Reef-carbonate diagenesis in the Pleistocene-Holocene of the well Xike#1, Xisha Islands, South China Sea: implications on sea-level changes

摘要

The Pleistocene was characterized by an unstable meteoric-marine diagenetic environment, influenced by high-amplitude global and regional eustatic variations. This paper focuses on the diagenetic characteristics of the Pleistocene-Holocene reef-carbonate rocks in the well Xike#1, drilled on the Stone Island (part of the Xisha Islands) in the South China Sea. A petrographic, cathodoluminescence and stable isotope study allowed the evaluation of the diagenetic evolution of the reef-carbonate rocks. The rock types present include framestones, packstones, wackestones and grainstones. Rock fabric analysis reveals the existence of skeletal grains, biodetritus, intraclasts, matrix and calcite cements. The upper part of the cored interval (0-180 m) has been heavily altered by meteoric fluids. Unequivocal evidence of meteoric diagenesis includes consistently negative delta O-18 values, well-developed subaerial exposure horizons and typical vadose cements. The deepest exposure surface is~80 m above the base of meteoric diagenesis, which means that the paleowater table could be extended to as deep as 80 m. The grainstone present at 7.74 m represents the boundary between the vadose and phreatic zones, which is marked by the first appearance of isopachous dogtooth cements and well-developed moldic pores. The reef-carbonate rocks between depths of 21.66 m and 180 m have undergone diagenesis in both meteoric and marine environments, in which the early-stage fibrous marine cements are surrounded by later meteoric drusy calcite cements. The end products of phreatic meteoric diagenesis are the limestones composed of low-Mg calcite with micrite envelopes, moldic pores, blocky spar calcite cementation, and aragonite neomorphism. The interval from 180 to 216 m has undergone a marine diagenesis, which is marked by cements with non-luminescent CL characteristic and a limited, relatively enriched C and O isotope composition. The study reveals that the diagenetic evolution of reef carbonate is mainly controlled by high-frequency eustatic sea-level changes, while the sedimentation rate variations, which were induced by regional subsidence and nutrient variations could also have had an impact on the diagenetic evolution.