Hirnantian Age Research Articles

Latest Ordovician–earliest Silurian acritarchs and chitinozoans from subsurface samples in Jebel Asba, Kufra Basin, SE Libya

Latest Ordovician–earliest Silurian Tanezzuft Formation shales recovered from core material of the shallow borehole JA-2 drilled in Jebel Asba at the eastern margin of the Kufra Basin, southeastern Libya, yielded well-diversified palynomorph assemblages with transparent and brownish to yellowish vesicles and organic matter (visual kerogen Types 1 and 2) from depth interval 46.20 to 67.82m. In addition, miospores including cryptospores, and Tasmanites sp. (“Tasmanites with nodules”), scolecodonts, and a stratigraphically significant palaeo-marker, the enigmatic, tubular organic structure Tortotubus protuberans, were also recorded frequently in most samples. Kerogen colour based on miospores (TAI <3) and chitinozoan reflectance indicate an immature facies for oil generation. The two uppermost samples (from 33.33m and 46.20m depths) and the lowermost ones (from 67.92 to 73.21m depth) contain rare palynomorphs and other organic remains and have been partially affected by oxidation.Furthermore, palynological and palynofacies analyses were carried out on cuttings from an old well (UN-REMSA well), ca. 530m towards the NNE from well JA-2. The composition of the organic residue is similar in both wells. However, the UN-REMSA well yields fairly numerous chitinozoans, scolecodonts and biofilms but lacks the “thread-like structures” and “Tasmanites with nodules” observed in well JA-2.All the investigated samples in well JA-2 are dominated by a single chitinozoan species, Euconochitina moussegoudaensis Paris (in Le Hérissé et al., 2013). Based on correlation with chitinozoan-bearing strata around the Ordovician–Silurian boundary, the analysed samples from well JA-2 and from the UN-REMSA well are regarded as post-glacial, but still of either latest Hirnantian age, or at least no younger than earliest Rhuddanian. A well-diversified acritarch, miospore and cryptospore assemblage recorded in well JA-2 supports a marginal marine (nearshore) depositional environment. This assemblage is no older than earliest Rhuddanian yet the latest Hirnantian age of the assemblage cannot be completely ruled out as our current knowledge on the post-glacial, latest Hirnantian acritarch and miospore assemblages is poorly documented in North Africa.

Did lingering ice sheets moderate anoxia in the Early Palaeozoic of Libya?

The Hirnantian glaciation of West Gondwana produced a glacially sculpted topography, which is draped by organic-rich latest Ordovician and early Silurian ‘hot shales’. Although these are the most important Early Palaeozoic source rock in North Africa, organic enrichment is distributed unevenly. For example, in Al Kufrah Basin, Libya, ‘hot shales’ are elusive, but outcrop analysis at the western basin demonstrates why this is the case. The topmost Mamuniyat Formation, of Hirnantian age, comprises glaciogenic sandstones, passing upward into mixed facies of the Tanezzuft Formation, which has a latest Ordovician–early Silurian age. The basal Tanezzuft Formation contains a shelly carbonate (cool-water deposits accumulated under oxygenating conditions) and bioturbated sandstone succession. Above, hummocky cross-bedded and graded sandstone intervals are intercalated with shale and siltstone (storm influx onto a muddy shelf). These are interrupted by several lonestone-bearing intervals (ice-rafted debris), a striated pavement (of subglacial origin), and manganese oxide crusts and concretions. The concretions and bioturbation imply oxygenation of the sea floor during transgression. These putative glacial deposits were deposited following the main phase of the Hirnantian glaciation, at the same stratigraphic level as ‘hot shales’ elsewhere in northern Gondwana. Lingering ice caps may have produced well-oxygenated marine waters precluding ‘hot shale’ deposition.

