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Chitinozoans from the upper Tremadocian (Lower Ordovician) Watch Hill Formation of the Lake District, northern England Chloé E. A. Amberg, Thijs R. A. Vandenbroucke, Stewart G. Molyneux & Thomas Servais To cite this article: Chloé E. A. Amberg, Thijs R. A. Vandenbroucke, Stewart G. Molyneux & Thomas Servais (2017) Chitinozoans from the upper Tremadocian (Lower Ordovician) Watch Hill Formation of the Lake District, northern England, Palynology, 41:sup1, 23-30, DOI: 10.1080/01916122.2017.1348721 To link to this article: https://doi.org/10.1080/01916122.2017.1348721

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Date: 19 December 2017, At: 04:24

PALYNOLOGY, 2017 VOL. 41, NO. S1, 23–30 https://doi.org/10.1080/01916122.2017.1348721

Chitinozoans from the upper Tremadocian (Lower Ordovician) Watch Hill Formation of the Lake District, northern England Chlo e E. A. Amberga, Thijs R. A. Vandenbrouckea,b, Stewart G. Molyneuxc and Thomas Servais

a,d

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a Univ. Lille, CNRS, UMR 8198 - Evo-Eco-Paleo, F-59000 Lille, France; bDepartment of Geology, Ghent University, Campus Sterre, Krijgslaan 281-S8, B-9000 Ghent, Belgium; cBritish Geological Survey, Keyworth, Nottingham NG12 5GG, UK; dVan Mildert College and Institute of Advanced Study, Durham University, Cosin’s Hall, Durham DH1 3RL, UK

ABSTRACT

KEYWORDS

The Global Stratotype Section and Point (GSSP) for the Lower Ordovician Floian Stage is defined by the first appearance datum (FAD) of the graptolite Tetragraptus approximatus at Diabasbrottet Quarry, Hunneberg, southern Sweden. Correlation of sections with the Floian GSSP when graptolites are absent, however, must rely on other criteria. Among other fossil groups with potential for biostratigraphical correlation at this level are chitinozoans. Here we describe a chitinozoan assemblage from the Watch Hill Formation in the Skiddaw Group of northern England. The Watch Hill Formation is dated on graptolite evidence as late Tremadocian, but the assemblage contains Euconochitina symmetrica, a species previously considered to indicate the eponymous lowermost Arenig (Floian) chitinozoan biozone in a long-established Gondwanan Ordovician biozonal scheme. The hitherto earliest Floian index species has also been reported recently from Tremadocian successions in South China and Morocco. We here confirm its extended range into the Tremadocian and its continued significance as an index species for the Tremadocian–Floian boundary interval.

chitinozoans; Tremadocian; Floian; Early Ordovician; biostratigraphy

1. Introduction The Ordovician System comprises three formally defined series (Lower, Middle and Upper Ordovician) and seven stages (Tremadocian, Floian, Dapingian, Darriwilian, Sandbian, Katian and Hirnantian). Global Stratotype Sections and Points (GSSPs) have been ratified for all seven Ordovician stage boundaries, and all are defined by the first appearance datum (FAD) of either a €m et al. 2009; Cooper graptolite or a conodont species (Bergstro & Sadler 2012). The question this poses is which other fossil groups can be used to correlate with the GSSPs, especially for those sedimentary successions that do not contain graptolites or conodonts. More specifically, in the context of this paper, how useful are chitinozoans for global correlation of Lower Ordovician strata? Chitinozoans have been studied extensively for biostratigraphical purposes. Webby et al. (2004) collated chitinozoan biozonations that parallel the graptolite and conodont biozones, based on detailed studies of chitinozoan biostratigraphy ~lvak 1999). from different palaeocontinents (e.g. Paris 1990; No One of these detailed studies (Paris 1990) described a chitinozoan biozonation for the ‘Northern Gondwana domain’ (actually located near the South Pole during the Ordovician; see Servais & Sintubin 2009). Paris’ (1990) Ordovician chitinozoan biozonation for the peri-Gondwanan margin in high southern latitudes has become a standard for international correlations and has been applied widely. Regardless of their widespread use, the chitinozoan biozones established by Paris (1990) remain in need of calibration

