中国科学院南京地质古生物研究所机构知识库

The Lower to Middle Ordovician Dawan Formation and the base of the overlying Guniutan Formation have been sampled for palynomorphs at Daping (Yichang area, South China) with the aim to discuss the environmental history of the lower Yushanian to lower Zhejiangian Stages (Arenig) from South China. Two samples from the Huanghuachang section (the same area) were also examined for comparison. Fifty-three samples from Daping proved to be palyniferous, while 17 of 70 were barren or nearly so. The resultant taxonomic database comprises 129 acritarch taxa, belonging to 41 genera and two species of prasinophycean algae. The morphology and the systematic status of most taxa are discussed in detail and their geographic and stratigraphic distribution are documented. Two new acritarch species (Dasydorus microcephalus, and Tenuirica? gradata) and four new varieties [Cymatiogalea granulata VAVRDOVA 1966 var. changjiangensis, Pacbyspbaeridium rhabdocladium (Lu 1987 emend. TONGIORGI et al. 1995) RIBECAI et TONGIORGI 1999 var. striatellum, Petemospbaeridium dissimile GORKA 1969 var. contractum, and Stelliferidium striatulum (VAVRDOVA 1966) emend. DEUNFF et al. 1974 var. radiatum] are instituted; Stelomorpha crassula (VARDOVA 1990b) is proposed as a new combination. A compositional analysis of samples from Daping, embracing the middle Oepikodus evae through the lower Lenodus variabilis conodont zonal interval, is based on relative percentages of acritarchs belonging to nine morphological classes (Veryhachid-, Diacromorph-, Sphaeromorph-, Micrhystrid-, Polygonid-, Peteinosphaerid-, Baltisphaerid-, Galeata, and Others). SIMPSON'S (unbiased) Dominance Index is then extrapolated from 45 of the more productive samples of the Dawan Formation. To understand the compositional fluctuations through the Daping section, an interpretative model is elaborated, by analogy with WALL et al.'s (1977) study of the distribution of dinoflagellate cysts in the sediments of modern seas, integrated with reported correlations between trophic levels and phytoplankton diversity. According to the integrated model, two principal concurrent ecological trends influence the distribution of phytoplankton diversity; the first lies in the inshore to offshore transect, while the second corresponds to a climatic (temperature) variation through time. The model predicts that phytoplankton diversity increases (and dominance decreases) from mesotrophic-eutrophic, cold (regressive), proximal habitats, towards warmer (transgressive), distal, oligotrophic environments. With this approach, a large dominance peak situated in the upper Paroistodus originalis conodont Zone is considered indicative of an important regression separating Middle and Upper Members of the Dawan Formation. The multivariate Correspondence Analysis proved very useful to investigate the acritarch biofacies, which characterize the various habitats on the platform during both regressive and transgressive events. This analysis enables identification of four main biofacies, each referable to a particular environment, as follows: Biofacies I (Galeata Biofacies), from cool waters of eutrophic, upwelling influenced, coastal habitats; Biofacies II (Baltisphaerid-Petemosphaerid Biofacies), from temperate waters of inshore, mesotrophic habitats; Biofacies III (Veryhachid-Diacromorph-Polygonid Blofacies) from temperate waters of offshore, moderately to highly oligotrophic environments; and, Biofacies IV (Barakella, Picostella, Striatotbeca Biofacies), from temperate, oceanic, highly oligotrophic habitats. Biofacies I corresponds to regression, while Blofacies II-IV correspond to a range of environments during transgressive episodes. Although acritarch biofacies are conceptually unrelated to inferred palaeogeographic settings, the more distal habitats (Biofacies III and IV), influenced by oceanic waters, would be expected to show more "Mediterranean" than "Baltic" acritarch characteristics. Conversely, "Baltic" attributes would be embodied by acritarch assemblages from proximal or internal situations (Biofacies II), having more restricted circulation of warmer inshore or estuarine waters. Consequently, the more substantial transgressions could be signalled by the occurrence of "Mediterranean"-type assemblages, while the essentially unaffected, more internal contemporaneous habitats could retain "Baltic" attributes. A more reliable indicator of sea-level fluctuation is the Diversity Index. The acritarch diversity curve measured at Daping can be correlated with the global sea-level curve reported for the Arenig, as follows: a) a large transgressive peak extends from the topmost Oepikodus evae to the entire Baltoniodus? triangularis-B. navis Zone, b) a negative trend comprises the lower two-thirds of the R originalis Zone, c) a transgressive tendency covers the upper third of the P originalis Zone and the lower Microzarkodina parva-Baltoniodus norrlandicus Zone, with a maximum in the upper part of the latter, followed by d) a short-term regressive trend, with a minimum in the middle Lenodus antivariabilis Zone, and by e) an apparent further positive trend in the upper L. antivariabilis Zone. The evolutionary consequences of these events are discussed, taking into account the importance of the different biofacies for the adaptative changes of the phytoplankton across repeated extinction (regressive) events. Abandonment of the current "palaeophytogeogeographic" terminology ("Mediterranean Province", "Baltic Province") is proposed, in favour of different terms ("Mediterranean palynoflora", "Baltic palynoflora") that are more closely related to both the environmental/ecological factors and the evolutionary adaptations of the phytoplankton in response to such factors.