Controlling factors and their geological significances of order degrees and unit-cell parameters of dolomites in the Middle Jurassic Buqu Formation in Qiangtang Basin

This paper discusses the characteristics, controlling factors and geological significances of the order degrees and unit cell parameters of the Middle Jurassic Buqu Formation dolomites in Qiangtang Basin. Basing on the detailed classification of dolomites in the early stage, the X-ray powder diffraction method is used to test the dolomite, and the Jade 6.5 software is used to process the test results. The maximum crystal plane spacing, order degrees, unit cell parameters and relevant information of dolomite are obtained, and the controlling factors and geological significance are discussed. The results show that: (1) The grain dolomites in Suona Lake area of the north Qiangtang Depression, which are with low to medium order degrees and concentrated unit cell parameters, develops in an evaporation lagoon environment from high magnesium calcites by the adjustment of dolomitization during the burial process; (2) To the dolomites of the Buqu Formation in the South Qiangtang Depression, the micro-powder crystalline dolomites with a large distribution range of order degrees and unit cell parameters are formed by rapid metasomatism of limestone in penesynchronous stage and recrystallization in burial stage; The fine-grained dolomites with a high degree of order and a positive correlation of unit cell parameters develop in a stable burial environment and the dolomite crystals grow freely in the pores of the inherited limestone; The medium-coarser-grained dolomite, whose degree of order is the highest, has experienced a long process of dolomitization in the middle-deep burial stage, and the narrow spaces limit the free growth of crystals. From micro-powder dolomite to fine-grain dolomite to medium-coarser-grain dolomite, the order degree of dolomites increases gradually, indicating that the stable burial process is conducive to the orderly transition of dolomites. (3) The “a” and “c” of unit cell parameters of the Buqu Formation dolomites shifted to the lower left of the ideal dolomite in different degrees, indicating that the Buqu Formation dolomites experienced a strong tectonic compression with an estimated pressure up to 1000 MPa after its formation, which is related to the tectonic movement of the late Cretaceous. It is suggested that the dolomite paleo-oil reservoir of Buqu Formation in southern Qiangtang Depression is caused by the thrust nappe tectonic movement.