Abstract

In order to evaluate the validity of Middle Miocene eustatic sea-level fluctuations on the Vail chart, coastal to shallow marine siliciclastic strata of northwestern Taiwan were selected for detailed facies and sequence analysis. The nature of these rocks can be well exemplified by the continuous sequences exposed in three stratigraphic sections, namely the Northern Coastal Highway, Chenfu, and Chuhuangkeng, from north to south. Based on lithology, sedimentary structures and fossil data, the depositional environments of these rock strata can be interpreted as a tide-influenced, wave-dominated shelf in which fourteen depositional facies can be recognized.

According to the stacking patterns of depositional facies, the sequence boundaries can be recognized by abrupt basinward-shifting of facies in shelf deposits and unconformities in coastal deposits. In shelf deposits, marine flooding surfaces are characterized by lag deposits, concentration of glauconite pellets and intensified bioturbation. Four parasequence types, each of which consists of a coarsening-upward facies succession, can be recognized in between marine flooding surfaces. The parasequences can be further stacked in aggradation-retrogradation-progradation successions which indicate the lowstand, transgressive and highstand systems tracts, respectively. In coastal deposits, marine flooding surfaces are identified by the erosion surfaces upon which subtidal sandstone facies scour into the underlying intertidal sand/mud interbeds. Another four parasequence types, each of which consists of a fining-upward succession, can be recognized. These parasequences can be stacked in aggradation-progradation successions which indicate transgressive and highstand systems tracts. On the basis of stacking patterns in systems tracts, the depositional facies successions of each section can be translated into relative sea-level changes.

Three biostratigraphically constrained maximum flooding surfaces can be correlated among the interpreted sequences in three outcrop sections. The relative sea-level changes of these sections are consistent with the third-order fluctuations on the Vail chart, except two minor discrepancies around 15.5 and 13.8 Ma. It appears that the Middle Miocene eustatic fluctuations are, in general, valid for Taiwan and had been a dominating factor in Miocene deposition.