Magnetic Susceptibility Changes and Paleoclimatic Significance during the Norian (Late Triassic) in the Baoshan Block, Yunnan, China

The Norian stage of the Late Triassic has traditionally been regarded as a period of climatic stability. However, as the longest continuous stage in the Phanerozoic, lasting over 20 million years—longer than most of the Cenozoic Era—its paleoclimatic evolution has long been lacking systematic investigation. This study focuses on the Xiquelin North section in the Baoshan area, integrating detailed sedimentological analysis, conodont biostratigraphy, and magnetic susceptibility measurements. Based on the occurrence of the marker fossil Mockina bidentata, the upper part of the section is assigned to the Sevatian substage of the late Norian. Sedimentological analysis reveals an overall pattern of “dolomite–limestone–dolomite–limestone” transitions throughout the section. The lowermost dolomite gradually passes into limestone, accompanied by increased silica, mud, enhanced terrigenous input, and deepening water depths. This is overlain by a thick dolomite interval, followed at the top by limestone with a significant increase in terrigenous detritus. Magnetic susceptibility shows a down-profile pattern of “low basal values–mid-section enrichment–upper variability.” The basal interval (0–20 m) has a mean magnetic susceptibility of 0.897×10⁻⁵ SI. In the enriched middle interval (20–84 m), values rise to 1.1–1.2×10⁻⁵ SI. Data are sparse for the upper dolomite (84–184 m), while the overlying limestone (184–240 m) yields a lower mean value of 0.776×10⁻⁵ SI. Overall, the paleoclimate recorded in the Xiquelin North section is divided into four phases: Stage I (0–20 m) reflects a relatively dry and hot environment; Stage II (20–84 m) is generally humid; Stage III (84–184 m) shows reduced terrigenous input and precipitation, indicating relatively arid and hot conditions; and Stage IV (184–240 m) marks a return to a relatively humid setting.