Didymograptus Murchisoni Research Articles

The Darriwilian Hiswah fauna of western Gondwana (Jordan): Biostratigraphy, palaeogeography and palaeoecology

The fauna and facies of the Hiswah Formation in southern Jordan were investigated. The formation indicates deposition under low energy, open-marine, siliciclastic shelf conditions and reflects the maximum of the first regional post-Cambrian marine transgression on the northern edge of the Arabian-Nubian Shield. This eustatic signal accompanied by a maximum flooding surface can be traced over the entire Arabian Plate and Turkey. The lower part of the Hiswah Fm. consists of pelagic mudstones with only occasional thin beds of rippled, hummocky cross-bedded siltstone and mudstone concretions, and contains a distinct fauna of low diversity. The upper part of the formation is characterized by sandstone-siltstone alternations with some hummocky cross-stratification in its lower portion and trough cross-bedding and ripple marks above indicating somewhat shallower conditions. A very detailed re-investigation of the fauna from the lower part of the Hiswah Fm. and extensive new fossil material indicates that the most remarkable faunal element is the planktonic graptolite Didymograptus murchisoni (Beck in Murchison, 1839in) which, in the past, has been determined erroneously as D. bifidus (Hall in Berry, 1962). Numerous specimens of small obolid brachiopod Palaeoglossa sp. cf. P. attenuata (Sowerby, 1839) represent the most common benthic element within the Hiswah fauna. The exceptional arthropod Hanadirella cf. bramkampi El-Khayal, 1985, is reported for the first time from Jordan, representing its fifth known region of occurrence in addition to Saudi Arabia, Algeria, Spain, and France. For the first time, cephalopod remains are described from the Ordovician of Jordan by the occurrence of ?Dideroceras sp. and a second, indeterminate species. The biostratigraphic age of the Hiswah fauna is revised to late Darriwilian (Dw 3, upper Middle Ordovician) according to our re-investigation of the graptolites. The Hiswah fauna represents the oldest body fossils of the Ordovician marine sedimentation in southern Jordan and is palaeogeographically strongly related to northern and northeastern Saudi-Arabia and to high/mid-latitude Gondwana margins in general.

On the integration of Ordovician conodont and graptolite biostratigraphy: new examples from Gansu and Inner Mongolia in China

Wang, Z.H., Bergström, S.M., Zhen, Y.Y., Chen, X. & Zhang, Y.D., 2013. On the integration of Ordovician conodont and graptolite biostratigraphy: New examples from Gansu and Inner Mongolia in China. Alcheringa 37, 510–528. ISSN 0311-5518.Few Ordovician successions in the world contain both biostratigraphically highly diagnostic conodonts and graptolites permitting an integration between standard biozones based on these fossil groups. The Sandbian Guanzhuang section in the vicinity of Pingliang in the Gansu Province has an outstanding graptolite record through most of the Nemagraptus gracilis and Climacograptus bicornis graptolite biozones. Calcareous interbeds in the succession yield biostratigraphically important conodonts, including some species used for biozonations in Baltoscandia and the North American Midcontinent. Likewise, the middle–upper Darriwilian Dashimen section in the Wuhai region of Inner Mongolia hosts both diverse graptolites of the Pterograptus elegans, Didymograptus murchisoni and lowermost Nemagraptus gracilis biozones, and conodonts of Midcontinent and Baltoscandic types. The distribution patterns of these index fossil groups provide an unusual opportunity to closely correlate conodont and graptolite biozones in the middle to upper Darriwilian to Sandbian interval. For instance, the base of the C. bicornis Biozone is approximately coeval with the base of the Baltoscandic B. gerdae Subbiozone and a level near the middle of the North American P. aculeata Biozone.Zhi-hao Wang [zhwang@nigpas.ac.cn] Xu Chen [xu1936@gmail.com], and Yuan-dong Zhang [ydzhang@nigpas.ac.cn], Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China; Stig M. Bergström [stig@geology.ohio-state.edu], School of Earth Sciences, Division of Earth History, The Ohio State University, Columbus, OH 43210, USA; Yong Yi Zhen [yongyi.zhen@austmus.gov.au], Australian Museum, 6 College Street, Sydney NSW 2010, Australia.

