J ournal of University o f Geosciences , Vol. 17 , No. 3 , p. 209 - 220 , September 2006 ISS N 1002 - 0705 Printed in China

Palynology of Yanchang Formation of Middle and Late in Eastern Gansu Province and Its Paleoclimatic Significance

Ji Liming*(吉利明) Lanzhou I nstitute o f Geology , Chinese Academy o f Sciences , Lanzhou 730000 , China ; Graduate School , China Univ ersity of Geosciences , Wuhan 430074 , China Meng Fanwei (孟凡巍) Nanjing I nstitute o f Geology and Palaeontolog y , Chinese Academ y o f S ciences , Nanjing 210008 , China

ABSTRACT :Xifeng (西峰) oilfield was recently found in the southwest of the Ordos basin. The oil source rocks are the Chang 7 Section of Yanchang(延长) Formation. In order to study the paleoclimate that controlled the formation of source rocks, a systemically palynological research on related beds in Yanchang Formation has been carried out. The core samples were analyzed with classical palynological techniques and the organic-walled sporomorphs from these samples were observed, identified and photo- graphed under a light microscopy and a fluorescence microscopy. Abundant sporopollen were found in drilling cores from Chang 8 and Chang 7 sections, and two assemblages were distinguished :the Aratisporites-Punctatisporites assemblage and the Asseretospora-Walchiites assemblage. Their charac- teristics are similar to those of the assemblage of Tongchuan (铜 川) Formation and the assemblage of Yanchang Formation in southeast Ordos basin , respectively. Their geological times are Ladinian of late Middle Triassic and Carnian of early Late Triassic , respectively. The correlation of palynoflora with their parent plants suggests the paleoclimate of eastern Gansu (甘肃) Province in the Middle and Late Triassic was warm and rainy with prosperous vegetation. The palynoflora indicated the area was in a temperate to subtropical zone then. Both the ecological types and differentiation degree curves of sporopollen indicated the period during Chang 8 and Chang 7 sections was warm and wet, and the phase accorded with large-scale lake transgression in Chang 8 Section and the largest lake area in Chang 7 Sec- tion. North China in Middle and Late Triassic was located in a warm and rainy, temperate and subtropi- cal zone. The palynofloras in Chang 8 and Chang 7 sections have the characteristics of North China flo- ra, however they also indicate wetter and warmer climate due to their proximity to the large lakes. The period of Chang 8 to Chang 7 sections is the climax of the expansion of the lake , and the bloom of fresh algae during the period , which helped form the high-quality source rocks in Chang 7 Section. KEY WORDS:Ordos basin, eastern Gansu Province, Yanchang Formation , source rock, sporopollen assemblage, paleoclimate.

INTRODUCTION Sea before the Mesozoic. The basin w as fo rmed dur- The Ordo s basin belongs to the m arginal terrace ing the Middle Triassic because o f the e xtrusion from of No rth China ancient land , e xtending to the Qinling w est to east on the w estern margin of the N orth Chi- na plate. It w as g radually depressed into a lake basin This paper is financially su pported by “ Prom inent You ngster in W estern phase in the Middle and Late T riassic , to form the C hina” Science Foundation of Chinese Academ y of S ciences (No. C AS 2002-404-01) and the Gan su Provin ce Young and Middle Scientists deeper so uthwestern part and shallow er no rtheastern S cience Foundation. part lacustrine-delta clastic sediments of o ver 1 000 *C orresp on ding author :jilimin @lzb. ac. cn m , know n as the Yanchang Fo rmatio n , w hich is the m ain oil so urce rock in the Ordo s basin (Edito rial M anu scrip t received March 10 , 2006. M anu scrip t accepted June 25 , 2006. Com mittee o f Petro leum Geolog y of Changqing Oil 210 Ji Liming and M eng F anw ei

Field , 1992). SETTING AND MATERIAL Xifeng oilfield w as recently fo und in eastern Geological Setting Gansu Province , in the so uthwest of the Ordos ba- Acco rding to the basin's structure , especially the sin. Studies have indicated that dominant oil sources structural differences since the M esozoic , the Ordos are mudsto ne w ith abundant org anic matter in Chang basin is divided into six units (Editorial Committee o f 7 Section of the Yanchang Formation (Fu et al. , Pe tro leum Geolog y of Chang qing Oil Field , 1992). 2004). A lgite m aceral in m udstones from Chang 8 The middle Yishan slope is the m ain pa rt of the Or- Sectio n to Chang 6 Section in the Yanchang Fo rma- do s basin , and the Yishan slope's no rthern par t and tion w ere found in previo us pe trog raphic studies. southern par t are Yimeng hunch and Weibei hunch Authors fo und many acritarchs and Botryococcus. respectively. The slope's western par t is the w estern The abundant alg ae are obviously important biologi- thrust belt and Tianhuan hollo w zone and the eastern cal sources for oil in the area. In order to study the part is the Jinxi flexural fo ld. The study area o f paleoclimate that controlled the formation of the source Xifeng oilfield is in the southw estern part of Yishan rocks , this paper presents sy stematic palynological re- slope , adjacent to Tianhuan hollow zo ne. search on related beds in the Yanchang Formation.

