The Global boundary Stratotype Section and Point (GSSP) of the Ladinian Stage (Middle Triassic) at Bagolino (Southern Alps, Northern Italy) and its implications for the Triassic time scale
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The Global boundary Stratotype Section and Point (GSSP) for the base of the Ladinian Stage (Middle Triassic) is defined in the Caffaro river bed (45°49'09.5''N,10°28'15.5''E),south of the village of Bagolino (Province of Brescia, northern Italy), at the base of a 15-20-cm-thick limestone bed overlying a distinct groove ("Chiesense groove") of limestone nodules in a shaly matrix, located about 5 m above the base of the Buchenstein Formation.The lower surface of the thick limestone bed has the lowest occurrence of the ammonoid Eoprotrachyceras curionii (base of the E. curionii Zone; onset of the Trachyceratidae ammonoid family).Secondary global markers in the uppermost Anisian include the lowest occurrence of conodont Neogondolella praehungarica and a brief normal-polarity magnetic zone recognized in closely correlated sections including the principal auxiliary section at Seceda in the Dolomites.The GSSP-level is bracketed by U-Pb single zircon age data from volcaniclastic horizons, indicating a boundary age of ca 241 Ma. Historical context of the Ladinian StageThe first formal recognition of a stratigraphic interval comprising what is now called Ladinian orginates from the subdivisions of the Triassic System proposed by E.v.Mojsisovics.Ammonoids served as the main biostratigraphic tool for these divisions.By 1874 and with later modifications (e.g., 1882), Mojsisovics used the name "Norian" for a stratigraphic interval including, at its base, the South Alpine Buchenstein Beds and siliceous limestones of Bakony (Hungary).Because Mojsisovics erroneously equated this interval with parts of the ammonoidrich Hallstatt-limestones, he used the term "Norian" as the stage name, which refers to the Norian Alps around Hallstatt near Salzburg (Austria).Later, these Hallstatt-ammonoids were found to be much younger.Bittner (1892) therefore proposed the term "ladinisch" (Ladinian, after the "Ladini"-people of the Dolomites area) as a new label for the stratigraphic interval comprising the South Alpine Buchenstein and Wengen Beds.Although not adopted by Mojsisovics et al. ( 1895), the Ladinian subsequently became the generally accepted stage name (e.g., Arthaber, 1906; for additional information and discussions of the history of the Ladinian see Brack &Rieber, 1994 andKozur, 1995).Only little progress was made in the following decades on the (bio)stratigraphy of the Buchenstein and equivalent intervals.In Mojsisovics' time, knowledge of ammonoid successions in the Anisian/Ladinian boundary interval was fragmentary and stratigraphic correlations rather speculative.For instance, the South Alpine Buchenstein Beds and siliceous limestones in Bakony were considered age equivalents and both attributed to the "T.reitzi" Zone.However, unambiguous evidence from recent biostratigraphic research on ammonoids, conodonts and radiolaria shows that the main ammonoid bearing interval of Mojsisovics' "yellow siliceous limestones of Bakony" (now called the "Vàszoly Fm.") partly predates and overlaps in age with only the lowermost parts of the South Alpine Buchenstein Formation.Starting in the 1960s numerous classical localities for Middle Triassic fossils of the Western Tethys were restudied and new schemes of ammonoid zones were sketched, with the Anisian/Ladinian boundary being placed at different stratigraphic positions.North American geologists meanwhile preferred to define their own stage boundary at yet another stratigraphic level (see e.g., schemes in Zapfe, 1983;Tozer, 1984).For a modern definition of the base of the Ladinian Stage, the perplexities associated with original concepts and historical usage of the Anisian/Ladinian boundary provide no useful basis for the positioning of the GSSP.However, the lower part of the South Alpine Buchenstein Formation is an adequate interval for the Ladinian GSSP because Bittner (1892) explicitly designated this stratigraphic unit as the oldest one of the stage.Keywords:
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这份报纸简短为寒武纪的系统的全球阶段总结正式批准的全球标准 Stratotype 节和点和建议 GSSP (GSSP ) 。Luoyixi 镇附近的 Luoyixi 节， Guzhang，西北的湖南为全球 Guzhangian 阶段的底作为边界 stratotype 被批准，它是一个未命名的系列的最高的阶段作为寒武纪的系列 3 暂时地称为。GSSP 位置谎言在在节的 Huaqiao 形成的底上面的 121.3 m，与大都会的 agnostoid 三叶虫 Lejopyge leavigata 的第一外观与一致。在 Duibian 村庄的 Duibian B 节， Jiangshan，西方的浙江，为是 Furongian 系列的第二个阶段的建议全球 Jiangshanian 阶段的底作为边界 stratotype 被建议。建议 GSSP 位置谎言在在节的 Huayansi 形成的底上面的 108.12 m，与大都会的 agnostoid 三叶虫 Agnostotes orientalius 的第一外观与一致。这根地平线也与大都会的 polymerid 三叶虫 Irvingella angustilimbata 的第一外观。
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This paper presents a brief review of the recent advances in the Triassic chronostratigraphic study and its related aspects. The Subcommission on Triassic Stratigraphy (STS), ICS has formally voted through the scheme that the Triassic is divided into three series and seven stages in 1990's. So far only the Global Stratotype Section and Point (GSSP) for the base of the Triassic, however, has been formally defined. The GSSP for the Anisian-Ladinian boundary will be hopefully clarified soon. But the other five boundaries still need much work for the GSSP definition. With the improvement of the isotope dating techniques and methods, the Triassic timetable has been updated considerably and all the boundary ages between the stages have been redefined recently. The substage is not a main work of the STS, but the subdivision of the stages into substages is an important aspect in the Triassic chronostratigraphy, and the Lower and Middle Triassic especially have a good basis for substages. A terrestrial Triassic chronostratigraphical system based upon tetrapods has been suggested and to be improved in application. A scheme of three series and six stages for the marine Triassic of China and a scheme of three series and seven stages for the terrestrial Triassic of China have been proposed by the Chinese Committee of Stratigraphy in 2000, providing a proper basis for the chronostratigraphic study in China.
