| dc.description.abstracteng | The South China Sea is a quite narrow Cenozoic ocean basin. It is located between China in the north, Vietnam in the west, Malaysia and Brunei in the southeast and the Philippines in the north and northeast. Rifting in the South China Sea initiated in the late Cretaceous and the extension, including the spreading, prevailed until the Miocene. The post-rift development of the southwestern part of the South China Sea, the Dangerous Grounds, especially their collision with Palawan and Borneo can be deduced from the investigation of Oligocene to Pliocene limestone formations. Of special interest in that area is the Oligocene-Miocene “Nido” limestone. This limestone forms a widespread carbonate platform offshore W-Palawan, Borneo and in the easternmost parts of the Dangerous Grounds. From seismic images it is clearly visible that this carbonate platform seals most of the extensional tectonics in the Dangerous Grounds.
Towards the southeast this carbonate platform dips downward and is overthrusted by the thrust wedges of Borneo and southern Palawan. Offshore northern Palawan the Nido limestone has not been overthrusted and it dips gently towards the west. The division line between these two occurrences can be drawn across
Ulugan Bay in the middle of Palawan Island. By analyzing the ages of this carbonates using biostratigraphic reports by Robertson Research the age for the top of the carbonate platform was assigned to 19 Ma, even though some of the reefs, growing on top of the platform continued to develop in some places up to recent times.
A second limestone developed on top of the thrusted wedge offshore SW-Palawan. This limestone formation was named “Tabon Limestone”. Since it is only slightly affected by the thrust tectonics, it is used to constrain the final phase of collision between the Dangerous Grounds and Palawan. The base and top of this limestone were investigated similar to the Nido and revealed a strong time- and space- transgressive nature. The age for the base of the carbonates ranges between 16 and 7 Ma, depending on the sampling point. There is a clearly recognizable younging trend towards the west. This led to the idea that the development of the limestone was strongly affected by the development of the wedge and that it is possible to use this limestone to date that development. While seafloor spreading is assumed to have ceased at 20.5 Ma (Barckhausen et al., 2014; Barckhausen and Roeser, 2004) the convergence in the Palawan area continued until around 5-7 Ma. A last uplifting event in the Pleistocene brought parts of southern Palawan above the sea level and exposed the Tabon limestone. Carbonate precipitates found on a marine bivalve in a cave near Quezon in the southern part of Palawan Island give indications to a working spleothem at 1.2 Ma (Rehm, 2002).
The following thesis presents the results of the investigation in the Dangerous Grounds and Palawan area, including the Reed Bank Block, as they were published in two papers: “Time constraints on the Evolution of southern Palawan Island, Philippines from onshore and offshore correlation of Miocene limestones”,
Journal of Asian Earth Sciences 2013 and “Oligocene-Miocene carbonates and their role for constraining the rifting and collision history of the Dangerous Grounds, South China Sea” Marine and Petroleum Geology 2014. | de |