Quote:
U-Pb Ages from the Neoproterozoic Doushantuo Formation, China
Daniel Condon,1* Maoyan Zhu,2 Samuel Bowring,1 Wei Wang,2 Aihua Yang,2 Yugan Jin2
U-Pb zircon dates from volcanic ash beds within the Doushantuo Formation (China) indicate that its deposition occurred between 635 and 551 million years ago. The base records termination of the global-scale Marinoan glaciation and is coeval with similar dated rocks from Namibia, indicating synchronous deglaciation. Carbon isotopic and sequence-stratigraphic data imply that the spectacular animal fossils of the Doushantuo Formation are for the most part younger than 580 million years old. The uppermost Doushantuo Formation contains a pronounced negative carbonate carbon isotopic excursion, which we interpret as a global event at circa 551 million years ago.
1 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
2 Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008, China.
* To whom correspondence should be addressed.
And an excerpt:
Quote:
The age of the uppermost Doushantuo Formation (Miaohe Member) is constrained by the age of a third ash bed, JIN-04-2 (Jijiawan Section) that occurs at the top of the black shale member containing the Miaohe biota. Zircons from this ash bed yield variably discordant dates (n = 10) that define a linear array on a concordia diagram anchored by two concordant analyses (fig. S2). All data yield a weighted mean 207Pb/206Pb date of 550.55 ± 0.75 Ma (MSWD = 0.48). The two concordant analyses yield a U-Pb concordia age of 551.07 ± 0.61 Ma (MSWD of concordance and equivalence = 0.48). This ash bed occurs about 85 cm below the base of the Dengying Formation (within the Miaohe Member) at the interface between a black shale unit and overlying carbonates that record a progressive increase in 13C values [from –4 to +0.5 per mil ()] over a thickness of 2.3 m. Negative 13C values occur both above and below the sequence boundary separating the Upper Sequence and Miaohe Member (fig. S1). Below this boundary a pronounced 13C excursion (with values as low as –8) occurs; in the Wuhe section it is characterized by invariant 13C values of –8, whereas in the Jijiawan section there is a return to positive 13C values before the base Miaohe Member sequence boundary is encountered. This local variation is most likely related to variable preservation below the sequence boundary and/or lateral variation in sediment accumulation and preservation. In both sections, the lowermost Dengying Formation dolomites have values of –3 to –1, which increase to about 3 over about 5 m. This trend is interpreted + as being the top of the pronounced 13C excursion (with values as low as –8) that characterizes the top of the Doushantuo Formation (Fig. 1), thus constraining the age of the sustained negative excursion to (just) older than 551.1 ± 0.7 Ma, the time at which 13C values increase to positive values. This interpretation assumes that any period of nondeposition across this sequence boundary has a duration that is less than the duration of the negative 13C excursion itself.