雾迷山旋回层的费希尔图解及其在定义前寒武纪三级海平面变化中的应用

The Fischer Plots of Wumishan Cyclothems as Records of Third-Order Sea Level Changes in Precambrian

天津蓟县剖面的中元古界雾迷山组是一套厚 3 3 0 0m的碳酸盐岩地层 ,其中广泛发育叠层石生物层及凝块石生物丘。这些生物层及生物丘与潮坪相白云岩和砂泥质白云岩一起构成具有近似对称相序组构的环潮坪型碳酸盐米级旋回层序 ,将其命名为“雾迷山旋回层”。这些旋回层常由泻湖相白云质泥页岩覆盖 ,在其顶常发育古土壤层 ,由此而表明雾迷山旋回层的界面多为瞬时暴露间断面。根据雾迷山旋回层在长周期三级层序中的有序叠加形式 ,可在雾迷山组中识别出 2 6个Ⅲ级层序 ,它们还可进一步归为 6个Ⅱ级层序。为了进一步研究三级海平面的变化幅度和Ⅲ级层序的大致形成时限 ,在露头发育良好的雾迷山组中上部共识别出 62 6个旋回层 ;考虑到雾迷山组的总体厚度 ( 3 3 0 0m)及其大致的总体形成时限 ( 10 3± 2 0Ma左右 ) ,可假定雾迷山组沉积期的总体地壳沉降速率为 2～ 4m/ 10万a。在上述假定的基础上 ,运用费希尔图解来估算三级海平面的变化幅度。

The Middle Proterozoic Wumishan Formation(1310±20～1207±10 Ma) in Jixian County of Tianjin represents a succession of 3 300 m carbonate strata, in which stromatolite biostromes and thrombolite bioherms are best developed. These biostromes and bioherms, together with tidal-flat dolomites and sandy-muddy dolostones, form Wumishan cyclothems with an approximately symmetrical lithofacies succession, belonging to a particular type of peritidal carbonate meter-scale cyclic sequences. The lagoon muddy shales with the caps of paleosoil layer constitute the top of Wumishan cyclothems, and a lot of freshwater diagenetic marks were formed in tidal-flat dolomites, which suggests that the boundaries of Wumishan cyclothems are mostly instantaneously-exposed punctuated surfaces. In the light of the regular vertical stacking patterns in long-term third-order sequences, 26 third-order sequences that may be incorporated into 6 second-order sequences can be discerned in Wumishan Formation. In order to study the changing extent of the long-term sea-level, the authors recognized 626 cyclothems in the middle and upper part of Wumishan Formation in which the outcrops are well developed. Considering the total thickness of Wumishan Formation and the general duration of its formation, the authors hold that the subsidence rate of the crust in the sedimentary period of Wumishan Formation was probably 2～4 meters in each interval of 100 thousand years. On the basis of this assumption, the Fischer plot might be used to calculate the changing extent of the third-order sea level. According to the Fischer plot, some meaningful conclusions can be reached and many useful data can be obtained on sequence stratigraphy of Precambrian.