Surface microscopic characteristics of quartz sand grains in the Paleogene Baoxiangsi Formation of Lijiang-Jianchuan area, northwestern Yunnan

Quartz sand grains have a variety of surface characteristics, which could be used as the main indicators of their depositional environment. In the present study, the surface characteristics of quartz sand grains were systematically analyzed on the basis of the samples from varying levels of the Baoxiangsi Formation (E2b) in Lijiang-Jianchuan area, Yunnan Province, by using electronic scan microscope. The results show that the grain surfaces of the samples from the lower part of the Baoxiangsi Formation have aqueous characteristics, those from the middle part have eolian characteristics, and those from the upper part have reworked eolian characteristics. It is thus concluded that the lower part is a proximal diluvial environment, the middle part is an eolian dune environment, and the upper part is a lake environment. Moreover, the quartz sand grains of the samples from the middle part are well sorted and rounded, and commonly have standard dish-shaped impact scars, crescent-shaped impact scars and frosted surface, suggesting typical desert sand sediments.