Geochronological and Paleomagnetic Constraints on the Lower Cretaceous Dalazi Formation from the Yanji Basin, NE China, and its Tectonic Implication

The Lower Cretaceous Dalazi Formation in the Yanji Basin, eastern Jilin Province is of particular interest because it contains key fresh water fossil taxa, oil and gas resources, a potential terrestrial Albian–Cenomanian boundary, and regional unconformities. However, the lack of a precise chronology for the non-marine strata has precluded a better understanding of the regional stratigraphic correlation and terrestrial processes. Here, we report magnetostratigraphic and U–Pb geochronologic results of a sedimentary sequence from the Xing’antun section in the Yanji Basin. Thirty-two zircons from the tuff sample were analyzed by secondary ion mass spectrometry (SIMS); the U–Pb zircon dating method yielded a weighted mean of 206Pb/238U age of 105.7 ± 0.8 Ma (2σ, internal error). Paleomagnetic results show that the Dalazi Formation is of normal polarity, which was correlated to the early chron C34n constrained by the SIMS U–Pb zircon geochronologic data, further demonstrating that the terrestrial sedimentary sequence of the upper Dalazi Formation is of late Albian age. The established geochronologic framework allows the regional correlation of the Dalazi Formation in the Yanji Basin to the strata from other terrestrial sequences in northeastern China. The similar geodynamic and geologic background between the Yanji Basin and other terrestrial rift basins in northeastern China suggests that the unconformity between the Dalazi and Longjing formations may represent syn-rift and post-rift stages in the Yanji Basin, and thus the switch from extension to contraction during the mid-Cretaceous, precisely constrained to ~106–101 Ma based on our new chronology and previously published high-precision U–Pb dating of the lower Longjing Formation. It is most likely attributable to the docking of the west Pacific plate along the East Asian continental margin

Depositional environment of the Late Santonian lacustrine source rocks in the Songliao Basin (NE China): Implications from organic geochemical analyses

The Songliao Basin (SLB) located in northeastern China is one of the largest Cretaceous continental sedimentary basins in the world. The SLB is filled with sediments deposited in the Upper Jurassic, the Lower Cretaceous and the Upper Cretaceous epochs. The Nenjiang Formation (K(2)n) is subdivided into five members, where Member 2 (K(2)n(2)) was deposited in the late Santonian in the lower part of the Nenjiang Formation. This member is characterized by a thick succession of organic-rich source rocks. However, the complexity of the depositional environment raises questions about the specific factors that drove this accumulation of organic material. Here, we present data on the total organic carbon (TOC) contents and their stable carbon isotope values (delta C-13(org)), as well as biomarker data from 50 outcrop samples collected from the Yuewangcheng (YWC) section, in the southeastern SLB. According to the variations of the bulk organic parameters (TOC and delta C-13(org)) and biomarker indices, the profile could be divided into three stages (stages I-III). The distribution of biomarkers (n-alkanes, steranes and hopanes) and delta C-13(org) values indicate that the organic matter (OM) in the Lower K(2)n(2) is derived largely from algae and macrophytes, with a minor input from bacteria and land plants. The water column was stratified, as indicated by the presence of gammacerane. Variations in the pristane/phytane (Pr/Ph) and aryl isoprenoids relative to phenanthrene ratio (A-i/P) suggest that bottom waters were anoxic during Stage I (31.5-26.0 m) and Stage II (26.0-16.3 m), with the anoxic layer impinging on the euphotic zone and a relative oxic environment at Stage III (16.3-0 m). This brackish environment persisted in the water over the interval represented by the section, as reflected by methytrimethyltridecyl chromans (MTTCs) Index (MTTCI) and alpha-MTTC/gamma-MTTC ratios. Marine transgressions, with subsequent seawater incursions, can be detected with the presence of 24-n-propyl-cholestanes and 24-isopropyl-cholestanes in the Lower K(2)n(2) sediments, especially during Stage I. These seawater incursions are closely correlated with anoxic conditions and the deposition of organic-rich source rocks. (C) 2018 Elsevier Ltd. All rights reserved

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

Cathodoluminescence images of representative zircons from sample XING988.

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