Palimpsest sediments in Sanniang Bay, Guangxi: evidence from end-member and rare earth elements

Relict sediments record the key information of transgression and regression, and it is also a powerful evidence for the reconstruction of paleogeography. In the marine geological survey in October 2019, a set of gravelly sediments was collected in the area in more than 10 m depth in the Sanniang Bay , Qinzhou, Guangxi. To understand the causes of formation and provenance, grain size analysis and geochemical test were conducted, and the end-member analysis model and rare earth element discrimination diagram were used. The information of Sanniang Bay sediment could be characterized by four end-member (EM). EM1 represents the characteristics of reciprocating tidal current in Qinzhou Bay, EM2 is the high-energy scouring environment of Dafeng River estuary, EM3 reflects the mixing action of relict sediments and wave, and EM4 signifies the original river deposition of relict sediments. The characteristics of rare earth elements (REE) show that the relict sediments has the characteristics of low ΣREE and high ΣLREE/ΣHREE. After the normalization of REE to the upper crust, it shows obvious δCe positive anomaly and δEu negative anomaly, which indicates the characteristics of the source of sand shale and sandstone. It is similar to the nearshore sedimentation in the northeast of Qinzhou Bay. Our comprehensive analysis showed that the palimpsest sediments in the Sanniang Bay is the relict product of the latest large-scale transgression in the Beibu Gulf, and is transformed by ocean current and tide. The result provides key information for the identification and evolution of ancient coastline in the Beibu Gulf.