Characteristics and origins of ridges and troughs on the top of the Middle Miocene strata in the Beijiao Sag of the Qiongdongnan Basin, northern South China Sea

摘要

Sedimentary structures generated by (deepwater) bottom currents are of interest to geologists. Based on the root mean square, coherence attribute, and 3D representations, ridges and troughs are imaged for the first time using 3D high-resolution seismic data and confirmed by a well, YL19-1-1, in the Beijiao Sag of the Qiongdongnan Basin. Combined with 2D seismic profiles, we define the flow direction of bottom currents resulting in ridges and troughs. The results indicate that ridges and troughs occur on the top of the Middle Miocene strata and dominantly present wave-shaped structures. Their magnitudes decrease toward the upper and lower slopes. They extend for tens of kilometers, dominantly parallel to one another, with an evenly spaced distribution, some of which locally merge and bifurcate. They are aligned obliquely to the regional slope. The internal ridges are characterized by internal mounded reflections and/or parallel underlying-strata reflections. The presence of polygonal faults and weak to moderate amplitudes within the ridges and troughs suggests that they consist of fine-grained mudstones, as confirmed by the well YL19-1-1. High amplitudes filled within troughs are probably composed of coarse-grained turbidite materials, where polygonal faults are inhibited. Truncations and onlaps occur along the thalweg of a trough and are also clearly observed on the flanks of ridges and the walls of troughs. We infer that the troughs are a product of the erosion of bottom currents and the ridges are remnant underlying (sediment waves) strata as a result of the redeposition of eroded sediments. In addition, troughs are filled by turbidite materials with high amplitudes in the southwestern area of the 3D seismic survey, where ridges and troughs are a combined result of the early erosion by bottom currents and the later reworking by turbidity flows. We have developed conceptual schematic models to indicate the evolutionary history of the ridges and troughs. It provides new insights into the further understanding of the erosional and depositional impacts of bottom currents.