Repositioning the Great Unconformity at the southeastern margin of the North China Craton

The Great Unconformity between the Precambrian basement rock and Cambrian sedimentary cover has been extensively documented in Laurentia and may provide insights into the environmental context of the Cambrian explosion. A similar unconformity has been known in the North China Craton (NCC) and it offers an opportunity to constrain the geographic extent and geochronological magnitude of the Great Unconformity. However, the placement and magnitude of the Great Unconformity in the NCC, particularly at the southeastern margin of the NCC, is uncertain. For example, in the Huaibei region of the southeastern margin of the NCC, this unconformity has been variously placed beneath or above the Gouhou Formation, which preserves critical micro- and macrofossil assemblages with great paleobiological and biostratigraphic significance. This uncertainty significantly hampers our ability to fully appreciate the geological history of the NCC. To resolve this issue, we carried out an integrated biostratigraphic, sedimentary petrological and detrital zircon geochronological investigation on the Gouhou Formation and the overlying Houjiashan Formation in the Huaibei region. Our data suggest that the Great Unconformity lies within the Gouhou Formation and represents a ca. 200-300 Myr depositional gap: the Lower Member of the Gouhou Formation is of Tonian age whereas the Middle-Upper members of the Gouhou Formation are of early Cambrian. Therefore, we propose that the Gouhou Formation be redefined to contain only the Lower Member, and the Middle-Upper members be included in the Houjiashan Formation. Our analysis also confirms that the Great Unconformity can be widely recognized along the southeastern margin of the NCC. The recognition of the Great Unconformity in the Huaibei region helps to clarify the tectonic history, basin development, stratigraphic correlation, and paleogeographic reconstruction of the southeastern margin of the NCC during the Tonian-Cambrian periods.