Abstract
Owing to remarkable facies changes, the definition of transgression-regression cycles in Oligocene to Miocene silisiclastic strata in northern Taiwan is often problematic, and highly changeable from place to place. In order to decipher detailed relative sea-level variations in these coastal to shallow marine deposits, we preliminarily apply sequence stratigraphic analysis to their outcrops in three northern coastal sections of Tawulungao, Batoutz and Pitouchiao. In a composite stratigraphic section, these strata, over 3,400 meters in thickness, evidently exhibit accumulation in a shallow shelf bordered by deltas and tidal flats. They are classifiable into eight facies associations, namely, the outer and inner shelves, shoreface, delta front, delta plain, subtidal, intertidal and supratidal flats. In shelf deposits, sequence boundaries are characterized by the sharp-based shoreface sandstones overlying inner/outter shelf deposits, while in coastal deposits, they are associated with paleosols and characterized by facies changes from the delta from to delta plain, or from the intertidal to supratidal flats. Nineteen Sequences are divided in the composite section. In each of them, lowstand, transgressive and highstand systems tracts and recognized by stacking patterns of aggradation, retrogradation and progradation in facies succession, respectively. Constrained by a chronostratigraphic framework delineated by biozone boundaries of P19-20/21-22, P21-22/N4, N8/9 and NN11/12, a sequence stratigraphic template is established as; 6 Oligocene Sequences, 6 Early Miocene and 6 Middle-Late Miocene ones (O-1~6, EM-1~6 and MLM-1~6, respectively). According to the sequence boundaries and stacking patterns of systems tracts, relative sea-level changes in each Sequence are divided into abrupt fall, gentle rise, significant rise and gradual fall. Thus, the facies succession of the studied sections are translated into 18 T-R cycles, in which LST of SQ MLM-2 is not preserved. Compared with the Vail's chart, the synthesized relative sea-level curve is generally concordant with its 3rd-order fluctuations, indicating that the Oligocene-Miocene sedimentation in the northern coastal area was influenced dominantly by the global eustacy.