| PDF: | |
| Time: | 2020 |
| Journal: | Acta Petrologica Sinica |
| Volumn: | 36 |
| Issue: | 9 |
| Pages: | 2893-2910 |
| Type: | SCI |
| Author: | Pujun Wang,Weidong Xu,Haichao Chen,Jian Yi,Hanfei Wang,Huafeng Tang,Wenhua Wang,Chengzhi Wu |
Abstract: | Lahars generated from the Tianchi crater lake in the Changbai Mountain area include coarse-grained lithic and fine-grained pumice deposits. They were transported along the Erdaobaihe River and Songhua River systems from the nearest Sanhe waterpower station being 40km away from the crater, through Fengman Dam (360km) and Jilin City (380km) to the far most Xiaobaiqitun 450km away from the crater lake. All the way along the lahar flows left behind widespread lahar deposits in the region. There are two explanations concerning formation of these volcanic lahar deposits. Firstly, they were formed by a sudden release of the giant lake water caused by dam break during the Millennium Eruption (ME) which occurred about CE 946±3 in the Changbaishan-Tianchi volcano, and all the pyroclastic materials were transported and deposited by the same off flood event. The second is that the lithic and pumice lahar deposits were separately formed by two episodes of transportation and deposition events after the ME. The question with the two models is that the water was not considered, i.e. whether or not there was any water in the crater lake at that time during the ME and how could the water accumulate. And more so, The latter scheme potentially avoided, to some extent, the sediment source, hydrodynamic condition and the mechanism on the transition that led to frequent inter bedding of the lithic and pumice lahars, those are, of course, important factors concerning depositional process of the lahars. We challenge the above explanations and researched in detail the well-known examples of the so-called classic lahar successions outcropped in the Shuitian Village which is 57.73km northwest from the Tianchi crater. We found that there are steady state of inversely back-striping relationship between the source denudation area and the sedimentary accumulation process. That is to say, both coarse debris in diameter 32~500mm and fine debris of 0.0625~16mm, their components change in upward sequence is as follows. The fragments in the lower part of the section are mainly pumices which are located on top in the source. Going up there is a significant increase in compositions of trachyte and basalt which lie under the pumice deposit in the source. And the detrital components in the upper part are vein diabase and basement rhyolite which are situated in the deeper horizon in the source part. In addition, Sedimentary structure and lithofacies associations show that the lower and upper pumiceous lahar deposits are fine-grained, well-sorted, well stratified and horizontally bedded, indicating fluvial and terminal fan facies formed in a relatively stable environment of downslope flow runoff. While the coarse-grained lithic lahars in the middle part are poorly stratified, and are diluvial event deposits that were most likely caused by sudden flooding. The frequently occurred scour surfaces in the sequence indicate that the water current may have periodically rapid increase. From bottom to top, the scale of the scour surfaces changes from small to large and then back to small, indicating paleo-current fluctuations of water flow regime. In order to figure out water storage of the Tianchi crater lake, We modeled and calculated the relationship amongst precipitation, evaporation, surface runoff, average permeability of the volcanic architecture as a whole and the water storage velocity in the lake based on the Darcy\\\'s Law and the principle of mass conservation. The results indicated that when the average permeability of the volcanic architecture was higher than 6mD (Millidarcy). Water would all leak out, and there was no water accumulation condition in the Tianchi crater lake, even if the precipitation minus evaporation went up to 2000mm/year in the region. When the precipitation minus evaporation was about 1500mm/year, the average permeability of the volcanic architecture needed to be below 4mD, and the Tianchi crater could collect the present volume of water in at least 200 years. When the precipitation minus evaporation was about 1000mm/year, the average permeability of the volcanos should be below 2.5mD, and the Tianchi crater could collect the present water volume within at least 200 years. By comparing sedimentary sequence of the lahar deposits with the calculated results above, we proposed an alternative explanation for the formation of the lahar sequence in the Tianchi region. (1) This is a post-ME interbedding sequence of fluvial background and diluvial event deposits. The fluvials were down flow runoff or river deposits and the event deposits could be caused by sudden release of the Tianchi crater lake water. (2) The background sediments on the lower part may correspond to the continuous water accumulation process in the Tianchi lake following the ME, which may last over 200 years. The upper background sedimentation may correspond to the stable stage of the river system and landform which were gradually close to the present situation. Taking into account of the growth age of Ao. Les lolophylla maxim (CE 1749-1768) recorded by the old tree rings on the north and the west slopes of the volcano. Because coniferous plants are fairly sensitive to stability of water system and geomorphology, we preferably inferred that the stable state of the environment in Tianchi region may be no later than CE1700’s after the ME. |