长江三角洲泥质潮坪沉积间断的定量分析

QUANTITATIVE ANALYSES ON DIASTEMS OF THE MUDFLAT DEPOSITS IN THE YANGTZE RIVER DELTA

现场观测获得了长江口开敞型泥质潮坪不同时间尺度的沉积速率及层偶保存率。每天观测结果表明 ,单个层偶的保存率约为 46 .6 %。随着观测时间间隔的增大保存率迅速减小 ,大小潮周期内层偶保存率降为 9.2 % ,季节性观测层偶保存率为 3.7%。现代长江三角洲建造一个完整的潮坪沉积层序约需 96 a。百年尺度潮坪层序层偶的保存率仅为 0 .74% ,99.2 6 %的潮汐周期由于侵蚀或无沉积而没有纹层保存下来 ,形成间断。因此 ,沉积间断占潮坪层序的比例应为 99.2 6 %。沉积速率和沉积时间之间存在对数线性负相关 ,拟合直线的斜率为 - 0 .39,由此推导出潮坪层序的完整性计算公式为 C=(t*/ t) +0 .3 9。虽然对泥质潮坪沉积中沉积间断的估计有很大的不确定性 ,但它是从现代沉积过程观测中得到的 ,可以帮助我们更深入地认识古代潮坪沉积的特征及其成因 。

In situ observation has given preservation potentials and deposition rates of non barrier mudflats in the Yangtze River estuary over different time scales.The preservation potential of a single couplet is ca.46 6% on daily scale and decreases with time interval extending,about 9 2% on semi lunar scale and 3 7% on seasonal scale.It takes 96 years to build up a complete tidal flat sequence in the modern Yangtze River delta.Couplet preservation rate is 0 74% on centennial scale,and 99 26% of tidal cycles have no laminations left because of non deposition and erosion.Therefore,sedimentary hiatuses could have covered 99 26% of geologic time in the Yangtze River open tidal flat sequence.A negative log linear relationship is proved to exist between sedimentation rates and the time spans,and the slope of the fitting line is -0 39,so the completeness of the tidal flat sequence can be given by C=(t */t) +0 39 .Although it is uncertain how long sedimentary hiatuses lasted during mudflat deposition,the present results are given by observations of modern deposition and can gain some further insights into the nature and origin of ancient mudflat deposits with emphasis on the sedimentary discordances.