Potash–Forming Regularity of the Ordovician Marine Northern Shaanxi Salt Basin: Insight from Review and Prospect of the Deep Geology of the Ordos Basin

Abstract

AbstractThe Ordovician Northern Shaanxi Salt Basin (ONSSB), located in the east–central Ordos Basin, western North China Craton (NCC), is one of the largest marine salt basins yet discovered in China. A huge amount of halite deposited in the Mid‐Ordovician Majiagou Formation, and potash‐containing indication and local thin layer of potash seam were discovered in O2m65 (6th submember, 5th member of the Majiagou Formation). This makes ONSSB a rare Ordovician potash‐containing basin in the world, and brings new hope for prospecting marine solid potash in this basin. However, several primary scientific problems, such as the coupling relationship between ONSSB and the continent nucleus, how the high‐precision basement fold controls the ONSSB, and how the basement faults and relief control ONSSB, are still unclear due to the limitations of the knowledge about the basement of the Ordos Basin. This has become a barrier for understanding the potash‐forming regularity in the continental nucleus (CN) area in marine salt basin in China. Up to now, the material accumulation has provided ripe conditions for the answers to these questions. Latest zircon U‐Pb ages for the basement samples beneath the Ordos Basin reveal that there exists a continental nucleus (Yi‐Meng CN) beneath the northern Ordos Basin. And this brings light into the fact that the ONSSB lies not overlying on the Yi‐Meng CN but to south Yi‐Meng CN. Both do not have superimposed relationship in space. And borehole penetrating into the basement reached Palaeoproterozoic meta‐sedimentary rocks, which suggests the ONSSB is situated in the accretion belt of Yi‐Meng CN during geological history. Basement relief beneath the ONSSB area revealed by seismic tomography and aeromagnetic anomaly confirms the existence of basement uplift and faults, which provides tectonic setting for sedimentary center migration of the ONSSB. Comparative research with various data sources indicates that the expanding strata in the ONSSB adopted the shape of the basement folds. We found that the orientations of the potash sags showed high correlation with those of several basement and sedimentary cover faults in the ONSSB. The secondary depressions are also controlled by the faults. Comparative research between all the global salt basins and continental nuclei distribution suggests that distribution of the former is controlled by the latter, and almost all the salt basins developed in or at the margin of the continental nucleus area. The nature of the tectonic basement exerts a key controlling effect on potash basin formation. And on this basis we analyzed in detail the geological conditions of salt‐forming and potash‐forming in the ONSSB.