Cambrian sequence stratigraphy of the middle Yangtze area

The present paper deals with the Cambrian sequence stratigraphic framework, sequence boundaries, sequence patterns and systems tracts, in the middle Yangtze area. Two second-order sequences and fourteen third-order sequences have been distinguished, including:(1) one second-order sequence (SS3) and six third-order sequences (SQ1-SQ6) for the Lower Cambrian strata, with an average time duration of about 5.0 Ma. The systems tracts are generally dark-coloured, with higher mud contents, and thus may serve as excellent source rocks, and (2) one second-order sequence (SS4) and eight third-order sequences (SQ7-SQ14) for the Middle and Upper Cambrian strata, with an average time duration of about 2.9 Ma. The systems tracts are generally light-coloured, with higher dolomite contents, and thus may serve as excellent reservoir rocks. Collectively, the H-type patterns are dominated; the T-type and TH-type are almost identical, indicating a moderate water depth during the Cambrian. The NS-directed differences are manifested in both second- and third-order systems tracts.