Late Early Devonian Research Articles

Was there more space in the late Early Devonian for marine biodiversity to peak than in the early Late Ordovician?: A brief note

After the so-called ?Cambrian explosion?, marine biodiversity peaked either in the early Late Ordovician (as shown by the ?classical? curves based on the extensive palaeontological data compilation) or in the late Early Devonian (as shown by the ?innovative? curve based on the sampling standardization). The brief review of the modern plate tectonic, palaeoclimatic, and eustatic reconstructions demonstrates that shelves, which likely provided the main space for biotic radiation, shrank, concentrated in the tropics, and were better connected in the late Early Devonian than in the early Late Ordovician. The results of the present analysis permit to hypothesize that there was more (or the same) space for marine organisms to reach their maximum in their number in the early Late Ordovician relatively to the late Early Devonian. This is the only particular hypothesis, and the other extrinsic and intrinsic factors should be considered in further discussions.

Depositional chemistry of chert during late Paleozoic from western Guangxi and its implication for the tectonic evolution of the Youjiang Basin

Continual deep-water sediments from the late Early Devonian to the Late Permian extended in wide areas of western Guangxi. We analyzed the major, trace, and rare earth elements of the Upper Paleozoic cherts in Badu, western Guangxi. High non-terrigenous SiO2 contents (Sinon-ter/Sibulk(%)> 80%) and pure chert components (> 70%) indicate a large extent of silicification in the Upper Paleozoic cherts, except for the Upper Devonian-Lower Carboniferous Luzhai Formation cherts, which have lower non-terrigenous SiO2 contents (avg. 71.8%) and pure chert components (40%–70%). The Al/(Al+Fe+Mn) ratios and Feter/Febulk(%) values of samples from the lowest horizon of the Pingen Formation are 0.05–0.26, 13.1%–14.5%, respectively, indicating hydrothermal origins. All other samples show high Al/(Al+Fe+Mn) ratios (0.39–0.81) and high Feter/Febulk(%) values (23.1%–186.8%), indicating non-hydrothermal origins. The Pingen Formation and Liujiang Formation cherts show slightly-moderately negative Ce anomalies (0.71±0.07, 0.81±0.08, respectively) and higher Y/Ho ratios (33.49±1.27, 36.10±2.05, respectively) than PAAS. This suggests that these cherts were deposited in the open marine basin, rather than in the intracontinental rift basin as previously assumed. The Luzhai Formation cherts may be deposited near the seamount or seafloor plateaus with no negative Ce anomalies (1.09±0.07) and no significant Y-Ho fractionation (Y/Ho=28.60±1.25). The Nandan Formation and Sidazhai Formation cherts were deposited in the open-ocean basin with moderately negative Ce anomalies (0.67±0.08, 0.73±0.11, respectively) and high Y/Ho ratios (36.01±1.00, 32.00±2.25, respectively). On the basis of our studies about cherts, we conclude that the Youjiang Basin originated as part of the Paleo-Tethys that controlled the depositional environments of cherts during late Paleozoic. The rift of the Youjiang Basin had occurred at least since the Early-Middle Devonian. The basin had a trend of evolving into an open-ocean basin during the Early-Middle Permian.

Tectonics of the North Qilian orogen, NW China

The Qilian Orogen at the northern margin of the Tibetan Plateau is a type suture zone that recorded a complete history from continental breakup to ocean basin evolution, and to the ultimate continental collision in the time period from the Neoproterozoic to the Paleozoic. The Qilian Ocean, often interpreted as representing the “Proto-Tethyan Ocean”, may actually be an eastern branch of the worldwide “Iapetus Ocean” between the two continents of Baltica and Laurentia, opened at ≥710Ma as a consequence of breakup of supercontinent Rodinia.Initiation of the subduction in the Qilian Ocean probably occurred at ~520Ma with the development of an Andean-type active continental margin represented by infant arc magmatism of ~517–490Ma. In the beginning of Ordovician (~490Ma), part of the active margin was split from the continental Alashan block and the Andean-type active margin had thus evolved to western Pacific-type trench–arc–back-arc system represented by the MORB-like crust (i.e., SSZ-type ophiolite belt) formed in a back-arc basin setting in the time period of ~490–445Ma. During this time, the subducting oceanic lithosphere underwent LT-HP metamorphism along a cold geotherm of ~6–7°C/km.The Qilian Ocean was closed at the end of the Ordovician (~445Ma). Continental blocks started to collide and the northern edge of the Qilian–Qaidam block was underthrust/dragged beneath the Alashan block by the downgoing oceanic lithosphere to depths of ~100–200km at about 435–420Ma. Intensive orogenic activities occurred in the late Silurian and early Devonian in response to the exhumation of the subducted crustal materials.Briefly, the Qilian Orogen is conceptually a type example of the workings of plate tectonics from continental breakup to the development and evolution of an ocean basin, to the initiation of oceanic subduction and formation of arc and back-arc system, and to the final continental collision/subduction and exhumation.

