Depositional environments of the organic-rich shales from the Lower Cretaceous Dalazi Formation in the Yanji and Luozigou Basins of Northeast China

Abstract

The organic-rich shales from the Lower Cretaceous Dalazi Formation in northeastern China are well known for their rich terrestrial fossils and significant hydrocarbon potential. However, the exact depositional environments of these shales remain unclear. Here, integrated geochemical proxies, including iron speciation (Fe_HR/Fe_T, Fe_py/Fe_HR), pyrite sulfur isotopes (δ³⁴S_py), and major- and trace-element enrichments were carried out on these organic-rich shales from the Zhixin section of the Yanji Basin and the Beigou section of the Luozigou Basin in Northeast China. This study aims to provide insights into the contemporaneous continental chemical weathering intensities accompanied with the lake redox conditions during the deposition of the Dalazi Formation shales. Low chemical index of alteration (CIA) values were detected in the shales, indicating low weathered continental environments during the depositional period of the shales. The reduced water availability under dry climate evidenced by plant fossils likely contributes to limiting the intensity of chemical weathering with low CIA values.

All samples exhibit Fe_HR/Fe_T ratios >0.38 and Fe_py/Fe_HR ratios far below 0.7, displaying the water column characterized by anoxic ferruginous conditions with limited HS⁻ availability during the shale deposition. The reconstruction of redox condition is consistent with the observations of the low pyrite contents, minimal well-formed framboidal pyrite crystals and covariation patterns of Mo_EF and U_EF in the shales. With low CIA values, the less continental input to the lake probably gives rise to the anoxic ferruginous deep-water conditions via limiting sulfate availability, which is consistently supported by high δ³⁴S_py values (12.2 ± 9.6 ‰), low pyrite contents, and the remarkable negative correlation between the CIA and δ³⁴S_py values. Considering the high content of organic matter up to 6.38 wt% and abundant fossil collections in the shales, this finding highlights the beneficial role of a sustained and stable anoxic ferruginous environment for preserving organic matter and carbonaceous fossils in the Dalazi Formation.