Biozone Age Research Articles

The Lower Ordovician Tøyen Shale succession in the Fågelsång-3 drill core, Scania, Sweden

The Ordovician Tøyen Shale Formation of the recently retrieved Fågelsång-3 drill core provides some important information on the graptolite biostratigraphy of the unit and its completeness in the region. The drill core reached downwards into the Kiaerograptus supremus Biozone of late Tremadocian age. Above a major fault zone, faunas with a number of specimens referred to Pseudophyllograptus densus are indicative of a late Dapingian age. Rare specimens of Azygograptus sp. also help constrain the age. The presence of Arienigraptus zhejiangensis indicates that the base of the Darriwilian Komstad Limestone Formation is of Levisograptus austrodentatus Biozone age. Specimens of Tetragraptus cor and Levisograptus sinicus in a shale bed within the lower part of the Komstad Limestone Formation can be referred to the upper subzone of the L. austrodentatus Biozone, the Levisograptus sinicus Subzone. This fauna represents the youngest graptolite fauna beneath the massive Komstad Limestone Formation. The Tøyen Shale Formation can be shown to be highly incomplete in Scania, possibly due to post-depositional tectonic deformation and faulting.

Palynology and correlation of Carboniferous–Permian glacigene rocks in Oman, Yemen and Pakistan

Palynological assemblages from sections in Yemen and Pakistan are similar enough to allow the application of biozones based on the Pennsylvanian glacigene sediments of interior Oman. Thirty four samples from the upper 84m of a 125m thick section of the Tobra Formation at Zaluch Nala, western Salt Range, Pakistan yielded palynomorph taxa including Microbaculispora tentula, abundant monosaccate pollen including Cannanoropollis janakii and Plicatipollenites malabarensis, Converrucosisporites grandegranulatus, Horriditriletes ramosus and Horriditriletes tereteangulatus indicating the late Pennsylvanian Oman 2165B Biozone. Eleven samples from the Yemen Kuhlan Formation, and 22 samples from the underlying Akbarah Formation from approximately 300m of a section near Kuhlan in northwest Yemen, suggest a 2165A Biozone age (also late Pennsylvanian). This correlation indicates the widespread nature of glacial sediments of a narrow biostratigraphic late Pennsylvanian age range (the Oman P5 unit) in basins across part of the north Gondwana margin. New data from three samples from the Tobra Formation at the Choa Road section near Khewra in the eastern Salt Range, Pakistan are assignable to the earliest Permian 2141B Biozone, indicating a possible correlation between the ‘Tobra shale’ and the Rahab Shale Member of Oman, which is considered to mark the final deglaciation sequence in Oman.

Stratigraphic Note: Update on the palynology of the Akbarah and Kuhlan formations, northwest Yemen

SUMMARY Following a preliminary palynological report of two samples from the lower part of the Kuhlan Formation (Unit A) near Kuhlan village, northwest Yemen (Stephenson and Al-Mashaikie, 2010), a further seven samples from Unit A, and an additional 22 samples from the underlying Akbarah Formation in the same locality are reported. The seven new samples from the Kuhlan Formation support the 2165A to 2141A Biozone age originally suggested by Stephenson and Al-Mashaikie (2010), and the new Akbarah Formation samples suggest an age not markedly different since Anapiculatisporites concinnus and Spelaeotriletes triangulus are also present in the Akbarah Formation (e.g. samples AK-11 and AK-12). This correlation confirms that the lower Kuhlan Formation and the Akbarah Formation, are likely to be late Carboniferous in age and equivalent to the lower parts of the Al Khlata Formation of Oman.

Provenance of chalk tesserae from Brading Roman Villa, Isle of Wight, UK

Thin section petrographical analysis of chalk tesserae at Brading Roman Villa, Isle of Wight, England, identifies a range of planktonic foraminifera and the calcareous algal cyst Pithonella that identify the Late Cenomanian Rotalipora cushmani Biozone (BGS Foraminiferal Biozones 4iii to 7). The local chalk crop to the north of the villa includes rocks of R. cushmani Biozone age, and indicates a likely local, rather than long distance, source for the tesserae. Microfossils provide a powerful tool for identifying the provenance of artefacts in Roman Britain.