Diagenetically-enhanced trilobite obrution deposits in concretionary limestones: The paradox of “rhythmic events beds”

Certain offshore facies with extraordinarily well preserved fossils, particularly trilobites, display distinctive motifs of rhythmically bedded, decimeter-scale, concretionary limestone and dark gray shale. Examples are known from Cambrian to Neogene age, but are especially common in the lower-middle Paleozoic. The Hollardops trilobite beds of the Lower Devonian (Emsian) Khebchia Formation, Morocco, provide the exemplar of this phenomenon. Comparative examples include trilobite beds from the Upper Ordovician Katian of Oklahoma, New York State, and Ontario, Canada, and the Lower to Middle Devonian (Lochkovian–Givetian) of Tennessee, Oklahoma, and New York State. These Lagerstätten are observed to crosscut facies belts, occuring in both sparsely fossiliferous, dysoxic facies and more abundantly fossiliferous sections. Articulated fossils occur in both limestones and interbedded shales, indicating episodic deposition of fine grained sediments in both instances, but those in the shales are highly compressed and difficult to extract. Conversely, limestones yield particularly well preserved fossil material, including in situ lingulid brachiopods, and bivalves, cephalopods and complete outstretched, and enrolled trilobites. Articulated, uncompressed fossils may occur in varied orientations relative to bedding. This suggests that the organisms were locally entrained in mudflows, although occurrence of exoskeletal molt ensembles in some beds suggests that in certain events organisms could be preserved in situ. Enclosing sediments are typically strongly bioturbated indicating prolonged periods of sediment starvation following mudflow events but prior to early diagenetic cementation; however, most burrows are small and evidently did not disrupt entombed organism remains. Pyritic and calcite spar fillings in internal spaces of fossils formed early and rarely preserve remnants of lightly sclerotized tissues. Carbonate cementation occurred during interludes of low sedimentation; association with pyrite suggests formation within the zone of sulfate reduction.These intervals are particularly well developed in relatively sediment-starved, transgressive intervals and constitute a type of time-specific facies; these rhythmic obrution beds are especially characteristic of the Late Ordovician, and late Early Devonian (Emsian), where they occur on at least four paleocontinents. These beds formed selectively in warm, in semi-arid climates in which minor siliciclastics supplied from a relatively low relief hinterland were episodically re-suspended in slurries and transported offshore; they were favored in systems poised between deposition of siliciclastic and carbonate muds. The commonly rhythmic bedding of these deposits records a regular, recurring cyclic motif with overall durations of 10s of Kyr of carbonate redistribution superimposed upon muds that include abundant obrution deposits.

The Salinic Orogeny in northern New Brunswick: geochronological constraints and implications for Silurian stratigraphic nomenclature1This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

