甘肃张掖早白垩世彩丘中古土壤的判别、类型和序列特征

Paleosol recognition,pedotypes and paleosol development sequences in Zhangye colorful hills,Gansu Province

古土壤记录了丰富的古气候和古环境信息,然而古土壤受后期地质作用影响常不易被正确地识别。甘肃张掖彩丘是白垩纪早期发育的彩色地层,目前详细的地层序列尚未建立。张掖彩丘表现为红色、黄色、灰色、紫色等多种颜色交替变化,通常被认为是河湖相沉积或洪积相沉积,然而野外考察发现大量的古土壤特征。野外观察识别古土壤主要从根迹、土壤结构和土壤发生层等方面,主要表现为红色根迹或青灰色晕状根迹,半干旱环境下形成的钙质淀积层,土壤在干湿交替环境下形成的粘土滑擦面,土壤发生层之间的过渡边界。本文通过对张掖彩丘地层的系统考察,建立了较为详细的厚760 m的地层序列,根据古土壤的发育特征,识别出14种不同类型的古土壤,整个彩丘地层共计156个古土壤剖面,是独特的古土壤序列。14种古土壤类型与现代土壤类型对照,并与国际土壤分类对比,类比的现代土壤类型主要出现在南亚的亚热带或热带气候条件,说明白垩纪早期比现在更为温暖。根据古土壤发育程度序列,结合古土壤特征和地层特点,将古气候变化分成4个阶段,依次是湿润(0~268 m)、半干旱(268~550 m)、湿润(550~740 m)、干旱(740~760 m)阶段。张掖彩丘古土壤的系统识别和发育程度序列的建立为进一步古气候研究奠定基础。

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 114.0~120.2 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;(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.