Abstract:

This paper uses the principle of sedimentology, petrology and geochemistry to carry on the comparative study of the grapeshaped hole between the Carboniferous formation in the Qaidam Basin and the Dengying Formation in the Sichuan Basin. And its formation mechanism and distribution characteristics are discussed in detail. Through the analysis, it shows that grapeshaped hole between the Carboniferous formation in the eastern Qaidam Basin and the Dengying Formation in the Sichuan Basin have the similar petrologic characteristics. There are petrologic symbols such as multilamina crust, mesh drainage channel, fresh water calcite, terrigenous sand, mud, and siliceous authigenic mineral. Geochemical analysis shows that diagenetic fluid of grapeshaped crust carbonate mineral could be seawater, diagenetic fluid of carbonate fillings in grapeshaped hole could be atmospheric freshwater. It is concluded that the grapeshaped hole is the product of atmospheric water, and the early forms of the grapeshaped hole on the Carboniferous formation of the Qaidam Basin is the slot hole on the Dengying Formation of the Sichuan Basin. The evolution process is as follows: due to periodic regression, tidalflat environment of carbonate rocks is exposed by dissolution, and stitches columnar calcite cementations take place in dissolution hole during the period of transgression. When the water evaporates further, the imitating replacement happens in needle cylindrical calcite cements in seawater, and forms syngenetic dolomite. The periodic regression and transgression lead to the formation of the multiple cycles of grapeshaped holes on the profile.

Key words: Qaidam Basin, Carboniferous, grape shaped hole, mixed clastic carbonate platform, atmospheric water dissolution