藏北改则--班戈地区下白垩统多巴组的高频旋回沉积特征与盆地性质

西藏北部改则——班戈地区的下白垩统多巴组中所发育的Ⅴ级旋回表现出3种垂向沉积序列:潮坪相碎屑岩沉积序列(Ⅰ)、浅海相碳酸盐岩沉积序列(Ⅲ)和碳酸盐岩、陆源碎屑岩混合沉积序列(Ⅱ)。其中序列Ⅰ-1和Ⅲ是正常的沉积水体向上变浅的沉积序列,而序列Ⅱ具有明显的陆源碎屑先进积后退积的特征,可进一步分成3个亚类:Ⅱ-1、Ⅱ-2和Ⅱ-3。横向上从南部物源区向北呈Ⅰ→Ⅱ-1→Ⅱ-2→Ⅱ-3→Ⅲ的变化过程。结合盆地构造背景和沉积特征,本文提出多巴组高频旋回的形成与弧背盆地南部冲断带的冲断作用相对应。冲断作用期,物源区粗碎屑物质大量进积,大于盆地基底沉降速率,沉积水体逐渐变浅;冲断间歇期,物源区碎屑物质的供应迅速减少,基底沉降速率逐渐大于碎屑物质的进积速率,沉积水体又有所加深。

DEPOSITIONAL FEATURES OF HIGH-FREQUENCY CYCLES IN THE DUOBA FORMATION, LOWER CRETACEOUS IN GERZE-BAINGOIN AREA, XIZANG

The three vertical depositional sequences, which were developed in the Duoba formation of Lower Cretaceous in the Gerze-Baingoin area, Tibet, were: (Ⅰ) clastic sequence deposited of tidal facies; (Ⅱ) combined depositional sequence of clastic and carbonate; and (Ⅲ) carbonate sequence of shallow-sea facies. Among which, the Ⅰ-1 and Ⅲ were depositional sequences characterized by shallowing water depth. However, the sequence Ⅱ was of typical terrigenous debris deposition with features of first progradation and then retrogradation, and therefore might be further divided into three sub-types: Ⅱ-1, Ⅱ-2 and Ⅱ-3. Laterally, the sequence changes might be from Ⅰ to Ⅱ-1 to Ⅱ-2 and Ⅲ from the south resource area to the north. Based on the basin tectonic background and depositional features, it was suggested in this paper that the form of high-frequency Duoba Formation might correspond with the upthrusting caused by upthrown fault block in the south of retroarc basin. During the upthrusting, the basin was deposited with abundant coarse debris sediments from the source area, and might surpass the basin subsidence, so as to shallowing the water body. In contrast, in the intermittent period of upthrusting, the water body was deeper due to the decreased input of the sediments which could not compensate the subsidence of the basin.