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Jixian County,an ancient county(previously Yuyang County)in North China,is located at the southern foot of the Yanshan Range and in the northern outer suburbs of Tianjin municipality.It is about 90 km east of Beijing and 100 km west of Tianjin.
Well-developed Proterozoic sequences crop out in the northern parto f the Jixian area(Fig.3-6).A NW-trending syncline with a core of Cambrian rocks occurs between Hongshuizhuang and Jing"eryu villages.A thrust fault parallel to the strike of the rocks between Yangzhuang and Chuancangyu villages caused a repetition of the strata.In addition,a large N-S trending fault from Huangyaguan to Yangzhuang with ENE-trending thrusts occurs along the front of the hill,north of Jixian(Fig.3-6).
The Proterozoic geology of the Jixian area and its vicinity was firsts tudied by Gao Zhenxi(C.C.Kao)et al.in 1934.These authors and many subsequent studies provided detailed descriptions of the lithology and subdivided the rocks into formations.They also properly identified the stromatolites occurring in the rocks.Based on these works,the Jixian section has been regarded as a typical section of the Middle and Upper Proterozoic in North China.
Fig.3-6 Geological map and cross-section of Jixian and adjacent areas
1—Quaternary;2—Cambrian;3—Jing"eryu Fm.;4—Luotuolingfm.;5—Xiamaling Fm.;6—Tieling Fm.;7—Hongshuizhuang Fm.;8—Wumishan Fm.;9—Yangzhuang Fm.;10—Gaoyuzhuang Fm.;11—Dahongyu Fm.;12—Tuan Shanzi Fm.;13—Chuanlinggou Fm.;14—Changzhougou Fm.;15—Archean;16—Pluton;17—Unconformity
At the beginning of the 1970"s,the Proterozoic sequences around Jixian and Yanshan Range have been studied by many geologists,and numerous papers and monographs have been published on the lithology,paleontology,geochemistry,geochronology,paleomagnetism and paleogeography of these rocks.
The Proterozoic sequences in the Jixian area are underlain by Archean rocks and overlain by lower Cambrian strata.These sequences have a total thickness of 9193 m,which can be divided into 12 formations that define 3 systems,the Changcheng,Jixian and Qingbaikou systems.
Geochronological results date the Proterozoic sequences in Jixian area as ranging ca.1800 to 850 Ma.The Changcheng System is dated at ca.1800-1 600Ma,and the Jixian System at ca.1600～1000Ma and the Qingbaikou System at ca.1000-850Ma,and these systems belong to the late Paleoproterozoic,Mesoproterozoic and Early Neoproterozoic,respectively.The stratigraphic succession of these units is described as below:
1.The Late Paleoproterozoic Changcheng System
The Changcheng system con Sists of four formations with a total thickness of 2762 m.The lowermost Changzhougou Formation rests unconformably over the Archean metamorphic Zunhua Complex-Group,and is chiefly composed of conglomerate,pebble-bearing sandstone and arkosic sandstone of fluvial origin in the lower portion and sandstone of marine facies in the middle-upper part.The second formation,the Chuanlinggou Fm.,is mostly composed of neritic shales grading upward into the dolomitic Tuanshanzi Fm.The fourth formation,the Dahongyu Fm.,co nsists o f littoral and neritic sandstone,shale and K-rich trachyte with cherty dolo stone in the upper part.
(1)The Changzhougou Formation
Location:North top of Changzhougou(40°13′03″N,117°30′47″E,494m)
Content:Unconformity between Changzhougou Formation(Chch)and Archean(Fig.3-7).
Fig.3-7 Unconformity between Changzhougou Formation and Archean in Changzhougou
A—the unconformity of Chch/Ar;B—the river conglomerate and cross-bed in Chch
The Changzho ugo u Fo rmatio n,the lo wermost unit of the Changcheng System,co nsists o f grayish purple,massive to thick-bedded co nglo merate and co arse-grained sandsto ne,pinkish and white quartz sandsto ne,sedimentary quartzite and so me thin-bedded sandsto ne,fine-grained sandsto ne,clayey and feldspathic sand Sto ne with a thickness o f 859 m.The quartzite and quartzitic sandsto ne of shallo w marine o rigin are the chief co mpo nents o f the fluvial conglomerate,and coarse grained sandstone dominates in the lower part.
So far,no stromatolites have been identitied in this formation.However,a numbero fa critarches,including ssme unicellular eukaryotes,have been identified from the intercalated beds of muddy siltstone in the upper part of the formation.
