| Summary: | 博士 === 國立臺灣大學 === 地質科學研究所 === 89 === Owing to ever-changing coastal to shallow marine environments, facies variations are drastic in the Oligocene-Miocene strata in western Taiwan. Although lithostratigraphy and biostratigraphy have been adjuested from sections to sections to deal with the facies changes. However the stratigraphic resolution and correlative precision remain inadequate due to unpunctuality of lithostratigraphic surfaces and discontinuity of fossil contents. Since sequence stratigraphy has been successful in unraveling the relative sealevel fluctuations which in turn play a predominant role in environment changes. Thus, the Oligocene-Miocene sequence stratigraphy of western Taiwan is established in this study to clarify the stratigraphic scenario among these continental margin deposits. The results make possible the unification of stratigraphic division throughout the studied area and the increase of correlative surfacesing with a leap. Also revealed are the details of facies distribution and relative sealevel changes in these strata.
Facies characteristics in the outcrops and electric logging of 6 stratigraphic sections and 10 exploration wells show that the Oligocene-Miocene rock strata are deposited in a shallow shelf influenced by storm waves. Coastal area is in the west and influenced by waves and tidal currents. Four depositional facies are recognized and represent 4 depth ranges with different sedimentary processes, namely the Outer Offshore, Inner Offshore, Nearshore (Shoreface and Tidal Channel) and Coastal Plain.
During relative sealevel falls, the depositional system moves westwards. Three types of sequence boundary are resulted and overlain by distinct facies successions. (1) The unconformity surface is characterized by paleosols in the emerged coastal area under the influence of weathering processes. (2) The forced regression surface is characterized by erosion surfaces in the inner shelf under the influence of wave and tide. (3) The conformity surface is characterized by stacking pattern changes in the outer shelf under the influence of storm wave. As relative sealevel ceases falling, lowstand systems tracts (LST) begin to be accumulated and stacked in aggradational successions. During relative sealevel rises, the depositional system moves eastwards. Transgressive systems tracts (TST) are accumulated and stacked in retrogradational successions. In the coastal area, LST are overlain by transgressive surface of erosion resulted from the tide and wave processes during transgression. In the inner shelf, LST are overlain by transgressive surface of lag deposits resulted from the repeated sediment starvation during transgression. In the outer shelf, LST are overlain by transgressive surface of condensed section resulted from the prolonged sediment starvation during transgression. As relative sealevel cease to rise, high stand systems tracts have been accumulates and stacked in progradational successions. On top of the TST, maximum flooding surfaces with significant trangressive features are recognized and characterized by stacking pattern changes from retrogradation to progradation.
In terms of biochronostratigraphy, 18 depositional sequences are recognized and correlated among the studied sections. The Oligocene to lower Lower Miocene strata below the NN2 and N5 biozones, deposited in the rifting stage, are characterized by drastic facies changes and wedge out to the west. They are composed of 8 depositional sequences with rapid changes in facies successions from coastal, inner shelf to outer shelf. The Miocene strata above the NN2 and N5 biozones, deposited in the post-rifting stage, are characterized by more consistent facies changes and some none deposition in the west. They are composed of 10 depositional sequences with subtle changes in facies successions.
The 18 depositional sequences are all in an age range of 1~2 million years, providing much improved constraints in the Oligocene-Miocene stratigraphic correlation and resolution in western Taiwan. They also manifest 18 transgression-regression cycles of depositional environments controlled by relative sealevel. Synthesis of the distirbuted outcrop sections rebuilds the relative sealevel curves, which are quantified by the depositional facies and dated by the sequence boundaries. Distnctive similarity in fluctuation numbers, ages and magnitudes emerges from the comparison among the synthesized western Taiwan curves, Exxon cycle chart, oxygen isotope records and New Jersey sequences. It suggests that the fluctuations of Global Eustacy have profound effects on the relative sealevels in western Taiwan during the Oligocene rifting stage and the Miocene post-rifting stage as well.
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