Characteristics and Tectonic Implication of Bottom Boundary of Upper Oligocene in Qiongdongnan Basin, Northwestern South China Sea
The study of key boundaries of tectonic sequences has been one of the most important subjects in the field of sedimentary basin dynamics. With the integrated analysis of latest geologic, geophysical and geochemical data, a new regional unconformity-SB7 was identified in Qiongdongnan basin, a significant petroliferous Cenozoic extensional basin in South China Sea. This sequence boundary lies at the bottom of Upper Oligocene, and can be traced in the whole basin.
The SB7 boundary is characterized by obvious changes in lithology, logging curve shapes and biological assemblages in wells, and local truncation and large-scale onlap or downlap on seismic sections. Moreover, different faulting systems with different fault activities, depocenters, and depositional systems are developed before and after the formation of SB7 boundary. Below this boundary, half-grabens and grabens are well-developed and mainly bounded by NE and NW-NWW trending faults with high activity rate. The depocenters are all close to the downthrown side of basin-bounding faults. The sedimentary environments evolve from lacustrine to semi-enclosed neritic facies before Late Oligocene. In contrast, above SB7 boundary, the stratigraphic distribution is mainly controlled by the nearly W-E and NE trending faults with relatively weak activity. The depocenters gradually migrated away from basin-bounding faults to the basin center. It is chiefly manifested as open neritic to bathyal sedimentary environment in Late Oligocene.
The above mentioned facts indicate that SB7 boundary is a tectonic and sedimentary transition surface. It divides the Paleogene rifting stage of Qiongdongnan basin into two episodes (i.e., faulted and fault-depression episodes) formed by different extensional tectonic stress fields, which mainly changed from NW-SE to N-S trending. The regional seismic stratigraphy and biostratigaphy correlations also show that SB7 boundary or its equivalent can be found in other basins of northern margin of South China Sea. It was formed in early Late Oligocene around 32 Ma ago. The forming time coincides with the initial spreading of South China Sea.
This research can therefore provide new information for dynamic setting of Qiongdongnan basin, and help understand the coupling effect of the spreading of South China Sea and the evolution of marginal basins in this area.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California