西藏日喀则地区早白垩世恰布林组辫状河-扇三角洲沉积

雅鲁藏布江蛇绿岩带北侧发育的早白垩世恰布林组以其杂色砂砾岩易于野外识别，岩石组成主要为陆源碎屑岩。根据不同的形成机制和岩相特征，将恰布林组岩相划分为7种砾岩相、3种泥岩相和1种灰岩相，由下至上组合成辫状河亚相、扇三角洲平原亚相、扇三角洲前缘亚相和前扇三角洲亚相，呈现急剧退积型扇三角洲层序。沉积的古盐度在低盐度-半咸水范围，古环境判别也落入未定义-海水区，认为恰布林组沉积环境为陆相辫状河-退积型扇三角洲。物源区分析显示，来自于北方近源再旋回造山带中的板块俯冲混杂岩和火山切割弧至过渡弧为恰布林组提供主要物源，而且该组砾岩砾石成分含有大量蛇绿岩成分组合，从而说明早白垩世时期恰布林组北侧不远处极可能发育一套残余的蛇绿岩。

Braided River Fan Delta Depositional System of the Early Cretaceous Giabulin Formation in the Xigaze Region,Xizang(Tibet)

The Early Cretaceous Giabulin Formation, which is distributed along the north margin of the Yarlung Zang bo ophiolite suite, is easy to distinguish in the field. It is characterized by a sequence of ter-rigenous clasticrocks, principally mottled sandstones and conglomerates. Seven types of conglomerate, seven types of sandstone, three types of shale, and one limestone may be classified according to formation and their lithofacies characteristics. They comprise foursedimentary subfacies, from bottom to top: braid-ed river subfacies, fan-delta plain subfacies, fandelt a front subfacies, and profan-delt a subfacies. This progressio n makes up a retrog radational sequence taking together the global Cretaceous transgress con-dition. Paleosalinity of the Giabulin Formation was 12.0‰-20.0‰, and its paleoenvironments were inter-mediate between semi-brackish water and salt water. The sedimentary environments of the braided river of terrestrial facies and the fan-deltas of transitional facies can be concluded from the discussion above. Sediments of the Giabulin Formation were derived chiefly from recycled subduction complexes, and from dissected transitional island arcs. Those provenances were all distributed nearby to the north of the Yarlung Zang bo River. During deposition of the Giabulin Formation, the Tectonic settings was relatively passive in the early stage and active in the late stage.