Characteristics of subaerial karstification and late reconstruction in the Dengying Formation, Sichuan basin, southwestern China

Abstract

Tectonic uplift at the end of Sinian resulted in the denudation of the Dengying (灯影) Formation on a different scale and the development of a great number of secondary dissolution vugs beneath the unconformity between Sinian and Cambrian. Geological and geochemical characteristics such as the development of caves, sinkholes, karst tubes and weathering crust, abrupt depletion and negative deflection of δ13C and δ18O in country rock and vug filling toward unconformity surface suggest that subaerial karstification is responsible for the development of secondary karst vugs within 150 m or so beneath the unconformity surface. This kind of secondary dissolution vug was partially or completely filled by different stage dolomite, calcite, quartz and bitumen during the later deep burial and uplift. Facts such as the obvious difference in δ13C and δ18O of the filling in the subaerial karst vug from the country rock and the value of δ13C and δ18O of the filling increasing or decreasing with that of the country rocks, suggest that the hydrothermal fluid charged in vugs is allogenic fluid. Subaerial karstification vug acted as a conduit for allogenic corrosive fluid which can create new secondary vugs and enlarge previous karst vugs. The dissolution, precipitant and recrystallization produced by allogenic corrosive fluid introduction and petroleum thermal cracking, caused contraction or enlargement of the subaerial karstification vugs and the development of new secondary vugs during the deep burial and uplift. The ultimate vug of deep burial dolomitite is determined not only by fluid charge, dissolution, mineral precipitant, and petroleum thermal cracking during the deep burial and tectonic uplift, but also by the amount of initial secondary dissolution vugs created by subaerial karstification.