limited to the sediment pore water. We conclude that the chemocline was either situated within the sediment or close to the sediment water interface. Spatiotemporally-restricted occurrence of organic matter enriched sediments and anoxic pore waters was promoted by restriction in water column ventilation in a local graben structure. • Organic matter-enriched strata in Tunisia predates but maximizes in the T-OAE. • Organic matter preservation occurred under anoxic but non-euxinic conditions. • Formation of oxygen deficient conditions was promoted by local paleobathymetry and basin restriction.

From shallow into deep sea: Sedimentary facies and U-Pb zircon ages in the early Paleozoic Noric Group at Veitsch (Eastern Greywacke Zone, Austria)

Abstract The low-grade metamorphic early Paleozoic basement of the Veitsch area presents a wide variety of sedimentary facies domains. The first domain consists of thick metadacites of Middle Ordovician age (Blasseneck Porphyroid), overlain by fine-grained metaclastics of the Rad Formation (Late Ordovician to Silurian) and Devonian limestones and calcitic marbles (Kaiserstein and Kaskögerl Formation, respectively). Rhyolitic to dacitic magmatism initiated at ca. 479 Ma (LAMC-ICP-MS U-Pb zircon data) and lasted until ca. 444 Ma. The second domain comprises metaclastics of the Stocker Formation (Early Ordovician to Silurian), characterized by thin volcanics and volcaniclastics of andesitic and rhyolitic composition. U-Pb zircon data give Middle Ordovician age (463 Ma – 468 Ma). The third domain, exposed northwest of Veitsch, consists of thick metadacites (Blasseneck Porphyroid, ca. 478 Ma), followed by (siliceous) phyllites which grade into turbiditic metasediments (Sommerauer Formation, Late Ordovician to Devonian?). Clastic sediments of the Stocker and Sommerauer Formations were sourced from northern Gondwana showing a prominent Pan-African detrital zircon peak at ca. 640 Ma. Middle to Upper Ordovician volcanics (ca. 462 Ma – 448 Ma) represent the second source. Tectonic reconstruction leads us to the arrangement of three facies domains. A shallow marine shelf facies is located in the present days southwest. A marginal basin with volcanic islands on a sloping continent, and a deep-water environment containing turbidites are situated further to the northwest. The present arrangement of these facies domains is explained by eo-Alpine and Variscan thrust tectonics.

The Gogo Formation Lagerstätte: a view of Australia's first great barrier reef

Discoveries from the Late Devonian Gogo Formation in the Canning Basin, Western Australia, have provided insights into the origin and evolution of many unique gnathostome features such as the origins of teeth, internal fertilization, air-breathing, transitional tissues between bone and cartilage, and insights into the fin-to-limb transition. Although vertebrate studies have dominated evolutionary work, invertebrate studies have added important insights into the palaeoecology of the site and demonstrated close faunal affinities along the margins of northern Gondwana and China. Geochemical analyses have broadened our understanding of the pathways involved in the exceptional preservation of this Devonian Konservat-Lagerstätte. Fossils from the Gogo Formation show extensive soft tissue preservation through phosphatization, recording anatomical details not normally obtained from fossil sites.

Mississippian southern Laurentia tuffs came from a northern Gondwana arc

Abstract A Paleozoic arc that formed by southward subduction of the Rheic oceanic plate beneath northern Gondwana has long been inferred, but its history and geochemical signatures remain poorly understood. New U-Pb ages, juvenile εHf signatures, and trace-element composition data of young zircons from tuffs at two southern Laurentia sites indicate their derivation from a continental arc that was active from ca. 328 to ca. 317 Ma and permit correlation of sedimentary sequences 800 km apart in southern Laurentia. These include the Stanley tuffs in the Ouachita Mountains of southeastern Oklahoma and southwestern Arkansas and the newly discovered Barnett tuff in the subsurface of the Midland Basin in west Texas (USA). The Barnett tuff has a zircon chemical abrasion–isotope dilution–thermal ionization mass spectrometry U-Pb date of 327.8 ± 0.8 Ma, similar to the oldest Stanley tuff in the Ouachita Mountains. Zircon Hf isotope depleted mantle model ages further suggest that the source was a continental arc on basement with both Grenville and Pan-African affinities, pointing to northern Gondwana or peri-Gondwana terranes. The new data link the tuffs to granitoids (326 Ma) of the Maya block in southern Mexico, which was part of northern Gondwana. Correlation of the Stanley-Barnett tuffs across southern Laurentia suggests the likely presence of Mississippian tuffs over a broad region in southern Laurentia, and their usefulness for constraining absolute ages of basin fills and characterizing the Gondwanan arc.

