Grid mapping revealed hidden geochemical lens and its chemostratigraphic bias in the middle‐upper Permian marine carbonate sequence in Laibin, South China

Abstract

Deep‐time chemostratigraphy provides important data for stratigraphy and palaeoenvironmental reconstruction. Geochemical analysis for the same proxy from the same outcrop section, however, often results in different data among various researchers. The dispute was ascribed to inappropriate screening of samples weathering, pre‐treatment, and analytical methods. An outstanding example is that a great variation of 1–3‰ in δ13CVPDB values from the same horizons at Auxiliary Global Stratotype of Section and Point for the Guadalupian–Lopingian (G‐L) boundary in Tieqiao, Laibin, South China was obtained by various research groups. The Walther's Law of Facies suggests that such differences in analytical results may be due to the diachronism of samples measured in high‐resolution stratigraphy. Although lithology and broad sedimentary facies are the same in a lithostratigraphic unit, heterogeneous distribution of bioclastics, δ13Corg, provenances, trace elements, and age‐controlling geochemical signals might have printed on sediments within the same bed. Here, we verified the variations by grid mapping and sampling, a new method for high‐resolution chemostratigraphy, and this practice is applied to the auxiliary GSSP at the Tieqiao section. A total of nine parallel columnar sections from the upper Guadalupian to lowest Lopingian were measured and sampled. In addition, four lateral profiles along four beds were also sampled, and two of them are close to the lower part of the section (upper Guadalupian) and near the G‐L boundary, respectively. These analytical results are plotted along the coordinate of X–Y axis and form the grid mapping on the outcrop. Contour maps of various proxies (i.e., CaO%, total Fe%, and δ13Ccarb) based on the grid mapping were constructed on the outcrop, instead of single excursions of various geochemical proxies throughout the upper Guadalupian to lowest Lopingian. The results show different negative excursions of δ13Ccarb coinciding with biotic extinction near the G‐L boundary and answered the differences of proxy reported in previous studies. Grid mapping of various proxies shows general trends of the proxies on the outcrop; however, there are invisible beddings across lenses in dozens meters size of geochemical abnormal. Analytical data of the same proxy from the same horizons at different columnar sections may present different excursions if sampling on the abnormal lenses or not. This is why the data are different among various studies when different columnar sections were sampled. Various lenses of geochemical abnormal which may be linked tectonic controlled diagenesis, and microhabitat et al., and resulted in the bias from different columnar sections were sampled.