comprising predominantly Proterozoic to Palaeogene
sediments, while the south-eastern part of the province
is situated on the Cathaysian Terrane, which is
mainly composed of crystalline rocks (Fig. 1A, B).
Nanning, which is the capital of the Guangxi Zhuang
Autonomous Region, is situated in the Nanning Basin
in the southern part of this province, approximately
150 km to the northeast of the border to Vietnam (Fig.
1B). The Nanning Basin is a late Mesozoic to early
Cenozoic pull-apart basin that is filled with an almost
700 m thick succession of Cenozoic sediments (Deng
& Wu 1992). Sedimentation started in the Palaeocene,
and the basin infill is dominated by deep to shallow
lacustrine facies (Deng & Wu 1992). During the Late
Eocene and Early Oligocene, the sediments of the
Yongning Group were deposited. Based on mammal
faunas (Chow 1958; Tang & Chow 1964), the group
has been subdivided into the Upper Eocene Nadu Formation
and the Lower Oligocene Gongkang Formation,
which has yielded the molluscs described below.
Later on, the discovery of several Anthracotheriidae
confirmed the Early Oligocene age of the Gongkang
Formation (Zhao 1983, 1993).