Permian ostracods from limestones associated with a coal deposit, Tak Fa Formation (Guadalupian), central Thailand
Résumé
This research aims to interpret depositional environment of limestones associated with coals found in Nong Phai District, Phetchabun Province, central Thailand based on microfacies and fossil ostracods. Core samples Tak Fa Formation, assigned to late Middle Permian, were obtained in course of subsurface prospecting. Most samples were muddy limestones. Two coal seams were found at 45- and 82-meters depth. Intercalations of limestones, sandstones, shales were found above the upper coal. Limestones classified as bioclastic wackestone and packstone contain brachiopod, ostracods, gastropods, dasyclad green algae, calcispheres and smaller foraminifers. Microfacies study suggest the inner part of homoclinal ramp, from shallow marine to shoal and restricted lagoon. Thirty-three limestone samples were processed by Hot Acetolysis: they provided thirty-four species belonging to 16 genera. The Bairdioidea is dominated while Hollinoidea, Kirkbyoidea, Kloedenelloidea occur in lower proportions. The occurrence of Paleodarwinula and Carbonita? at 13 meters depth suggests the influence of freshwater influence within a shallowing sequence. In conclusion, these limestones and coals from the Tak Fa Formation seem to have accumulated in shallow to transitional marine waters. These observations may relate to the regional sea level drop during late Middle Permian (Wordian-Capitanian), coals being deposited in lagoons developed during this period.