Ordovician Cephalopod Biofacies of South China and Its Provincialization

Home , Michelinoceras

J ournal of China University o f Geosciences , Vol. 17 , No. 3 , p. 221 - 230 , September 2006 ISS N 1002 - 0705 Printed in China

Xiao Chuantao*(肖传桃) Faculty o f Earth Sciences , China University o f Geosciences , Wuhan 430074 , China ; Geosciences College , Y angtze University , J ingz hou 434023 , China Chen Zhiyong (陈志勇) Faculty o f Earth Sciences , China University o f Geosciences , Wuhan 430074 , China ; Qinghai Oil f ield Com pany , Dunhuang 736202 , China Liu Li (刘莉) He Youbin (何幼斌) Geosciences College , Y angtze University , J ingz hou 434023 , China

ABSTRACT :The research on biofacies and its provincialization is of important significance not only for the increasing of precision of stratigraphic subdivision and correlation in South China, the reconstruction of ancient environment and paleogeography and even the guiding of oil and gas exploration, but also for the study of paleobiogeogrphy and sea level changes of southern China in Ordovician. On the basis of the studies of the ecological characteristics of Ordovician cephalopods from South China , eighteen cephalopod biofacies are recognized and described :(1)Open platform Proterocameroceras biofacies ;(2) Restricted platform Pseudoectenolites-Xiadongoceras biofacies; ( 3 ) Open platform Retroclitendoceras- Pararetroclitendoceras biofacies;(4)Open platform Prona jaceras-Mamagouceras biofacies;(5) Shelf slope-basin Cyclostomiceras biofacies;(6) Open platform Cameroceras-Cyrtovaginoceras biofacies ; (7) Open platform Coreanoceras-Manchuroceras biofacies;(8) Shelf slope-basin Kaipingoceras- K yminoceras biofacies;(9) Inner shelf Bathmoceras-Protoc ycloceras biofacies;(10) Middle shelf Dideroceras-Ancistroceras biofacies;(11) Deep-water shelf Lituites-Cyclolituites biofacies;(12) Stag- nant basin Lituites-Trilacinoceras biofacies;(13) Deep-water basin Paraendoceras-Sactorthoceras biofacies;(14) Deep-water shelf Sinoceras-Michelinoceras-Disoceras biofacies;(15) Deep-water shelf Be loitoceras-Jiangshanoceras biofacies;(16) Deep-water shel-f basin Eurasiaticoceras biofacies ;(17) Shelf-slope Jiangxiceras-Yushanoceras biofacies;(18) Deep-water basin Michelinoceras biofacies. The cephalopods of these biofacies, their ecological characteristics, and living conditions are elucidated in this article. The association law of cephalopod biofacies in time and space shows that there were three cephalopod biofacies provinces in South China during the Ordovician, i. e. , Yangtze biofacies province , East Guizhou (贵州)-West Hunan (湖南) biofacies province (mixed-type biofacies province), and Central Hunan-West Zhejiang(浙江) biofacies province. It is suggested that differentiation of cephalopod biofacies was mainly controlled by sea level changes and tectonic evolution. The differentiation is obvious during lower sea level and not developed during high sea level. KEY WORDS:cephalopod biofacies, cephalopod biofacies province , sea level changes, Ordovician , South China.

INTRODUCTION Bio facies refers to an association of org anism- This paper is suppo rted by the National Natural Science Foundation of eco logy and sedim entary characteristics , w hich can C hina (No. 40272060). reflect a specific ancient environment. Research o n *C orresp on ding author :ctxiao188 @yahoo. com. cn biofacies and its provincialization is significant no t o nly for increasing the precisio n of stratigraphic sub- M anu scrip t received March 10 , 2006. M anu scrip t accepted June 25 , 2006. divisio n and co rrelation , reconstructing ancient envi- 222 Xiao Chuantao , Chen Z hiyo ng , Liu L i and He Yo ubin ronments and paleo geog raphy , and oil and gas ex plo- ance to fo rw ard mov em ent , and a small siphuncle and ration , but also fo r the study of interco ntinental pa- large chambe r are favorable to reg ulate the fluids leobiogeog raphy and sea level chang es. w ithin them . In contrast , cephalo pods w ith a hig h Altho ug h the term biofacies w as established expansion rate , large siphuncle , and sediments w ith- mo re than 30 y ears ago by Bouco t (1981) and Schafer in chambers are po or sw immers and suitable for a (1972), detailed studies have only beg un to em erge shallo w-w ater enviro nment (Fig. 1). in the last few years. There is little research on the paleoecolo gy o f O rdovician cephalopo ds (Zhou , CEPHALOPODS BIOFACIES 2000 ;Xiao and Pan , 1997 ;Xu and Xu , 1986 ). Open Platform Proterocameroceras Biofacies Therefo re , m ore studies on biofacies a re required. It is mainly found in the Xinchang S tage (China This ar ticle presents new research concerning the O r- Stratig raphic Group , 2002) in the Upper Yang tze dovician