Basic types and geologic significances of “truncation and onlap” unconformities

Abstract

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 superimposing styles of overlying and underlying strata vary under different tectonic settings. Under extensional settings, there are three basic superimposing styles and unconformities, such as parallel superimposing style and unconformity with breaking outward, parallel superimposing style and unconformity with breaking inward, as well as superimposing and unconformity style with rotation. Under compressional settings, there are four basic types, such as overlap superimposing style and unconformity on fold, syn-depositional fold superimposing style and unconformity, anticline-syncline superimposing style and unconformity, as well as superimposing 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., tilting, rotating, folding, and migration. Controlled by the migration of basin depocenters, it is shown that there are unconformities formed under the transition of paleogeomorphy or subsidence center, under the migration of fault-sag, and under the slipping of detachment fault. “Truncation and onlap” unconformities are important positions for traps and significant pathways for hydrocarbon migration, and they are favorable sites for ore deposit and resources occurrence. Key words: truncation and onlap, unconformity, hydrocarbon accumulation, ore deposit occurrence, intra-continental deformation, geologic significance