Keywords: 
• stable isotopes /
• carbonates /
• lacustrine sediments /
• Palaeogene /
• Sanshui Basin

Abstract: The lacustrine environmental evolution of early Palaeogene Sanshui Basin has been established based on stable isotopic data from bulk carbonates from the Upper Xinzhuang Formation and the Honggang Member of the Buxin Formation (major oil source bed of the Basin). Three stages, when the basin had connection with the sea, have been recognized. With the transgression of sea waters and increased regional aridity, δ ¹⁸ O values showed a sharp positive shift, accompanied by the deposition of dolomites, whereas δ ¹³ C displayed lower values due to decomposition of organic matter that occurred at this time, which resulted in water rich in ¹³ C-depleted CO ₂. During the intervals without connection with the sea, the δ ¹⁸ O and δ ¹³ C pattern showed a positive covariance (r = 0.65). This moderately high covariance, although poorer than that reported from typical closed lakes, suggests that the basin had, like most closed lakes, relatively long residence times. Gypsum deposits that occurred frequently in the core section also indicate hydrological closure conditions. This means that the Sanshui Basin was probably a periodic hydrologically closed lake during these times. The stable isotopic compositions of carbonates were mainly influenced by variations in evaporation/precipitation balance. Higher δ ¹⁸ O values indicate overall increase in E/P ratios, low lake levels and concentrated waters.Lower δ ¹⁸ O values suggest decrease in E/P ratios and a large fluvial inflow and a subsequent high stand of water level. δ ¹³ C shows more variability than δ ¹⁸ O, which implies that δ ¹³ C was also controlled by organic activity and usually reflects the productivity, burial vs. reoxidation of organic material.