Deciphering the Early Cretaceous transgression in coastal southeastern China: Constraints based on petrography, paleontology and geochemistry

The occurrence of a transgression in coastal southeastern China during the Early Cretaceous has been debated. This question was addressed here through the use of petrographic, paleontological and geochemical studies of ten representative Lower Cretaceous outcrop sections in the study area. The petrographic results suggest that limestones in the Shipu section of northeastern Zhejiang Province were predominantly deposited in a tidal flat environment. The paleontological study reveals marine red and brown algae in black shales and mudstones of the Yongʹan and Chongʹan sections of the western Fujian Province. In the biomarker study of black shales and mudstones, the widespread detection of gammacerane, in combination with ratios of tricyclic terpane C26/C25 < 1.3, hopane C35S/C34S > 0.8 and hopane C29/C30 < 0.8, demonstrates that the study area was influenced by a transgression. A carbon isotopic study provides additional evidence of the transgression, including positive carbon isotopes of the majority of calcareous mudstone and limestone samples (approximately 2.5‰), and correlation of the values between saturated (− 30.01‰ to − 22.87‰) and aromatic (− 29.42‰ to − 21.35‰) hydrocarbons of shale and mudstone samples. Thus, transgression did take place in coastal southeastern China during the Early Cretaceous, and was broadly simultaneous (from 119 ± 3 Ma to 99 ± 3 Ma) in different depositional regions based on zircon U–Pb dating. Under this isochronous framework, a paleogeographical limit of the transgression was tentatively proposed for the first time. The northern boundary extends at least to 29° N, whereas the western boundary is limited to the southeastern side of the Wuyi Mountains. Regional tectonic subsidence together with the overall high sea level may be the main driving force for the transgression. These results have broad implications for regional studies and for Cretaceous paleogeographical studies in general.