Features of the Paleogene Shahejie Formation in the northern Dongpu Sag

Based on paleontology,petrology and well-logging,the authors have studied the stratigraphic features,division and correlation of the Paleogene Shahejie Formation in northern Dongpu Sag.Member 2 and lower Submember of Member 4 of Shahejie Formation are mainly composed of a set of red sandstones and mudstones.There is an unconformity surface between the lower Submember and the upper Submember of Member 4 of Shahejie Formation.There exists a hiatus at the bottom of Member 2 of Shahejie Formation.Member 1,Member 3 and upper Submember of Member 4 of Shahejie Formation are mainly composed of a set of gray sandstones,mudstones and shales with four sets of salt rocks.The stable calcareous mudstones and shales of Member 1 and Member 3 of Shahejie Formation constitute fairly ideal key beds for regional stratigraphic correlation in the Dongpu Sag.Fossils are rare in the lower Submember of Member 4 of Shahejie Formation.The local ostracodes,called Cyprinotus altilis assemblage,began to develop in Member 4 of Shahejie Formation.Member 3 of Shahejie Formation is characterized by very flourishing ostracodes,such as Huabeinia and Candona,called Huabeinia chinensis assemblage.The dinoflagellates of Member 3 of Shahejie Formation are called Bohaidina-Parabohaidina assemblage.Member 2 of Shahejie Formation is characterized by the most prosperous stonewort called Charites producta assemblage.The ostracodes of Member 2 of Shahejie Formation are called Cyprinotus xiaozhuangensis assemblage.Member 1 of Shahejie Formation is characterized by the most prosperous gasteropods called Valvata(Cincinna) rehetaiensis assemblage.The ostracodes of Member 1 of Shahejie Formation are called Phacocypris huiminensis assemblage.The standards adopted by previous researchers were the same as those in dividing each Member of Shahejie Formation in northern Dongpu Sag.However,there existed remarkable differences in determining the specific boundary.What is more,the previous researchers adopted the standards of extinction of Cyprinotus altilis and the lower boundary of Huabeinia acme-zone in dividing the boundary of Member 3 and Member 4 of Shahejie Formation.However,because of the limited accuracy of palaeontologic standards in dividing the strata,the lack of geological data from wells at that time and the existence of complex faults in the Dongpu Sag,the boundaries of different areas defined by the same researcher are different.It is pointed out in this paper that strengthening researches on sedimentology in such aspects as the sequence stratigraphy and variation regularity of sedimentary facies constitute the key to resolving the problems of stratigraphic division and correlation in the Dongpu Sag.