cephalopo d biofacies o f southern China. area , this biofacies is low in diversity , dominated by O n the basis of the studies on the paleoecological P roterocameroceras and associated with Camero- characteristics o f Ordovician cephalopods from parts ceras, Dayongoceras , and Ellesmeroceras. Cephalo- of South China and the systematic co llectio n and pods of this biofacies a re characterized by a medium- analysis of O rdovician cephalo pods fro m m ore than sized shell , hig h expansion ra te and large siphuncle , ten provinces o f So uth China (Xiao and Pan , 1997 ; therefore , they are no t able to sw im rapidly . Addi- Bureau of Geolog y and M ine ral Resources of H unan tionally , they are mo stly preserved in bioclastic lime- Pro vince , 1997 ;Bureau o f Geo logy and Mineral Re- stone and intraclastic limestone , indicativ e of an sources o f Zhejiang Province , 1997 ;Bureau of Geolo- open-platform enviro nment. g y and Mineral Resources of Jiangxi Province , 1997 ; Bureau of Geolog y and M ineral Resources of Guizhou Restricted Platform Pseudoectenolites-Xiadongoceras Pro vince , 1997 ;Committee o f Chinese Stra tig raphic Biofacies Dictionary , 1996 ;Wang et al. , 1987), the authors This biofacies occurs mainly at the low er par t of have presented the paleoeco logical characteristics the Xinchang Stage (Xiling xia Form ation ) in the pro vided by the da ta and their relationships with sed- Yichang , Songzi , and Tong shan areas of H ubei imentary facies. As a result , 18 cephalopod biofacies Province. It has a high dive rsity of elements do mina- and three cephalopo d biofacies provinces have been ted by Pseudoectenolites and characterized by X ia- recog nized. dongoceras. The co mpo nent members include Changjiangoceras , Dakeoceras , Paraeburoceras , ECOLOGY OF CEPHALOPODS Neoeburoceras, Zhangnanoceras , Anguloceras , Cephalopods are m arine zoo phagous inverte- Clarkoceras , P araconoceras , E llesmeroceroides , and brates , mo st of w hich are nekto nic , w ith so me P arahuaiheceras apart fro m Pseudoectenolites and plankto nic and benthonic org anism s. The lifesty le of X iadongoceras. T hey hav e comm on characteristics , cephalopods depends largely on their shapes and namely , a small-to-medium shell , high expansio n structures. Of the planospiral cephalopods , tho se rate , and a siphuncle lo cated near to the par abdomi- with tig ht spiral , sharp abdo minalis , flattened and nalis , w hich indicates that cephalopods of this bio fa- streamlined shell , sm ooth shell , and complicated su- cies are unable to get used to rapid sw im ming . In ad- ture m ostly adjust to rapid sw imming in deep-w ate r. dition , they are m ostly preserved in dolo rudite. A s a In co ntrast , cephalopods with loose spiral , ventri- result , this biofacies represents a restricted shallow- cous shell , w ell-developed o rnamentation , and simple w ate r enviro nment. suture are unable to become accusto med to rapid sw imming and deep-w ater life , therefore , mo st of Open Platform Retyoclitendoceras-Pararetroclitendo- them are benthonic or saltant sw im in shallow-w ate r. ceras Biofacies H ow ev er , or thomo rphic cephalopo ds are situated dif- This bio facies is mo stly distributed in the m iddle ferently. Cephalopo ds w ith a lo ng shell , low ex pan- part of the Xinchang S tage (N anjing guan Fo rm ation) sion rate , small siphuncle , and no sediments w ithin in East Sichuan , Yichang , Songzi , and Tong shan ar- chambe rs are g ood swim mers and suit a deep-w ater eas of H ubei Province. It is com posed of Retrocliten- enviro nment , because low expansio n rate and no sed- doceras , Pararetroclitendoceras, Sanxiaoceras , Pro- iments in chambers are favo rable fo r reducing resist- terocameroceras , and Escharendoceras. M ost of the O rdo vician Cephalo pod Bio facies o f South China and Its P rov incializa tion 223 genera are w idespread and found in many lo calities a- expansion rate , w hich show s that the cephalopods o f round the w orld. The cephalopods of this biofacies this biofacies are able to become accustomed to mo re are characte rized by a dorsiventrally co mpressed si- rapid sw im ming . M oreover , they are mo stly phuncle and abdominal , curved do rsal part , and low preserved in thick calcsparite intraclastic limesto ne.

Figure 1. Environmental distribution of important Ordovician cephalopods in South China. 224 Xiao Chuantao , Chen Z hiyo ng , Liu L i and He Yo ubin

Therefo re , this biofacies is indicative of fo rd environ- features indicate that cephalopo ds of this biofacies are m ent , open platfo rm , favo rable to reserving oil and suitable for shallow w ate r and bentho nic sw imming gas. and represent an open platform environment.

