Lumuwan Gr
Type Locality and Naming
Hainan Island and adjacent part of Guangdong. The No.764 Geological Team of the Guangdong Bureau of Geology and Mineral Resources erected the Lumuwan Group in 1964. The section for the designation is in the river valley of Lumuwan about 150 m northeast of Lumuwan of Danxian, Guangdong. Comprises a lower Nanmei Fm and an upper Hongmao Fm.
Synonym: Lumuwan Fm
Lithology and Thickness
The Lumuwan Formation is mainly represented by purple red, purple gray and grayish green conglomerate, sandstone, siltstone and mudstone with andesite and dacitic pyroclastic rocks, forming a set of rhythms. The formation is 736 m thick.
Relationships and Distribution
Lower contact
Its base marked by conglomerate shows an angular discordance with the underlying strata. In the Sanya area (southern Hainan island), it lies unconformably on the Upper Ordovician Gangoucun Fm. In Baisha Basin, it is a disconformity onto the Nanbigan Gr (metamorphosed silty slate).
Upper contact
Its top is unknown. In the Sanya area, it lies disconformably under the Miocene Foluo Fm.
Regional extent
The formation mainly occurs in the Baisha-Ledong, Yangjiang-Qionghai, Changpo-Dongjiang, Leiming and Sanya basins of the Hainan Island. It is varied in the development of volcanic partings, for example, 1-20 partings in the Baisha-Ledong basin, 5 partings in the Leiming basin, and no volcanic partings are found in the Leiming and the Yangjiang-Qionghai basins. It varies considerably in thickness, for example, 736 m in the Baisha-Ledong basin and 2417 m in the Lleiming basin.
GeoJSON
Fossils
The formation yields ostracods Cypridea wenmingsiensis, C. curta, C. epimeco, Eucypris debiloides, E. angulata; Charophytes Nesochara symmetrica- Euaclistochara assemblage.
Age
Depositional setting
It is of fluvial-piedmont basin facies.
Additional Information
"Cretaceous strata are widely distributed in a series of basins on Hai- nan Island. The Baisha Basin is the largest, occurring as a faulted- depression containing red bed sequences, which are also widely present in South China. It is mainly filled by a set of lacustrine facies clastic rocks, and the main stratum is the Lumuwan Formation. The Lumuwan For- mation unconformably overlies the Ordovician strata and Indosinian granites, and mainly consists of light grey-purple conglomerate, sand- stone, with partly intercalated silty mudstone. Those rocks contain subrounded to angular detrital grains (Dong et al., 1991; Wang et al., 1991a). Previous paleontological studies from the Lumuwan Formation have recognized plant, ostracoda, stonewort and sporopollen fossils, suggesting that it was deposited in the Aptian to early Albian stages (Dong et al., 1991; Huang, 2010)." [Quoted from Xiao-Yan Jiang et al., 2015. U-Pb age and Hf-O isotopes of detrital zircons from Hainan Island: Implications for Mesozoic subduction models. Lithos 239: 60-17.