Cambrian sedimentary basins of northern Gondwana as geodynamic markers of incipient opening of the Rheic Ocean

Diachronous opening of the Rheic Ocean and separation of Avalonian–Cadomian terranes from Gondwana began with a change from an active to passive margin in the late Ediacaran to early Cambrian. During the Cambrian, extension within these terranes was recorded by magmatism and by the development of sedimentary basins. However, the timing, style, and kinematics of this transformation still remain poorly understood and plate-scale models vary significantly. To address this issue, the Příbram–Jince basin in the Bohemian Massif was chosen as a case study since it preserves an excellent record of Cambrian rifting. Here, after Cadomian subduction ceased at ∼527 Ma, extension initiated and a thick pile of continental siliciclastics was deposited in the basin between ∼515 Ma and ∼499 Ma, interrupted by marine transgression at ∼506–503 Ma. Our field, paleocurrent, and rock-magnetic data suggest that the source areas were located to the ∼ESE and to the ∼SW of the basin during the deposition of the lower and upper formations, respectively. Sediment sources changed accordingly from distant metamorphic basement (Gondwana?) and Cadomian volcanic arcs and an accretionary wedge underlying the basin. This redirection marked a change in the tectonic evolution of the basin from orthogonal to dextral oblique extension that enlarged the basin into a pull-apart structure. Integrating this depositional and tectonic record into a large-scale picture, we suggest that strike-slip movements along the former Avalonian–Cadomian belt controlled the diachronous opening of the Rheic Ocean. An inherited suture in the Avalonian ribbon terrane facilitated complete rifting and rift–drift transition while the Cadomian terranes remained attached to Gondwana. The kinematics of this event remains controversial. Either it was opposite along the westerly (sinistral) and easterly (dextral) segments of the belt, which may be explained by interaction with an intervening spreading center, or it was the same dextral transtension.

Hf isotopic constraints and detrital zircon ages for the Austroalpine basement evolution of Eastern Alps: Review and new data

The Alps, as part of the Alpine-Mediterranean Mountain chain, are one of the classical localities for orogenic studies, where the Mesozoic-Cenozoic tectonic evolution is well known, and many classical models have been proposed to explain the tectonic evolution from Mesozoic rifting and breakup to Late Mesozoic-Cenozoic subduction, plate collision, and exhumation. However, the pre-Mesozoic tectonic evolution of the pre-Alpine basement remains poorly known because of the lack of sufficient age data due to complex polyphase deformation and multiple metamorphic overprints. New data from the mainly amphibolite-facies pre-Alpine basement of the Austroalpine mega-unit indicates that this basement is composed of a heterogeneous series of continental units, island arcs, ophiolites, subduction mélanges, accretionary wedges, and seamounts affected by different metamorphic grades. This study presents new results of LA-ICP-MS U-Pb dating and MC-ICP-MS Lu-Hf isotopic tracing of zircons from three key areas of Austroalpine basement, including the: i) Wechsel Gneiss and Waldbach Complexes, and Wechsel Phyllite Unit, (ii) Saualpe-Koralpe-Pohorje, and (iii) Schladming-Seckau areas. We determine the Wechsel Gneiss Complex to be a continental magmatic arc formed during 500–560 Ma in the proximity to a continental block with a ‘memory’ of Late Archean to Early Proterozoic continental crust. The Wechsel Gneiss Complex has U-Pb ages of 2.1 to 2.2 Ga and 2.5 to 2.8 Ga that indicate a close relationship to northern Gondwana, with depleted mantle Hf model ages as old as 3.5 Ga. The Wechsel Phyllite Unit structurally overlying the Wechsel Gneiss Complex has partly different sources, including juvenile crust formed at ca. 530 Ma. In contrast, the Waldbach Complex constantly added new crustal material during the 490–470 Ma period and bears considerably more positive εHf(t) values than the underlying Wechsel Gneiss Complex and gives relatively young, depleted mantle model ages of 700 to 500 Ma. The Waldbach Complex is, therefore, interpreted to be part of a magmatic arc that formed during closure of the Prototethys and was metamorphosed during Variscan orogenic events at ca. 350–330 Ma. The Schladming-Seckau and Wechsel Complexes represent a Cambro-Ordovician magmatic arc system formed by Prototethys subduction processes with the associated Late Neoproterozoic to Early Ordovician ophiolitic Speik complex having formed in its back-arc basin or as Prototethyan lithosphere. The Plankogel Complex and structurally overlying micaschist and amphibolite units represent accreted ocean, ocean island, and continent-derived materials, interpreted to be an accretionary complex formed during the Permo-Triassic closure of the Paleotethys. Many granites of the Austroalpine mega-unit with Permian ages (e.g., porphyric granite called Grobgneiss and other granite gneisses and associated pegmatites) were likely formed in an extensional environment that culminated in the opening of the Middle-Late Triassic Meliata oceanic rift. These granites formed by partial remelting of crust with mainly Middle Proterozoic Hf model ages. Taken all these data together, we find that the Austroalpine basement is heterogeneously composed and includes complexes of different ages, different tectonic evolutionary histories and different remolten sources representing different locations before final accretion. The composite of pre-Alpine complexes in the Austroalpine mega-unit likely assembled not earlier than Late Permian or Early Triassic.