Open Platform Pronajaceras-Mamagouceras Biofacies Open Platform Coreanoceras-Manchuroceras Biofacies Developed chiefly in the upper par t of the Xin- This bio facies is m ainly distributed in the chang S tage (Fenxiang Formation) in the Middle Daobaow an Stag e of East Sichuan , Yichang and Yang tze a rea such as East Sichuan and the Yichang , Tongshan areas o f H ubei , and the Lo we r Yang tze S ong zi , and To ng shan areas of H ubei Province , this district. Co mpo nent membe rs of the biofacies include biofacies is com po sed of Pronajaceras , Mama- Coreanoceras , Manchuroceras , Clitendoceras , P ara- gouceras, Proterocameroceras , Recurvoceroides, clitendoceras , Cassinoceroides , Oderoceras , Chao- Hopeioceras, a few Coreanoceras, etc. . Cephalopods huceras , H em icircoceras , P seudopliendoceras , D id- of this biofacies have commo n characteristics , name- y moceras , Reniceras , Daobaowanceras , Belemnoc- ly , a straight siphuncle , round to ov al cross section , eras , Teratoceroides , and H opeioceras. This bio fa- and a w ide ring (Wang et al. , 1987), w hich are dif- cies has a hig h diversity and the cephalo pods are ferent from that of Retyoclitendoceras-Pararetrocli- characterized by a bulky siphuncle near the abdomi- tendoceras biofacies. Mo st cephalopo ds o f this biofa- nalis , a small chambe r , and a high ex pansion rate , cie s are obvious in thick calcsparite o olitic limesto ne indicating that they are unable to swim quickly and and biohermal limestone , imply ing an open platform live in a deep-w ater enviro nment. They often occur in and ford environment favo rable to reserving oil and biohe rm al limesto ne (Xiao e t al. , 1993 ;Zhu , 1993) gas. and thick-bedded biolithite , show ing the shallow- w ate r enviro nment of open platform . Shelf Slope-Basin Cyclostomiceras Biofacies This bio facies occurs mainly in the Xinchang Shelf Slope-Basin Kaipingoceras-Kyminoceras Biofa- S tage of M ajiang in East Guizhou , and in Yuanling cies and Cili of West H unan and Central H unan to West It is found ex tensively in the Daobaow an Stage Zhejiang . I t is characterized by Cy clostomiceras , ac- of East Guizhou , West and Central H unan , East companied by Ellesmerocerass and Bactroceras. The Jiangx i , and West Zhejiang , this biofacies includes com mon characteristics of these cephalopods are that component membe rs such as K aipingoceras , K y mi- they have a straight siphuncle , a low ex pansion rate , noceras , and Dideroceras. They are characterized by round cro ss section , and a small siphuncle near the a straig ht shell, a low expansion rate , and a sm all si- abdominal , show ing that they are suitable fo r rapid phuncle , sho wing that they are suitable for quick sw imming and deep-w ater environment. They occur m ovements and capable of swim ming in deep-w ater. in thin-banded limesto ne and argillaceo us limesto ne They occur in middle-thick bedded knotty lime stone , with ho rizontal bedding , g raded bedding , and flute limesto ne , thin-bedded siltstone , and argillite. cast , indicating a shelf slope-basin environment. These rocks have such structures as g raded bedding , sandy lam ination , and erosio n surface on the bottom Open Platform Cameroceras-Cy rtovaginoceras Biofa- of the ro cks , indicating that this bio facies represents cies a shelf slope-basin enviro nment. It is m ostly fo und in the Daobao w an Stage (Chi- na Stratig raphic Group , 2002) o f the Ebian , Nan- Inner Shelf Bathmoceras-Protocycloceras Biofacies chuan , and Junlian areas of South Sichuan , the Zuny i This is a widespread biofacie s , fo und in the area of Nor th Guizho u , and Central Guizho u , this Daow an Stag e of Junlian in South Sichuan , biofacies is dominated by Cameroceras and character- Zhenxiong in East Yunnan , Sinan and Jiang ko u in ized by Cyrtov aginoceras, accom panied by H opeio- nor theastern Guizho u , Wushan and Yo uy ang in East ceras , Oderoceras, Allocotoceras, and Tri f urcato- Sichuan , Long shan in West H unan , Yichang in Hu- ceras. Cephalo pods of this bio facies are characterized bei , and the Low er Yangtze district. Its components by an elliptical cro ss section , a high expansio n rate , are dominated by P rotocycloceras and characterized and a big siphuncle near the oute r shell. They occur by Bathmoceras , associated with Dideroceras , in thick limestone and biohermal limesto ne. These Chisiloceras, Changy angoceras , Vaginoceras , An- O rdo vician Cephalo pod Bio facies o f South China and Its P rov incializa tion 225 cistroceras, Cochioceras , and Pesudoandistroceras. Michelinoceras and m ost of them are similar to sal- They have comm on characteristics such as a smaller pingiform w ith a small siphuncle and a large air siphuncle than tha t of the Daobaow an Stag e , lo cated chamber , indicating tha t cephalo pods of this biofacies at or near the cente r of the