Abstract
Shallow marine sequences of the northern South China Sea (SCS) are uplifted and exposed by plate convergence in the Taiwan mountain belt. These deposits provide detailed geological information about the rifting event, stratigraphy, sedimentology, paleoclimate and paleoceanography of the shallow SCS to compare with what are recorded in the ODP 1148 deep-sea core. Seismic surveys and marine micropalentological studies show that Eocene sequences in the offshore Taiwan Strait and onland Taiwan mountain belt are all deposited in rifting basins and are covered unconformably by the Late Oligocene-Neogene post-rifting strata. Between syn-rifting and post-rifting sequences, there is a regional break-up unconformity throughout the island. Early Oligocene and Late Eocene strata are missing along the break-up unconformity equivalent to the T7 unconformity in the Pearl River Mouth Basin off south China. This may suggest that the SCS oceanic crust could have initiated between 33 and 39 Ma. Neither obvious stratigraphic gap nor slumping features are found in the Oligocene-Miocene transition interval of Taiwan. This observation highly contrasts with what has been documented from the ODP 1148 deep-sea core. This suggests that the stratigraphic gap and slumping features could only be recorded in the SCS deep sea region, but not in the shallow shelf near Taiwan. Compared to the Middle Miocene paleoceanographic re-organization events in the SCS deep sea, the geological history of the Taiwan shallow sequence shows changes of in sedimentation and faunal composition. Due to the Antarctic glacial expansion at ∼14 Ma, Middle to late Miocene strata of the Western Foothills show progressive regression sedimentations associated with a decrease of benthic foraminiferal abundance and a sharp faunal turnover event. Many Early-Middle Miocene endemic benthic foraminifers were extinct in 14-13 Ma and new benthic foraminifers of the Kuroshio Current fauna appeared from 10.2 Ma, comparable with new occurrence of Modern benthic foraminifers at 9 Ma in the Java Sea area. This reveals that the Western Boundary Kuroshio Current in the North Pacific could initiate from 10-9 Ma due to closures of the Indo-Pacific seaways by convergent tectonics between the Australian Continent and the Indonesian Arc in 12-8 Ma. Subduction of the SCS oceanic lithosphere since the Middle Miocene resulted in formation of the Hengchun Ridge accretionary prism and the North Luzon Arc. Occurrence of these two bathymetric highs (−2400 m) since the Middle Miocene and closures of the inter-arc passages in the North Luzon arc in the last 3.5 Ma would control the water exchanges between the West Pacific and the deep SCS. Accordingly, the tectonic evolution in the Central Range-Hengchun Peninsula accretionary prism and the arc-forearc Coastal Range not only control directly the route for water exchanges between the West Pacific and the SCS, but also indirectly shows a great influence on the geochemistry of deep SCS waters. The latter is best shown by much negative carbon isotope values of benthic foraminifers in the ODP 1148 deep-sea core than the West Pacific records in the last 14 Ma.
Highlights
The South China Sea (SCS) represents one of the large marginal seas in the low latitude West Pacific
After the formation of syn-rifting basins, due to thermal upwelling from mantle magma, basalts are erupted along E-W-trending MOR and new oceanic crust is born in the SCS deep basin [3]
Integrating positive evidence of (1) Nummulites junbarensis in the Szeleng Sandstone (Pei-I Highway section, core PH-29-1) and in the Paileng Formation, (2) Nummulites sp. in the Szeleng Sandstone of the Nanshichi section, (3) Assilina formosensis associated with calcareous nannofossils of Zones NP14-15 in the Paileng Formation of the Jun-Keng section, and (4) Eocene planktic foraminifers in the upper part of the stratotype Szeleng Sandstone, the Szeleng Sandstone or Paileng Formation can only be assigned to Middle Eocene in age (Figure 6)
Summary
Seismic investigations over the offshore Taiwan Strait and the onland Coastal Plain show that there are Paleogene rifting basins bounded by normal faults [8,16]. Drillings in the offshore Penghu Basin further prove that the Paleogene synrifting sequence is comprised of marine shale dominant sediments of the Middle Eocene age (Zones NP15-16 of calcareous nannofossils; Figure 3) and thick volcanics. The Eocene syn-rifting sequence is unconformably covered by the Miocene post-rifting strata (Figure 3). In the Peikang Basement High (Figure 1; NE extension of the Dongsha Uplift in the northeastern SCS), Early Miocene post-rifting strata overlie unconformably on the Middle Eocene volcanics (44.1±2.2 Ma by K-Ar dating) [17] and sandstone-shale (Well PC-1; Figure 1), or on the Paleocene shallow- marine strata (Well WG-1 [18]; Figures 1 and 3), or even covered directly on the Cretaceous pre-rifting basement of the Peikang. Basement High [8](Well PK-3, Figure 1)
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