Three-dimensional seismic characterization of tidal sand ridges in submerged low-profile backgrounds and simulation of their depositional processes—An example from the Pinghu Formation of the Xihu Depression

Tidal sand ridges are widely distributed in the Pinghu Formation in the Xihu Sag. Previous studies were carried out on single-factor analyses and speculations regarding both the formation process and the controlling factors of tidal sand ridges; however, the combined influence of multiple factors on the genesis of tidal sand ridges was largely ignored. Using drilling and logging data and seismic data, we studied the morphological characteristics and spatial distribution of the tidal sand ridges in the Pinghu Formation of the Xihu Sag, and numerically simulated the formation process and controlling factors of the tidal sand ridges. Results show that: ① The tidal sand ridges are developed during the recession period above T33, and several sand ridges are superimposed in the longitudinal direction. The tidal sand ridges are isolated and distributed in strips, and their directions are similar to that of the tidal currents; ② The formation of tidal sand ridges is mainly affected by tidal action. The river action transports the sand bodies to the estuary bay and forms a delta, providing material sources for the study area. At ebb tide, the sand bodies are transported to the sea, and at flood tide, the tidal current on the gentle slope is blocked by thicker sand bodies and deflected; the tidal hydrodynamics scour and erode the weak points between the sand bodies, transforming the sand bodies into isolated, long, and thin tidal sand ridges; ③ The formation of tidal sand ridges is also affected by tidal hydrodynamics and underwater low rises. At weak tidal hydrodynamics, the tide spreads the sand bodies onto the underwater low rises. With the tidal hydrodynamics increase, the sand bodies are carried to deeper areas and eroded and transformed into long tidal sand ridges. The underwater low rise play a blocking role. When the height of the underwater low rise is low, river action is strong, and the tidal action does not transform the sand bodies much; when the height increases, the river action is blocked from continuing to transport the sand bodies to the sea, the tidal action transports the sand bodies to the gentle slope, and finally form tidal sand ridges.