Middle-Late Ordovician iron-rich nodules on Yangtze Platform, South China, and their palaeoenvironmental implications

Abstract

Iron‐rich nodules occur on the Yangtze Platform, South China, especially along the marginal area, during the late Middle and early Late Ordovician (Darriwilian–Sandbian). Based on the study of samples from the Daling section, Anhui Province, these nodules include: (1) oncoids; (2) coated bioclasts; and (3) coated micritic intraclasts, all of which potentially have a similar biological origin. The nodule‐bearing strata were deposited in a midouter ramp setting, indicating sediment starvation and open marine conditions, possibly combined with anoxia in deeper basinal areas and cool water upwelling during the formation of the nodules, rather than terrestrial input or local tectonic movement. The widespread distribution of iron‐rich nodules at this time was probably favoured by high sea level and cooling climate, conditions that simultaneously favoured the evolutionary radiations of the GOBE in South China. Thus, these nodules form a time‐specific signature of a critical time in the evolution of the marine ecosystem in the context of local environmental background along the marginal Yangtze Platform.

Middle to Late Ordovician carbon isotope chemostratigraphy of the Lower Yangtze Platform: Implications for global correlation

“…Based on conodont biostratigraphy, the Datianba Formation is largely Sandbian in age, comprising the Pygodus anserinus, Baltoniodus variabilis, B. alobatus, Amorphognathus tvaerensis biozones (An, 1981(An, , 1987An et al, 1985;An & Ding, 1982;Ding et al, 1993). In the studied sections, the Datianba Formation is characterized by grey nodular limestones (Figure 3e) and characteristically contains abundant iron oncolites (Figure 3d,f) which indicate sediment starvation and open marine conditions, potentially combined with anoxia in the adjacent deeper basinal areas (Luan et al, 2018). Biostratigraphically, the Datianba Formation in the Daling section comprises the Yangtzeplacognathus jianyeensis/B.…”