SEQUENCE STRATIGRAPHY OF THE MIDDLE ALBIAN LACUSTRINE SUCCESSION (JINJU FORMATION), SOUTHEASTERN KOREA

In many extensional basins, tectonism has been considered to primarily control the stratal patterns of sediments and the formation of sequences, but its relative importance can be minimized in non-marine closed basins. The Gyeongsang Basin was formed by extensional tectonism during the Early Cretaceous in the southeastern part of the Korean Peninsula and is filled with a non-marine deposit. In this study, facies analysis was performed on the outcrops of the over 1,000 m thick middle Albian lacustrine succession (the lower Jinju Formation) in the southern part of the basin to test the controlling factors for lacustrine sedimentation. Based on the facies transition, four stages of the stratal evolution associated with the lake-level changes are interpreted in the framework of sequence stratigraphy. The lowermost stage 1 starts with the deposition of shallow saline lake sediments with the distributary channel sandstones of low-sinuous braided fluvial style, indicating relatively low accommodation condition at the early lake-level rise (lowstand systems tract). Continuing lake-level rise led to the deposition of offshore lake and channel mouth bar deposits during the stage 2 (transgressive system tract). The following stage 3 is marked by the onset of several occasions of channel progradation. The development of channel sandstones of high-sinuous meandering fluvial style suggests relatively high accommodation condition during maximum lake-level (highstand systems tract). The relatively thin interval of stage 4 is characterized by the presence of playa-lake deposits and frequent occurrences of sedimentary features indicative of subaerial exposures and erosion, such as mudcracks, slikensided joints, and carbonate-intraclast gravels. (falling-stage systems tract).

The stratal evolution of the studied Jinju lacustrine succession seems to follow the marine sequence stratigraphic models, in response to gradual increase/decrease of accommodation space. The fluctuations of precipitation might be a possible cause of sequence development, rather than tectonic forcing such as faulting at the basin margins. Regional warm temperatures and semi-arid conditions might have constrained the lake-level below the basin-sill, and thus climate became the major controlling factor for Jinju succession development.