淹没不整合型碳酸盐三级旋回层序——兼论碳酸盐台地的“凝缩作用”

THE THIRD-ORDER CARBONATE CYCLIC SEQUENCES OF DROWNED UNCONFORMITY TYPE WITH DISCUSSIONS ON “CONDENSATION” OF CARBONATE PLATFORMS

关键词:  淹没不整合  旋回层序  凝缩作用

Key words:  drowned unconformity  cyclic sequence  condensation

摘要:

与层序地层学定义的类型Ⅰ层序(LST+TST+CS+HST)及类型Ⅱ层序(SMT+TST+CS+HST)不同,淹没不整合型碳酸盐三级旋回层序以凝缩段直接覆盖在层序界面之上形成的,"CS+HST"序列为特征,而且其底界面为一淹没不整合面或加深饥饿间断面。该类型三级旋回层序是碳酸盐台地淹没事件产生的凝缩怍用过程的产物,它以总体向上变浅的非对称相序为特征。
华北北部晚寒武世地层中发育的四个淹没不整合型三级旋回层序的相序特征表明,台地淹没事件及其相关的凝缩作用,只要沉积背景适合,并非三级旋回层序所独有,高频率海平面变化也常产生"瞬时淹没",因而凝缩段并非都发育在最大海侵期,并且凝缩段也存在级别归属问题。

Abstract:

Different from the type Ⅰ (LST+TST+CS+HST)and type Ⅰ sequences (SMT+ TST+CS+HST) which are defined by sequence stratigraphy,the third-order carbonate cyclic sequences of drowned unconformity type is marked by "CS-(-HST" successions in which a condensed section lies directily the sequence boundary,and the basal sequence boundary is a drowned unconformity surface or a deepened starved punctuated surface distinguished from the type Ⅰ and type Ⅱ sequence boundary created by the exposure-punctuation events.This type of carbonate cyclic sequences is the products of the condensation processes on the carbonate platforms that is produced by the platform drowning event,and is characterized by a generally asymmetrical upward-shallowing facies sequence.
The characteristics of the four Late Cambrian third-order carbonate cyclic sequences of drowned unconformity type in the northern part of North China indicate that the platform drowning events and concomitant condensation are not typical of third-order cyclic sequences,they often occur in the high-frequency sea-level changes.It follows that the condensed sections ought to belong to the cyclic sequences of various orders,which are not only a genetically composite unit of the third-order cyclic sequences.