High‐Resolution Eocene Magnetostratigraphy of the Xijigou Section: Implications for the Infilling Process of Xining Basin, Northeastern Tibetan Plateau

Abstract

The Eocene was the initial stage of the Cenozoic global climatic transition from greenhouse to icehouse conditions. Our understanding of the Eocene climate is based mainly on marine records, and comparison of marine and terrestrial climate records of the Eocene has revealed inconsistencies. Thus, there is a need for more continuous, high‐resolution Eocene sedimentary records from land. Xining Basin in the northeastern Tibetan Plateau contains a sequence of well‐exposed and complete Eocene strata. Extending previous research, we have conducted a new high‐resolution lithostratigraphic and magnetostratigraphic study of the Xijigou section in Xining Basin. The results indicate that the Xijigou section spans the interval from ~34.2 to 53 Ma, representing an average sediment accumulation rate of ~1.4 cm/kyr. The basal ages of the Mahalagou and Honggou Formations are ~39.9 and ~52.4 Ma, respectively. A comparison of all available Eocene sedimentary records from Xining Basin indicates that the strata deposited during ~53–35 Ma developed conformably and are well correlated over space. We conclude that sediment deposition was not affected by large‐scale tectonic activity at the time and rather was controlled by the global climate. After ~35 Ma, the sediment accumulation rate increased to ~4 cm/kyr, which may reflect the far‐field influence of the India‐Eurasia collision on Xining Basin.

Provenance of Cenozoic Sediments in the Xining Basin Revealed by Nd and Pb Isotopic Evidence: Implications for Tectonic Uplift of the NE Tibetan Plateau

“…With a thickness of more than 800 m, the well‐developed Cenozoic stratigraphy which is unconformable on the late Cretaceous sedimentary rocks or older basement rocks in the Xining Basin has been divided into the following formations, proceeding upward: Qijiachuan, Honggou, Mahalagou, Xiejia, Chetougou, and Xianshuihe (QBGMR, ; Xiao et al, ; Yang et al, ; Figure ). The stratigraphy consists of basal mudstone with intercalated gravelly sandstone and gypsiferous layers (Qijiachuan Formation), overlain by brownish‐red mudstone with intercalated cyclical thick gypsum layers (Honggou and Mahalagou formations), and in turn overlain by light brownish‐red to yellow mudstone with occasional sandstone layers (Xiejia, Chetougou, and Xianshuihe formations; Dai et al, ; QBGMR, ; Xiao et al, ).…”

“…The stratigraphy consists of basal mudstone with intercalated gravelly sandstone and gypsiferous layers (Qijiachuan Formation), overlain by brownish‐red mudstone with intercalated cyclical thick gypsum layers (Honggou and Mahalagou formations), and in turn overlain by light brownish‐red to yellow mudstone with occasional sandstone layers (Xiejia, Chetougou, and Xianshuihe formations; Dai et al, ; QBGMR, ; Xiao et al, ). Magnetostratigraphic investigations have been intensively conducted on the Xining Basin sedimentary sequence (e.g., Dai et al, ; Xiao et al, , ; Yang et al, ). Recently, Fang et al () synthesized the paleontological, lithological, and magnetostratigraphic records of the Xining Basin, showing that the fluvial‐lacustrine sedimentary sequence of the basin was deposited from ~54 to ~4.8 Ma.…”

“…The age, stratigraphy, lithology, and sedimentary facies of the Xijigou and Tashan sections. The magnetostratigraphic age of the Xijigou section is from Yang et al (), and that of the Tashan section is from Xiao et al (, ). (a) Mudstone of the Xianshuihe Formation, (b) sandstone of the Chetougou Formation, (c) mudstone with intercalated gypsiferous layers (~1–3‐m thick) of the Xiejia Formation, (d) mudstone of the Xiejia Formation, (e) mudstone with intercalated gypsum nodules of the Mahalagou Formation, (f) mudstone with gypsum of the Mahalagou Formation, (g) mudstone with gypsum of the Honggou Formation, (h) sandstone of the Honggou Formation, (i,j) mudstone of the Qijiachuan (QJC) Formation.…”

“…The measured 278‐m‐thick Xijigou section consists of the uppermost parts of the Qijiachuan and Honggou formations and the lower‐middle part of the Mahalagou Formation (Yang et al, ). The uppermost part of the Qijiachuan Formation (278–269 m) consists of variegated mudstone with intercalated a few gypsiferous layers (Figures i and j), indicating a shallow brackish lake environment.…”

“…At the northeastern margin of the Tibetan Plateau, the Xining Basin (Figure a) has accumulated a sequence of early Eocene‐early Miocene sediments, which are an outstanding archive for understanding the tectonic and climatic history of the NETP (Dai et al, ; Xiao et al, ; Yang et al, ; Zhang, Xiao, et al, ). Basin sediments transported by rivers from the adjacent mountains can record fundamental information about the uplift, exhumation, and regional climate history (Carrapa et al, ; Wang, Zhang et al, ).…”