Study On The Formation Of Dolphin And Majiagou Formation Dolomite In The Ordovician Liangjia Mountain Formation In Shimenzhai Area, Qinhuangdao

Ordovician marine carbonate rocks were widely distributed in Shimenzhai area, Qinhuangdao, North China. This paper selected Lower Ordovician Liangjiashan Formation and Middle Ordovician Majiagou Formation in Liangjiashan section as research interests. By using the field data, microscopic analysis, X-ray diffraction patterns, scanning electron microscope (SEM), electron microprobe analysis, fluid inclusions, and cathodoluminescence, the petrologic features, sedimentary micro facies and diagenesis influences of Liangjiashan mottled limestones and Majiagou dolostones were studied systematically to discuss the possible dolomitization mechanisms in these two kinds of rocks. The main results include following points:1. Liangjiashan Formation is consisted of bioclast limestones, flat-pebble conglomerates (gravel-sized intraclast grainstones and floatstones), wackestones, fine-grained dolostones, mottled limestones, bioclast-pelletal grainstones; Majiagou Formation is mainly composed of dolomitic mudstones, fine to medium-grained dolostones, wheat-shaped dolostones and vey fine-grained dolostones.2. Six microfacies were recorgnized in Liangjiashan Formation. They are RMF18:Bioclastic wackestone with ostracods in restricted marine of inner ramp; RMF11:Marls with intraclasts and limestone pebbles in mid-outer ramp; RMF9-A:Floatstones with intracalsts in mid-outer ramp; RMF9-B:Intraclast-bioclast wackestone/ floatstones in mid-outer ramp; RMF22:Finely dolostone/ micrite in peritidal zone of inner ramp; RMF26:Bioclastic packstones in sand shoals of inner ramp; RMF28-A:Bioclastic grainstones and packstones with disorganized fabrics (mottled limestone) in sand shoals and banks of inner ramp; RMF28-B:Bioclastic-pelletal grainstones in sand shoals and banks of inner ramp.3. Two main microfacies were identified in Majiagou Formation. They are RMF19-A:Non-burrowed dolomitic mudstones in pertidal zone and RMF19-B:vey fine-grained dolostones in intertidal-supratidal zone of the inner ramp.4. Most carbonate rocks in Liangjiashan Formation endured the modification of eogenetic meteoric diagenesis, and the dolostones in Majiagou’ Formation were reconstructured by multi-diagenesis during the burial history.5. The dolomitization in Liangjiashan mottled limestones was happened shortly after the extensive bioturbations within the carbonate sediments. The sediments in biological burrows were filled by the mixed fluid, and laterly changed to dolomite. At last the irregular dolomitic stripes and mottles were formed in the limestones. The dolomite with clear rings structure in mottled limestones indicates the influences of multi-diagenetic fluids.6. The order degree of dolomite is decreasing and its dolomitization which transforming from burial dolomitization to penecontemporaneous dolomitization models is gradually weakened from the bottom to the top in Majiagou formation. The fine to medium grained dolostones presented the reconsturctions of multi-dolomitizations during the burial period on the basis of the early dolomization. "Wheat grain" dolostones were also suffered the transformation during the burial diagenetic period. The crystal growth was controlled by its growth habit and the diagenetic solutions, and finally formed the dolomite with the characteristics of single phase extension and its orientation perpendicular to c axis.