Hirnantian Age Research Articles

Geological Heritage of the Três Barras Farm section, Ordovician-Silurian record in the Paraná Basin, Brazil

The Três Barras Farm section is an outcrop located at the northern border of the Paraná Basin, in Midwest Brazil. Rocks of the Ordovician-Silurian age are found in this section, represented by the Iapó and Vila Maria formations of the Rio Ivaí Group, which directly overlay the crystalline basement. The Devonian rocks of the Furnas Formation are also observable at the top of the Três Barras section. Although the first fieldwork conducted there dates back to 1985, the section has been visited by different generations of researchers, often executing paleontological studies. The fossil record in this section comprises invertebrates such as mollusks and brachiopods, mineralized microfossils such as ostracods, and palynomorphs such as acritarchs and cryptospores. The different analyses conducted until the 2000s yielded a Lower Silurian age to the section's complete Vila Maria Formation strata, and no invertebrate fossil records were reported. Since then, much of the knowledge regarding the Rio Ivaí Group and the Três Barras Farm section has evolved. Recent work has questioned the assigned Silurian age, and fossil ostracods and brachiopods indicate the Hirnantian age to the Iapó and Vila Maria formations strata. Recent palynological research reported palynomorphs in the Iapó Formation for the first time, and the overall assemblage endorses the post-glacial paleoenvironment interpretation. Significant knowledge gaps persist despite notable advancements in understanding the palaeobiodiversity of the Iapó and Vila Maria formations, particularly due to recent discoveries at the Três Barras Farm. International correlation studies, especially in coeval basins of the Gondwana, need to be conducted to reinforce the chronobiostratigraphic repositioning of the upper portion of the Rio Ivaí Group on the north flank of the Paraná Basin.

Orbital forcing and paleoenvironmental changes across the upper Ordovician glaciation-lower Silurian hot shale in the Risha gas field, northeast Jordan

This paper presents a comprehensive analysis of sedimentary successions and paleoenvironmental indicators in two wells, Rh3 and Rh46, located in NE Jordan, within the context of the Ordovician-Silurian boundary transition. By integrating spectral analysis, cyclostratigraphy and sedimentary architecture, significant shifts and cyclical patterns are identified, shedding light on paleoenvironmental changes and orbital forcing dynamics. The study reveals distinct lithological and geochemical signatures at the Risha/Mudawwara (Hirnantian/Rhuddanian) boundary, indicative of major shifts in depositional environments. Analysis of Milankovitch cycles suggests orbital forcing as a driver of sedimentary cyclicity, with implications for understanding climatic variations during the Ordovician-Silurian transition. Spectral analysis revealed significant cycles with different periodicities. Testing the sediment accumulation rates (SAR) revealed an optimum SAR at 5.9 and 4.6 cm/kyr in Rh3 and Rh46 wells, respectively. A new refined astronomical time scale (ATS) is created based on the 100 kyr eccentricity cycle. The new ATS assigns the Risha Formation to the Hirnantian age and revealed a probable time gap at the base of Risha Formation of about 0.3 myr in Rh46 well; deposition of the Risha Formation commenced at 444.88 myr and 444.58 myr in Rh3 and Rh46 wells, respectively, with sedimentation continuing up to the Ordovician/Silurian boundary at 443.07 myr. The Ordovician/Silurian transition experienced shortening in the obliquity and precession cycles. The tuned data show that the 41, 36 and 31 kyr obliquity cycles, in addition the 21, 19 and 16 kyr precession cycles, were recognised. Furthermore, 740, 425, 250, 147 and 100 kyr eccentricity cycles were also recognised. Notably, the study identifies the glacial/fluvioglacial Risha Formation is bounded by g4-g3 eccentricity nodes coinciding with Hirnantian stage boundaries that correspond to significant geological events, such as the Late Ordovician Mass Extinction and the Hirnantian glaciation events. The g4-g3 eccentricity cycle is estimated to be 1.8 myr. Surprisingly, the lower Silurian OAE coincided with the g4-g3 eccentricity node at the Ordovician/Silurian transition. Additionally, the application of DYNOT modelling has elucidated sea-level fluctuations and their relationship with sedimentary sequences, highlighting the role of tectonic and climatic factors in shaping the sedimentary architecture. Furthermore, the investigation of Th/U and Th/K ratios provides insights into marine and terrestrial depositional settings.

