Thermal records of carbonate cements: Deciphering the hydrothermal imprint on Eocene Shahejie Formation sandstones in the northern Dongying Depression, China

Abstract

The carbonate cements in the sandstones of the Eocene Shahejie Formation in the Dongying Depression record crucial temporal and thermal information related to hydrothermal activity. This study presents a comprehensive investigation of these carbonate cements, detailing their diagenetic process, chronological formation, and material origins. Sedimentological evidence indicates that the sandstones in the study area were predominantly deposited in deltaic and marginal lacustrine settings, with a subdivision into matrix-rich and matrix-poor types at the microscale. Petrographic observation and electron probe microanalysis (EPMA) reveal that calcite is the predominant authigenic mineral phase, exhibiting variability in both abundance and morphology depending on sandstones maturity. Matrix-poor sandstones, characterized by higher maturity, predominantly exhibit blocky calcite cement, whereas those with a rich matrix and lower maturity tend to develop scattered patchy calcite. Fluid inclusion homogenization temperatures indicate that these calcite cements precipitated under abnormally high-temperature conditions, ranging from 85 to 350 °C. And the carbonate cements exhibit low δ¹³C (−11.8 to −3.1 ‰ VPDB) and δ¹⁸O (clustered tightly from −14.0 to −11.1 ‰ VPDB) values, interpreted to reflect a mixed source comprising magmatic CO₂, air CO₂, and a potential contribution from soil CO₂. In situ U-Pb isotopic dating of calcite constrains the timing of carbonate cement precipitation, with ages ranging from 39.4 ± 1.9 Ma to 42 ± 7.6 Ma. Ultimately, the results indicate that the calcium source for the carbonate cements in the Eocene Shahejie Formation sandstones of this region is primarily a mixture of primary alkaline formation waters and hydrothermal fluids. The carbon source is dominated by a mixture of magmatic CO₂ and air CO₂, with a potential minor contribution from soil CO₂. This study suggests that the hydrothermal activity influencing carbonate cementation in the study area was likely associated with the Cenozoic subduction of the New Pacific Plate. Through the integration of multiple advanced analytical techniques, this study elucidates the material origins and precipitation timing of hydrothermal derived carbonate cements, validating the potential application of advanced techniques such as in situ U-Pb isotopic dating of calcite in clastic rock research. Additionally, the hydrothermal activity age determined for the Shahejie Formation in this study provides crucial data support for the research on tectonic-magmatic activity in the Dongying Depression.