EARLY JURASSIC MOLASSE WEDGE SEDIMENTS AND OIL POTENTIALS IN THE CHUXIONG BASIN,YUNNAN

The Chuxiong Basin is a Mesozoic strike-slip peripheral foreland basin.The authors contend,in the light of the detailed study and comparison of the sediments from the Lower Jurassic Fengjiahe Formation,that the Early Jurassic is an important stage of intense downwarping and subsidence of the basin,spaning from early to late stages the foreland basin filling processes of suddenly deepening and then shallowing.Because of the continue thrust loading of the foreland thrust blocks,the Early Jurassic sediments in the western part of the Chuxiong Basin were derived from the Ailaoshan uplift in the southwest,and their sedimentary environments are dominated by littoral and shallow lakes or lacustrine deltas;those in Zhonghe of Yongren and Longjie of Dayao in the northeastern part of the basin originated from the Yuanmou uplift,and were deposited in a similar sedimentary en-vironment,and those in the Huijiguan region,Chuxiong in the central part of the basin rapidly increase in sediment thickness and lay down in the deep-lake sedimentary environ-ments with turbidite sandstone bodies on the basis of well logs,thin sections and grain-size analysis.The discovery of the deep-lake marls or lime mudstones in the Dieshemo zone, Xinping has revealed that the subsidence and depositional centres of the Early Jurassic Chuxiong Basin were migrated during the Late Triassic from the frontier of the Ailao Mountains northeastwards to the Huijiguan-Dieshemo zone,where the sediments were in-filled in the form of foreland wedges.Furthermore,the first discovery of asphaltic sand-stones in the third member of the Fengjiahe Formation in the Longjie region,Dayao has al-so disclosed the oil potential in the Chuxiong Basin.The asphaltic sandstones may be clas-sified into two types in terms of occurrence:the crack and pore-space filling type and inter-granular pore-space filling type,all of which occur in the lake-delta sequences where the reversed rhythmic stratifications are well-developed.It follows that care should be taken to distinguish the Lower Jurassic crack and pore-space filling type traps from the lithologic traps in the study area.