Environmental changes revealed by Lower–Middle Ordovician deeper-water marine red beds from the marginal Yangtze Platform, South China: Links to biodiversification

Abstract

The origin of reddish nodular limestone in the Zitai Formation, a deeper-water type of marine red beds (MRBs) distributed along the marginal Yangtze Platform, South China, is enigmatic. Here we focus on the red colorations through sedimentological, mineralogical and geochemical analyses, as exemplified by the Zitai Formation at the Xiangshuidong section, Songzi City, Hubei Province and the Daling section, Shitai City, Anhui Province. Limestones studied here can be gray, green or red. The grayish limestones are generally below the reddish and/or greenish limestones, whereas the greenish layers are associated with reddish layers in forms of homogenous, banded or mottled limestone. Hematite that is fine and poorly crystalized could be the coloration mineral for the Zitai MRBs, which is largely caused by increased terrigenous iron, when the bottom seawater was oxic. Except that the irregularly banded and mottled reddish-greenish limestone are of a possible diagenetic origin, the primary greenish limestones are related to relatively reducing conditions compared with reddish limestones, as a result of eustatic changes or other fluctuating environmental factors. During this time interval, the Great Ordovician Biodiversification Event (GOBE) was facilitated by highly oxic seawater indicated by MRBs in South China, Baltica and Sibumasu, but the diachroneity of radiations in South China could be ascribed to increased terrigenous supply, emphasizing that regionally environmental factors might be critical for biological radiations at the early stage of the GOBE.