Lipid biomarkers record fundamental changes in the microbial community structure of tropical seas during the Late Ordovician Hirnantian glaciation

The Late Ordovician mass extinction was linked to climate cooling and glaciation of Gondwana during the terminal Ordovician Hirnantian Age (444.7–443.4 Ma). Extinction patterns have been well described for many marine taxa, but much less is known about marine microbial communities through this interval. To elucidate the structure of microbial communities in tropical marine basins through the Late Ordovician, we analyzed lipid biomarkers in thermally well preserved strata from the Taconic foreland (Anticosti Island, Canada), the Cincinnati Arch (midwestern United States), and the western continental margin (Vinini Formation, Nevada, United States). Despite clear oceanographic differences, lipid biomarker profiles show similarities between these three localities. Major shifts in biomarker distributions of Anticosti Island and the Vinini Formation, mainly hopane/sterane ratios, record changes in the balance of bacterial versus algal primary production. Bacterial contributions to sedimentary organic matter were highest during warm intervals, both before and after Hirnantian cooling. In particular, 3β-methylhopanes, likely sourced from aerobic methanotrophic bacteria, occur in high relative abundance (many times the Phanerozoic average) across Laurentia throughout most of the interval studied. 3β-methylhopane abundances also reveal an overall positive relationship with paleotemperature proxies, implying increased methane cycling during warm intervals. These results suggest that enhanced methane cycling could have provided an important positive feedback on climate during extended intervals of early Paleozoic time.

The end-Ordovician chitinozoan zones of Anticosti Island, Québec: Definition and stratigraphic position

The Ellis Bay Formation cropping out in the central and western parts of the Island of Anticosti, is one of the thickest uppermost Ordovician carbonate shelf successions. Three new species of Hercochitina: Hercochitina florentini, Hercochitina changi and Hercochitina longi, are described from the lower part of the formation. These species are associated with Clathrochitina concinna, Belonechitina concava and Ancyrochitina longispina, and define the new H. florentini–C. concinna Biozone. This zone is present at the Anse aux Fraises section in the western part of the island and in the lower Lousy Cove and probably the Prinsta members of the Ellis Bay Formation in a section on the northeastern coast. The H. florentini–C. concinna Biozone follows the Hercochitina crickmayi Biozone of the Vauréal Formation and is overlain by the B. gamachiana Biozone, which ranges through most of the Ellis Bay Formation in western Anticosti. The B. gamachiana Biozone is succeeded by the Spinachitina taugourdeaui Biozone just below the reefal Laframboise Member. In eastern Anticosti the B. gamachiana Zone has only been recognized in the upper Lousy Cove Member of the Ellis Bay Formation.The presence of a Hirnantian shelly fauna, including the typical genus Hirnantia, the comparison of the Anticosti chitinozoan assemblages with those from other sections dated by graptolites and the available stable isotope geochemistry information suggest that the Ellis Bay Formation is of Hirnantian age.

Cryptospore assemblages from Upper Ordovician (Katian–Hirnantian) strata of Anticosti Island, Québec, Canada, and Estonia: Palaeophytogeographic and palaeoclimatic implications

Rich palynological assemblages have been recovered from deposits of Hirnantian age in Anticosti Island (Québec, Canada), and in borehole Valga-10 in southern Estonia. The assemblages are well preserved, and include acritarchs, chitinozoans, and cryptospores. The age of the deposits is well constrained by means of palynomorphs (acritarchs and chitinozoans) as well as sequence stratigraphic and chemostratigraphic correlations. Cryptospore assemblages from the two localities are similar and are also broadly comparable to the few known coeval assemblages described elsewhere. They include 11 genera and 20 species, and testify to the presence of an extended and diverse flora during Hirnantian times in Laurentia and, for the first time, also in Baltica. The present findings contribute to an improved knowledge of origin and early development of vegetative cover. The recovery of diverse and abundant cryptospores in Hirnantian deposits may be related to increased input of land-derived sediment during the global sea-level fall linked to the Late Ordovician glaciation, but it also demonstrates that the early land plants may have tolerated a wide range of climatic conditions.