A refined graptolite biostratigraphy for the late Ordovician–early Silurian of central Wales

Morphometric analysis of graptolites from the persculptus and acuminatus biozones of central Wales identifies four successive morphospecies of normalograptids. These graptolites can be used for biostratigraphical subdivision of these strata as follows: (i) an early persculptus Biozone interval containing broad forms with geniculate thecae that have the morphology of Persculptograptus persculptus with an early insertion point for the full median septum (theca 11); (ii) a supra-adjacent level of early persculptus Biozone age, with narrower, parallel-sided forms, that have been referred to as Normalograptus? aff. parvulus and have a slightly later insertion point for the full median septum (theca 12); (iii) a third interval, encompassing the later part of the persculptus Biozone to the early acuminatus Biozone, with Normalograptus? cf. parvulus, which has the full median septum delayed to the level of theca 3–7; and (iv) a younger interval, in the mid-acuminatus Biozone with Persculptograptus cf. persculptus specimens that do not display the median septum on its reverse side. These taxa can be used for refined biostratigraphy and correlation in the late Ordovician and early Silurian of central Wales. The progressive delay in the insertion of the median septum in these taxa may have wider application for the correlation of the interval immediately after the Hirnantian glacial maximum.

Integrated Upper Ordovician graptolite–chitinozoan biostratigraphy of the Cardigan and Whitland areas, southwest Wales

To help calibrate the emerging Upper Ordovician chitinozoan biozonation with the graptolite biozonation in the Anglo-Welsh, historical type basin, the graptolite-bearing Caradoc–Ashgill successions between Fishguard and Cardigan, and at Whitland, SW Wales, have been collected for chitinozoans. In the Cardigan district, finds of Armoricochitina reticulifera within strata referred to the clingani graptolite Biozone (morrisi Subzone), together with accessory species, indicate the Fungochitina spinifera chitinozoan Biozone, known from several Ordovician sections in northern England that span the base of the Ashgill Series. Tanuchitina ?bergstroemi, eponymous of the succeeding chitinozoan biozone, has tentatively been recovered from strata of Pleurograptus linearis graptolite Biozone age in the Cardigan area. The T. ?bergstroemi Biozone can also be correlated with the type Ashgill Series of northern England. Chitinozoans suggest that the widespread Welsh Basin anoxic–oxic transition at the base of the Nantmel Mudstones Formation in Wales, traditionally equated with the Caradoc–Ashgill boundary, is of Cautleyan (or younger Ashgill) age in the Cardigan area. In the broadly time-equivalent, graptolite-rich Whitland section, also in SW Wales, two Baltoscandian chitinozoan biozones and a subzone have been recognized (again using accessory species), namely the Spinachitina cervicornis Biozone?, the Fungochitina spinifera Biozone and the Armoricochitina reticulifera Subzone. The new chitinozoan data provide a more precise means of correlation between the Whitland and Cardigan successions and suggest that the Normalograptus proliferation interval of the Whitland section is at least partly attributable to the Dicellograptus morrisi Subzone of the Dicranograptus clingani Biozone, rather than equating with the overlying Pleurograptus linearis Biozone.

The Tamar Trough revisited: correlations berween sedimentary beds, basalts, their ages and valley evolution, North Tasmania

The Tamar Trough, an Early Palaeogene fault structure, contains sedimentaty beds and interleaved basaltic flows that infill the structure along its 70 km length. These infills represent a complex interplay between sedimentation, channel erosion, eruptive dislocations, and even 'out of trough' diyersions of the ancestral Tamar drainage. Several areas of resistant basalt flows remain in the south, upper, middle and lower Tamar reaches. Although some palynological control was known, radiometric dating of previously untested basalts now allows close integration and age-pegging for observed palynological biozones. The K-Ar and Ar-Ar ages of the basalt bodies indicate eruptive events at 47, 33-37 and 25 Ma, correlating with Proteacidites asperopolus-Malvacipollis diversus, Nothofagites asperus and Proteacidites tuberculatus biozone age sedimentary beds respectively. Basanite, alkali basalt and hawaiite flows dominate basalt lithology with lesser olivine nephelinite, transitional olivine basalt, olivine tholeiite and quartz tholeiite. Basalt geochemistry suggests derivation from different degrees of partial mande melting (from 7 to 35%), with alkaline and tholeiitic basalts being derived from separate source regions. Most alkaline basalts have high-jl (HIMU) related trace element signatures, which are absent in the tholeiitic rocks. A basalt plug on the trough margin at Loira gave a Jurassic age and has Jurassic dolerite-like geochemistry. The Tamar sequence suggests that the initial fluvio-Iacustrine and later channel-fill sedimentation from 65(?) to 24(?) Ma was then punctuated in places by periods of alkaline volcanism between 47 to 33(?) Ma, and alkaline and tholeiitic volcanism between 33 to 24(?) Ma. No Neogene fossils are known, so this later period was probably one of net erosion. These contrasting quiet sedimentary and more volcanic intervals are related here to a tectonic model that involves northerly drift of Victorian and Tasmanian lithosphere over several former Tasman metasomatised mantle plume sources.