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Middle Triassic beds exposed along the road between Mišji Dol and Poljane pri Primskovem (Posavje Hills) comprise marlstone, tuff, volcaniclastic sandstone, and thin- to medium-bedded limestone and dolostone. The succession was logged and sampled for conodonts. A relatively rich conodont assemblage was determined, consisting of Budurovignathus gabrielae Kozur, Budurovignathus sp., Cratognathodus kochi (Huckriede), Gladigondolella malayensis Nogami, Gladigondolella tethydis Huckriede, Gladigondolella sp., Neogondolella balkanica Budurov & Stefanov, Neogondolella cf. excentrica Budurov & Stefanov, Neogondolella constricta (Mosher & Clark), Neogondolella cornuta Budurov & Stefanov, Neogondolella sp., Paragondolella excelsa Mosher, Paragondolella liebermani (Kovacs & Kozur), Paragondolella trammeri (Kozur), Paragondolella cf. alpina (Kozur & Mostler), and Paragondolella sp. The assemblage correlates with the upper Anisian and lowermost Ladinian assemblages from the Global Boundary Stratotype Section and Point (GSSP) of the Ladinian at Bagolino in the Southern Alps in northern Italy. Along with conodonts, numerous specimens of benthic foraminifera Nodobacularia? vujisici Urošević & Gaździcki were recovered from the lowermost part of the succession. Previous research on this taxon is critically evaluated.
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Abstract: This paper briefly summarizes an officially ratified Global Standard Stratotype‐section and Point (GSSP) and a proposed GSSP for global stages of the Cambrian System. The Luoyixi section near Luoyixi town, Guzhang, northwestern Hunan is ratified as the boundary stratotype for the base of the global Guzhangian Stage, which is the upmost stage of an unnamed series termed provisionally as Cambrian Series 3. The GSSP position lies 121.3 m above the base of the Huaqiao Formation in the section, coinciding with the first appearance of the cosmopolitan agnostoid trilobite Lejopyge leavigata . The Duibian B section at Duibian village, Jiangshan, western Zhejiang, is proposed as the boundary stratotype for the base of the proposed global Jiangshanian Stage that is the second stage of the Furongian Series. The proposed GSSP position lies 108.12 m above the base of the Huayansi Formation in the section, coinciding with the first appearance of the cosmopolitan agnostoid trilobite Agnostotes orientalius . This horizon is also with the first appearance of the cosmopolitan polymerid trilobite Irvingella angustilimbata .
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Recent excavation and sampling in the upper part of the Felsőörs section (Balaton Highland, Hungary) yielded important ammonoid findings, identified as belonging to the genera Nevadites, Chieseiceras, Eoprotrachyceras and Falsanolcites. Several specimens of Chieseiceras chiesense were found in a dm-thick clay layer with limestone nodules, regarded as the equivalent of the "Chiesense Groove" of the Bagolino and other sections in the Giudicarie area (North Italy). The immediately overlying limestone bed yielded Eoprotrachyceras cf. curionii and Falsanolcites cf. rieberi. Together these reliably prove the base of the Curionii Zone, corresponding to the base of the Ladinian Stage, the GSSP of which was recently established at Bagolino. The new data further improved the excellent correlation between the ammonoid records of the Felsőörs section and the Global Stratotype Section at Bagolino. Felsőörs may be considered the most important reference section for the Anisian to Ladinian boundary interval.
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