THE MECHANISM OF RETROGRESSIVE EROSION AND KARSTIFICATION: A CASE STUDY OF CAVE FORMATION IN EARLY HERSINIAN WEATHERED CRUST IN TAHE OILFIELD

Retrogressive erosion refers to the continued upward and head-ward erosion towards groundwater source produced by the flow of groundwater in cave under the water table after the water table in inner land on the verge of karst basin slope rises during transgression. The erosion results in multi-cycled collapse sequences and aggradational caves being developed over the original cave. The collapse sequence is composed of huge collapsed breccias and underground fluvial deposits. The aggradational cave is characterized by the filling of fluvial deposit and aggradational distribution. The complex huge cave is composed of the water table cave and the above aggradational cave. In this paper, the methods for the comparison of paleo-cave layers in the Early Hersinian weathered crust in Tahe oilfield is introduced, the result from the comparison of paleo-cave layers is showed, the mechanism of retrogressive erosion and karstification is illustrated through the analysis on configuration of the deposits filled in the huge cave and the dynamic model of underflow, and some key problems associated with retrogressive karstification are discussed.