Ashgill Age Research Articles

Strain analysis of Late Ordovician tectonic events in the In-Tahouite and Tamadjert Formations (Tassili-n-Ajjers area, Algeria)

The Tassili-n-Ajjers area in southern Algeria is located in the south-east of the Saharan platform. It is bounded to the east by the Tihemboka N–S trending structural system, the Hoggar shield to the south and to the north by the Illizi Basin. During the Palaeozoic, the Saharan Platform was part of the northern passive margin of the Gondwana supercontinent and the tectono-stratigraphic evolution of sedimentary systems in the Algerian Sahara was typical of the Gondwana stable cratonic platform. During the Late Ashgill (Hirnantian), the North African platform was located near the South Pole and an icecap developed over much of Africa and South America. Field observations, in the study area, together with integrated strain analysis and previous studies provide evidence for a number of Late Ordovician deformation phases. An extensional event of probably Late Caradoc–Early Ashgill age (?) was associated with the development of planar domino-style normal faults and listric faults that occurred prior to the infill of glacial palaeovalleys. This is observed in the pre-glacial In-Tahouite Formation. Glaciotectonic over the Tassilli-n-Ajjers is related to the Late Ashgillian (Hirnantian stage) Taconic event. Probably, the Taconic unconformity is a combination of traditional tectonic movements and glaciotectonics. Many soft-sediment deformation structures are attributed to glaciotectonic process. A systematic description of these deformation structures and strain analysis show that there are two types of deformation, pure shear at the top of the Hirnantian syn-glacial Tamadjert Formation and simple shear in the basal part and there are two types of glaciotectonic deformation. The first type at the base is the subglacial deformation and is characterized by simple shear strain and extensional tectonics. The second type at the top is characterized by pure shear strain and compressional tectonics and consists on proglacial deformation. The glacial palaeovalleys show a gravity-sliding mechanism affecting the unconsolidated deposits.

The Ordovician sponge Palaeomanon cratera (Roemer, 1848) as an erratic on Gotland (Sweden)

Recent cataloguing of collections of Late Ordovician erratic sponges from Gotland (Sweden) in Swedish museums has revealed the presence of Palaeomanon cratera (Roemer, 1848), previously known only from Silurian strata in Tennessee (USA) and the Northwest Territories (Canada). The species forms part of a rich sponge assemblage occurring on Gotland, representing one of three associations in north and northwest Europe, and originating from unknown source areas in Baltica. Palynomorphs extracted from adhering sediment document a Late Ordovician (Ashgill) age; thus, they are the oldest representatives of the species known to date. Naturally, this has implications for the palaeobiogeography of Palaeomanon, in relation to faunal exchange between Baltica and Laurentia; a possible migration from Baltica to Laurentia is discussed.

The Highland Border Complex, Scotland: a paradox resolved

A previously inexplicable difference between the Highland Border Complex, Scotland, and its correlative in Ireland, the Clew Bay Complex, is that rocks of Caradoc–Ashgill age occur only in the former. We reject evidence from supposed chitinozoa for this dichotomy: no sedimentary rocks of proven age younger than Arenig occur in the Highland Border Complex. Consequentially, the stratigraphy must be totally recast, and the ‘exotic terrane model’ replaced by one in which a largely autochthonous Highland Border Complex in stratigraphical continuity with the Dalradian (Grampian terrane), was overridden by the Highland Border ophiolite (Midland Valley terrane). Tectonic models for SE Laurentia are thereby considerably simplified.

Late Ordovician glaciation in southern Turkey

ABSTRACT We present a new survey of several Palaeozoic sections in both the Taurus range and the Border Folds that documents typical glacial features including a glacial pavement and striated dropstones (Halevikdere Formation) and demonstrates the former presence of an ice sheet in southern and south‐eastern Turkey. Evidence for the late Ashgill (Hirnantian) age of this episode is provided by macro‐ and microfossils found within the glacial formation. The extension of ice‐related deposits into the northernmost part of the Arabian Platform (Mardin region) implies a much wider distribution of the Ordovician ice sheet than was previously believed, and strongly suggests that southern Turkey lay close to Egypt during the Lower Palaeozoic.

