Controls on carbonate cementation in early syn-rift terrestrial siliciclastics: The Lower Cretaceous of the Bayindulan Sag in Er'lian Basin, China

Carbonate cementation of sandstones at channel bases is commonly interpreted to be a result of solutes generated from surrounding rocks in a burial-diagenetic regime. However, in rapidly buried and under-compacted conglomerates presented in this study, we interpret carbonate cements formed during early-diagenesis (1000–1600 m) to be precipitated directly from Ca- and CO2-rich saline formation water, derived from hypersaline lake water and driven by gravity and compaction. In the Bayindulan Sag of the Er'lian Basin, which is a continental rift basin, the sandstones and conglomerates of the A'ershan Formation (K1ba) were deposited in fan delta front settings during the syn-rift phase. Within these, tightly non-ferroan calcite-cemented conglomerates are present at the bases of distributary channels. In addition, some Fe-dolomite cements are present in sandstones with high present-day porosity, which are located in the upper parts of distributary channels. A petrographic analysis and fluid-inclusion microthermometry study show that the calcite cement predates the Fe-dolomite. Stable isotopic values indicate that both calcite and Fe-dolomite are characterized by enrichment of 13C and depletion of 18O (δ18OV-PDB: −15‰ to −8‰), and the value of δ13C V-PDB in calcite (+1.6‰ to +3.8‰) is less positive than that in Fe-dolomite (+3.1‰ to +8.4‰). Integrated analyses imply that the studied carbonate cements were probably formed within an open diagenetic system and were influenced by multiple diagenetic fluids. During deposition of the A'ershan Formation (K1ba), the lake water was brackish to hypersaline (Sr/Ba>1 and Sr content > 300 ppm). During deposition of the Tenggeer Formation (K1bt), the K1ba progressed rapidly into a deep burial phase (90 m/Ma), diagenetic fluids were controlled by hypersaline lake water, incursion of meteoric water occurred along faults, and methanogenic fermentation. Calcium was probably derived from the saline formation water and the transformation of smectite to illite in associated mudstones, leading to the precipitation of calcite (54–78 °C). In the Saihantala period, the K1ba experienced a slight uplift, the fault activity ceased, and the diagenetic fluids were derived from the saline formation water and were influenced by methanogenic fermentation, and Fe-dolomite began to precipitate. In deeper burial detrital feldspar grains and some calcite cement were probably dissolved by organic acid discharged from mudstones. In conclusion, the early diagenetic characteristics in the continental rift basins were controlled by the composition of the depositional waters and tectonics that controlled the opening and closing the diagenetic system thus influencing fluid composition.