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

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.