Lithofacies Palaeogeography Of Miocene-pliocene In Qianhe Basin

The Qianhe Basin is located on the southern margin of the Ordos block,and the southern part is connected with the Qin-qi orogenic belt by a fault.Its sedimentary-tectonic evolution in the early stage of the fault depression proceeded simultaneously with the Weihe Basin,until the Pliocene was cut off by the Weibei Uplift on the west side of Fengxiang,and independent geological tectonic evolution began.The structural evolution of the Qianhe Basin is not only related to the Weihe Basin,but also unique.The study of the MiocenePliocene sedimentary facies and environmental evolution of the Qianhe Basin is a necessary supplement to the geological-geomorphological evolution of the Late Cenozoic basin on the southwestern margin of Ordos.The Miocene-Pliocene strata in the Qianhe Basin were carried out by means of field reconnaissance,field surveying of sections,collection and sorting of drilling sections,and sample collection and analysis in the study area,combined with scientific theoretical methods such as sedimentology,petrology,and paleontology The determination and division were followed by dividing the sedimentary facies developed in different evolution stages of the Qianhe Basin and their distribution ranges,and finally the paleogeographic restoration of the Qianhe Basin was completed.The main conclusions as below:(1)The Gansu Group at the bottom of the Qianhe Basin is mainly composed of red clay with partial sandy gravel layers.The red clay is underdeveloped,with calcareous nodules and grain size characteristics similar to typical loess,which should belong to an aeolian sedimentary system.The sandy gravel layer belongs to flowing water sedimentation,which represents a relatively warm and humid paleo-climatic environment.(2)The lower part of the Sanmen Formation is a rhythmic deposit composed of gray fine sand-silt-clay,with horizontal bedding and water-bearing animal fossils,which belong to lacustrine deposits;the upper part is an interbedded layer of clay and sand and gravel,which belongs to the interaction between rivers and lakes.Deposition.(3)During the Late Miocene-Morning Pocene,the Weibei Uplift experienced denudation and flattening during the Late Cretaceous-Early Miocene,and its height was greatly reduced,eventually forming a gentle hilly landform.At the same time,due to climate change caused by the uplift of the Qinghai-Tibet Plateau,dust accumulation began to appear extensively in northwestern my country,forming the Miocene-Pliocene red clay at the bottom of the basin.(4)In the middle of the Pliocene,due to the long-range effect of the uplift of the Qinghai-Tibet Plateau,the Longxian-Qishan-Mazhao fault occurred in a left-strike-slip fault,and a faulted lake was formed in the Longxian-Qianyang area.(5)In the late Pliocene,affected by the Qinghai-Tibet Movement A(3.4 Ma),the southwestern edge of Ordos rose rapidly,the lakes in the Qianhe Basin disappeared,and alluvial fans and rivers gradually developed.The Qinghai-Tibet Movement B(2.5Ma)closed the Qianhe Basin and began to accept Quaternary loess deposits.