Magnetostratigraphy of the Cenozoic Lulehe section in the Qaidam Basin: Implications for the tectonic deformationon the northeastern Tibetan Plateau

Abstract

<p indent="0mm">The Qaidam Basin is the largest Cenozoic basin located on the northeastern margin of the Tibetan Plateau. The extremely thick Cenozoic sedimentary strata in the basin recorded regional tectonic deformation and associated climate change when the Tibetan Plateau grew outward to its margins. The Cenozoic strata in the Qaidam Basin have been historically subdivided into seven primary stratigraphic units: The Lulehe, Xia Ganchaigou, Shang Ganchaigou, Xia Youshashan, Shang Youshashan, Shizigou, and Qigequan formations. Although many studies have been performed in recent decades, the Qaidam Basin’s inception, development and extinction are still unclear, which requires more work, such as high-resolution magnetostratigraphies. This paper focuses on the Lulehe section, which is the type section of the Lulehe Formation on the northern margin of the Qaidam Basin. The Lulehe section encompasses the Lulehe to Shizigou Formations, and the section was previously estimated to have been initially deposited 52 Ma. We collected paleomagnetic samples from 2041 sites in the Lulehe section to establish magnetic polarity sequences of the section. Constrained by the regional stratigraphic correlation, the recognized magnetic polarity sequences of the Lulehe section were anchored to chrons C12n–C3n.4n (31–5 Ma). Our correlation places the age of the Lulehe Formation at 30.9–23.7 Ma, suggesting that the onset of sediment accumulation in the Qaidam Basin occurred ca. 31 Ma. Overlying the Lulehe Formation, the Xia Ganchaigou Formation was deposited from 23.7 to18.1 Ma, the Shang Ganchaigou Formation spans from 18.1 to 12.9 Ma, the Xia Youshashan Formation spans from 12.9 to 9.9 Ma, the Shang Youshashan Formation spans from 9.9 to 6.9 Ma, and Shizigou Formation is younger than 6.9 Ma. To characterize the sedimentary environments of strata within the Lulehe section, we undertook a detailed stratigraphic analysis. At the bottom of the section, the Lulehe Formation is characterized by alluvial facies (30.9–23 Ma). Upward, the Xia Ganchaigou Formation and Shang Ganchaigou Formation (23–13 Ma) are interpreted to have formed in braided river, meandering river, and lacustrine environments. The sedimentary environments of the Xia Youshashan Formation and Shang Youshasahn Formation are interpreted as delta-fan and braided river facies. An abrupt sedimentary environmental change from lacustrine to delta-fan occurred ca. 13 Ma in the Lulehe section. This lithofacies change in the section may represent tectonic deformation of the South Qilian Shan in the northern Qaidam Basin. From ca. 11 Ma, coarse-grained clasts significantly increased upward in the section, and the sedimentary environment changed to braided river. This phenomenon suggests that the northern Qaidam Basin fault system propagated southward to the region adjacent to the Lulehe section. At the top of the Lulehe section, the Shizigou Formation is characterized by an alluvial fan system. Sediment accumulation rates in the Lulehe section from ca. 31 to ca. 5 Ma were calculated based on the Lulehe magnetostratigraphy. We found that the Lulehe section almost maintained a stable accumulation rate from ca. 31 to ca. 15 Ma with an accumulation rate of 170.6 m/Ma. From ca. 15 Ma, the accumulation rate in the study area increased significantly to 271.2 m/Ma, and the accumulation rate in the section decreased slightly to 204.3 m/Ma from 11 Ma. The variations in the accumulation rate ca. 15 and ca. 11 Ma in the Lulehe section are consistent with the sedimentary environment changes ca. 13 and ca. 11 Ma. The increase in the sedimentation rate ca. 15 Ma again suggests that the northern Qaidam Basin fault system began to be active. The decrease in the sedimentation accumulation rate ca. 11 Ma indicates that the Lulehe section was involved in tectonic deformation related to southward propagation of the northern Qaidam Basin fault system, which may have reduced the accommodation space in the Lulehe region on the northern basin margin. Overall, our results indicate that the Qaidam Basin experienced two-stage tectonic deformation beginning ca. 31 Ma and subsequently ca. 15 Ma. The Oligocene (31 Ma) deformation led to the formation of the Qaidam Basin. The northern basin margin has been intensively deformed by anticlines and synclines as a result of fault thrusting since the middle Miocene.