Discovery of Lower Cretaceous unconformity in Linhe Depression of Hetao Basin and its implication to structural evolution

The Linhe Depression of Hetao Basin is a NE-SW trending Meso-Cenozoic arc-shaped strike-slip pull basin. Based on the combination of microfauna fossils, logging response characteristics (including the method of acoustic wave time difference to determine unconformity) and seismic response characteristics of unconformity, an unconformity within the Lower Cretaceous was identified. The unconformity is distinguished by the overlying Guyang Formation and underlying Urta Formation. The former is generally developed throughout the basin, and its thickness does not change much, and the latter with thickness decreasing from northwest to southeast as a whole is mainly distributed in two small sags close to the Langshan fault. The time and depth conversion results of the seismic data show that the thickness of the Urta Formation in the south is greater than that in the north. The thickest formation in the south is about 800 meters, and the thickest formation in the north is about 600 meters. After the deposition of the Urta Formation, the basin was affected by the Middle Yanshan Movement and subjected to the northwest-southeast compression. Due to the squeezing effect, the strata were uplifted and suffered erosion, resulting in the internal unconformity within the Lower Cretaceous. The unconformity is characterized by gradually changing from parallel unconformity to angular unconformity from northwest to southeast. Combined with the causes of the unconformity and the change of the thickness of the strata, the structural evolution of the basin in the early Cretaceous can be divided into three stages: the fault depression stage in the early Early Cretaceous, the uplift and erosion stage in the middle Early Cretaceous and the weak extension depression stage in the late Early Cretaceous.