Dolomitization mechanism of Cambrian carbonates in the Bachu area, Tarim Basin, NW China

Abstract

Geochemical characteristics of the Cambrian carbonates are studied through systematic sampling in the Bachu area of the Tarim Basin. The dolomitization mechanisms are compared and analyzed of the dolomites above and below salt beds and limestone between the salt beds. The δ18O values of dolomites in the limestone between salt beds are larger than −6‰, the δ13C values are normally larger than −1‰, the 87Sr/86Sr values are smaller than ancient sea water plus Mn and Na element enrichment. All these features suggest that the dolomites developed due to early sabakha dolomitization. For the dominantly medium- and coarse-grained dolomites above the uppper salt bed, the δ13C values of samples are lower than zero, the δ18O values are normally −9‰–0, the 87Sr/86Sr values are close to ancient sea water. These features show that the dolomites developed in a shallow burial environment. For the dominantly powdered and fine dolomites below the lower salt bed, the δ13C values of samples are larger than zero, the δ18O values are normally lower than zero, the 87Sr/86Sr values are larger than the above mentioned dolomites, plus Fe and Sr element enrichment. These features indicate that the dolomites developed within an evaporate environment on a restricted platform, and they were of penetration reflux dolomitization and affected by the deep burial hyperthermal environment later. Key words: Tarim Basin, Cambrian, Bachu area, carbonate rock, dolomitization