TERRESTRIAL MOLLUSK EVIDENCE FOR THE ORIGIN OF A LATE TERTIARY LOESS-PALEOSOL SEQUENCE AT QIN′AN IN THE WESTERN CHINESE LOESS PLATEAU
黄土高原西部秦安新近纪风尘堆积的蜗牛化石证据

In the Loess Plateau of Northern China, the Late Tertiary aeolian deposits are mainly composed of two parts in spatial distribution: the eastern Red Clay Formation (also called the Hipparion Red Clay Formation) and part of western Gansu Group, both being thought to contain much information about East Asian environmental changes. For a long time almost all of the Late Tertiary research work were from the eastern Red Clay Formation. The origin of the Red Clay Formation had long been controversial. Previous studies regarding the origin of the Red Clay Formation had many different conclusions such as “relic deposits”, “creep and rain wash deposits”, “eolian origin in the upper part and lacustrine origin in the lower part”, “with water influence, neither aeolian origin nor relic deposits”, “multiple origins with being chiefly of eolian origin”, and “aeolian origin”, etc. Systematic studies of the western Gansu Group were very scarce in the past. Recently, a Miocene aeolian sequence (also called Miocene loess-soil sequence) from Gansu Group at Qin′an was found. Sedimentological and geochemical evidence for its origin have been found. However, biological evidence, especially direct and continuous bio-record, is still scarce. In this study, a new continuous mollusk record of 310 samples from the Qin′an Miocene loess-soil sequence, the western Loess Plateau, is reported. 24 species of terrestrial mollusks were identified in the Qin′an loess-paleosol sequence. The preservation of mollusk fossil assemblages in the Qin′an section shows the original population. Most of mollusk fossil individuals maintain at an original living habit generally with carbonate shells and the characteristic of mixed mature and young shells. The imprint of suffered current transportation and selection has not been found in fossil populations. Results of mollusk fossil record from the studied section show that all of identifiable mollusk species reported here are composed of terrestrial taxa, such as Gastrocopta,Pupilla,Vallonia,Metodontia,Cathaica and Punctum, highly consistent with the well-known Quaternary loess-paleosol sequences. The terrestrial mollusk record supports the view that Gansu Group at Qin′an contains a loess-paleosol sequence, which has the same origin as the overlying Quaternary loess-paleosol sequence, both being of wind-blown origin. The further study of mollusk fossils can provide detailed and reliable history of palaeoenvironmental evolution during this geological period.