Revision of the Conodont Zonation of the Wenlock–Ludlow Boundary in the Prague Synform
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Estonian Journal of Earth Sciences, 2014, 63, 4, 305–311 doi: 10.3176/earth.2014.35 Revision of the conodont zonation of the Wenlock–Ludlow boundary in the Prague Synform Ladislav Slavík Department of Paleobiology and Paleoecology, Institute of Geology AS CR, v.v.i., Rozvojová 269, 16500 Praha, Czech Republic; [email protected] Received 18 June 2014, accepted 9 October 2014 Abstract. The regional zonation of the Wenlock–Ludlow boundary is established for the Prague Synform using refined data from updated conodont records. The following conodont zones have been recognized in the Prague Synform: the Ozarkodina sagitta sagitta Zone, the Ozarkodina bohemica Interval Zone, the Kockelella crassa Zone, the Kockelella variabilis variabilis Interval Zone and the Ancoradella ploeckensis Zone. The Ozarkodina bohemica longa and Kockelella ortus absidata zones are used herein at subzonal level only because the entries of index taxa cannot be precisely detected. The Bohemian conodont zonal scale is correlated with the recently proposed standardized zonation. The established conodont zones are tentatively correlated with global graptolite zonation and matched against generalized eustatic and carbon isotope curves. Key words: Late Silurian, Wenlock, Ludlow, conodonts, global correlation, stratigraphy. INTRODUCTION only slight refinements and modifications: Aldridge & Schönlaub (1989) included the Ozarkodina bohemica Z. The definitions of the series and stages of the Silurian into the Homerian. Later, a more complete zonation of System and their correlation are traditionally based on the Wenlock–Ludlow boundary interval was established relatively well developed graptolite zonation. The Silurian in Sardinia (Corradini & Serpagli 1999; Corriga et al. conodont zonation is rather a complementary correlation 2009) (see Fig. 1). It comprises conodont zones based tool that is, in general, more complicated because of on the most relevant taxa described by Walliser: the facies constraints and a high degree of provincialism Oz. s. sagitta and Oz. bohemica zones (in the Homerian) at some stratigraphic levels caused by diverse palaeo- and the K. crassa Z., Kockelella variabilis Interval Zone environmental settings. It is, however, essential for (I. Z.), Wurmiella hamata Z. and A. ploeckensis Z. (in correlation of carbonate-dominated successions. The the Gorstian). The most detailed refinement of conodont Wenlock–Ludlow boundary is correlated globally with zonation was made by Jeppsson (1997). Later it was the first appearance of the graptolite Neodiversograptus finally modified by Jeppsson et al. (2006) who included nilssoni and the graptolite zonation (especially for a number of new conodont zones based on fauna from the Homerian Stage) can be directly applied almost Gotland. With the exception of the East Baltic (e.g. universally. The basis of the conodont zonation of the Märss & Männik 2013), the conodont zonation of the Wenlock and Ludlow series has been established by Homerian Stage based on Gotland has not yet been Walliser (1964). The regional conodont zonations of tested in other regions. Two zones, Ozarkodina bohemica parts of this time-span have later been provided, e.g., by longa and Kockelella ortus absidata, were, however, Corradini & Serpagli (1999), Viira (1999), Jeppsson incorporated to the generalized global conodont zonal et al. (2006), Corriga et al. (2009) and Slavík & Carls scale by Cramer et al. (2011). Their zonation for the (2012). Recently, an updated Silurian correlation chart Homerian and Gorstian was consequently adopted by was published by Melchin et al. (2012). It combines Melchin et al. (2012) into the recent Silurian Time graptolite and conodont zonations plotted against geo- Scale. Presently, this zonal scale seems to be of general chemical and sea-level trends. The primary conodont use. The proposed Homerian zones bear, however, zonation established by Walliser (1964), which includes apparent limitations. It is due to the conodont Mulde the Ozarkodina sagitta sagitta Zone (Z.) for the Homerian Event (Jeppsson & Calner 2003), which is related to and the Kockelella crassa and Ancoradella ploeckensis perturbations in the global carbon cycle (cf. Cramer zones for the Gorstian, has been used for decades with et al. 2012) and negatively influenced the evolution in 305 Estonian Journal of Earth Sciences, 2014, 63, 4, 305–311 conodont lineages already in the early Homerian. The BIOSTRATIGRAPHIC OVERVIEW OF THE scarcity of the globally distinguishable taxa with the WENLOCK–LUDLOW BOUNDARY IN THE potential to define short time intervals is then a direct PRAGUE SYNFORM consequence of the event that is accompanied by depositional bias of the carbonate