DIATOM IN ZENGKOUHE BURIED OYSTER REEF, TIANJIN PLAIN AND ITS PALEOENVIRONMENTAL IMPLICATIONS

Diatom was used to reconstruct the palaoenvironment of the muddy sediment infilling and the muddy layers overlying Zengkouhe oyster reef along the section HD303, Tianjin plain. Three diatom zones, namely Ⅰ, Ⅱ and Ⅲ from bottom to top, have been identified in the oyster reef, corresponding to the environments of the middle-lower intertidal zone, the lower intertidal zone to the subtidal zone, and the upper-middle intertidal zone often influenced by storm surge or strong waves respectively. There are also three diatom zones, Ⅳ, Ⅴ and Ⅵ from the bottom to the top in the overlying muddy layers. Diatom assemblage indicates that they were deposited in the upper part of the intertidal zone of a lagoon, the intertidal zone near the mean high level of a spring tide and the supratidal zone successively. The influence of river on the overlying layers was weak and the salinity in the zones of Ⅳ and Ⅴ was higher than that required by the oyster reef. It is speculated that the closing of the lagoon and the reduction of river influence were the reason to stop the growth of the oyster reef. The shallower water depth made the reef environment more vulnerable to storm and/or strong wave. The reworked clastic layers consisting of shell fragments appeared in the upper part of the reef provided the evidence for the evolutionary history. And the mean water level of the spring tide could be as high as 1.3 m above the reef top then.