Abstract:

Under the impact of strong tectonic uplifts of Tongwan Ⅰ and Ⅱ in the early Caledonian orogeny, the top of the second and fourth members of the Sinian Dengying Formation underwent intensive longterm unconformity karstification, respectively. In addition to the karst associated with unconformity on the top, there are also some stratiform shortterm karst layers within the second and fourth members of dolomite. The shortterm karst layers were developed under fourthorder sequence boundaries and were associated with multiple shortterm tectonic upliftsubside cycles during the sedimentation of the second and fourth member of dolomite. Under the impact of shortterm tectonic uplifts as well as the fall of relative sea level, the dolomite exposed to surface and underwent contemporaneous/penecontemporaneous karstification which formed some layers of several meters with a large number of dissolution pores and caves within the second or fourth dolomite members. The sediment between two karst layers is always the compact cryptocrystalline dolomite, argillaceous dolomite or siliceous dolomite formed in deep water environment during tectonic subsidence. Four and eight shortterm karst layers were respectively identified in the second and fourth members of the Sinian Dengying Formation in the Yingshuiba outcrop in the north of the Sichuan Basin; and six were identified in the second member in the LeshanLongnüsi uplift by well correlation. The multiple shortterm contemporaneous / penecontermporaneous karst layers within dolomite members stacked each other vertically, which made the present Dengying Formation dolomite reservoir with relatively big thickness. The reservoir property associated with shortterm karst was further enhanced by the longterm unconformity karstification associated with tectonic uplifts of Tongwan Ⅰ and Ⅱ in the early Caledonian orogeny.

Key words: Dengying Formation, dolomite, karst, unconformity, sea level