The Salinic Orogeny is defined to encompass tectonic interactions that affect all elements of Ganderia involved in the closure of the Tetagouche–Exploits back-arc basin between the Late Ordovician and Early Devonian. Hence, the D1 and D2 deformations in the Miramichi Highlands and Elmtree Inlier of northern New Brunswick are Salinic events, and onlap of Lower Silurian rocks onto exhumed parts of the Brunswick Subduction Complex represents the earliest (Salinic A) of three Silurian unconformities in the region. Upper Ordovician to Lower Silurian rocks of the Matapédia successor basin contain widespread evidence of Middle Silurian tectonism (e.g., disconformities, angular unconformities, and fold interference patterns) created by Devonian overprinting of Silurian folds lacking axial planar cleavage (Salinic B). Recent U–Pb radioisotopic dating of chemically abraded zircon from rhyolite just above the Salinic B unconformity has yielded an age of 422.3 ± 0.3 Ma; combined with late Early Silurian fossil ages just below the unconformity, this indicates a ca. 5 million year Middle Silurian hiatus. Finally, Upper Silurian (Ludfordian) rocks are locally disconformably overlain by polymictic conglomerates that form the base of the Devonian section (Salinic C). All Silurian rocks in northeastern New Brunswick have historically been included in the Chaleurs Group; however, unconformities and local stratigraphic variations (especially compared with the type locality) support the introduction of new higher rank names in New Brunswick. Hence, the Quinn Point Group is introduced to incorporate Lower Silurian rocks, the Petit Rocher Group to include Upper Silurian sedimentary rocks in the Nigadoo River Syncline, and the Dickie Cove Group for Upper Silurian volcanic rocks in the Charlo – Jacquet River area. Upper Silurian rocks west of Campbellton that are contiguous with the Chaleurs Group in Quebec, will remain part of the Chaleurs Group.

Late Emsian Rutoceratoidea (Nautiloidea) from the Prague Basin, Czech Republic: morphology, diversity and palaeoecology

Abstract: Nautiloids of the superfamily Rutoceratoidea from the late Emsian (late Early Devonian) of the Prague Basin (Czech Republic) are commented upon. Species recognized include the hercoceratids Hercoceras mirum, H.? transiens, Ptenoceras proximum, P. nudum, P. minusculum and Anomaloceras anomalum, as well as the rutoceratids Adelphoceras bohemicum, Homoadelphoceras devonicans, Pseudorutoceras bolli and Goldringia? devonicans. In addition, four new species are described: Parauloceras regulare sp. nov., Roussanoffoceras chlupaci sp. nov., Otomaroceras sp. nov. and Goldringia sp. nov. Morphology and distribution patterns of Pragian and late Emsian rutoceratoid faunas from the Prague Basin are compared. They show that an increased diversity was accompanied by a higher level of specialization of rutoceratoids, which manifested itself in low abundance, increased facies dependence and greater variation in shell size during the Early Devonian. The evolution of sculpture and a contracted aperture, both regarded as protective adaptive features, was also examined, but no adaptive trend towards more pronounced sculpture and constriction of the aperture was found to have occurred in the Early Devonian. A more distinctive sculpture was, however, observed in shallow‐water assemblages of P. proximum in comparison with deeper‐water faunules, and two additional cephalopod species were examined in order to obtain comparative data. The presence of distinct sculpture patterns in coeval shallow‐ and deeper‐water assemblages suggests limited migration between them and consequently reflects some degree of territoriality in Devonian nautiloids. New data on early shell development in P. proximum are presented. During the Choteč Event, rutoceratoid generic diversity dropped dramatically, one family became extinct and the Early Devonian diversification of the group came to an end. The recovery of nautiloid faunas was slower than that of other cephalopods and associated, unrelated invertebrates. The absence of change in abundance patterns between Pragian and late Emsian rutoceratoid faunas, i.e. prior to and subsequent to ammonoid radiation, suggests that the appearance and radiation of the latter group in the early Emsian did not affect the structure of nautiloid assemblages, i.e. these two clades did not occupy the same niches.

Turbidite pathways, pore-fluid pressures and productivity in the Central Wales Orefield

Abstract: Lodes in the Central Wales Orefield are mineralized oblique-slip normal faults that postdate the regional joints. They developed shortly after the (Acadian) deformation of the Welsh Basin in the late Early Devonian and many were reactivated with further mineralization in the early Carboniferous. Although orefield extent is clearly centred on the structural culmination around the Plynlimon Dome the major areas of lead–zinc production are strongly off-centered. These areas lie on the depositional pathways of three horizons of late Ordovician–early Silurian massive turbidite sandstones. It is argued that these basin-floor sand bodies are preferential sites of lode nucleation and that their distribution controls the size and abundance of the lodes and hence their chances to link laterally and at depth. This in turn controls their ability to capture and transmit mineralizing fluids and thus their chances of hosting large ore deposits. The prime cause of high fluid pressure gradients during the first phase of mineralization was rapid uplift of ‘tight' rock with dehydration fluids from Acadian metamorphism. Second phase fluids probably originated as seawater and were driven by convective overturn at essentially hydrostatic pressures. The stratigraphic control of lode productivity has implications for mineral exploration in such terranes.

