Pigmentation of the Early Silurian shallow marine red beds in South China as exemplified by the Rongxi Formation of Xiushan, southeastern Chongqing, central China

The origin and pigmentation of red beds have long been investigated, but mainly on continental settings and carbonate facies (including deep marine red beds). This paper focuses on the shallow marine clastic red beds developed in South China during the Silurian. Based on the sedimentological and geochemical analyses on red layers, green layers and laminae of the Rongxi Formation (Llandovery, Lower Silurian) of Xiushan, southeastern Chongqing, known as the ‘lower red beds’ (LRBs), the following observations are made: (1) the LRBs are primary red beds whereas the green layers are secondary (i.e., formed during early diagenesis); (2) the color differentiation of sediments shows little relevance to the bottom redox potential shift but has a close relationship to grain size and sediment sorting which may be caused by sea level changes and tides. The controlling factors of LRBs pigmentation are significantly different from those of deep marine red beds, and some coeval red beds in North America and Europe. It is noted that, among the shallow marine clastic red beds, the sedimentological and geochemical signatures from the laminae seem to better indicate the environmental conditions than those derived from the layers.