CONTACT Chloe E. A. Amberg © 2017 AASP – The Palynological Society

[email protected]

against both the British chronostratigraphical scheme, historically used in many parts of the world (Fortey et al. 1995), and against the new international chronostratigraphy for the Ordo€m et al. 2009). This calibration has been vician System (Bergstro partly completed for the Upper Ordovician Series, with studies focussing on chitinozoan biostratigraphy of the historic type areas of the British Caradoc and Ashgill series in the AngloWelsh Basin (Vandenbroucke 2008; Vandenbroucke et al. 2005, 2008a, 2008b, 2009; Challands et al. 2014) as well as on some of the new Upper Ordovician GSSPs (Vandenbroucke 2004; Goldman et al. 2007; Tessitore et al. unpub. pers. comm.). However, no such calibration has been attempted for the Lower and Middle Ordovician series. A few putative chitinozoans are found in the Cambrian Stage 5 (Shen et al. 2013), but the group only widely appears in the Tremadocian, well above the base of the series (Grahn & Paris 2011), as a zooplanktonic element of the Ordovician Plankton Revolution (Servais et al. 2016). Although the scarcity of Cambrian and Tremadocian records limits their usefulness for correlation at the base of the Ordovician System, they do have potential applications for correlation of the bases of the higher stages of the Lower and Middle Ordovician series. Detailed chitinozoan investigations at the Tremadocian– Floian boundary are rare, although palynological studies of acritarchs have been conducted. Few chitinozoans have been recovered from this interval, and their diversity is low (Achab & Paris 2007, fig. 2). In the context of a continuing refinement of Lower Ordovician chitinozoan ranges and biozones, and their positioning against the international and British

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C. E. A. AMBERG ET AL.

€m et al. 2009; chronostratigraphies (Fortey et al. 1995; Bergstro Cocks et al. 2010), the objective of this paper is to document chitinozoan assemblages from the upper Tremadocian Watch Hill Formation of the English Lake District.

2. Geological setting and stratigraphy

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Rocks at the base of the Ordovician succession in northern England comprise the Skiddaw Group, which consists of mainly mud-rich turbidite lithologies, deposited in an extensional basin (or basins) along the continental margin of Avalonia as it rifted from Gondwana in the Early to early Mid Ordovician (Tremadocian–mid Darriwilian). The main Skiddaw Group outcrop is in the Skiddaw Inlier of the northern Lake District (Figure 1), where the group is at least 5 km thick and contains evidence for largescale slumping. There, the group is divided into a Northern Fells

succession and a Central Fells succession by the WSW–ENE trending Causey Pike Fault (Cooper et al. 1995, 2004). The Watch Hill Formation is one of five formations in the Skiddaw Group of the Northern Fells (Figure 1). In ascending order, these are the Bitter Beck, Watch Hill, Hope Beck, Loweswater and Kirk Stile formations. Turbidite sandstone beds occur throughout the succession, but are more common in, and differentiate, the Watch Hill and Loweswater formations. The rest of the succession consists predominantly of turbidite mudstone with thin turbidite sandstone interbeds. The Bitter Beck, Watch Hill and lowermost Hope Beck Formation are late Tremadocian in age (murrayi graptolite Biozone; Figure 2). The rest of the Hope Beck Formation and the lower part of the Loweswater Formation are placed in the Floian Stage (phyllograptoides and varicosus graptolite biozones; Figure 2), and the upper part of the Kirk Stile Formation in the

Figure 1. Geological sketch map of the area around Watch Hill showing sample localities. Inset maps show the location of Watch Hill in the Lake District of NW England. Based on the relevant parts of British Geological Survey 1:50,000 (England and Wales) sheets 23 (Cockermouth) (British Geological Survey 1997) and 29 (Keswick) (British Geological Survey 1999). Contains British Geological Survey materials © NERC 2017.