A new species ofHirnantia(Orthida, Brachiopoda) and its implications for the Hirnantian age of the Ellis Bay Formation, Anticosti Island, eastern Canada

AbstractAnticosti Island, eastern Canada, records an exceptionally thick and well-exposed Ordovician/Silurian boundary section that hosts a series of diverse marine invertebrate faunas across the Late Ordovician mass extinction. However, the base of the terminal Ordovician stage, the Hirnantian, has been difficult to identify on Anticosti due to the lack of a traditionalHirnantiafauna within the Upper Ordovician Ellis Bay Formation. Previously, the eponymous taxon of theHirnantiafauna, and type species of the genusHirnantia,H. sagittifera(M'Coy, 1851) has been reported only from the uppermost Ellis Bay Formation, leading to uncertainty as to the age of the lower Ellis Bay Formation. Here we reportHirnantia notiskuanin. sp. from the lowermost Ellis Bay Formation. This new species is similar to the type species,H. sagittifera, but is distinguished by its strongly dorsibiconvex shell in mature forms and variously developed uniplicate anterior commissure. Occurrences of these two similar species ofHirnantia,H. notiskuaniandH. sagittifera, within the lower and uppermost Ellis Bay Formation, respectively, indicate a Hirnantian age for the entire Ellis Bay Formation, a finding that is supported by recent palynological and chemostratigraphic studies. Brachiopod assemblages within the Ellis Bay Formation therefore are best characterized as a unique and diverseHirnantiafauna, consisting of genera from both the typicalHirnantiafauna and the epeiric seas of Laurentia.UUID:http://zoobank.org/1c1dff6a-ee38-4cd3-b8bd-3d803a2774ef

The first record of Hirnantian Ostracoda in South America: implications for the biostratigraphy and paleozoogeography of the Paraná basin

AbstractHerein is reported the first occurrence of ostracodes from the Iapó Formation, an uppermost Ordovician unit of the Rio Ivaí Group in the Paraná basin, Brazil. Two ostracode species were identified in the Três Barras Farm section:Harpabollia harparum(Troedsson, 1918) andSatiellina paranaensisAdôrno and Salas in Adôrno et al., 2016 were recovered from dropstone-bearing shale overlying glaciogenic diamictites, a feature typical of Hirnantian (uppermost Ordovician) strata throughout Gondwana. The taxonomy of the GenusHarpabollia, as well as its type speciesHarpabollia harparum, was reviewed, and emended and new diagnoses were respectively proposed for each taxon. Occurrences ofHarpabollia harparumandSatiellinaspecies were common in areas influenced by cold waters. Additionally, the occurrence ofHarpabollia harparum, an index species to the uppermost Ordovician of several stratigraphic units in Baltica and southern Gondwana, allowed us to infer a Hirnantian age for the deposits of the Iapó Formation. Other than being associated withHarpabollia harparumin Iapó Formation of the Paraná basin,Satiellina paranaensisis also found in lower levels of the Vila Maria Formation; therefore, these are also considered Hirnantian in age. Above these lower levels of the Vila Maria Formation, a well-dated Rhuddanian (lowermost Llandovery, Silurian) palynomorph assemblage is observed within the formation. These occurrences are evidence of a continuous process of sedimentary deposition during the Ordovician–Silurian transition in the Paraná basin.

Carbon isotope chemostratigraphy and sea-level history of the Hirnantian Stage (uppermost Ordovician) in the Oslo–Asker district, Norway

AbstractWe present a δ13Ccarb chemostratigraphy for the Late Ordovician Hirnantian Stage based on 208 whole-rock samples from six outcrops in the Oslo–Asker district, southern Norway. Our data include the Norwegian type section for the Hirnantian Stage and Ordovician–Silurian boundary at Hovedøya Island. The most complete record of the Hirnantian Isotope Carbon Excursion (HICE) is identified in a coastal exposure at Konglungø locality where the preserved part of the anomaly spans a c. 24 m thick, mixed carbonate–siliciclastic succession belonging to the upper Husbergøya, Langåra and Langøyene formations and where δ13Ccarb peak values reach c. +6 ‰. Almost the entire HICE occurs above beds containing the Hirnantia Fauna, suggesting a latest Hirnantian age for the peak of the excursion. The temporal development of the HICE in southern Norway is associated with substantial shallowing of depositional environments. Sedimentary facies and erosional unconformities suggest four inferably fourth-order glacio-eustatically controlled sea-level lowstands with successively increased exposure and erosion to the succession. The youngest erosional unconformity is related to the development of incised valleys and resulted in cut-out of at least the falling limb of the HICE throughout most of the Oslo–Asker district. The fill of the valleys contains the falling limb of the HICE, and the postglacial transgression therefore can be assigned to the latest part of the Hirnantian Age. We address the recent findings of the chitinozoan Belonechitina gamachiana in the study area and its relationship to the first occurrence of Hirnantia Fauna in the studied sections, challenging identification of the base of the Hirnantian Stage.

