Characteristics and Formation of Salt Structures in Baicheng Sag, Kuqa Fold-and-Thrust Belt
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摘要: 关于沉积差异负载对褶皱-冲断带中塑性盐岩变形的影响仍然不清楚，库车褶皱-冲断带拜城凹陷受到的构造挤压变形微弱，凹陷中沉积了巨厚的盐上覆层，是研究沉积差异负载如何影响塑性盐岩变形的天然实验室.根据野外地表调查结果、钻井资料和二维地震剖面，约束库车褶皱-冲断带拜城凹陷盐构造特征，并探讨盐构造成因机制.盐顶构造图揭示拜城凹陷盐上覆层发育东、西2个沉积中心，东沉积中心位于拜城县附近，西沉积中心位于BZ1井南侧，二者之间(大宛齐乡附近)形成大宛齐盐枕和拜城凹陷鞍部.钻井资料和地震剖面显示：(1)东、西沉积中心下伏盐岩流动减薄，二者之间盐岩聚集加厚；(2)大宛齐盐枕核部盐岩聚集发生于上新世晚期-现今；(3)上新世末期-现今，拜城凹陷鞍部下伏盐岩聚集加厚，盐岩厚度往东、西沉积中心逐渐减薄，形成东西向背形构造.基于盐构造特征和前人研究成果，提出库车褶皱-冲断带拜城凹陷盐构造相关的盐岩流动变形模型，认为沉积差异负载在库车褶皱-冲断带盐岩变形中发挥着重要作用：(1)在挤压应力和沉积差异负载作用下，以东、西沉积中心为圆心，盐岩向四周放射状流动；(2)盐岩变形以沿挤压方向(N-S)流动为主，但剖面间(E-W)流动也很明显.Abstract: The role of differential sedimentary loading in deformation of viscous rock salt in contractional setting is unclear. Combined with field observation, well and seismic data, salt structures in Baicheng sag, Kuqa fold-and-thrust belt were constrained, and their formation mechanisms were investigated. The structural map of top salt shows that two depocenters were developed in Baicheng sag, eastern depocenter near Baicheng County and western depocenter to the south of well BZ1, respectively. Dawanqi salt pillow and saddle of Baicheng sag were developed between two depocenters. Seismic and well data reveals that: (1) the salt layer below two depocenters is very thin, while thick salt layer has accumulated between two depocenters; (2) Dawanqi salt pillow formed from the Late Pliocene to present; (3) the saddle of Baicheng sag, which is an antiform structure in E-W trend, formed from the end of Late Pliocene to present. Based on our investigations and previous studies, we propose a model showing salt flow related to salt structures in Baicheng sag, Kuqa fold-and-thrust belt, and it suggests that differential sedimentary loading plays an important role in Kuqa fold-and-thrust belt: (1) under bulk shortening and differential sedimentary loading, salt flew out of depocenter in three-dimensional; (2) salt flew dominantly along the direction of contraction (N-S) and also obviously along strike (E-W).
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图 1 库车褶皱-冲断带构造单元和拜城凹陷(研究区)
库车褶皱-冲断带位置见(b)插图；数字高程模型(DEM)数据来自USGS SRTM 90 m(http://srtm.csi.cgiar.org)，垂向比例尺放大3倍
Fig. 1. A sketch map showing the structural units in the Kuqa fold-and-thrust belt and the location of Baicheng sag
图 2 库车褶皱-冲断带地层综合柱状图(据Chen et al., 2004; Tang et al., 2004a修改)
Fig. 2. Comprehensive stratigraphic column in Kuqa fold-and-thrust belt
图 3 库车褶皱-冲断带拜城凹陷盐顶构造
Fig. 3. Structural map of top salt in Baicheng sag, Kuqa fold-and-thrust belt
图 5 (a) 过大宛齐盐枕的南北向解释剖面C-C'，标注了与剖面F-F'(图 6b)相交的位置；(b)过大宛齐盐枕的东西向解释剖面D-D' (剖面位置见图 1, 3)
Pre-Cz.前新生界；E1-2km.古-始新统库姆格列木组；E3s.渐新统苏维依组；N1j.中新统吉迪克组；N1k.中新统康村组；N2k.上新统库车组；Q.第四系(图 6同)
Fig. 5. (a) Interpreted North-South trend seismic section C-C' across Dawanqi salt pillow, and marked the location of intersection between section C-C' and section F-F' (Fig. 6b); (b) interpreted East-West trend seismic section D-D' across Dawanqi salt pillow
图 6 (a) 过秋里塔格构造带和拜城凹陷的南北向解释剖面E-E'；(b)过拜城凹陷鞍部的东西向解释剖面F-F'；(c)拜城凹陷鞍部生长地层特征(剖面位置见图 1，3；图例说明见图 5)
Fig. 6. (a) Interpreted North-South trend seismic section E-E' across Qiulitage structural belt and Baicheng sag; (b) interpreted East-West trend seismic section F-F' across saddle of Baicheng sag; (c) patterns of growth strata related to the saddle of Baicheng sag
图 7 在挤压应力和沉积差异负载作用下，盐岩二维剖面内流动变形特征(a)和平面上流动变形特征(b)
二维剖面内盐岩从向斜下伏流出减薄，流入背斜核部聚集加厚；平面上以沉积中心为圆心，盐岩向四周放射状流动
Fig. 7. Illustrations showing that lateral salt flow in two-dimensional cross section (a), and in map view (b) under bulk shortening and differential sedimentary loading
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