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Figure 2. Stratigraphy of the Tremadocian and lower Floian succession in the English Lake District showing the position of the symmetrica-destombesi chitinozoan assemblage (this study) in relation to stage and time slices, lithostratigraphy, graptolite zones and acritarch zones in the lower part of the Skiddaw Group, and the ‘North Gondwanan’ chitinozoan biozonation. Sources indicated on the figure.

Dapingian (victoriae–gibberulus graptolite biozones) and lower Darriwilian (cucullus–artus graptolite biozones) stages. Crucially for this study, placement of the bases of the Floian Stage and the phyllograptoides graptolite Biozone within the lower part of the Hope Beck Formation (Cooper et al. 2004, fig. 6) means that all the samples considered herein are from the upper Tremadocian part of the succession (Figure 3). The chitinozoans recorded herein are from palynological samples that have previously yielded the distinctive and stratigraphically important Cymatiogalea messaoudensis-Stelliferidium trifidum acritarch assemblage (Cooper et al. 1995). This acritarch assemblage was first described from the lower part of the Skiddaw Group succession, where it was originally called the ‘Watch Hill assemblage’ (Molyneux & Rushton 1988). The assemblage was divided into five sub-assemblages by Cooper et al. (1995), and these were subsequently used to define the C. messaoudensis-S. trifidum Assemblage Biozone, with five constituent subzones numbered 1–5 (Cooper et al. 2004). The biozone and its subzones enable correlation along the southern margin of Gondwana with successions in Ireland, Wales, the Isle of Man, Belgium, Germany, Spain and Turkey (Molyneux et al. 2007). The messaoudensis-trifidum Biozone in the Lake District correlates with the Araneograptus murrayi and Tetragraptus phyllograptoides biozones, the lower four of the five subzones correlating with the upper Tremadocian murrayi Biozone, and the fifth and highest with the lower Floian phyllograptoides Biozone (Figure 2; Cooper et al. 2004; Molyneux et al. 2007).

3. Material and methods Eight previously collected samples were selected from the micropalaeontological (MPA) collection of the British Geological Survey (BGS) at Keyworth, Nottingham, UK, and were subsampled for chitinozoans. Sample numbers are MPA 28677, 28680, 28681, 28682, 28685, 28704, 28706 and 28707. The samples are from the type area of the Watch Hill Formation on Watch Hill, about 3 km ENE of Cockermouth in the Lake District

(Figures 1, 3), and were collected during the detailed lithostratigraphical, biostratigraphical and structural study of the Skiddaw Group of the English Lake District, published in the BGS Memoir of Cooper et al. (2004). The samples were processed and analysed in the palaeontology research unit of the Evo-Eco-Paleo research department at the University of Lille. For a complete account of the extraction methods used, see Amberg et al. (2016). The residues were filtered at 51 mm and the fraction retained by the filter was handpicked and examined under the Scanning Electron Microscope for identification. All figured specimens are housed in the collections of the Evo-Eco-Paleo research department at the University of Lille (building SN5, av. Paul Langevin).

4. Results All samples were productive, although the uppermost sample, MPA 28706, produced only a very few unidentified fragments. In about 60 g of rock dissolved, 163 specimens were recovered in the most productive sample and 23 specimens in the least productive one. As expected, diversity is low, in common with other Lower Ordovician chitinozoan assemblages, in which usually only a few species are present. Nevertheless, a total of three genera and six chitinozoan species were identified (Figure 3). Due to poor preservation and problems of classification, three of these taxa are provisionally assigned at the species level. The chitinozoans recovered from samples MPA 28677 and MPA 28682 were abundant and relatively well preserved, whereas those from MPA 28704 and MPA 28707 were much altered. The six species identified are: Euconochitina symmetrica, Euconochitina paschaensis, Lagenochitina destombesi, Lagenochitina cf. L. obeligis, L. ?esthonica, and Conochitina sp. aff. C. decipiens. Lagenochitina destombesi, Lagenochitina cf. L. obeligis and Euconochitina symmetrica are all present in the lowermost sample (Figure 3), corresponding to Subzone 2 of the messaoudensis-trifidum Biozone (Cooper et al. 2004; Molyneux et al. 2007), i.e. the middle part of the A. murrayi Biozone (Figure 2).

