Sea-level changes recorded by cerium anomalies in the late Jurassic (Tithonian) black rock series of qiangtang basin, North-Central Tibet

The Upper Jurassic black rock series of the Amdo area of north-central Tibet have attracted attention because of its organic-rich matter and oil seepage in the rock series. Abundant ammonites of Aulacosphinctes and Virgatosphinctes as well as bivalves including Buchia and Chlamys, are well preserved in the Qiangmuleiqu Formation of the Middle to Late Tithonian (Late Jurassic). The total rare earth elements (ΣREE) abundance of the formation varies from 18.814 to 46.818 ppm and is lower than that of an average in marine shales. Inter-element correlations suggest that the shale-normalized REE patterns provide the information about the origin of sedimentary rocks (i.e., not affected by diagenesis), and can be used as a potential indicator for eustatic sea-level changes. The Ce_anom values gradually increased from -0.015 to -0.238 from the lower to middle part of the series, indicating a rise in sea level. The Ce_anom then dropped to -0.081, suggesting a lowering of sea level. From the middle to upper part of the section, there are two more sea-level fluctuations indicated by variations of cerium anomaly values. Thus, sea-level fluctuations occurred three times in this Late Jurassic section, with major episodes of eustatic rise took place in the lower-middle part of this section. These episodic cycles show that the sediments in the Amdo area of north-central Qiangtang basin were deposited in continental shelf or deep-water continental slope environments, which is not in agreement with the previous views that there are no deep-water sediments in Qiangtang basin. Our results call for further studies to be undertaken on Jurassic stratigraphic framework and tectono-sedimentary evolution in Qiangtang basin, northern Tibet.