晋陕峡谷北段晚新生代河流演化初步研究

晋陕峡谷黄河形成的时代是地学界十分关注的科学问题,长期存在比较大的意见分歧,至今没有得到比较好的解决。山西保德地区发育一套晚中新世-早上新世的河湖相沉积,年代为>8.3～3.7Ma, 地貌与地层分析表明,它们是唐县期夷平面的相关沉积。河湖相地层所含砾石层的组构和岩性统计揭示,当时汇入古湖的河流是发源于周围抬升区的短小河流,形成一个局地规模的湖泊水系。晋陕峡谷北段唐县期夷平面之上分布两期红粘土覆盖的河流砾石层,形成年代分别在4.9Ma前和3.7Ma前; 砾石组构和岩性统计揭示,它们所指示的河流流向向北,与黄河流向相反,是一个不同于现代黄河的古河流,它将保德古湖水系与河套盆地联系起来。3.7Ma前,强烈的地面上升导致河湖相沉积结束和唐县期夷平面抬升,风成红粘土堆积其上,地形起伏增大,河流溯源侵蚀加剧,水系进入重新组织阶段; 1.2Ma前,黄土高原南部的河流将原北流水系袭夺,黄河形成; 最近1.2Ma的强烈地面上升,导致黄河不断下切并发育多级阶地,逐渐形成现代壮观的深切峡谷地貌。

TIME-SLICE OF THE FLUVIAL EVOLUTION IN THE NORTHERN JINSHAAN GORGE DURING LATE CENOZOIC

The formation time of the Yellow River in the Jinshaan Gorge is generally regarded as a significant scientific question,attracting wide attention in earth sciences.However,some views for it diverge so greatly that we can not reach a consensus for this challenging issue until now.Late Miocene and Early Pliocene fluviolacustrine sediments with chronological framework >8.3～3.7Ma were accumulated in the Baode region(Shanxi Province).Geomorphic and stratigraphic analysis suggests that they are correlated sediments of the Tangxian Planation Surface.The statistics of gravel fabric and lithology in these fluviolacustrine sediments reveals that a paleo-lake occupying the Baode area was fed by some local streams from surrounding uplifted regions,and gradually growing into a local lacustrine drainage.In the northern Jinshaan Gorge,two fluvial gravel layers covered by Red Clay were stacked on the Tangxian Planation Surface.Their formation times were dated prior to 4.9Ma and 3.7Ma respectively.The statistics of gravel fabric and lithology indicates that these gravel layers represent a northward flowing paleo-river,which is different from the Yellow River which flows southward.It linked the paleo-lake occupying the Baode region and the drainage in the Hetao Basin.A dramatic surface uplift initiating prior to 3.7Ma not only interrupted the fluviolacustrine sedimentation,but also leaded to lifting of the Tangxian Planation Surface.These exposed surfaces were then immediately covered by continuous aeolian Red Clay.Owing to this drastic uplift,hypsographic relief was enlarged,and resulting in fluvial head erosion and reorganization of river system.In the northern Jinshaan Gorge,the northward flowing river linking the Baode paleo-lake and drainage in the Hetao Basin was pirated by the river in the southern Loess Plateau,and the Yellow River appeared prior to 1.2Ma.An episode of drastic uplift initiating at 1.2Ma forced the Yellow River to continuously downcut along the Jinshaan Gorge,and developing a series of fluvial terraces.The modern spectacular down-cutting valley has been shaped gradually by the Yellow River since this uplift.