泰国、老挝南部和越南中部的非海相白垩纪双壳类生物地层学

泰国、老挝和越南白垩纪沉积全属非海相,它们广泛分布于泰国北部、东北部、西南部和南部,老挝北部、中部和南部,越南东北部、西北部、中部和南部。泰国南部的白垩纪地层分为Lam Thap,Sam Chom和Phun Phin组;其他地区,特别是西北部的呵叻高原地区的白垩系属呵叻群的Phra Wihan,Sao Khua,Phu Phan,Khok Kruat,Maha Sarakham及Phu Thok组。在老挝,万象盆地的白垩系由老挝呵叻群的NamSet,Phu Phanang,Ban Ang,Champa和Ban Thalat组与丰洪群的Thangon和Saysomboun组组成;沙湾拿吉(东兴)盆地的白垩系由南通(河)群的Nam Phouan,Nam Xot,Nam Noy和Nong Boua组组成。越南的白垩系在西北部分为Nam Na,Yen Chau,Van Chan和Ngoi Thia组,东北部为Ban Hang组,中部分为NhaTrang,Mu Gia和Dong Dzuong组,南部则为Phu Quoc组。三国白垩系的岩性普遍以红棕色至浅灰色砂岩、砾(岩质)砂岩、粉砂岩、泥岩和砾岩为主。泥岩中具有钙质结核和硅结砾岩,但盐和石膏仅见于Maha Sarakham,Saysomboun和Yen Chau组内。过去,这些白垩纪地层的生物地层没有经过详细研究。根据地层层序和化石对比,在泰国东北部呵叻高原和南部半岛地区及老挝南部沙湾拿吉(东兴)盆地中识别出了2个标记非海相白垩纪的类三角蚌类化石组合:阿普特期(但可上延至阿尔必期)的Trigonioides(Trigonioides)kobayashi-Plicatounio(Guangxiconcha)suzukii组合,阿普特阿尔必期(但主要为阿尔必期)的Trigonioides(Diversitrigonioides)diversicostatus-Pseudohyria subovalis组合。越南中部可能也产早白垩世阿普特阿尔必期的类三角蚌类双壳类Plisatouniosp.-Trigonioidessp.组合。通过双壳类组合,并结合包括孢粉和恐龙在内的其他化石的对比,泰国北部和南部、老挝南部和越南中部的非海相白垩纪地层得到了定年和对比,从而揭示了东南亚非海相白垩纪盆地的演化历史。

NON-MARINE CRETACEOUS BIVALVE BIOSTRATIGRAPHY OF THAILAND,SOUTHERN LAO PDR AND CENTRAL VIETNAM

The Cretaceous sedimentary rocks in Thailand,Lao PDR,and Vietnam consist of entirely non-marine facies.They are widespread in the northern,northeastern,southeastern and southern parts of Thailand;northern,central and southern parts of Lao PDR;and northeastern,northwestern,central and southern parts of Vietnam.In Thailand,theybelong to the Lam Thap,Sam Chom and Phun Phin formations in the southern part and the Phra Wihan,Sao Khua,Phu Phan and Khok Kruat,Maha Sarakham and Phu Thok formations of the Khorat Group in the other parts,particularly in the Khorat Plateau of the northeastern part.In Lao PDR,the rocks are represented by the Nam Set,Phu Phanang,Ban Ang,Champa,and Ban Thalat formations of the Lao Khorat Group,and Thangon and Saysomboun formations of the Phon Hong Group in the Vientiane basin,and the Nam Phouan,Nam Xot,Nam Noy,and Nong Boua formations of the Nam Theun Group in the Savannakhet(Donghen) Basin.In Vietnam,the rocks belong to the Nam Ma,Yen Chau,Van Chan and Ngoi Thia formations in the northwestern part,Ban Hang Formation in the northeastern part and Nha Trang,Mu Gia and Don Dzuong formations in the central part,as well as Phu Quoc formation in the southern part.Generally,reddish brown to light grey sandstones,conglomeratic sandstones,siltstones,claystones and conglomerates are the main lithologies of these rocks;calcrete nodules and silcretes are also present in claystones,but salts and gypsum are found only in the Maha Sarakham,Saysomboun,and Yen Chau formations.Previously,the biostratigraphy of these rocks was not studied in detail.On the basis of the stratigraphic sequences and fossil correlation,two assemblages of the trigoniodid bivalves,the indicators of the non-marine Cretaceous,have been recognized from the Khorat Plateau of the northeastern and the Peninsular Region of southern Thailand,and Savannakhet(Donghen) Basin of southern Lao PDR:Aptian,but could extend up into Albian Trigonioides(Trigonioides) kobayashi-Plicatounio(Guanxiconcha) suzukii assemblage,and Aptian Albian,but mainly Albian,Trigonioides(Diversitrigonioides) diversicostatus-Pseudohyria subovalis assemblage.Central Vietnam probably yields the Early Cretaceous(Aptian Albian) trigonioidid bivalve Plicatounio sp.-Trigonioides sp.assemblage.By means of the correlation of the bivalve assemblages,together with the other fossils,including the palynomorphs and dinosaurs,the non-marine Cretaceous strata were dated and correlated from Berriasian Barremian to Late Cretaceous in northern and southern Thailand,southern Lao PDR and central Vietnam,resulted in revealing the history of the non-marine Cretaceous basins in Southeast Asia.