Analysis On Sedimentary Environment And Silicified Wood Preservation In The Second Member Of Upper Jurassic Tuchengzi Formation In Yanqing,Beijing

The study area of this paper is situated in the Qianjiadian Area of Yanqing UNESO Global Geopark,Beijing,China(hereafter referred to as Yanqing Global Geopark).The major research objective is the second member of Upper Jurassic Tuchengzi Formation containing many silicified woods.This paper,based on the detailed field study on two stratigraphic sections,combined with the analysis of depositional sequences,sedimentary structures,rock and mineral properties and grain-size distribution data,deals with the Late Jurassic sedimentary environment in the study area,and discusses the preservation conditions of petrified woods in Tuchengzi Formation.In addition,combined with the participation of the silicified wood conservation project,some effective methods and techniques for silicified wood conservation are summarized.The major results of this paper are as follows:Based on the analysis of depositional sequences,structures,lithological and mineral properties,and grain-size distribution of Q0055 and Q0058 sections,this paper indicates that the Upper Jurassic Tuchengzi Formation was formed in the fluvial and lacustrine environments The Q0055 section is dominated by coarse-grained sandstones,interbedded with pebbly coarse-grained sandstones,low in the composition and textural maturity.Many silicified woods are well preserved with distinct heterochthonous burial.Three-phased braided channel deposits are superposed vertically and contain a lot of detrital grains,indicating strong hydrodynamic force and fast proximal deposition.The short distance transportation and fast burial provide a favorable buried condition for the large tree trunks to form petrified woods.There is no volcanic material found in the Q0055 section,indicating the formation of the silicified wood has no direct connection with volcanic activity,and the Si O₂ for forming the petrified woods probably comes from the underground water heated by magma activity.The Q0058 section is dominated by the fine-and medium-grained sandstone with low composition and textural maturity,and parallel beddings.The sand bodies are outspread and extend gently to both sides.All these features indicate a shore-lacustrine environment.The grain-size distribution analysis shows these sediments are river-originated,indicating the large amount of terrigenous loads by river filled into the lake.Silicified woods are easily affected by environmental factors,including the variation of humidity and temperature,and plant growth,so each silicified wood should take specific conservation methods based on its real situation.The general conservation procedures for silicified woods are as follows:1.Measuring the dimensions of the silicified woods and recording their conservation status by photos;2.Cleaning,including mechanical cleaning,chemical cleaning,liquid cleaning;3.Original location identification and fixation of the broken fossil pieces;4.Aesthetic restoration;5.Hydrophobation;6.Documentation of the conservation-monitoring and review.