ASHES AND TRILOBITES IN THAILAND: THE KEY TO DATING THE CAMBRIAN-ORDOVICIAN BOUNDARY

Sibumasu (the Shan-Thai Block) was a peri-Gondwanan terrane that flanked equatorial Gondwana’s northern margin during the late Cambrian and early Ordovician. Along with the North and South China, Lhasa, Qiantang, Tarim, and Indochina blocks, Sibumasu lay outboard of the eastern Gondwanan core comprising India-Antarctica-Australia in some configuration, the details of which are yet to be determined. The Tarutao Group outcropping in southern-most Thailand is Sibumasu’s best exposed, most continuous and accessible unit spanning the Cambrian-Ordovician boundary and uniquely comprises a sequence of interbedded fossiliferous sandstones and rhyolitic tuffs. Age-equivalent units elsewhere in Sibumasu include the Machinchang Formation of Malaysia, the Molohein Group and Pangyun Formation of the Shan States of Myanmar, and the Baoshan Formation of western Baoshan in southwest Yunnan, China. Recent work on the Tarutao Group has recovered new late Cambrian and early Ordovician trilobite taxa, including a new genus, Satunarcus, and other genera not previously recovered from Sibumasu including Haniwa, Apatokephalus, and Wuhuia. The expanded collection of taxa and the inclusion of new field localities and stratigraphic units facilitate improved biostratigraphic and paleogeographic correlation between Sibumasu and associated Gondwanan terranes. Sibumasu’s multiple datable ashes collectively span a significant portion of the latest Cambrian and the Cambrian-Ordovician boundary. They will permit age calibration of the Gondwanan biostratigraphic record in what is currently one of the most poorly dated intervals of Phanerozoic history. They will also allow assessment of the rates of evolutionary change during the transition from the Cambrian fauna to the establishment of the Paleozoic fauna in the Ordovician. The genus-level similarities of the Tarutao fauna with both Korean fauna and Australian aids in stratigraphic correlation and also finally resolving the configuration of peri-Gondwanan terranes, the reconstruction of which is supported by detrital zircon records.