The Characteristics And Cause Analysis Of Basal Conglomerate Of Changzhougou Formation In Qianan

The Changzhougou Formation was mainly developed in the southwest and westin Qianan of Hebei province. The cause of the basal conglomerate which developedwidely between the Changzhougou Formation and Qianxi Group and their surfacecraters haven’t been explained yet. The sedimentary environment of the basalconglomerate is analysis through the research for its characteristics, distribution rangeand thickness variation. The environment of the early Changzhougou Formation hadbeen confirmed through regional correlation. According to the research, the Qiananbasal conglomerate was of intermontane river genesis. In the early ChangzhougouFormation, there were intense crustal movements in Qianan. Terrain here changerapidly, the weather here is dry and hot. The basal conglomerate is river subphase,vertical rhythm composite of jet coarse gravel micro facies and sustained coarse sandfacies alternatively which represent the sedimentary of the river in flood period anddry period. Fine-grained sediments cannot be found. The lithology of gravels in thebasal conglomerate are quartz sandstone, quartzite and siliceous. Sedimentary hastransported short distance and stacking quickly. Coarse sand in the basal conglomeratecontains a great deal of feldspar, the types of the probability cumulative grain sizecurves are mainly two-segment style, jump components in the majority, the diametersof clastic are thick, suspension components in minority. The C-M image mainlyconsist of PQ,RQ and RS parts which stand for high velocity flow environment, theimage reflect the characteristics of traction current. The tendency of ab surface ofgravel and laminas of cross bedding reveals that the paleocurrent direction was fromSoutheast to Northwest, we can conclude that the provenance is the Shanhaiguan oldland which in the Southeast direction. In the early Changzhougou Formation, theYanshan basin was under the control of the Xinglong-Lianyuan fracture in the westand the Qianan-Funing fracture in the southeast. The main course of the river was inthe area of Jixian-Xinglong，there are two branches in its southeat as well as itsnorthwest area. At last, through both the observation of outcrop and the analysis ofsedimentary environment, we eliminate that glaciation and tectonism has contributedto the surface craters. We conclud that stratum static load has accelerated the mutualetching of gravels, under the great pressure surface craters developed.