LATE QUATERNARY EUSTATIC AND CLIMATIC RECORDS OF THE TAIPEI ESTUARY, TAIWAN

Estuaries contain continuous Holocene sediment records that usually provide important repertories of past eustatic and climatic information. During the lowstands, however, estuarine deposits are prone to erosion such that pre-Holocene sediment records are often obliterated and become hard to interpret. Only in rapidly subsiding estuaries like the Taipei Basin of northern Taiwan could early sediment records be preserved and reveal past environmental responses to varying climate and eustacy. The Taipei Basin is a filled estuary that accumulated up to 350-m-thick siliciclastic sediment in the past 200 ky. Near the depocenter in the northwest Taipei Basin, the sediment comprises two unconformity-bounded sequences, each of which consists of an aggradational alluvial fan gravel, a retrogradational fluvio-estuarine sand/mud and a progradational fluvial sand/mud. The aggradational and progradational successions were deposited in cold and warm stages shown by pollen assemblages, and the retrogradational successions during the cold-warm transitions. These successions can be tied to the deep-sea oxygen isotope curve through palynostratigraphy and radiometric datings and correlated with glaciation-modulated eustatic cycles. The result shows that the aggradational, retrogradational, and progradational successions were accumulated during the lowstands, transgressions, and highstands respectively. Outside the depocenter, the sediment becomes thinner, the transgressive systems tracts are less well developed and the highstand systems tracts often absent. The sediment is packed with amalgamated alluvial gravel and sand of the lowstand systems tracts. These sediment records show that the Taipei Basin was subjected to intense erosion during the sea level fall and only began to accumulate alluvial fan gravels during the late lowstand. During the sea level rise, the Basin was inundated as an estuary and filled with retrogradational fluvial and estuarine sediments. As the sea level neared the highstand, fluvial sediment prograded into the Basin and filled it up as a floodplain. In view of the Taipei Basin stratigraphy, pre-Holocene deposits tend to be strewn with hiatuses even in rapidly subsiding estuaries, and the sediment record is inevitably biased toward cold stages and lowstands.