Depositional Sequences and Storm Deposition on Low-energy Coast of China

Two different scale dipositional micro-and small sequences are distinguished to de essential componentsof the sequences of low-energy coast. Micro-sequences, with the thickness of mm scale, are mainly formedby tidevs, and can be distinguished as mud laminae and sand laminae. Small sequences are composed ofmicro--sequences and are generally several to 20cm thick. According to the relative ratio of mud and sandlaminaes inside, the small sequences can also be classified as fine (muddy) small-sequences and coarse(sandy) ones which are belived to be formed under fair weather and storm weather respectively, and can alsobe regarded as normal and event sequences respectively. Grain size and sand contents in both normal andevent layers increase from low tidal level both seaward (downward) to subtidal zone and upward (landward)to upper intertidal flat. Near the lower water level the sand contents in the normal layers can be higher thenthat of the event layers in the upper tidal flat, and the sands are also coarser. The differences in sand contentsand the. grain size between adjacent normal and event deposites are obvious. The seaward charges from thelower tidal flat to the wave base are similar to the landward changes that occured from the lower tidal flat tothe uwner tidal flat Comparision among the presen，subrecent and aneclent sectiuns in many areas demonstrated that the ba-sic sequences of the low-wergy coast of China is a sequence shat fines from lower tidal flat both upward tothe upper tidal deposits and downward to the wave base deposits. In the tidal flat where the tidal channels aredeveloped the tidal sequence fines only upward with a ubvions erosional surface at the bottom and withunderlying mud deposits, which is similar to the classic sequence described by Klein(1977).In the areaswhere river mouth sand bars and tidal sand ridges ace developed, thc; sequence also fines only upwards butwith underlying sand deposits. Adjacent coastal rocky hills or man-made dams can make the slope of the up-per tidlal Rat or supertidal steepen, which subsequently cause the sedunents coarser at the top of the low-en-ergy coast sequence. Shell layers can someUmes be found in the coarsened upper tidal deposits. Such sequencehad been detected in the Yangtae Delta and the Paleozoic tidal deposits in western Zhejiang Province.