shell , and a round or o val are suitable fo r floating instead of rapid swim ming. cro ss sectio n , indicating that cephalopods of this bio- This arg ument can be dem onstrated by lo w diversity facies develop in the direction of w eight-reduction and and few benthonic org anism s associated with this bio- rapid-mov em ent. Cephalopods o f this biofacies often facies. Cephalopo ds o f this bio facies occur in black occur in kno tty lim estone , and limestone represen- shale w ith a high content of org anic carbon , w hich is ting an inne r shelf enviro nment. favorable to producing oil and , therefore , show s a stag nant basin enviro nment. Middle Shelf Dideroceras-Ancistroceras Biofacies This is also an ex tensive biofacies found in the Deep-Water Basin Paraendoceras-Sactorthoceras Bio- Darriwilian Stage (China S tratig raphic Group , 2002 ; facies Chen et al. , 2000) of Central Guizhou , No rth and It is fo und mo stly in the Daw an Stage to the East Sichuan , Day ong in nor thwestern H unan , low er par t of the Aijiashanian Stage in East Guizhou , Yichang in H ubei , and the Low er Yang tze district. West and Central H unan , East Jiang xi and West Co mponent members of this biofacies include Zhejiang , this biofacies includes com po nent members D ideroceras, Ancistroceras , Vaginoceras , Chisilo- such as Sactorthoceras , P araendoceras , Talass- ceras , Michelinoceras , Paradnatoceras , Sinoceras, oceras , Maraendoceras , P rotocyclocer , Dideroceras , Richardsonocerass , Troedssonella , etc. . They have and Michelinoceras. S actorthoceras is the character- a w idespread distribution and are characterized by a istic mem ber of the biofacies. Cephalo pods of this big shell with a low ex pansion rate (ex cept for Ancis- biofacies are characterized by a straig ht shell with a troceras), a small siphuncle , and a large air cham- small siphuncle , a larg e air chambe r , a low expan- ber. These features show that cephalopods of this sio n rate and little o r no sediment , and undevelo ped biofacies have a goo d ability to reg ulate the fluid endocone. These features indicate that the cephalo- within them , w ith little resistance to m ovement , and pods could be deep-w ater ecolog ical ty pes o f org an- are suitable for rapid sw imming in deep w ater. Mo re- ism s. These cephalopods mo stly o ccur in rock assem- o ver , they occur m ostly in kno tty lime stone , and , blag es o f black carbonic shale , shale , silicic shale , therefo re , represent a middle shelf enviro nment. and kno tty limestone (Bureau o f Geology and M ineral Resources of Guizhou Province , 1997). Therefore , Deep-Water Shelf Lituites-Cyclolituites Biofacies this biofacies represents a deep-w ater basin enviro n- It is found e xtensively in the Miaopo Stage of ment. southw estern Sichuan , East Yunnan , Nor th Guizhou , East H ubei , and the Low er Yangtze dis- Deep-Water Shelf Sinoceras-Michelinoceras-Disoceras trict , com po nent member s o f this bio facies include Biofacies Lituites, Cyclolituites , Mysterioceras , and Micheli- This is an extremely w idespread bio facies , fo und noceras , and are cha racterized by a small siphuncle , from the uppe r part of the Aijiashanian Stage to the one eig hth to one tenth the diam eter of their shell. low er part of the Qiantang jiangian Stage in South and They occur mainly in limestone and po ly go nal lime- East Sichuan , East Yunnan , Central and East stone. T he cephalopo ds of this biofacies have fea- Guizho u , West H ubei , and West H unan. Its compo- tures that indicate they are suitable fo r sw imming o r nent members include Sinoceras , Michelinoceras , flo ating in a deep-w ater shelf environment. Disoceras , Orthoceras, Richardsoceras , Parormo- ceras, Dideroceras , Winnipegoceras , Eosomicheli- Stagnant Basin Lituites-Trilacinoceras Biofacies noceras , Cy clolituites , Ancistroceras , and Dongkal- This biofacies mainly occurs in the low er part of aceras. It is dominated by S inoceras and Michelinoc- the Aijiashanian S tage of No rth and East Sichuan , eras and characterized by D isoceras. They have co m- and West and No rth Hubei. It is coeval w ith differ- mo n characteristics such as straig ht shells with small ent facies from deep-w ater shelf Lituites-Cyclolituites siphuncles near the center o f the shell , large air biofacies. Component member s of this biofacies in- chambers , and a lo w ex pansion rate. These features clude Lituites, Trilacinoceras, Cy clolituites , and sho w that cephalopods o f this bio facies have a go od 226 Xiao Chuantao , Chen Z hiyo ng , Liu L i and He Yo ubin ability to regulate the fluid w ithin them and little re- and straight shell with a middle-big siphuncle , a hig h sistance to m ovement , and they are suitable fo r rapid expansion rate , and sediments w ithin the air cham- sw imming in a deep-w ater enviro nment. This arg u- ber , indicating they