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Objective The Luojiawopeng Formation is a set of purplish red sand and gravel accumulation strata in the Lower Pleistocene Series in the southeastern margin of Songnen plain. The clay mineral analysis of this formation is conducive to understanding the paleoclimate characteristics in this region and defining its stratigraphic age. Methods Whole rock geochemistry， X-ray diffraction （XRD）， Scanning electron microscopy （SEM）， and High-resolution transmission electron microscopy （HRTEM） analysis of clay minerals in Luojiawopeng Formation were conducted to reveal their composition and transformation characteristics of clay minerals. Comparisons were performed on clay mineral composition between the red clays in the Chinese Loess Plateau and Luojiawopeng Formation. Results The clay mineral composition of Luojiawopeng Formation is dominated by kaolinite （content of 57%⁃73%）， followed by illite （ranging from 25% to 41%）， and the content of biotite /vermiculite mixed layer minerals is the least （average 1%）. Kaolinite shows pseudo hexagonal morphology， and its high degree of euhedral crystallinity indicates its authigenic origin， as does its high illite crystallinity （average 0.55） and illite chemical index （average 0.50）. These results， combined with the high clay to quartz ratio， indicate intense weathering characteristics. The transformation process of clay minerals is illite → kaolinite and biotite → biotite /vermiculite mixed layer minerals → kaolinite， which is consistent with the significant kaolinization trend of constant element 4Si-M+-R2+ ternary diagram. combined with the high chemical index of alteration（CIA） and hematite-to-goethite ratio of the formation， this process indicates that these minerals formed in a humid and hot environment with increasingly strong weathering. Conclusions Thus， the Luojiawopeng Formation was likely formed in a humid and hot climate， rather than through moraine accumulation as previously reported， and the stratigraphic chronology of the formation may be in the humid and hot period when the East Asian summer monsoon prevailed before the Early Pleistocene.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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