Morphologic and Hydrodynamic Controls on the Occurrence of Tidal Bundles in an Open-Coast Macrotidal Environment, Northern Gyeonggi Bay, West Coast of Korea

Abstract

Tidal dunes with well-defined rhythmic tidal bundles are documented from the lower intertidal zone of an open-coast macrotidal environment in Gyeonggi Bay, Korea. Based on combined dataset of morphology, sedimentology and hydrodynamics of the dunes, this study aims to characterize factors that govern the temporal and spatial variability of the tidal bundles in a non-barred, unconfined macrotidal environment. Tidal dunes are flood-asymmetric with long wavelength (10–20 m) and small ebb caps on the upper bank, and symmetric to slightly ebb-asymmetric with relatively short wavelength (5–10 m) and larger ebb caps on the lower bank. The upper-bank dunes are characterized by more steeply dipping flood-directed tabular cross beds and thinner mud drapes than the lower-bank dunes. Each tidal bundle contains single mud drape that is stratified to cross-stratified, rich in silts and very fine sands and overlies the ebb-directed ripples, representing a dynamic mud deposition during ebb tide. The presence of strong rotary currents (up to 0.25 m/s) and low suspended-sediment concentration of flood currents prevent deposition of mud drapes during the high-tide slack-water period. Distinct peak velocity asymmetry (up to 1 m) resulted in the preferential preservation of flood-directed cross beds in the lower intertidal zone, where ebb currents are stronger than flood currents but have a shorter duration that is insufficient to reverse a dune profile. Pronounced time-velocity asymmetry at higher elevation combined with distinct peak velocity asymmetry leads to the better preservation of hierarchical tidal cycles in the upper-bank dunes. The present study suggests that the persistent occurrence of single, stratified to cross-stratified mud drape that reflects dynamic mud deposition during ebb currents, and the dominance of flood-directed cross beds are diagnostic features of tidal bundles formed in the intertidal zone of an unbarred, open-coast macrotidal environment. A proposed model for mud drape deposition provides new perspective on the origin of tidal bundles and useful criteria for reconstructing paleo-depositional environment.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017