are not suitable fo r swimming in m ent can be demo nstrated by few bentho nic o rgan- a deep-wa ter environment. In addition , cephalo pods isms associated with this bio facies. M oreover , they of this bio facies are mo stly pre served in a rock assem- occur mostly in polyg onal limesto ne (Zhou et al. , blag e of thick-bedded limestone , siltstone , and sand- 1993 ;Wang et al. , 1987) and , the refore , represent stone develo ped with an erosio n surface , g raded bed- a deep-w ater shelf environment. ding , and w ith a Bouma sequence in it (Zhou e t al. , 1993), indicating that this biofacies represents a Deep-Water Shelf Beloitoceras-Jiangshanoceras Bio- shelf-slope enviro nment. facies It is m ainly fo und at the low er part of the Qian- Deep-Water Basin Michelinoceras Biofacies tangjiangian S tage in Central H unan , N orth Jiangxi , This is an ex trem e widespread biofacies fo und in and West Zhejiang , this biofacie s includes J iangsha- the upper part of the Qiantangjiangian S tage in the noceras , E urasiaticoceras, B eloitoceras , Michelinoc- Yang tze district and West and Central H unan. Mich- eras , Chisiloceras , D ideroceras , and Kotoceras , and elinoceras is the unique com po nent member o f the is characte rized by J iangshanoceras. They have com- biofacies and the associated bentho nic o rg anism s are m on features such as middle-size shells , small si- also very few . Instead , sev eral planktonic g rapto lites phuncles , and large air chambers and they are pre- are developed in this bio facies. Michelinoceras has a served in thin-bedded knotty limesto ne and argilla- straig ht shell w ith a small siphuncle near the center ceous limestone. Additionally , associated benthonic of the shell , a low expansio n rate , and round cro ss org anism s are very few in the biofacies. These fea- sectio n , and these features indicate that Michelinoc- tures show that these biofacies could represent a eras is suitable for rapid sw imming in a deep-w ater deep-w ater shelf environment. environment. Additio nally , Michelinoceras is preserved in silicic shale and shale , suggesting a deep- Deep-Water Shelf-Basin Eurasiaticoceras Biofacies w ate r basin enviro nment (Zhou et al. , 1993). This biofacies chiefly occurs at the lowe r pa rt of the Qiantangjiangian S tage in Central H unan , Nor th PROVINCIALIZATION OF CEPHALOPOD BIOFA- Jiang xi , and West Zhejiang , and is dominated by CIES membe r Eurasiaticoceras asso ciated o nly with J iang- The provincializatio n o f biofacies show s paleo- shanoceras. Eurasiaticoceras has a pan-like shell w ith ecolo gical and paleobiogeog raphical feature s , and also a sm all siphuncle and a larg e air chamber , and these dem onstrates the paleog eo graphical and the tectonic features indicate that this org anism is able to swim paleog eog raphical cha racteristics of the lithofacies. rapidly in a deep-w ate r environm ent. Cephalo pods of South China is divided into three cephalo pod biofacies this biofacies are mo stly preserved in silicic shale and provinces in this ar ticle , based o n the associatio n shale (Bureau of Geo logy and Mineral Resources of law s of cephalopo d biofacies in time and space , and Jiang xi Province , 1997). M oreover , associated ben- paleoecological and paleogeog raphical distributio n thonic organism s are very few in the biofacies. The features (Fig. 2). Different biofacies pro vinces have features mentio ned above sho w that this biofacies diffe rent associations o f biofacies. could represent a deep-w ater shelf-basin environ- m ent. Yangtze Biofacies Region This biofacies pro vince cover s East Yunnan , Shelf-Slope Jiangxiceras-Yushanoceras Biofacies Central and No rth Guizhou , mo st of Sichuan , Hu- It is m ainly fo und in the upper part of the Qian- bei , Jiang su , and Anhui. A straig ht shell with a big tangjiangian Stage in Daw an , Jiang xi , and Jiangshan siphuncle , a sm all air chamber , and a high expansio n in Zhejiang , this biofacies is com po sed o f J iangx- rate cha racterizes Early Ordovician cephalopods o f iceras , Yushanoceras , Ormoceras , Orthonybyoceras, this bio facies province , and these features indicate Endoceras , Ly ckholmoceras, Antip letoceras , and that the cephalopods lived with a benthonic sw im- Actinoceras, and is characterized by members J ian- ming or jumping sw imm ing mo de. From the M iddle g x iceras and Yushanoceras. They mo stly have a large Ordovician till the Late Ordovician , cephalo pods o f O rdo vician Cephalo pod Bio facies o f South China and Its P rov incializa tion 227 this biofacies province we re characterized by a sio n rate , a sm all siphuncle , and a la rg e air cham ber , straight , half-curved , and pan-like shell com bined indicating a prog ressive increase in swim ming ability with prog re ssive evolutio n tow ard a g radual ex pan- and adaptation to a deep-w ater enviro nment.