Stratigraphical and palaeoecological importance of Caradoc (Upper Ordovician) graptolites from the Cardigan area, southwest Wales

Graptolites from more than 60 horizons in the basinal Caradoc succession of southwest Wales, between Fishguard and Cardigan, allow recognition of the multidens, clingani and linearis biozones. The biostratigraphy permits recognition of major differences in the sedimentary rock-sequence north and south of structures associated with the Fishguard–Cardigan Fault Belt. The Penyraber Mudstone Formation, disconformably overlying the Fishguard Volcanic Group (Llanvirn), is partly of multidens Biozone age. It is succeeded south of the Newport Sands Fault by the Cwm yr Eglwys Mudstone Formation of clingani to linearis biozones age. North of the fault the Cwm yr Eglwys Mudstone Formation is replaced laterally by the northwards-thickening, sandstone turbidite-dominated Dinas Island Formation (clingani and linearis biozones). Graptolite stratigraphical distribution indicates that Dicranograptus clingani occurs only rarely within the caudatus Subzone of the clingani Biozone and that Climacograptus antiquuss.l. also does not range above the lower part of the clingani Biozone. The first occurrence of Dicellograptus morrisi, within the upper clingani Biozone, confirms its value as a marker for the morrisi Subzone, and is associated with the first occurrences of Diplacanthograptus dorotheus and Normalograptus minimus. Dicellograptus flexuosus, used to indicate the morrisi Subzone elsewhere, occurs throughout the clingani Biozone in the Cardigan area. The linearis Biozone is recognized by Climacograptus tubuliferus. Oxic bottom conditions in early and early mid-Caradoc times largely precluded the influx of, or preservation of, graptolite faunas in the Penyraber Mudstone Formation. Anoxic mudstones of the Cwm yr Eglwys Mudstone and Dinas Island formations preserve graptolite assemblages of 21 and 26 species, signalling strong open marine influences which persisted in this area until late Caradoc times. This contrasts with the shelfal faunas in the Whitland area (south Pembrokeshire), where the late Caradoc is dominated by low-diversity Normalograptus-dominated assemblages.

The graptolitic age of the type Ashgill Series (Ordovician), Cumbria, UK

SUMMARY The Rawtheyan Stage of the type Ashgill Series, in Westerdale, Cumbria, is shown to be of linearis Biozone age, largely on the basis of extensive new graptolite collections. This is the only reliable graptolite dating of the type Ashgill, although it is possible that the underlying Cautleyan Stage is also of linearis Biozone age. There is no direct evidence for the complanatus, anceps, extraordinarius and persculptus biozones, although these could equate roughly with the top of Ingham’s (1966) zone 7 and his zone 8, the latter embracing the Ashgill (Shales) Formation. A total of 26 graptolite taxa have been identified in the type Ashgill Series, from 26 localities, many of them new. Dicellograptus praeanceps sp. nov. is described.

Trilobite faunas and palaeoenvironmental setting of the Silurian (early Ludlow) Melbourne Formation, central Victoria

