Mesozoic Stratigraphic Sequences In Shanwanzi Basin And Its New Age Constraints,north Hebei Province

The Mesozoic Sanwanzi basin is located in the north margin of the North China Block,covering the Dam edge zone of Saihanba Dam in the Hebei-Inner Mongolia border.It contains two sets of stratigraphic system of Hebei and Inner Mongolia,characterized by felsic volcanic rocks interbeded with sedimentary rocks.The lower member consists of mainly acidic volcanic rocks bearing sedimentary rock,the middle member dominated by fossil-bearing sedimentary rock with medium-acidic volcanic rock,and upper member featured mainly by neutral volcanic rock with sedimentary rock.Several methods(tracking marker layer in horizontal,isotopic dating of volcanic rocks and profile control et al.)were taken to ascertain the Mesozoic stratigraphic sequence in Shanwanzi Basin and its new age constraints.New results in this paper include: 1)four stable sedimentary layers were found in the basin,the sedimentary environment of which were alluvial fan facies and lacustrine facies;2)two parallel unconformity surfaces were discovered;3)twelve zircon LA-ICP-MS U-Pb ages(126~155Ma)of volcanic rocks were obtained;4)reconfirmed the existence of Upper Jurassic(143.1 ± 0.7Ma ~ 154.7 ± 0.6Ma)in the area,and divided it into Manketou Obo Formation(150 ± 0.6Ma ~ 154.7 ± 0.6Ma)and Manitu Formation(143.1 ± 0.7Ma ~ 149.7 ± 0.7Ma);5)re-determined the lower Cretaceous(129.5 ± 0.7 ~ 140.06 ± 0.59Ma)Zhangjiakou Formation,Dabeigou Formation and Yixian Formation.Combined with the new discovery in regional survey,based on regional tectonics,lithological assemblages,rock geochemistry,and contact relationships,the key geological problems—the distribution and stratigraphy of Late Jurassic-Early Cretaceous stratigraphic in the different basins in northern Hebei province—was discussed.This is of great geological significance for the division of Mesozoic strata in northern Hebei Province.