The Dawangou auxiliary GSSP (Xinjiang autonomous region, China) of the base of the Upper Ordovician Series: putting global chitinozoan biostratigraphy to the test

Abstract. The Dawangou section, an auxiliary Global Stratoype Section and Point (GSSP) for the base of the Upper Ordovician Series, was sampled for chitinozoans. The 32 samples taken from the black shales of the Saergan Formation, spanning the Pterograptus elegans, Didymograptus murchisoni and Nemagraptus gracilis graptolite biozones, yielded 6536 chitinozoan vesicles. In addition to a diverse endemic assemblage, species important for international correlation were recovered, which enables the subdivision of the Saergan Formation into two chitinozoan biozones, one subzone and one provisionally recognized biozone: the Baltoscandian Laufeldochitina striata Biozone and the Laurentian Lagenochitina sp. A (sensu Achab, 1984) (/Belonechitina hirsuta) Biozone, and the Belonechitina tuberculata Subzone are recognized based on the presence of the index species, while the Baltoscandian Laufeldochitina stentor Biozone is provisionally recognized on the presence of Laufeldochitina sp. A aff. striata sensu Grahn et al., 1996 which has a range confined to the latter biozone. One new chitinozoan species is described: Cyathochitina giraffa sp. nov. and two species were assigned to a different genus: Belonechitina tuberculata comb. nov. and Hercochitina seriespinosa comb. nov.

Darriwilian graptolites of the Shihtien Formation (Ordovician) in west Yunnan, China

West Yunnan in southwestern China formed an important part of the Sibumasu Terrane during the Ordovician. The Ordovician rocks of the region are exposed at many localities but have been strongly affected by several phases of tectonic activity, especially by the most recent Himalayan phase that commenced in the Paleogene. For these reasons, together with the difficult access to the localities, the Ordovician graptolites and biostratigraphy of this region are rather poorly known. Herein, we describe a Darriwilian graptolite fauna from the Shihtien Formation at Baoshan and Shidian, western Yunnan. The fauna comprises 11 genera and 15 species, including Didymograptus artus, D. murchisoni, D. spinulosus, Pterograptus sp., Hustedograptus vikarbyensis, H. teretiusculus, Archiclimacograptus angulatus, A. riddellensis, Haddingograptus oliveri, Proclimacograptus angustatus, and the dendroids Dictyonema, Palaeodictyota, Dendrograptus, Callograptus and Ptilograptus, indicating an age of middle to late Darriwilian. Two graptolite biozones are recognized: the Didymograptus artus and Didymograptus murchisoni biozones, in ascending order. The graptolite fauna is substantially similar to those of Baltica and South China.

Graphic correlation of Middle Ordovician graptolite shale, southern Appalachians: An approach for examining the subsidence and migration of a Taconic foreland basin