Table 1 Stratigraphic divisions of the Yanchang Formation in eastern Gansu Province

The Middle and Late Triassic Yanchang Fo rma- and M iddle Triassic sediments are m ainly fluvial and tion o f Ordo s basin is m ainly a series of fluvial and la- lacustrine clastic sediments because the environment custrine clastic sediments formed in a large-scale in- w as arid during that time , how ever in the M iddle and land basin. The base o f the Yanchang Formation lies Late Triassic thick , deep to half-deep lake , oil source unco nform ably over the Middle Triassic Zhifang Fo r- ro ck sedim ents w ere fo rmed because the arid enviro n- m ation , and its top is eroded to different deg rees and ment became w et and entered into a lake development is unco nformably o verlain by the Lo we r phase. The uplift of the Ordos basin in the Late Tri- Yanan Formation or Fuxian Fo rmation. The Early assic resulted in the disappearance of the paleo-lake Paly nology of Yanchang F ormation of M iddle and La te T ria ssic in Eastern Gansu and Its Paleo clima tic Significance 211 basin. The co re samples we re analyzed w ith classical Yanchang Form ation can be divided into 5 litho- paly nological techniques , and the o rganic-w alled ho rizons (T 2 y1 - T 3 y 5 ) fro m bottom to top , and in- spo romo rphs fro m these sam ples we re observed , cludes 10 oil laye r sections (Chang 10 Section to identified and photog raphed under a light microscopy Chang 1 Section) (Table 1). Previous studies on and a fluorescence microscopy. 66 sam ples have been plants , fish and bivalves have constrained g eological analy zed , and the results indicate there are abundant time from Chang 10 Section to Chang 8 Section to the spo ropo llen micro fo ssils in Chang 8 Sectio n to Chang Middle Triassic , and from Chang 7 Sectio n to Chang 7 Section. Over 65 % o f sam ples are found to have 1 Section to the Late T riassic (Institute of Geolo gy , spo ropo llen. Over 50 % are suitable fo r statistics Chine se Academy o f Geolo gical Sciences , 1980). (m ore than 100 spo res and po llens in o ne sample), In this paper , the Yanchang Formation includes particula rly sam ples from Chang 8 Section , over 85 % w hat has been previously called the Yanchang Fo ma- of w hich contain fossils , with 75 % suitable fo r sta- tion and the Tongchuan Fo rmatio n. The To ng chuan tistics. Only Chang 7-3 Sectio n co ntains rare fo ssils ,

Fo rmatio n includes T 2 y 1 - T 2 y2 par ts , namely Chang w ith 20 % of the samples fro m this section suitable 10 Sectio n to Chang 8 Section. N arrow Yanchang for statistics. T he lack of spo ropo llen in Chang 7-3 Fo rmatio n includes T 3 y 3 - T 3 y5 par ts , namely Chang Section is probably due to a half-deep and deep lake 7 Section to Chang 1 Section. environment in the area , w here less spo ropollen w ould have been deposited. T here is also a hig h Materials and Methods abundance o f o rg anic sedimentary rocks that are un- The explo red strata in eastern Gansu Province suitable for the preservation of spo ropollen fo ssils. are fo cused o n Chang 8 Section to Chang 6 Section. Because the cores are limited in w ells , we co llected RESULTS AND DISCUSSION samples fro m different w ells in the region in orde r to Sporopollen Assemblages get mo re complete info rmatio n. The majority of sam- A total of 93 g enera and 131 species of sporopol- ples came fro m wells X17 , X30 , X36 , X44 and N4 len fossils w ere identified from the Chang 8 Sectio n to (Fig. 1), and some key samples from Chang 7-3 Sec- Chang 7 Section in eastern G ansu Province. We could tion also came from wells X40 , X43 , Zh42 , Zh44 and disting uish tw o spo ro pollen assemblag es acco rding to Y22 , M 9 and Zh5 in the adjacent region. The stratig- m ain spo ropo llen types and their distributions : raphies of all sam ples are co nfirmed by the means of Aratisporites-Punctatisporite assem blage in Chang 8 co rrelation betw een each section (Fig. 2). Section and Asseretospora-Walchiites assem blag e in Chang 7 Sectio n. The main sporo pollen genus and their distributio ns are show n in Fig. 3 and Fig . 4.

Aratisporites-Punctatisporites assemblage This assemblage includes 80 gene ra and 113 spe- cies , the main characteristics are very high abundance and diversity of spo res and pollens. Fe rny spo re has predominant abundance , at 52. 5 % - 88. 7 %, after w hich comes g ym no sperm pollen at 11. 3 %- 45. 7 %. Aratrisporites is the mo st hig hly abundant o f ferny spo res , at 6. 8 %- 66. 2 %;Punctatisporites is 4. 0 % - 50. 4 %;o ther spo res with high abundance are Osmundacidites and Verrucosisporites , w ith 3. 5 %- 25. 6 % and 2. 3 % - 18. 0 % respectively . Com mon spo res are Calamospora , Conv errucosis- porites , Apiculatisporites and Asseretospora , w ith 4. 8 %- 10. 2 %, 2. 2 %- 13. 3 %, 2. 2 %- 12. 0 % and Figure 1. Position of main sampling section in 1. 0 % - 6. 0 % respectively. Other low abundant eastern Gansu Province. ferny spores include Concavisporites, Leiotriletes , Todisporites , Cy clogranisporites, Lophotriletes , 212 Ji Liming and M eng F anw ei

Figure 2. Strata correlation of main sampling section in eastern Gansu Province.

