Basic types and geologic significances of "truncation and onlap" unconformities

Based on geologic, seismic and drilling data of basins in China, the basic types of "truncation and onlap" unconformities are examined and their temporal and spatial variation and geologic significances are explored. The "truncation and onlap" unconformities and stacking styles of upper and lower strata vary under different tectonic settings. Under extensional settings, there are three basic stacking styles and unconformities, such as parallel stacking style and unconformity with breaking outward, parallel stacking style and unconformity with breaking inward, as well as unconformity and stacking style with rotation. Under compression settings, there are four basic types, such as overlap stacking style and unconformity on fold, syn-deposition fold stacking style and unconformity, anticline-syncline superimposing stacking style and unconformity, as well as stacking style and unconformity in overlap-filling incised valley on top of folds. "Truncation and onlap" unconformities always superimposed each other in space, and exhibit three kinds of superimposed unconformities, i.e., superimposed unconformities formed under continuous uplifting, under transition of uplifting, and under propagation of deposition center. They may also change with time too, i.e., tilt, rotate, fold, and migrate. Controlled by the migration of basin depocenters, it is shown that there are unconformities formed under the transition of paleogeomorphy or subsidence center, or under the migration of fault-sag, and or under the slipping of detachment fault. "Truncation and onlap" unconformities are important positions for traps and significant pathways of hydrocarbon migration, and they are favorable sites for ore deposit and resources occurrence. © 2018, The Editorial Board of Petroleum Exploration and Development. All right reserved.