Characteristics of Jurassic sequence boundary surfaces on the northeastern margin of Ordos basin and their constraints on the spatial-temporal properties of sandstone uranium mineralization

On the basis of outcrop, drilling cores seismic, logging and geochemical data, the main sequence boundary surfaces of the Yan'an and Zhiluo Formation of Jurassic on the northeast margin of Ordos basin were systematically studied. On outcrop profiles and drilling cores, the sequence boundary surfaces show characteristics of iron weathering crust, truncation and lithologiclithofacies transformation. On seismic profiles, the boundaries are featured by onlap, downlap and truncation. The rock-electricity relation indicates different types of mutations. The trace elements also show mutation characteristics. Ten different orders of sequence boundaries were identified, including three I-type(TSB1-TSB3)and seven Ⅱ-type(SB1-SB7)sequence boundary surfaces. Ⅰ-type boundaries include TSB1 sequence boundary surface between Yan'an(J_1-2y)and Yanchang Formation(T₃y), corresponding to T_y in seismic profiles; TSB2 sequence boundary surface between Yan'an(_1-2y)and Zhiluo Formation(J₂z), corresponding to T_z-1 in seismic profiles; TSB3 sequence boundary surface between Jurassic and Lower Cretaceous, corresponding to T_k in seismic profiles. As regional unconformities, Ⅰ-type sequence boundaries including TSB1, TSB2, TSB3 reflected tectonic activity during Mesozoic period. The Ⅱ-type sequence boundaries were main climate transformation surfaces and showed the cyclic changes in climate factors. The vertical variations of values of Sr, Cu, Sr/Cu, FeO/MnO and Al₂O₃/MgO indicate that the paleoclimate and paleoenvironment experienced a changing process from warm-humid climate to dry-wet alternation, arid and semi-arid climate; TSB2 and SB6 were main climate transformation surfaces. Vertical zonation of stratigraphic structure is obvious between Yan'an and Zhiluo Formation on the 3D models of the study area. The uranium bodies mainly lie in the large sandstone overlying TSB2 sequence boundary surface, in the form of plate on the 3D models of sandstone and uranium mineralization bed. The uranium bed was obviously influenced by the vertical zonation of stratigraphic structure and paleoclimate transformation. The integrated identification of sequence boundaries will not only reduce the casualty of artificial empirical recognition but also provide a basis for the division of Jurassic sequences and the establishment of isochronous sequence stratigraphic framework. The research on space-time attributes and the indicated geological meanings of the main sequence boundary surfaces will be helpful to revealing the process of "Yanshan movement" and its influence on the paleoclimate transformation and sandstone-type uranium mineralization.