The optical textures of pyrobitumen in nature and its geological significance: revealed from the Ediacaran pyrobitumen in the central Sichuan Basin, SW China

Abstract

Some natural pyrobitumens exhibit optical anisotropy similar to mesophase asphalt, potentially recording critical geological information. However, the significance of their distinctive optical textures remains understudied. Anisotropic pyrobitumen in the Ediacaran Dengying Formation (central Sichuan Basin, southwestern China) provides a key example. This study investigates controls on pyrobitumen optical texture evolution using petrography, scanning electron microscopy, and U–Pb dating. Then, it discusses the relationship between these textures and anomalous thermal events by analyzing their spatial distribution and their geological significance. Results reveal that anisotropic pyrobitumen in Dengying Formation formed from oil cracking caused by Late Permian hydrothermal activity. These pyrobitumen can be divided into two categories based on the optical textures: fibrous and mosaic, reflecting distinct internal physical structures. Fibrous pyrobitumen exhibits more ordered aromatic layers than the mosaic type. Temperature primarily controls optical textures by regulating polycondensation reactions and pyrobitumen viscosity. Although the Gaoshiti and Moxi areas share similar burial histories, Gaoshiti is dominated by highly evolved fibrous pyrobitumen, while Moxi contains predominantly low-evolved mosaic pyrobitumen. This disparity indicates formation of anisotropic pyrobitumen was unrelated to normal burial temperatures but to hydrothermal activity. This study demonstrates that anisotropic pyrobitumen cannot form below 240 °C, even over extended durations. Consequently, in situ anisotropic pyrobitumen in deep strata serves as an indicator of paleo-thermal anomalies, with varying textures acting as proxies for relative temperatures within these events. Under identical geological settings, fibrous pyrobitumen typically records higher temperatures than mosaic pyrobitumen.