Fossil diversity and taphonomy of glacial and post-glacial lower paleozoic strata, NE Paraná Basin, Brazil

Unlike other South American basins, the boundary between Ordovician and Silurian in the Paraná Basin, Brazil, has yet to be studied in detail. This article reports the occurrence and taphonomic analysis of a fauna composed of the organophosphatic brachiopods Kosoidea australis and ?Palaeoglossa sp., the mollusks ?Paleoneilo sp. and ?Cuneamya sp., besides rynchonelliform brachiopods and the ostracods Harpabollia sp. and Satiellina paranaensis. This assemblage indicates a Hirnantian age for the top of the Iapó and the base of the Vila Maria formations, northeastern border of the Paraná Basin. Taphonomic signatures and facies associations point to a pro-glacial environment, at the transitional shoreface, represented by the last sedimentary facies of the Iapó Formation. The overlying transgressive strata represent the beginning of the Vila Maria Formation, in which fossils and lithofacies indicate predominant post-glacial conditions in an offshore setting. Two taphofacies were delineated. One features size- and type-selected fossils, deposited in rain-out facies originated from the melting glacier. The second, with autochthonous to parautochthonous fossils, represents a post-glacial offshore setting. The low faunal diversity points to marine connections between North and South African basins, and with South American basins.

Constraining the biotic transitions across the end‐Ordovician mass extinction in South China: Bio‐ and chemostratigraphy of the Wulipo Formation in the Meitan area of northern Guizhou

The richly fossiliferous succession of the Wulipo Formation in the Huangjiaba area near Meitan in northern Guizhou, SW China, represents one of the very rare records of shelly fauna across the Ordovician and Silurian transition worldwide. This area is therefore crucial for understanding the pattern and dynamics of the end‐Ordovician mass extinction (EOME). Historically, the Wulipo Formation was dated as middle Rhuddanian (early Silurian). However, its fauna shows a close affinity with Transitional Benthic Fauna 3 (TBF 3), now known to be confined within the late Hirnantian (latest Ordovician) in well‐constrained successions globally. Here we present for the first time, chemostratigraphic data from the Wulipo Formation which confirm the presence of the Hirnantian Isotope Carbon Excursion. A critical review of faunal evidence further indicates a late Hirnantian age for this formation, and thus the hitherto only known anomalous TBF 3 record documented from South China is convincingly redated. The important implication is that the substantial biotic recovery after the EOME commenced globally at the very beginning of the Silurian with an overall amelioration of physical conditions. The new findings also suggest a much wider distribution of postglacial warm‐water benthic faunas on the Yangtze Platform during the late Hirnantian than previously envisaged.

Palynological analysis of Upper Ordovician to Lower Silurian sediments from the Diyarbakir Basin, southeastern Turkey

ABSTRACT This paper reports on a palynological analysis of 41 core and 21 cutting samples from a well drilled through an Upper Ordovician–Lower Silurian sequence, belonging to the Bedinan and Dadas formations, of the Diyarbakir Basin of southeastern Turkey. The samples yield abundant and well-preserved marine palynomorphs (acritarchs, chitinozoans and scolecodonts) although non-marine palynomorphs (spores/cryptospores) are extremely rare or absent. The Upper Ordovician sediments of the Bedinan Formation have low organic content but contain abundant palynomorphs whereas the Lower Silurian sediments of the Dadas Formation have high organic content, dominated by amorphous organic matter, with relatively rare palynomorphs. Three chitinozoan assemblages are identified and attributed a late Katian (merga Biozone), Hirnantian (moussegoudaensis Biozone) and Llandovery (?alargada Biozone) age. Two acritarch assemblages are identified and attributed a Katian–Hirnantian and Llandovery (Aeronian–Telychian) age. The chitinozoan and acritarch age determinations are compatible and suggest that the Bedinan Formation is of Katian–Hirnantian age and is separated by an unconformity from the Dadas Formation that is of Llandovery (Aeronian–Telychian) age. These findings confirm the presence of an unconformity at the Ordovician–Silurian transition in the southeastern Turkey. Palynofacies analysis suggests that the Bedinan Formation accumulated on an offshore shelf that was initially well oxygenated but became increasingly anoxic, whereas the Dadas Formation accumulated in an offshore basin that was anoxic. Palynomorph assemblages recorded in the Bedinan and Dadas formations indicate northern Gondwana affinity.

