Oriented graptolites in the Uppermost Ordovician Wufeng black shale in southern China

Abstract

Graptolites are elongated fossils that can be used to decipher the current direction to evaluate paleocurrent conditions. Abundant graptolite fossils are well-preserved and clearly exhibit strong alignment in the black shales of the Upper Ordovician Wufeng Formation on the southern edge of the Upper Yangtze Sea, South China. The orientations of these graptolites indicate that the remains of graptolites were deposited on the seafloor under the influence of currents, instead of under calm water conditions. Based on the measurements of the long-axis directions of graptolites, the distributions of graptolites preserved on the bedding planes display discernable orientations and exhibit systematic changes throughout the stratigraphic section. In the western region of the study area, the paleocurrent directions in the stratigraphic sections gradually change from NE–SW in the Dicellograptus complexus biozone to N–S in the Paraorthograptus pacificus and Normalograptus extraordinarius–N. ojsuensis biozones. However, in the eastern region of the study area, the paleocurrent directions are more variable, ranging from multi-directional (NNE–SSW and NNW–SSE) in the P. pacificus biozone to NW–SE in the N. extraordinarius–N. ojsuensis biozone. The changes in the orientations of graptolites in the D. complexus and the P. pacificus biozones are consistent with the southern configuration of the Upper Yangtze Sea and indicate that a coastal current existed. In addition, some of the contemporaneous deformation structures were caused by tectonic forces observed in the lower Graptolite Shale Member of the Wufeng Formation in the eastern region of the study area. Some of the changes in the orientations of graptolites in the N. extraordinarius–N. ojsuensis biozone can be interpreted to have resulted from a change in tectonic activity (the Caledonian orogeny) before the N. extraordinarius–N. ojsuensis biozone. Furthermore, Lingula brachiopods, which live on the seafloor in shallow water, have been reported in the black shale of the Wufeng Formation, which indicates that the paleoenvironment was at least partially oxic; this supported the growth, development and reproduction of planktonic graptolites and other organisms. These results support the interpretation that the remains of organisms were quickly buried and that the amount of exchange between pore water and the water column gradually decreased. H2S gas generated by the decay of organisms during deposition consequently generated a strongly reducing environment in these sediments, which resulted in the formation of black shales.