Recognition of cryptic bounding surfaces in macrotidal estuarine successions by multi-proxy based OSL-dated sequence stratigraphic analysis from SW Korea

High-resolution stratigraphic frameworks from modern coastal-to-shelf analogues extend our understanding of the stacking patterns variability associated with Milankovich-driven cyclothems, providing a stratigraphic basis for predicting reservoir heterogeneity. A fundamental key in sequence analysis, especially in the stratigraphic sections of a coastal zone, is the identification of time-stratigraphic bounding surfaces represented by unconformities. The boundary records a break in sedimentation and may juxtapose markedly different facies assemblages above and below the boundary. Major surfaces particularly in shallow-marine environments may be characterized by erosional truncation and draped by gravel lags, but not all sequence boundaries show these features. Here we consider invisible sequence boundary from SW Korea as a case study to discuss practical problems in mapping high-frequency sequence from a coastal-shelf region. We bring together OSL chronology, geochemical and micropaleontological tool to characterize facies-tract architectures of multi-sequential order sequences and to recognize bounding surfaces. We show that our multi-proxy based, OSL-dated sequence analysis reveals that the presence of fifth-order sequences nested within fourth-order systems tracts, which are difficult to distinguish in chirp seismic data taken in subtidal area. Moreover, the placement of bounding surface is equivocal because it lies on homogeneous mud facies with a lack of changes in faces—the fact that it represents a significant time gap between MIS 5a and MIS 3. Ultimately, facies criteria, while providing useful clues, are inadequate in mapping higher-frequency sequences with variables symmetry and thus leading to overlook complex and composite erosional time-break facies boundary.