and siliciclastic sedi- In the Prague Synform the late Wenlock and Gorstian mentation systems on Gotland (cf. Calner & Jeppsson are developed as a volcanosedimentary complex that is 2003). The occurrences of conodont taxa are largely replaced by largely carbonate sedimentation from the controlled by local facies changes which are not Ludfordian. The facies development of the Wenlock– synchronous in different areas. Accordingly, synchroneity Ludlow boundary in that region changes in different of conodont indexes involved is also often uncertain. parts of the former basin (Kříž 1991) and is greatly As mentioned above, some of the zones introduced influenced by volcanic activity. It reflects a regressive– on Gotland have not yet been extended to or even tested transgressive eustatic regime combined with tectonically in other regions. The problematic biozonal correlation unstable basin settings. may also be caused by sampling bias (i.e. insufficient According to Kříž et al. (1993), the boundary interval size of samples taken in comparison with Gotland) or by is characterized by three main facies: (1) volcanoclastics the absence of corresponding intervals in the studied and basaltic rocks with subordinate shallow-water sections. This is, for example, also the case of the limestones, (2) shales and cephalopod limestones and classic Silurian Cellon section where the newly suggested (3) mostly shale facies with tuffite layers without Homerian zones (Oz. b. longa and K. o. absidata) are carbonates. missing because of a gap and/or large condensation The pioneer biostratigraphic studies of the boundary (see Corradini et al. 2014). On that account, before interval were based on graptolites and date back to the worldwide acceptance, there is an urgent need to verify end of the 19th century. For a summary see Kříž et al. the application of the Baltic conodont zonation in other (1993), who carried out the major reinvestigation of the areas outside northeastern Europe. boundary interval. The graptolites have been restudied The purpose of this paper is the biostratigraphic by H. Jaeger and J. Kříž, conodonts by H. P. Schönlaub, revision of conodont faunas from the Wenlock–Ludlow chitinozoans and sporomorphs by P. Dufka. The local boundary in the Prague Synform and their correlation graptolite zonation can be directly plotted to the present with the recently proposed standardized conodont global standard. The conodont studies by Schönlaub zonation. The revision is based both on previous and provided a database of stratigraphically important taxa newly obtained conodont data. with defined local ranges that were directly or tentatively Fig. 1. A correlation chart for the Homerian–Gorstian interval. Graptolite zonation for the Prague Synform is modified according to Štorch et al. (2014) and juxtaposed with conodont zonal scales: (1) standardized – Geological Time Scale, The Silurian Period (Melchin et al. 2012) (‘GTS 2012’); (2) Sardinian zonation (Corradini & Serpagli 1999); (3) zonation for the Prague Synform (this paper). The zonations are matched against the generalized carbon isotope and eustatic curves – Geological Time Scale, The Silurian Period (Melchin et al. 2012) (‘GTS 2012’). 306 L. Slavík: Conodonts of the Wenlock–Ludlow boundary correlated with graptolite zones. In the Homerian (e.g. Barrick & Klapper 1976; Cramer et al. 2006; Schönlaub (in Kříž et al. 1993) recognized the following Barrick et al. 2009). In the Prague Synform it has index taxa: Oz. s. sagitta, Ozarkodina sagitta rhenana been recognized in the Arethusina Gorge and Lištice and Oz. bohemica; in the early Gorstian: K. variabilis, sections in strata corresponding to the Testograptus testis K. crassa and Ozarkodina inflata. The taxa Ozarkodina Subzone of the Cyrtograptus lundgreni graptolite Z. excavata (= Wurmiella excavata s.l.) and K. absidata Ozarkodina s. sagitta has also been newly recorded in were reported to cross the Wenlock–Ludlow boundary. the Nad Hostímí Section. Associated conodonts are These data enable recognition of the Oz. s. sagitta and W. excavata s.l., K. o. absidata, Delotaxis ex gr. silurica, Oz. bohemica zones for the Homerian and the K. crassa Z. Dapsilodus obliquicostatus and transitional forms between for the Gorstian in the Prague Synform. Recently, Frýda Oz. s. sagitta and Oz. bohemica. Denticles in these & Frýdová (2014) detected a characteristic Homerian transitional specimens are not fused but the basal cavity double-peaked positive carbon isotope excursion in the is broader than in Oz. s. sagitta. Prague Synform that is partly correlated with the Mulde Event. The stratigraphic position of the excursion is still Ozarkodina bohemica Interval Zone tentative due to insufficient biostratigraphic data and The long-ranging Oz. bohemica Z. had characterized the unknown range of the late Homerian conodont