Micropalaeontology of a Moroccan Ordovician deposit yielding soft-bodied organisms showing Ediacara-like preservation

A newly discovered locality of an hitherto unknown age, but now estimated to the Late Ordovician, in south-eastern Morocco has yielded numerous specimens of problematic soft-bodied metazoans preserved in coarse sandstones. The excellent preservation of the recovered specimens permits an improved understanding of both the palaeobiology of these Ordovician metazoans and the taphonomic processes associated with them. This style of soft-tissue preservation is closely comparable to that of the late Neoproterozoic problematic Ediacaran organisms, and is virtually absent in the Phanerozoic. Palynological analysis (chitinozoans and acritarchs) of shaly intercalations in an ophiuroid-bearing sandstone slab facilitates accurate identification of the site as belonging to the Upper Ktaoua Formation and thus a relative dating of the associated fossils. The chitinozoan assemblage is relatively well preserved, moderately diverse and closely comparable to the previously described chitinozoan fauna from the Upper Ktaoua Formation (Rawtheyan, late middle Ashgill age). Acritarchs are abundant but mostly ill-preserved; nonetheless they clearly indicate an undifferentiated Ashgill age, consistent with the chitinozoan chronostratigraphic result. These datings prove that the newly discovered Moroccan metazoan assemblage contains one of the oldest known paropsonemids, and possibly some of the youngest protolyelloids and aglaspidids. The new finds also show that Ediacara-like coarse-clastic preservation did not completely vanish with the onset of the Phanerozoic.

Mineralogy and geochemistry of Late Ordovician phosphate-bearing oolitic ironstones from NW Sardinia, Italy

¶In the Nurra region, NW Sardinia, oolitic ironstones are interlayered within coarsening upward metasedimentary sequences of siltstone, sandstone, breccia, and conglomerate. A Caradoc-Early Ashgill age is suggested by the analogies with metasediments of Central-Southern Sardinia following the Sardinian tectonic phase. The sequences including oolitic ironstones are overlain by black metapelite of inferred Hirnantian to early Silurian age. The ooids consist of chamosite, siderite or magnetite or, rarely, stilpnomelane. Chamosite ooids consist of up to 30 lamellae and sporadically show clustering of magnetite grains in core and rim, and widespread late replacement by siderite. The alternation of chamosite and Fe-oxide observed in a few samples points to an ooid transport over the crests and hollows of megaripples in a continental shelf at 0–60 m depth, and/or a random displacement of the littoral environments in a rapidly evolving continent-sea transition zone producing an alternation of oxidizing and reducing conditions. Black phosphate clasts, including older Fe-ooids, frequently occur. The oolitic ironstones of Nurra are variable mixtures of an Al-Si-Ti rich- detrital component and a Fe-rich chemical component.

Caradoc bivalves from the Argentine precordillera: A local radiation-extinction event

Pebbles with a diverse shelly fauna, including bivalves, brachiopods, gastropods, and bryozoans havebeen found in the basal diamictite of the Don Braulio Formation, cropping out in the Sierra de Villicum, Argentine Precordillera. The Don Braulio Formation is of Ashgill age, but according to the brachiopods the fauna of the pebbles is of early Caradoc age. On the basis of this age and some elements in common, pebbles probably come from the underlying La Cantera Formation. Bivalves include nine genera, seven of which are new. The relatively high diversity and endemism of this fauna contrasts with the low diverse and widespread Caradocian associations from other regions of South America. The life span of this association is short. The Ashgillian beds of the Don Braulio Formation have yielded a different, low diverse association of bivalves. Causes of this brief radiation-extinction event affecting the bivalves are not clear. According to the recently proposed palaeobiogeographic reconstructions, during the early Caradoc the Precordillera attained the “pre-accretion stage”, and was placed near to the Gondwana continent. Consequently, a geographic isolation cannot be advocated to account for the high endemism of the bivalves. The principal cause of the endemism could be linked to climatic conditions. According to the global and local temperature curves, during the Caradoc an increase in water temperature occurred. Caradoc units of the Precordillera (Las Plantas and Las Aguaditas formations) include some carbonate deposits suggesting temperate waters. This could be the reason for the rapid diversification of the bivalves. Conversely, the dominantly clastic sedimentation of other Gondwanan areas such as the Central Andean basin of NW Argentina and Bolivia suggests that water temperature could have been cooler than in the Precordillera basin and that cold waters could have been a climatic barrier precluding expansion. The rapid extinction of the fauna could be linked to the progressive cooling during the Ashgill which culminates with the latest Ordovician glacial event.

