Character istic and GeneticMechan ism of Pyroclastic Base-Surge Deposits of Volcanic Swarm in Longgang Area, Jilin Province
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摘要: 吉林龙岗火山群火山碎屑基浪堆积是中国少数保存较好的、近代喷发的低平火山区之一。基于岩性、岩相与相序的识别与分析,火山碎屑基浪堆积序列由分选性和磨圆度较差的玄武质砂、砾和火山灰构成的毫米级—厘米级厚高频率韵律有序叠置而成,堆积物中发育大量的块状层理、似丘状层理、低角度板状交错层理、槽泊层理、平行层理、冲蚀槽等堆积构造。横向上低平火山由内至外其碎屑粒度、堆积构造、厚度存在着一定规律变化,与易混淆的火山岩区地面流水沉积和火山碎屑流堆积物存在明显的差别。岩浆射汽喷发晚期往往伴随斯通博利式喷发和夏威夷式熔岩流,三者构成一个完整火山活动旋回。Abstract: Pyroclastic base-surge deposits of volcanic cluster in the Longgang region of Jilin Province is one of the few neoteric maar depositswhich are preserved in good condition in China. Based on the lithological characters, lithofacies and sequences, pyroclastic base-surge sequence consists of millimeter-scale to centimeter-scale frequent cyclothem that constitute a regularly vertical stacking pattern, which is made of sub-edge angle and worse graded basaltic sand, lap illus and tuff ash. Mass bedding, dune-like structure, low angle cross-bedding, chute and pool structure, parallel bedding and scouring through are widely developed in base-surge deposits. Pryoclastic granularity, de- posit structures and thickness are developed with a regularly transverse discip linarian from inner to external of maar. The characteristic of pyroclastic ejecta ismore differentwith ignimbrite and alluvial and /or diluvial deposits from vol- canic cone. B its of strombolian-type scoria and Hawaii-type effusive lava may accompany lately after Phreatomagmatic erup tion, they constitute a erup tive activity cycle in its entirely.
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Key words:
- maar /
- base-surge deposits /
- genetic mechanism /
- Longgang area in J ilin Province
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