Chitinozoan biostratigraphy of a new Upper Ordovician stratigraphic framework for Anticosti Island, Canada

Thickness and facies variations in the Upper Ordovician succession of Anticosti Island have made correlations between the western and eastern ends of the island difficult. This chitinozoan study reveals that most strata of the Ellis Bay Formation at the eastern end of the island correlate with the Vaureal Formation at the western end. In the eastern end, chitinozoans typical of the Ellis Bay Formation occur only in the upper part of the Lousy Cove Member, whereas chitinozoans diagnostic of the mid-upper Vaureal occur in the rest of the formation. Moreover, the eastern Schmitt Creek and Mill Bay members of the Vaureal Formation are older than the uppermost Vaureal eponym members in the western end. New east-west correlations, supported by geochemical, sedimentological, and sequence stratigraphic data, are proposed. Comparison with coeval Upper Ordovician successions suggests a Hirnantian age for the gamachiana Zone that occurs in the lower members of the Ellis Bay Formation. It also indicates that the Hirnantian Isotopic Carbon Excursion (HICE) on Anticosti begins in the D. anceps Zone, just below the base of the Hirnantian Stage, reaches a first peak in the lower N. extraordinarius Zone, a second modest peak high in the N. extraordinarius Zone, and a third, more important one, in the lower part of the N. persculptus Zone. These results are in agreement with the biostratigraphic and chemostratigraphic data at the Global Boundary Stratotype Section and Points (GSSPs) for the base of the Hirnantian and Silurian in south-central China and at Dob9s Linn in Scotland.

Provenance of the Lower Paleozoic Balcarce Formation (Tandilia System, Buenos Aires Province, Argentina): Implications for paleogeographic reconstructions of SW Gondwana

Lower Paleozoic moderately sorted quartz–arenites from the Balcarce Formation deposited in eastern Argentina (Tandilia System) comprise mainly detrital material derived from old upper crustal material. The sources were magmatic, sedimentary, and subordinated felsic metamorphic terranes. High concentrations of tourmaline and Ti-rich heavy minerals, including zircon and nearly euhedral chromite, are common. Trace element concentrations (Nb, Cr) on rutile indicate pelitic and metabasaltic sources, respectively. Major element analyses on chromites indicate a basic volcanic protolith of mid-oceanic ridge origin, which was exposed close to the depositional basin. The delivery of chromite may be associated with convergent tectonics causing the consumption and obduction of oceanic crust during pre-Upper Ordovician times. The oblique/orthogonal collision of the Precordillera Terrane with the western border of the Rio de la Plata Craton, west of the Balcarce Basin or source further to the east from a Lower Palaeozoic extensional basin are possibilities. Geochemical and petrographic data exclude the underlying Precambrian and Cambrian sedimentary rocks as dominant sources, and favour the basement of the Río de La Plata Craton, including Cambrian rift-related granites of South Africa and the Sierras Australes (eastern Argentina), as main suppliers of detritus. Trace element geochemistry of recycled pyroclastic material, associated with the quartz–arenites, also suggests volcanic arc sources. The provenance of the pyroclastic material may either be the Puna–Famatina arc, located in north and central Argentina, or a hypothetical active margin further to the south. These ash layers are equivalent in age to volcanic zircons found in the Devonian Bokkeveld Group in western South Africa. The deposition of a glacial diamictite of Hirnantian age (Sierra del Volcán Diamictite) is interpreted as a member of the Balcarce Formation. Based on the stratigraphic re-location of the glacial diamictite and trace fossils, the Balcarce Formation is considered here to be Ordovician to Silurian in age. The Balcarce Formation can be correlated with similar rocks in South Africa, the Peninsula Formation, and the upper Table Mountain Group (Windhoek and Nardouw subgroups), including the Hirnantian glacial deposit of the Pakhuis Formation.