First record of a Hirnantia Fauna from Spain, and its contribution to the Late Ordovician palaeogeography of northern Gondwana

AbstractA new occurrence of the Hirnantia brachiopod fauna is documented from the Criadero Quartzite of Almadén, Ciudad Real Province, Spain. This unit is the regional development of a largely unfossiliferous sandy facies that frequently overlies the typical Late Ordovician diamictitic glaciomarine formations in the Iberian Peninsula and the Armorican Massif. The new occurrence establishes palaeontologically the latest Ashgill age of the quartzite, at least for its lowest horizons, and adds new data on a fauna that, although widespread, has been very rarely documented from peri-Gondwanan Europe. The new collection contains only Hirnantia sagittifera and Plectothyrella crassicosta chauveli. The subspecific status of the latter and its inclusion within Plectothyrella crassicosta is discussed herein, based on the continuous variation in rib thickness of several samples of both forms. The extremely low diversity and the occurrence of the key form P. c. chauveli, are both typical of the Bani Province that developed on the subpolar margins of Gondwana. This contrasts with other occurrences of the Hirnantia Fauna in peri-Gondwanan Europe, such as those from Sardinia and the Carnic Alps, which are characteristic of the more temperate Kosov Province.

Late Ordovician (Ashgill)Foliomenafauna brachiopods from northeastern Maine

Foliomena folium(Barrande) and other very small, thin-shelled brachiopods from the Pyle Mountain Argillite in Aroostook County, Maine, provide the only record of the presence of theFoliomenafauna in North America. Associated fossils include numerous small trilobites and a few specimens of other groups. An Ashgill age is established by the presence of graptolites of theClimacograptus spiniferousZone in the underlying Winterville Formation.Most of the 190 specimens are small strophomenaceans and plectambonitaceans. One of the most numerous is the christianiidNubialba forbesin. gen. and sp. that has ribs and elaborately developed internal features. In the absence of evidence to contradict previous interpretations that theFoliomenafauna records cold-water environments, the presence of one or more deep basins along the southeastern margin of the trans-equatorial North American paleoplate occupied by cool peri-Gondwanan waters during a short segment of Ashgill time is implied.

Late Ordovician rugose corals of the northern Sierra Nevada, California

The shoo fly Complex of Late Devonian and older Paleozoic age is a regionally extensive rock assemblage in the northern Sierra Nevada of northern California. It consists chiefly of a coherent unit of phyllite, quartzose sandstone, and chert, and a melange unit (Hannah and Moores, 1986). Several limestone lenses in the Taylorsville area comprise the Montgomery Limestone (Diller, 1892, 1908; McMath, 1958; Figure 1). The Montgomery was long considered to be Silurian, largely on the basis of corals, brachiopods, and cephalopods (Diller, 1892, 1908; McMath, 1958; Berry and Boucot, 1970; Merriam, 1972). However, recent analyses of the biota indicate an Ashgill (middle Maysvillian–Gamachian) age (Boucot and Potter, 1977; Harris, personal commun. cited in Hannah and Moores, 1986, p. 790; Potter et al., 1990b; present study).

Geochemistry of the Sunnfjord Melange: sediment mixing from different sources during obduction of the Solund–Stavfjord Ophiolite Complex, Norwegian Caledonides

AbstractThe Sunnfjord Melange is a strongly tectonized sedimentary assemblage occurring tectonostratigraphically below the Solund–Stavfjord Ophiolite Complex of Ashgill age, and upon continental margin deposits of Wenlock or older age. The Sunnfjord Melange, which probably started to form within an oceanic transform fault, developed further during obduction of the Solund–Stavfjord Ophiolite Complex. The Sunnfjord Melange is divided into two units, the lower Øyravatn Unit of predominantly fine grained calcareous quartz-bearing chlorite–muscovite schists, and the upper Markavatn Unit, mainly metagreywackes. Both units contain blocks of metabasalts, marble and meta-arkose. Serpentinite blocks are restricted to the Markavatn Unit. The appearance of serpentinite detritus is marked by much higher contents of Cr and Ni in the metasediments of the Markavatn Unit than in those of the Øyravatn Unit. The geochemistry of the metasediments from the Øyravatn Unit and the Markavatn Unit is compatible with mixing of detritus from continental and ophiolitic sources. Despite the strong and pervasive deformation of the Sunnfjord Melange, its geochemical composition from the lowest to the highest tectonostratigraphic levels reflects that of the inverse pseudostratigraphy of the Solund–Stavfjord Ophiolite Complex. Thus, only metabasalts of the Solund–Stavfjord Complex and continental meta-arkoses yielded detritus to the schists of the Øyravatn Unit. The compositions of the schists and metagreywackes of the Markavatn Unit, on the other hand, show that a multicomponent detritus derived from metabasalt and serpentinite of the Solund–Stavfjord Ophiolite Complex, mixed with the detritus from continental meta-arkoses.