Figure 3. Distributions of main spores in Chang 8 Section and Chang 7 Section.

Lycopodiacidites, Densoisporites, and Laev igato- are infrequently incom plete Saccites and co state. A l- sporites , and they are classic Mesozoic microfossils. isporites has the m ost abundance in pollens of the as- Gy mnosperm pollens have many types with Sac- semblage , at 4. 0 % - 14. 1 %. Abietineaepollenites , cites dominating and abundant Spruce types. T here P inuspollenites , P iceaepollenites , Walchiites and Paly nology of Yanchang F ormation of M iddle and La te T ria ssic in Eastern Gansu and Its Paleo clima tic Significance 213

Cycadopites are 4. 8 % - 10. 2 %, 2. 2 % - 13. 3 %, dant and co mmo n po llens in the M esozoic are Pro- 2. 2 %- 12. 0 % and 1. 0 %- 6. 0 % respectively . Pro- topinus , Podocarpidites , Chordasporites , Lueck is- topicea , Piceites, Cay tonipollenites, Cordaitina and porites and Paleoconi ferus. Protconi ferus also often occur. The other low abun-

Figure 4. Distributions of main pollens in Chang 8 Section and Chang 7 Section.

Asseretospora-Walchiites assemblage lamospora , A piculatisporites and Laev igatosporite This assem blage includes 79 g enera and 105 spe- also o ften occur. The other low abundant and co m- cie s. Its main characteristics are hig h abundance and mo n spo res in the M esozoic are Concav isporites , diversity of spo res and pollens. Gy mnosperm pollen Cy athidites, Todisporites , Dictyophy lid ites , Loph- has the predominant abundance at 53. 3 %- 84. 1 %; otriletes , Baculatisporites, Neoraistrick ia , Ly copo- ferny spore is also abundant , at 15. 9 %- 44. 7 %. diacid ites, Poly cingulatisporites and Marattis- Asseretospora is the mo st highly abundant in porites. ferny spores , at 5. 2 %- 20. 2 %;and Aratrisporite is The abundance and diversity o f Gy mnosperm also abundant , w ith 4. 0 % - 15. 8 %. Compared to pollens are higher than those of the assemblag e in the assem blage in Chang 8 Section , the assem blage in Chang 8 Sectio n , and the spores of Saccites are mo re Chang 7 Sectio n is characterized by hig h Duplexis- obvious. A mong them , Abietineaepollenites and P i- po rites abundance , how ever Asseretospora , Crassi- nuspollenites have the highe st abundance , at 8. 4 %- tudisporites and Duple xisporites also o ccur. P unc- 34. 0 %. Walchiites has a content of 4. 0 %- 22. 2 %. tatisporites , Osmundacidites , Converrucosisporites P iceaepollenites , Protopicea and Alisporite also and Verrucosisporites also have a little hig h abun- have hig h contents at 3. 8 % - 14. 8 %, 3. 0 % - dance , ho wever their abundance is obviously low er 17. 9 % and 4. 7 %- 15. 6 %, respectively. The hig her than that of the assem blage in Chang 8 section , and abundant spores o f Saccites are Podocarpidites , their contents are 2. 6 % - 12. 6 %, 2. 4 % - 5. 6 %, P seudowalchia , Taeniaesporites , Paleoconi f erus 1. 0 %- 3. 0 % and 1. 1 % - 6. 9 %, respectively . Ca- and P rotoconi ferus. Their contents are higher than 214 Ji Liming and M eng F anw ei the assem blage in Chang 8 Sectio n , at diffe rence is that Aratrisporites has higher abundance 2. 0 %- 3. 8 %, 1. 1 %- 5. 4 %, 1. 9 %- 7. 1 %, 1. 6 % than Punctatisporites , and is the dominating kind in - 3. 2 % and 1. 8 % - 6. 0 % respectively. The con- our assemblag e. The spo res o f Marattisporites still tents o f com mon Piceites , Cay tonipollenites and Cy- spo radically occur. Osmundacid ites in trilete spores cadopites are similar to tho se of the assem blage in also has hig h abundance. The gym no spe rm po llens in Chang 8 Sectio n. The low abundant M esozoic pollens our assem blages are subordinate , as in Q u's assem- include P rotopinus , Dacrycarpites , P laty saccus, blag e (1980). Picea type pollens of disaccites are Lueckisporites, and Araucariacites. The archaic comm on kinds , including P iceaepollenites , P rotopi- ty pes such as Cordaitena and Edosporites occur in cea and P iceites. There a re also costate Chordas- the assemblage by chance. porites and Lueckisporites , and o ccasional ascon Cy c- adopites in our assem blage. The m ain difference be- Stratigraphy Correlation tween our assemblage and that o f Q u is that Abietin- Qu (1980) previo usly did sporopollen studies of eaepollenites , Alisporites and Walchiites o f disaccites the T riassic outcro p in the To ngchuan-H ancheng area have rather hig her abundance. of the Ordos basin. The sporopollen assem blages