Biostratigraphical constraints on the disconformity within the Upper Ordovician in the Baoshan and Mangshi regions, western Yunnan Province, China

The Ordovician and Silurian of the Baoshan and Mangshi regions in western Yunnan Province, SW China, have long been regarded as being continuous and representative of the Sibumasu terrane. However, our recent biostratigraphical investigations of three Katian to Llandovery sections of predominantly clastic facies reveal a significant, largely correlative disconformity within the Upper Ordovician. These sections include the Laojianshan and Feida Storehouse sections in Baoshan, and the Shaodihe section in Mangshi. In the Baoshan region, the disconformity lies at the base of the overlying Jenhochiao Formation and spans a time interval from earliest late Katian (Dicellograptus complexus Biozone) to early Hirnantian (lower Metabolograptus extraordinarius Biozone), as constrained by the trilobites and brachiopods from the underlying strata and the graptolites from the overlying strata. In the Mangshi region, the disconformity lies at the base of the overlying Wanyaoshu Formation, spanning nearly the entire late Katian (Dicellograptus complexus Biozone to lower Paraorthograptus pacificus Biozone), as constrained by graptolites, trilobites and brachiopods from underlying and overlying beds. A considerably diverse, typical Hirnantia fauna occurs in the Wanyaoshu Formation, which is correlated with the same fauna from the Kuanyinchiao Formation in the Yangtze region, South China, and its equivalents elsewhere. A weathering crust of 10-20cm in thickness occurs ubiquitously in both regions, in association with the disconformity. This stratigraphical gap within the Upper Ordovician in western Yunnan is probably resulted from the abrupt sea-level drop induced by Late Ordovician Glaciations, or alternatively and less likely result from a regional tectonic uplift in northern Sibumasu terrane.