Devonian-Carboniferous paleosols and tectonics of northwest China

Abstract

Paleosols in the Late Devonian (Famennian) Shaliushui and Qianheishan formations near Pingchuan, in northeast Gansu, China, are characterized petrographically and chemically to reconstruct paleoenvironments early in the evolution of forests on Earth. The Devonian paleosols, analogous to Neogene paleosols in the Lesser Himalaya of India, formed within the outwash plain of an extremely high (5–8 km elevation) mountain range on the North Qilian Block resulting from continent–continent collision in North China. Such high paleoaltitude is evidenced by Devonian paleosols exhibiting unusually thick horizons of subsurface calcareous nodules, as evidence of inferred mean annual precipitation range of 107 ± 22 mm. These conditions are comparable to those observed in paleosols from India, where monsoonal seasonality is driven by extreme elevation differences. Depth to Bk (calcic horizon) corrected for burial compaction of the paleosols reveals semiarid mean annual precipitation (MAP) of 361 ± 147 mm, with some subhumid outliers of 563 ± 147 mm. Chemical composition of the paleosols is evidence of subhumid (MAP 572 ± 182 mm, but up to 936 ± 182 mm) and temperate climates (MAT 12.5 ± 4.4 °C, but up to 14.9 ± 4.4 °C). Such cool temperatures at low paleolatitude (5-20°) are additional evidence of high elevation of at least 1 km. The vegetation consisted of Devonian pteridophytic progymnosperms, evidenced by rare foliar spurs and common woody root traces.