Middle Pleistocene To Holocene Sedimentary Records In The Gas Hydrate Drilling Hole QK-6 From The Northern Qiangtang Area And Their Significances

In this paper,Quaternary sediments in the Gas Hydrate Drilling Hole QK-6 recovered by the National Natural Gas Hydrate Exploration and Exploitation Project in permafrost area of Northern Qiangtang Area in 2014 were studied.Based on using 2 ESR dating and 5 biological or sedimentary age marker records,this study assigned a middle Pleistocene age of 1311 ± 174 ka for the 246.40 m sedimentary sequences and established a stratigraphic framework for the studied well.Records of particle size of middle Pleistocene to Holocene sediments and its vertical change were obtained for this permafrost area.Furthermore,lacustrine facies and fan delta facies were determined and they were subdivided into 6 microfacies.The gravel composition in this hole was further studied.Based on change of gravel content,3 sedimentary intervals were recognized: low gravel content interval(246.40– 172.00m),gravel increasing interval(172.00-55.25m),and high gravel content interval(55.25-0m).Among them,roundness values of gravel particles in the lowest interval(246.40– 172.00m)are higher than the upper intervals.The petrologic and chemical composition of gravels were analyzed preliminarily4 sporopollen assemblages were identified,and they indicated that the process of gradual vegetation evolution from Middle Pleistocene dry herbs dominated woodland Steppe to Holocene arid grassland?It was found that the increase of gravel content in this sedimentary sequence can be compared with the tectonic uplift event(0.9~0.8Ma)of “the Kunlun-Yellow River Movement”on the Qinghai Tibet Plateau.Furthermore,although gas hydrates were not observed by this drilling,this study suggested that the Quaternary frozen soil characterized by loose,with abundant coarse gravel grain components and thick sedimentary sequence in this hole,should meet favorable sedimentary conditions for hydrate accumulation.