Magnetostratigraphy of Borehole CJ-3 in the Dalangtan Area, Northwestern Qaidam Basin and Its Tectonic Implications

The Qaidam Basin, on the northeastern margin of the Tibetan Plateau, preserves abundant evaporite resources and provides a key archive for investigating basin evolution, climatic change, and brine mineralization. In the Dalangtan area of the northwestern basin, a thick Cenozoic gravelly succession has recently been recognized as the principal host of a newly identified gravel-type potash-bearing brine deposit. Here, we present detailed rock-magnetic and magnetostratigraphic results from borehole CJ-3 to constrain the Late Cenozoic sedimentary evolution of the Altyn Tagh piedmont and its relationship to potash-bearing brine reservoir development. The magnetic polarity sequence consists of 12 pairs of normal and reversed zones, which can be correlated with subchrons C2An.2n–C3r of the geomagnetic polarity timescale (GPTS 2012). A major sedimentary facies shift at 370.75 m yields an age of 3.08 Ma, indicating the onset of a high-energy piedmont alluvial-fan depositional system. This transition was primarily driven by intensified tectonic activity along the Altyn Tagh fault during the Late Pliocene, whereas increasing regional aridity favored the accumulation and preservation of coarse clastic sediments. This age further constrains the initial development of the principal sand–gravel brine reservoir in the Dalangtan area to the latest Pliocene, providing a new chronological basis for understanding sand–gravel-type potash-bearing brine reservoir formation in the northwestern Qaidam Basin.