（2)The Chuanlinggou Formation
Location:Chuanglinggou(40°11′52″N,117°28′50″E,295m).
Content:Boundary of Chuanglinggou Formation/Changzhougou Formation(Chc/Chch).
The Chuanlinggou Formation consists chiefly of silty and illitic lenses.It has a thickness of 889m in Jixian area,but much less in the eastern and western parts of the Yanshan Range.
This formation can be subdivided into three sections based on the quantity of sandstone and the color of rocks.The lower section of supratidal to intertidal origin is characterized by a large amount of yellowish-green sandstone,and the middle section is composed of black shale of subtidal to lagoonal facies with many intercalations of intertidal sandstone.Mud cracks are spectacularly developed and sand veinlets are common in the sandstone of the lower section.
Acritarchs are abundant in this formation and they include the following groups:Sphaeromorphida(e.g.Leiominuscula,Margominuscula,Dictgosphaera,Kildinellaand Leiosphaerida），Scaphomorphida(e.g.Dictgosphaera,Kildinellaand Schizovalvia），Versimorphida,including blade-like,lobe-like,valve-like and band-like forms(e.g.Pholimorphaaa,Lobomorphaand Valvimorpha）：and Nematomorphida(e.g.Qingshaniaand Changchengonema).In addition,some macroscopic,ribbon-shaped fossil algae have been reported.
There are no stromatolites in this formation in Jixian area,but two horizons containing stromatolites have been found in the western Yanshan Range.
(3)The Tuanshanzi Formation
Location:40°10′32″N,117°27′59″E,291m.
Content:Tuansanzi Formation/Chuanglinggou Formation(Cht/Chc).
The Tuanshanzi Formation is composed mainly of muddy and silty,micritic dolomite with a total thickness of 518 m(Chen et al.,1980).The lower member of this formation contains abundant muddy,silty and carbonaceous material,with pyrite impregnations and black carbonaceous films on the bedding planes.In the upper member,the content of clastic material increases considerably resulting in micritic sandstone,sandy dolostone,thin-bedded sandstone and small stromatolite bioherms.
The lower member of the Tuanshanzi Formation is characterized by even bedding indicating deposition in a quiet,weakly reducing,subtidal environment(lagoonal facies）,whereas the upper part formed in an unstable environment probably between the intertidal and supratidal zones.Previously,the Tuanshanzi Formation was regarded as being poor in fossils,containing only a few of stromatolites,microfossils and doubtful megafossils dominated by Tyrasotaenia(Hofmann and Chen,1981).However,a large collection of megafossils represented by Iongfenshanids(Antiqufolia,gixianfolium and others)have been identified in the lower part of this formation in recent years,and interpreted as Paleoproterozoic megasc Opic eukaryotes Or metaphytes.
(4)The Dahongyu Formation
Location:40°10′28″N,117°28′00″E,258m.
Content:Dahongyu Formation/Tuansanzi Formation(Chd/Cht).
The Dahongyu Formation can be subdivided into two units.The lower unit consists mainly of white quartz sandstone and sandy dolostone,intercalated With purple and green tuffaceous siltstone or tuffaceous silty shale,locally accompanied by beds of volcanic breccia,tuffaceous sandstone and high-K lava.The upper section consists of micritic dolomite and cherty micritic dolomite with abundant stromatolites.A fe W poorly preserved acritarches have also been found mainly in the lower section of this formation.Some microscopic algal fossils occur in the cherty beds of the upper unit.
2.The Mesoproterozoic Ji Xian System
Disconformably overlying the Changcheng System,the Jixian system consists of five conformable formations with a total thickness of about 6088 m.The Gaoyuzhuang Formation at the base,is composed of littoral-neritic dolostone and dolomitic limestone.The overlying Yangzhuang Formation i S chiefly made up of alternating red-white marine dolostone with minor limestone.In the middle,the Wumishan Formation is composed of thick beds of siliceous dolostone intercalated with abundant cherty bands,and it grades upward into the Hongshuizhuang Formation composed dominantly of black shale.The uppermost Tieling Formation consists of shale and limestone with abundant stromatolites.
(5)The Gaoyuzhuang Formation
The Gaoyuzhuang Formation disconformably overlies the Dahongyou Formation of the Changcheng Group.Because it has sedimentary and paleontological characteristics typical of the“Jixianian System”.
