Petrology and organic geochemistry of the Middle Devonian cutinitic liptobioliths from Luquan region, Yunnan Province, China

The majority of Middle Devonian coal samples from Wujing (WJ), Damo (DM), and Batang (BT) areas in the Luquan region of China have cutinite content exceeding 50%, classifying them as typical cutinitic liptobioliths. These coals are notable for their thinly-bedded texture, which allows them to split readily into leaf-like laminae. However, coals from these areas display distinct macroscopic, microscopic, and geochemical features. The split laminae from WJ and DM coals are black and ribbon-like, with those from WJ being notably longer and wider. In contrast, BT coal laminae are irregular, small fragments with golden or brown hues. Correspondingly, the cutinite in WJ coals is predominantly thick-walled, DM coals primarily contain a medium-walled type, and BT coals are characterized by a thin-walled type. Additionally, BT coals are rich in sporinite, with some classified as sporinite-rich durain or sporinitic liptobiolith. Abundant tetracyclic diterpenoids were detected in the maltene fraction of all the coal extracts. The concentration and composition of these diterpenoids are influenced by the content and type of cutinite present in the coals. The presence of tetracyclic diterpenoids, with beyerane, atisane and kaurane skeletons, had already evolved in early land plants. This implies that the precursors of these compounds were likely significant components of the cuticles of early land plants or, at least coexisted alongside plant cuticles. The spore assemblages in coals and the microscopic features of cutinite suggest that the coal-forming plants of the Luquan cutinitic liptobioliths were early land plants, predominantly herbaceous lycopsids, rhyniopsids, Orestovia and Spongiophyton. However, it is important to note that the specific types of these coal-forming plants may vary by areas. This variation leads to the distinctive macroscopic and microscopic characteristics observed in coals from different locations.

Cutinitic liptobiolith appears to have been exclusive to the Middle Devonian period, especially during the Givetian. The emergence of this distinct coal type is likely due to the unique structure of Devonian land plants. Specifically, the combination of well-developed cuticles and less developed woody tissues in these early land plants might have played a key role in the formation of cutinitic liptobioliths.