Microbial Carbonates of Upper Triassic Doi Long Formation, Lampang Group: A Study of New Outcrop Localities in Northern Lampang, Central North Thailand

The Doi Long Formation is the youngest record of Triassic deposits of the Lampang Group, Sukhothai Zone, central northern Thailand. The evolutionary history behind the presence of microbial limestone remains unknown, despite its high diversity and abundance in reef environments. This study documents new observations on the microbial carbonates of the Doi Long Formation in the northern Lampang Province. Eight new outcrop localities have been discovered and are analyzed in terms of sedimentological details. Microfacies description and fossil identification allow us to interpret the specific depositional environments of the bioconstructors of these mound-like build-ups. In the study localities, the Doi Long Formation consists of predominantly microbial carbonates, containing microproblematic organisms, microbialites (stromatolites), algae, and sponges. Five main microfacies are distinguished and several microproblematic organisms have been identified. The study localities yield microencrusters and microproblematica fossils, including Girvanella-like microstructures, Cladogirvanella, Garwoodia, Cayeuxia, Tubiphytes-like microorganisms, and shell fragments. Depositional environments corresponding to reef (mound-like microbial/microbialite build-ups) settings and lagoon environments have been proposed. Understanding the diversity within the Doi Long Formation is essential for clarifying the fossil assemblage and biological processes in reef ecosystems during the Late Triassic.