First report of Hirnantian (Upper Ordovician) high-latitude peri-gondwanan macrofossil assemblages from Portugal

AbstractThe first report of theHirnantiahigh-latitude peri-Gondwanan Fauna from Portugal (Upper Ordovician, Ribeira do Braçal Formation) is presented here. The described macroassemblages are fairly diverse containing fossils of brachiopods, trilobites, echinoderms, machaeridians, and ostracodes. Among the brachiopods, the most abundant isMirorthis mira.Plectothyrellacf.P.libyca,Paromalomenacf.P.polonica,Plectoglossa? sp., and a small indeterminate discinoid are also present. The trilobites are represented by abundant sclerites ofMucronaspiscf.M.mucronata, and an isolated cranidium and pygidium assigned toFlexicalymene. The occurrence ofMucronaspisandFlexicalymenerepresents the first record of these genera in Portugal. Echinoderms are dominant in the basal bed of the formation; the columnal plates tentatively ascribed to morphogenusPentagonocyclicusare the most abundant, followed by the echinosphaeritids. Abundant disarticulated machaeridian plates, of the genusPlumulites, associated with trilobites are present in one of the localities. Ostracodes were found in one single locality and have been assigned tentatively to the genusHerrigia. Ramose and massive bryozoans also occur in the assemblage. This new macrofossil assemblage supports the assignment of an Hirnantian age for the Ribeira do Braçal Formation. Most of the brachiopod species and the dalmanitidMucronaspisare commonly present in Hirnantian deposits globally, but the presence in the assemblage of a brachiopod close toPlectothyrella libyca, a cold-water species, previously reported only from the Hirnantian of Libya and Morocco, is noticeable. This strengthens the case for a high latitudinal setting of the present-day territory of the Portuguese Central Iberian Zone during the Late Ordovician.

Stratigraphy and facies analysis of the La Chilca Formation, Central Precordillera: Insights on the postglacial Ordovician–Silurian boundary and Early Silurian deposits from Argentina

The La Chilca Formation is exposed at the Poblete Norte and Ancha sections, Talacasto, Central Precordillera. Detailed facies analysis on field outcrops suggests a tide‐dominated tidal plain model on the Silurian siliciclastic ramp, integrated by supratidal to subtidal plain deposits. The lower part of the La Chilca Formation (upper supratidal plain) is herein attributed to reworked glaciomarine deposits that occurred during interglacial Hirnantian age, under conditions of sea rise and the action of tidal amplification. The intertidal–supratidal depositional environments are represented by tidal channels, scour and fill, tidal dunes, and hummocky cross‐stratified sandstones. The estimated tidal conditions for the tidal plain were mesotidal to macrotidal, under action of combined flows, attributed to the interaction of tidal currents and orbital low‐velocity waves. The identification of microbially induced sedimentary structures (MISS) in these deposits, such as petee polygonal structures, pitted texture, imploded sand volcanoes, Astropolithon sp., Manchuriophycus sp., domal build‐ups, biofilms, elephant skin, and trace fossils such as Chondrites sp., Zoophycos sp., Undichnia sp., Planolites sp., Monomorphichnus sp., and gastropod traces, may have played a protection and stabilization role on the clastic sediment surfaces against erosion of the La Chilca Formation deposits. The prolific growth of microbial mat on the intertidal to supratidal zone occurs explicitly at the turning points of transgression–regression cycles in shallow‐water environments under stable base‐level conditions. The middle‐upper section of the La Chilca Formation shows non‐cyclic rhythmites that are replaced by the cyclic rhythmites, suggesting an open tidal plain subject to an orbital/climatic rather than a tectonic control of cyclicity (Milankovitch cycles). The aggradation in this section indicates a shallowing towards the top of the formation. The La Chilca Formation records two third‐order transgressive events and a shallowing event. The transgressive events are found in the base and top of the formation and are marked by the presence of Fe‐oolites, while the shallowing event is determined by the presence of MISS.