Figure 2. Ordovician biofacies and their regionalization in South China.

Early Ordovician beg an to take place in this biofacies region. Never- Xinchang age theless , the diffe rence betw een the eastern and w est- In the Xinchang age , an ex tensive transg ression ern parts is minor in paleogeog raphic terms , and they 228 Xiao Chuantao , Chen Z hiyo ng , Liu L i and He Yo ubin belong to the same platform environment. The dif- biofacies in the Daw an ag e w as B athmoceras-P rotocy- ferentiation of biofacies w as because of the differ- cloceras , w hereas , the Guniu ag e w as characterized ences o f paleo topog raphy and source. As the upper by D ideroceras-Ancistroceras. Yang tze area was near the old East Yunnan land and had a high content o f terrigenous sediment , cephalo- Late Ordovician pods developed , and only the Protocameroceras bio- During the Late Ordovician , because of the fur- facies occurred in this biofacies region. Because the ther rising of the sea level and northwa rd subductio n paleotopography w as not flat and the seco nd-o rder of the South China pla te , this biofacies region became cycles w ere in the process of sea-level change , the a deep-w ater shelf enviro nment and finally a deep sea Lo we r Yang tze area w as characterized by restricted basin at the late age of Late Ordovician , and g radual- platfo rm and open platform alternately. Restricted ly merged w ith the East Guizho u-West H unan bio fa- platfo rm environment w as characterized by Pseudoec- cies province and the Central H unan-West Zhejiang tendites-X iadongoceras biofacies and open platform biofacies province. At the early period of the Late by Retroclitendoceras-Pararetroclitendoceras biofa- Ordovician , a curved shell characterizes the cephalo- cie s. A s the transg ressio n ex panded in the late Xin- pods o f this biofacies regio n during infancy , and chang ag e , the Low er Yang tze area w as characterized straig ht shell w ith sm all siphuncle during holaspis by P ronajaceras-Mamagouceras biofacies. perio d. These features show eco logical characte ristics of deep-w ate r floating. From the early to the m iddle Daobaowan age perio d of the Late Ordo vician , because of the further As the Dao baow an age advanced , the transg res- rising of sea level , this biofacies region w as charac- sion ex panded and reached high sea level (Xiao et terized by the sw imming type of cephalopod w ith a al. , 1996). The paleo to pog raphy o f this biofacies re- small siphuncle , and by deep-w ater cephalopods w ith g ion tended to be little different , co mbined w ith a a ve ry sm all siphuncle at the late period of the Late tropical clim ate. As a result , o rganic reefs developed Ordovician , as the biofacies region becam e a deep-sea e xtensively on the large Yang tze platfo rm (Xiao et basin. al. , 1993 ;Zhu , 1993). Althoug h the differentiation of paleog eo graphy w as mino r , there w as a differenti- Early Aijiashanian age a tion of bio facies by different genera. The Upper Because of the influence of a co ntempo raneous Yang tze region w as characterized by Cameroceras- fault (Tanlu fault) (Xiao et al. , 1996 ;Zhou e t al. , Cy rtov ag inoceras biofacies and the Lo we r Yangtze 1993), the diffe rentiation of paleo geog raphy occurred region by Coreanoceras-Manchuroceras biofacies. during the Miaopo age. As a result , this biofacies region w as differentiated into tw o types of enviro n- Middle Ordovician ments , i. e. deep-wa ter shelf and stag nant basin , During this tim e , the second big transg ression w hich o ccur red as facies diffe rentiation of Datianba occurred in the Yang tze bio facies region. This trans- Forma tion and Miaopo Fo rmatio n , and develo ped g ressio n made the bio facies region change from a w ith Lituites-Cyclolituites bio facies and Lituites- platfo rm enviro nment to a shelf environment. Mean- Trilacinoceras biofacies , respectively. w hile , distinctive change s o ccur red in the component membe rs of cephalopods , w hich we re characterized Late Aijiashanian-early Qiantangjiangian age by a straight shell with a small siphuncle nea r the During this age , because of the furthe r rising o f center of the shell , a larg e air chambe r , and a low sea level and no rthw ard subductio n of the South Chi- e xpansio n rate (Xiao and Pan , 1997). These features na plate (Xiao et al. , 1996 ;Zhou et al. , 1993 ), the indicate that the cephalopods m ade further pro gress Yang tze plate bent dow nw ard. As a result , this bio- by adapting to their