Soft-Sediment Deformation Structures and Their Tectonic Significance in Carbonate Rocks of the Mesoproterozoic Guandaokou Group, North China Craton

The heterogeneity of carbonate ooze in plane-slice and vertical-slice orientations is usually less than that of clastic ooze and has been reported in fewer types of soft-sediment deformation structures in carbonate rocks. Most soft-sediment deformation structures reported in the literature developed in clastic rocks. Soft-sediment deformation structures developed in the platform-facies dolomites of the Mesoproterozoic Guandaokou Group along the southwestern margin of the Ordos Block in the western North China Craton (NCC). On the basis of their genesis, these structures in the Guandaokou Group can be divided into two major types: (1) soft-sediment deformation structures formed by loading and compaction during diagenesis, including load casts, flame structures, sandstone balls, sandstone pillow structures, and convolute bedding, and (2) soft-sediment deformation structures caused by seismic activity, namely, seismic structures, including structural types such as microfaults, self-clastic breccias, ball-pillow structures, and quasi-ball-pillow structures. Horizontal comparison of the sedimentary facies indicates that the study area should be near the boundary of the Ordos Block. Seismic structures indicate that the area was strongly disturbed by tectonic activity in the surrounding area at 1600-1400 Ma. On the basis of field data and regional geological data analysis, we suggest that the seismic structures in the Guandaokou Group reflect strong extension and that such an extensional tectonic setting is consistent with the synsedimentary faults within the Mesoproterozoic strata under the Mesozoic-Cenozoic cover rocks, the spreading Kuanping Ocean at the southern margin of the Ordos Block, and the homochronous magmatism in the adjacent area west of the Ordos Block. The early Mesoproterozoic Columbia breakup event resulted in different structural responses in the carbonate rocks of the Guandaokou Group in the southern NCC. In the Xiaoqinling area, the presence of acidic tuff indicates continental rifts, whereas soft-sediment deformation structures are observed in the study area.