| According to the observation of the field outcrop profiles at Fandian, Yankong and Sanhuichang, and the sequence boundary characters and succession analysis of two connecting well sections cross Sichuan Basin, the Cambrian strata can be divided into 12 third-order sequences, named as SQ1 to SQ12 from lower to upper, which include SQ1 to SQ6 in lower Cambrian Series and SQ7 to SQ12 in middle to upper Cambrian Series. Several shallowing-upward cycles develop in the middle-upper Cambrian strata, which include lagoons to platform inner shoals, lagoons to tidal flats, and platform inner shoals to tidal flats. The middle-upper Cambrian sediments are the tidal flat-platform inner shoals-lagoon facies from west to east and the thickness of sediments show a varying pattern that is thick in the east and thin in the west, which is complementary to Lower Cambrian sediments. The total thickness of HST is thicker than that of TST in middle-upper Cambrian strata (SQ7-SQ12) so that it is generally characterized of fast transgression and slow regres sion. Semi-closed shallow-water carbonate platform gradually formed in the east and shoreline developed in the west in the SQ7 during middle Cambrian epoch. The middle Cambrian sedimentary model was inherited in SQ8 to SQ12 during
upper Cambrian but more platform inner shoals developed in the west due to the weakening influence of the terrestrial detritus input and shallowing water. The platform inner shoals generally develop in the middle to top parts of HST in vertical and in the center and eastern parts of Sichuan Basin in plan. |
