Cisuralian Brachiopod Biostratigraphy and Palaeobiogeography of the Sibumasu Block

The tectonic evolution of the Sibumasu Block during the Permian remains controversial. Permian faunas and biostratigraphy provide valuable insights into its paleogeographic and tectonic histories. In this study, a new brachiopod fauna is described from the Taungnyo Group in the Taungnyo Range, eastern Myanmar. This fauna comprises 21 species and is dominated by Cimmeriella, Strophalosia, Etherilosia, Costatumulus, Taeniothaerus and Trigonotreta, along with other characteristic elements, such as Bandoproductus and Spirelytha. Owing to the unclear stratigraphic horizon of this fauna, its age is difficult to be determined precisely. Given that the topmost part of the Taungnyo Group is dated as late Kungurian, its age cannot be younger than the Cisuralian. In this study, machine learning methods including Random Forest, k-Nearest Neighbor and XGBoost are applied for age prediction, based on global Cisuralian brachiopods from the PBDB database supplemented with some data from our own database. The high-confidence results obtained from different methods indicate that the age of this fauna is Sakmarian. Furthermore, the age of other Cisuralian brachiopod faunas from the Sibumasu Block that lack precise chronostratigraphic control are also reassigned based on the same methods. Therefore, two independent stratigraphic assemblages are recognised, the lower (earlier) Bandoproductus-Spirelytha Assemblage of a Sakmarian age, and the upper (younger) Spinomartinia-Retimarginifera Assemblage of a Kungurian age. The Sakmarian assemblages are mainly composed of Gondwanan-type genera, and widely found in association with glacial-marine diamictites throughout the Sibumasu Block, indicating that the Sibumasu Block was attached to the northern Gondwanan margin during the early Cisuralian.