Foraminiferal biostratigraphy and facies analysis of the Permian Longge Formation in the Rongma Area, Tibet, China: implications for the palaeogeography of the South Qiangtang Block

Abstract

Wang, Z-B., Gao, J-H., Wang, G-H. & Kang, Z-H., June 2018.2018. Foraminiferal biostratigraphy and facies analysis of the Permian Longge Formation in the Rongma Area, Tibet, China: implications for the palaeogeography of the South Qiangtang Block. Alcheringa XX, x–x.The Permian Longge Formation in the South Qiangtang Block, Tibet, is overwhelmingly composed of carbonate rocks with various skeletal (brachiopods, foraminifers, gastropods, echinoderms, ostracods, corals, bivalves, algae, and bryozoans) and non-skeletal (intraclasts and ooids) components. Three stratigraphic sections of the Longge Formation in the Rongma area of north-central Tibet—known as South Yibug Caka, Niushan and East Yibug Caka—were selected for this study, which examined both sedimentary facies and foraminiferal assemblages. The foraminifers in these sections consist of at least 38 species belonging to 18 genera. Based on the distribution of the foraminifers throughout the composite section, two foraminiferal assemblages were established, and the age of the Longge Formation was determined to be late Kungurian to Capitanian. During lithological studies, ten microfacies were identified using depositional textures, petrographic analysis and faunal content: mudstone, bioclast wackestone, bioclast perforated-foraminifera packstone, bioclast crinoid grainstone, intraclast wackestone, breccia, intraclast grainstone, ooid grainstone, fine crystalline dolostone and residual-grain dolostone. These microfacies are interpreted to represent four depositional environments—restricted lagoon, open marine, shoal and slope—which together suggest a shoal-rimmed carbonate platform. The non-fusuline foraminifers show transitional palaeobiogeographic affinities (Tethyan Cimmerian subregion), and the assemblage is considered to be influenced by the northward drift of the South Qiangtang Block, the climatic warming after the Late Paleozoic Ice Age and warm-water oceanic currents caused by the newly formed Neotethys Ocean. This indicates that the South Qiangtang Block was located in a relatively warm-water, low-latitude area during the middle Permian. The Permian depositional sequences in the Rongma area were also influenced by the palaeogeographic evolution of the South Qiangtang Block.Zhong-Bao Wang [zbwang@cugb.edu.cn] and Zhi-Hong Kang [kangzh98@cugb.edu.cn] School of Energy Resource, China University of Geosciences (Beijing), 29 Xueyuan Road, Beijing 100083, PR China; Jin-Han Gao* [wang_zhongbao@126.com] and Gen-Hou Wang [wgh@cugb.edu.cn] School of Earth Sciences and Resources, China University of Geosciences (Beijing), 29 Xueyuan Road, Beijing 100083, PR China.