of The assemblage of Yanchang Formation estab- Chang 8 Section and Chang 7 Sectio n are very similar lished by Qu (1980) has a little predom inance of fern to the assem blages of Tongchuan Form ation and spo res. The spo res o f Apiculatisporites have the Yanchang Form ation established by Qu (1980), and highest abundance , follow ed by the spores o f Punc- the characteristics of the assem blage s can be com- tatisporites. The hig he r abundance spo res are Ca- pared. lamospora , Verrucosisporites , D ictyophy llidites and Fern spores are predo minant in the To ng chuan Osm undacidites. The co mmo n spo res include Granu- Fo rmatio n assemblag e. Trilete spores of Punctatis- lasporites , Cy clogranisporites, Conv errucosis- porites have the hig hest abundance , fo llow ed by Ver- porites , Lophotriletes , Acanthotriletes , Neoraist- rucosisporites. The other fern spores that have high rickia , Baculatisporites , and D up lex isporites. The abundance include Lophotriletes , Apiculatisporites m ain m ono lete spo res are Marattisporites , and other and Calamospora. The re are a few fe rn spores of m ono lete spo res are Monolites and Aratrisporites , Granulasporites and Duple xisporites, and sporadic w ith spo radic Torispora. The percentage of gy mno- fern spores o f K luk isporites. The re are a few fern sperm pollens is a little low er than the percentag e o f spores of Marattisporites , Monolites , Aratrisporites fern spo res in the assemblage. P iceaepollenites and in m onoletes. The g ym no sperm po llens have the sec- P rotopicea of Picea ty pe in disaccites a re the main o nd predom inance in the assemblage o f To ng chuan pollens. Podocarpidites and ascon Cy cadopites also Fo rmatio n , dominated by Disaccates w ithout costate. have high abundance. Pinuspollenites , Cay toniale- The high abundant pollens include Piceaepollenites, spollenites, P iceites , Pseudopinus have low abun- Protopicea and P iceites. The comm on pollens are dance , and co stat disaccites also has lo w abundance , Pseudop inus and P inuspollenites. The costate po l- w ith comm on kinds of Chordasporites , Taeniaes- lens are Chordasporites, Striatites and Lueckis- porites and Lueck isporites. porites , and ascon of Cy cadopites also occur fre- The percentag e of fern spo res in Asseretospora- quently in the assem blage. Walchiites assemblag e from Chang 7 Section in east- The assemblag e in Chang 8 Sectio n in eastern ern Gansu Province is smaller than that of the assem- Gansu Province contains g reater abundance and di- blag e from Chang 8 Sectio n. Among the fern spo res , ve rsification than the assemblages of Tongchuan Fo r- Ap iculatisporites and P unctatisporites of trilete m ation established by Qu (1980). The Aratisporites- spo res have a hig h percentage. Calamospora , Verru- Punctatisporites assemblage in Chang 8 Section in cosisporites , Dictyophy llidites and Osmund acidites this paper also has the fern spores predominance. are also highly abundant. Converrucosisporites , Lo- A piculatisporites and Calamospora of trilete spo res photriletes , Neoraistrickia , Baculatisporites and also have high abundance. There are seve ral com mon Dup lex isporites often occurred , and Marattisporites spores of Lophotriletes, Apiculatisporites and Ca- and Torispora of m ono lete spores occurred occasio n- lamospora. The differences are that our assemblage ally. The differences are that Asseretospora of du- has m uch highe r abundance of m ono lete spo re and plex isporites in the assem blage fro m Chang 7 Section low er abundance of trilete spores. The o bvio us is the m ost abundant , how ever the percentage o f Paly nology of Yanchang F ormation of M iddle and La te T ria ssic in Eastern Gansu and Its Paleo clima tic Significance 215 monolete spo re Aratrisporites is low er , and Laev i- gy mnosperm pollens mainly consist of Alisporites , gatosporites also often occurred. The gy mnosperm Chordasporites, and o ther pollens of disaccites. pollens also hav e the predominance in this assem- The Duple xisporites-Chordasporites subzo ne is blage , and Piceaepollenites and Protopicea of Picea characterized by a high predominance of gy mnosperm ty pe in disaccites have hig h abundance. Podocarpid- pollens. The obvious characteristic is hig h abundance ites , Cycadopites , Pinuspollenites , Cay tonialespol- of D uplex isporites. The o ther co mmo n trilete spores lenites and P iceites are also comm