Fossil microphytoplankton dynamics across the Ordovician–Silurian boundary

A critical review of all available data on acritarch biostratigraphy and diversity dynamics across the Late Ordovician through the early Silurian, permits a better appreciation of the potential of acritarchs for the recognition of the systemic boundary. This analysis also reveals the response of marine microphytoplankton populations to the Late Ordovician palaeoenvironmental crisis (Hirnantian glaciation). Previous zonal schemes are improved, and an updated acritarch biostratigraphic chart is proposed, plotted against the most recent chronostratigraphic subdivisions. Sections from Anticosti Island (Québec, Canada), Algeria, Morocco, and Estonia preserve the best palynological record for the investigated interval. The present analysis shows that no true mass-extinction event occurred in latest Ordovician times in connection with the well known glacial event. “Pre-glacial” Ashgill acritarch suites are dominated by species of Baltisphaeridium, Multiplicisphaeridium, Ordovicidium, Orthosphaeridium, and netromorph acritarchs. An important proportion of these taxa (excluding Ordovicidium and Orthosphaeridium) survive the onset of glacial conditions in Hirnantian (latest Ordovician) times and continues through the early Silurian. The development of morphological polymorphism appears as a response (a survival strategy?) to the establishment of glacial conditions. In glacial-related sediments of Hirnantian age in North Africa, acritarch assemblages display a burst of relative abundance and intra-specific morphological variability (polymorphism) of long-ranging taxa such as Veryhachium, Multiplicisphaeridium, Dactylofusa, Poikilofusa, and Evittia. The extinction of several species characteristic of Upper to uppermost Ordovician strata occurs near the boundary, in “post-glacial” Ashgill (uppermost Hirnantian). This extinction event is counterbalanced by the almost contemporaneous (within the limits of stratigraphic resolution) appearance of several new taxa showing already a clear “Silurian affinity”, e.g., Tylotopalla, Cymbosphaeridium, and Visbysphaera. This origination event is observable in, and correlatable between the North African, the Bohemian and the Anticosti sections, making it global in extent. The completion of the palynological turnover and the establishment of a diverse Silurian acritarch suite occurs well above the base of the Silurian, during Aeronian times. The strong survival capability of the oceanic plankton through periods of palaeoenvironmental crisis in latest Ordovician times (but also throughout the Phanerozoic) could have played an important role in the post-extinction rebounds of metazoan clades, by assuring the continuity of marine trophic resources to consumers and avoiding irreversible disruptions of the trophic chains.

Early Silurian glaciation along the western margin of Gondwana (Peru, Bolivia and northern Argentina): Palaeogeographic and geodynamic setting

The record of the Ordovician–Silurian transition within the southern Peru–Bolivia Palaeozoic basin (extending across Peru, Bolivia and northern Argentina) is characterized by a diamictite-bearing unit which overlies several different Ordovician units, and underlies mid Silurian shales. We review the lithostratigraphy, sedimentology and biostratigraphy of this unit, revising stratigraphic relationships and several ideas and interpretations previously proposed. Analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and environments involved during its deposition. Turbidite and shale interbeds within the diamictites indicate a deep marine environment, with interbedded mud flows, debris flows, slumps and large displaced slabs providing evidence for sediment instability and resedimentation. Chitinozoan biostratigraphy indicates a Llandovery age, in contrast with previous proposals of a Hirnantian age, and suggests the need for more detailed sedimentologic and biostratigraphic studies, and a reassessment of this diamictite unit in relation to adjacent regions in South America. The resedimented character of the deposit explains some of the Ordovician fauna previously described, which must be considered as recycled from underlying units. In situ fauna needs to be reassessed, as it may be endemic, diachronic and/or indicative of migrations within Gondwana. Glacially-faceted and striated clasts, as well as large granitoid boulders within the resedimented materials, provide evidence for glaciation of the source area, and are interpreted as recycled from former glacigenic deposits. The evidence found in the Central Andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late Llandovery (Telychian). The precise age of glaciation in this part of Gondwana cannot be confirmed do to a lack of true tillites. Tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western Gondwana during the Late Ordovician and Early Silurian. The new data imply that the Ordovician–Silurian boundary was not preserved in some areas of the basin, and that an erosional hiatus (disconformity with variable regional significance) is present between different Ordovician units and the diamictite unit. Late Ordovician (?late Caradoc–?early Ashgill) euxinic black shales locally present in the central Altiplano and Eastern Cordillera of Bolivia may represent the youngest Ordovician unit in the region, and more research should be focused on them. Paleogeographic reconstruction of the Early Silurian Peru–Bolivia Basin depicts the extension of the diamictite unit southwards into northern Argentina and western Paraguay, connecting with the Paraná basin through the Asunción Arch. To the north it continued into northern Peru and Ecuador, connecting with the Silurian record in Colombia and Venezuela, and to the northeast into Brazil (Solimões and Amazonas basins). The sedimentary record of glaciation in the Peru–Bolivia basin was due to the development of local ice fields to the west of the basin, related with tectonism and resulting reliefs along the active margin of Gondwana, and probably did not coincide with the development of the North African late Ashgillian ice cap.

