Using cyclostratigraphic evidence to define the unconformity caused by the Mesoproterozoic Qinyu Uplift in the North China Craton

Abstract

In the Yanliao Basin of the North China Craton, the Mesoproterozoic Xiamaling Formation contacted unconformably with the underlying Tieling Formation. This unconformity was caused by the Qinyu Uplift. Underneath this unconformity, the Tieling Formation experienced variably weathering with more extensive erosion of carbonates in the west than in the northeast of the basin. To constrain the timing of the Qinyu Uplift and the duration of subsequent unconformity, cyclostratigraphic analysis was conducted on the Tieling carbonate rocks. Referencing the periodicities of the eccentricity, obliquity and precession obtained from the Mesoproterozoic Xiamaling and Hongshuizhuang Formations, Milankovitch cycles during the Tieling Formation were recognized. A bentonite located at the middle of the Tieling Formation was recognized from the core at the Beizhangzi section, and was considered to be contemporaneous with those found at the Jixian and Liujiagou sections, which had a consistent age of 1441 Ma. Then a deposition rate from the bentonite layer to the top interface of the Tieling Formation was calculated to be 1.65 ± 0.22 cm kyr−1, and the duration time were ca. 9 million years. So the minimum age of the Tieling Formation was ca. 1432 Ma. Combined with our new 207Pb/206Pb weighted average age of 1418 ± 14 Ma obtained from the bottom of the Xiamaling Formation, the Qinyu Uplift was constrained to occur between 1432 Ma and 1418 Ma, older than the previously suggested age of 1400 Ma. The duration of this unconformity was suggested to be no less 14 million years, and might be longer in the western part.