The early Palaeozoic evolution of northwest England

AbstractThe Lake District and smaller Craven inliers of northwest England contain a Lower Palaeozoic sequence deposited on the Gondwanan side of the Iapetus Ocean, close to the junction with the Tornquist Sea. The Tremadoc to Llanvirn Skiddaw and Ingleton groups are deep water assemblages of turbidite, olistostrome and slump deposits, formed at a continental margin. They experienced uplift and erosion as a precursor to the eruption of two largely subaerial Llandeilo-Caradoc volcanic sequences: the tholeiitic Eycott Volcanic Group in the north and the calc–alkaline Borrowdale Volcanic Group in the central Lake District. The volcanic episodes are the earliest part of a major episode of magmatism, extending through to the early Devonian and responsible for a major batholith underpinning the Lake District. Subsidence in an intra-arc rift zone preserved the subaerial volcanic sequences. A marine transgression marks the base of the Windermere Group, which comprises a mixed carbonate–clastic shelf sequence of Ashgill age, passing upwards through the Silurian into a thick, prograding foreland basin sequence of Ludlow turbidites. Deformation of the Lower Palaeozoic sequences was possibly diachronous from north to south. It is attributed to the late Caledonian (Acadian) Orogeny and resulted in folding, cleavage and thrust development. Granitic intrusions, including those at Shap, Skiddaw and beneath the hydrothermal Crummock Water Aureole, are partly syntectonic and were therefore penecontemporaneous with this deformation event. Some thrust faulting post-dates the intrusive phase. Post-deformation Devonian conglomerates are also present locally.

Quelques assemblages de Chitinozoaires de l'Ordovicien du Maroc, Bassin de Tadla

Palynological analysis of 24 subsurface samples (cores) from Ordovician rocks of the Tadla Basin, western Méséta, Morocco, has permitted us to identify five chitinozoan assemblages of late Arenig, latest Arenig, early Llandeilo, early Caradoc and late Ashgill age. Two new species Laufeldochitina protolardeuxi nov. sp. and Lagenochitina spinatostoma nov. sp. are proposed in this paper. Comparison of these assemblages with contemporary assemblages from North Africa and southwestern Europe show their stratigraphic position and identifies taxon with a stratigraphic potential.

Ashgill greywackes in the Southern Uplands of Scotland: an extension of the Ordovician succession in the Northern Belt

ABSTRACTIn the Southern Uplands of Scotland, the youngest greywackes along the SE margin of the Northern Belt are shown to be of Ashgill age. Mudstones interbedded with the dominantly quartzose greywackes of the Shinnel Formation have yielded Dicellograptus anceps, indicating the anceps Biozone. In addition, the newly-defined mafic-rich Glenlee Formation, lying to the SE of the Shinnel Formation in the New Galloway-Moniaive area, has yielded fossils of the pacificus Subzone, the upper subzone of the anceps Biozone. Since the oldest greywackes of the Central Belt, separated from the Northern Belt by the Orlock Bridge Fault, are now confirmed as of basal Silurian acuminatus Biozone age, the stratigraphical break across the Orlock Bridge Fault is greatly reduced, lending support to the ‘single terrane’ model for the Southern Uplands.

Ordovician provincial signals from Appalachian‐ Caledonian terranes

ABSTRACTStatistical analysis of mainly mid to deep‐shelf Ordovician brachiopod faunas from localities within and marginal to the Appalachian‐ Caledonian Orogen reveals biogeographical patterns not entirely consistent with those presumed for the progressive orthogonal closure of the Iapetus Ocean. Correspondence analysis of faunas of late Arenig‐early Llanvirn, late Llandeilo‐earliest Caradoc and late Ashgill age emphasize the importance of marginal and island faunas, the role of regressions and transgressions on faunal distributions and the effects of climatic change in the definition of biogeographical trends during the Ordovician.

Microbrachiopods and the end‐Ordovician event

Millimetre sized chitinophosphatic brachiopods ("microbrachiopods") largely, but by no means entirely, centred around the family Acrotretidae are commonly regarded as being in decline after the early Ordovician. Work on Irish Upper Ordovician limestones however shows that this is not the case, material recovered showing both numerical abundance and taxonomic diversity in beds of Ashgill age. Although forms are known from the Devonian, the published record of these neglected fossils from the Silurian is sparse, so that the effects, if any, of the end‐Ordovician event on this ecologically enigmatic group cannot as yet be determined.

