Sequence stratigraphy of the Late Oligocene carbonate system on the Xisha Islands in the South China Sea

Abstract

Sedimentological, paleontological, and mineralogical data from core XK-1 in the Xisha Islands allow for a detailed lithologic and sequence stratigraphical analysis on the Late Oligocene interval in the Xisha Islands. In this study, 13 facies are differentiated and a depositional model of a carbonate-dominated system is established. Variation in facies stacking patterns, changes in composition and grain size, and fluctuation in abundance of terrigenous matter are analyzed, and sequence division is interpreted at two different hierarchal levels. In the Late Oligocene interval of core XK-1, a total of thirteen small-scale sequences and three medium-scale sequences are divided out, with one medium-scale sequence mainly composed of four small-scale sequences. Based on the available timing constraints, the durations of the small- and medium-scale sequences are estimated to be 100 kyr and 400 kyr, respectively. These small-scale and medium-scale sequences are thus inferred to be primarily controlled by orbital forcing. The long-term evolution between 24.3 and 23.6 Ma was controlled by the eustatic sea level, while it was more influenced by a tectonic subsidence linked to the ridge jump event in the South China Sea after 23.6 Ma. Our study demonstrates that eustatic sea level fluctuations had a significant impact on the initial carbonate development at various scales in the northern South China Sea, while tectonic movement also played an important role in shaping the secular carbonate evolution.