The North Devon Basin: a Devonian passive margin shelf succession

The North Devon Basin, situated in a more proximal passive margin regime than the rift basins to the south, is not constrained but its succession is thought to represent in large part the sediments debouched from a northerly hinterland. Rather than that immediate source being South Wales an original location of the basin well to the south-east and west of the Ardennes massif is considered probable, with its present position being attained by Carboniferous displacement along the Bristol Channel-Bray Fault. The basin's thick (6000 m) succession comprises terrestrial and marine deposits that form two major sedimentary cycles, which are apparently closely linked to rift basin formation to the south. The GCR sites span a relatively straightforward shelf succession that extends from the late Early Devonian to the Carboniferous. The sedimentology, palaeontology, and depositional environments of terrestrial and marine facies lithostratigraphical units are detailed, some sites providing the macrofossil assemblages important in the identification and definition by Sedgwick and Murchison of the Devonian System.

Jardamarekia enigma, a new Early Devonian tryblidioidean from Royal Creek area (Yukon Territory, Canada), and paleobiogeography of the Early Devonian of northwestern Canada

The Tryblidia (= Monoplacophora) represents the conchiferan class with the fewest Recent taxa in the phylum Mollusca (Haszprunar 2008) and its phylogeny is still poorly known. This group is known already in Cambrian strata (Early Paleozoic) more than 500 Ma ago. Present-day tryblidian species are known mainly from hadal environments (Schwabe 2008, but see also Wilson et al. 2009) in contrast to Paleozoic species, which have been described only from shallow environments of continental shelves of many paleocontinents (e.g., Horný 1962). A typical feature of fossil as well as living tryblidian species is their rarity. The vast majority of species are known only from several specimens (Haszprunar 2008). Furthermore, description of Paleozoic tryblidian molluscs is strongly underrepresented in the literature, despite the existence of diverse material. This is also true for fossils described in the present study based on a diverse silicified molluscan fauna of mostly gastropods collected from Lower Devonian strata of the Royal Creek area, Yukon Territory (Fig. 1) by Alfred C. Lenz and David G. Perry from 1970–1980. Lists of all hitherto described molluscan species as well as detailed information on their age and locality can be found in Lenz (1977a), Blodgett et al. (2001, 2010) and Frýda et al. (2008). Prior to our studies of the Royal Creek tryblidian and gastropod fauna, no descriptions or illustrations were available for Lower Devonian molluscs of north-western Canada, although a short discussion and faunal lists were provided by Blodgett et al. (1988) for Lower Devonian tryblidians and gastropods from the relatively nearby Delorme Formation of Northwest Territories, and early Emsian (late Early Devonian) tryblidians and gastropods from the Mt. Lloyd George area, northeastern British Columbia. The poor knowledge of Paleozoic tryblidians and gastropods faunas of Laurentia (North America) caused difficulties in the evaluation of Early Devonian paleobiogeography (Blodgett et al. 1999). The present paper is focused on the taxonomy of a new Devonian tryblidian limpet, but it provides also useful data for paleobiogeography and biostratigraphy of the Lower Devonian of western Canada.

Cross-Basin Variations in Magnetic Susceptibility Influenced by Changing Sea Level, Paleogeography, and Paleoclimate: Upper Devonian, Western Canada Sedimentary Basin