Trilobites are common faunal elements in the Melbourne Formation, a unit of early Ludlow (upper nilssoni Biozone) age, which crops out extensively in the Darraweit Guim Province of the Melbourne Zone, central Victoria. New diagnoses are given for species previously described, including Maurotarion euryceps (McCoy, 1876; = Cyphaspis spryi Gregory, 1901), Raphiophorus jikaensis (Chapman, 1912; = Ampyx yarraensis Chapman, 1912), Cromus simpliciculus (Talent, 1964), Cromus spryi (Chapman, 1912), Sthenarocalymene kilmorensis (Gill, 1945; = Gravicalymene hetera Gill, 1945) and Trimerus harrisoni (McCoy, 1876). A new phacopid genus, Orygmatos is described, represented by the species O. yanyeani gen. et sp. nov. Other species newly described include Cromus melbournensis sp. nov., Arcticalymene australis sp. nov., “Ananaspis” woiwurrungi sp. nov. and Kettneraspis hollowayi sp. nov. Species composition of the trilobite fauna varies spatially, and a number of distinct assemblages can be defined. Abundant trilobite moult configurations are conclusive for interpretation of the benthic fauna as autochthonous, inferring depth estimations based on benthic community distribution to be valid. A depth-related succession of communities is recognised and indicate the Melbourne Formation was deposited at relatively shallow depths on a broad, eastwardly deepening shelf, with deposition dominated by storm processes. The palaeoenvironment comprised a BA-1 community including the Arcticalymene australis trilobite assemblage, restricted to very shallow depths (∼20 m) on the SW coastal margin of the shelf and preserved in proximal tempestite lithologies; and a BA-5 community group containing three distinct trilobite assemblages dominated by species of Cromus and a deeper water fauna, preserved in distal tempestite lithologies and ranging widely over the shelf at depths in the range of maximum storm wave base (∼50 – 80 m).

The Silurian fauna (graptolite and nautiloid) of the Niarbyl Formation, Isle of Man

Abstract The discovery of a mid-late Wenlock (Silurian) graptolite and orthoconic nautiloid fauna at Traie Dullish Quarry, Peel Hill, in the Niarbyl Formation (Dalby Group) of the Isle of Man, disproves all earlier correlations between the Niarbyl and Lonan Flags on the west and east coasts of the island, respectively. All previous structural hypotheses require re-examination. The graptolites comprise Cyrtograptus cf. lundgreni, Monograptus flemingii cf. warreni and Monograptus ex gr. flemingii . They suggest, but do not prove, a lundgreni Biozone age, thus indicating a possible correlation between the Niarbyl Formation and the Birk Riggs Formation of the English Lake District, and the Denhamstown Formation of the Balbriggan Inlier, Southern Ireland.

Uppermost Telychian graptolites from the North Esk Inlier, Pentland Hills, near Edinburgh

Synopsis The North Esk Inlier is an important outcrop of Silurian strata, well known for the abundance, quality and variety of fossiliferous material present. It records a significant segment of the sedimentary and tectonic history of the Midland Valley of Scotland. Numerous graptoloid specimens have recently been discovered and identified, and this has enabled the graptolite biostratigraphy of these deposits to be determined more accurately. The marine sediments of the North Esk Group have for many years been referred to as Upper Llandovery (Telychian), Monoclimacis crenulata Biozone age. The species now known to be present, are better and more precisely accommodated within the biozonal scheme of Bohemia, which is considered to be a refinement of the existing ‘British’ scheme, and correlation of relevant graptolite biostratigraphical schemes is considered. The sediments of the North Esk group are shown to range in age from the middle of the upper Llandovery spiralis graptolite interval, to lowest Wenlock. The graptolite species present are illustrated.

The Blea Wyke Sandstone Formation (Jurassic, Toarcian) of Rosedale, North Yorkshire, UK

SUMMARY Revision of the stratigraphy and field relationships of the late Toarcian succession of Rosedale in general, and of Hollins Mines in particular, has clarified the stratigraphy of the Blea Wyke Formation and given new evidence for the nature of the pre-Dogger Formation deformation. Four new members of the Blea Wyke Sandstone Formation are introduced for the succession in Rosedale: the Yew Grain Member, the High House Ironstone Member, the Low Baring Member and the Rosedale Ironstone Member. The Yew Grain Member comprises a coarsening-up succession, dominated by siltstones, but with bioturbated sandstones at its top. The siltstones yield ammonites indicative of a fallaciosum Subzone age. The ironstones of the High House Ironstone Member are bioturbated silty berthierine ooidal mud- and wacke-ironstones, with abundant sideritic and phosphatic concretions, and are of dispansum Subzone age. The Low Baring Member comprises bioturbated fine-grained sandstones and siltstones of dispansum Subzone age, and is largely equivalent to the ‘Serpula Sandstone’ of previous authors. The Rosedale Ironstone Member is characterized by ooidal grain-ironstones, with berthieroidal ooids and a siderite cement. The Rosedale Ironstone Member is of uncertain age, constrained by the dispansum Subzone age of the Low Baring Member below and the probable opalinum Biozone age of the unconformable base of the Dogger Formation above. The magnetitic ironstones of the Hollins Mines are referred to the Rosedale Ironstone Member. Their different mineralogy reflects a different diagenetic evolution. The magnetitic ironstones are locally more than 25 m thick and are not channel-fills, as previously believed, but are contained within truncated synclines, probably roll-overs associated with extensional growth faulting. East of a north–south line passing just east of Hollins Mines the lowest part of the Dogger Formation is the Glaisdale Ironstone Member (of Aalenian, opalinum Biozone age). West of this line the Low House Sandstone Member is the lowest element of the Dogger Formation. The Low House Sandstone Member and the western part of the outcrop of the Glaisdale Ironstone Member are overlain by the ‘Blakey Series’, forming the upper part of the Dogger Formation (of Aalenian age, murchisonae Biozone), which also contains ooidal grain ironstones in its upper part in the south-east of Rosedale. These were also worked, although on a smaller scale, in the area of Hollins Mines and a little to the north near Abbey Bank.

