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Göttinger Arbeiten Zur Geologie Und Paläontologie GÖTTINGER ARBEITEN ZUR GEOLOGIE UND PALÄONTOLOGIE Nr. 59 J oachim Ebert Globale Events im Grenz-Bereich Mittel-/ 0 ber-Devon 1993 Im Selbstverlag der Geologischen Institute der Georg-August-Universität Göttingen Göttinger Arb. Geol. Paläont. 106 S., 25 Abb., 12 Tab., I Taf. Göttingen, 04.10.1993 -"59 Joachim Ebert Globale Events im Grenz-Bereich Mittel-/Ober-Devon Als Dissertation eingereicht am 27.05.1993 bei den Mathematisch-Naturwissenschaftlichenl Fachbereichen 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 Palaeo~tology Institute of Geology and Dynamlcs of tbe Litbospbere Redaktion Dr. Helga Uffenorde Institut und Museum für Geologie und Paläontologie Goldschmidt-Straße 3 0-37077 Göttingen ISS N 0534-0403 @) Geologische Institute, Universität Göttingen Offsetdruck KINZEL, Göttingen EBERT, J. (1993): Globale Events im Grenz-Bereich Mittel-IOber-Devon. [Global events near the Middle/Upper Devonian boundary.]- Göttinger Arb. Geo1. Paläont., 59: 106 S., 25 Abb., 12 Tab., 1 Taf.; Göttingen. Drastic facial and faunal changes are detectable at the top of the Maenioceras Stufe (late Givetian) and of the Pharciceras Stufe (end Givetian). They are of global extent. The lithofacial development of investigated sections in Germany and Morocco allows the correlation with T -R cycles of JOHNSONet a1. (1985). High-resolution diversity pat- terns (species-level) of various marine faunal groups representing different biofacies are presented. They demonstrate coincidence of high rates of extinction with lithologi- cal and facial changes. Possible mechanisms that could have caused extinctions are discussed. Givetian. Frasnian, Germany, Morocco. lithofacies. diversity patterns, goniatites, brachiopods. corals. stromatoporoids. conodonts, biofacies, evolution. Joachim Ebert, lnst. Geo1. Paläont., Goldschmidt-Str. 3, D-37077 Göttingen. SUMMARY Distinct global events are detectable at the top of the Maenioceras Stufe (late Givetian) and of the Pharciceras Stufe (end Givetian). They are documented as consequent effects of significant and rapid transgressions, both by drastic lithological and faunal changes. Investigations on pelagic/hemipelagic cephalopod limestones were carried out in Germany (sections 'Blauer Bruch' and Syring Quarry) and Morocco (sections Bou Tchrafine I, Hamer el Khdad SW). They correlate exactly with transgressive-regressive (T -R) cycles IIa and IIb sensu JOHNSON et al. (1985). Whereas the lithological and facial effects at the base of T-R cycle IIa (Taghanic Onlap) are negligible, they are exceedingly significant at the begin- ning of T-R cycle IIb where black shales are present. The investigated sections correspond precisely with regard to sedimentological and facial development. Two characteristic horizons of approximately the same thickness can be distinguished already in the field. They are called Lower and Upper Pharciceras Horizon because of the abundant appearance of goniatites of the genus Pharciceras within the Moroccan sections. Together they represent the time-specific facies (TSF) of the Pharciceras Limestones, that begins at the base or within the Upper hermanni-cristatus Zone and ends at the top of the Upper disparilis Zone. The Pharciceras Limestones document regressive phases with maximum regression within the Upper Pharciceras Horizon. They are termed regression R IIa land R IIa2 with reference to the terminology of JOHNSON et al. (1985). The beginning of a regressive phase within the Upper hermanni-cristatus Zone is detectable in agiobai scale, as confirmed by data from USA and Canada. The facies change at the base of the Lowermost asymmetricus Zone is an abrupt and drastic event and represents the most significant sea-Ievel rise within the Givetian. The resulting black-shales indicate the beginning of T -R cycle IIb combined with dysaerobic or anaerobic conditions, and thus aperiod of increasing life-hostility. The term 'Ense Event' is suggested as an alternative to other designations for this globally distributed facies shift. Styliolinites within the black shales are, similar to the Pharciceras Limestones, markers of time-specific facies. Their origin can be explained by mechanisms that are connected with high-energy sedimentation. Analogies with the pumilio Events give reason for their interpretation as tsunamits. High-resolution diversity studies (species-level) allow the documentation of diversity patterns of various marine faunal groups representing different biofacies. The examined period covers a time-span of approximately 7.2 Ma (Middle varcus Zone to Upper asymmetricus Zone), including nine conodont-biochronologic units (CBCE) covering 0.8 Ma each, However, primary literature-sources are appropriate for the compilation of extensive data-sets. For the present study, goniatites, brachiopods, corals, stromatoporoids and conodonts proved to be the most suitable taxa for such investigations because their distributions are weil established by conodont-chronology. Different 'parameters such as extinction-, speciation-, diversification-, and turnover-rates were plotted into simple X/Y -diagrams, so that dimensions of faunal changes could be described. 