Late Neogene Geology And Environment Evolution At Zoig(?) Basin Of Northeast Margin Of Qinghai-Tibet Plateau

The deposits of Zoig(?) Basin in Late Neogene is one natural geological record for thestudy on uplift evolution history of previous northeast margin and its surroundingenvironment response and water system change of Qinghai-Tibet Plateau, formationfeature of loess, space-time evolution of ancient sand hills, and modern grasslanddesertification etc. and has significant scientific meaning. Relying on the projects of ChinaGeology Investigation Bureau, Investigation and Data Update of Geological Backgroundof Resources and Zoology of Sichuan, Yunnan and Guizhou Provinces and RegionalGeology Investigation and Revision of 1: 250000 Zoig(?), Hongyuan, Luhuo and MaerkangCounties (coded 1212010510304), this paper selects geology in Late Neogene and itsenvironment evolution of Zoig(?) Basin as study project, which obeys geological eventswith supported evidences, and remodels geology and environment evolution process ofLate Neogene at northeast margin of Zoig(?) Basin of Qinghai-Tibet Plateau.Through several methods like field geological mapping, investigation and sampling, indoor sample test and analysis, and remote sensing interpreter etc., using landform andmulti subject knowledge including quaternary geology, multiple geology, tectonics, climate and environmental science, geochemistry, remote sensing, chronology, andglaciology etc., we have carried out fully, systematic and composite study upon divisionand contrast of late Neogene multiple stratigaphic and its environment evolution of thestudied region, structural evolution and formation mode of the basin, space-time evolutionof ancient channel of Yellow River, space-time feature and change of ancient sand hill ofHolocene Epoch, investigation of basin and grassland desertification, formationinvestigation upon basin loess, distribution features of geochemical elements of the basin, landform zoning features of the basin and its relationship with peat land etc. The mainachievements are showed as below: 1.Making use of multidisciplinary theory, completely and systemically takingneotectonic movement as the background, basing on paleomagnetism, ¹⁴C, thermoluminescence, optically stimulated luminescence dating data, referring tooryctocoenose species, original horizon, and climatostratigraphy, depending on multiplestratigraphic classification, we have carried out further study on the contrast andclassification upon lithostratigraphy, biostratigraphy climatostratigraphy, chronostratigraphy, magnetostratigraphy of Zoige Basin in Pleistocene series andHolocene series of late Neogene. We have put forward the stratigraphic classification oflate Neogene of the studied region, and further completed stratigraphic sequence anddistribution rule of late Neogene stratum of Zoige Basin. And we also learned thefollowing points.We also learned that accumulative statrum of the rock of the river courses of trunkstream of Yellow River, tributary stream of Bai River and tributary stream of Hei Riverhas different features in different developing stages from Pleistocene series to Holoceneseries under the background of neotectonic movement, and the deposits of different watersystems of the same period is not totally the same as one another, which shows thedifference of stratigraphic developing process and the spatial difference of late Neogene ofthe basin. Except glacial accumulative formation of equivalent Pleistocene series in boringholes, ground mainly outcrops at Muhalong in northeast corner, Yanggonghaimountainous area in southeast corner and Nianbaoyeze mountainous area in northwesternpart, and reflect several relatively intensive cold events of the last Glacial Epoch andHolocene Epoch of terminal moraine dating. The peat sporo-pollens profile and RM holesporo-pollens data shows that climate in Zoig(?) region displays extreme instability andtemperature rises and falls frequently from late Glacial Epoch to post Glacial Epoch.Geological period of vertebrate fossils at stratum of the trunk stream of Yellow River, HeiRiver and Bai River belongs to Late Pleistocene epoch. Age boundary of stratigraphicclassification of late Neogene of the investigated area is proposed according to dating dataof isotope of ¹⁴C, TL, OSL, ESR etc., combining with geological base of rock bed andbiostratigraphy. Climate stratum evolves alternatively between Glacial Epoch or inter Glacial Epoch or dry period and humid period that can be classified as Glacial Epoch ofhighland 4366 of mid Pleistocene epoch and Glacial Epoch of Gongbalake, Muhalong icestep, Zhaerji ice step, Yanggonghai ice step and Ximencuogou ice step of the early stageof last Glacial Epoch of Late Pleistocene epoch, and Annaier ice step of the late stage oflast Glacial Epoch as well as post Glacial Epoch of Holocene Epoch.2.Loess origin of Zoig(?) Basin has been discussed basing on synthetic study in graincomposition of the loess, surface texture of quartz sand, as well as other geologicalphenomena. On the probability cumulative curve of loess, saltation mass and suspensionare obvious, and the later is dominant, and matrix method is adopted to compute grain sizeof the loess. The statistic analysis on quartz sand surface textures of loess throughscanning electron microscope shows the features of air transportation, the main symbol ofdeveloped dishing pit, and combined profile features of Aeolian environment. In addition, sporo-pollens are scarce in the loess and ice-wedges are discovered in the neighboringloess strata. As above, it appears that Zoig(?) Basin loess is Aeolian sediment in theperiglacial environment, loess formation age mainly are Chazhenliangzi loess of Malanperiod and Suokezang Temple loess of Lishi period.3.Through field investigation of landform features of ancient river course of YellowRiver (identified valley margin profile, relatively good valley slope remained locally, smoothly connection between two sides of ancient river course and terrace of gradeâ…¡ofmodern Yellow River, unclogged and open ancient valley etc.), combining with traceanalysis by satellite pictures (it presents obvious deep gray reverse 'S' image features inplane pattern), thus