Improved efficiency of the biological pump as a trigger for the Late Ordovician glaciation

The first of the ‘Big Five’ Phanerozoic mass extinctions occurred in tandem with an episode of glaciation during the Hirnantian Age of the Late Ordovician. The mechanism or change in the carbon cycle that promoted this glaciation, thereby resulting in the extinction, is still debated. Here we report new, coupled nitrogen isotope analyses of bulk sediments and chlorophyll degradation products (porphyrins) from the Vinini Creek section (Vinini Formation, Nevada, USA) to show that eukaryotes increasingly dominated marine export production in the lead-up to the Hirnantian extinction. We then use these findings to evaluate changes in the carbon cycle by incorporating them into a biogeochemical model in which production is increased in response to an elevated phosphorus inventory, potentially caused by enhanced continental weathering in response to the activity of land plants and/or an episode of volcanism. The results suggest that expanded eukaryotic algal production may have increased the community average cell size, leading to higher export efficiency during the Late Katian. The coincidence of this community shift with a large-scale marine transgression increased organic carbon burial, drawing down CO2 and triggering the Hirnantian glaciation. This episode may mark an early Palaeozoic strengthening of the biological pump, which, for a short while, may have made eukaryotic algae indirect killers. Enhanced algal productivity during the Late Ordovician may have led to carbon drawdown and the inception of the Hirnantian glaciation, according to sediment geochemistry and carbon cycle modelling.

87Sr/86Sr evidence from the epeiric Martin Ridge Basin for enhanced carbonate weathering during the Hirnantian

A pronounced positive δ13C excursion in the Hirnantian Age has been documented globally, reflecting large perturbations of carbon cycling in the Late Ordovician oceans. Increased organic-carbon burial or enhanced carbonate weathering during glacioeustatic sea-level regression has been proposed to account for this anomalous C-isotope excursion. To test the two competing hypotheses, we measured 87Sr/86Sr and δ13C of carbonates from the Copenhagen Canyon section in Nevada, USA. Our data reveal two rapid negative 87Sr/86Sr shifts that coincide with two prominent positive δ13C excursions and glacial advances. Numerical model simulations suggest that enhanced weathering of carbonates driven by glacio-eustatically controlled sea-level fall is required to produce the observed drops of 87Sr/86Sr and the coeval large positive δ13C excursions, possibly with or without increased organic carbon burial.

Chitinozoan biostratigraphy and carbon isotope stratigraphy from the Upper Ordovician Skogerholmen Formation in the Oslo Region. A new perspective for the Hirnantian lower boundary in Baltica

The end-Ordovician has received wide attention because it hosts major global events including mass extinctions, glaciations, significant sea-level fluctuations, and large-scale perturbations of the Earth's carbon cycle. Knowing the order and timing of these events and their components is crucial for understanding these environmental changes. Here, we use stable carbon isotope stratigraphy in combination with chitinozoan biostratigraphy to correlate the Upper Ordovician Belonechitina gamachiana chitinozoan Biozone. Its position has long been a matter of debate; some argue that it is of late Katian age and others that it is of early Hirnantian age. The Skogerholmen Formation from the Oslo-Asker District in Norway has been correlated with the lower-middle Pirgu Baltic Stage, hitherto believed to correspond to the international upper Katian Stage. Our study, however, reveals the presence of B. gamachiana, diagnostic of the eponymous Biozone, in the descending trend of a modest carbon isotope excursion in the lower part of this formation. This is strikingly similar to data from coeval end-Ordovician sections in North America, where the prevailing evidence suggests an early Hirnantian age for the B. gamachiana chitinozoan Biozone. This new correlation suggests that the lower Hirnantian boundary may be positioned within the Pirgu Baltic Stage.

