AGES OF THE KEY BOUNDARIES DURING THE PRECAMBRIAN-CAMBRIAN INTERVAL IN SOUTH CHINA

The Precambrian-Cambrian (PC-C) transition is a key interval during Earth history, most notably because of its effect on the evolution of life. A number of important events occurred during this time period, from the Snowball Earth event to the biological radiation in the Early Cambrian (the so-called Cambrian Explosion). In order to better constrain these events and make correlations between different sections in different continents, precise dating of marker layers and key boundaries in the PC-C strata is of utmost importance. Here we summarize all the available age information and report new data for the PC-C interval in south China.

(1) the Ni-Mo sulfide marker layer in the Lower Cambrian Niutitang Formation: the age window for this layer should be 537-521Ma, from Re-Os ages of Ni-Mo sulfides (537±10Ma, Jiang et al., 2003) and black shales (535±11Ma, Jiang et al., 2007), and Pb-Pb ages of Ni-Mo sulfides (521±54Ma, Jiang et al., 2006) and black shales (536±39Ma, Chen et al., 2009; 531±24Ma, Jiang et al., 2006), and the best estimate of the age should be slightly younger than 532.6 Ma (zircon SHRIMP U-Pb dating of volcanic ash bed several meters below this layer, Jiang et al., 2009).

(2) Bed 5 in the lower Cambrian Meishuchu section in Yunnan province gives a U-Pb date on an ash bed of 536.5±2.5Ma (nano-SIMS, Sawaki et al., 2008), 535.2±1.7Ma (SIMS, Zhu et al., 2009), or more precisely, 533.85±0.13Ma (TIMS, this study).

(3) the Liuchapo chert is believed to represent the deep water facies of the Ediacaran Dengyi Formation of dolomite, however, no age constraint for it before. We dated an ash bed about 5 meters below the top of the Liuchapo cherts in Hunan province with an age of 546±12Ma, so we suggest the top of the chert layer should represent the PC-C boundary (542±0.3Ma) in south China.

(4) the zircon U-Pb age of an ash bed at the uppermost Liantuo Formation of Neoproterozoic is 730.7±8.2Ma (SHRIMP, this study), indicating that the Liantuo Formation is comparable to the uppermost Banxi Group. This age also places good constraints on the bottom boundary of the Cryogenian.