Transcript
ORIGINAL PAPER Microhabitat complexity — an example from MiddleDevonian bryozoan-rich sediments in the BlankenheimSyncline (northern Eifel, Rheinisches Schiefergebirge) Andrej Ernst & Peter Königshof & Paul David Taylor & Jan Bohatý Received: 14 July 2011 /Revised: 19 September 2011 /Accepted: 20 September 2011 # Senckenberg Gesellschaft für Naturforschung and Springer 2011 Abstract A Middle Devonian section in the BlankenheimSyncline (northern Eifel, Rheinisches Schiefergebirge)contains a rather diverse fauna dominated by bryozoans, brachiopods, corals and other taxa, such as calcimicrobes.The section covers sediments of the Junkerberg Formationand the transition to the Freilingen Formation. Thetransition between the Junkerberg and Freilingen forma-tions coincides with the base of the otomari Event Interval.The section displays an overall transgressive trend inshallow subtidal and deeper subtidal environments belowfair-weather wave base. In contrast to the general trans-gressive trend, one horizon is interpreted to represent aregressive trend of local srcin. Sedimentation rates werequite low, as indicated by the abundant occurrence of suspension-feeding organisms. According to StandardMicrofacies Types (SMF), the carbonates represent mainlywackestones and floatstones and can be assigned to SMF 5and SMF 8, respectively. Complex fossil communities insome horizons suggest excellent living conditions duringdeposition. Large abundances of calcifying taxa, such as Girvanella , occur in some layers, particularly at the base of the Junkerberg Formation. Other encrusting/calcifyingorganisms are present in various layers, suggesting deposi-tion during supersaturation of CaCO 3 minerals in a calcitesea in a low-energy environment. The studied bryozoanfauna is very diverse throughout the entire section, erect branching colonies being dominant. Five new taxa aredescribed: the trepostomes Trachytoechus globosus sp. n., Multihemiphragma minor sp. n. and Eridotrypella multae- formis sp. n., and the rhabdomesine cryptostomes Isostylusvulgaris n. sp. and Bigeyella prima n. sp. Lenaporidae fam.n. is erected for the genera Lenapora and Bigeyella . Keywords Bryozoans.Eifelian.Microfacies. Otomari Event Interval.Blankenheim Syncline.RheinischesSchiefergebirge Introduction The Eifel area, in particular the Middle Devonian sequencesof the Eifel carbonate synclines, contains well-known fossillocalities which have been investigated for many decades(Goldfuss1826 – 1844; Römer 1844; Steininger 1848). As a result of this long-term research, a large number of publications are available on the brachiopods, stromatopor-oids, rugose corals, echinoderms (crinoids) and trilobites of the Middle Devonian of the Eifel Synclines, as well as onthe stratigraphy and regional geology. Brachiopods have A. Ernst ( * )Institut für Geowissenschaften der Christian-Albrechts-Universität zu Kiel,Ludewig-Meyn-Str. 10,24118 Kiel, Germanye-mail:
[email protected]. Königshof Senckenberg Forschungsinstitut und Naturmuseum Frankfurt,Senckenberganlage 25,60325 Frankfurt am Main, Germanye-mail:
[email protected]. D. Taylor Department of Palaeontology, Natural History Museum,Cromwell Road,London SW7 5BD, UK e-mail:
[email protected] J. BohatýInstitut für Geologie und Mineralogie der Universität zu Köln,Zülpicher Straße 49A,50674 Köln, Germanye-mail:
