The Salt-forming Basin Evolution And Salt Dome Structure In Western China
Article type :
Proceed ings of Symposium on DIAPIRISM
General: I. The Evolution of Salt-forming Basin
The salt-forming basin in the region mainly developed and formed in the superimposed seg of Indchinese geosyncline fold.
1. Brief Introduction to The Basement Of Salt-Bearing Basin
Tibet-western Yunnan para-geosyncline region:Owing to the expanding of northern Tibet-Jinsha river-Tengtiao river rift,volcanic sedimentation started in Late Devonian on the Tethy s oceanic crust in the early stage of Hercynian movement. In Carboniferous-Permian, submarine volcanism intensified by the subduction and submerge of the oceanic crust, and the sediment are mainly of limestone and middle-basic volcanic rocks. In the late stage of Hercynian movement, though the region were influenced by folding, Triassic trough continued the development, and deposited large quantity of middle-basic volcanic rocks and carbonate, and developed into Indochinese geosyncline.
Gondis-Tengchong paraplatform region: In the eraly stage of Ordovinian-Carboniferous, it had sedimentary character of platform overburden. Bangong-Nujiang river rift occoured subduction by Hercynian movement, during Carboniferous-Permian, middle-basic volcanism were frequent. There occoured fluvia- glacial deposit and cool water fauna from Gondwana land,and had the character of geosynclinal deposition. In the late stage of Hercynian movement, the sea basin in the region elevated steadily, with the deposition of mainly of carbonate and less quantity of middle-basic volcanic rocks.
Late in the Middle Triassic, influenced by the Tndo. chinese tectonic movement, the western China geosyncline were in overall fold inversion, thus ended the development of the geosyncline.
2. The Evolution Of Salt-Forming Basin
The salt-forming basin in western China, developed and formed continuously mainly by recieving sediments of all stages from Late Triassic to Paleocene on the basement of fold inversion of Indochinese geosyncline, and affection from Yenshan movement. The first phase of Himalayan movement brought the evolution of salt-forming basin to a stop. Tibet-western Yunnan region is the main salt-forming area, which is the focal point in this paper.
Late Triassic-Early Jurassic: During Late Triassic Kaneck stage, the crust in Tibet-western Yunnan region kept constant descending, and a molasse formation and a sand-shale interlayered with volcanic rocks and carbonate formation deposited in the sea basin. In the early Noric stage, it was mainly of middle-basic volcanic rocks and carbonate formation, while in the late Noric stage, crust elevation and subsidence were frequent, weather were humic and hot, and composed a paraflysh formation with pyroclastic rock, sand-shale and coal. It was burning hot and dry in the early Jurassic, sea water concentrated and the basin reduced by degrees. The red sand-shale were predominant in deposition, and gypsum and salt enriched in the center of the basin. It is the earliest salt-forming stage in the region. Gypsum and salt occurrence are found both in southern Qinghai and Simao basin,Yunnan.
Middle-Late Jurassic: The Tethys sea basin, to the east of Bangong-Nujiang rift zone, closed due to the early stage affection of Yenshan movement, It is mainly of terrestrial and shallow sea facies of sand-shale interbedded with littoral flat carbonate deuosition. It was humic and hot in the early stage and burning hot and dry in the late stage.
The sedimentary basin reduced,sea water became concentrated, and gypsum and salt deposited locally. There are fossils of Myophorus, Protocardia strickland, etc. in limestone. It was still burning hot and dry in late Jurassic, sea basin reduced, and mainly are red terrestrial sand-shale formation.
Gondis-Tengchong region: Yaluzambu river,and Tethys basin started expanding. The Middle Jurassic is predominant of red sand-shale and limestone sediment, interbedded with middle-basic volcanic rocks, and locally enriched of gupsum and salt, while the Late Jurassic are of volcanic and siliceous rocks deposition.
The Middle Jurassic is the second salt-forming stage in the region, and gypum and salt occurrences bf this stage are found in Leiwuqi of Tibet, Yanshiping Group of Oinghai, Simao Hepingsiang formation of Yunnan.
Early Cretaceous: Tibet-western Yunnan region predominantly show a disconformity between Cretaceous and Jurassic owing to the affection of second elevation and subsidence stage of Yenshan movement. It was humic and hot in the Early Cretaceous, and mainly of yellow and purple sand-shale deposition, interbedded with coal-bearing clastics and coal seam, as well as interlayered marine marlite. There are zooliths marine of Mytilus, Inoceranus, etc. and zooliths terrestrial of Trigonioides-Nippononaia-Nakamuranaia as the main compound, and phytoliths of Cupressinacladus, etc. It belongs to alternative marine-terrestrial facies, and recieved no salt deposit.
Gondis-Tengchong region: This is igneous flysh formation, interbedded with radiolarian limestone and siliceous rock deposition, locally of sand shale with coal formation.
The Middle-Late Cretaceous: The third stage of Yenshan movement in the region was still elevation and subsidence, with the paraconformity contact between Middle and Lower Cretaceous. It was burning hot and dry in the Middle-Late Cretaceous. In northern Tibet-westemYunnan region, there were two sedimentary cycles composed of fluviolacustrine interbedding of grey purple sandstone with purplish red sand-shale, with the yellow marlite in the upper part of each cycle, which might be interbedded marine layers. Gypsum and sulfide deposited locally. The upper part have terrestrial Lamelli-branchia fossils of Nakamuranaia (Chingshanpensis; (Gr),Yunnan- oconcha pupengnesis: Ma, Nippononaia Carinatd Kob,etc.