C. E. A. AMBERG ET AL.

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Figure 3. Vertical section through the Bitter Beck, Watch Hill and Hope Beck formations of Watch Hill showing sample positions, chitinozoan occurrences, and correlation with acritarch and graptolite biozones and chronostratigraphic divisions.

Lagenochitina cf. L. obeligis and Euconochitina symmetrica are present in all samples except MPA 28706. Lagenochitina ?esthonica has its FAD slightly higher in the succession, in sample MPA 28680, just above the base of the messaoudensis-

trifidum Subzone 3. Conochitina sp. aff. C. decipiens and Euconochitina paschaensis have their FADs higher in the succession, in sample MPA 28681 from the upper part of the messaoudensistrifidum Subzone 3 (Figure 3). All six taxa are present in the

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uppermost part of the investigated sequence, from Subzone 4 of the messaoudensis-trifidum assemblage (Figure 3).

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5. Discussion The records of Euconochitina symmetrica from the Watch Hill Formation of the Lake District are of interest because they corroborate upper Tremadocian records of the species in recent studies of chitinozoan assemblages elsewhere on the Gondwanan margin, notably South China, where E. symmetrica occurs in the murrayi Biozone with elements of the messaoudensis-trifidum assemblage (Wang et al. 2013a, 2013b), and Morocco (Nowak et al. 2016). The significance of this is that Paris (1990) established Euconochitina symmetrica (Taugourdeau & de Jekhovsky) as the index fossil of the symmetrica total-range Biozone, considered at the time and subsequently (Webby et al. 2004) to be indicative of an early Arenig (Floian) age, while Lagenochitina destombesi is indicative of the eponymous Tremadocian biozone, also originally designated a totalrange biozone (Paris 1990). The base of the symmetrica Biozone is correlated with the base of the Floian Stage in recent compilations (e.g. Cooper & Sadler 2012). It is important to note, however, that Paris (1990, p. 190) acknowledged stratigraphical control at the type locality of E. symmetrica to be poor, and that the type strata (in Algeria) were only tentatively referred to the ‘lower Arenig’ by Legrand (1985). The ranges of index species Lagenochitina destombesi and Euconochitina symmetrica significantly overlap in our section, so we refrain from identifying formal biozones in the Watch Hill succession and our limited set of samples, awaiting a more comprehensive revision of the chitinozoan biozonation. Euconochitina symmetrica was first described (as Conochitina symmetrica) from the Floian of Algeria by Taugourdeau & de Jekhowsky (1960). The holotype is 250 mm long and has a membranous collarette at the end of the neck. Achab (1980) reported occurrences of E. symmetrica (also as Conochitina symmetrica), with dimensions from 200–275 mm, from the lower Arenig of Laurentia. She also noted (Achab 1980, p. 228) that comparable forms had been encountered in beds attributed to the Tremadocian Stage at Lauzon and Saint Michel de Bellechasse, Quebec, raising the possibility that the species ranges down into the Tremadocian there. Paris & Mergl (1984) reported the species from the Czech Republic, where occurrences are from the lowermost part of the Klabava Formation, at a level that is also attributed to the lower Arenig (i.e. Floian), but the specimens are smaller. ~lvak (2006) described Euconochitina In Baltica, Hints & No symmetrica from Estonia, from beds of the Leetse Formation that they considered most likely to be of Tremadocian age (lower part of the Paroistodus proteus conodont Biozone). Specimens from the Leetse Formation are between 130 mm and 230 mm in length, and occur with specimens of Euconochitina primitiva between 218 mm and 359 mm in length, which have a very similar shape to E. symmetrica. Consequently, Hints & ~lvak (2006) discussed the difficulty of differentiating these No species. In Baltica, E. primitiva is considered to be diagnostic of the eponymous chitinozoan biozone that straddles the Trema~lvak 1999; docian–Floian (Tremadoc–Arenig) boundary (No ~lvak et al. 2006). No

