The Cretaceous Sedimentation On Both North And South Of Mount Langshan And Their Constraints On Its Uplift

Mount Langshan district includes Mount Langshan which belongs to western part of Yinshan Mountain,Celaomiao Depression on north Mount Langshan and the south foothill area of Mount Langshan.With southwestern-northeastern extension,Mount Langshan is composed of Precambrian metamorphic rocks and several subsequent periods' granite.Located on north Mount Langshan,with similar southwesternnortheastern extension,Celaomiao Depression is mainly covered by Cretaceous clastic rocks.Unlike the north side,multiple roughly parallel and narrow banded thrust sheets,comprising Precambrian meraporphic rock sheets and Cretaceous sedimentary rock sheets,are distributed over the south foothill area of Mount Langshan.According to former studies on the sedimentary facies and paleogeography in the Celaomiao Depression,it was believed that the clastic rocks of the Early Cretaceous Bayingebi Formation were mainly supplied by the neighboring orogenic belt,Mount Langshan,to its southeastern margin.It means that Mount Langhshan had already been uplifted in the Early Cretaceous.For decades,this model exerted profound influence on Celaomiao Depression's mining work as a guiding thought,however,no satisfactory results were reported.This paper's research indicates that the source area of the Early Cretaceous clastics in the Celaomiao Depression was a fossil orogenic belt situated to the north Mount Langshan had not been uplifted yet.Through field profile measurements,paleocurrent analysis,palaeostress field recovery,stratigraphic contact relationship analysis and strata dating work,this paper achieves below result:(1)According to the result of field profile measurement,a compelet sedimentary cycle of Lower Cretaceous Bayingebi Formation,in Celaomiao Depression,from bottom up,can be concluded as: alluvial fan facies,lower fluvial facies,lacustrine facies,upper fluvial facies,alluvial fan facies,with a max stata thickness of 2255 m.Through stratigraphic correlation,during Early Cretaceous lake flood period,Celaomiao Depression developed a lake ranging over a large area including whole current Celaomiao Depression and part of Mount Langshan,thus deducing that Mount Langshan had not uplifted at least until Early Cretaceous.(2)This paper conducted field expeditions in the Mount Langshan area.Using the foreset beddings/laminations and ripple marks as the major indicators of the waterflow direction,we measured the attitudes of the sedimentary beds and sedimentary structures on the outcrops of the Early Cretaceous Bayingebi Formation and Lisangou Formation on both north and south side of Mount Langshan,recovering their original paleocurrent directions through steorographic projection.All the measurements indicate that the paleocurrent directions maily from northwest to southeast with a range of 140°—210° in Celaomiao Depression,and with a major direction of 135° in the south foothill area of Mount Langshan,perpendicularly pointing to the extension trending of Mount Langshan.(3)Through deformation structure analysis,regional early(the end of Early Cretaceous)maximum principal stress of the Mount Langshan district shear similar features with its later(from late Early Cretaceous to present)ones,for example,they both have a predominant direction,symmetrically distributing in NW and SE direction regions.The late stress field in the Mount Langshan district is the result of continuous development,strengthening and progressive deformation on the basis of the early stress field.The Mount Langshan district has been in the NW-SE compressive stress field since the end of the early Cretaceous.(4)Influenced by the thrust uplifting from northwest to southeast,Mount Langshan district had developed multiple thrust sheets and 5 contact boundaries between them,including thrust fault boundary ?,?,and ?,and overlap unconformity boundary ?and ?.On boundary ?and ?,metamorphic rock sheets form Mount Langshan thrusts over red beds of Lisangou Formation.While on boundary ?,sheets of Bayingebi Formation thrusts over red beds of Lisangou Formation.Therefore,inference can be made that Mount Langshan had not uplifted at least until the late diagenesis stage of Bayingebi Formation and Lisangou Formation.(5)Some typical components of the Jehol Biota,including conchostracans Eosestheria sp.,the mayfly Ephemeropsis trisetalis,the aquatic beetle Coptoclava longipoda,are discovered in the Bayingebi Formation,Celaomiao Depression,indicating that the middle Upper Member of Bayingebi Formation can be correlated with the upper Yixian and the lower Jiufotang formations in western Liaoning Province.Hence we can infer that the Upper Member of Bayingebi Formation has a Barremian–early Albian age(~129.4Ma—~113Ma).Similarly,5 minimus zircon single grains U-Pb age of Lisangou Formation is 129.4 Ma,showing that its oldest age is no more than Early Cretaceous.(6)Through analysizing the geomorphic features such as incised meanders and multistage terraces,we can infer that Mount Langshan had a tectonically active period since Cenezoic,revealing characteristics of a young(compared to Early Cretaceous)mountain.