MULTI-PROXY EVIDENCE FOR RAPIDLY SHIFTING SEDIMENT SOURCES TO THE TAIWAN WESTERN FORELAND BASIN AT THE MIOCENE-PLIOCENE TRANSITION

Collision between the Philippine Sea Plate and the Eurasian Plate in the late Miocene-early Pliocene resulted in the uplift of the Taiwan orogen, and lithospheric flexure to the west formed the adjacent Western Foreland Basin (WFB). Based on petrographic studies of late Miocene to Recent sedimentary strata, the WFB fill is thought to have been sourced mainly from Eurasia until the late Pliocene-early Pleistocene, after which, Taiwan became the dominant source. However, uplift of Taiwan began significantly earlier, and sediment derived from the island should reflect its onset and acceleration of uplift and subsequent erosion.

To resolve the timing of changes in sediment sources in the WFB, we performed a multi-proxy analysis using clay mineralogy, δ¹³C_org and C/N of organic matter, and mass-specific magnetic susceptibility of late Miocene-early Pliocene strata of the Kueichulin Formation in Taiwan. Clay mineralogy shows an upward increase in illite and illite crystallinity, and a decrease in chlorite and kaolinite after the late Miocene, which is attributed to the rapid erosion of Taiwan. Measurements of δ¹³C_org and C/N ratios show evidence of Taiwan-derived sediment in the early Pliocene, where there is a marked change from dominantly marine- to dominantly terrestrially sourced organic matter. Finally, a rapid decrease in magnetic susceptibility during the early Pliocene indicates a dilution of magnetic minerals deposited in the WFB by the high flux of non-magnetic minerals delivered from Taiwan. Together, these datasets record a major shift in sediment source to the WFB during the late Miocene to early Pliocene, and that Taiwan became the dominant source by the early Pliocene, approximately two million years earlier than previously thought. In addition, because the morphology, tectonic setting, and evolution of Taiwan has remained largely unchanged since the late Pliocene, the processes that eroded and delivered sediment from Taiwan to the WFB are likely similar to the present-day, where sediment is delivered to the Taiwan Strait largely during tropical cyclones.