Cyclic-Sequences of Composite Sea -Level change Developed in Zhangxia Formation of Middle-Cambrian in North-China

Key words: 
• cyclic-sequence /
• variation of composite sea-level /
• Zhangxia Formation /
• Middle Cambrian /

Abstract: Zhangxia Formation of the Middle Cambrian in North China is particularly famous for the well-de-veloped oolitic limestone.Many kinds of meter-scale cyclic-sequences, belonging to subtidal carbonatetype are discerned in Zhangxia Formation, and they are characteristic of "upward thickening of rock-bed" and "upward coarsening of sedimentary grain" as well as "upward shallowing of depositional enviroment".The stacking pattern of 1 ∶ 4 indicates that one meter-scale cyclic-sequence is genetically relatedto short-precession cycle of the Milankovitch cycle.In strata of Zhangxia Formation, various order cyclic-sequences and their regularly vertical stacking patterns constitute a typicall carbonate cyclic-sequence ofcomposite sea-level changes.On basis of the nature of the carbonate cyclic-sequence of composite sea-level changes developed inZhangxia Formation,it could be concluded as follows:① Cyclic-sequence ought to be a genetically relative facies-succession at first, the meaning of whichis that this facies-succession can reflect a process of relative sea-level changes.There are various ordersfor cyclic-sequences, which are genetically related to various periods of sea-level changes.② The surface of cyclic-sequence is a facies-changing surface that generally coexists with punctuatedsurface, which includes exposing punctuated surface and deepening-starve punctuated surface.③ In the research of cyclostratigraphy and sequeuce stratigraphy, especially the study of outcrop aswell as rock-core section, "lithology and lithofacies as well as facies-succession" are the key to discerncyclic-sequences, various order cyclic-sequences and their regularly vertical stacking patterns are thecore.④ Condensed section that is defined by stratigraphy, does not exist in the third-order cyclic-se-quence.In some depositional environment, there is also "condensed section" in the fourth-order cyclic-se-quence(subsequence).So it must be careful to pay attention to the belonging of coudensed section in var-ious order cyclic-sequences.⑤ There are two important factors for cyclic-sequences: regularity of facies-succession in space andsimultarity of environment change in time, both of which endow with particular meanings on stratigraphyand sedimentology to cyclic-sequences.