Magnetostratigraphy and Biostratigraphy of the Jurassic sedimentary sequences, Qiangtang Basin, revealed the initial time of the Lhasa-Qiangtang collision

Abstract

The Qiangtang Basin, located in the Tibetan Plateau, is an appropriate area to verify the Lhasa–Qiangtang collision, which was recorded by the middle-upper part of the Yanshiping Group (the Xiali and Suowa Fms) in the basin. However, the chronology of the Xiali and Suowa Fms is HYPERLINK “javascript:;” controversial, which limits comprehending the timing of the Lhasa–Qiangtang collision. More importantly, HYPERLINK “javascript:;” oil HYPERLINK “javascript:;” shale and salt springs were exposed in the Xiali and Suowa Fms in the basin. 544 paleomagnetic samples were collected from the Yanshiping section in the basin in order to reveal the timing of the Lhasa–Qiangtang collision from the view of paleomagnetic ages of the two formations. However, we did not give credible magnetostratigraphic ages of the two formations because of ammonite fossils, a global primary standard for the Jurassic strata correlation, without being found in the last study. Yin (2016) revised the long-term HYPERLINK “javascript:;” controversial paleontological age of the Suowa Fm. from a Tithonian age of the Late Jurassic or a Berriasian age of the Early Cretaceous, to a Middle Bathonian–Middle Callovian age of the Middle Jurassic based on new ammonite fossils. Considering ammonite fossils as a powerful tool and a global primary standard for the Jurassic strata correlation, we attempted to correlate the last magnetostratigraphy with the GPTS 2012 again. Magnetostratigraphic ages of 164.0–160.2 Ma and 160.2–156.8 Ma for the Xiali and Suowa Fms are suggested, respectively. The timing of the Lhasa–Qiangtang collision (156.8–154.9 Ma) is revealed from the magnetostratigraphic ages and the zircon U–Pb age of the Xueshan Fm, 154.9 (+6.8/−1.6) Ma, overlying the Suowa Fm in the Yanshiping section.