Contrasting Tectonic and Hydrodynamic Controls on the Infill of the Toukoshan Formation: A Seismic Stratigraphic Study Offshore Central Taiwan

Understanding the origin of high-frequency stratigraphic heterogeneity in active orogenic basins is essential for distinguishing the relative contributions of regional tectonics and local environmental forcings. In the offshore areas of central Taiwan, the Early Pleistocene to present Toukoshan Formation exhibits complex architectural variations that challenge singular tectonic interpretations. This study utilizes multichannel seismic reflection profiles and borehole data to dissect the evolutionary mechanisms driving these stratigraphic shifts. While the underlying Late Miocene to Early Pleistocene sequences exhibit architectural stability as well-stratified reflections, the Toukoshan Formation marks a transition to highly discontinuous geometries, reflecting a switch in dominant drivers toward localized hydrodynamic forcing. The lower Toukoshan Formation features co-existing parallel and progradational clinoform geometries, indicating significant lateral variations. These progradational structures are vertically overlain by continuous, sub-parallel reflections, recording a low-to-high-to-low energy transition. While tectonic subsidence typically produces laterally continuous stratigraphic geometries, the observed progradational sets in this study exhibit marked vertical and lateral discontinuities. This suggests that localized stratigraphic architecture is decoupled from the gradual tectonic trend, reflecting a switch in dominant drivers toward rapid hydrodynamic forcing. Such features likely record wave-driven sediment redistribution and the development of localized barrier complexes under high-energy conditions during relative sea-level fluctuations, rather than being a direct response to tectonic loading. Correlation of key time horizons across multiple seismic profiles reveals a southward migration of the depocenter within the Toukoshan Formation. This spatial pattern is consistent with the southward propagation of the orogenic belt and the resulting higher subsidence rates in the south as noted in previous studies, indicating that such regional-scale sediment redistribution is primarily governed by foreland basin subsidence. Our findings reveal a decoupling of stratigraphic drivers: while isopach maps confirm sustained tectonic control over regional accommodation, the internal architecture of the Toukoshan Formation marks a switch to localized hydrodynamic forcing. Wave-driven sediment supply and reworking overrides the tectonic signal, creating high-frequency heterogeneity and proving that even under active tectonics, environmental energy can be the primary sculptor of the depositional landscape.