A Late Devonian plant assemblage from New South Wales, Australia: Diversity and specificity

Gondwanan floras of Late Devonian age are poorly known. In Australia, the rare studies that have been published on Late Devonian plants are old and need reinvestigation. This paper is an account of the plant macro- and micro-remains found in the Mandowa Mudstone at Barraba, New South Wales. According to the miospores, plants are late to latest Famennian in age. The record of anatomically preserved specimens is diversified, with nine taxa assigned to the Lycopsida, Cladoxylopsida, Iridopteridales and Archaeopteridales. One specimen is referrable to the spermatophytes. Several taxa are specific to Barraba, i.e., the lycopsid genera Cymastrobus and Lycaugea, the iridopteridalean genus Keraphyton, the cladoxylopsid species Polyxylon australe, and possibly a plant represented by a large Hierogramma branch showing exarch protoxylem strands. The adpression record is dominated by axes of the cosmopolitan lycopsid genus Leptophloeum. It also includes specimens interpreted as seed plants such as a possible ovule resembling Pseudosporogonites, and two types of foliage differing by their petiole width. One of this foliage consists of delicate fronds broadly comparable to those of Cosmosperma. The closest flora from Barraba is the late Famennian–earliest Tournaisian flora of the New Albany Shale in eastern USA, suggesting floral connexion and comparable environmental conditions between Northern Gondwana and Southern Laurussia.

Controls on the compositional framework and petrogenesis of Early Cretaceous first cycle quartzose sandstone, North Gondwana

Recent advances in sedimentary research have shown that quartz sandstones are scarce in the global geologic record and prolonged sediment transport does not automatically produce mineralogically mature sediments. Published data are based mainly on Cambro-Ordovician arenites. This paper discusses the formational controls of early Cretaceous syn-rift quartzose from northern Gondwana based on integrated petrographic, mineralogic, and electrofacies proxies on the Upper Sarir Sandstone Formation, Sirte Basin, Libya. The study aimed to understand the compositional framework and construe the sedimentological processes that produced the quartz-rich sandstones. Acquired samples (n = 40) from three boreholes (N8-97, N7-97, and N4-97) were subjected to petrographic analysis, x-ray diffractometry, and variable pressure scanning electron microscopy with energy dispersive x-ray spectrometry. Qualitative and quantitative datasets indicate that the mean framework composition (Q97F0L3) of the sample is quartz-dominated and primarily detrital, although diagenetic imprints, mostly quartz cementation, quartz overgrowth, and other authigenic precipitates, are present. Evidenced by the predomination of monocrystalline quartz grains (99.10%) over the polycrystalline type (0.90%), the dominance of non-undulatory extinction (72.50%), and noticeable euhedral crystals, the compositional framework of the studied quartzose reflects a predominant plutonic provenance with some metamorphic inputs. The possibility of significant sediment reworking is ruled out based on short to moderate transport distance (~100 km) interpreted from the interplay of textural parameters, depositional model, and paleogeography. A humid paleoclimate alongside its associated rigorous chemical weathering is constrained as the dominant control in the formation of the Upper Sarir quartzose sandstone from a dominant craton interior provenance. These conditions probably produced other early Cretaceous arenitic sandstones in other reaches of North Gondwanaland.

