The Genesis Of Lacustrine Sediments In Menghuocheng, Tuowan Mountain At The Dadu River-Anning River Watershed

For nearly a century there has been a hypothesis that the Paleo-Dadu River once flowed to the South.One of the strongest pieces of evidence is a group of fluvial sediments around Menghuocheng,the watershed between the Anning River and the Dadu River,suggested to be the wind gap deposits of the paleo-Dadu River.However,no detailed study about the sediments' provenance and characteristics has ever been carried out.In this thesis,using the methods of geomorphological investigation,sedimentology,palynology and geochronology,we have studied why,how and under what climatic condition this group of sediments was formed.When in the field,we investigated the Menghuocheng River from upstream to downstream,and found a set of sendiments on the right bank,from Menghuocheng to Lizishu,which have a depositional cycle:gravel-sand-lacustrine.So,selecting nine profile(P1?P9)and making inclination measurement on the gravel layer,delta foreset,horizontal bedding structure,then plot the pictures of sediment longitudinal variation and the ancient flow.We get the following conclusions:(1)When the sediment began to form,the hydrodynamic conditions constantly change from weak to strong,then from strong to weak,making the sediment appear lacustrine-gravel-lacustrine-sand-lacustrine-gravel.The gravel size on each layer is tapering from upstream to downstream on P1-P7 profile;In the P9 profile,there appear a large number of diabase breccia,showing proximal deposit.(2)The investigation in the field found the downstream of sediment point to the upper and the sediment of the branch also decline to menghuocheng River.The imbricate structure of gravel layer and forese bedding in middle and upstream show that the main direction of Paleo-direction is from east to west,consistenting with modern Menghuocheng River,.In downstream,the direction of Paleo and modern river is opposite,from west to east.In conclusion,Menghuocheng was a lake basin when the sediment formed.(3)From all above characteristics,the set of sediments conform with dammed lake model.We have investigated the ancient lake dam near the Lizishu,which is made up of landslide cataclastic rocks.In addition,according to the geological map in the study area,the lithology of suspected mountains on either side of the dam body is different.The mountains on the East bank of the river are composed of Triassic Baiguowan group:sandstone,shale,coal carbonaceous shale clip;while those at the West Bank with diabase.In the small tributaries along the East Bank near the dam no diabase can be found,whereas on the west bank in a small tributary we found a lot of ginkgo bay group rocks.This is the evidence that the dam body came from a landslide on the east bank.(4)In P2 section,we collected two buried thick branches in the top and bottom,using C-14 and getting the ages were older than 40000BP years,which showed that the lake deposite have ended about 40000 years ago.Using TCN dating method,we determined the age(0.34±0.10 Ma)at the bottom of P2.Considering the fact that that the Menghuocheng River still flows in the sediments,not in the bedrock,we speculate that there is a considerable thickness of sediments at the bottom of river.Therefore,the age of the sediments is Middle Pleistocene-Late Pleistocene.In addition,the sporopollen analysis of P2 section,in combination with glaciers evolution in source area Xiaoxiangling,we believe that the coarse-fine 'of sediments cycle change is controlled by glacial and interglacial.Finally we propose that the sediments at Menghuocheng were formed after the landslide damming.The conclusion is consistent with the research in detrital zircon of Dadu River.The Menghuocheng fluvial sediment cannot be cited as the evidence of the south flowing Dadu/Anning River and don't support the hypothesis that the Paleo-Dadu River once flowed to the South.