Thermally altered microbialites of the terminal Ediacaran Dengying Formation, Sichuan Basin, South China

Abstract

In the Sichuan Basin and surrounding areas, the terminal Ediacaran Dengying Formation hosts diverse microbialite forms. Interest in this formation has mainly centered around oil and gas reservoir development, but recent research aims to characterize its paleobiology via petrographic and molecular fossil analysis. This study aims to characterize the paleobiology and diagenetic history of samples from the Moxi area (central basin), Micangshan Uplift (NW basin margin), and Dabashan thrust belt (N basin margin). Moxi samples are all core samples of Dengying Member 4. Michangshan and Dabashan samples are from outcrop; the Micangshan samples are from Dengying Member 3 and the Dabashan samples are from unknown members of the Dengying Formation and underlying the Doushantuo Formation. All samples were photographed using a petrographic microscope in plane-polarized light (transmitted and reflected), with crossed nicols and in blue-violet light. Hydrocarbon geochemistry and extractable biomarkers were isolated from outcrop samples and characterized. Microbialites in the core samples are mostly stromatolitic and occasionally thrombolitic in texture, while outcrop microbialites are oncolitic and thrombolitic. All samples show putative evidence of hydrothermal alteration and tectonic strain. Extractable biomarkers were limited to a low concentration of n-alkanes with an abnormal distribution of chain lengths. Kerogen geochemical data confirms the samples’ low amenability to biomarker extraction; the maximum hydrogen index (HI) is 51 for all Dengying samples and among the samples with acceptable total organic carbon ( 0.5%) the maximum HI is only 6. Based on mass spectra and free oil measurements, extracted hydrocarbons can be attributed to contamination. The Dengying Formation represents a microbially-dominated system that preserves no convincing evidence of eukaryotes, despite the presence of putative metazoans in nearby sections of the Doushantuo. Extraction of saturated alkanes produced poor yields in the outcrop samples and would likely not be feasible for the core samples based on petrographic analysis showing thermal and tectonic stress. Literature has drawn mixed conclusions about the Dengying Formation’s potential as a source of geologic biomarkers; this study provides more evidence that the Dengying is unlikely to yield any significant biomarker information.