Diagenesis and Evolution of the Holocene Coquinite from the Haishan Island,Eastern Guangdong,China

The coastal hard rock with a thickness of over 5 m and a distribution area of nearly 200 ha in the Haishan Island, south China, has long drawn researchers’attention. However, there were controversies over its formation and classification, and these controversies in turn lead to the dispute of sea level changes and coastal uplift‐subsidence of this area. To investigate its diagenesis and evolution, petrographic analysis, elemental geochemistry, isotopic analysis, and radiocarbon dating were used in the present study. Radiocarbon dating indicates that the deposition of the Haishan Coquinite commenced in the mid Holocene and lasted to ～600 a B.P. Petrographic analysis shows that the Haishan Coquinite is cemented by low‐Mg calcite, indicating that the cementation occurred in a meteoric environment. The elemental geochemistry and isotopic values demonstrate that the coquinite suffered strong leaching, which was thought to be responsible for the meteoric cementation of the coquinite. According to these results, the diagenesis of the coquinite is revealed: deposition of the Haishan Coquinite commenced in the mid Holocene in a shoal environment, initial cementation occurred and cement may be high‐Mg calcite or aragonite; latterly the coquinite exposed to meteoric environment as a result of lowering of relative sea level, and the cement altered to low‐Mg, which took the morphologies of bladed calcite rim and equant spar. A four‐stage evolution model is proposed: (1) deposition stage, (2) initial cementation, (3) exposure to and cementation in meteoric environment, and (4) erosion stage. The published reports indicate that the hard rock should be designated as coquinite. Based on these studies, mid‐Holocene sea level in this area was discussed, and the Haishan Island was proposed to uplift with a rate of ～5 mm/a in the last ～600 a.