The Taiwan Western Foreland Basin is thought traditionally to have received sediment mainly from Eurasia until the late Pliocene–early Pleistocene, after which time, the Taiwan orogen became the dominant source. However, a combination of clay mineralogy, δ13Corg and C/N of organic matter, and mass-specific magnetic susceptibility of late Miocene to early Pliocene strata of the Kueichulin Formation indicate that onset of major sediment contributions from Taiwan occurred much earlier, and correlates closely to the uplift and initial emergence of the Taiwan orogen. Clay mineralogy shows an upsection increase in illite and illite crystallinity, and a decrease in chlorite and kaolinite after the late Miocene, and this is attributed to rapid erosion of the Taiwan orogen. Results from δ13Corg and C/N analyses show that organic material in the Kueichulin Formation changed from dominantly marine to dominantly terrestrial in the early Pliocene, and this is linked to the delivery of large quantities of terrestrial organic material from the Taiwan orogen to the adjacent Taiwan Strait. Magnetic susceptibility also decreases significantly during the early Pliocene, resulting from dilution of magnetic minerals through the influx of non-magnetic minerals delivered from the Taiwan orogenic belt. The establishment of the growing Taiwan orogen as a major sediment source to the Western Foreland Basin occurred at the Miocene–Pliocene transition, about two million years earlier than previously recognized.