Characteristics and source-sink response of the Lahar deposits in Changbaishan-Tianchi volcano and adjacent area following the Millennium Eruption

源于长白山天池地区的火山泥石流沉积可分为粗碎屑岩块(岩屑)泥石流和细碎屑浮岩泥石流,它们沿二道白河和松花江水系搬运的路径为从距天池火山口40km的三合水电站经过丰满大坝(360km)和吉林市(380km)到小白旗屯(450km),形成广泛的沉积区域。这两类火山泥石流的沉积成因有两种解释:一是形成于千年大喷发同期,是由一次性洪水事件搬运和沉积形成的;二是形成于千年大喷发期后经过多次搬运和沉积的产物。两个模式的共同问题是都没有考虑天池当时是否有水及其蓄水过程。后一模式在某种程度上,还回避了导致岩屑与浮岩两类泥石流频繁互层的沉积物源和水动力条件以及二者的转换机制,而这恰恰是关于泥石流沉积成因的基本要素。通过重新研究火山泥石流经典剖面(位于天池西北57.73km的水田村),作者发现本区火山泥石流沉积存在明显的物源剥蚀区与沉积堆积区的反剖面关系。即无论是粒径32~500mm的粗碎屑还是0.0625~16mm的细碎屑,成分自下而上(或沉积早期到晚期)呈现规律性变化:剖面下部的碎屑成分以浮岩为主(浮岩在物源区位于顶部),向上粗面岩和玄武岩明显增多(在源区它们位于浮岩之下),而沉积序列上部的碎屑成分是在物源区处于较深层位的岩脉辉绿岩和基底流纹岩。整个序列碎屑成分的沉积分异特征明显。沉积构造和岩相组合特征显示,该火山泥石流剖面的下部和上部碎屑粒度细、分选较好、成层性好、水平状层理发育,主要表现为环境较为稳定的以地面径流为主的河流相和末端扇相背景沉积;中部粒度粗、成层性差、主要表现为突发性洪水作用导致的洪积相事件沉积。沉积序列中频繁出现的冲刷面构造指示水流强度曾出现周期性的快速增加。自下而上冲刷面规模由小变大再变小,指示水流强度由弱变强再变弱。为了探讨天池的积水条件和蓄水过程,作者基于达西定律和质量守恒原理,模拟计算降水量、蒸发量、地表径流量、火山机构整体的平均渗透率和天池积水速率之间的关系。结果显示,当天池火山机构平均渗透率高于6mD(毫达西)时,天池地区降水量减蒸发量即使高达2000mm/y,水亦会全部渗流而出,因此天池不存在积水环境。当降水量减蒸发量小于1500mm/y时,则天池火山体平均渗透率需要小于4mD,天池才可能在200年之内集满现今的水量。当天池降水量减蒸发量小于1000mm/y时,天池火山体平均渗透率需要小于2.5mD,天池才可能在200年之内集满现今的水量。将水田村火山泥石流沉积序列与天池蓄水过程计算结果加以对比,我们提出本区火山泥石流沉积序列的另一种成因解释:(1)这是形成于千年大喷发之后的以地面径流或河流为主的背景沉积与洪水导致的突发性事件沉积互层的序列;上部和下部的细碎屑层主要表现为背景沉积,中部的粗碎屑岩块泥石流主要表现为洪流事件沉积。(2)下部的背景沉积可能对应于天池千年大喷发之后的持续积水过程,时间可能不少于200年;而上部的背景沉积则对应于本区的水系和地貌逐渐稳定并接近于现今条件的稳定型河流沉积。结合天池北坡和西坡古老树木年轮指示的沙松冷杉生长年代(公元1749-1768)同时考虑松柏类植物对水系和地貌稳定性较为敏感等因素,推测上部沉积环境趋于稳定的时间应该不晚于公元十八世纪初。;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.