210 / 2019-06-20 10:15:30

Early Cretaceous paleosols in Zhangye Danxia National Geopark: paleosol identification, pedotypes and its paleoclimatic implication

Zhangye colorful hills; paleosols; root traces; Early Cretaceous

Fossil soils (paleosols) document significant records of paleoenvironments and paleoclimates they formed. However, pre-Quaternary paleosols are sometimes not correctly recognized due to post digenesis and lack of diagnostic features. Early Cretaceous colorful hills in Zhangye Danxia national geopark (38.97436 N, 100.02969 E) in Gansu Province is well-known for its brilliantly colored landscapes with alternative colors of red, green, yellow, purple, brown, and gray. Zhangye colorful hills include Xiagou Formation and Zhonggou Formation of Early Cretaceous, where Xiagou Formation was constrained in 114-120.2 Ma. Although it was previously studied with respect of landforms and ages, the entire stratigraphy (760 m) and age of these special strata is not reported yet. Based on sedimentary beddings and sediment structures, the colorful hills were previously regarded as lacustrine or alluvial-fluvial sediments. But, massive pedogenic features, such as root traces, pedogenic calcite nodules, are observed in field, indicating lots of paleosol sequences. The occurrence of paleosols indicates sediment incompleteness or unconformity. In this study, based on field observations and mapping, we report: (1) 760-m-thick stratigraphy of Zhangye strata which is dominated by silt with small amount of sand and gravel, (2) Paleosols were recognized in field by root traces, soil horizons, calcareous nodules, soil structures, indicating paleosol sequences rather than alluvial-fluvial sediments. (3) 156 paleosol sequences of 14 different pedotypes were identified in field, and (4) paleoclimate stages indicated by paleosol analogues to modern soils (USDA, FAO-Unesco). Most pedotypes found in Zhangye colorful hills are analogous to modern soils in South Asia, indicating warmer paleoclimate in Early Cretaceous than present. Paleosol development patterns display four stages of paleoclimate and sediment sequence completeness: 0~268 m (humid, weak development paleosols, good sequence completeness), 268~550 m (arid, moderate development paleosols, bad sequence completeness), 550~740 m (humid, weak development paleosols, good sequence completeness). Paleosol recognition and paleosol development sequence are significant for further paleoclimate reconstruction.