distribution features of ancient courses of Yellow River is confirmedmacroscopically. Through contrast and analysis of deposit grain size test and grain sizedistribution curve of ancient courses and present Yellow River, we can learn that itsdeposit media and deposit environment is coincident. Therefore we have achievedmicroscopic evidence for the existence of ancient courses of Yellow River from theconsistency of mineral constituents from the result of mineral constituent analysis(showing the consistency of their oxidation and deposit environment). Taking neotectonicmovement as the background, depending on dating data, we have confirmed that ancient courses of Yellow River was formed in Late Pleistocene epoch. Variation of Yellow Riverresults from temporal uplift of the north of the basin, water system variation under theaction of bradyseism and stream capture of Bai River and Tangke settlement center thatlies at the severest settlement point of the basin.4.Combining with the digital geochemical map of Zoig(?) Basin based on 1: 200000regional geochemical exploration results, we learn that the overwhelming majority ofgeochemical elements including Zn, Sb, Hg, Pb, As, Au, Ag etc. shows obviouslow-background distribution in ancient lake zone of Zoig(e) Basin, which is one typicalgeochemical zone lack of surface genetic elements in the west of Sichuan province. InZoig(?) Basin, variation factor CV of most elements is less than 30%, in dispersed state, andlack of superposition of late geochemical action. However variation factor of obviousunevenly distributed elements including Hg, As, Sb, Mn, Au, Sr etc ranges from 50.5 to92.8, reflects concentrated distribution trend, and has certain relation with metallization ofgold, and uranium etc. According to special nature environment features of Zoig(?) Basin, we find out that main factor which can affect elements distribution features are: thetemperature and humidity of the climate and the intensive reducing environment oforganic substances like marsh and peat.5.Through field investigation and sample analysis, we have discussed desertificationprocess of Zoig(?) Basin since Holocene Epoch. Grain size and mineral analysis andscanning electron microscopy analysis shows that sand source of ancient (present) aeoliansand of Zoig(?) Basin comes from outwash drift of terrace of Yellow River and plateauregion, however under the action of wind, sorting degree and extreme resistant mineralcontent is enhanced more or less. In term of extreme resistant mineral, present aeoliansand＞ancient aeolian sand＞fluvial faces sand, relative resistant light mineral quartzand sand sorting approximately shows same sequence, and this displays that non resistantand resistant minerals of sand are weathered and decomposed with prolonged deflation, however extreme resistant minerals are accumulated and sand well sorted. Since lateHolocene Epoch, desertification at the studied region shows fluctuant development undergeneral situation of global environment variation, and it is basically consistent with Chinese climatic changes and desertification process of west neighboring region, andmainly experienced four periods. Ages of active sand dunes in much older time of thisregion includes 5000-7000a, 11500-15400a, and before 47000a.6.Through the field investigation and remote sensing image analysis and contrast of1999 (previous stage) and 2005 (present stage), we can learn that potential desertificationland of Zoige region has reduced 20189.4hm², however, desertification farmland, stabilized sand, semi-stabilized sand and wandering sand have increased 57.4hm².3572.9hm², 194.2hm² and 2816.3hm² respectively, and land with obvious desertificationtrend has increased 3.6hm². Comparing with 1999, Aeolian accumulation of 2005increases 36204.9hm². From this, it can be seen that Aeolian accumulation at Hongyuan-Zoig(?) region is still in a rising trend. From Aeolian accumulation variation of the studiedregion from 1999 through 2005, we can learn that potential desertification land anddesertification farmland are reducing year after year through returning farmland to forestry, ecological environment treatment projects and natural forestry reserve projects, however, stabilized sand, semi-stabilized sand, wandering sand, and naked sand are increasing yearafter year. From this, it can be seen that desertification trend at the studied region ischanging from overall deterioration state to local improvement. Grassland desertificationcauses are analyzed through combing with field investigation. Nature factors play oneequivalent role as human factors in the process of grassland desertification. Geologicalfactors and climate change are dominant in nature factors;geological factor is the maininternal element leading to grassland desertification, and climate change is the mainexternal element; human factor mainly comes from oversized cattle population, channeling and discharging, unreasonable grassland utilization, exploitation abuse, etc.7.On the basis of chronosequence, deposit features, lithology combination and cyclestructure of boring data, as well as variation of deposit rate, combining with combinationfeatures of cold-warm and dry- humidity environment temperature, we can identify threeobvious structural accelerated uplift periods (namely 800ka, 360ka, 160ka) at the east ofthe plateau and three stages of plateau environment evolution after 900ka. And we alsohave studied features of lower basin self rupture system at different uplift stages of the plateau including activity, structural evolution and formation mode.8.Through the field investigation and remote sensing interpreter, we have studiedbasin landform subzone and morphological feature, geomorphologic type of peat. Welearn that peat formation all have relation with negative geomorphology (lake basindepression, dale with flat bed, valley, piedmont depression, and cleuch floor etc.), andgeomorphologic process has close relation with peat formation, evolution, occurrence anddistribution. Geomorphology interpreting is helpful to offer correct comprehension uponpeat metallographic rules. And this is meaningful for the prospect evaluation of peat andresources in the studied region.