U–Pb ages of tuff from the Triassic Yanchang Formation in the Ordos Basin: Constraints on palaeoclimate and tectonics

Abstract

The Chang 7 member of Triassic Yanchang Formation in the huger Ordos Basin in North China is important economically as a source of oil in the Triassic, but dating the Triassic successions has proved problematic, especially important oil-bearing units such as the Yanchang Formation. Here we analyze depositional ages, palaeoclimate conditions and palaeoenvironments of the Chang 6 to Chang 10 members of Yanchang Formation using SHRIMP U–Pb isotope dating, trace element composition and palynological analyses for samples from well F75 in the northwestern Ordos Basin. The geochronological and palynological data synthetically indicate that the Chang 7 member was deposited in the Ladinian (Middle Triassic) and the Chang 8 to Chang 10 members were deposited in the Anisian (Middle Triassic). Based on the trace element analysis, the palaeoenvironment abruptly changed in the early sedimentary stage of the Chang 7 member, representing warm and humid palaeoclimate, anoxic, brackish lake water and stronger chemical weathering. Therefore a humid paleoclimate event was recorded during the Chang 7 sedimentary period of the Middle Triassic, which provides a basis for the comparison with the global Triassic paleoclimatic event. It was inferred that the paleoclimate event was referred to palaeotopographic (the basin was surrounded by mountains on three sides and low terrain in the east) of the Ordos Basin, which was controlled by the Qinling orogeny and low-latitude warm and humid airflow of the paleo-Pacific Ocean associated with monsoons.