Silurian trilobites from the G. B. Schley Fjord region, eastern Peary Land, North Greenland

A trilobite fauna of Pterospathodus celloni Biozone age (late Llandovery) is described from a horizon near the top of the Odins Fjord Formation. The commonest trilobite is the phacopid Acernaspis sp., with which are associated the proetid Cyphoproetus? cf. C. alyo, the harpetid Scotoharpes?, a second phacopid (Ananaspis?), the encrinurid Briania and an undetermined encrinurid. The llilobile species compare closely with others already described from various localities in the mid-late Llandovery of Greenland and Canada. The overall composition of the trilobite fauna, however, contrast with that previously known from the Odins Fjord Formation; this may partly reflect the more offshore position of the Schley Fjord fauna on the Peary Land shelf. However, drowning of the shelf produced a possibly diachronous decrease in fauna! content and diversity as deepening and muddying progressed southwards over time. Thus, faunal contrasts may simply reflect sampling of different levels within this muddying-upwards succession.

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.

Graptolite dating of Ordovician vulcanicity in the Arenig area, North Wales

AbstractGraptolite assemblages of multidens Biozone age have been recovered from mudstones immediately above and below the Llyn Conwy Formation, the local representative of the regionally extensive acid tuffs that mark the end of vulcanicity in the south and east of the Harlech Dome. They include several taxa new to Wales and are markedly different to contemporaneous faunas from the Welsh Borders. One new species, Pseudoclimacograptus isknos, is described.

A revision of the age of the hen‐dy‐capel ooidal ironstone (ordovician), llanengan, N. Wales

AbstractPendent didymograptid graptolites are described from mudstones immediately overlying the Hen‐dy‐capel ironstone (St. Tudwal's Peninsula, Lly̌n, North Wales). The graptolites indicate a Llanvirn age for these beds, implying a Llanvirn or Arenig age for the ironstone, in contrast with the previous interpretation of a gracilis Biozone age. The redating of the ironstone horizon suggests that the Pen‐y‐gaer mudstones may be at the base of the local Nant Ffrancon Formation, rather than being equivalent to the lower Caradoc Tyddyn Dicwm beds above the Tremadog ironstone as widely supposed. The ironstones at Trefor, Betws Garmon, and Llandegai, which have been correlated with the Hen‐dy‐capel ironstone on the basis of acritarch assemblages, may also have a similar age. A reinterpretation of the age of these localities as Llanvirn would remove the stratigraphic problems introduced by their previously suggested early gracilis Biozone age. The Hen‐dy‐capel ironstone forms one of a large number of ironstones recorded from close to the Arenig‐Llanvirn boundary in N. Wales, and elsewhere at high southern palaeolatitudes.

The biostratigraphic age of the Ordovician Skiddaw Group in the Black Combe Inlier, English Lake District

SUMMARY Graptolites and acritarchs from several localities on the NW side of the Whicham Valley, Black Combe, show that the beds there (the Whicham Blue Slates of Helm 1970) are of late Arenig Didymograptus hirundo Biozone age. Acritarch floras show that the beds underlying the Borrowdale Volcanic Group on the SE side of the Whicham Valley are of latest Arenig age. Probable latest Arenig acritarchs have also been recorded from a tuffite at the base of the volcanic sequence of Millom Park. The possible age of this tuffite is discussed.