Geochemistry of the trace elements and rare-earth elements at the boundary between Cambrian Series 2 and Series 3 at Jianshan, South China: Paleoenvironmental and stratigraphic implications

Abstract

The Jianshan section in Jianhe County, Guizhou Province, is a very important auxiliary section to the candidate stratotype at the boundary between Cambrian Series 2 and Series 3. Twenty four samples collected from the Jianshan section of the Kaili Formation were analyzed for the concentrations of trace element and rare-earth elements (REEs) by inductively coupled plasma mass spectrometry. The trace element and REE geochemical characteristics of the Kaili Formation at the Jianshan section show that trace element composition and REE distribution patterns across the section are covariant. Notably, at 44 m above the base of the Kaili Formation, most trace element contents and REEs concentrations are lower than those values observed below 44 m. Above 44 m, an increasing trend in the concentration of both trace elements and REEs is observed. These geochemical data indicate important, but subtle, changes at the 44 m horizon. If this is, in fact, verified by work in progress, geochemical criteria can provide additional supports for defining the top surface of Cambrian Series 2, especially in sections where Oryctocephalus indicus is absent. Trace element ratios, specifically Ni/Co, V/Cr, V/(Ni+V) and Th/U as well as Ce and Eu anomalies indicate that the sedimentary environment of the Kaili Formation at the Jianshan section was oxidizing, that benthic redox conditions were less oxic, and may have been influenced by freshwater as compared to the Balang area. Moreover, the Eu anomalies observed in the Kaili Formation are similar to the negative Eu anomalies observed in post-Archean sedimentary rocks, and reveal fluctuations in oxygen content from the bottom to the top of the Kaili Formation.