Graptolite-rich shale in the Valley and Ridge province of Alabama, Georgia, Tennessee, and Virginia records subsidence and migration of the Sevier foreland basin, which formed in response to arc-continent collision during the Middle Ordovician Blountian phase of the Taconic orogeny. Rapid, tectonically induced subsidence of what had been a shallow carbonate platform led to pelagic deposition of graptolite shale in a deep basinal setting. The contact of the shale with the underlying carbonate rocks is a distinct record of initiation of subsidence. Graptolites were collected extensively and systematically from the shale at 50 localities in the southern Appalachians. Correlation with zonal biostratigraphy demonstrated that the shale sequence is diachronous. In localities to the southeast, in Alabama and Georgia, it correlates with the Didymograptus murchisoni Zone. To the northwest, across strike in Alabama, Tennessee, and Virginia, the shale becomes progressively younger and correlates sequentially with the Glyptograptus teretiusculus , Nemagraptus gracilis , and Climacograptus bicornis Zones. This diachronism records the northwestward, cratonward migration of the foreland basin, which was driven by postcollisional plate convergence. Biostratigraphic data from 22 localities were amenable to analysis by graphic correlation. At these localities, graptolites were collected from measured sections, and ranges of all species in the 22 sections were compiled into a CSRS (composite standard reference section) that correlates with the biostratigraphic interval from the G. teretiusculus Zone to the C. bicornis Zone. On the basis of the most reliable radiometric ages correlated into the graptolite zonation, the duration of the composite standard reference section is estimated to be 5 m.y. It is divisible into 89 CSUs (composite standard units), each with an approximate duration of 56 000 yr. Recorrelation of each of the 22 measured sections with the composite standard reference section allowed the stratigraphic base of the shale in each section to be expressed in terms of composite standard units, and such correlations permitted the diachronism of the basal shale contact to be measured with much greater precision than with zonal biostratigraphy. In addition, age differences of the basal shale contact between sections can be expressed in terms of time. With the temporal information provided by the composite standard reference section and original distances between sections determined from a palinspastic reconstruction, the rate of migration of the foreland basin as recorded in the diachronous basal shale contact was determined. Between sections in northeastern Tennessee and adjacent Virginia, calculated rates averaged 13 mm/yr and decreased northwestward from 40 to 9 mm/yr. This cratonward decrease in rate may represent deceleration as foreland basin migration, and the arc-continent collision that drove it, slowed to a halt. The rate of 40 mm/yr may be considered as the maximum rate of plate convergence following arc-continent collision. Precollisional plate convergence rates may have been significantly greater. The rate of 40 mm/yr is, however, well within the range of modern rates of plate convergence. In addition, the deceleration recorded in the Sevier foreland basin is similar to the deceleration of postcollisional plate convergence at the Timor Trough.

Time-scale calibration by high-precision U [sbnd]Pb zircon dating of interstratified volcanic ashes in the Ordovician and Lower Silurian stratotypes of Britain

High initial parent/daughter element ratios and a unique dual decay scheme make U Pb zicron ages more precise and reliable than most isotopic ages, and thus inherently superior for time-scale calibration. Employing improved techniques to the conventional method of U Pb dating, we have analyzed microgram-size (2–12 × 10 −8 g) zircon fractions from biostratigraphically controlled volcanic ashes and dated key Paleozoic time-markers with a precision better than 1% (±2Ma). Four of the stratotype samples from Britain for which fission-track ages [ 1] were previously reported have yielded improved ages of:438.7 ± 2.0Ma for the lower Silurian zone of Coronograptus cyphus from Llandovery strata at Dob's Linn, southern Scotland;457.5 ± 2.2 Ma for a Middle Ordovician Caradoc (Longvillian) ash near Bala, North Wales, and;465.7 ± 2.1and464.6 ± 1.8 Ma for the Didymograptus artus Zone and the type Didymograptus Murchisoni Zone, respectively, of the Llanvirn Series at Arenig Fawr and Abereiddi Bay, Wales. Another sample from the zone of Dicellograptus anceps ( P. pacificus Subzone) of the Ashgill Series at Dob's Linn has been dated at445.7 ± 2.4Ma, suggesting placement of the Ordovician-Silurian time boundary at approximately 441 Ma. A sixth bentonite from Caradocian age strata of North America (Spechts Ferry Shale, Decorah Formation, Missouri) is453.7 ± 1.8Ma old, indicating that the Rocklandian Stage of the Mohawkian Series is only slightly younger than the Longvillian Stage of the Caradoc Series in Britain.

The Ordovician graptoloid Didymograptus murchisoni in South Wales and its use in three-dimensional absolute strain analysis

ABSTRACTThe dimensional statistics and proximal development characters of the Llanvirnian pendent didymograptid D. murchisoni are defined using the type material from near Builth Wells (Powys) and other collections from Abereiddi Bay (Dyfed). Population variability (as expressed by variation coefficients) in the undeformed faunas amounts to about 10% for the sicula lengths and thecal spacings, but is over 20% (up to 200%) for stipe lengths and divergences. Characters with low variability have greater species-diagnostic value. Proximal development in the species involves a dicalycal th 11 and a low origin for that theca from the sicula. The taxa Didymograptus amplus, D. nanus, D. geminus, D. geminus latus and D. acutus are varieties within populations of D. murchisoni and are therefore junior synonyms.The resulting rationalised taxonomy allows advances to be made in the use of the species in stratigraphy and tectonics. For example, an occurrence of undeformed (pyritised) D. murchisoni in full relief at Abereiddi in the same beds as flattened and deformed faunas, allows a calculation of the absolute three-dimensional strains to be made for the slates there. The strains are strongly flattening (K < 0 · 3) and involve volume changes of about −56%. The high dilatation values imply that stylolite and grain-boundary pressure solution operated during the deformations to take the changes in volume beyond what is possible with simple compaction (i.e. porosity reduction).

