High-Resolution Sequence Stratigraphy of Da'anzhai Formation, Lower Jurassic in Sichuan Basin

Key words: 
• high-resolution sequence stratigraphy /
• base-level cycle /
• parasequence set /
• isochronos-tratigraphic correlation /
• stratigraphic framework /
• Da'anzhai Formation /
• Sichuan Basin

Abstract: According to the data of rock cores and well logging interpretations, three order base-level cycles were recognized from Da'anzhai Fo rmatio n, Lower Jurassic in Sichuan Basin, w hich consist of 41～47 short-term base-level cycles, 5 mid-term base-level cycles and 2 long-term base-level cycles from bottom to top of the formation. The short-term base-level cycles could be divided into two cycle types of nonsymmetry and symmetry, of which the former developed fining upward and the latter developed fining upward coarsening. The 5 mid-term base-level cycles were composed of several short-term base-level cycles in different stacking pattens and represented respectively the low stand progradation sequence set, lake transgressive aggradational-retrogradational sequence set, lake flood aggradational parasequence set, early highstand aggradational to weak progradational sequence set and later highstand strong pragradational sequence set. The 2 long-term base-level cycles consisted of 5 mid-term base-level cycles, and demonstrated separately the changes of the lake level rise and fall in long term during the Da'anzhai Epoch, Early Jurassic, and constituted the complete depositional sequence of lacustrine transgressive-regressive cycles. According to the high-resolution isochronostratigrapnic correlation of the turnround points of the multi-order based-level cycles, the Da'anzhai Formation high-resolution sequence stratigraphic framework can be built and it showed the relationship among the evolution of the multi-order base-level cycles and the distribution of the reservoirs, source rocks and caprocks of the formation.