柴西红沟子地区晚新生代沉积特征及其对阿尔金山隆升的约束

柴达木盆地是青藏高原东北缘的一个山间盆地，发育了巨厚的中-新生代地层，详细记录了盆地及周缘山地的构造活动历史。本文通过对柴达木盆地西北部厚约1040m并具有精确古地磁年代控制的红沟子剖面（包括下油砂山组、上油砂山组、狮子沟组和七个泉组）开展精细沉积学研究，包括岩性特征与沉积相、砾岩与砂岩含量变化、剖面沉积速率变化特征以及沉积区物源分析等，结果表明：1）红沟子剖面晚新生代沉积相经历了下油砂山组（>17.0Ma） 的冲积扇或扇三角洲平原相到上油砂山组（17.0~9.8Ma）扇三角洲前缘-半深湖-扇三角洲前缘相，再到狮子沟组（9.8~5.0Ma）扇三角洲前缘相，最后转为七个泉组（16.5Ma、9.8Ma以及16.5~5.0Ma）沉积物可能主要来自北部的阿尔金山地区。基于上述认识，将红沟子剖面晚新生代沉积划分为5个演化阶段：1）>16.5Ma 主要以粗颗粒砾岩为主的冲-洪积相沉积；2）16.5~9.8Ma 沉积环境主要为半深湖或前扇三角洲，以泥岩或泥灰岩等细颗粒为主，沉积速率较低；3）9.80~8.53Ma 沉积环境为扇三角洲前缘，岩性变粗且有大套砾岩堆积，沉积速率达到剖面最高值，并在9.8Ma存在不整合面；4）8.53~7.48Ma沉积环境基本与上一阶段类似，砾岩含量减少，砂岩沉积相对增加，沉积速率呈现低值特征；5）7.48~5.00Ma 主要是扇三角洲前缘沉积环境，砂岩含量较高，沉积速率较高。因此，进而分析指出构造活动是影响研究区沉积特征演化的主要因素，并建立了研究区及周缘新生代构造与沉积特征演化模式图。综合对比前人资料，发现红沟子剖面沉积特征在9.8Ma的变化，特别是沉积速率的陡增在阿尔金山具有普遍性，与前人不同方法捕获的晚新生代阿尔金山乃至整个高原东北部地区的构造隆升时间基本一致，认为红沟子剖面沉积特征在9.8Ma的突变可能是对阿尔金山乃至青藏高原东北缘约10Ma一次大范围隆升事件的响应。

Late Cenozoic sedimentation characters of Honggouzi area in the western Qaidam Basin and its constraints on the uplift of the Altyn Tagh

As an intermountain basin in the northeastern margin of the Tibetan Plateau, the Qaidam Basin developed thick Mesozoic-Cenozoic sediments that have recorded the tectonic evolution history of the basin and the peripheral mountains. This study presents a high-resolution sedimentary records from the Middle Miocene to the Early Pliocene 1040-thick sediments of the Honggouzi(HGZ)section(38°34'N, 91°03'E), located in northwestern Qaidam Basin. The oldest strata developed in HGZ section are Jurassic clastic rocks, covered by the Xia Youshashan Formation, Shang Youshashan Formation, Shizigou Formation and Qigequan Formation in an upward sequence, which were well-dated to be >17.0Ma, ca.17.0~9.8Ma, ca.9.8~5.0Ma, and 17.0Ma) to the fan delta front/semi-deep lake/fan delta front environment of the Shang Youshashan Formation(17.0~9.8Ma), and transferred from fan delta/shallow lake/fan delta of the Shizigou Formation (9.8~5.0Ma)to the diluvial fan of Qigequan Formation (16.5Ma, 9.8Ma and 16.5~5.0Ma. Based on the above, the Late Cenozoic sedimentary evolution of the HGZ was divided into five stages : (1) >16.5Ma, the alluvial-pluvial deposition mainly composed of coarse conglomerates; (2) 16.5~9.8Ma, the semi-deep lake/fan delta depositional environment with fine grained mudstone or marlstone and relatively low sedimentation rate; (3) 9.80~8.53Ma, in the fan delta front environment, the lithology became coarsen with massive conglomerate, the sedimentation rate of the section reached the highest and the unconformity developed at 9.8Ma; (4) 8.53~7.48Ma, the similar sedimentary environment as the previous stage with the decreasing of the conglomerate, increasing of the sandstone and the lowering sedimentation rate; (5) 7.48~5.00Ma, the fan delta front environment with high sandstone percentage and relatively high sedimentation rate. Those results indicated that the tectonic activity was the main driving factor for the sedimentary evolution in the studied area. We further proposed an evolution model showing the relations between the Cenozoic tectonic evolution and the sedimentary characters of the studied area and surroundings. Compared with other published results, the event of the sedimentary changes of HGZ at 9.8Ma, especially, the widespread sharp increase of the sedimentary rate in Altyn Tagh ranges, was in accordance with the tectonic uplift of the northeastern Tibetan Plateau, which suggested that the northeastern Tibetan Plateau and the Altyn Tagh Mountains has experienced significant tectonic activities around 10Ma.