Microbialite fabric evolution controlled by marine redox conditions in the Qigebrak Formation, NW Tarim Basin: Implications for hydrocarbon exploration

Abstract

Microbialites constitute key archives for reconstructing the co-evolution of early life and Earth surface environments. Although the environmental controls on microbialite formation are widely acknowledged, the response of specific microbialite architectures to changes in pelagic ocean chemistry still remain poorly constrained. This paper integrates detailed field mapping, sedimentary facies analysis, and geochemical profiling of the Qigebrak Formation at the Xigou section on the northwestern margin of the Tarim Basin, to characterize microbialite fabrics and assess the role of contemporaneous marine redox conditions in shaping their development during the late Ediacaran. The microbialites at Xigou are well preserved and exhibit diverse fabrics, deposited in mid- to inner-ramp settings. Member II is dominated by laminated microbialites with subordinate clotted and binding fabrics. In contrast, Member III is typified by foam spongy fabrics, marking a significant shift in microbialite architecture. Bulk-rock δ 13 C values record a pronounced positive excursion (∼+5‰ to +6‰) at the base of the formation, followed by a prolonged interval of isotopic stability. This isotopic variation exhibits global correlation significance and, when combined with the Dengying Formation age constraints, limits the depositional interval to approximately 551.1 ± 0.7 Ma to 538.8 Ma. Trace element and rare earth element data from microbial components indicate that both δCe values and V/(V+Ni) ratios in Member III are markedly lower than those in Member II, reflecting a significant marine redox transition from anoxic to more oxic conditions during the late-Ediacaran ocean. This redox shift exerted a primary control on microbialite fabric development: anoxic conditions favored microbial mats accretion and preservation of laminated fabrics, whereas subsequent oxygenation enhanced organic matter degradation, facilitating the emergence and dominance of foam spongy fabrics. The fabric transition may also reflect increased ecological disturbance linked to the rise of metazoans, pointing to a potential coupling between microbialite decline and biotic innovation in the terminal Ediacaran. Our study underscores that changes in marine redox chemistry, coupled with ecological interactions, were primary controls on the evolution of microbialite in the late Ediacaran, and further elucidates the implications of these fabric transformations for hydrocarbon exploration. • Terminal-Ediacaran microbialites shift from stromatolitic to foam spongy fabric. • Ce anomaly and V/(V+Ni) document a redox shift from reducing to oxidizing marine conditions. • Redox evolution controlled the formation and preservation of microbial fabrics. • Ediacaran oxidation and emerging metazoans jointly suppressed stromatolite formation. • Redox evolution controlled the source-reservoir integrated petroleum system of the Qigebrak Formation.