Development history and activity characteristics of typical debris flows in the Grand Bend of the Yarlung Zangbo River since the Holocene
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摘要:
雅鲁藏布江大拐弯附近晚更新世末次冰期—全新世发育多期次泥石流, 组合形成了现代大规模扇形堆积体。以派镇蹦嘎沟泥石流为例, 采用地面调查、钻孔及14C测年等方法, 研究泥石流形成年代序列、堆积深度、冲出范围等特征, 分析结果表明: 现代蹦嘎沟依然有小规模的支沟泥石流发育且广泛堆积于沟道内, 现存堆积扇区域尚未发现泥石流堆积; 距今8500年左右为蹦噶沟全新世泥石流活跃期, 单期次累积堆积深度约10.9 m; 滨湖浅水相沉积(河流相)形成的浅灰色粉细砂中的两处碳样表明雅鲁藏布江现代河床在40~100年左右沉积深度约0.4 m, 年平均沉积速率4~10 mm; 海拔2906.1~2896.7 m及2849.4~2848.2 m处钻孔依次揭露厚度为9.4 m和1.2 m饼状青灰色粉质黏土, 推测发生两次堵江事件。上述结果可为该区域全新世以来泥石流活动性特征研究提供参考。
Abstract:Multi-period debris flows have been developed in the last glacial period of the late Pleistocene-Holocene near the Grand Bend of the Yarlung Zangbo River in southeast Tibet, which combined to form a modern large-scale fan-shaped accumulation. The debris flows in the Bangga gully, Pai Town, were explored by ground survey, borehole, and 14C dating methods to investigate the chronological sequence of formation, accumulation depth, and outrush range. The analysis results show that there are still small-scale debris flows in the tributaries of the Bengga gully, and they are widely accumulated in the channel, but no debris flow accumulation has been found in the existing accumulation fan area. The Holocene debris flows in the Bunga gully were active around 8500 years ago, and the cumulative accumulation depth of a single period is about 10.9 m. The two carbon samples in the light gray silt sand formed by the shallow lake facies (fluvial facies) show that the modern riverbed of the Yarlung Zangbo River was deposited at a depth of about 0.4 m in 40 to 100 years, and the annual average deposition rate was about 4~10 mm. The boreholes at 2906.1~2896.7 m and 2849.4~2848.2 m above sea level reveal a thickness of 9.4 m and 1.2 m cake-like bluish-gray clay in turn. It is assumed that two river-blocking events occurred. The above results could provide a reference for the study of the debris flow activity characteristics since the Holocene in this region.
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图 6 蹦嘎沟新近泥石流特征
a—堆积扇形态;b—扇顶稀性泥石流沉积;c—流通区漂木及漂石;d—主沟流通区泥石流堆积
Figure 6. Photos showing the features of recent debris flows in the Bengga gully
(a) Morphology of the alluvial fan; (b) Debris flow sediments at the fanhead; (c) Driftwood and boulder in the inundation area; (d) Debris flow accumulation in the inundation area
表 1 测年取样样品及其位置统计表
Table 1. Statistical table of the samples for dating and their locations
样品位置及取样编号 样品照片 表 2 漂木样品测年结果
Table 2. Dating results of the driftwood samples
样品编号 埋深/m 放射性碳年龄 公历校正 测年概率/% ZK135-1 12.6 7120+/-30 a B.P. 6062—5977 cal.B.C. 76.0 5948—5919 cal.B.C. 19.4 ZK135-4 13.0 7220+/-30 a B.P. 6100—6011 cal.B.C. 76.7 6219—6135 cal.B.C. 18.7 ZK135-3 38.5 7810+/-30 a B.P. 6696—6568 cal.B.C. 93.4 ZK139-2 61.0 9370+/-40 a B.P. 8753—8547 cal.B.C. 93.7 ZK139-1 61.8 9420+/-30 a B.P. 8792—8622 cal.B.C. 95.4 表 3 蹦嘎沟泥石流物源分布及时间变化特征
Table 3. Source distribution and temporal variation characteristics of the sources in the Bengga gully
编号 类型 面积/m2 厚度/m 体积/×104 m3 影像日期 BH01 崩滑物源 95734 4.21 40.29 2007-9-17 BH02 崩滑物源 26254 3.13 8.21 2007-9-17 BH03 崩滑物源 44733 3.54 15.81 2007-9-17 BH04 崩滑物源 51883 3.66 18.98 2007-9-17 BH05 崩滑物源 12012 2.61 3.14 2007-9-17 BH06 崩滑物源 76095 3.99 30.39 2007-9-17 BH07 崩滑物源 483453 6.10 294.98 2013-10-9 BH08 崩滑物源 67118 3.88 26.04 2013-10-9 BH09 崩滑物源 126595 4.49 56.81 2013-10-9 BH10 崩滑物源 134525 4.55 61.21 2014-11-8 BH11 崩滑物源 5820 2.21 1.29 2017-12-4 BH12 崩滑物源 8658 2.43 2.10 2017-12-4 BH13 崩滑物源 7208 2.33 1.68 2018-12-26 BH14 崩滑物源 15242 2.76 4.21 2021-1-23 GD01 沟道堆积 4149 2.05 0.85 2007-9-17 GD02 沟道堆积 16190 2.80 4.53 2007-9-17 GD03 沟道堆积 47282 3.58 16.93 2014-11-8 GD04 沟道堆积 71395 3.94 28.09 2018-12-26 -
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