Abstract Upper Devonian rocks in western Canada were deposited as carbonate-platform and surrounding slope and basinal rocks during a second-order sea-level cycle, characterized by long-term climatic warming, that ended with cooling associated with the Frasnian–Famennian mass-extinction event. In this contribution, we assess the hypothesis that high-resolution magnetic susceptibility (MS) data records cross-basin variations that are intimately tied to sea level and associated climatic change, the depositional environment, and the paleogeographic location of siliciclastic sediment sources. Four main trends are apparent in the MS data: (1) the magnitude of the MS signature increases from west to east across western Canada; (2) platform facies generally have low MS signatures compared with off-platform facies; (3) the overall MS signature increases and then decreases throughout the stratigraphic profile, largely in response to changes in depositional setting (i.e., item 2); (4) MS is generally high during third- and fourth-order sea-level lowstand and early transgression and lower during late transgression and highstand. During the late Middle (Givetian) to early Late (Frasnian) Devonian, siliciclastic sediments (silt and fine sand size) sourced from the west Alberta arch and Peace River arch, contributed to relatively high MS signatures in facies deposited proximal to these source areas. Other important detrital sediment sources were the Ellesmerian orogenic belt in northern Canada and the Laurussian continental landmass to the east. These latter sources supplied fine-grained sediments that were dispersed basinwide and resulted in a nearly order-of-magnitude increase in MS values to the east. This trend and fine grain size implies that some detrital sediment was delivered from the east as eolian dust from the “Old Red Continent.” By the early Frasnian, the west Alberta arch was submerged, and by middle to late Frasnian time the Peace River arch was largely covered. During the late middle Frasnian a pronounced rise in MS signatures can be tied directly to deposition of the clay- and locally silt-rich Mount Hawk Formation. During the late middle to late Frasnian, carbonate-ramp facies prograded from east to west, resulting in seaward migration of the shoreline and increased siliciclastic input to the ramp slope that influenced the pronounced increase in MS signatures to the east. By the late Frasnian, cross-basin MS profiles record uniformly low susceptibilities indicating that high MS siliciclastic detritus was either bypassed seaward, was swamped by carbonate input, or was dominated by diamagnetic quartz. Individual MS events can be accurately correlated across much of the basin at a resolution that is higher than single conodont zones despite the local influences on variability of the signature. Variations in the MS signature can thus be explained partly by depositional environment (platforms with low MS, basins with higher MS) and partly by third- and fourth-order sea-level and associated climatic changes. Fourth-order changes in sea level are routinely interpreted to result from variations in paleoclimate in the Milankovitch band and recent work indicates that third-order sea-level changes may be linked to longer-term modulation of Milankovitch-band orbital variations. The increase in MS during prolonged Frasnian warming thus appears to be directly linked to paleoclimatic change, and comparison of the MS signature with more direct measures, like oxygen isotopes, would serve as a test of its utility as a paleoclimate proxy.

Palaeozoic Foraminifera: Systematics, palaeoecology and responses to global changes

Because the Foraminifera are very sensitive to various environmental parameters (e.g., water temperature, salinity, light, etc.), there are important proxies used for palaeoenvironmental and palaeogeographic reconstructions. The evolution of the structure, shape and size of the mineralized tests of Foraminifera can directly reflect the variation of these parameters through geological time. Furthermore, their biostratigraphic value has been widely demonstrated. In this context, the systematics, evolution and ecological behaviour of the first mineralized Palaeozoic Foraminifera are important to discuss in order to have a clearer picture of former shallow marine environments, and finally understand their distribution through space and time. The systematics of the fossil group of Foraminifera that first developed a mineralized test remains under discussion. These early foraminifers are considered as Textulariata (as generally admitted), recrystallized Fusulinata or an independent group, sometimes called Astrorhizata. In this paper, we argue to assign the early foraminifers to the Fusulinata, and to subdivide this class into six orders: Parathuramminida, Archaediscida and Earlandiida (forming together the subclass Afusulinana n. subcl.), and Tournayellida, Endothyrida and Fusulinida (subclass Fusulinana nom. translat.). These subdivisions are discussed and linked to the first occurrences of the later classes: Miliolata, Nodosariata and Textulariata. The environmental living conditions of the first fossilized foraminifers remain enigmatic during the Early Palaeozoic (Cambrian-Silurian). During the Late Silurian, the unilocular Parathuramminida started to colonize the inner parts of ramps and platforms. The first plurilocular microgranular foraminifers (Semitextulariidae, Nanicellidae, and Eonodosariidae) developed in back-reefal systems and in deeper-water environments (“griottes”-type nodular limestone) from the late Early Devonian to the early Late Devonian. The Moravamminida, another group of possible Protista, are typical markers of Devonian inner ramp systems. The Semitextulariidae, Nanicellidae, and Eonodosariidae did not survive the Frasnian/Famennian crisis. From the Tournaisian to the Serpukhovian (Mississippian subsystem or Early Carboniferous), numerous new genera and species of Archaediscida, Tournayellida and Endothyrida flourished but remained confined to inner ramp environments. In deeper water depositional systems (i.e. coral thrombolite microbialites and/or nodular limestones), a few opportunistic Foraminifera were living up to the disphotic zone. During the Pennsylvanian (Bashkirian to Gzhelian), the habitats extended to more confined, shallower areas of the inner ramp (with Staffelloidea). During the Late Carboniferous and Permian, the larger Fusulinida (Schwagerinoidea) reached the outer platform as they have been commonly reworked in calciturbidites. During the Late Permian, some taxa were even able to live in hypersaline environments such as sabkhas and hypersaline lagoons. Two major biotic crises occurred during the Permian (post-Middle and post-Late Permian crisis), but the number of survivors after the PTE (Permian/Triassic Extinction) is probably higher than previously admitted. From the Cambrian to the Serpukhovian, the Foraminifera were probably all infaunal or living at the sediment/seawater interface. The TROX and TROX-2 models are consequently applicable. Anoxia, often suggested as triggering environmental crises, was likely not systematically lethal for many infaunal foraminifers. The late Tournaisian-Changhsingian Tetrataxis genus was probably the first epiphyte foraminifer, because of its conical, limpet-like test. The Tetrataxidae (e.g., Tetrataxis, Pseudotaxis and Abadehella) constituted the unique trochospirally coiled plurilocular foraminiferal family of the Palaeozoic. The Bradyinoidea, Ozawainelloidea, Staffelloidea, and the Pseudoschwagerinidae (Schwagerinoidea) are other examples of Pennsylvanian-Permian epiphytes but cannot be considered as planktonic taxa. All the other Schwagerinoidea are related to high-energy environments and coarse-grained substrates. Their history, as well as that of the Neoschwagerinoidea, was likely subject to the vicissitudes of their endosymbiotic algae.

