Biostratigraphic and chemostratigraphic constraints on the age of early Neoproterozoic carbonate successions in North China

Abstract

Thick carbonate successions of Proterozoic age were deposited in the Huaibei and Huainan regions along the southern margin of the North China Block. These successions, including the Huaibei Group in the Huaibei region and the Huainan and Feishui groups in the Huainan region, have been correlated with the Qingbaikou Group, which outcrops in the Jixian area of North China and is the eponymous succession of the Qingbaikouan System (traditionally regarded as 1000–800Ma). Recently, it has been shown that lower Qingbaikouan strata in the Jixian area may be as old as 1300–1400Ma. By correlation, the Huaibei, Huainan, and Feishui groups may include Mesoproterozoic strata, but these units have also been interpreted as Tonian or Cryogenian–Ediacaran–Cambrian in age. These contradictory age interpretations hamper our ability to explore the rich paleontological and sedimentary records preserved in these rocks and to assess paleogeographic reconstructions. In this report, we describe new microfossils and stable isotope data (δ13Ccarb, δ18Ocarb, δ34SCAS) from the Huaibei, Huainan, and Feishui groups. Possible vase-shaped microfossils from the Jiayuan Formation of the lower Huaibei Group and characteristic early Neoproterozoic acritarchs (particularly Trachyhystrichosphaera aimika) from the Gouhou and Liulaobei formations are consistent with a Tonian age for the Huaibei and Huainan groups. Similarly, carbon isotope trends in the Huaibei, Huainan, and Feishui groups are comparable with Tonian δ13Ccarb patterns in South Australia and Laurentia, with a plateau of moderately positive (1–5‰) values followed by pronounced negative δ13Ccarb excursion in the Gouhou Formation, which may be equivalent to the Bitter Springs anomaly in South Australia. δ34SCAS values of the Huaibei Group are mostly 25–40‰, similar to values from the stratigraphically equivalent Jingeryu Formation in the Jixian area. Overall, the combination of biostratigraphic, chemostratigraphic, and available radiometric data suggests that the Huaibei, Huainan, and Feishui groups are early Neoproterozoic (Tonian) in age.