on. The o bvio us in the Meso zoic , especially armillate ty pes , are o ften difference is that Abietineaepollenites , P inuspolleni- found in the assem blage. Some armillate spo res can tes , Walchiites and Alisporites of disaccites are the also be found in the Asseretospora-Walchiites assem- most abundant , and Paleoconi f erus and Protoco- blag e in eastern Gansu Province. T hey have a low ni f erus become co mmo n po llens in the assemblage percentage but high diversificatio n , including Den- fro m Chang 7 Section. soisporites , Poly cingulatisporites , S tenozonotri- The Xifeng area in eastern Gansu Province is lo- letes , P atellisporites , and Cingulatisporites. Assere- cated in southw estern Ordo s basin , and the area w as tospora (probably a synony my of D uplex isporites) o f in a deep part o f the lake during the Middle and Late duplexispo rites occurs in high abundance. The disac- T riassic. H ow ever , the To ng chuan-H ancheng area cites pollens in bo th assem blages are very similar and studied by Qu (1980) w as in a shallow slo pe area of both of them have P iceites , Alisporites , P rotoco- the lake , developing delta and plains during the Mid- ni ferus , Abietineaepollenites , Podocarpidites and dle and Late T riassic. The differences betw een bo th Cy cadopites of mo no sulcate. areas are induced by the different paleogeog raphies Although the assem blages in easte rn Gansu and sedim entary environments. H ow ev er the assem- Province in our w o rk a re limited in stratig raphy , blages from the tw o areas all reflected tw o phases of their facies and bio logical stratig raphies are very the Middle Triassic and Late Triassic , and probably clear , providing proof of g eological time. had the same geological times. The Aratisporites- P unctatisporites assem blage sho uld be Ladinian of Paleoclimate La te M iddle Triassic , and the Asseretospora- A sporopollen assemblage can offer impor tant Walchiites assemblage sho uld be Carnian of early info rmatio n on the features of a plant comm unity . Its La te Triassic. g row th , development , flourishing and decline have a Three pollen assem blage s have been established close relation to the environment and clim ate , so we in the Yanchang Fo rmatio n of the so uthern Ordos ba- can deduce the paleovegetatio n and paleoclim ate o f sin (Zhang et al. , 1982). The Yan 1 to Yan 2 sec- the tim e by researching the paly noflo ra. Some M eso- tions containing the Todisporites-Lophozonotriletes zoic veg etation ty pes still exist now aday s ;their pro- assemblage are com pared to the Chang 10 Section to genito r can be deduced from the modern parent Chang 8 Section. The Yan 3 Sectio n co ntaining the plants. M eanw hile , quite an am ount of plant species D uple xisporites-Chordasporites subzone o f armillate have become ex tinct , but their so urce plants and tax- assemblage is com pared w ith Chang 7 Section to onomic positions can be identified w ith in-situ spores Chang 4 +5 sections. Although these assem blages (Van Ko nijnenburg-van Cittert , 1989 , 1981 ;Po t- are rough , their characte ristics and development can onie , 1962). With the developm ent of studies o f in- also be com pared with o ur assem blages. situ spo res , our kno w ledge on the bo tanical attribute The Todisporites-Lophozonotriletes assemblage of M esozoic dispersed spores is continuously impro- is characterized by highly predominant fern spores , ving (Balm e , 1995 ;Van Konijnenburg-van Citter t , with Todisporites mo st hig hly abundant. Lophozo- 1993 ;Litw in , 1985). notriletes also has high abundance , m ainly co nsisting Acco rding to the know n genetic relatio nships o f of Cycloverrutriletes and Verrucosisporites. The oth- dispersed spo ropo llen , this paper has identified the er trilete spo res occurring in the Aratisporites- taxo nom ic po sitio n of the main mem bers o f the M id- P unctatisporites assemblage can be found easily in dle and Late Triassic palynoflora in eastern Gansu the assemblage in eastern Gansu Province. Mo no le te Province , w hich establishes a basis for reco nstructing spores are scarce , but Aratrisporites has dominance the features of the plant com munity and developing in the assemblage in eastern Gansu Province. The paleoclimatic research. Figure 5 show s the contents and stratig raphic distributions of the sporopollen classes w ith know n parent plants from Yanchang 216 Ji Liming and M eng F anw ei