A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia

The Late Ordovician mass extinction (LOME), one of the five largest Phanerozoic biodiversity depletions, occurred in two pulses associated with the expansion and contraction of ice sheets on Gondwana during the Hirnantian Age. It is widely recognized that environmental disruptions associated with changing glacial conditions contributed to the extinctions, but neither the kill mechanisms nor the causes of glacial expansion are well understood. Here we report anomalously high Hg concentrations in marine strata from south China and Laurentia deposited immediately before, during, and after the Hirnantian glacial maximum that we interpret to reflect the emplacement of a large igneous province (LIP). An initial Hg enrichment occurs in the late Katian Age, while a second enrichment occurs immediately below the Katian-Hirnantian boundary, which marks the first pulse of extinction. Further Hg enrichment occurs in strata deposited during glacioeustatic sea-level fall and the glacial maximum. We propose that these Hg enrichments are products of multiple phases of LIP volcanism. While elevated Hg concentrations have been linked to LIP emplacement coincident with other Phanerozoic mass extinctions, the climate response during the LOME may have been unique owing to different climatic boundary conditions, including preexisting ice sheets. Our observations support a volcanic trigger for the LOME and further point to LIP volcanism as a primary driver of environmental changes that caused mass extinctions.

Sedimentology of Hirnantian glaciomarine deposits in the Balkan Terrane, western Bulgaria: Fixing a piece of the north peri-Gondwana jigsaw puzzle

Glaciomarine deposits of late Hirnantian age in the western part of the Palaeozoic Balkan Terrane have persistent thickness (~7m) and lateral uniformity in rock colour, bedding pattern, lithology, and sedimentary structures. Four lithofacies are distinguished from base to top: lonestone-bearing diamictites, interbedded structureless mudstones, crudely laminated diamictites, and finely laminated mudstones. The diamictites are clast-poor to clast-rich comprising muddy to sandy varieties. Their compositional maturity is evidenced by the very high amount of detrital quartz compared to the paucity of feldspar and unstable lithic grains. Other textural components include extraclasts derived from the local Ordovician basement, mudstone intraclasts, and sediment aggregates. Turbate structures, grain lineations, and soft sediment deformation of the matrix below larger grains are locally observed. Sedimentological analysis reveals that deposition occurred in an ice-intermediate to ice-distal, poorly agitated shelf environment by material supplied from meltwater buoyant plumes and rain-out from ice-rafted debris. Remobilization by mass-flow processes (cohesive debris flows and slumps) was an important mechanism particularly for the formation of massive diamictites. The glaciomarine deposits represent a typical deglaciation sequence reflecting retreat of the ice front (grounded or floating ice sheet), relative sea-level rise and gradually reduced sedimentation rate with increasing contribution from suspension fallout. This sequence was deposited on the non-glaciated shelf of the intracratonic North Gondwana platform along the southern margin of the Rheic Ocean. The Hirnantian strata of the Balkan Terrane can be correlated with similar glaciomarine deposits known from peri-Gondwana terranes elsewhere in Europe showing clear ‘Armorican affinity’. Several lines of evidence suggest that the provenance of siliciclastic material was associated mainly with sedimentary recycling of mature sands which had been deposited across North Gondwana in Cambrian and pre-glacial Ordovician times.

Refining 2 km of Ordovician chronostratigraphy beneath Anticosti Island utilizing integrated chemostratigraphy

New high-resolution chemostratigraphy, in combination with updated biostratigraphy, refines the chronostratigraphic resolution of the nearly 2 km thick Ordovician section below Anticosti Island. A total of 1414 horizons, spaced at 1.0–1.5 m intervals, were sampled from the New Associated Consolidated Paper (NACP) drill core and analyzed for major and trace elemental composition by portable X-ray fluorescence analyzer (pXRF). Select micrite and calcareous shale powders were then analyzed for δ13Ccarb (number of samples, N = 364) and 87Sr/86Sr (N = 25). Our results indicate a Floian to early Darriwilian (F3–Dw1) age for the Romaine Formation, a middle Darriwilian to Sandbian (Dw2–Sa1) age for the Mingan Formation, a lower to mid-Katian (Ka1–Ka2) age for the Macasty Formation, an upper Katian (Ka3) age for the lower Vauréal Formation, an uppermost Katian (Ka4) age for the upper Vauréal, and a Hirnantian (H1) age for the Ellis Bay Formation. This integrated chemostratigraphic and biostratigraphic synthesis establishes the position of numerous unconformities, the duration of the intervening depositional sequences, and rates of sedimentation.