Crustal seismic velocity structure of southern Poland: preserved memory of a pre-Devonian terrane accretion at the East European Platform margin

AbstractThe updated geological and potential fields data on the East European Platform margin in SE Poland confirm the existence of several regional units differing in Ediacaran to Silurian development: the Upper Silesian Block, Małopolska Block and Łysogóry Block. All the blocks are characterized by a distinct crustal structure seen in Vp velocity models obtained from the seismic refraction data of the CELEBRATION 2000 Programme. The first two units are interpreted as exotic terranes initially derived from Avalonia-type crust and ultimately accreted before the late Early Devonian. The Łysogóry Block is probably a proximal terrane displaced dextrally along the Baltica margin. The sutures between the terranes do not precisely match lateral gradients in Vp models. This is partly explained by a limited resolution of refraction seismic data (20 km wide interpretative window). Most of the difference is related, however, to a post-accretionary tectonism, mainly Variscan transtension–transpression. The latter processes took advantage of lithospheric memory recorded earlier as zones of rheological weakness along the former suture zones. The course of the East European Platform margin (= Teisseyre–Tornquist Zone) corresponds most likely to the Nowe Miasto–Zawichost Fault marking the NE boundary of the proximal Łysogóry Terrane.

Palaeohighs: their influence on the North African Palaeozoic petroleum systems

Abstract We present new insights for the characterization of the petroleum system evolution in North Africa based on a review of the stratigraphic architecture description of some selected North African Palaeozoic basins. During Palaeozoic time, the Gondwana platform was divided into sub-basins bounded by structural highs. Most of the highs were inherited from north–south and SW–NE Pan-African crustal faults which were reactivated during the Palaeozoic and later, in the Austrian and Alpine tectonic phases. We studied the stratigraphic architecture of the Palaeozoic succession around four main highs showing a clear tectonic activity during the Palaeozoic sedimentation. The Gargaff Arch, in Libya, is a major SW–NE broad anticline which slowly grew up during the Cambrian and Ordovician and stopped rising during the Silurian. The activity resumed during Late Silurian and early Devonian and during the Late Devonian. The Tihemboka High is a north–south anticline in between Libya and Algeria. The uplift started during the Cambro-Ordovician then stopped during most of the Silurian. The activity resumed during the Late Silurian and continued until the Lower Carboniferous. The Ahara High, separating the Illizi and Berkine basins in Algeria, has continuously grown during the Cambro-Ordovician, stopped rising during the Silurian, and grew again continuously during the Devonian. The Bled El-Mass High is a part of the Azzel-Matti Ridge separating the Ahnet and Reggane basins in Algeria. The high mostly rose during the Cambro-Ordovician then subsided relatively less quickly than the surrounding basins during the Silurian and Devonian. The uplift timing and chronology of each palaeohigh partly controlled the petroleum systems of the surrounding basins. Topographic lows favoured the occurrence of anoxic conditions and the preservation of Lower Silurian and Frasnian source rocks. Complex progressive unconformities developed around the palaeohighs form potential complex tectonostratigraphic traps. Finally, hydrocarbons could have been trapped around the highs during pre-Hercynian times, preserving reservoir porosity from early silicification. Mixed stratigraphic–structural plays could then be present today around the highs.