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Following this taxonomic consideration, de la Puente & Rubinstein (2009) created a new species named Euconochitina paschaensis, which has a similar shape and length (from 134 mm to 209 mm) to E. symmetrica, but which “does not display the wide flaring collarette” (de la Puente & Rubinstein 2009, p. 75) diagnostic of the latter species. According to de la Puente & Rubinstein (2009), E. paschaensis is present at levels that are equivalent to the upper Tremadocian Lagenochitina conifundus chitinozoan Biozone of Gondwana (Paris 1990; Webby et al. 2004) and is considered to be indicative of the upper Tremadocian in Argentina (de la Puente & Rubinstein 2009). More recently, Nowak et al. (2016) described chitinozoans from the uppermost Tremadocian of Morocco, from sections in which correlation between acritarchs, chitinozoans and graptolites is possible. Nowak et al. (2016) classified E. paschaensis and E. symmetrica together as the E. paschaensis-symmetrica group in their Moroccan material, with lengths of about 150 mm. In the Moroccan assemblages, only a single specimen identified as E. symmetrica displays a wide opening of the collarette (Nowak et al. 2016). In particular, Nowak et al. (2016) reported overlapping ranges of chitinozoan taxa that were previously used to distinguish separate Tremadocian and Floian totalrange chitinozoan biozones in the biozonation of Paris (1990). In fact, some of the chitinozoans considered until now to be indicative of the Floian Stage (i.e. of the lower Arenig Series of British chronostratigraphy) are present in the Moroccan successions in rocks that have yielded Tremadocian graptolites and acritarchs. Similarly, Wang et al. (2013a) identified chitinozoans considered hitherto to be characteristic of the base of the Floian (Paris 1990; Webby et al. 2004) in the Tremadocian of China. The specimens from the English Lake District recorded here range from 130 mm to 250 mm in length, and have a more or less pronounced flexure. Some specimens display a wide open neck (e.g. Plate 1, figure 4), whereas others have a narrow opening (Plate 1, figure 5). Consequently, two taxa, E. symmetrica and E. paschaensis, are identified in the Watch Hill assemblage, but not E. primitiva. Molyneux & Dorning (1989) and Molyneux et al. (2007) reported an occurrence of the upper Tremadocian–lower Floian messaoudensis-trifidum acritarch assemblage in the ‘Login Beds’ of the Carmarthen area, South Wales. The Login Beds underlie the basal Arenig Allt Cystanog Member of the Ogof H^ en Formation, but there is no independent control on their age. The chitinozoan assemblage of the Login Beds is currently under investigation (Amberg, unpub.) and contains a different assemblage from that described here, with both Lagenochitina destombesi and L. brevicollis, i.e. taxa from the eponymous Tremadocian chitinozoan biozones of Gondwana (Paris 1990; Webby et al. 2004). This might indicate that the Login Beds occupy a slightly different, perhaps lower stratigraphic level than the Watch Hill Formation. The occurrence of L. destombesi with E. symmetrica, as seen in the Watch Hill Formation, has been observed less frequently elsewhere. Nowak et al. (2016) noted an occurrence of Lagenochitina cf. L. destombesi rather high in the Fezouata Formation in the upper Tremadocian Araneograptus murrayi Biozone, accompanied by the species Euconochitina paschaensis and Euconochitina symmetrica, grouped together in the

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Plate 1. SEM micrographs of selected chitinozoans from the Watch Hill Formation. 1, 2, 4, 6, 7. Euconochitina symmetrica. 3, 5. Euconochitina paschaensis. 8. Conochitina sp. aff. C. decipiens. 9. Lagenochitina destombesi. Scale bar = 100 mm.

E. paschaensis-symmetrica group. As noted above, E. paschaensis was described by de la Puente & Rubinstein (2009) in Argentina (‘Western’ Gondwana) as corresponding to the Tremadocian Lagenochitina conifundus chitinozoan Biozone of ‘Northern’ Gondwana. Nowak et al. (2016) also found chitinozoan species that are typically considered to be of Floian age just above these levels, but still in the murrayi Biozone, notably Eremochitina brevis.

