西藏南部江孜盆地的演化特征

Development of Jomzi Basin in Southern Tibet

位于西藏南部江孜盆地的沙拉岗矿区发育一套晚侏罗世至早白垩世的碎屑流、滑动与浊流形成的碎屑岩夹硅质岩与灰岩的沉积。在野外识别出 8个三级层序 ,包括 2个Ⅰ类层序和 6个Ⅱ类层序 ;建立了层序地层格架。在此基础上将处于斜坡背景的沉积演化大致划分为 4个阶段 :(1)相对陡的陆源斜坡阶段 ;(2 )相对平缓的陆源斜坡阶段 ;(3)硅质海底扇发育阶段 ;(4 )陆源海底扇发育阶段。

The gravity flow depositions were formed by the debris, sliding and turbidity flows in Late Jurassic Early Cretaceous in Shalagang ore area 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, including two type 1 and six type 2 sequences. The fromework 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 about 40 Ma and the estimated average time of every three order sequence was 5 Ma. Sequence 1(SQ1) 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 calcious slide bodies and minor turbidity 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 stratigraphy, depositional developments of slope setting are divided into 4 stages: (1) Relatively steep terrigenous slope, corresponding 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) Relatively 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 sandy sliding 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 gravity flow sediments, and slope deepened again. (4) Terrigenous submarine fan, corresponding to SQ5. Similar to SQ5, SQ6, SQ7 and 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.