Non-Impact Origin of the Baisha Structure in Hainan Province, China

The Baisha Structure,with a rim-to-rim diameter of ~3.7 km,in the center of the Hainan Province,southern China has been considered to be an impact crater. Field investigation and petrological study are presented in this paper to investigate the impact hypothesis for this structure. The ~600-m-thick Lower Cretaceous feldspathic quartz sandstones from the Lumuwan Formation are the major outcrops both within and outside of the structure. The amphitheater-shaped rim of the structure is composed of granite porphyries that are intruded in the Lumuwan Formation. Previously interpreted impact breccia and impact melt rocks are actually granite porphyries different cooling rates and weathering status. Rocks from locations that most likely have recorded shock metamorphic signatures are sampled,but petrographic analyses reveal no indications of shock metamorphism. While subtle structural deformation occurs at the contact boundary between the granite porphyries and the feldspathic quartz sandstones,the feldspathic quartz sandstones exhibit uniformed dipping strata across the crater floor and walls. All the evidence suggests that the Baisha Structure was not formed by impact cratering. It most likely has been shaped by a combination of magmatic intrusion and long-term differential erosion.