峡东地区成冰系南沱组新认识

前人普遍认为峡东地区成冰系仅发育南沱组，缺失古城冰期和大塘坡间冰期相应地层。笔者发现峡东地区青林口剖面原先划定的南沱组包含上、下两套冰碛岩及之间的页岩夹含锰灰岩，应解体为古城组、大塘坡组和南沱组。本文在剖面实测的基础上，对比了青林口剖面含锰灰岩与长阳古城村剖面菱锰矿层的岩石学特征和稀土元素特征，分析了CIA、CIW和PIA等化学风化强度指标的垂向演化。结果表明，青林口剖面和古城村剖面成冰系均由两套冰期地层夹一套间冰期地层组成；青林口剖面大塘坡组含锰灰岩与古城村剖面大塘坡组菱锰矿具有相似的岩石学和稀土元素特征，指示两者应为同时期地质作用的产物；青林口剖面与古城村剖面大塘坡组在形成过程中所遭受的化学风化强度显著高于冰碛岩地层。因此，峡东青林口剖面成冰系包含成冰纪古城冰期、大塘坡间冰期及南沱冰期的沉积记录，为完整的成冰纪地层序列。

New insights into the Cryogenian Nantuo Formation in the East Yangtze Gorges area

It is generally believed that the Cryogenian Nantuo Formation in the East Yangtze Gorges area is only the record of Nantuo Glaciation as the deposits of Gucheng Glaciation and Datangpo interglaciation are absent. This study found that the primary Nantuo Formation at Qinglinkou section contains two sets of tillite strata and one set of shale strata intercalated with manganiferous limestone between them, which should be divided into the Gucheng Formation, the Datangpo Formation and the Nantuo Formation. Based on the comparison of field measurements, this paper compared the microscopic and REE characteristics of the manganiferous limestone from the Qinglinkou section and the rhodochrosite from the Guchengcun section, and the vertical evolution of chemical weathering indexes such as CIA, CIW and PIA of the Qinglinkou and the Guchengcun sections. The results show that both the sections consist of two sets of glacial strata and one set of interglacial strata. Also, the manganiferous limestone from the Qinglinkou and the rhodochrosite from Guchengcun sections show similar microscopic and REE characteristics indicating that these could be the product of geological action in the same period. Besides, compared to the moraine strata, the Datangpo Formation of Qinglinkou section and Guchengcun section suffered stronger chemical weathering. Accordingly, the strata of Cryogenian at the Qinglinkou section is a complete stratigraphic sequence, which contains the deposits from the Gucheng Glaciation to the Nantuo Glaciation.