Taking the layered argillaceous limestone in the upper Es4 in the Dongying Sag as the research object, the geochemical analysis of major, trace, and rare earth elements (REEs) established the response relationship between REE distribution characteristics and sedimentary diagenesis. The average values of total light REE ( Σ LREE )/total heavy REE ( Σ HREE ) of micrite calcite and argillaceous laminae are 6.75 and 4.06, respectively. The LREEs and HREEs are differentiated, consistent with the distribution pattern of REEs in the crust. Th and U elements are more enriched in the sediments in the lacustrine sedimentary environment than in the diagenetic calcite veins. In primary sediments (argillaceous clay and micrite calcite laminae), LREEs are more enriched, HREEs are depleted, and Eu shows positive anomaly-enrichment characteristics. The LREEs and HREEs of the sparry calcite veins are lower than those of the original sediment argillaceous clay and micrite calcite, showing characteristics of a negative anomaly depletion. Sparry calcite veins originate from diagenetic fluid crystallization and precipitation and have the characteristics of low Th and U, evident positive anomalies of Sr and Eu, and substantial depletion of La. The distribution patterns of REEs within the four components of the laminated argillaceous sparry limestone reflect the order of REE distribution from primary sediment laminae (argillaceous clay and micrite calcite) to diagenetic laminae (calcite veins). Compared with the North American shale, the four components of the contact surface between the argillaceous and bright crystalline laminae, the micrite calcite, the calcite veins, and the argillaceous laminae all showed weak negative δ Ce anomalies and positive δ Eu anomalies. The fractionation degree between LREEs and HREEs reflected by La / Sm cn and Gd / Yb cn is in descending order: the interface between the argillaceous lamina and sparry calcite lamina, micritic calcite, calcite vein, and argillaceous lamina. The argillaceous laminar material has the characteristics of basalt REEs, indicating that the terrestrial debris and argillaceous lacustrine shale in the upper Es4 member of the Niuzhuang subsag are primarily derived from the basic extrusive rocks of the Qingtuozi bulge. REE differentiation is most noticeable at the interface between the argillaceous lamina and calcite vein, proving the directionality of REE differentiation from the original sedimentary lamina to the diagenetic lamina. Shale in the study area is primarily deposited below the redox interface of water at a certain depth, and the deposition rate is stable and slow, providing good conditions for preserving organic matter.
Sedimentological and chronological analyses of coastal dunes can determine their sedimentation processes and yield information that can reflect changes in local marine environments (e.g., information on climate, biology, and sea-level changes in tectonically stable areas). In this study, Late Quaternary deposits on Shidao Island, one of the Xisha Islands, South China Sea, were analysed using techniques such as U/Th dating to obtain the following results: (1) Four lithofacies associations and two unconformities were identified from the deposits on Shidao Island.On this basis, the Shidao Island sediments can be divided into three stages of aeolian sediments and one stage of beach rock sediments. (2) U/Th dating revealed that the aeolian deposits on Shidao Island formed during three periods, namely, marine isotope stages 5.2, 5.1, and 4, and that the beachrocks were formed in the Holocene when the sea level rose to its present height. (3) The aeolianites on Shidao Island all formed in an environment in which the sea level was relatively stable or slowly falling. This study determined for the first time the specific sedimentary ages of the aeolianites on Shidao Island, and reconstructed the sedimentation process of the deposits on Shidao Island based on palaeo-sealevel changes. These results can provide more reliable geochronology data for the future study of Late Quaternary marine environments across the East Asian region.
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