Quantitative stratigraphy of the Wufeng and Lungmachi black shales and graptolite evolution during and after the Late Ordovician mass extinction

Graptolite-bearing black shales are widely distributed on the Yangtze Platform in South China. Based on 19 sections systematically studied recently, we conducted a quantitative stratigraphic analysis of the graptolite biodiversity change during and after the Late Ordovician mass extinction. The combination of graphic correlation and rarefaction methods make it possible to evaluate the effect of sampling intensity on biodiversity estimates. Graptolite diversity reached its peak in the Dicellograptus complexus Chron, and then a gradual decline to the P. pacificus Chron. It was followed by a two-step mass extinction, the first and major phase occurred in the Metabolograptus extraordinarius Chron, and the second and minor phase in the latest M. persculptus Chron. The second phase of the extinction was mostly represented by the elimination of the Diplograptina survivors from the major extinction event, but did not show significant decline in standardized diversity curve. Thereafter, a moderate increase in richness appeared in the early and mid Parakidograptus acuminatus Chron, which was followd by a considerable decrease of graptolite richness in the late P. acuminatus Chron and early C. vesiculosus Chron. The present study also indicates slow and uneven sedimentation rates in the studied sections from mid-Katian to early Rhuddanian, including a relatively slower sedimentation rate during the Hirnantian Epoch. The latter, and the hiatus at the Honghuayuan and Guanwu sections, probably reflect the effects of the Hirnantian glaciation and regional tectonic movements.