[email protected] PalaeoenvDOI 10.1007/s12549-011-0060-6 been extensively studied by Struve and described in anumber of publications (see bibliography in Weddige andZiegler 2000). Rugose and tabulate corals, crinoids andstromatoporoids have also been extensively studied (e.g.Bohatý2005a,b,2009,2010; Byra1983; Brühl1999; Coen-Aubert and Lütte1990; Lütte1987,1990; Salerno 2008; Schröder and Salerno2001). Trilobites have been studied by Richter and Richter, and Richter and Struve(summarized in Struve1996) and, more recently, by Basse(e.g., Basse2002,2003,2006). Studies on the stratigraphy, regional geology and correlation of different formations inthe Eifel Synclines have been summarized by Meyer (1986), Ribbert (1998) and Struve et al. (2008). Knowledge about bryozoan diversity in the Eifel areawas restricted to a few papers from the last century (e.g.Kräusel1953,1957,1981; Solle1937,1952,1968; Toots 1951) until recent papers were published by Ernst (2008a, b), Ernst and Bohatý (2009) andErnstandSchroeder(2007). Bryozoans are very often common components of mixedcarbonates and siliciclastics, and they are generally well preserved due to the low maturity of the Eifel area (e.g.Helsen and Königshof 1994; Königshof and Werner 1994; Teichmüller and Teichmüller 1979).Here we describe an example of a complex microhabitat community from Middle Devonian strata in the Blanken-heim Syncline, which is dominated by diverse bryozoans.The section is also interesting in terms of Devonian events.The lowermost part of the section contains the KlausbachEvent (sensu Struve1992), and the upper part of thesequence described in this paper represents the transition between the Junkerberg and Freilingen formations. Thistransition can be correlated with the ostiolata ExtinctionEvent ( ‘ Great Gap ’ of Struve1982), which may be the baseof the otomari Event Interval (e.g. Struve et al., 1997). Geological setting The Rheinisches Schiefergebirge (RSG) is generally assignedto the Avalonia terrane, which was separated from GondwanaintheEarlyOrdoviciananddriftednorthwards(e.g.Onckenet al.2000; Romer and Hahne2010; Tait et al.2000). Avalonia collided with Baltica in Late Ordovician – Early Siluriantimes, leading to closure of the Tornquist Sea. Due to thiscollision the Iapetus Ocean was also closed and Laurussiawas formed (Kroner et al.2007; Linnemann et al.2008; Nance et al.2010). According to Kroner et al. (2007) and Nance et al. (2010), the closure of the Rheic Ocean began inthe Late Silurian/Early Devonian and continued until theEarly Carboniferous by successive closure from west to east.In the RSG east of the river Rhine are several par-autochthonous (e.g. Meischner 1991; Schwan1991; Wachendorf, 1986) and allochthonous units (e.g. Engel et al.1983; Franke2000; Huckriede et al.2004; Oczlon1992, 1994; Salamon2003; Salamon and Königshof 2010), the latter recognized by Kossmat (1927) who proposed thenappe tectonic concept for the Rhenohercynian.The Old Red Continent (ORC) delivered siliciclasticsinto the southern shelf area during the Devonian. Accordingto Erben (1962), marine Devonian sediments can generally be assigned to two facies, the ‘ Rhenish ’ and ‘ Hercynian ’ facies. The Rhenish Facies is characterized by thick siliciclastic successions, which are referred to as deltaicshallow marine environments (e.g. Stets and Schäfer 2002;Wehrmann et al.2005cum lit.). Typical sediments of theHercynian Facies are pure limestones and argillaceousshales, whereas sandstones are rare. Starting in the EarlyDevonian, the Rhenohercynian Basin developed as anarrow (about 250 – 350 km) but rather elongate (more than2000 km) trough south of the ORC. Towards the south, thetrough was confined by the Mid-German High during theEarly Devonian (Gedinnian and Siegenian). Detrital supplycame from northern (Wierich1999) as well as southernsource areas (Hahn1990; Hahn and Zankl1991). Lower Devonian sequences in the RSG are characterized mainly by sandstones and siltstones, which were rapidly depositedin sedimentary troughs ( ‘ depocenters ’ ) and on swells (e.g.Meyer and Stets1980,1996; Mittmeyer 1982) due to a rift