The Kellwasser events in the Upper Devonian Frasnian to Famennian transition in the Toc Tat Formation, northern Vietnam

• Published in: The island arc Vol. 28; no. 1
• Main Authors: Komatsu, Toshifumi, Urakawa, Ryota, Inada, Toshitaka, Yamauchi, Kazuki, Maekawa, Takumi, Takashima, Reishi, Williams, Mark, Nguyen, Phong D., Doan, Hung D., Nguyen, Minh T., Niko, Shuji, Tanaka, Gengo, Yamaguchi, Tatsuhiko
• Format: Journal Article
• Language: English
• Published: Melbourne John Wiley & Sons Australia, Ltd 01.01.2019; Wiley Subscription Services, Inc
• Subjects: Biostratigraphy; Carbon isotopes; Carbonates; conodonts; depositional environments; Devonian; Devonian Kellwasser events; Frasnian–Famennian boundary; Isotopes; Platforms (geology); Survival; Total organic carbon; Vietnam; Vietnam
• ISSN: 1038-4871; 1440-1738
• DOI: 10.1111/iar.12281

Upper Devonian carbonates of the Toc Tat Formation in the Si Phai Pass area of Dong Van District, northern Vietnam were deposited in carbonate platform, slope, and basin environments. These carbonates yield abundant conodonts indicative of the Palmatolepis nasuta, Pa. linguiformis and Pa. triangularis zones, the Frasnian–Famennian stage boundary being identified by the first occurrence of Pa. triangularis. Two positive carbon isotope excursions are recognized, the lower excursion peaking in the interval of the lower to middle Pa. nasuta Zone, whilst the upper excursion peaks just above the local Frasnian–Famennian boundary. Based on the biostratigraphy, these excursions equate to the Lower and Upper Kellwasser events. Locally, tentaculitoid taxa (Nowakia, Styliolina, Homoctenus, and Metastyliolina?) are abundant in the interval of the Pa. nasuta Zone, but show a drastic decline in abundance before the Lower Kellwasser Event, and only two taxa survived into the Famennian. Carbonates in the lower Toc Tat Formation, northern Vietnam, consist of gravity‐flow deposits and hemipelagites formed in slope to basin plain settings. These carbonates are rich in Upper Devonian microfossils including ostracods, conodonts and tentaculitoids. Frasnian Palmatolepis nasuta, Pa. linguiformis and Famennian Pa. triangularis conodont zones are recognized in the formation. Two positive d13Carb excursions representing Kellwasser events are recognized in the late Frasnian to Famennian transition. The numbers of tentaculitoid species and individuals decreased drastically before the Lower Kellwasser Event. 北部ベトナムのシーファイ峠地域に分布するトクタット層の上部デボン系の堆積環境とコノドント生層序,  炭酸塩岩の安定炭素同位体比,  産出化石について検討した.  この地域のトクタット層下部は,  炭酸塩プラットフォームの海底斜面から海盆底で堆積した炭酸塩岩類からなる.  これらの堆積物からは,  Palmatolepis nasuta帯,  Palmatolepis linguiformis帯,  Palmatolepis triangularis帯に特徴的なコノドントが多産し,  Pa. triangularisの初産出で定められるフラニアン/ファメニアン境界を挟んでいる.  安定炭素同位体比は,  2層準で明瞭な正のシフトを示し,  下位の正のピークは下部–中部Pa.  nasuta帯に挟まれており,  上位の正のピークはフラニアン/ファメニアン境界の直上にある.  これらの事実から,  安定炭素同位体比の2つの正のシフトは,  大量絶滅で知られている下部・上部ケルワッサー事変層準に対比されることが分かった.  NowakiaやStyliolina,  Homoctenus, Metastyliolina?からなるテンタキュリトイド類は,  Pa. nasuta帯で多産するが,  これらの産出量は下部ケルワッサー事変よりも下位の層準で劇的に減少し,  StyliolinaとHomoctenusのみがファメニアン階まで産出する.