HIGH-RESOLUTION SEQUENCE STRATIGRAPHY FOR THE JIGUNSAN FORMATION

Delineation of sequence stratigraphic key surfaces is important. However, fine-grained sedimentary rocks are formed generally homogeneously so that there is no critical lithologic changes implying stacking patterns and also those key surfaces can be made differently if there are carbonate successions. Consequently, it makes us hard to find sequence stratigraphic key surfaces reflecting relative sea-level changes in those outcrops. In this study, high-resolution sequence stratigraphic framework for the Jigunsan Formation is constructed by detailed outcrop description coupled with geochemical and mineralogical analysis including TOC, XRD, and XRF.

Jigunsan Formation consists of mudstones and siltstones as background deposits and limestones as interbedded deposits, showing mixed succession of carbonate and siliciclastic deposits. Through previous studies, It has been interpreted as TST (Transgressive Systems Tracts) deposit from the uppermost in the Makgol formation (under the Jigunsan Formation) which has Sauk-Tippecanoe boundary which is a transgressive surface in 2nd or 3rd order scale. Middle to upper part of the Jigunsan formation is widely accepted as HST (Highstand Systems Tracts) deposit in also same order scale. However, there are inherent sequence stratigraphic surfaces for the extent of high order scale. This study focuses on finding those inherent key horizons at stratigraphic level and defining their systems tracts of the Jigunsan Formation.

Integrated analysis including detailed outcrop description, geochemical, and mineralogical date suggests that it can be subdivided into early and late stage of transgressive systems tract and subdivided into lower and upper part of highstand systems tracts. Late TST starts with rapid back-stepping surface and high peak of TOC value following gradual decreases upward and ends with maximum flooding surface. Lower and upper of HST is divided by the lithological transition zone which changes in matrix grain size, rock color, nodule shape, and pyrite proportion. As a consequence of this study, it is expected that we understand better on the source rock in the mixed carbonate siliciclastic successions for hydrocarbon within high-resolution stratigraphic framework especially in unconventional oil field like shale gas and oil shale.