stage. Middle Devonian successions in the RSG are predominantly composed of shales, marls, platy limestonesand biostromal limestones. Siliciclastic sedimentationchanged to predominantly carbonaceous sediments when biostrome development began to flourish during the MiddleDevonian. In the open shelf area further to the south onlyislands could sustain biostrome growth. The development of biostromes in this area is closely related to volcanismand volcaniclastic deposits (Königshof et al.2010cum lit.).West of the river Rhine, the Eifel area (Eifel Synclines)is the dominant structural unit interpreted as a N – S trendingaxial depression of the RSG (Fig.1). Siliciclastic sedimentswere delivered from the north during the Early Devonianand early Middle Devonian (Eifelian) but diminishedduring Givetian times when shallow subtropical carbonateswere established over much of the region. Struve (1963)established a depositional model for the Eifel area with a N – S trending basin surrounded by landmasses, which heconsidered as the so-called ‘ Eifel Sea Street ’ . In contrast tothis model, Winter (in Meyer et al.1977) defined threefacies belts in the Eifel Synclines (Fig.2). Facies A in thenorthern synclines mainly represents the accumulation of siliciclastics in a marginal marine setting. Facies B, typicalof the eastern Eifel Synclines, is characterized by carbonate platform facies and biostromal reef deposits (including theMid-Eifelian High) with interbedded siltstones and mud-stones. Further to the south, Facies C is mainly composedof marls and shales due to the increasing clay content. Palaeobio Palaeoenv Faber (1980) subsequently modified this model, based ondetailed microfacies studies which provided evidence of rhythmic development of a carbonate platform during theearly Eifelian and of a flat shelf lagoon during the lateEifelian and early Givetian, affecting the eastern part of theEifel Synclines. Paproth and Struve (1982) distinguished between N-, W- and S-Eifel biofacies based on faunaldifferences. During the Givetian this facies differentiation broke down to some degree and mainly stromatoporoid/ coral biostromes extended over the entire area. Krebs(1974) characterized this environment as a broad shelf lagoon bordered to the south by a carbonate rim or barrier.The study area lies within the Blankenheim Syncline(Fig.2), between the villages of Blankenheim in the SE andBlankenheimerdorf in the SW, and comprises shallow shelf mixed carbonate and siliciclastic facies of Middle Devonianage (Eifelian) deposited on the southern margin of theformer Avalonia microcontinent.The outcrop exposes deposits the Junkerberg Formationand transition to the Freilingen Formation and can beassigned to the Polygnathus australis Conodont Biozone(Table1). Material and methods Bryozoans from this section are abundant and relativelydiverse. The studied fauna includes 12 bryozoan species.Seven species are described in the taxonomy section of this paper: the trepostomes Leptotrypa prima (Duncan, 1939), Trachytoechus globosus sp. n., Multihemiphragma minor sp. n. and Eridotrypella multaeformis sp. n., and therhabdomesine cryptostomes Isostylus vulgaris sp. n., Bigeyella prima sp. n. and Acanthoclema distilum Bigey,1988. Five additional bryozoan species were identified but arenot described here: the cystoporate Canutrypa francana Bassler, 1952, the trepostome Microcampylus regularis Ernst,2008a, the cryptostome Intrapora variabilis Ernst, 2008a, andthe two fenestrates Anastomopora sp. and Spinofenestella sp.Microfacies analysis is based on a detailed study of thinsections. In order to obtain a broad overview we used thinsections of both a ‘ normal ’ (24×48 mm) and large size(50 ×50 mm). Samples for sectioning were taken