Figure 5. Distributions of main sporopollen families in Yanchang Formation.

Formation in eastern Gansu Province. M odern Ptero psida are w idely distributed aro und the w orld and are the richest in the tropics and sub- Plant ecology tropics. M ost of them are the main inhabitants under Modern Coniferales comm only like w arm and w arm and moist forests or marshlands (Botany Insti- coo l , and are suited to a middle humid enviro nment. tute of Chinese Academy of Sciences , 1976). M ost o f They usually g row in low-altitude uplands , and are ex tant Osmundaceae and Selaginellaceae are terrestri- the main co mpo nents of a tem perate zone fo rest. al m oderate-scale ferns distributed in the temperate , Pinacea m ostly are evergreen arbor , rarely tropic and subtropic zones , w hile some are herba- shrubbery , and are distributed widely in the no rthern ceous elements g ro wing in dam p ma rshland or along temperate zone (Wang et al. , 1995). Picea of Pina- rivers under forests. Bo th Lycopodiaceae and Fontin- cea are also everg reen arbo r. Mo dern Picea m ostly alaceae prefer a humid clim ate and acid soil and al- grow in the northern hem isphere , and are distributed w ay s grow in low-lying and moist ground. Cya- in high-altitude uplands of northeastern , so uthwest- theaceae a re mo stly w oods and the only ex tant v ariety ern , nor thw este rn and Taiw an Pro vince in China. favors the transitio nal enviro nment from humid to Podoca rpaceae are everg reen arbor and shrubbery , m oderate moist zones , and living Cyatheaceae are and today g row m ostly in the we t reg ion of the found along rive rs and lakes and in low-lying areas. southern hemisphe re. Podocarpus is mainly distribu- Ex tant Equisetaceae with the sole Equisetum are ted in So uth China and Taiw an Pro vince. Cycad are w idely fo und in frig id , tem perate and tropical zones. classical tropical and subtropical evergreen plants , Dipte ridaceae ferns are distributed in subtro pical w hich g row in rainy areas. Araucariaceae are also zo nes. As for the m odern Marattiales , they w idely mainly distributed in tropic and subtropical areas. inhabited glo bal tropical and subtro pical forests , in Classpopollis are extinct dry-resistant plants , w ere w hich Angiopteris are large-scale terrestrial ferns. eve rg reen arbo rs , and mainly g rew in ho t and dry Cheiro pleuriaceae g row o n the lim estone under tro pi- surrounding s on highland slopes. cal and subtropical forests. Paly nology of Yanchang F ormation of M iddle and La te T ria ssic in Eastern Gansu and Its Paleo clima tic Significance 217

Paleoclimate of palynoflora da and the spores of Cheiropleuriaceae and Cyatheace- According to the know n affinities , mo st spo res ae that belo ng to Filicopsida. The uncertain spores o f discovered from the M iddle and Upper T riassic in Filicopsida also have the highest abundance. The pol- eastern Gansu Province could be assigned to Filicales lens of Pinacea and Pte ridospexm opsida have domi- belonging to Lepo to sporangiatae and Pteropsida. nant abundance in the gy mnosperm pollens of the as- Co mmo n Osmundacidites , Todisporites and Cy- semblages , and the pollens o f Cy cado psida and Cor- clogranisporites belo ng to Osmundaceae , Concav is- daiopsida have the second highest abundance. The porites belongs to Cheiropleuriaceae , Cy athid ites be- pollens o f Podo carpaceae and Cay to niaceae that be- long s to Cy atheaceae , Leiotriletes belo ng s to Schizae- long to Conifero psida o ccur in low abundance. The aceae , and D ictyophy llidites and Converrucosis- uncertain po llens of Coniferopsida also have ce rtain a- porites com monly are tho ug ht to be included in Dipte- bundance. The Asseretospora-Walchiites assem blage ridaceae. Marattisporites , A piculatisporites and Ac- in the early Late Triassic has dominant gy mnosperm anthotrilees belo ng to Marattio psis o f Euspo rangiate pollens. H ow eve r , both the kinds of pollens and the in Filicales. Other spores w ith uncertain biological kinds of spores are very similar to the assem blage relations that belong to Filicales mainly include from low er beds. The main differences between them Punctatisporites, Verrucosisporites , Asseretospora , are the abundance changes of spores and pollens. D uple xisporites and Laev igatosporites. Ly