environm ent and evolved tow ard facies reg ion merged w ith the East Guizhou-West the direction of decreasing resistance (a low ex pan- H unan biofacies pro vince and became a deep-w ater sion rate) and regulating the movement up or dow n shelf basin. As the na ture of the bottom and the eco- (a small siphuncle and a larg e air chamber). Because logical environment tended to integ rate , the cephalo- of the mino r differentiatio n of paleogeog raphy and pod biofacies had obvious differentiation , character- similarity of ecological enviro nment , the re w as little ized by cephalopod assemblag es w ith a straig ht shell differentiatio n in the biofacies. The representa tive o r plate-shaped and sm all siphuncle indicating deep- O rdo vician Cephalo pod Bio facies o f South China and Its P rov incializa tion 229 w ater ecolog ical features. Bo th this region and tral H unan-West Zhejiang biofacies pro vince in paleo- Guizhou-West H unan biofacies develo ped w ith geog raphy , but the re w as no differentiatio n in cepha- Sinoceres-Michelinoceras-D iscoceras biofacies. lopod biofacies. Both these bio facies pro vinces and the Central H unan-West Zhejiang biofacies province Late Qiantangjiangian age develo ped Cyclostomiceras-Bactroceras biofacies w ith In the late Qiantangjiangian age , the sea level a small siphuncle , a large air cham ber , and a low ex- continued to rise and reached its highest level. Com- pansio n rate. bined w ith the no rthw ard subduction o f the So uth In the Daobaow an ag e , this biofacies province China plate (Xiao et al. , 1996 ;Zhou et al. , 1993), inherited the paleog raphical framew ork o f the Xin- this bio facies region became a deep-w ater basin envi- chang age , but developed different deep-w ater type ronment and merged with the East Guizho u-West genera of cephalopods , com po sed of K aip ingoceras- H unan biofacies province and w ith the Central E llesmeroceras biofacies. H unan-West Zhejiang biofacies pro vince. Thin-bedded silicite and siliceo us shale w ere developed and Middle Ordovician-earliest Late Ordovician contained several flo ating g raptolite s with very few Because of the sea-level rise during this pe rio d , benthonic org anism s , w hereas , cephalopo ds of this this biofacies province becam e a deep-w ater , shelf-to- biofacies reg ion w ere characterized by Michelinoceras basin enviro nm ent tog ether with the Central H unan- biofacies with a very small siphuncle. West Zhejiang biofacies province. Cephalopods did not change much in shape and structure com pared to East Guizhou-West Hunan Biofacies Province (Mixed the previous perio d. They w ere characterized by a Biofacies Province) small siphuncle and a low ex pansion rate. The chan- This biofacies pro vince mainly includes the areas ges of this biofacies province fo cused on the differen- that are east of the line alo ng M ajiang-Tong ren in tiation o f species and genera of cephalo pods , and de- East Guizho u and Tongshan in So utheast H ubei , and veloped w ith P araendoceras-Sactothoeeras biofacies. west of the line along Chunx i-Taoy uan in West H u- nan. During the Ordovician , these areas we re in a Early period of Late Ordovician-late period of Late Or- shelf slo pe-deep w ater basin environment , w hich in- dovician clined eastw ard and w ere the main part o f the So uth During this period , the northw ard subductio n of China paleo-co ntinental margin. This tecto nic and the South China plate (Xiao et al. , 1996 ;Zhou et paleog raphical framew ork de termined the bio facies to al. , 1993) caused a rise in sea level and the merging be a mix of Yangtze-type and Jiang nan-type o rgan- of this biofacies province into the Yang tze biofacies isms. The feature s of the mix ed bio facies could be regio n , and became a deep-w ater , shelf-to-basin en- e xplained by having the sam e cephalopod biofacies vironment. As a result , deep-w ater type biofacies , bo th in the East Guizhou-West Hunan bio facies prov- S inoceras-Michelinoceras-Discoceras and Michelinoc- ince and the Central H unan-West Zhejiang biofacies eras bio facies characterized this period. pro vince during the Early Ordovician to the earliest La te Ordo vician , and having the same cephalo pod Central Hunan-West Zhejiang Biofacies Province biofacies both in the East G uizhou-West Hunan bio- This bio facies province included Yuanlin , A n- facies pro vince and the Yang tze biofacies province hua , T aoy uan , East Jiang xi , and West Zhejiang. during the Late Ordovician. Cephalopods o f