天津蓟县雾迷山旋回层基本模式及其马尔柯夫链分析

津蓟县的中元古界雾迷山组，是一套碳酸盐岩地层，具近似对称相序组构的环潮坪型碳酸盐米级旋回层序特别以育。其近似对称的相序组构及普遍的1：4叠加形态，表明它们与短偏心率旋回具有成因关联，被命名为雾迷山旋回层来代表真正的碳酸盐沉积旋回。雾迷山旋回层是与高频率产面变化相关的环境加深及环境变浅过程的产物。马尔柯夫链分析的结果表明了雾迷山旋回层的基本相序模式是客观存在的，由于它们以瞬时暴露间断面为界，因而与Vail等（1977）的层序地层概念体系中所定义的“准层序”存在明显的差异。尽管难以断定前寒武纪地球轨道效应旋回的周期安全与显生宙一致，但是，七级韵律层、六级旋回层及五级准层序组之间的垂直叠置形态，以及由它们所指示的周期时限，与显生宙温室效应时期的轨道效应旋回周期是大体一致的。

The Basic Model of Wumishan Cyclothems and Their Markov Chain Analysis, Jixian Section of Tianjin

The Mesoproterozoic Wumishan Formation in Jixian section of Tianjin is composed of 3 300m thick carbonate strata with a duration of about 100Ma(1 310±20Ma-1 207±lOMa)．In this set of strata, particularly carbonate meter-scale cyclic sequences belonging to peritidal type with approximately symmetrical lithofacies-succession fabric are developed very wel1. The widespread development of superimposed patterns of 1:4 shows that these meter-scale cyclic sequences are genetically related to the short-eccentricity cycles, which are called figuratively Wumishan cyclothems and could represent true sedimentary cycles. Generally, massive and thick-bedded limedolostones and dolomitic limestones of stromatolite biostromes and thrombolte bioherms constitute the central part of Wumishan cyclothems, the bottom and upper part are consisted of tidal flat dolostones, the top part is composed of lagoon-facies dolomitic shales with a cap of paleosol layers. Therefore, the approximately symmetrical lithofacies-succession fabric is formed. Many features such as the clear deepening and shoaling vectors of cyelothems, and all kinds of fresh-water diagenesis marks suggest that the Wumisham cyclothems are the products ofautocyclic sedimentation governed by allocyclic high-frequency sea-level changes. The results of Markov chain analysis reaffirmed the objective existence of 1ithofacies-succession fabric of Wumishan cyclothems. The boundaries of Wumaishan cyclothems are marked by instantaneous exposed punctuated surface, which lead to the discrepancy between the cyclothem and the parasequerce of sequence stratigraphy terminology system defined by Vail et al. (1977). It is difficult to judge that the time-span of Milankovitch cycles in Precambrian is certainly equal to those of the Phanerozoic, but the regularly vertical stacking patterns of seventh-order rhythms, sixth-order cyclothems and fifth-order partasequence sets stil1 indicate their consistence with the period duration of Milankovitch cycles in the Phanerozoic.