Revised stratigraphy and relationships of Lower Palaeozoic rocks, eastern Taurus Mountains, south central Turkey

AbstractLithostratigraphic terminology is revised for rocks of Ordovician age in the eastern Taurus. The Seydişehir Formation, of late middle Cambrian to early Ordovician age, is shown to be applicable in the eastern and western Taurus as well as in south-eastern Turkey. Near Degˇirmentaş, northeast of Adana, the Seydişehir Formation is overlain, with inferred unconformity, by clastic strata of Ashgill age referred to the Şort Tepe Formation, first defined south of Hakkâri, in the Border Folds of southeastern Turkey. Palaeontological evidence is cited for the early Silurian (Llandovery) age of the unconformably overlying Halityayla Formation farther south, on the coast near Ovacik, where the Şort Tepe Formation is as yet unrecorded. A general model is proposed showing relationships of Cambrian and Ordovician rocks and faunas in southern and southeastern Turkey.

A review of the trinucleid trilobites of China

Trinucleid trilobites from China assigned to twenty-five species representing seven genera are reviewed. Their occurrence is restricted mainly to the Yangtse region of China, and two stratigraphically distinct faunas are present. A hanchungolithine-Yinpanolithus fauna of Arenig age shows affinities to the trinucleid faunas of central Europe and northern Gondwanaland. ANankinolithus- reedolitne fauna of Caradoc to Ashgill age possesses species of Caradoc age that show affinity to the trinucleid faunas of Europe, whereas the species of Ashgill age show cosmopolitan affinities. No trinucleids are yet known from the Llandeilo of China and records from north of the Yangtse region are as yet unpublished. One new species,Nankinolithus yanhaoi is erected and a full description given for the first time of “Trinucleus”richthofeni Kayser, 1883 which is assigned to the genusHanchungolithus. Lectotypes are selected forNingkianolithus welleri, Ningkianolithus honghuayuanensis, Hanchungolithus xiangyangensis andYinpanolithus yinpanensis.

Trilobites from the Upper Ordovician Tangtou Formation, Jiangsu Province, China

ABSTRACTForty-one species assigned to thirty-seven named genera of trilobites are described from the Tangtou Formation at Tangshan and Lunshan, in Jiangsu Province, SE China.Nankinolithus nankinensisLu, the Zone fossil of the upper part of the Formation, is abundant. Many Tangtou Formation species occur also in the Huangnehkan, Linhsiang and Chiencaokou formations, and all are considered to be of early Ashgill age. The trilobites are closely related to those of theStaurocephalus clavifronsZone (Rawtheyan) of Poland, demonstrating the long duration and wide extent of deeper water faunas. There are also close links with the Ashgill faunas of Central Asia.One new species,Amphitryon cheni, is established. The following morphological features are described: a small remopleuridine hypostome is unlike any other in construction;Nileus transversusretains genal spines to full grown size in some specimens, but loses them in others; the hypostome lacks the posteromedian cusp of other species. The hypostome ofParaphillipsinellais strongly styginid in conformation, and unlike that ofPhillipsinella. Telephina convexais the only species known with three spines on the librigena. The arrangement of pits inNankinolithus nankinensis, the type species, is described, including illustrations of the irregularities which occur in every fringe. A small bulb at the tip of the hypostome ofOvalocephalus(=Hammatocnemis) bears two backwardly pointing spinules, one above the other, on the vertical posterior margin. Three types of protaspides, and numerous meraspid parts are described.

Arenig-Llandovery stratigraphy and faunas across the Scandinavian Caledonides

Summary The nature and development of Arenig-Llandovery sedimentary environments is reviewed with reference to 18 allochthonous-parautochthonous sections in the Scandinavian Caledonides. The faunas suggest that age constraints are imprecise, and direct correlations with sequences on the adjacent platform are difficult. However, it is possible to recognize a northerly transition from near-shore to offshore facies on Hardangervidda and a platform to slope transition in Jämtland and E Jotunheimen during the early Ordovician. There is no direct evidence of a distinct Finnmarkian orogenic event (late Cambrian-early Ordovician) in the lower Allochthon, while the central Scandinavian upper Allochthon, containing N American shelly fossils, has a pre-Caradoc history far removed from that of the Baltic platform. In the Bergen area no fossils older than Ashgill age are known and the youngest marine rocks are early Llandovery. Locally-important features of the platform sequences, including breaks in sedimentation, the deposition of bentonites, basement faulting and the early termination of marine sedimentation (late Llandovery in the N and W), cannot be directly related to discrete tectonic events in the orogen.