西藏南部中白垩世旋回地层学

Cyclostratigraphy of Mid-Cretaceous in Southern Tibet

根据旋回特征可把西藏南部的中白垩纪地层分为四个等级。第一级旋回主要是大套的碎屑岩与碳酸盐岩的交替，以及生物种类的变化，表现了当时海平面周期性变化。第二级旋回代表了古生产率和氧化还原的韵律性变化，通过对ＣａＣＯ３含量的ＡＲ—ＦＦＴ处理得出的周期（ＺＭａ）与Ｂｅｒｇｅｒ等人所计算的理论周期基本相符。因此，可以认为这种旋回根本原因是因轨道参数变化所引起的古环流改变所造成，如地中海型环流与河口型环流的互换，必将导致古生产率以及海洋水体性质的改变。第三级旋回表现为层偶的变化，如泥岩与灰岩互层以及黑色页岩与灰岩互层，在本区主要有三个周期：４．３、５．２和９．２万年，它们分别相当于斜率（ε）和偏心率（Ｃ）的理论周期值，旋回的周期主要是因为气候的变化引起陆源物质的稀释作用发生周期性变化，气候的周期性变化是由于轨道参数的改变而引起，因此，这种族回具有全球的可对比性。第四级旋回是由于海水里季节性变化所引起的纹层状层理。

Cycles of mid-cretuceous in southern Tibet are divided into four ciders by cyclic characteristics. The first order is shown in the up-and-down of sea level. The second order is represented by pelaeoproductivity and redo and five cycles of which periods (2Ma) obtained by the methed of AR-FFT corresPOnding in theoretically orbital parameter (2. 03Ma) Caculated by Berser (1988) are due to rhythmical changes of polaeocurrent. The third cycle is represented by con pletS (Limestone / Marl and black shale / limestone ), and three im portan t periods of cycles are 43,52 and 92 thousand years respectively, which corresponds to the periods of obliquity and eccentricity and were origined from palaeoproduotivity and dilution by climatic variations which are resulted from the changes of orbital paameters. The fourth order cycle is represented by fine horizontal laminations caused by seasonal variations.