DISCOVERY OF PALYNOMORPHS AND THEIR SIGNIFICANCE IN THE TUNTIANYING FORMATION OF BAOYUAN,LONGJING CITY,JILIN PROVINCE
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Abstract:
The Tuntianying Formation,named in 1959,is a series of volcanic rocks intercalated with volcaniclastic rocks under neath the Changcai Formation in the Yanji Basin.It distributes mainly in Mingyue Town of Antu County, Laotougou, Tianbaoshan, Shilipu and Wangqing areas in Longjing City.Because of the insufficiency of study on volcanic movements and deficiency of biostratigraphic data,the definition,geological age and subdivision of the Tuntianying Formation remain debatable.The geological age of the Tuntianying Formation has been regarded as the Middle or Late Jurassic for a long period before Li(2001) assigned it to Berriasian of the Early Cretaceous based on palynomorphs in the Tuntianying Formation of Mingyue Town, Antu County. We studied palynomorphs in the Tuntianying Formation of Baoyuan,Longjing City to discuss the geological age and subdivision of the Tuntianying Formation further,and to provide more evidence for subdivision and correlation of Mesozoic volcanic strata inside and outside the Yanji Basin.The 2 samples were collected from Bed 4 in the section from Baoyuan railway station to the Laotougou coal mine which was first measured by Zhang Chuan-bo in 1980.There are 31 genera and 51 species in the palynomorph assemblage,including 16 genera and26 species of spores of Pteridophyta and 15 genera and 25 species of pollens of Gymnospermae.Pollens of Gynmospermae are more than spores of Pteridophyta in proportion,the former accounting for 58.64% and the latter 41.36%.The main components of pollens of Gymnospermae are dissacate pollens of conifers accounting for 32.81%of the total number,among which developed bisaccate pollens of Pinaceae and Podocarpaceae are more than undeveloped bisaccate pollens of Coniferales. There are a certain amount of monocolpate pollens of Ginkgoaceae or Cycadaceae(5%) and ring-grooved pollens of Cheirolepidiaceae(2.61%).Among the pollens of Gymnospermae incertae sedis, monosaccate Callialasporitesaccounts for 2.59%.The rest palynomorphs account for less than 2% except Psophosphaera.Among spores of Pteridophyta,Leiotriletes is the most numerous,accounting for 11.21% of the assemblage. The second are the spores of Lygodiaceae accounting for 10.24%,among which Cicatricosisporites accounting for 7.03% is the most numerous.There are some important spores of Pteridophyta accounting for about 2% such as Pilosisporites,Aequitriradites and Lygodiumsporites.This palynomorph assemblage can be named as Cicatricosisporites-Pilosisporites-Aequitriradites Assemblage.Among the Cicatricosisporites-PilosisporitesAequitriradites Assemblage,developed bisaccate pollens are slightly more than undeveloped bisaccate pollens.Spores of Lygodiaceae reach 3 genera and 8 species and account for more than10% of the total number, which makes them important in the assemblage.In addition,with some typical Early Cretaceous taxa such as Aequitriradites and Pilosisporites,the assemblage shows a feature of the Early Cretaceous.In the current palynomorph assemblage occurredsome important Early Cretaceous palynomorphs such as Cicatricosisporites,Pilosisporites, Lygodiumsporites and Aequitriradites whose proportions are close to those in early Early Cretaceous.The other taxa are common in the Jurassic and Cretaceous,and developed bisaccate pollens are more than undeveloped bisaccate pollens, which is also a feature of the Early Cretaceous. Hence,the geological age of the palynomorph assemblage in the Tuntianying Formation of Baoyuan should be Berriasian—Valanginian of the Early Cretaceous.The palynomorph assemblage in the Tuntianying Formation of Antu County(Li,2001)is characterized with numerous bisaccate pollens of conifers and fewer Schizaeoisporites,Fixisporites, Cicatricosisporites. The current assemblage is similar in a certain content with that in Antu County. However,their difference is obvious. For example, there are more Cicatricosisporites accounting for 7.03% of the current assemblage,while spores of Pteridophyta accounts for 5% with Cicatricosisporites decreases greatly in the assemblage of Antu County.Furthermore,Pilosisporites,Aequitriradites and Lygodiumsporites account for about 2% in the current assemblage. Therefore, the current assemblage should be younger than that in the Tuntianying Formation of Antu County.According to the feature of the palynomorph assemblage in the Tuntianying Formation of Baoyuan,we can infer that during Berriasian—Valanginian of the Early Cretaceous,plants of Pinaceae,Podocarpaceae and Cycadaceae with tall trunks and pteridophytes of Cyatheaceae and Lygodiaceae stood on the river or lake banks or the high lands beside intermountain basins,herbs of Osmundaceae,Selaginellaceae and Lycopodiaceae and bryophytes grew in the shady areas of low lands and humid areas of river valleys and lake depressions,and plants of Cheireolepidiaceae grew in some areas with a sunny exposure or arid zone.During early Early Cretaceous, the palaeoclimate in Baoyuan area of the Yanji Basin was probably warm and humid subtropical,a drier interval might also have existed because of volcanic movement.