The age of the Euconochitina symmetrica Zone and implication for Lower Ordovician chitinozoan and graptolite zonations of Laurentia

An Euconochitina symmetrica Zone was defined in the G-locality at Lévis, Quebec, within an interval belonging to Zone A of Raymond that contains Paratetragraptus approximatus. The interval was consequently attributed to the Floian. E. symmetrica was also recognized in Northern Gondwana, and the species was considered an undisputed marker for the base of the Floian. During the past decades, several occurrences of E. symmetrica have been reported from Avalonia, southern China, and even from Northern Gondwana in Tremadocian strata casting doubt on the Floian age of the Zone.Revisions of the graptolites in the G locality and N section at Lévis provides a frame on which the age of the E. symmetrica Zone can be precisely documented. At both localities the E. symmetrica chitinozoan assemblage is older than the base of the Paratetragraptus approximatus graptolite Zone. Therefore, the presence of E. symmetrica can no longer be correlated with the graptolite P. approximatus Zone as it was initially suggested. It rather characterizes the upper Tremadocian Sagenograptus murrayi Zone, its upper part could however reach the Floian.An older assemblage characterized by a Lagenochitina gigas n. sp. assemblage (previously designated Lagenochitina maxima) occurs in strata correlated with the Tremadocian Aorograptus victoriae graptolite Zone.Correlation of the biostratigraphic ranges of the chitinozoan and graptolite faunas in the lower part of the Lévis Formation allowed for a better positioning of the Lower Ordovician Laurentian chitinozoan zones on the Ordovician time scale.

Low-latitude ?upper Barremian–lower Aptian palynoflora and paleovegetation of the Biyadh Formation (Arabian Plate, eastern margin of northern Gondwana): evidence for a possible cold snap

A palynological analysis of core and cuttings samples from successions assigned to the Biyadh Formation of Saudi Arabia in the Arabian Gulf (eastern margin of northern Gondwana) revealed diverse and well-preserved palynological assemblages including bryophyte and fern spores, gymnosperm and flowering plant pollen, dinocysts, and acritarchs. Palynological data corroborates earlier geological and stratigraphic models, indicating a ?late Barremianearly Aptian age. The fluctuations of dinocyst abundance and diversity proved extremely useful to detect fine-scale sea-level changes in carbonate platforms with episodic terrigenous inputs. In agreement with current understanding of dinocyst paleoenvironmental distribution trends, Cribroperidinium and Aptea? vannophora are restricted to inner neritic/?fluvial-influenced settings, while Spiniferites and Trichodinium have been confirmed as more open marine water (middle/outer neritic) indicators. The composition of the terrestrial assemblages is dominated by fluvially-transported spores, mainly released by ferns and probable tree ferns. The overall floral composition is considered to reflect a seasonal, humid, climate punctuated by episodic droughts. Recurrent increases in abundance of wind-pollinated cheirolepid and Araucariaceae pollen, observed in assemblages otherwise characterized by low dinocyst content, are interpreted to represent sea-level falls and basinward expansion/migration of conifer-dominated coastal vegetation. The abundance of the dinocysts Cepadinium variabilis and Pseudoceratium nohrhansenii sp. nov., previously recorded at higher latitudes, is interpreted to reflect the invasion of cooler/colder water masses, confirming a significant ?latest Barremian/early Aptian global cold episode, possibly related to the OAE1a event. The following new dinocyst taxa are described: Pseudoceratium nohrhansenii sp. nov. and Protobatioladium alhajrii sp. nov.