Magnitude of Cenozoic erosion from mean sonic transit time, offshore Taiwan

Abstract

Mean sonic transit times from eight stratigraphic units in forty-four wells in western and northern offshore Taiwan were plotted against depth. Apparent erosion (displacement on the depth axis from the normal compaction) was determined for each unit in each well. Cretaceous and Eocene units show greater apparent erosion than Miocene and Pliocene units in the same well, which in turn show more erosion than Pleistocene units. Apparent erosions derived from two different units are plotted against each other for correlation analysis. Based on the analysis, the main periods of erosion were during the Oligocene and Plio-Pleistocene.The timing of erosion inferred from sonic interval transit is consistent with recognized unconformities, and its areal distribution is consistent with the observed structural evolution of the region. The maximum true erosion (corrected for post-erosion burial) at the Oligocene unconformity was 3500 m on the Penghu Platform. Approximately 2500 m of true erosion occurred on the Kuanin Ulift and the Pengchiahsu Platform. Due to moderate-to-minor erosion and a great amount of post-erosion burial, true magnitude of erosion is masked at the Miocene unconformity in most of the area.Erosion of up to 500 m on the eastern side of the Penghu Island at the Pleistocene unconformity defines the shape of the present-day Penghu Waterway. In the northern offshore area up to 1000 m of Plio/Pleistocene, true erosion occurred in the Taiwan-Sinzi folded zone where Miocene folds are observed. The magnitude of true erosion decreased west of the Taiwan-Sinzi folded zone to 500 m in the Pengchiahsu Basin. A maximum of 500 m of true erosion is also observed in the west offshore area along Hsing-Chu to Miaoli coast. ᭧

Tectonic Implications of Stratigraphy Architecture in Distal Part of Foreland Basin, Southwestern Taiwan

“…The characteristics of periodic coarsening upward sequences indicate a strong tectonic signature that can be comparable to other examples in some tectonically active belts [9,10,11]. The tectonic origin for the unconformities is also supported by the calculated magnitude of erosion of the unconformities in the Plio-Pleistocene [98], which scale is larger than that could be caused by eustatic sea level fall during the same period. The well bore data was correlated to a seismic line tied to the well and the interpreted seismic profile shows that the sequence boundaries are characterized by prominent canyon morphology to the south of the well site ( Figure 9).…”

Review and New Insights on Foreland Tectonics in Western Taiwan

“…From the Late Eocene to the Oligocene the foreland area in western Taiwan encountered differential uplifting and formed a large-scale regional unconformity. During this period, the 3000-3500 m thick Paleogene strata were eroded on the Peikang-Penghu Basement High (Fuh, 2000), and the greater uplift to the south resulted in the complete erosion of Paleogene strata in that area. From the beginning of the Late Oligocene, the foreland area in western Taiwan encountered differential subsidence that formed two Neogene basins now separated by the Peikang-Penghu Basement High (Meng, 1971;Sun, 1982).…”

Numerical modeling on the relationship between thermal uplift and subsequent rapid subsidence: Discussions on the evolution of the Tainan Basin

“…The eroded sediment thickness based on seismic data increases from north to south, which is about 250 m in the Wenzhou Sag, lying to the north of the Lishui Sag, and about 600 m in the Lishui Sag [ Wang , 1991]. On the basis of the sonic velocity versus depth relationship, Fuh [2000] concluded that on the Pengchiahsu Platform, the maximum erosion at the Oligocene unconformity was 2500 m, while it was 2000–3500 m in the West Taiwan basins. On the basis of the same kind of data, Lin et al [2003] estimated that the erosion in the area ranged from 1000 m to 3000 m, and the maximum erosion could accumulate to 4500 m on the Penghu Platform.…”

“…The uplift and erosion event is not simply the result of eustasy, but caused by tectonic events. The change of global sea level in the Oligocene is about 200 m, which would at most cause 1000 m erosion based on local isostasy and cannot account for more than 3 km erosion, as mentioned above [ Fuh , 2000; Lin et al , 2003; Wang et al , 1995]. As shown in Figure 2, in the late Eocene and Oligocene, there were some strong tectonic events, for example, the Zhuqiong II event (in the late Eocene) and the Nanhai event (∼30 Ma, Breakup unconformity) in the Pearl River Mouth, the Puli event (37–30 Ma [ Lin et al , 2003]) in the West Taiwan Basin, and the Yuquan event (in the late Eocene) and Huagang event (in the late Oligocene) in the East China Basin.…”