Sedimentology of a wave-dominated and tide-influenced deltaic system, upper Middle Miocene, southwestern Ulleung Basin, Korea

Abstract

Abstract Integrated sedimentological and ichnological study of the gas-bearing, upper Middle Miocene strata in the southwestern Ulleung Basin has identified five main lithofacies, arranged in order from offshore and offshore transitional environments to coastal-plain channel settings. In each of repeated successions, the basal mudstone-dominated, offshore to offshore transitional facies are sharply overlain by upper shoreface sandstones that are, in turn, erosively overlain by distributary channel sandstones. In the open-marine environment, interbedded storm and interstorm deposits are predominant with sporadic flood-event beds containing a navichnia trace-fossil suite. Bioturbation style in this facies tends to reflect depositional energy variations, alternating between dwelling structures in the storm beds and feeding and grazing trace-fossil assemblages in the interstorm beds. In the channel deposits, flood-interflood couplets are commonly expressed as cross-bedded, almost non-bioturbated sandstones that alternate with bioturbated, tidally-formed, interbedded sandstone and mudstone. From this, the co-existence of storm-wave and tide-generated structures in the studied interval suggests that the depositional environment is a wave-dominated and tide-influenced deltaic setting, with the recorded tidal influence restricted to the channels, which have a higher level of heterogeneity because of tidal sedimentation during periods of low river discharge. Stratigraphically, the 400-m thick, upper Middle Miocene section shows an overall, coarsening-upward succession, representing 3rd-order falling-stage to lowstand deposits as a result of long-term basinal uplift. This succession can, in turn, be broken into four, 4th-order stratigraphic packages, each of which comprises a transgressive (T)-regressive (R) succession that is separated by a transgressive surface. These packages can be further subdivided into small-scale, upward-coarsening successions (5th-order stratigraphic packages of parasequence scale). Each small-scale succession is sharply overlain by offshore mudstone which reflects a rapid vertical facies transition, representing a marine flooding surface. Internally, each parasequence is characterized by a progradational facies succession wherein offshore mudstones are erosively overlain by upper-shoreface sandstones, forming sharp-based shoreface deposits that are likely to be produced during a forced regression, an interpretation supported by the presence of an omission trace-fossil suite on the erosion surface. In this context, the high-frequency parasequences represent short-term relative sea-level falls superimposed on the longer-term falling sea level due to a compressional tectonic regime.