The type specimen of Didymograptus murchisoni

D. murchisoni (Beck 1839), the types species of Didymograptus M'Coy, has been the subject of considerable confusion as a result of the extreme taxonomic splitting within the didymograptids. An attempt is made here to show the range of variation in this graptolite, based on the population on the original type slab, and to suggest ways in which studies on populations can lead to better understanding of the stratigraphic range of these Ordovician forms. Gruptolithina, Didymograptus murchisoni, taxonomy.

Correlation of the late Lasnamägian Stage (Middle Ordovician) with the graptolite succession

Abstract Shaly strata of the Didymograptus murchisoni Zone at Killerod, Skane, southern Sweden, have yielded elements of the conodont species Pygodus serrus (Hadding) and Eoplacognathus reclinatus (Fahraeus) which are characteristic of the Folkeslunda Limestone of late Lasnamagian age. These species provide the first conclusive evidence that at least part of the late Lasnamagian is coeval with the Didymograptus murchisoni graptolite Zone. The presence of Pygodus anserinus Lamont and Lindstrom and Prioniodus prevariabilis Fahraeus throughout the “Bronni Beds” at Killerod proves that this unit is older than has been believed previously and is equivalent to part of the Glyptograptus teretiusculus Zone rather than to the Nemagraptus gracilis Zone. Other evidence for the correlation between the North Atlantic graptolite and conodont zonal successions is also reviewed.

Conodont-Based Correlations of Lower and Middle Ordovician Strata in Western Newfoundland

Lower and Middle Ordovician conodont faunas from the Table Head Formation and the Cow Head Group in western Newfoundland to a large extent, or in some cases exclusively, contain forms that occur in the Balto-Scandian sequence, thus facilitating a correlation between the northern Applachians and Balto-Scandia. The lower boundary of the Table Head Formation probably lies within strata corresponding to the British Didymograptus bifidus Zone and corresponds to a level tentatively regarded as at the base of the Aluoja Substage. The upper boundary of the middle Table Head corresponds to a rather high level in the Lasnamagi Stage (in the upper part of the Didymograptus murchisoni Zone). The upper Table Head consists of black graptolitiferous shales, which have not been searched for conodont elements. Studies of conodonts from the Table Head, the Cow Head Group, and the uppermost part of the St. George Formation, and comparisons with published and nonpublished accounts of conodonts from the Fort Pena, the Joins and the Levis Formations, indicate that the North American bifidus Zone is at least partly equivalent to the British bifidus Zone, hence supporting the correlation suggested by Berry (1960).

CLASSIFICATION OF ORDOVICIAN CHAZYAN SHELLY AND GRAPTOLITE SEQUENCES FROM CENTRAL NEVADA

The Antelope Valley Limestone, nearly 1000 feet thick in the Toquima Range in central Nevada, contains typical zones of the “Whiterock Stage.” Trilobites and cephalopods in the lower Antelope Valley seem older than those in the middle (Crownian) and upper (Valcourian) type Chazyan and are like those in the Table Head Limestone of Newfoundland; upper Antelope Valley-Whiterock faunas resemble some in the higher Chazyan stages. Thus, the lower Antelope Valley-Whiterock has the stratigraphic position of the lower type Chazyan Dayan Stage. The Petes Summit Formation in the Clipper Canyon Sequence of argillites, cherts, quartzites, and volcanic rocks has a succession of graptolite faunas through some 600 feet. Assemblages near the base resemble those in the Didymograptus extensus and Didymograptus hirundo zones of the Arenigian Series and those in the upper Deepkill of New York; they may extend into the lower Llanvirnian Zone of Didymograptus bifidus. Well above these in the Petes Summit is an exceptionally rich assemblage with Pterograptus elegans and forms associated in the upper Llanvirnian Zone of Didymograptus murchisoni. The highest faunules in the Petes Summit have Glyptograptus teretiusculus, suggestive of the Llandeilan Series. The succession supports the view that the Deepkill and Levis Didymograptus bifidus-bearing beds are Arenigian, older than the named zone in Europe. As Table Head faunas lie below graptolites like those low in the Petes Summit, the lower Antelope Valley Limestone and Whiterock Stage seem to be highest Arenigian, and the higher part Llanvirnian and Llandeilan.

