Spore Color Index Study of the Hongsa Sediments, Lao PDR: Implications for Thermal Maturity and Paleoenvironment

Abstract

Palynological analyses of samples from Neogene organic-rich fine-grained sediments in the Hongsa coal deposit, Lao PDR, have yielded a high count of palynomorphs, including abundant sporomorphs with varying colors. This study aims to evaluate the thermal maturity of the sediments and their paleoenvironment using the Spore Color Index (SCI). Microscopic analysis of sporomorph color variation provides a quick method to reconstruct the thermal alteration of sediments. Only monolete spores were selected to investigate spore color variation. The results show that almost 50% of the lightest colored monolete spores are pale yellow to light yellow, indicating a thermally immature zone within the early diagenetic stage of organic matter transformation. This group represents in situ materials, consistent with coal rank and vitrinite reflectance. Another group shows yellow to brown sporomorphs, suggesting the catagenetic stage, with coal ranks ranging from subbituminous to low volatile bituminous. The latter group is considered reworked sporomorphs transported to the Hongsa deposit area along with in situ palynomorphs. These reworked materials possibly originated from the Mesozoic bedrock surrounding the Hongsa basin, influenced by the basin’s development related to the interaction between the Indian and Eurasian plates.