Sedimentary facies and models for the Palaeogene-Neogene deposits on the northern margin of the Qaidam Basin,Qinghai

Sedimentary facies and models for the Palaeogene- Neogene deposits on the northern margin of the Qaidam Basin,Qinghai are explored on the basis of field outcrops and core observation,thin section examination and well logs. These Palaeogene- Neogene deposits are grouped into the alluvial fan,braided stream,braided delta and littoral-shallow lake depositional systems,and associated sedimentary models for the braided stream and intermittent lake deposits are constructed. In the braided stream deposits,there developed two successions of strata: the coarse-grained river bed subfacies deposits in the middle-lower parts,and the fine-grained overbank subfacies deposits in the middle-upper parts,which are attributed to the hydrodynamic variations on the alluvial fans and in the rivers due to the variations in precipitations and palaeoclimates. These variations also resulted in the changes in sediment supply rates and lake-level fluctuations,lake strandline migration and delta depositional areas, and finally in the frequent alternations in grain sizes of the depositional sequences and facilitate the development of favourable reservoir rocks and cap rocks in the study area.