Geological characteristics and age of Nanhao Formation in Nanhao Area, Hainan Province

Abstract：The Nanhao Formation is a rock-stratigraphic unit at the bottom of Neopaleozoic transgressive sequence. The rock association characteristics of the lectostratotype section is alternating beds of quartz fine sandstone, sandstone, sericite slate and silty slate with different thicknesses， whose bottom is pebbled inequigranular sandstone with Brachiopoda, Gastropod and crinoidal fossils. The Nanhao Formation is in angular unconformity contact with the underlying Lower Silurian Zusailing Formation, with the thickness over 156 m, and the top is unobserved. The top of the second bed of the lectostratotype section is silty slate, having Fusella sp.，Leptagonia sp. and Chonetipustula sp. of Brachiopoda （Ⅲ31）, and the section also contains Eochiristites sp. of Brachiopoda in Lower Carboniferous Yanguan stage. As for the bottom of the second bed, there are siltstones with Tenticospirifer cf. supervilis, Ptychomaletoechia sp., Fusella sp. of Brachiopoda （Ⅲ17） and crinoidal caudex. The first two, belonging to Upper Devonian Xikuangshan stage, are associated with the last two which are of Lower Carboniferous period. Since there is no Devonian fossil in either the Xinshan section or the east adjacent Fenggaoling section in Nanhao area, the Nanhao Formation is classified as Carboniferous period. Recently, some researchers collected Silurian fossils in the second bed of the lectostratotype section, and they thus hold that the lectostratotype section belongs to Silurian instead of Carboniferous, that the Nanhao Formation is not in angular unconformity contact with the underlying Strata, and that the Nanhao Formation should be abandoned. The authors here point out that the Silurian fossils in the second bed of the lectostratotype section are actually collected from the Silurian Strata that underlie the Nanhao Formation, and hence the Nanhao Formation should not be abandoned.