11 The data demonstrated a correlation between the diversity patterns and lithological and facial changes. High rates of extinction coincided with or occurred shortly after the beginning of a transgressive pulse. High rates of extinction at the base of the Upper hermanni-cristatus Zone approved to be connected with world-wide regressions. Uniform tendencies in range distribution were recognizable within the investigated taxa and allowed a differentia- tion between long- and short-lived species. The new terms 'perennial species' for long-lived and 'ephemeral species' for short-lived species are suggested. In addition, the new term 'intercedering species' is introduced for taxa which represent species or faunal associations transitional between two characteristic associations, in this case Middle and Upper Devonian. Evidently, those taxa are recognizable within the brachiopods, corals and stromatoporoids. Remarkable is a conformable trend of different diversity curves especially between representatives of the neritic facies and the reef facies. High rates of extinction occur briefly after the beginning of T -R cycle IIa (corals, stromatoporoids), during the regressive phase of the same cycle (R IIa I; brachiopods, corals, stromatoporoids), and at the beginning of T-R cycle IIb (brachiopods, corals). The extinction of typical Middle Devonian taxa seems to be a stepwise process, which is terminated before the beginning of the Upper Devonian. High rates of speciation are due to the appearance of intercedering species at the base of the Lower hermanni-cristatus Zone, the main specia- tion of ephemeral species within the Lowermost asymmetricus-Zone, and the radiation of the first typical Upper De- vonian species close to the base of the Lower asymmetricus Zone. The diversity patterns of goniatites show a different progression in comparison with other investigated faunal groups. Middle Devonian goniatite taxa such as Maenioceratidae, Agoniatidae, Pinacitidae and Sobolewiidae disappear in the upper part of the Middle varcus Zone. Their extinction coincides with the facies shift at the beginning of T -R cycle IIa. The diversification of multilobat pharciceratids starts close to the base of the Lower hermanni-cristatus Zone and culminates. in highest species diversity during the TSF of the Pharciceras Limestone. The extinction of most pharciceratids coincides with the rapid facies shift at the beginning of T -R cycle IIb. Only one species, Petteroceras feisti, barely reaches the base of the Upper Devonian but disappears in the lower part of the Lower asymmetricus Zone. The diversification of the gephuroceratids takes place shortly after the extinction of the last pharciceratid species and starts in the lower part of the Lower asymmetricus Zone with rapid radiation of manticoceratids. Global extinction events around the Middle/Upper Devonian boundary may be the consequence of rapid and drastic deterioration of living conditions. Evidently, the primary reasons for the extinction of the investigated taxa are transgressions connected with dysaerobic or anaerobic conditions. In addition, regression (R IIa 1) may have caused the extinction of numerous perennial species of the neritic facies and the reef facies. The species-area effect seems to be a possible mechanism considering observed lithofacial features. ZUSAMMENFASSUNG Im Bereich der Mitte1-IOber-Devon-Grenze zeichnen sich signifikante biologische Ereignisse ab, die global und in allen Faziesräumen feststellbar sind. Es sind dies Faunenschnitte, die jeweils am Ende der Maenioceras- (Pharciceras-Ereignis sensu WALLISER 1982, 1984; Taghanic Event sensu HaUSE 1985) und der Pharciceras-Stufe zu drastischen Veränderungen der Faunenzusammensetzung führen. Untersuchungen in den pe1agisch/hemipe1agischen Cephalopodenkalken Deutschlands (Schwellenfazies) und Marokkos (Schwellenäquivalente) dokumentieren einen nur wenig auffälligen lithologischen Wechsel am Top der Maenioceras-Stufe. Dagegen ist der lithologische und fazielle Wechsel am Ende der Pharciceras-Stufe als global
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