EOCENE PALYNOFLORAS FROM THE DAINAN AND SANDUO FORMATIONS IN NORTH JIANGSU WITH SPECIAL REFERENCE TO EOCENE CLIMATIC CHANGES IN SOUTHEAST CHINA
Zhang Yi-yong;Qian Ze-shu Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing 210008 Institute of Geology, Jiangsu Oil Exploration and Development Corporation, Yangzhou 225261
The geolgical age of the Dainan and Sanduo Formations widely developed in the subsurface ofnorthern Jiangsu and in the South Yellow Sea Basin has been in dispute since the 1950's. In the past,most geologists and micropaleontologists regarded it as Oligocene. However,after sampling and makinganalysis of the cored samples from the Dainan and Sanduo Formations,the writers now consider thatthe palynological assemblages under study from both formations may be compared with the Eocene pa-lynofloras from the strata yielding planktonic mierofossils or mammals. At present,three palynological assemblages have been distinguished from both formations; theirmain elements and occurrences are shown in Table 1 (see Chinese text). These assemblages may be briefly described as follows: A. The Ulmipollenites-Inaperturopollenites-Pterisisporites Assemblage This assemblage obtained from the Dainan Formation is characterized by the presence of rich triporate-type pollen andconiferous pollen grains, with the common occurrences of some extinct species,such as Parcisporites,Anizonosaccites, Bhardwa japollenites, Punctamonoaperturites, Navisulcites, Pentapollenites and Paraalnipollen-ites; Aquilapollenites and Proteacidites, which appeared frequently in Late Cretaceous and the Palaeocene,became rather rare. The assemblage is early Eocene in age and suggests a subtropical climate. B. The Taxodiaceaepollenites-Caryapollenites Assemblage This assemblage found from theLower bed of the Sanduo Formation is characterized by the occurrence of a great number of Caryapol-lenites triangulus, with some extinct components assigned to Parsisporites, Bhardwajapollenites, Puncta-monoaperturites, Paraalnipollenites and Pentapollenites. This assemblage indicates an early Eocene age anda subtropical climate. C. The Nitrariadites-Retitricolpites-Euphorbiacites Assemblage The assemblage form theupper part of member Ⅱ of the Sanduo Formation is very different from the underlying two assem-blages in the abundance of tricolpate and tricolporate pollen. Saccate coniferous pollen rarely occurs,whereas Ephedripites increases in number. This palynoflora is late Middle Eocene to/or early LateEocene in age and indicates a subtropical dry climate. Based on the present palynofloral data and sedimentary facies of Southeast China, the followingthree climatic events may be proposed during the Paleocene--Eocene in this area. 1. Oceurrence of Late Paleocene--Early Eocene optimum: During this time interval SoutheastChina was under the influence of comparative warm and humid climate, with wide distribution offlourishing vegetation and further development of dark sediments in major depressions, indicating animportant period for generation of oil and coal. The palynoflora is dominated by the triporate pollen ofUlmaceae ,Betulaceae,Juglandaceae,associated with some Fagaceae. The palynoflora seems to be simi-lar in aspect to that of Sabinan, Wilcox group of North America. 2. Invasion of dry-hot climate during middle Eocene:At the beginning of Middle Eocene ,dry-hotclimate invaded this area ,causing the development of poor vegetation and red-beds in Southeast China.But on the southern and northern borders of this area,luxuriant vegetation and dark sediment deposi-tion continued, owing to the weak influence of the dry climate. The palynoflora is marked by domi-nance of pollen of Fagaceae,which may be compared with that of the Claiborne Formation in NorthAmerica. 3. Strong invasion of dry-hot climate during late Middle Eocene to/or early Late Eocene: In thisperoid the arid area extended eastwards to as far as the shelf of East China Sea and Yellow Sea; the dryclimate covered a vast latitudinal area of China, with the development of red sediments intercalatedwith gypsum beds and arid palynofloras in the Chinese mainland and shelf. In the palynological assem-blage xerophytic pollen such as Ephedripites and Nitraridites often occur. Therefore,this event may beconsidered as a peroid under the driest climate since the Tertiary in SE China.