The lowermost Silurian of Jämtland, central Sweden: conodont biostratigraphy, correlation and biofacies

ABSTRACTThe Late Ordovician–Early Silurian succession in Jämtland includes the marine Kogsta Siltstone, which is unconformably overlain by the shallow-water Ede Quartzite that grades into the open-marine Berge Limestone. A Hirnantia shelly fauna dates the uppermost Kogsta Siltstone as Hirnantian, and shelly fossils indicate an Aeronian age for the Berge Limestone. Biostratigraphically highly diagnostic conodonts of the early-middle Aeronian Pranognathus tenuis Zone provide the first firm date of the Upper Ede Quartzite and the lowermost Berge Limestone. The Lower Ede Quartzite has not yielded fossils, but sedimentological data suggest it to be of Hirnantian age and reflect the glacio-eustatic low-stand. The contact between the Lower and Upper Ede Quartzite, here taken to be the Ordovician–Silurian boundary, appears to be an unconformity associated with a stratigraphic gap that at least includes the Rhuddanian Stage. The biostratigraphically important conodonts Pranognathus tenuis, Kockelella? manitoulinensis, and Pranognathus siluricus are recorded from Sweden for the first time, and these and other conodonts are used for correlations with coeval units in Europe and North America. In a regional review of Aeronian conodont faunas, three intergrading, apparently depth-related, conodont biofacies are recognised, the Jämtland conodonts representing the one characteristic of the shallowest water.

Cryptospore Assemblages from the Ordovician/silurian Boundary in the Puna Region, North‐west Argentina

Palynomorph assemblages have been recovered from deposits believed to straddle the Ordovician/Silurian boundary, from the upper member of the Salar del Rincón Formation, in the Puna region of north‐west Argentina. The palynomorph assemblages are dominated by terrestrial cryptospores, but also contain marine elements (acritarchs, prasinophycean algae and chitinozoans). The cryptospore assemblages are similar in composition to those described from coeval deposits worldwide, suggesting that the producers were cosmopolitan and tolerated a wide range of climatic conditions. They are correlated with the Imperfectotriletes spp. Interval Biozone (sub‐biozone α) of the Imperfectotriletes spp.‐Laevolancis divellomedia Assemblage Biozone, indicating a Hirnantian (latest Ordovician) age. Acritarchs include late Ordovician species such as Eupoikilosusa striata and Villosacapsula cf. setosapellicula, that coexist with the Llandovery species Dactylofusa estillis. Based on lithological and palynological evidence, an early Llandovery, or a late Hirnantian (post‐glacial) age is proposed for the basal part of the upper member of the Salar del Rincón Formation.

The effects of the final stages of the Late Ordovician glaciation on marine palynomorphs (chitinozoans, acritarchs, leiospheres) in well Nl-2 (NE Algerian Sahara)

Palynomorph assemblages, especially chitinozoans and acritarchs, from the Upper Ordovician of well Nl-2 (north-east of the Algerian Sahara) are studied in order to precisely date the ultimate effects of the Late Ordovician glaciation and to document the impact of this major climatic stress on the diversity of the palynoplankton. References are made to stable isotope excursions and to global eustatic sea level variations in order to improve the local age determination. The Hassi el Hadjar Formation, i.e. glacio-marine diamictites, is interpreted as a transgressive event resulting from the melting of the northern Gondwana ice cap. It yields poorly preserved and moderately diverse chitinozoans of late Hirnantian age. Acritarchs are more abundant in the lower part of these “microconglomeratic clays”, but display a low diversity and are badly preserved throughout the whole formation. Reworked individuals are recorded in both groups. The marine sediments of the M'Kratta Formation of latest Hirnantian age contain better preserved, more abundant and more diverse palynomorph assemblages, especially in the Upper Member. The composition of this palynoplankton indicates a fairly good faunal and phytoplankton recovery after the early Hirnantian climatic stress. The extinction of the Ordovician forms, and the appearance of Silurian type taxa occur only in the uppermost Hirnantian, i.e. following with a slight delay the glacial event. The overlying black shales of Wenlock age (lower part of the Oued Mehaiguène Formation) are indicative of marine anoxic environments. They yield a virtually exclusive, but enormous number of Tasmanacea. The very peculiar composition of this palynoplankton seems to be independent of the Late Ordovician glaciation and is most likely related to the factors that, later, generated and maintained anoxic conditions in this area. A new species of chitinozoan, Spinachitina oulebsiri sp. nov. from the latest Hirnantian M'Kratta Formation, is described and illustrated.

