Biostratigraphie, Mikrofazies und Diagenese tertiärer Karbonate aus dem Südchinesischen Meer (Dangerous Grounds—Palawan, Philippinen)

This study is based on dredged carbonate samples from the Dangerous Grounds (South China Sea). They cover an area of approximately 130 000 km2. Carbonate samples from wells at the NW-shelf of Palawan and from the St. Paul's complex exposed on the island of Palawan are compared. Most samples contain abundant larger foraminifera. They are Upper Oligocene—Lower Miocene (often more precise Lower Miocene; Te, mostly Te5 referring to the Southeast Asian Tertiary letter classification) for all three sample groups. Because of their abundance in the investigated material, larger foraminifera are studied in more detail. Compilation of existing literature on the (paleo-) ecology shows that foraminifera associations can be used for sophisticated facies interpretation. This is especially helpful for evaluation of the isolated dredge samples. Facies interpretations are checked using four known Oligocene-Miocene facies distribution models, resulting from detailed investigations in the Philippines, N-Borneo (2 studies) and Australia. These models describe the distribution patterns for larger foraminifera. Samples can be assigned to various shallow-water facies zones. Facies and faunal characteristics of each sample are given in Chapter 4.3. To establish a consistent ‘model’ showing the facies distribution pattern for the S. China Sea would need more intensive sampling. It can be shown that the samples originally had been deposited in the euphotic depth zone (0–120 m water-depth). Based on their comparable age, the geographical distribution of sampling sites and an extensive MCS survey these carbonates represent a drowned Oligocene-Miocene carbonate platform. This platform is now encountered at waterdepths up the 2400 m. These results are in agreement with the recently confirmed interpretation of the Reed Bank—Dangerous Grounds area as foundered continental fragment (HINZ & SCHLUTER 1985; KUDRASS, WIEDICKE et al. 1986). The platform age gives a maximum age for the onset of rapid (?) subsidence. The metastable mineralogy of shallow-water carbonates can be used to compare diagenetic changes in the three sample groups (offshore-wells-onshore): The offshore samples (dredged and piston-cored) show only moderate loss of primary textures—the St. Paul's samples suffered most. This can be demonstrated with fossil shells and foram tests. Comparison of samples established a stability ranking of the different taxonomic groups: (Lowest stability) aragonitic skeletons (corals, bivalves) —soritids—lepidocyclinids/miogypsinids—Cycloclypeus —coralline algae-echinoderms—amphisteginids (stable). Typical diagenetic changes of shell-structures mainly of foraminifera groups are described in Chapter 6.1. From detailed examination of textural elements such as biogenic components, primary sedimentary structures, solution phenomena and different cement generations, relative age-relations could be established (Chapter 6.5). The analysis of trace elements revealed very low Sr yields in the samples (680–180 ppm) regardless of sampling group. In contrary to some older interpretations in Sr-literature, this low Sr content can be achieved assuming solely ‘marine’ diagenesis (marine porewater). Oxygen and carbon stable isotope analysis confirm this interpretation. Late diagenetic dog-tooth cement, often interpreted as resulting from frehwater diagenesis, and larger foraminiferal tests revealed unusually heavy oxygen isotope values (∂18O = O to 2.5%) in the offshore samples. These heavy values are interpreted as secondary values of diagenetically stabilized metastable components. Comparable heavy values are seen in some bathyal cemented sediment samples (fp-limestone; Chapter 6.7). This diagenetic shift of isotope values is attributed to (late diagenetic) lower water temperatures resulting from the drowning and subsidence of the shallow-water carbonates. The combined textural and geochemical indications were used to propose several diagenetic stages defining the diagenetic history of the investigated sample material (Chapter 6.8).