An early Ordovician shore-line in Radnorshire, near Builth Wells

Summary This paper deals mainly with the western margin of the Builth-Llandrindod Ordovician inlier and is concerned with the phenomena associated with the unconformity between the Newmead Series and the underlying Builth Volcanic Series. The rocks dip westwards and strike approximately north and south. They are extensively faulted and, in particular, the outcrops are displaced by powerful tear-faults. The Newmead Series consists of Grey Felspar Sands, Pyritous Felspar Sands and the flinty mudstones of Tan-lan. Within the sands, and especially in the lower part of them, there are many impersistent boulder-beds; both these rocks are beach deposits. There is progressive overlap within the Newmead Series, so that ultimately the flinty mudstone rest directly upon the Builth Volcanic Series. The Newmead Series is followed unconformably by the Glyptograptus teretiusculus shales. A rhyolitic series emerges from beneath the G. teretiusculus shales near Cwm-amliw and can be traced into the northern part of the inlier. The Newmead Series oversteps different divisions of the Builth Volcanic Series, the succession in the latter being most completely developed in the southern part of the inlier. The contact between the two series reveals the characteristics features of a coast-line, with wave-worn surfaces, sea-cliffs, screes and stacks as well as the beach deposits which occur in the Newmead Series. It is possible to reconstruct in detail the features of the area exposed to sub-aerial and marine erosion which was subsequently buried beneath the Newmead sediments. It consisted of horizontal layers of spilitic rocks, making trap topography, and in one locality this was overlooked by rounded hills of keratophyre. The presence of several outliers of Newmead sands to the east of the main outcrop indicates that the deposition of the Newmead sediments was probably widespread over most of the southern half of the inlier. From our observations, it is possible to interpret the general sequence of events, in particular of elevation, erosion and depression. It is shown that all these events took place within a relatively restricted part of the time-interval represented by the Didymograptus murchisoni zone. Finally, attention is called to the remarkable preservation of these ancient Ordovician physiographical features.

Notes on the Geology of the District West of Caermarthen

In a paper published in the Society's Journal in the year 1885 a complex synclinal of Lower Palaeozoic rocks occurring in the neighbourhood of Haverfordwest was described. Owing to the occurrence of a fault at the northern side of that synclinal, no strata of Arenig age were noticed in the paper, the oldest Ordovician beds referred to being the Didymograptus Murchisoni -shales below the Llandeilo Limestone in the neighbourhood of Narberth, at the south-eastern end of the synclinal. It seemed important, therefore, to work the strata lying east of Narberth in order to try and discover Arenig Beds in the district between that town and Caermarthen. This was accordingly done in some of the vacations between 1886 and 1890, and a complex anticlinal was discovered lying north-east of the Haverfordwest synclinal, having its oldest beds exposed north of Whitland. The accompanying map shows the general distribution of the beds in this anticlinal, no attempt being made to trace accurate boundary-lines ; but two approximate boundaries are drawn at the top of the Tetragraptus -beds (to be presently described) and along the line of outcrop of the Llandeilo Limestone respectively, in order to show the general distribution of the strata and the nature and direction of the anticlinal. The beds may be described, in ascending order, as follows :— 1. Tetragraptus-beds.—These beds consist of alternating conglomerates, shales, and grits, seen at Talfan, :pen-parc-uchaf, and probably near Whitland Abbey, all places lying generally north of Whitland. The shales have yielded graptolites, kindly determined by