this bio- During the Early Ordovician to the earliest Late Or- facies province w ere characterized by a small siphun- do vician , this biofacies pro vince , to gether w ith the cle , a large air cham ber , and a low expansion rate. East Guizho u-West H unan biofacies province , formed an environment sy stem composed o f shelf Early Ordovician slope-deep basin. The cephalopod bio facies differen- During the Xinchang age , this biofacies province tiations w ere no t clear. Both biofacies provinces de- carried o n the Cambrian paleog raphical framew ork veloped w ith Cy clostom iceras, K aip ingoceras- and fo rm ed a shelf slope-deep w ater basin environ- K y minoceras , and Paraendoceras-S actorthoceras m ent sy stem togethe r w ith the Central H unan-West biofacies during the Early O rdovician to the ea rliest Zhejiang bio facies province. The re w as little differen- Late O rdovician. tiatio n betw een this bio facies province and the Cen- A t the beginning of the early age of the Late Or- 230 Xiao Chuantao , Chen Z hiyo ng , Liu L i and He Yo ubin dovician , no rthw ard subduction of the South China (in Chinese) plate led to the hig hest sea-level rise , and this biofa- Che n, X. , Rong , J. Y. , Z hang , Y. D. , et al. , 2000. A cie s province went fro m a deep-w ater shelf-basin en- Commentary o n O rdo vician Chronostratig raphy . J ourna l o f S tratigraphy , 24:18 - 26 (in Chinese with Eng lish vironm ent to a shelf-slo pe environment. Therefo re , A bstract) biofacies ev olved from deep-w ater type , Beloitoceras- China S tratig ra phic G roup, 2002. Directio n o f China Regional J iangshanoceras and Eurasiaticoceras biofacies , to Chro no stratig raphic T able (G eologic Chro no lo gy). Geo- the shelf type , J iangx iceras-Y ushanoceras biofacies , logical P ublishing Ho use , Beijing. 51 - 54 (in Chine se) characterized by a middle siphuncle and a hig h ex pan- Committee of Chine se Stra tig raphic Dictio nary , 1996. China sion rate. Additio nally , the w estern pa rt of this bio- St ratig raphic Dictionary (O rdovician). Geo lo gical Pub- facies pro vince , the Central H unan area , m erged into lishing H ouse , Beijing. 4 - 104 (in Chinese) the Yangtze and the East Guizho u-West H unan bio- Schafe r, W . , 1972. Eco lo gy and Paleo eco lo gy of M arine Envi- facies pro vince during the Wufeng age. ronme nts. U niv. Chicago P ress, Chicag o. 561 - 578 W ang , X. F. , Xiang , L. W. , N i, S. Z . , et al. , 1987. Bio s- tratig raphy of the Y ang tze Go rg e A rea (2):Ea rly P alaeo- SUMMARY zoic Era. Geo log ical Publishing House , Beijing . 43 - 137 The evolutio n process of the Ordovician cephalo- (in Chinese) pod biofacies and provincializatio n o f So uth China Xiao , C. T . , Jiang , Y . W. , Liu , B. L. , et al. , 1993. A Dis- demonstrates the uninterrupted rise o f Ordovician sea cove ry o f Batostoma Genus o f Early-M iddle Ag e o f Early level in the Yangtze district , the continuous ex pan- O rdovician and a Study o n Its Geo log ic Functio n and sion of the Yang tze platfo rm , and the g radual decrea- Ecolog y in M iddle Yang tze A rea. Chinese Science B ulle- sing process of the transitional district and the Jiang- tin , 38 :1314 - 1316 (in Chinese) Xiao , C. T . , Jiang , Y. W . , Z hu, Z . D. , e t a l. , 1996. A nan area. I t also show s the escalating actio n of the Study o n O rdovician Sequence Stratig raphy in Yichang S outh China platfo rm. Distric t, Hubei and Sedime nta ry Environment of the W ufeng Fo rmatio n in Y ang tze A rea. Geological J ournal ACKNOWLEDGMENTS o f China Universities , 2:339 - 347 (in Chinese w ith The authors are g rateful to M r. Yao H ailin fo r English A bstract) his help. This study w as suppo rted by the N ational Xiao , C. T . , Pan , Y. T . , 1997. O rdovician Cephalo po d Bio- Natural Science Foundatio n of China ( No. facies in the Western M a rgin of the Jiang han Basin. Ex- 40272060). ploration of N ature , 16 :83 - 85 (in Chinese w ith Eng- lish Abstract) Xu , G . H. , Xu , A . W. , 1986. T he Ecology and Enviro nment REFERENCES CITED o f Cephalo pods f rom the Honghuayuan F ormation and the Boucot , A. J. , 1981. Principles o f Benthic M arine Paleoecolo- Baota (P ago da)— Lo wer-M iddle Linx iang F ormatio n in gy . A cademic P ress , N ew Yo rk. 454 - 465 the Eastern Yang tze Go rge s A rea , China. Geological Re- Bureau of Geo log y and M ineral Resources o f G uizhou P rovince , v iew , 34:97 - 104 (in Chinese w ith English A bstract) 1997. 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