Discovery of Tidal Sand Ridges and Its Significance of Pinghu Formation in Xihu Depression

Key words: 
• tidal sand ridges /
• sedimentary facies /
• Pinghu Formation /
• Xihu Depression

Abstract: Pinghu Formation in Xihu Depression is obviously affected by tide, but no tidal ridges are documented which are the typical sedimentary type in the tide-dominated environment. Through comprehensive application of cores, logs and seismic data, tidal sand ridges are discovered in Pinghu Formation, Xihu Depression. In the RMS amplitude attribute map, the topographic expression of ridges is banded. In the core, the tidal sand ridge is typically coarsing upwards. In the upper part, tabular cross bedding and scour which show the strong hydrodynamic force during the sedimentation are common; while in the lower part, the sediments are fine, and tidal beddings and bioturbation structures are frequent. There is a sharp base of the ridge in well logs and the heavy minerals are rich in ridges area. Tidal ridges are excellent quality reservoir and always surrounded by mud, so the ridges are good stratigraphic trapping potentials. Whether Xihu Depression is connected with the southern open sea or not can be known by tidal ridges coverage, so the study of tidal ridges are also helpful in palaeogeographical reconstruction.