Abstract:

Sedimentary filling characteristics of the basin are different under different paleogeomorphic backgrounds. For the Lower Cretaceous Qingshuihe Formation (K₁q) of Moxizhuang-Yongjin area (Junggar Basin), we studied the stratigraphy, sediment source and depositional system on the basis of paleogeomorphic evolution via integrated field outcrop, core logging, and 3-D seismic data analysis. The results show that K₁q is a whole 3^rd order sedimentary sequence: the 1^st member can be classified into LST and TST, while the 2^nd member is HST. In the Early Cretaceous, the Chepaizi-Mosuowan paleouplift was largely buried, and the local highs of the paleouplift were still exposed. As the whole basin was steadily deposited, the paleouplift gradually disappeared. During the LST period, the depositional center of the basin was located north of the study area. From the TST to HST period, the basin geomorphology was gentle and the depositional center moved southward gradually under tectonic uplift and tilting in the north. With tectonic evolution, the provenance system of the north basin was supplied to the study area from the northeast and northwest. In shallow water environment of the gentle slope of the basin, the far braided river delta and shallow lake depositional system was developed. During the LST period, due to the combined influence of paleouplift residual geomorphology, lake-level changes and hydrodynamic conditions, the sediments debris were mostly deposited in the low-potential geomorphologic area, whereas the shallow-water braided-river delta was locally developed. In the study area, the sediments were mainly deposited in the inner delta front, and underwater distributary channels were formed in succession. The sedimentary system developed in the LST period may have flattened the primitive geomorphology, and the shallow lake system was inherited in the TST period. The shallow water braided river delta sedimentary system was developed in the HST period. The HST period was dominated by the outer delta front deposition at Moxizhuang-Yongjin, widely developed sheet sand and locally developed underwater distributary channel. Inheritance geomorphology may have controlled the sedimentary accommodation in different parts of the basin, determined the sediment transport and dispersion path, and controlled the basinal sedimentary filling types and characteristics.

Key words: Junggar Basin, Moxizhuang-Yongjin area, Cretaceous Qingshuihe Formation, Chepaizi-Mosuowan paleouplift, geomorphic evolution, sedimentary system