fromcarbonate intervals through the sequence. Facies andmicrofacies types were mainly compared using the classi-fications of Wilson (1975) and Flügel (2004), with special Fig. 1 Geological map of the Rheinisches Schiefergebirge and Ardenne (slightly modified from Wehrmann et al.2005). Rectangle demarcatesstudy area (Fig.2)Palaeobio Palaeoenv focus on the Junkerberg and lower Freilingen formations.Thin sections are stored at the Senckenberg, Forschungsin-stitut und Naturmuseum Frankfurt (SMF 21.500 – SMF21.665, and SMF 70.501 – SMF 70.508). Systematic palaeontology Phylum Bryozoa Ehrenberg, 1831Class Stenolaemata Borg, 1926Order Trepostomata Ulrich, 1882Suborder Amplexoporina Astrova, 1965Family Atactotoechidae Duncan, 1939Genus Leptotrypa Ulrich, 1883[= Calacanthopora Duncan, 1939] Type species: Leptotrypa minima Ulrich, 1883. Cincinna-tian (Upper Ordovician), North America. Diagnosis: Colonies encrusting, thin. Autozooecia with polygonalapertures.Autozooecialwallsirregularlythickened,throughout the colony or near the periphery, indistinctlylaminated, usually integrated in initial parts and merged near the colony surface. Autozooecial diaphragms absent or rare.Exilazooecia rare. Acanthostyles small to moderately large,common to abundant (modified after Astrova,1978). Occurrence: Middle Ordovician to Middle Carboniferous;worldwide. Remarks: Leptotrypa Ulrich, 1883 differs from Anomalo-toechus Duncan, 1939 in having thin encrusting coloniesand rare diaphragms. Leptotrypa prima (Duncan, 1939) (Fig.3a – e;Appendix) 1939 Calacanthopora prima Duncan, p. 236, pl. 15,figs. 4 – 7. Material: SMF 21.500-21.510. Description: Colonies encrusting, often hollow ramose,0.14 – 0.30 mm thick. Autozooecia budding from a thinepitheca, for a short distance oriented parallel to thesubstrate, then bending sharply to the colony surface.Autozooecial apertures polygonal. Autozooecial dia- phragms absent. Acanthostyles large, protruding above thecolony surface, 4 – 7 surrounding each aperture, srcinatingat the base of exozone, having distinct calcite cores anddark, laminated sheaths. Exilazooecia common, small.Autozooecial walls in endozone granular, 0.010-0.015 mmthick; in exozone finely laminated, merged, without visible boundaries, 0.030-0.055 mm thick. Remarks: Leptotrypa prima (Duncan, 1939) differs from L. pulchra Morozova and Weis, 2006 (in Morozova et al.,2006) from the late Givetian of Poland [Ja ź wica Formationof the Kowala Association (Member B)] in having moreabundant acanthostyles (4 – 7 acanthostyles per aperture vs.3 – 4 in L. pulchra ). Leptotrypa prima (Duncan, 1939) differsfrom L. donensis Lavrentjeva, 1970 from the Late Devonian(Frasnian) of Russian Plate in having smaller autozooecialapertures (0.10 – 0.17 vs. 0.12 – 0.24 mm in L. donensis ). Occurrence: Traverse Group, Middle Devonian (Givetian);Michigan, USA. Klausbach Member, lower JunkerbergFormation (Eifelian), Middle Devonian; Blankenheim,Eifel, RSG. Fig. 2 Idealized facies model of the Middle Devonian of the Eifel(modified from Winter in Meyer et al.1977). Facies type A : faciesdominated by clastic input; facies type B : facies characterized bycarbonate platforms and biostromal reefs (including the Mid-EifelianHigh); facies type C : facies characterized by reduced clastic input andincreasing carbonate. Asterisk Area of section Fig. 3 a – e Leptotrypa prima (Duncan,1939). a , b Longitudinalsection of the encrusting colony, SMF 21.506, scale bars : ( a ) 1 mm,( b ) 0.2 mm. c Tangential section, SMF 21.506, scale bar : 0.2 mm. d , e Tangential section, SMF 21.501, scale bars : ( d ) 0.5 mm, ( e ) 0.2 mm. f Trachytoechus globosus sp. n.: oblique section of the colony, holotypeSMF 21.511, scale bar : 1 mm Palaeobio Palaeoenv Palaeobio Palaeoenv
