PROBLEMS ON THE ASSIGNMENT OF THE STRATA AT THE WESTERN END OF THE LONGSHOU MOUNTAINS

Two pieces of Middle Carboniferous common plant fossils of Neuropteris of.gigantea and Mesocalamites sp.were collected from the black slate on the southern slope of Mount Qinglong at the western end of the Longshou Mountains when the author undertook regional geological surveys of J-47-XI（Zhangye Sheet） in 1970.It was considered that the fossil-bearing horizon showed a conformable transitional relation with the migmatite on the southern side and the siliceous limestone phyllite on the northern sheet.On that basis,the Proterozoic strata covering an area of a few hundreds of square kilometers at the western end of the Longshou Mountains were all assigned to the Middle and middle Upper Carboniferous.On the Geological Map of Gansu Province（scale 1:500,000） published in 1976,their ages were changed to the Middle Carboniferous and Upper Carboniferous-Lower Triassic.Thus there appears a great difference in the age assignment of the strata at the western end of the Longshou Mountains and the corresponding strata in other parts of the Longshou Mountains. Through serious in-situ observations and studies of biostratigraphy,it has been confirmed that the black slate where the above-mentioned fossils were collected is in faulted contact with the strata on the northern and southern sides.The strata were traced eastwards along the strike of the Middle Carbon- iferous black slate,quartz sandstone and shale;as a result,late Early Carbon- iferous to Late Permian strata were recognized in the middle and eastern sectors of the Longshou Mountains,and a relatively complete Permo-Carboniferous stratigraphic system is thus established,In the system,the late Middle Carboniferous-Late Permian strata can be well correlated with the type section of the Benxi Formation-Shiqianfeng Formation in the North China region,while the late Early-early Middle Carboniferous strata can be correlated with the Couniugou Formation-Jingyuan Formation.It has been confirmed through tracing that the distribution of the Permo-Carboniferous strata in the Longshoushan area is strictly controlled between the WNW-trending thrust faults.They occur in the form of a graben,covering an exposed area of about 35 sq km.