Redox Condition Variations During Deposition of Lacustrine Mixed Sediments for the Lower Ganchaigou Formation (Oligocene) in Yingxi Area, Qaidam Basin, China: Evidence From Pyrite Framboids and Geochemical Data

Abstract

The typical lacustrine mixed sediments were developed in the lower Ganchaigou Formation (Oligocene) in Yingxi area of Qaidam Basin. The mineral assemblage of lacustrine mixed sediments has vital significance for reconstructing paleo-lake environment. However, little research has been done in the study area. In order to reconstruct the redox conditions of paleo-lake bottom water, the pyrite framboid size distributions, sulfur isotopes of pyrite (δ³⁴S_py) and redox-sensitive element ratios (Th/U, V/(V+Ni), V/Cr) of authigenic carbonates in mixed sediments were studied.

Three different types of mixed sediments were distinguished by petrology and XRD analysis: mixed terrigenous clast, mixed carbonate and mixed evaporate. There are abundant pyrite framboids in different types of mixed sediments, and the size of pyrite framboids is positively correlated with its δ³⁴S_py. Large mean diameters (5.45~8.43μm), wide size distributions (standard deviation≥2.05), large maximum diameters (~25μm) and large average δ³⁴S_py (-1.7‰) in mixed terrigenous clast indicate diagenetic origin and dysoxic/oxic water-column condition. Medium mean diameters (3.70~6.37μm), normal size distributions (SD≤2.28), medium maximum diameters (~15μm) and medium average δ³⁴S_py (-6.1‰) in mixed carbonate suggest syngenetic origin and euxinic-anoxic water-column condition. Small mean diameters (2.32~3.13μm), narrow size distributions (SD≤1.51), small maximum diameters (~7μm) and small average δ³⁴S_py (-9.4‰) in mixed evaporate indicate syngenetic origin and euxinic-anoxic water-column condition. The redox-sensitive element ratios (Th/U, V/(V+Ni), V/Cr) demonstrate the same conclusion: different types of mixed sediments develop in divergent stages of lacustrine basin, corresponding to different redox conditions. Therefore, three models of the sedimentary period of the mixed sediments in saline lacustrine basin are proposed. With the strengthening closure of the basin, the climate gradually goes dry and cold, and the evaporation and salinization have enhanced, thus mixed terrigenous clast (Model I), mixed carbonate (Model II) and mixed evaporate (Model III) are precipitated successively. Correspondingly, the lake water gradually develops salinity stratification, and in the bottom water, the degree of hypoxia has gradually raised; the degree of reduction has gradually strengthened; the role of microbial action (bacterial sulfate reduction) has gradually enhanced.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018