FOSSIL PLANTS FROM THE LATE MIDDLE JURASSIC IN YABULA BASIN,WESTERN NEI MONGOL,CHINA

The plant fossils described in this paper were collected by the authors in 1998 from the upper part of the Xinhe Formation of Hongliugou section in Yabula Basin, western Nei Mongol (Inner Mongolia), China (Text fig. 1). The Xinhe Formation, which rests directly on the Proterozoic bojite and is overlain conformably by the Upper Jurassic Shazaohe Formation, is characterized by gray, grayish green coarse grained sandstone, muddy siltstone, calcic mudstone in the lower part, and brown or red medium grained sandstone alternated with gray or grayish green muddy sandstone, mudstone and intercalated with very few coal streaks in the upper part. The plant fossils were collected from the gray or grayish green colored rocks in the upper part.thedistributionofthechiefelements ,theflorashould beattributedtothelateMiddleJurassic ,probably Bathonian Callovian .Amongtheknownspecies ,Co niopterishymenophylloide ,C .burejensisandO tozamitesgramineusappearedintheMiddleJurassic YorkshireFloraofEnglandandotherflorasofthe sameageintheworld .Theappearanceofthese speciesstronglysuggeststhatthepresentfloraisMid dleJurassicinage .Coniopterisangustilobaisfound frequentlyintheMiddleJurassicinMiddleAsia .Ac cordingtotheliteratures ,theJurassicflorainNorth Chinareachedthepeakofitsdevelopmentduringthe earlyMiddleJurassic ,andthengraduallydeclinedin BathonianandCallovian .TheflorasofAalenianand Bajocianare generallycomposedofmorethan 1 0 0 species,whichisdominatedbyFilicopsidaand Ginkgopsida ,whiletheBathonianflorasusuallyde creasestoonly 2 0 - 30species,whichischaracterized byCycadopsidaandscaleleafconifers .Thepresent floraisaveryreducedonethatconsistsofonlya dozenofspecies,andischaracterizedbycycado phytes .ThismeansthatthepresentfloraislateMid dleJurassicinage (Bathonian Callovian) . Asmentionedabove ,theXinheFormationis characterizedby grayishmudstone ,muddysiltstone andbrownred brownsandstone ,withoutcoalbeds intercalatedexceptveryfewcoal streaks .Thiscanbe consideredasatypicalupperMiddleJurassicstratum . StudieshaveshownthatinthewholeNorthChina , theJurassicisdividedverticallyintotwoparts .The loweroneincludingtheLowerandlowerpartofthe MiddleJurassicisdominatedbycoal bearingsedi ments;whiletheupperoneincludingtheupperpart oftheMiddleJurassicandUpperJurassicchieflycon sistsofgray ,brownandredcoloredsediments ,usu allywithoutindustrialcoalbedsexceptsomecoal streaks .TheUpperJurassicischaracterizedbyase riesofredcoloredrocks ,usuallywithrareplantfos sils.Therefore ,theXinheFormationisreliablyas signedtotheupperpartoftheMiddleJurassic ,and theupper partoftheXinheFormationshouldbe BathoniantoCallovian . ThepresentfloracompareswiththoseofSub tropicalPhytogeographicalRegion (Euro Asianre gion)incomposition .Themajorcontentoftheflora iscycadophytes ,especiallyOtozamites ,agenusthat usuallyoccursinSouthChina ,MiddleAsiaandEu rope ,butishardlyfoundinNorthChinaandSiberia duringtheJurassic .Therefore ,thepresentflorabe longstotheSouthChinatypethoughitoccursinvery northarea .Thissuggeststhattheboundarybetween theSouthandNorthPhytogeographicalRegionsis notalongtheKunlun Qilian DabieMountainsasin theEarly MiddleJurassic (beforeBathonian) .It greatlymovednorthwardsduringthelateMiddle Jurassicduetotheclimatechanging ,andwouldlo catedinnorthofNeiMongol (possiblyinMongolia) inwestandattheYinshanMountainsineast.

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Q914.2