Morphological Structure And Environmental Evolution Of Coastal Alluvial Fan Along The Northern Coast Of Haitan Island, Fujian

Alluvial fan could provide valuable information in recording local climate change,reflecting neotectonic movement and rebuilding paleogeographic environment.Compared with inland areas,coastal zone as the interface of three layers of atmosphere,ocean and land has distinct differences in sedimentary environment,resulting unique material sources and dynamic condition.Thus,alluvial fan in coastal zone may have special geological landscape process and sedimentary structure characteristics.There is a great sandy hill which covered by grayish yellow eolian sand layer in the northern Junshan along the coast of Haitan island,Fujian Province.It seems to be a big aeolian dunes,but in fact is alluvial fan shape.In this paper,we used the ground penetrating radar(GPR)to detect the internal sedimentary structure of the great sandy hill,and grain-size characteristics and magnetic measure to analyse the evolution of sedimentary environment.Lastly,according to the Quaternary strata of Haitan Island and climate change moder besed the late Pleistocene,we tried to get the answer about the Morphological evolution of Coastal Alluvial.The primary conclusions of the research have followed:(1)Detection results shown that in the middle-upper part of the great sandy hill,there developed sedimentary cycle composed of angular gravel or coarse to fine sand and groove depositional structure,which presents the depositional characteristics of traditional alluvial fan;in the middle-lower part of the great sandy hill,there developed curved depositional structure presented alternately in the layer.Based on the results,we also also found that detecting depth of the alluvial fan record the wide range of three to five times of piedmont ancient flood sedimentary history.(2)GPR data interpretation was based on classification into nine distinct radar facies.Nine radar facies were grouped into unconformities and bed structure,which completely reflect that the mega-dune consists of two alluvial fans.We consider that the mega-dune deposition consists of debris flow?channel and part of sheetflood deposition,and what's more,contains the characteristics of aeolian sediments along the coastal zone.(3)There is a distinctive difference between the old red sandy sediment in bottom of the mega-dune and the alluvium and diluvium sediment in the middle and top of the mega-dune.Firstly,the old red sandy sediment suggest the grain-size characteristics have clearly eolian transportation,and then suffer chemical weathering causing the increase of clay and silt.The alluvium and diluvium sediment suggest the particles size properties is flood current transportant under the circumstance of unsteady turbulent flow.Particle size characteristics in the study area show that,the size composition and the frequency distribution curve of the aeolian sand are similar with beach sand.The modern aeolian sand have the particle character combing strong erosion and slump by gravity in the lee slope.The C-M plot suggest the alluvium and diluvium sediment is mainly transported in rolling action and appear to the existence of hydrodynamic force.(4)We have measured Room temperature magnetic parameters and magnetic analysis shows that megetic particle of all the main sedimentary facies in the mega-dune are dominated by ferrimagnetism,and contain a few of antiferromagnetic mineral such as hematite.The existence of hematite suggest that when the old red sandy sediment pile up,the climate character of the Fujian Province is under dry-heat circumstance environment.(5)As the mention above,we assumed that the development process of such coastal alluvial fan can be divided into four stages:1)During the last interglacial period of late Pleistocene,"Old Red Sand" formed "Red Desert" in the coastal zone;2)During the late glacial,weathering clastic materials,derived from the mountainous regions,poured and stacked on the coastal zone,and then formed alluvial fan covered above the erosion surface of "Old Red Sand";3)During the early-middle period of Holocene,the river cut down the alluvial fan to form the basic shape of the great sandy hill;4)During the late Holocene,wind-sand activity shaped the geomorphic feature of the modern great sandy hill.