It is marine sand-shale,volcanic rock and siliceous rock formation to the north of Yaluzambu river zone.
Paleocene: It recieved sediments continuously on the basis of Middle-Late Cretaceous sedimentary basin. The lake basin in northern Yibet-western Yunnan reduced as the paleo-weather was burning hot and dry-Water in the lake underwent multiple concentrations, and developed toward salt-forming stages. Lake facies sediment developed graduously from red clastics into salt-bearing formation of carbonate, sulfate, oxidate, etc. It is the important salt-forming stage in the region. The salt-bearing system commenly have boulder clay structure, and have multiple cyclothem character. This salt- forming stage can be divided into three substages, with the first substage composed of shale and salt-bearing boulder clay of random deposition, Where only a few places have salt deposit. The second and third substages consist of lake facies calcareous siltstone with boulder clay, gypsum, chlorate. Salt layers are generally thick, large and stable, have the excellent economic value. In places of Fenghuoshan of southern Qinghai, Gongjue and Yanjing in Changdu area of Tibet, Lanping, Jinggu, Simao basins of Yunnan, important salt deposits are found. There are also sylvinite enrichment locally. Subsequent of salt-bearing system deposition, it was still Durning hot and dry, deposited upto thousand meter thick red terrestrial sand-shale formation.
The area to the north of Yaluzambu river zone may have recieved sedimentation of that stage.
The first phase of Himalayan movement closed lake basin and ended the evolution of salt-forming basin.
II. The Developing Mechanism Of Salt Dome Structures
The formation and development of the salt dome structures in the region closely relate with the Himalayan movement.
1. Multiple Developing Stage Feature Of Salt Dome Structures
After salt formation in Paleocene, salt layers were horizontal. During the first phase of Himalayan movement (middle Late Eocene), the salt basin mantle formed wide and smooth folds, and during the folding of the second phase of Himalayan movement (before Miocene), the salt basin was again under the affection of lateral force from both sides, salt layers folded again, and deformed and flowed plastically, thus thickened the layers at the axes of anti- and synlines, and thinned the layers at their limbs, while locally formed salt dome structures (see Fig.2).During the third phase of Himalayan movement, mintensive folding and smooth moving occoured in the region, resulting in a series of structures of nappes and nappe outliers, and salt dome structure subjected to compression again, and resulted in piercement structures (see figure 3 and 4).
2. Salt Dome Structures Occoured Owing To The Differences On Rock Physical Feature
The lower part of the salt system in the region is a gigantio thick sandstone layer of Pashahe formation, with the thickness raging from some tens of meters to hundreds of meters, which are rigid beds. The upper part is a plastic layer of mud stone, gompholite, gypsum salt layers, etc. In Paleocene, salt system and salt layers occoured horizontally. They were compressed laterally from both sides of the basin in the folding of the first phase of Himalayan movement, and both rigid and plastic layers formed wide and smooth folds at the same time. In the folding of second phase of Himalayan movement, the layers were again subjected to compression from both sides of the basin, often formed the sancstone in dome shapes. The plastic layer of salt system, under the force of upward compression by the sandstone, slided downwardly. During the folding and sliding compression of the third phase of Himalayan movement, sandstone continued spining upward, salt system and salt layers slided and cantorted, thus thinned salt system. Sandstone contacted with the salt system and salt layers at different levels in para-unconformity. Salt layers thinned at domes and thickened at subsynclines. When tectonism were intensive, the rocks fractured and dome structures destroied (see figures 5 and 6).
3. Strike Slip Faults Are the Important Causes In Forming Diapires
During the first and second phases of Himalayan movement in the region, the basin were subjected to lateral compression from both sides. The direction of structural line of the salt-forming basin was in line with that of structural line on both sides of the basin, as the structural lines of anticline, syncline, and faults were mainly of NWN trend.
During the folding and strike slip faulting in the third phase of Himalayan movement, produced folds that cut with the direction of slip force, resulted in nearly EW striking anticlines, synclines, and nappes. Comparatively, there occoured a series of salt dome structures, and produced diapires and piercements. When slip force was intensive, nearly EW trend faulting occoured, and on the faulting side, diapires are extrimely developed( see Fig.7).
Salt-forming basin in the region is a superstratum on a seg,fonned on the basis of Indochinese geosyncline inversion through evolution history by recieving deposition of all stages of Late Triassic-Paleocene.
The formation of salt-bearing system experienced processes of all stages of Yenshan movement. The frequent elevation and subsidence of sea/land alternations, long period of burning hot and dry weather, multiple concentration of sea water, and finally the salt basin minimized in Paleocene, and brine were highly concentrated into salt. The deposition history of the salt-forming basin ended with the first phase of Himalayan movement.
The formation and development of salt dome structure are closely related to the Himalayan movement and the rocks physical features of the salt system, and have a multistage character.
Salt-forming basin in the region formed in persistent evolution by recieving sedimentations of stages from Late Triassic to Paleocene and influence of Yenshan movement mainly on the basement of Indochinese geosyncline fold inversion,whereas the formation and development of salt dome structures are closely related with the Himalayan movement and physical characters of salt-bearing system.
"Western China ",mentioned in this naper, refers to the regions of rift zone from northern Tibet in the north, through Mani,Yushu regions,to the Jinsha river and Tengtiao river,as well as the large wide area to north of Yaluzambu river. This area involves two structural elements of Tibet-western Yunnan para-geosyncline, and Gondis-Tengchong para-platform( see Fig.1).