潮坪层序的粒度特征与沉积相划分——以杭州湾庵东浅滩为例

GRAIN-SIZE CHARACTERISTICS AND THEIR APPLICATIONS TO THE INTERTIDAL SUBFACIES DIVISION: A CASE STUDY FROM ANDONG TIDAL FLATS IN THE HANGZHOU BAY

现代潮滩依据多年平均潮位线划分高、中、低潮滩,这对于古代潮坪沉积相研究无实际意义。本文尝试用潮坪层序粒度参数剖面来划分沉积相。潮坪沉积物以粉砂为主,但敏感地反映沉积动力变化的却是粗、细尾部分。因此,在沉积相划分时主要依据粘土、砂含量及平均粒径的变化特征。分选系数是一重要依据,但要考虑絮凝沉积的影响。偏态和峰态变幅小、较复杂,在相划分中只具参考价值。利用粒度参数剖面进行潮坪沉积相划分可广泛应用于古代潮坪层序研究,而且根据中潮坪沉积厚度(x)与平均潮差(y)存在的显著性相关关系:y=1 8314x+157 31(单位cm),推算古潮差。

Intertidal mudflats are subdivided into upper, middle and lower sections on the basis of long-term mean tidal levels, which however are unknown and cannot be applied to subfacies analyses on ancient tidal deposits. Vertical changes of grain-size parameters in each tidal-flat sequence were employed to define intertidal subfacies boundaries in this study. Although silt is prevalent in tidal-flat deposits, it is coarse and fine tails that change sharply in their percentages after sedimentary dynamic variations. Mean grain-size and sandy and clayey fractions are key parameters to intertidal subfacies analyses. Sorting coefficient is another important parameter for subfacies analysis, but for physical environmental interpretation, influence by fine-grain flocculation should be paid attention to. Changes of Skewness and Kurtosis are complex and have their limited value in subfacies division. The method, discriminating intertidal facies boundaries on the basis of vertical variations in grain-size parameters, is useful in the study of ancient tidal deposits where tidal data are inaccessible. An apparent linear relationship was examined between thickness (x) of the defined middle intertidal deposits and mean tidal ranges (y). The developed equation, y=1.8314+157.31 (x and y in cm), can be employed to estimate ancient tidal ranges from thickness of the middle intertidal deposits defined by a trend analysis on the grain-size parameters.