Palynological and Geochemical Records of the Karamay Formation in the Junggar Basin and the Ladinian Shift

The Dalongkou section on the southeastern margin of the Junggar Basin preserves continuous and complete Triassic sediments. Multiple sets of dark mudstone and shale are developed within the Karamay–Huangshanjie Formations, showing considerable significance for hydrocarbon exploration. The authors recently conducted detailed palynological and geochemical investigations. In the lower Karamay Formation, spores are slightly more abundant than pollen. The spore assemblage is predominated by Todisporites, with common occurrences of Leiotriletes, Calamospora and Aratrisporites, whereas other spore taxa are low in abundance. Pollen is dominated by non-striate bisaccate grains, accompanied by frequent striate bisaccates and monosulcate pollen, along with minor spherical and monosaccate type. The upper Karamay Formation and the Huangshanjie Formation share highly similar palynological assemblages, which are both pollen-dominated with low spore contents. Pollen compositions are generally consistent with those in the lower Karamay Formation. Although total spore abundance remains low, spore diversity increases notably, with the first appearance of Late Triassic prospering taxa such as Dictyophyllidites and Concavisporites. Another critical shift is the absence of any overwhelmingly dominant spore species. These discrepancies indicate a distinct palynological turnover within the Karamay Formation, reflecting rapid terrestrial vegetation replacement. Geochemical analyses also reveal an obvious trend transition of multiple proxies in the middle Karamay Formation. Ratios of CaO/MgO, FeO/MnO, phosphorus content and total organic carbon (TOC) all increase upward, reach synchronous peaks in the middle Karamay Formation, and then decline stratigraphically upward. The FeO/MnO ratio additionally displays a high-value interval near the base of the overlying Huangshanjie Formation. The CaO/(CaO+Fe) ratio maintains a stable low value throughout the lower Karamay to lower Huangshanjie Formations, with only a small peak in the middle Karamay Formation. The synchronous inflection of these geochemical proxies demonstrates that a major geoenvironmental shift occurred during the deposition of the middle Karamay Formation. Latest high-precision zircon U–Pb ages (Zhao et al., 2025) indicate that the boundary between the Karamay Formation and Huangshanjie Formation is roughly equivalent to the Middle–Upper Triassic boundary, meaning that the entire Karamay Formation belongs to the Middle Triassic. Based on the above evidence and the latest chronostratigraphic framework, this study proposes a major geoenvironmental shift in the Junggar Basin that initiated in the early Ladinian. The paleoclimate shifted to warm and humid conditions from the early Ladinian and persisted through the Carnian. The deposition of black shale at the base of the Huangshanjie Formation, elevated FeO/MnO ratios, and a pronounced reduction in spore content may represent local responses to the Carnian Pluvial Episode (CPE).