Penghu Fan and Gaoping Fan: The depositional systems developed during passive margin–active margin transition offshore SW Taiwan

Abstract Two submarine fan systems have been identified in the folded strata off SW Taiwan, named Penghu Fan and Gaoping Fan. These two submarine fans are characterized by different seismic expressions, revealing their different natures and properties. Penghu Fan is an elongate submarine fan consisting of multiple leveed channels up to 150 km long with limited sand deposits, interpreted as mud-rich system. Gaoping Fan has concentrated sand stacks with fine-grained overbanking deposits as well, which reveals a system of sandier content. The combined volume of the known part of these two submarine fans is nearly 1/3 of the volume of the present Taiwan island, which should be considered as a significant geological record. Based on the seismic stratigraphy observations, these two submarine fans were first deposited before folding and thrusting, indicating a passive margin setting, in front of the coeval deformation front. At some point, these submarine fan deposits started to be folded. The uplifting resulted in the incision of sediment pathways into the previous Penghu Fan and Gaoping Fan deposits and form the submarine canyons. On the anticlinal ridges, the deposition is limited. In the piggyback basins, clear growth strata mark the pre-growth and syn-growth units. The sediments deposited in piggyback basins are generally spilled from the canyons and interbedded with some local mass transport deposits from the tilted flanks. Three tectonostratigraphic units were identified representing the passive and active margin transition. Post-rift to early syn-orogenic passive margin deposition, syn-orogenic pre-growth orogenic sediment injection, and syn-orogenic syn-growth deposition and erosion on the fold-and-thrust belt. This study provides a stratigraphic framework of the frontal part of the orogenic margin. Further discussions have been made for their sediment dispersal systems, their initiation timing, and their responses to the mountain building. The identifications of the Penghu and Gaoping Fan systems and relevant depositional elements are remarkable, that is helpful for understanding the geological history of this region and the developments of the depositional systems during the passive margin-active margin transition.