New faunal data from the highest Ordovician rocks at Pomeroy, County Tyrone, Northern Ireland

Synopsis Two new shelly faunas are reported from the Tirnaskea Formation in the Pomeroy inlier, County Tyrone. The middle part of the formation contains species of the brachiopods Dedzetina, Sericoidea, Proboscisambon and Protozyga ? indicative of a deep-water biofacies and suggesting correlation with the Rawtheyan Stage of the Ashgill Series although an Hirnantian age cannot be excluded. At higher levels within the unit Dysprosorthis occurs with Eostropheodonta and the trilobite Mucronaspis, confirming an Hirnantian age for the upper part of the Tirnaskea Formation. Deep water facies persisted at Pomeroy during the Hirnantian in contrast to the upward shallowing sequence at Girvan, SW Scotland, suggesting a more offshore setting for the Irish deposits.

Gradient Analysis: A Quantitative Technique for Biostratigraphic Correlation

Gradient analysis has previously been used by ecologists and paleoecologists to quantify spatial gradients in the distribution of taxa. Temporal gradients can also be quantified by these techniques, which makes them potentially useful in biostratigraphy. Case studies of the methods of detrended correspondence analysis and reciprocal averaging are given in this paper, demonstrating their potential for ordinatingfaunas by age. Detrended correspondence analysis successfully ordinated the Ordovician sequence of Estonia on the basis of its btyozoan fauna, establishing an on the first axis. Previously undated bryozoan faunas of Baltic affinity from erratic boulders in Poland and the Naungkangyi Formation of Burma were dated by establishing their position along the Estonian gradient. The erratic boulder fauna from Poland was determined to be of Hirnantian (Latest Ashgill) age. The Naungkangyi fauna was dated as ranging from Late Caradoc to Early Ashgill in age. Reciprocal averaging was used to group Late Ordovician and Silurian bryozoan faunas into three age categories based on first axis scores and into clastic vs. carbonate facies categories based on second axis scores. These results signify that gradient analysis can be a useful technique for quantitative biostratigraphic correlation and that bryozoans, although rarely used in biostratigraphy, are potentially significant index fossils. INTRODUCTION

New evidence on the South African Lower Palaeozoic: age and fossils reviewed

AbstractTrilobites from the Soom Shale, Cedarberg Formation, of the Table Mountain Group, South Africa, have been identified asMucronaspis oliniTemple, indicating a latest Ordovician (Rawtheyan–Hirnantian) age. This new data is taken as an opportunity to present new records of some non age-diagnostic brachiopods and molluscs from the area, and also to review the substantial shelly fauna from the overlying Disa Member of the Cedarberg Formation. It is concluded that that fauna is of Hirnantian age and not early Llandovery as stated by some previous authors.

The Ordovician–Silurian boundary at Keisley, northern England

AbstractThe relationships of the Keisley Limestone, an isolated carbonate mudmound in the Cross Fell Inlier of northern England, have long been problematical with respect both to the other upper Ordovician as well as the Silurian strata of the inlier. Bedded limestones, separated by gaps from the Keisley mudmound below and fromatavusBiozone graptolite shales above, yielded a prolific shelly fauna to Temple in 1968 which he regarded as being of lower Llandovery age. This age is controversial. To try to establish the precise ages of the rock units, the position of the Ordovician–Silurian boundary, and the detailed nature of the changes which took place at this boundary, a critical section was excavated. Bed by bed lithological and faunal collections reveal from beneath the knownatavusoutcrop (a) five bentonite bands, unexpected in the context of northern England, (b) graptolites of bothacuminatusandpersculptusBiozonesabovethe bedded limestones and (c) a similar but distinct assemblage from siltstones associated with these limestones which containsHirnantiaelements. From the palaeontological evidence there is little doubt that the topmost part of the carbonate sequence is of Hirnantian age.