Character istic and GeneticMechan ism of Pyroclastic Base-Surge Deposits of Volcanic Swarm in Longgang Area, Jilin Province

Key words: 
• maar /
• base-surge deposits /
• genetic mechanism /
• Longgang area in J ilin Province

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.