co podiales The assemblage in Chang 8 Section is character- have the second hig hest abundance , and Aratris- ized by especially abundant spores o f Lycopsida. Dur- porites among them belongs to Pleuromeiaceae , Ca- ing its deposition Aratisporites of Pleuromeiaceae is marozonosporites and Ly copod iumsporites belong to the representative , and the spores of O smundaceae Ly co podiaceae , Densoisporites and Neoraistrick ia and other Filicopsida also have rather high abun- belong to Selaginellaceae. The spo res of Spheno psida dance. A lthoug h the above fe rn spo res also have hig h only have Calamospora of Equisetaceae. abundance in the assemblage of Chang 7 Section , The M iddle and Late Triassic gy mnosperm po l- their propo rtio ns have o bvio usly low ered. H ow ever , lens in the area mainly are Coniferopsida and Pterido- the proportions o f pollens of Pinacea , Coniferales , spex mopsida. Podocarpidites and P laty saccus be- Pterido spexmo psida and the Walchiites w ith uncer- long to Podocarpaceae , Abietineaepollenites , Pinus- tain relation hav e obvious accretio ns. We think that pollenites , P iceaepollenites and Piceites belong to the chang es from Chang 8 Section to Chang 7 Sectio n Pinacea. There are a few Cay toniapollenites of Cay- are induced by lake level fluctuations. During the to niaceae , Araucariacites and Callialasporites be- deposition o f Chang 8 Section , the area w as adjacent long to Araucariaceae. Other Co nifero psida pollens to the shore of the lake , and the large scale w et low- with uncertain relations mainly include Palaeoco- lands and sw amps are suited to the flourishing o f ni f erus , Protopicea and P rotopinus. The Pterido- ferns , preserving abundant spores. H ow ever , during spex mopsida pollens include Taeniaesporites , Chor- the depo sition of Chang 7 Section , lake transg ressio n dasporites and A lisporites. The secondary pollens in submerged the area entirely , beco ming a deep-w ater the area are Cycadopsida including Cy cadopites that lake. A s a result , the preserved spo res in the area belong s to cycad o r gingkgo. There are also Cordait- cam e mainly from a terrene area. The face o f the as- ena of Cordaio psida. The Middle and Late Triassic semblage has been so rted out because saccate pollens paly noflo ra in the area consist of these kinds of are more easily transported to the middle lake , and spores and pollens. Their percentages and strata dis- the saccate po llens have obvious predominance in the tributions are show n in Fig. 5. assemblag es. This pheno menon has been observed in Fern spores have o bvious dominance in the m any studies (Huang et al. , 2003 ;Gong and Bi , Aratisporites-P unctatisporites assem blag e of the late 2001). Middle Triassic in the area. Am ong them , the spo res The main generes of the Middle and Late Trias- of Pleuromeiaceae and Osmundaceae have the highest sic paly noflo ras in eastern Gansu Province are tro pi- abundance. The spores of Marattiaceae and Dipteri- cal and subtropical everg reen , based o n the current daceae that belong to Filicales and the spores of Equi- environment of their po ssible parent plants. Ex cept- setaceae that belong to Sphenopsida have the seco nd ing Pinacea , the o ther parent plants are mo stly from hig hest abundance. T here are a few spores of Selag- tropical and subtropical zo nes. The dominant floras inellaceae and Lycopo diaceae that belong to Lycopsi- are plants that like w etness and w arm th , and second- 218 Ji Liming and M eng F anw ei ly plants that like we tne ss and hea t , indicating a semblages are devoid o f Classopollis and E phaedrip- w arm and w et climate. Ferns that like we tness are ites that are commo n in the M iddle and Late T riassic pro spe ro us in the area , and are dominated by Pleu- and show a dry environment. Therefo re w e conclude romeiaceae of Lycopsida and Osmundaceae of Filicop- that eastern Gansu Pro vince had prosperous plants in sida , w hich mainly are distributed in tropical and the Middle and Late Triassic , and the palynofloras subtro pical we t areas. There are also spo res of Cy a- indicate the paleoclimate w as w arm and rainy in a theaceae , Dipteridaceae and Selaginellaceae , w hich temperate to a subtro pical zone. indicate w et and mildly we t surrounding s. The as-

