Re-inundation of the North China epeiric platform: A cyclic peritidal succession above a Lower-Middle Ordovician unconformity in southern Korea

초록

The depositional record of epeiric platforms is typically interrupted by unconformities, reflecting repetition of sedimentation and non-deposition. The Ordovician epeiric platform on the North China includes a Lower to Middle Ordovician unconformity, and this study documented early Darriwilian re-inundation events in the Makgol Formation from four sections over a distance of 25 km in the Taebaek area, eastern North China Craton. The Makgol Formation mainly comprises micritic limestone with subordinate laminite and grainy facies, exhibiting subaerial exposure features in its lower part, and is interpreted as a micrite-dominated inner platform deposit. The re-inundation pattern is characterized by accumulation of metre-scale peritidal and subtidal cycles, which show an overall deepening upward trend, marked by a decrease in exposure features and an increase in subtidal cycles. However, this deepening trend was not gradual and occurred stepwise. Seven cycle sets, each consisting of one to six cycles, are described along with eight correlated key flooding surfaces. Each flooding event caused retreat of the facies belt to varying degrees, while cycles within cycle sets exhibited shallowing upward stacking patterns. These deepening upward trends are similar to punctuated transgressions observed in continental shelves, which suggest the applicability of the sequence stratigraphic concept from continental margins to inner epeiric platforms. Lateral variations in the number of cycles between sections are recognised in seven intervals, four of which are interpreted as the result from erosion and non-deposition associated with subaerial exposure features. This finding suggests the incompleteness in transgressive deposits in inner epeiric platform conditions and highlights the importance of identifying missing cycles for the interpretation of stacking pattern and origin of metre-scale cycles.

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