Abstract: Recently,four fossil ice-wedges have been found at the outcrop excavated,at the north end of the Plio-Pleistocene Dayi Conglomerate Profile on the west bank of the Xiejiang River near Dayi Nitrogenous Fertilizer Factory,west of the Chengdu Plain.Its co-ordinates are 30°35'N,103°31'E,and 530m above sea level.
These ice-wedges have been developed on the top of the outcrop of Dayi Conglomerate.The largest wedge is 3m wide at the top,and extends 2.5m vertically.The wedges extend vertically downwards,and obliquely intersect the bedding planes of the Dayi Conglomerate.They thus postdate deposition of the Dayi Conglomerate and appear to be epigenetic.The sediments in the ice-wedges are different from the Dayi Conglomerate.The former are brownish yellow,well-sorted,with 5cm~15cm clasts in a fine-grained matrix.The latter is grey-white or yellow-grey,poorly-sorted,with 20cm~30cm clasts.The ice-wedge sediments have a higher percentage of gravel(95%) compared to the Dayi Conglomerate(85%).In addition,the distribution of ab planes for pebbles in the ice-wedge sediments is random,whereas ab planes in the Dayi Conglomerate are dominantly orientated 170°~210°∠30°~40°.According to these differences,the ice-wedges can be distinguished from outcrops of the Dayi Conglomerate.The age of sediments in the ice-wedges is 0.171 Ma from ESR data.This suggests that these ice wedges might correspond to stage 6(0.195Ma~0.128Ma) of the oxygen isotope temperature curve from deep sea-core V28-238.This is similar to the climatic fluctuations recorded by L_2~5 (0.195Ma~0.180Ma) and L_2~4(0.180Ma~0.167Ma) of Chinese loess,and the penultimate glacial stage in Qinghai-Xizang Plateau.It is generally considered that ice-wedges are indicative of permafrost,therefore the samples we obtained suggest that permafrost developed in the Chengdu Plain during the Pleistocene.
It has been reported that permafrost can only develop below -6℃ in high-latitude areas,and the southernmost extent of permafrost occurrence is equivalent to the -2℃ to -4℃ isotherm.However,the annual air temperature in Dayi is about 16℃ now,which means the temperature during ice wedges formation may have been at least 18℃ less than present.