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Wissenbacher Schiefer (Rheinisches Schiefergebirge, Harz, Devon)

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Wissenbacher Schiefer (Rheinisches Schiefergebirge, Harz, Devon) GÖTTINGER ARBEITEN ZUR GEOLOGIE UND PALÄONTOLOGIE Nr.68 Michael Schubert Die dysaerobe Biofazies der Wissenbacher Schiefer (Rheinisches Schiefergebirge, Harz, Devon) 1996 Im Selbstverlag der Geologischen Institute der Georg-August-Universität Göttingen Göttinger Arb. Geol. Paläont. 131 S., 28 Abb., 6 Tab., 3 Taf. Göttingen, 16.12.1996 CdP68 Michael Schubert Die dysaerobe Biofazies der Wissenbacher Schiefer (Rheinisches Schiefergebirge, Harz, Devon) Als Dissertation eingereicht am 26.09.1996 bei den Mathematisch-Naturwissenschaftlichen Fakultäten der Georg-August-Universität erscheinen in unregelmäßiger Folge im Selbstverlag der Geologischen Institute der Georg - August - Universität Göttingen: Institut und Museum für Geologie und Paläontologie Institut für Geologie und Dynamik der Lithosphäre are issued irregularily by the Geological Institutes (until 1985: Geol.-Paläont. Inst.) of Göttingen University: Institute and Museum of Geology and Palaeontology Institute of Geology. and Dynamics of the Lithosphere Redaktion Dr. Helga Uffenorde Institut und Museum für Geologie und Paläontologie Goldschmidt-Straße 3 D-37077 Göttingen ISS N 0534-0403 @ Geologische Institute, Universität Göttingen Offsetdruck KINZEL, Göttingen SCHUBERT,M. (1996): Die dysaerobe Biofazies der Wissenbacher Schicfer (Rheinisches Schiefergebirge, Harz, Devon). [The dysaerobic biofacies of the Wissenbach Shale (Rhenish Schiefergebirge, Harz, Devonian)). - Göttinger Arb. Geol. Paläont., 68: 131 S.; 28 Abb., 3 Taf.; Göttingen. For many years, the Wissenbach Shale has been considered to be an example of a black shale including anoxic conditionshostile to benthic life. In the present study, detailed palaeontologic and taphonomic analyses of the Wissenbach Shale from different sites lead to a modified ecological interpretation. Benthic life was possible for a great time interval of the sequence because of dysoxic bottom waters. Alleged pseudoplanktonic organisms are re-interpreted as benthic in origin. The new term 'dq-benthos' is suggested for organisms living in dysoxic and quiet environments such as the Wissenbach Shale. Emsian, Givetian, Germany, ecology, taphonomy, pyrite, pseudoplankton, brachiopods, pelec)pods, gastropods,hyolithids, corals, trilobites. Michael Schubert, Helvesanger 18, D - 37081 Göttingen. SUMMARY Following global transgression in the late Early Devonian, the thick series of the "Wissenbach Shale" was deposited seaward of the Rhenohercynian shelf. Stratigraphically, the series ranges from the upper Emsian through the lower Givetian stages. For many years, this unit has been considered to be an example of a black shale deposited in a deep, open ocean basin with conditions hostile to benthic life. Deposits that correlate in part to those of the "Wissenbach facies", and which have been deposited simultaneously under oxygen-depleted conditions, are known from Thuringia, Moravia, Morocco, and China. Palaeontologic, taphonomic, and geochemical analyses of the monotonous Wissenbach Shale from different sites (SW-Lahn Syncline, NW-Dill Syncline, Oberharzer Diabaszug) lead to a modified ecological interpretation of the "Wissenbach facies". Benthic life was possible for a great time interval of the sequence, and alleged pseudoplank- tonic organisms are re-interpreted as benthic in origin. A comparison with the recent Sargassum community supports this interpretation. Although it is generally poor in fossils, extensive faunal material was collected from the Wissenbach Shale at several different localities. Faunal lists of previous workers were included in the study as the former Dachschiefer mining project had created better outcrops and sampling conditions within the Wissenbach Shale than exist today. Thus, the fauna collected in former times exceeds by far the one that can be collected today (e.g. Olkenbach Syncline) and must be taken into consideration in the overall interpretation. A pelagic fauna (cephalopods and thin-shelled tentaculites) predominates; however, numerous typically benthic groups (especially brachiopods, pelecypods, gastropods, hyolithids, anthozoans, and trilobites) can also be found. The state of preservation of the fauna differs: there are pyrite steinkerns (often alte red to goethite or limonite), sediment steinkerns and/or casts. Preservation of shells is rare in the Wissenbach shale. The preservation of fossils is determined by the taphofacies, particularly by dissolution and disintegration processes at the sediment/water interface, and the mineralogy of their shells. Because of this a considerable selective taphonomic loss in some parts of the sequence must be attributed to the dissolution of shells. Aragonite shells