Storm-flood-dominated delta succession in the Pleistocene Taiwan Strait

Abstract

Storm-flood-dominated deltas are sedimentary systems in which a complex interplay of hydrodynamic processes occurs during storms (e.g. tropical cyclones) due to the coeval action of continental and oceanic processes. This paper reports on a superbly exposed, 135.5 m thick stratigraphic succession of the Pleistocene Cholan Formation exposed along the Da'an River, Taiwan. The sedimentary succession comprises alternating mudstone and sandstone, is mostly fine-grained, and exhibits multiple event beds that record deposition during tropical cyclones and post-depositional deformation features produced during earthquakes. Detailed facies analyses reveal that deposition towards the base of the succession occurred in the palaeo-Taiwan Strait in storm-flood-dominated prodelta and delta-front environments passing upwards into delta-plain environments. Tropical cyclone beds are encountered throughout the subaqueous storm-flood delta successions, and are identified by (i) trough cross-stratified sandstone bedsets with erosive bases that contain both mud clasts and mudstone beds, (ii) sandstone with aggrading wave ripples and (iii) hummocky cross-stratified sandstone with rare gutter casts filled with coal fragments and shell remains. Tropical cyclone deposits are either top-down burrowed or capped by massive or laminated mudstone. Seismites are rare and are mainly recognised through soft-sediment deformation of beds; they do not show evidence of slope failure. Compared to storm-flood delta successions described elsewhere, the Cholan Formation shows significantly fewer oscillatory-generated sedimentary structures and gutter casts. This difference is attributed to the Cholan Formation being deposited in and along the margin of a strait characterised by strong shore-parallel currents and relatively small storm waves due to its position between Taiwan and mainland China. This study refines depositional process interpretations of the Cholan Formation, provides criteria for recognising storm-flood delta deposits in tectonically active straits with multiple sediment sources fed by steep drainages and short river catchments, and provides additional criteria for recognising tropical cyclone deposits in shallow-marine settings.