Abstract: The No. 47 borehole is located at Jiangling County in the Jianghan Plain, Hubei Province, China. There are abundant sedimentary diatoms (about 26 genera and 148 taxa) in the upper section (46.6 – 2.2 m) of the borehole. Most of them belong to the existent freshwater diatoms, being mainly epiphytic, littoral and shallow-water-inhabitant species. Among them some gerera are rich in taxa such as Navicula (28 taxa), Cymbella (23 taxa),Achnanthes (15 taxa), Gomphonema (14 taxa) and Eunotia (13 taxa). The dominant species are Gomphonema tropic& var. nonpunctatum, Cyclotella comta, Cocconeis placentula var. lineata and Stephanodiscus dubius, etc. From the lower to the upper section of the sediment, species number (Sp./strat. ) and amount ( Ind./g dry sed. ) of the diatoms underwent a change from scantiness to plenty, then gradually decline to final disappearance. The change of the diatoms obviously reflects a paleoenvironmental change of water bodies in this area, i. e. :at the early stage, this area was possibly a river (might have been an obsolete course of the Yangtze River), then it gradually became a side bank in the river, which later turned into a depression retaining water perphaps in the form of a lake or a swamp in a flood plain. Then it gradually became deepened and widened with final disappearance of the retained water. According to the diatom assemblages in different sedimentary phases, it may be inferred that the pH range in these water bodies were basically stable, ranging from neutral (pH = 7) to slightly alkaline (pH > 7). Combining pollen amdysis (mainly the pollens of Pinus and Betula ) and mineral analysis with diatoms,it might be deduced that the paleoclimate of this area during the period of luxuriant growth of diatoms was somewhat colder than the present time. In the sediment, the amount and species number of the diatoms all significantly correlated with the mean grain size of the sediment.

Key words: 江汉平原, 化石硅藻, 古环境