Abstract
In order to better understand the sea-level fluctuations of the middle to upper Miocene strata in northern Taiwan, sequence stratigraphic analysis is applied to three stratigraphic sections, namely the North Coast, Chenfu and Chuhuangkeng. The chronostratigraphic framework of these 'wave- and tide-dominated shoreline' siliclastic strata can be established by integrating the available micropalaeontologic data. In these sections, parasequences can be divided into facies successions of 5-20m thick. Each parasequence is bounded by marine flooding surfaces which are characterized by intensive bioturbation, shelly lags, or erosion surfaces. Sequence boundaries can be identified by the sharp-based, shoreface sandstones overlying offshore interbeds, and by the top of upward-shoaling facies successions from 'subtidal sandstones' to 'intertidal interbeds' which are associated with mud cracks, rootlets or paleosols. These features represent the abrupt basinward shifting in facies characteristics and unconformities which display the large-scale falls of relative sealevel. Ten, eight and 10 sequences are divided by sequence boundaries in the three studied sections respectively. Three sequence types are recognized by different facies succession. In shelf or coastal sequences, parasequences can be stacked in the aggradational-progradation, retrogradation and progradation successions which represnt lowstand, transgressive and highstand systems tracts, respectively. A complete, large-scale transgression-regression cycle of relative sea-level is displayed by the sequence boundary and systems tracts in each sequence. However, the parasequences in the coastal sequences, which directly overlie shelf deposits, can only be stacked in the aggradation and progradation succession which represent transgressive and highstand systems tracts. An incomplete, large-scale transgression-regression cycle of relative sea-level is implied by the missing of lowstand systems tracts. Constrained by the chronostratigraphic framework, the sequences of three sections appear to be mutually consistent. The sequence stratigraphy of the middle to upper Miocene is established and provides a new scheme of nine correlatable units. Furthermore, the derived relative sea-level changes of the studied sections fluctuate with the 3rd-order eustatic curves on the Vail's chart with only two discrepancies around 15.5 and 13.8 Ma. Consequently, it is inferred that sedimentation of the middle to upper Miocene was dominated by eustacy in northern Taiwan.