New SIMS U-Pb age constraints on the largest lake transgression event in the Songliao Basin, NE China.

The largest lake transgression event (LTE) associated with lake anoxic events (LAE) and periodic seawater incursion events (SWIE) in the Songliao Basin, northeastern China, occurred during deposition of the Cretaceous Nenjiang Formation. The Yaojia-Nenjiang Formation boundary (YNB) marks the beginning of the LTE, as well as LAE and SWIE. However, there is an absence of direct radioisotopic dating, and therefore the age of the YNB, as well as the beginning of LTE, together with their relationship with other geological events, is strongly debated. Here we present a new SIMS U-Pb zircon age from the lowermost Nenjiang Formation. The bentonite bed located 9.88 m above the YNB of the X1-4 borehole was analyzed. Twenty-five analyses of 25 zircons were conducted, which produced a weighted mean age of 85.5±0.6 Ma (MSWD = 0.87). Based on the average sediment accumulation rate, the age of the YNB is suggested to be 85.7 Ma, indicating that the LTE began in the Early Santonian. The new ages provide a precise chronostratigraphic framework for climatic and geological events. Our new results imply that the beginning of the LTE, LAE and SWIE occurred almost simultaneously with short-term sea level rise, and probably had a close relationship with OAE3

U-Pb concordia diagram showing secondary ion mass spectrometry analytical data for zircons from sample XING988 obtained from a bentonite layer in K₂n.

<p>Data-point error ellipses are 2σ.</p

Stratigraphy of borehole X1-4.

<p>Fig 2 was created by CorelDRAW X8 (<a href="http://www.coreldraw.com/us/pages/free-download" target="_blank">http://www.coreldraw.com/us/pages/free-download</a>). Copyright (c) 2018 [Huaiyu He] and its licensors. All rights reserved.</p

Secondary ion mass spectrometry zircon U-Pb data.

<p>Secondary ion mass spectrometry zircon U-Pb data.</p

Cathodoluminescence images of representative zircons from sample XING988.

<p>Ellipses on the analyzed zircon grains show the positions of U-Pb analytical sites.</p