The boundary between the Middle and Upper Cambrian strata in the Tarim Basin, NW China: Regional to global SPICE event correlation

Abstract

The subdivision and correlation of the Cambrian System have long been controversial globally with multiple schemes. This is especially the case for the Tarim Basin, western China where regional and global correlations of the Middle-Upper Cambrian boundary are hampered by incomplete biostratigraphy. The Middle-Upper Cambrian boundary (T81) in the Tarim Basin is characterized by an abrupt switch of lithological and sedimentary facies, a sudden change of wireline log responses, and the contrasting seismic features, structural styles and carbonate platform architectures. The T81 boundary manifests in various forms and styles including syn-sedimentary unconformity, overlapping unconformity, tectonic unconformity, multi-stage superimposed unconformity, parallel unconformity and conformity. Each manifestation type of the T81 boundary was influenced by at least one of the factors including paleo-geomorphology, tectonic movement, eustacy, and climatic change. Based on the correlation between carbon isotopes and sedimentary characteristics in different regions globally, we suggest that the δ13C weak negative shift that follows the strongly positive shift of the Late Cambrian SPICE event can serve as a reliable isotope stratigraphic marker for identifying the T81 boundary of the western Tarim Platform. The top of the Awatag Formation (the top of the Middle Cambrian) in the western Tarim Platform should thus be correlated to the top of the Mohershan Formation in the eastern Tarim Basin, corresponding to the top of the Miaolingian Stage. The absence of the SPICE event in the western part of the Tarim Basin may have been caused by a regional depositional hiatus associated with the T81 boundary.