would have especially been affected by this process (in particular pelecypods). The rclationship between oxygen content in the lower water column and pyritization of fossils plays an important role in interpreting the ecology of the fauna. There is a causal relationship between oxygen/rate of sedimentation and the pyrite crystal mineralogy as shown in the "Pyrite taphofacies model" by BRETT & BAIRD (1986). Pyritized steinkerns can only form if sulfidic microenvironments exist within the shell. Naturally, these microenvironments do not form under euxinic, i.e. anoxic-sulfidic conditions, but rather in bottom water low in oxygen, i.e.under dysoxic conditions. Sequences within the Wissenbach facies that contain pyritized steinkerns are ecologically interpreted as gyttja. Examples are the Rupbach shale of the Pit Langscheid, the Schmale Bruch in the Haiger Hütte section, many occurrences in the Upper Harz Mountains. and the Olkenbach Syncline. This interpretation is supported by geochemical da ta from the literature. In order to determine the degree of oxygenation within the sequence, several sampies taken from a core close to the type section of the Wissenbach Shale were analyzed. The II total VICr ratio in the sam pIes averages I, which is typical for gyttjas. Taphonomic and geochemical parameters within a great portion of the sequence evidence a dysoxic environment and support the conte nt ion that benthic life was possible. However, oxygen conte nt of the bottom water was f1uctuating and anoxie phases presumably also existed. Other important ecological factors that were effective within the Wissenbach shale environment are soft bottom, low CUTTent velocities, low light penetration, and presumably temporarily turbid water. The pseudoplankton problem is of major importance for the ecologic interpretation of the faunal elements. Even though benthonic Iife was possible in principle, the autecological (functional morphology, physiology, and larval ecology) and demecological (population parameters and distributional patterns) characteristics of each taxon have to be evaluated. Poor preservation of fossils and lack of knowledge of the fauna also hamper the analysis. Similar morphological adaptations are to be expected for pseudoplankton and DQ (dysoxic, quiet environment) benthos such as thin shells and sm all body size. Thus, the lack of an attachment mechanism is the only evidence against a pseudoplanktonic life mode of the concerned taxa. However, this can rarely be proven because of the infrequent preservation of soft body parts. Further considerations on functional morphology often bear a high factor of uncertainty but nonetheless can provide a conclusive overall picture. Considerations on physiology and larval ecology give valuable information for an ecologic interpretation despite their theoretical character. An important criterion for the differentiation between pseudoplankton and benthos is facies distribution. Great palaeo- geographie distribution of a species is often the only argument for pseudoplanktonic life mode. This argument is rejected if a significant dependence between facies and taxon can be recognized. Pseudoplanktonic organisms should be independent of the facies. A comparison with the recent Sargassum community, which frequently is considered to be the recent analog of fossil pseudoplankton, shows that the significance of pseudoplanktonic life mode among the Wissenbach fauna is low at best. Analogy of the two communities is doubted because of the different composition of the biocoenoses (low diversity in the Sargassum fauna as opposed to high total diversity in the Wissenbach Shale), the different strategies of attachment (predominantly encrusting as opposed to attachment by pedicle or byssus), the origin of the organisms (nearshore taxa as opposed to offshore taxa), and the diverging distributional pattern (restricted geographical distribution regardless of facies changes as opposed to high geographical distribution with a distinct dependence upon facies). Another argument against a great significance of pseudoplanktonic organisms in the Wissenbach Shale is the lack of those associations. Articulate brachiopods are predominant. They are basically considered to be unsuited for a pseudoplanktonic mode of life because of their energy-saving organisation and their presumably nonplanktotrophic larvae. The most important "Wissenbach" brachiopods such as Plectodonta (D.) minor ssp., "Retzia" novemplicata, Bifida lepida, the Lissatrypidae ("Peratos"), and Prokopia cf. bouskai are considered to be benthonic on ac count of varied observations. Among the pelecypods, which are especially common in the sections of Ziegenbacher Teich and Huttaler Wider- waage, the mode of li fe of Buchiola remains unclear. "Cardiomorpha", "Panenka", the Nuculoides, and the highly abundant, tiny. inc. sed. pelecypods
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