Upper Ordovician (Sandbian–Katian) graptolite and conodont zonation in the Yangtze region, China

Abstract

The graptolite and conodont biostratigraphy of the latest Darriwilian, Sandbian and early and middle Katian succession on the Yangtze Platform is reassessed based on numerous new fossil collections and previously published data. At least at some localities, the lowermost Miaopo Formation is of pre-Nemagraptus gracilis Zone age (uppermost Pygodus serra and lowermost P. anserinus conodont zones). The rest of this formation (except for the uppermost part, which lacks diagnostic graptolites) has a diverse graptolite fauna of the N. gracilis Zone. The uppermost part of the Miaopo Formation contains few biostratigraphically diagnostic graptolites but the occurrence of conodonts of the Baltoniodus alobatus Subzone of the Amorphognathus tvaerensis Zone suggests equivalence with part of the Climacograptus bicornis graptolite Zone. The conodont succession of the Datianba Formation is virtually identical with that of the Miaopo Formation, confirming that these units are coeval. The lowermost part of the overlying non-graptolitic Pagoda Formation represents the upper A. tvaerensis Zone, and its upper part represents the A. superbus Zone. The occurrence of Dicellograptus elegans in the overlying Chientsaokou Formation (equivalent to the Linhsiang Formation in Yangtze Gorges region) suggests equivalence with the Pleurograptus linearis Zone in Scotland, which is consistent with the relatively non-diagnostic conodont fauna in these Chinese units. The biostratigraphic data are in good agreement with the δ13C chemostratigraphy and permit the establishment of precise correlations with the Baltoscandic and North American successions.

Darriwilian (Middle Ordovician) δ13Ccarb chemostratigraphy in the Precordillera of Argentina: Documentation of the middle Darriwilian Isotope Carbon Excursion (MDICE) and its use for intercontinental correlation

“…In the Honghuayuan succession the lower Darriwilian U. austrodentatus Zone is well developed in the uppermost Meitan Formation. This unit is overlain by the Shihtzupu Formation that has yielded conodonts of the Yangtzeplacognathus foliaceus (formerly E. foliaceus) Subzone (An, 1987) as well as graptolites, such as G. linnarssoni, which indicate the uppermost Darriwilian Dicellograptus vagus (formerly H. teretiusculus) Zone (Chen et al, 2011). Hence, an interval corresponding to at least two graptolite zones (N. fasciculatus and P. elegans zones) are missing and this absent interval corresponds to most, if not all, of the range of MDICE.…”

Emended Sandbian (Ordovician) conodont biostratigraphy in Baltoscandia and a new species ofAmorphognathus

“…The variation in morphology of P and M elements of A. tvaerensis has been noticed by many authors (Dzik, 1990, 1994; Stouge, 1998; Viira, 2008; Xu et al 2010; Paiste et al 2022). The first illustrations of the elements described here as A. viirae sp.…”

“…The study interval is dominated by marl and argillaceous limestone of shelf origin. A geochemical comparison of δ 18 O phos and δ 13 C carb curves, along with litho-and biostratigraphy from the Mehikoorma core, were published recently (Männik et al 2021), but conodonts from the Mehikoorma core are visualized in only a few samples (Xu et al 2010;Paiste et al 2020).…”

“…Occurrences of A. tvaerensis in the sections of Poland and Scotland are also known (Bergström, 1962). Several authors have discussed variations in elements of A. tvaerensis (Dzik, 1990, 1994; Viira, 2008; Xu et al 2010; Paiste et al 2022) but have not drawn any conclusions, probably because of a lack of representative material for such a study. In the present study, which is based on a rich collection of conodonts from the Mehikoorma drillcore section from Estonia, the morphological variation of the P and M elements of the species of genus Amorphognathus in the Sandbian and lower Katian interval is analysed in detail, with the aim of clarifying species variation and the resulting implications to stratigraphy.…”