Development of Jomzi Basin in Southern Tibet

Key words: 
• sliding sediment /
• submarine fan /
• sequence stratigraphy /
• depositional development /
• Jomzi basin

Abstract: The gravity flow depositions were formed by the debris,sliding and turbidity flows in Late Jurassic-Early Cre-taceous in Shalagang ore afea of Jomzi basin located at southern Tibet．which consisted of the slope clastic rocks and interbeds of silicalites and limestones．8 three-order sequence stratigraphies are divided,induding two type-1 and six type-2 sequences．The framework of sequence stratigraphy are established．These sequences,with clear three-division structures,were characterized by the gravity-flow sediments of deep-water slope,developed within about40 Ma and the estimated average time of every three-order sequence was 5 Ma．Sequence l(SQl)of type 1 consisted of the gravity-flow sediments of deep-water slope．The SMST of SQ2 of type 2 was composed of the prograded and aggraded sequences of lower slope channel sandstones and conglomerates to upper slope black shales which interbeddded with sliding sandy-sediments and turbidity sandstones．The TST consisted of the retrograded black shales interbedded with turbidity fine-siltstones of the lower slope．the CS consisted of thin-bedding lamella limestones,and the HST consisted of the progradded black shales interbedded with sandy-and calcios-slide bodies and minor turbidty siltstones of the upper slope．The sliding sediments characterized the SQ3 of type 2．The SQ4 of type 2 consisted of siliceous turbidities．SQ5．SQ6 and SQ7 of type 2 were composed of sediments of terrigenous submarine fan．The SQ8 of type 1 were also composed of sediments of terrigenous submarine fan,and extending and reactivating of channel of inner fan towards basin was indicator of the sequence． As a whole．the formation of sequences was closely related to sea-level changes,type 1 or 2 boundaries and sequences were respectively created,corresponding to fast and slow falling．Based on study of sequence stragraphy,depesitional development of slope setting are divided into 4 stages： (1)Relatively steep terrigenous slope,corresonding to SQ1 and SQ2．In early time,calcious and terrigenous debris-flow sediments were accumulated in relatively steep slope toe．In late time,some parts of slope were cut by channel,and slope channel filling-sequence with debris-flow conglomerates and conglomerate-bearing sandstones was formed．(2)Reatively gentle terrigenous slope,corresponding to SQ3.with rising of sea level and deepening of water body,debris-flows lacked or slightly developed． It implied that slope was becoming gentle,but sandyslid bodied characterized slope．(3)Siliceous submarine fan,corresponding to SQ4.With continuos rising of sea level and halting of terrigenous supply,siliceous submarine fan was dominated over formal terrigenous gravety-flow sediments,and slope deepened again．(4)Terrigenous submarine fan,corresponding to S05．Similar to SQ5,SQ6,SQ7and SQ8 were also characterized by the development of terrigenous submarine fan．In the end of the forth stage,with large-size fast falling of sea-level once again,greatly-thick slope escallication which switched 500--600 Km and distributed widely along Shalagang-Jomzi-Zhongba areas and stopped the growth of submarine fan．After Late Cretaceous,Jomzi basin was gradually filled out by shallow-water sediments and disappeared．