An approach to the stratigraphic ages of the Upper Triassic Sanhedong Formation in Lushi, Yunnan

The Upper Triassic Sanhedong Formation in Lushi, Yunnan occurs as the main host horizons of the Jinding supergiant lead-zinc-silver deposit and Weishanzhacun gold deposit. However, there is still debate about the stratigraphic ages of the Upper Triassic Sanhedong Formation. The authors contend, according to lithostratigraphic division, organic fossils, microfossils and zircon U-Pb ages, that the bioclastic limestones, micritic limestones and dolomitic limestones in the Sanhedong Formation exhibit conformable contacts with the carbonaceous slates in the Waluba Formation, and thrust fault contacts with the sandy slates in the Waigucun Formation. The organic fossils including Halobia cf. Superbescens Kittl, Halobia fallax Mojsisovics, Posidonia sp., Halobia yandongensis chen, Halobi arugosoides Hsu, Halobi symmetrica Smith, Halobia cf. ganziensis che, Halobi moluccana J. Wann., Halobi alaskana Smith, and Mytilus sp. from the carbonaceous slates in the Waluba Formation and the microfossils including Hindeodus parvus, Ozarkodina sp., Eucycloscala sp., and Suicocythere vongovata Wei from the limestones in the Sanhedong Formation are both dated at the Late Triassic in age. In addition, the detrital zircons from the siltstones in the Waluba Formation give a minimum U-Pb age of 218 Ma, indicating that the deposition of the Waluba Formation may be traced back to the Late Triassic. It is concluded that the age of the Sanhedong Formation should be the Late Triassic in age.