Reconstruction Of South Ningxia-Liupanshan Original Basin During Yan’an Period

Original basin reconstruction is to remove the influence of late reformation and recover the original basin face in main development stage and each evolutionary phase, which is the significant content in basin study and also the difficult and key point of basin research. Original basin reconstruction has great significance on the evolution of sedimentary basin and exploration of energy mineral resource.This paper chooses the Ningnan-Liupanshan area to the west of Ordos Basin as the study area and emphasizes on the relatively weak strata—Early-Middle Jurassic Yan’an Formation. Based on the guiding theory of "original basin reconstruction" and various materials of outcrops, data from oilfields and coalfields and corresponding analytical test, and by comprehensive means of sedimentology, tectonics, geochemistry, geophysics and so on, this paper systematically analyses the stratigraphic age and distribution, sedimentary environment and primary sedimentary boundary, and resumes the original basin face by studying periphery simultaneous geologic feature. The main knowledge and conclusions are as follows:(1) Reclassify the controversial coal measure strata (Upper Triassic or Middle-Lower Jurassic) in Tanshan, Yaoshan, Mahuanggou as Middle-Lower Jurassic strata.(2) There are many deposition-subsidence centers in study area. Yan’an Formation was controlled by many faults and the depressions distributed in beaded shape in Liupanshan where the thickness of Yan’an Formation is much bigger than that of Ordos west margin and periphery intermountain basins.(3) It turns out that Yan’an provenance of South Ningxia-Liupanshan was mainly from its north orogenic belt(Yinshan Orogenic Belt)and northwest ancient land(Alxa Block),while the North Qilian Oroginic Belt on its south and southwest contributed relatively less.(4) South Ningxia-Liupanshan suffered widespread Indosinian movement which was stronger in its west and weaker in its east. This caused undulating topography which was higher in the north and lower in the south and multi-palaeohighs when Yan’an Formation initially deposited. The study area was in stretching environment and was distinctly controlled by faults.(5) The primary southwestern boundary was to North Qilian Orogenic Belt by Qingtongxia-Guyuan Fault and Haiyuan Fault; its northwestern boundary was to Weiningbeishan palaeohigh and Alxa Block by Zhongning-Zhongwei Fault and Western Helanshan Fault respectively; its northern boundary was to Yinshan Mountain and Hetao palaeohigh. It’s connected to Ordos Basin on its east side and Hexi Corridor on its west side.(6)At the beginning of Yan’an period, palaeohighs and sags coexisted in South Ningxia-Liupanshan area,where fluvial facies prevailed and several minitype shallow lakes existed in front of North Qilian Oroginic Belt.The rivers in the study area eventually flew in Ganshan Ancient River in Ordos Basin on its east side. During early-middle Yan’an period, the palaeohighs were gone and the minitype lakes developed into one big faulting lake with the enhancing faulting.As the large lake in Ordos Basin expanding, the early fluvial facies evolved into fluvial-delta facies.In the late of Yan’an period, fluvial facies prevailed in the study area again.