An in situ shelly fauna from the lower Paleozoic Zapla diamictite of northwestern Argentina: Implications for the age of glacial events across Gondwana

A shelly fauna from the upper part of the Zapla glacial diamictite includes the lingulate brachiopod Orbiculoidea radiata Troedsson, the rhynchonelliforms Dalmanella cf. testudinaria (Dalman) and Paromalomena sp., the bivalve Modiolopsis? sp., and the trilobite Dalmanitina subandina Monaldi and Boso. Both taphonomic and paleoecologic data indicate a lack of transport reflecting the original community. The assemblage is closely comparable to the widespread latest Ordovician Hirnantia-Dalmanitina fauna. The Hirnantian age of the Zapla diamictite is further corroborated by the record of the northern Gondwana chitinozoans Spinachitina cf. oulebsiri Paris and Desmochitina minor typica Eisenack. The graptolites and chitinozoans from the overlying Lipeón Formation indicate that the postglacial transgression took place in the earliest Llandovery (Parakidograptus acuminatus Biozone). According to the tectonosedimentary evidence, the Early Silurian age of the Cancañiri and San Gabán diamictites of north–central Bolivia and south Peru based on their palynological record is more likely the age of posglacial gravity flows and not that of the glaciation. We support the hypothesis that the weakly lithified glacigenic deposits of Hirnantian age were reworked and redistributed by high-energy marine processes during the postglacial transgression and then transported to the adjacent deep-marine trough. Iron-rich horizons have been recognized in many basins of southern South America reflecting eustatic and paleoclimatic fluctuations. Most of them formed during the early stages of the postglacial transgression at the Ordovician/Silurian transition and are associated with low sedimentation rates and condensed intervals. The mild maritime postglacial climate, the increasing atmospheric CO2, and possibly the presence of incipient vegetated areas led to extensive weathering of glacigenic sediments supplying iron into the marine system to form ferruginous deposits. The sea level fall related to the peak of glaciation is recorded by both paleovalley incision and a sharp subaerial to subglacial unconformity. The transgressive systems tract starts with fluvio-estuarine deposits within incised valleys followed by widespread deposition of subtidal to open marine organic-rich shales onlapping regionally the basement rocks. The recognition of key stratigraphic markers (e.g. sequence boundary, flooding surface, ferruginous beds), alongside reliable micro and macropaleontological evidence allow a more accurate correlation between the Central Andean Basin of Peru, Bolivia and NW Argentina, the W Puna region, the Paraguayan and Brazilian sectors of the Paraná Basin, the Precordillera Basin of W Argentina, and the Cape Basin of South Africa.

Late Hirnantian (latest Ordovician) carbonate rocks and shelly fossils in Shiqian, northeastern Guizhou, Southwest China

The Kuanyinchiao Formation has generally been regarded as representing the only carbonate sediments deposited during the Hirnantian (Late Ordovician) interval in South China, where the Hirnantian GSSP is situated. Reinvestigation of several key Ordovician-Silurian boundary sections in Shiqian, northeastern Guizhou Province, Southwest China, reveals a lithologically distinct carbonate unit with abundant shelly fossils, including tabulate and rugose corals, conodonts, brachiopods, trilobites and stromatoporoids, many showing close Silurian affinities and suggesting a level stratigraphically much higher than the Kuanyinchiao Formation. This observation, together with further evidence that graptolites indicative of the Akidograptus ascensus biozone are present in immediately overlying rocks at a nearby section, implies that these shelly fossiliferous strata are of latest Hirnantian age (possibly straddling the Ordovician-Silurian boundary). Hence they were most likely deposited after the Hirnantian glaciation, rather than representing glacial cool water sediments (the Kuanyinchiao Formation) as was previously thought. Ordovician-Silurian boundary sequences in South China typically reflect global glacio-eustatic sea-level changes. Owing to their unique lithology and palaeontology, these newly recognized carbonate rocks, here formally named the Shiqian Formation, add substantially to our knowledge of Ordovician-Silurian stratigraphy in this region.