Devonian (Eifelian) disparid crinoids from the Zeravashan range, southern Tien Shan, Uzbekistan

No Devonian disparid crinoids have been described from Uzbekistan, although parahexacrinid thecae and assorted columnals have been described in several papers in the past 50 years. Discovery of a cup of Pisocrinus and a thecae of Haplocrinites are the first of these genera known from Uzbekistan. The stratigraphic range of Haplocrinites is revised to be late Early Devonian (Emsian) to Early Carboniferous (Tournaisian) because Silurian species assigned to the genus are based on loose ossicles that morphologically do not belong to the genus. The paleogeographic ranges of Pisocrinus and Haplocrinites are extended into Uzbekistan. Haplocrinites uzbekistanensis n. sp. is described.

Late Silurian and Early Devonian Beyrichioidea from Gondwana and Perigondwanan terranes and their palaeobiogeographical implications

Abstract The Late Silurian to Early Devonian palaeogeography and the question whether there has been a large and deep Rheic Ocean between Laurussia and Gondwana is still a matter of discussion. It has been assumed in many papers that the Rheic Ocean formed a palaeogeographical barrier and, therefore, Gondwana had no common beyrichioidean ostracode faunas with Laurussia before the Emsian. The appearance of ostracodes (particularly the beyrichioideans) in Gondwana at this time should testify the closure of the Rheic Ocean. Although some palaeogeographic reconstructions consider common ostracodes in Laurussia and the Perigondwanan areas Armorica and Perunica in the Lochkovian, they do not refer to occurrences in Gondwana. However, several publications describe late Silurian to Pragian beyrichioideans from Gondwana and adjoined Terranes (Perigondwana), thus questioning the existence of the Rheic Ocean at this time. This paper aims to summarize the occurrences of common beyrichioidean ostracodes from (Peri-)Gondwana and Laurussia in an attempt to provide a reliable data source for future work. The occurrences of beyrichioideans from North Africa, South America, southeastern Turkey, Ibero-Armorica, and the Ossa Morena Zone (Spain) are reviewed and their ages are partially re-dated through correlation with other faunas. The presented results indicate that shallow marine migration paths must have existed, thus strengthening the arguments against the Rheic Ocean.

Emsian (Late Early Devonian) sponges from west-central and south-central Alaska

Relatively common specimens of the hypercalcified agelasiid sponge Hormospongia labyrinthica Rigby and Blodgett, 1983 and specimens of associated species of Hormospongia have been previously reported from Emsian and Eifelian stratigraphic units at several localities in south-central and southeastern Alaska (Rigby and Blodgett, 1983). Those sponges were first described from the type section of the Eifelian Cheeneetnuk Limestone in the McGrath A-5 quadrangle. Since then several additional specimens of Hormospongia labyrinthica have also been collected from a new locality in the Talkeetna C-6 quadrangle in south-central Alaska (Figs. 1, 2.1), and are documented here.

The Earliest Land Plants

Considerable progress has been made in documenting evidence of very early plants starting in the basal Ordovician employing dispersed spore, phytodebris, and mesofossil data. Macrofossil evidence is sparse until Late Silurian, but recent new data are improving our understanding of aspects of earliest plants. The considerable information about the possible source of cryptospores and trilete spores especially from the well-preserved mesofossils of the Late Silurian and Early Devonian is summarized. Promising avenues of research are the study of spore ultrastructure, and neo-paleo comparisons between newly discovered resistant components of extant bryophytes and fragmentary fossil remains. Recent macrofossil discoveries in the Late Silurian advance our understanding of early events in plant evolution and raise new questions about the timing of evolution or relationships among earliest (mostly vascular) plants.

