Characteristics of palynological assemblies and depositional age of bauxite deposits in southern Guizhou
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Abstract:
The study of bauxite depositional patterns is of utmost importance for China’s resource security and mineral explorations, as China relies heavily on imports to meet its demand. Numerous factors, such as tectonic background, paleoclimate, erosion rate, parent rock, age, paleovegetation, and paleotopography, influence the formation of bauxite. Therefore, accurate age estimate is essential in understanding the impacts of these factors on bauxite formation and providing theoretical support for future exploration efforts. Resolving the depositional age of bauxite deposits presents a formidable challenge due to the intricate processes encompassing pedogenesis and reworking. The scarcity of plant root fossils or trace fossils further complicates the estimate of the ultimate depositional age. Presently, the most successful approach to ascertain the depositional age of bauxite deposits involves a comprehensive examination of preserved palynological assemblages. In China, documented depositional ages of bauxite deposits are predominantly within the late Paleozoic. The bauxite deposit in Guizhou represents such an example in South China. However, the age of the Guizhou bauxite deposit has been controversial. In order to address the aforementioned controversy, we conducted palynological investigations on two sections containing bauxite in Longshan Town, Longli County, and Gaopo Miaozu Village, Huaxi District, Guizhou. The findings indicate that the palynological assemblages observed in the two sections align with the standard Lycospora pussila (Pu) palynological assemblage zone of the Mississippian in Western Europe. These results suggest that the depositional age of the Fengyuan bauxite deposit is no younger than the early-middle Visean age of the Mississippian. Therefore, the Fengyuan bauxite deposit represents the earliest known karstic bauxite in Guizhou with unequivocal fossil evidence. The palynological assemblages from the Fengyuan bauxite deposit are generally preserved in lower stratigraphic horizons when compared with those from the bauxite deposits in Qingzhen, central Guizhou, and Zunyi, northern Guizhou. The palynological assemblages of the Fengyuan bauxite deposit is separated from the palynological assemblages in Qingzhen and Zunyi by one palynological assemblage zone (Perotriletes tessellatus-Schulzosopra camplylotera assemblage zone) or one conodont zone (Gnathodus bilineatus zone). This suggests that the depositional age of the Fengyuan bauxite deposit predates those of the bauxite deposits in Qingzhen and Zunyi by 4.2 million years. The frequent sea-level changes caused by the Late Paleozoic Ice Age (LPIA) led to gradual submergence and deposition and likewise, gradual age changes of the bauxite deposits from older in the south to younger in the north of Guizhou.
