长江三峡地区泥盆纪层序地层及海平面变化

DEVONIAN SEQUENCE STRATIGRAPHY AND SEA LEVEL CHANGES IN THE YANGTZE GORGES AREA

该区泥盆纪地层仅发育中泥盆世晚期云台观组,晚泥盆世早期黄家磴组和晚期写经寺组。在岩石地层、生物地层和沉积特征总结的基础上,根据对准层序组的研究,分别于云台观组近顶部、黄家磴组中部和近顶部、写经寺组中段的顶部识别出四个Ⅱ型界面。据此,将长江三峡地区泥盆纪地层划分为一个Ⅰ型层序（SQ_1）和三个Ⅱ型层序（SQ_2、SQ_3、SQ_4）,其中SQ_1、SQ_3、SQ_4、由TST和HST组成,海侵面与层序界面重合名;SQ_2由SST、TST和HST组成。系统研究了各层序的发育特征和演化过程,讨论了海平面的变化特点,将海平面变化分为四个T-R旋回,它们共同构成一个高级次的海平面升降旋回。

There are only Yuntaiguan formation of the late stage (Givetian Stage) of Middle Devonian, Huangjiadeng formation of the early stage (Frasnian Stage) and Xiejingsi Formation of the late stage (Famennian Stage) of the Late Devonian to the Tournaisian Stage of the Early Carboniferous in the Yangtze Gorges area. Based on the overall summaries of the lithostratigraphy, sedimentary features and biostratigraphy, by means of the integrated analysis of the Parasequence beds, four type - II sequence Boundaries (SB2) are distinguished in near top of the Yuntaiguan Formation, the middle and the near top of the Huangjiadeng Formation, and the top of the middle section of the Xiejingsi Formation. So the Devonian Stratum in this area is subdivided into four third-order sequences, i.e., one type -Ⅰ depositional sequence (SQ1), and three type -Ⅱ ones (SQ2, SQ3 and SQ4). The transgressive surfaces (TS) of the SQ1, SQ3 and SQ4 are consistent with their sequence boundaries (SB). And the SB of the SQ1, which is the interface between YuntaiguanFormation and the shamao Formation and is a regional parallel unconformity type and is resulted from tectonization, is type - Ⅰ sequence boundary (SB1). And the four SB of others are conformity types and are type - Ⅱ sequence boundary (8B2), SQ1, SQ3 and SQ4 are composed of the transgressive system tracts (TST) and Highstand system tracts (HST), and the TS of the SQ4 is consistent with the chronostratigraphic boundary between the Frasnian and the Famennian. The SQ2 is composed of the shelf margin system tracts (SST), TST and HST, its TS is identical with the lithostratigraphic boundary between the Yuntaiguan and the Huangjiadeng Formation, and the chronostratigraphic boundary between the Givetian and the Frasnian. Authors systematically studied structural pattern and characteristics of the depositional sequences and development. In addition, this paper discussed the sea-level changes, which are divided into four transgression -regression (T- R) cycles. Altogether, they form one higher-grade cycle, And the scale of regression is the largest in the fourth T-R cycle, and it coincides with the event of the eustatic fall of sea level between the Devonian and the Carboniferous. And there are two structures of the T- R cycle which are fast transgression to slow regression and fast transgression to fast regression.