Functional morphology of chonetidine (Brachiopoda) spines: biomechanical tests of a potential key innovation

Chonetidine brachiopods were a significant component of Paleozoic marine life; chonetidines experienced a major adaptive radiation during the Late Silurian and Early Devonian. The Chonetidina clade is united by the presence of spines on the hingeline but the function of these spines has not been clearly demonstrated. The present study performs a biomechanical experiment, using specimens of Neochonetes granulifera with and without artificial spines in a recirculating flume, to test if the spines may have inhibited entrainment in higher energy settings. Specimens with spines were less likely to be overturned or transported than those without spines, and were overturned or transported at significantly higher velocities than specimens without spines. Performance improved with the addition of another pair of spines. In addition, spinose specimens reoriented such that their commissures faced upstream. The results suggest that chonetidines could have survived in higher-energy environments, even if the brachiopods were not physically attached to the substrate. As such, it is functionally plausible that the evolution of hinge-spines may have possibly facilitated the adaptive radiation of the clade by enabling chonetidines to inhabit previously unoccupied habitats.

Silurian–Devonian active-margin deep-marine systems and palaeogeography, Alai Range, Southern Tien Shan, Central Asia

Analysis of Mid-Palaeozoic successions in the northern part of the Alai Range (Kyrgyzstan and bordering Uzbekistan), Southern Tien Shan, Central Asia, has identified a Silurian–Devonian deep-marine depositional system of basin-slope facies-associations. Here, we document the stratigraphy and sedimentology of a region in Central Asia that, through conflict, has become inaccessible for geological research. The turbidite-dominated Pul'gon Formation (Silurian) accumulated in sea-floor depressions and within the inferred basin axis. The large-scale, coarse clastic lenses of the Dzhidala Formation (mostly Devonian) represent the fills of submarine channels, canyons or gullies of the palaeoslope; other slope apron processes include sediment slides, debris flows and olistoliths. The partly time-equivalent condensed sequences of the Mid- to Late Silurian Kursala Formation and Early Devonian Tamasha Formation represent graptolitic mudstone and chert accumulation, respectively, together with thin dolomitic limestones, that accumulated over c . 10–15 Ma, probably on seamounts in the Turkestan (Fergana) Ocean between the continental margin of Kazakhstania and the Alai microcontinent. The graptolitic shale-rich Chakush Formation (early Silurian) is geographically and petrographically different from the Pul'gon and Dzhidala formations and suggests a different provenance (opposing continental margin or seamount talus).

Early Archean to Middle Jurassic Evolution of the Korean Peninsula and Its Correlation with Chinese Cratons: SHRIMP U‐Pb Zircon Age Constraints

U‐Pb zircon ages of tuffs and sandstones of the Daedong Supergroup (Bansong and Nampo groups) in the Korean Peninsula were determined using a sensitive high‐resolution ion microprobe (SHRIMP) in order to constrain their age of sedimentation and to unravel discrete geologic events as recorded in detrital zircons. The ages of four tuffaceous samples from the Bansong Group imply that the Daedong Supergroup formed at ca. 187–172 Ma in association with the Early‐Middle Jurassic orogeny. These data are in marked contrast with paleomagnetic arguments suggesting that the Bansong and Nampo groups are precollisional Early‐Middle Triassic deposits that are correlative with the North and South China blocks, respectively. Detrital zircons of the Daedong Supergroup define seven age components: (1) Early‐Middle Archean (3.64–2.97 Ga), (2) Late Archean–middle Early Proterozoic (2.63–2.33 Ga), (3) late Early Proterozoic (1.98–1.75 Ga), (4) Middle‐Late Proterozoic (1.2–0.6 Ga), (5) Devonian (400–355 Ma), (6) Early Permian (280–255 Ma), and (7) Middle Triassic–Early Jurassic (240–180 Ma). These age distributions, together with available geochronological data, suggest that crustal growth of the Korean Peninsula has continued since ca. 3.6 Ga and culminated at ca. 2.5 and 1.9–1.8 Ga. Major age populations of detrital zircons of the Bansong and Nampo groups are similar, except for the presence of Middle‐Late Proterozoic ages in the latter. Inasmuch as these ages are characteristic for the South China Block, the Gyeonggi massif, or at least the local source of the Nampo Group, is most likely a correlative of the South China Block.