Figure 6. Differentiation degree curves of sporopollen fossils.

The No rth China plate , including the Ordos ba- mate conditions maybe w ere the dete rm inant o f the sin , w as near the paleoequato r and lay in the tropic- w et-hot clim ate pattern in the Middle and Late Trias- subtro pics in the nor thern hemisphere in the T rias- sic in eastern Gansu Province. sic. After the maxim um reg ression during the fo rma- Fig ure 6 is the differentiation degree curves o f tion o f Pangea in the Late , an ex tensive and spo ropo llen fossils from Chang 8 Section to Chang 7 frequent transg ressio n period came in the M iddle and Section. Both sim ple differentiation and com plex dif- La te Triassic (G olonka and Fo rd , 2000). The fo rma- fe rentiation deg rees have ra ther high v alues. T hey tion and evolution of the Ordo s basin had ex perienced imply the time was chronically w arm and w et and a mig ration from a low er latitude to a higher latitude. condign to overg row th of many plants , and there are The basin lay in a low er-latitude area near the paleo- no obvious clim ate fluctuations and vege tatio n alter- equato r in the -Permian , and it had nations in the tim e. So the relative proportion o f continuously moved northw ards. Based on the rela- spo ropo llen canno t be ex plained by climate changes tional survey and data , the paleolatitude values of the and veg etation alteration , and can only be explained O rdos basin w ere 18. 3°, 21. 4°and 25. 4°respectively by lake transg ressio n. in the Early , Middle and Late Triassic (Zhu e t al. , The curve of simple diffe rentiation degree o f pol- 1998). The refore , the paleo geog raphy and paleocli- lens is rather smoo th , indicating that the vege tatio n Paly nology of Yanchang F ormation of M iddle and La te T ria ssic in Eastern Gansu and Its Paleo clima tic Significance 219 dominated by Coniferales is rather steady. This phe- F eatures of Rese rvoir Fo rma tion and Ex ploratio n Strateg y nomenon implies that the mountains near the lake o f Xifeng O ilfield. Acta Petrolei S inica , 25(2):24 - 29 had rathe r hig h altitudes to av oid the influence of cli- (in Chinese with Eng lish A bstract) G olo nka , J. , F o rd, D. , 2000. Pangean (Late Ca rbonifero us- m ate and season , and also im plies that there are no M iddle Jurassic) Paleoenvironment a nd Lithofacies. large-scale clim ate events betw een the la te Middle Palaeogeography , Palaeoclimatology , Palaeoecology , T riassic and early Late T riassic in the area. The low 161 :1 - 34 and sm ooth curve of predominance degree also show s G ong , S. L . , Bi, L. G . , 2001. Palyno log ical Sedimentatio n persistently suitable conditio ns and profuse vegeta- and Its Relationship with Neo gene Sedimenta ry Env iron- tion. ment in PL 19-3 A rea. China O f f shore Oi l and Gas (Ge- Palynoflora and ecological research also indicates ology), 15(6):388 - 392 (in Chine se with English A b- that there are no o bvio us changes of vege tatio n and stract) climate in Chang 8 Section to Chang 7 Sectio n (late H ua ng , K . N . , Zhan , J. Z . , Z ou , Y . S. , e t al. , 2003. Sedi- mentary Env ironments and P alaeoclimate of the T ria ssic Middle Triassic-early Late Triassic), and implies a and Jurassic in K uqa Riv er A rea , Xinjiang. J ournal of long-term w et and w arm climate fro m Chang 8 Sec- Palaeogeography , 5(2):197 - 208 (in Chinese w ith tion to Chang 7 Sectio n. This co nclusion is also in ac- English A bstract) cord w ith the lake transg ressio n in Chang 8 Section Institute of G eolo gy , Chinese A cademy of Geolog ical Sciences , and w ith the highest lake area in Chang 7 Section. 1980. M eso zo ic S tratig raphy and Paleontolog y of Shaa n- The lo ng-term wet and w arm climate and bro ad deep G an-N ing Basin. V ol. 1. Geo log ical Publishing House , lake provided the essential conditions for fresh algae Beijing . 2 - 29 (in Chinese) to flourish during that time. T he hig h pro ductivity of L itw in , R. J. , 1985. Fertile O rg ans and Insitu Spo res of Ferns f rom the L ate T riassic Chinle Fo rmation of A rizona and algae provided a g ood biolog ical source fo r the high N ew M exico , with Discussio n o f the A sso ciated Dispe rs- quality so urce rocks o f Chang 7 Sectio n in eastern ed Spo res. Rev. Palaeobot. Paly nol. , 44:101 - 146 Gansu Province. P otonie , R. , 1962. Sy nopsisder Spo rae Insitu. Beih Geol. J b, 52:1 - 204 ACKNOWLEDGMENTS Q u , L. F . , 1980. T ria ssic Spore s and Pollen. In:M esozoic The author s are g rateful to Academician Zhang St ratig raphy and P alaeo nto log y of the Shaan-Gan-Ning Benren and Professo r Yin Leiming for their co nstruc- Basin. Vo l. 1. G eolog ical P ublishing H ouse , Beijing . tive comm ents and to Advanced Engineer Li Jianfeng 115 - 143 (in Chinese) fo r his assistance in collecting sample s. This w ork is V an Ko nijnenburg-van Citte rt , J. H . A . , 1981. O smunda- ceous Spo res Insitu fro m the M iddle Jurassic o f Y ork financially suppo rted by a g rant from “ Prominent Shire , Eng land. Rev. Palaeobot. Paly nol. , 33:169 - Young ster in Western China” Science Fo undation of 181 Chine se A cademy of Sciences in 2002 (No. CAS2002- V an K o nijne nburg-v an Cittert , J. H. A . , 1989. Dickso nia- 404-01) and the item “M esozoic Oil Source Correla- ceous Spo res Insitu f rom the Jurassic o f Yo rk Shire , Eng- tion and Resource Evaluation in Xifeng Oil Field , land. Rev. Palaeobot. Paly nol. , 61 :273 - 301 Easte rn Gansu Province” of Gansu Province Young V an Ko nijnenburg-van Cittert, J. H. A . , 1993. A Review of and Middle Scientists Science Foundation in 2003. M a toni-A ceaebasedo n Insitu Spo res. Rev . Palaeobot. Palynol. , 78 :235 - 267 REFERENCES CITED W ang , F . X. , Qian, N. F. , Z hang , Y . L. , et al. , 1995. Balme , B. E. , 1995. Fo ssil in situ Spores and Po llen G rains: P ollen F lo ra of China. Second Editio n. Science P ress , A n A nno tated Catalo gue. Rev. Pa laeobot. Paly nol. , Beijing . 164 (in Chinese) 87 :81 - 323 Zhang , Z . H . , F u, Z . Y . , Luo , K . Q . , e t al. , 1982. M e so- zoic Spo ropollen A ssemblag es f rom the Southern Shaa n- Bo ta ny Institute o f Chinese A cademy of Sciences , 1976. Spo re P te ridophy to rum Sinicorum. Science P ress, Beijing. 451 G an-N ing Basin. In:Selected Wo rks o f the F irst A ca- (in Chinese) demic M eeting of Sporo po llen So ciety o f China. Science Edito rial Committee o f Petroleum Geo log y o f Changqing O il P ress , Beijing . 116 - 124 (in Chinese) Fie ld , 1992. Petroleum G eo lo gy of China (V ol. 12), Zhu , R. X. , Yang , Z. Y. , Wu , H . N . , e t al. , 1998. P aleo- mag netic Co nstraints o n the T ectonic Histo ry of the M a- Changqing O il Fie ld. P etro leum Industry P ress , Beijing. 124 - 145 (in Chine se) jo r Blocks of China during the Phanero zoic. Science in China (S eries D), 28(S1):1 - 16 (in Chinese) Fu , J. H. , L uo , A . X. , Y u, J. , et al. , 2004. Geo log ical 220 Ji Liming and M eng F anw ei