Late Ordovician brachiopods from the Chingiz Terrane, Kazakhstan, and their palaeogeography

The Late Ordovician Akdombak Formation of the Chingiz-Tarbagatai Terrane, Kazakhstan, has yielded brachiopods from four successive horizons, three Katian and the upper one of Hirnantian age. Comparable faunas of an earlier Katian age are also described from the upper part of the Taldyboi Formation, also within the Chingiz-Tarbagatai Terrane, but there are no Hirnantian rocks there. The new subfamily Alpeisinae is erected within the Wangyuiidae (Plectorthoidea), and the following new genera are introduced: Holmerglossa (Linfguloidea), Wrightiops (Craniopsoidea), Buminomena (Strophomenoidea), Alpeis, Ashinaorthis and Enbektenorthis (all Orthoidea) and Rongatrypa (Atrypoidea), as well as 10 new species: Bokotorthis minuta, Craniops pristinus, Chonetoidea enbektenensis, Dalmanella kotyrzhalica, Diambonioidea koknaiensis, Enbektenorthis molesta, Kassinella kasbalensis, Kozlowskites botobaicus, Leangella (Leangella) bakanasensis Phragmorthis eximia and Strophomena (Tetraphalerella) namasensis. Eleven ecological associations are recognized, which inhabited a spectrum of depths ranging from Benthic Assemblage Zone (BA) 1 to BA 5 depth ranges. Comparison of the faunas with the neighbouring Kazakh terranes is chiefly with the Chu-Ili, North Tien Shan and Boshchekul terranes since, although others have Late Ordovician faunas, their brachiopods are unknown or unrevised. There is much in common between the brachiopods of the Kazakh terranes, but individual faunas contain different species, many of which are endemic to one terrane. The Chingiz-Tarbagatai brachiopods show some similarities to contemporaneous faunas from the equatorial sectors of Gondwana, including Australia, and some with North China and South China, but have very little in common with those from Baltica and Laurentia. Thus the Chingiz-Tarbagatai Terrane was probably part of an archipelago which lay in a subequatorial position to the north-west of the Australasian sector of Gondwana and the adjacent South China continent in the Late Ordovician.http:zoobank.org/urn:lsid:zoobank.org:pub: C502E307-C67D-4216-869D-17D199812088

Nd isotope records of late Ordovician sea-level change—Implications for glaciation frequency and global stratigraphic correlation

Detailed records of ice-sheet advances and retreats are reconstructed for the Hirnantian and Katian ages of the Late Ordovician using Nd isotopes (εNd) as a sea-level proxy in three study sections from the western margin of Laurentia: two shallow water platform sections located south and north of the paleo-equator, and one deep water section located in a continental slope-rise setting. When sea-level was high and paleo-shorelines had migrated eastward, the εNd value of seawater in the vicinity of each of the study sections shifted toward the εNd value of the eastern Panthalassa Ocean (~−4.0). By contrast, when sea-level was low and paleo-shorelines had migrated westward, the εNd value of seawater shifted toward the εNd value of the continental weathering flux from Laurentia (−8.5±0.2, 2σmean). These stratigraphic patterns of changing εNd values are interpreted to reflect the eustatic sea-level fluctuations that previous studies have documented in response to Gondwanan ice-sheet advances and retreats, thus linking the εNd sea-level proxy to Late Ordovician global-scale climate changes. The εNd profiles for the two platform sections yielded similar proxy sea-level curves with five cycles of oscillation recorded during the latest Katian and Hirnantian. Three additional cycles of late Katian sea-level change are recognized in the εNd profile of the deep water continental slope-rise section.The combination of εNd, δ13C and graptolite biostratigraphic data facilitates a precise interregional correlation of the Hirnantian Age and the paleoclimate changes that took place during this interval. The new correlations support previous findings that the Hirnantian ice age comprised two major glacial periods separated by a minor interglacial during the early part of the Metabolograptus persculptus Biozone. The younger glacial (confined to mid M. persculptus Biozone time) led to more extensive sea surface cooling than did the earlier one, and resulted in extensive eustatic sea-level drawdown and C-cycle changes. It records the strata most often recognized as HICE (the Hirnantian Carbon Isotope Excursion) in sedimentary successions worldwide, such as Anticosti Island, Scotland, Estonia, Siberia, and South China. The results of this study support and strengthen the view that glaciation predated the Hirnantian Age in the Late Ordovician, and that the records of small positive δ13C excursions in Katian successions from Baltica and eastern North America are themselves proxy indicators of glaciation frequency and eustatic sea-level changes.