This formation consists chiefly of carbonates and is subdivided into four members:
Member 1 is characterized chiefly by dark-gray cherty micritic dolomite,containing lenses of muddy and sandy dolomite.It also contains intercalations of thin-bedded,clayey,micritic dolomite and shale or silty shale.In addition,a bed of sandstone,about three meters thick,with ripple marks occurs at the base,where it overlies an erosional surface.
Member 2 could be further subdivided into two parts.The lower part consists of thin-bedded manganiferous,sandy micritic dolomite intercalated with dolomitic siltstone.Thick-bedded to massive dolomite with wavy bedding surfaces and large-scale ripple marks makes up the upper part of this Subformation.
Member 3 i S characterized by dark-gray muddy dolomite,calcareous micritic dolomite and dolomitic and micritic limestone with platy bedding,intercalated with some clastic layers and nodular limestone.
Member 4 begin S with dark,thick-bedded,calcareou S dolomitic micrite and dolomite,interca-lated with black,bituminous and muddy dolomite.This passes upward into gray,micritic dolomite with siliceous layers.All of these rocks in the upper part are thin-bedded and contain wavy bedding and cherty concretions.
The Gaoyuzhuang Formation contains various fossils.Stromatolites occur mainly in member 1 and are represented by Confusoconophytonmultiangulum,Gaoyuzhuangiacrassibrevis,G.Bulbosa,G.gaoyuzhuangensis,Tabuloconigera paraepiphyta,Conophyton garganicum and C.cylindricum.Microfossils interpreted as microscopic algae occur mainly in the cherty rocks of member 1.Although the microfossils are poorly preserved in the Jixian area,they are abundant in the same horizon in adjacent areas.Acritarchs are generally sparse and poorly preserved in the formation,and occur only member 1 and 3.The carbonaceous megafossils are represented by G.rvpaniaspiralis(namely Sangshuaniaspiralis,S.sangsuanensis and S.buccinata)and they occur only in the lower part of member 3.
(6)The Yangzhuang Formation
The Yangzhuang Formation consists of brick-red argillic,silty micritic dolomite with alternating white layers.Near both the base and top of the unit,there are thick layers of siliceous micritic or sparry dolomite.Many of the red layers have small,white spots.
This formation is subdivided into three members based on lithology.Both the lower and middle members contain ripple marks,mud cracks,cross-beds,and gypsum and salt casts indicating that they formed in supra-and intertidal lagoonal environments in a hot,dry climate.However,these features are absent in the upper member,which consists of bituminous dolostone.The Se rocks probably formed in a hot,wet,lagoonal environment.
Fossils are sparse and poorly preserved in the Yangzhuang Formation.Only some small stromatolites(Microstylus,Scyphus,Yangzhuangia)and a few achritarchs have been found in the lower and upper members.
(7)The Wumishan Formation
The Wumishan Formation,with a thickness of3336m,makes up one third oft he total Proterozoic section in the Jixian system.The rocks of this formation are rich in organic matter and show many cycles,suggesting deposition in a tidal environment.This formation includes four members:
1)The Lozhuang Member
The lower part of this unit contains alternating layers of white,muddy to silty,micritic dolomite,banded cherty micritic and sparry dolomite with intercalations of bituminous dolostone.The upper part is composed mainly of gray and dark-gray,medium-to thick-bedded,sparry dolomite with bands of chert and black,bituminous dolostone.These microbial deposits are characterized by small,wavy,laminated and columnar stromatolites.
2)The Mopanyu Member
This member consists chiefly of white silty to shaly micritic dolomite and thick-bedded stromatolitic,sparry dolomite.Chert layers,chert concretions and flat-pebble cherty conglomerates occur in the lower part of the unit.The upper part contains some thick-bedded,bituminous sparry dolomite with wavy laminae of algae,with several large,conic stromatolitic and silicified oolitic beds atthe top.
3)The Ershilipu Member
This member is subdivided into a lowera nd an upper parts.The lowerp art consists of white,rhythmically layered shaly to silty micritic dolomite,gray calcareous to muddy dolomite and gray，calcareous micritic dolomite with layers of black chert.Red,shaly,silty or sandy micritic dolomite occur in the base of the lower unit.The upper part generally consist S of dark-gray and thick-bedded calcareous,micritic dolomite.
4)The Shanpoling Member
This member can be subdivided into al owera nd an upperp arts.The lowerp arti s composed of gray to grayish-white,calcareous,micritic dolomite and cherty micritic dolomite intercalated with bituminous dolostone.The upper part consists mainly of light gray,thin-bedded calcareous,micritic dolomite without chert.In addition,a bed of white dolomitic sandst
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