University, UK. TS also thanks David A.T. Harper (Van Mildert College, Durham) for hosting his visit and for many hours of stimulating discussions. This work is a contribution to the IGCP projects 591 (The Early to Middle Paleozoic Revolution) and 653 (The Onset of the Great Ordovician Biodiversification Event). This is a contribution to the French ANR (‘Agence Nationale de la Recherche’) ‘SeqStrat-Ice.’ The samples were collected by CEAA at the BGS, Keyworth, UK, courtesy of SGM. SGM publishes by courtesy of the Executive Director, British Geological Survey, NERC, UK.

Disclosure statement 6. Conclusion The present study provides a first, preliminary description of a chitinozoan assemblage in the Watch Hill Formation of the English Lake District, and has revealed the presence of Euconochitina symmetrica in rocks that are attributed to the Tremadocian Stage on the basis of graptolite and acritarch biostratigraphy. These results corroborate previous findings of E. symmetrica in the Tremadocian of China (Wang et al. 2013a) and Morocco (Nowak et al. 2016). E. symmetrica, considered as an indicator of the lower Floian by Webby et al. (2004), based on the study of Paris (1990), can no longer be used as an unqualified index species for the Floian Stage, but instead marks the wider upper Tremadocian – lower Floian interval.

Acknowledgements Part of TS’s contribution for this manuscript was written whilst undertaking a Visiting Fellowship at the Institute of Advanced Study (IAS), Durham

No potential conflict of interest was reported by the authors.

Funding This work is part of a PhD study, financed by the Ecole Doctorale 104 Sciences de la Matiere, du Rayonnement et de l’Environnement (SMRE) of Lille University. TRAV and CEAA acknowledge financial support from the CNRS (INSU, action SYSTER), the French ‘Agence Nationale de la Recherche’ through [grant ANR-12-BS06-0014] ‘SeqStrat-Ice’, and the SMRE doctoral school of the University of Lille.

Notes on contributors CHLOE AMBERG obtained her MSc at Geneva University in Switzerland and received her PhD from Lille 1 University, France in June 2016. Her interest includes chitinozoans and palaeoclimate of the Ordovician, as well as biostratigraphy and general micropalaeontology. THIJS R.A. VANDENBROUCKE obtained his MSc and PhD degrees at Ghent University in Belgium. Following postdoctoral positions in Durham (UK),

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Leicester (UK), Ghent and an appointment as researcher at the French CNRS, he now is an Assistant Professor in Biostratigraphy at Ghent University. His early studies focussed on Ordovician chitinozoan biostratigraphy, while current projects include a wide range of aspects related to the evolution of the early to middle Palaeozoic oceans and palaeo-environment.

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STEWART MOLYNEUX received his degrees of BSc and PhD from the University of Sheffield, UK, where his PhD research on Ordovician palynology was supervised by Professor Charles Downie. He joined the British Geological Survey (BGS) as a palynologist in 1979, and remained there throughout his career, working initially on Mesozoic successions of the North Sea, but then increasingly to support BGS mapping of Lower Palaeozoic outcrops in Wales, northern England, the Southern Uplands of Scotland and Devonian successions in SW England. He also worked on various international projects for BGS and under contract for other organisations, coupled with editorial work and management responsibilities. He retired from BGS in 2014, but remains active in both research and consultancy work, with a focus on Lower Palaeozoic palynology. THOMAS SERVAIS is a Research Director at the French Centre of Scientific Research (CNRS). Trained as a geologist at the universities in Namur and Li ege (Belgium), his PhD (Liege, Belgium) and post-doctoral studies at Liege, Berlin (Germany) and the British Geological Survey (BGS, Keyworth, Nottingham, UK) were focused on the biostratigraphy, palaeobiogeography and palaeoecology of Ordovician acritarchs. His current research is mostly dedicated to the understanding of Ordovician Plankton Revolution. He is currently co-leader of the IGCP project 653 (The Onset of the Great Ordovician Biodiversification Event) and a Vice-President of the International Paleontological Association.

ORCID Thomas Servais http://orcid.org/0000-0002-4089-7874

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