Cambrian and earliest Ordovician fauna and geology of the Sông Dà and adjacent terranes in Viet Nam (Vietnam)

Abstract

Later Cambrian and earliest Ordovician trilobites and brachiopods spanning eight horizons from five localities within the Sông Mã, Hàm Reng and Dông Son formations of the Thanh Hóa province of Viet Nam, constrain the age and faunal affinities of rocks within the Sông Dà terrane, one of several suture/fault-bounded units situated between South China to the north and Indochina to the south. 'Ghost-like' preservation in dolomite coupled with tectonic deformation leaves many of the fossils poorly preserved, and poor exposure precludes collecting within continuously exposed stratigraphic successions. Cambrian carbonate facies pass conformably into Lower Ordovician carbonate-rich strata that also include minor siliciclastic facies, and the recovered fauna spans several uppermost Cambrian and Lower Ordovician biozones. The fauna is of equatorial Gondwanan affinity, and comparable to that from South China, North China, Sibumasu and Australia. A new species of Miaolingian 'ptychopariid' trilobite, Kaotaia xuanensis, is described. Detrital zircon samples from Cambrian-Ordovician rocks of the North Viet Nam and Sông Dà terranes, and from Palaeozoic samples from the Trueng Son sector of Indochina immediately to the south, contain a predominance of ages spanning the Neoproterozoic period and have a typical equatorial Gondwanan signature. We associate the Cambrian and Tremadocian of the Sông Dà terrane with areas immediately to the north of it, including the North Viet Nam terrane and the southern parts of Yunnan and Guangxi provinces of China.