The implication of the geophysical data and drilling records for the formation and paleoenvironment of the Hobq Desert, China

The Hobq Desert (HD) locates at the northern margin of the Ordos Plateau, China and the transition zone to the Hetao Basin. It is one of the main proximal deserts of the Loess Plateau, and its formation was controlled by the evolution of the Yellow River and the Ordos Plateau. It is still worthy of discussing the regional structure to enhance the understanding of the desert formation and paleoenvironment, especially with the additional works in the eastern part. Geophysical methods including gravity, magnetic, EM (electromagnetic) are often used to analyse the regional and near-surface structure. As for drilling records, they could show the lithology sequence directly and be applied for the environmental study (e.g., OSL dating). Therefore, in this paper, we used the satellite gravity data, GPR (Ground Penetrating Radar), TEM (Transient electromagnetic method) and drilling records with borehole resistivity data to analyse the desert structure from different scales and the paleoenvironment. Trough the upward continuation and Park-Oldenburg calculation for the gravity data, the thickness of Quaternary deposits is 0.85–1.55 km. The inferred faults indicated the performance of the Hetao Basin and the Ordos Plateau's uplift in Pleistocene, which also contributed to HD's formation and existing topography. A three-part structure within 350 m was revealed in the profile at eastern HD by EM data. The upper layer within 80–90 m was the aeolian dry sand, the middle one was mainly fluviolacustrine saturated sand with the occurrence of clay and grey calcareous root tubes, and the lower one might be the aeolian sand. This near-surface structure was consistent to the western part, and the forming period for the sand in the upper zone was thereby estimated no earlier than 80 Ka. Moreover, during this period, millennial-scale changes in paleo-effective moisture occurred although it was dominated by the arid climate.