Clay mineral assemblages of the Luojiawopeng Formation red bed in the Songnen Plain, NE China and their paleoenvironment implications

The Luojiawopeng (LJWP) Formation red bed is proluvial fan deposits and distributed in the eastern Songnen Plain, NE China. Its stratigraphic age has been assigned to the early Early Pleistocene based on lithologic stratigraphic correlation. However, the paleoenvironmental characteristics of the red bed remain poorly resolved, which limits our in-depth understanding of how the red sediments were formed during the global cooling period, particularly in northeastern China. To address this gap, we report on the clay mineral assemblages of the LJWP Formation red bed and the associated paleoenvironment during its sedimentation period. Based on the analysis of XRD, SEM, EDS and HRTEM, the results indicate that the clay minerals in the LJWP Formation strata are primarily composed of kaolinite (57–73 %), illite (25–41 %), and biotite/vermiculite (0–4 %). The pseudo-hexagonal plate-like shapes suggest an authigenic origin of the kaolinite, indicating that the LJWP Formation strata underwent intensive chemical weathering. The hydrolysis processes in the LJWP Formation are thus deduced as follows: illite → kaolinite, biotite → biotite/vermiculite → kaolinite. Significant amounts of iron were released from both the crystal structure of biotite and its secondary clay minerals during weathering processes, resulting in nanoscale hematite precipitating on the surfaces and within the pores of particles, as shown by the HRTEM and petrographic examination. Accordingly, based on these findings, we conclude that the secondary hematite minerals imparted a reddened staining to the LJWP Formation deposits, and that the LJWP Formation red bed accumulated during a period of warm and humid conditions. Recent research based on geochemical and heavy mineral contents supports the presence of the warm climatic conditions, challenging the previously held view that this red bed originated from glacial tills formed under cold climatic conditions.