Study of an Early Silurian–Early Permian paleo-weathering profile in Zunyi, north Guizhou Province, China

Abstract

To understand and help settle the controversy around the living time of Pinnatiramosus qianensis Geng, a paleo-weathering profile situated in the town of Yongle near the city of Zunyi, north Guizhou Province, China, was analyzed. The profile formed during a break in sedimentation between the Early Silurian and the Early Permian. Paleosol developed with a depth of several meters. The fossil plant P. qianensis Geng is present, but only in the lower portions (Layer 2) of the paleosol. Another plant with an irregularly branching system is found in Layers 2 and 3. The distinct geochemical characteristics of the lower and upper portions of the Gaojiayan paleosol indicate a compound genesis. Its lower portions (Layers 1 and 2) resulted from in situ weathering of silty mudstone of the lower Silurian Hanjiadian Formation. The upper portions (Layer 3) are allochthonous. Transgression brought substantial concentrations K and Na, and led to K- and Na-enrichment in the profile. Pumping of vascular plants and downward leaching enhanced the K enrichment in the middle portions. A superior preservation of P. qianensis Geng was observed in an exposure of Layer 2. Mass balance calculation indicates a great K enrichment related to bioaccumulation in the top of Layer 2 and a K loss in Layer 3. Fossil plants (e.g., P. qianensis Geng) preserved in the paleosol are Permian rooting systems growing down into the lower Silurian rocks.