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冰川冻土, 2021, 43(3): 853-863 doi: 10.7522/j.issn.1000-0240.2021.0026

黄河中游响应气候变化和地表相对抬升发育阶地研究

潘保田,, 胡振波

兰州大学 资源环境学院 西部环境教育部重点实验室,甘肃 兰州 730000

The study on the terrace development by the middle reaches of Yellow River as a response to the climate change and relative surface uplift

PAN Baotian,, HU Zhenbo

Key Laboratory of Western China’s Environmental Systems,Ministry of Education,College of Earth and Environmental Sciences,Lanzhou University,Lanzhou 730000,China

编委: 周成林

收稿日期: 2021-05-10   修回日期: 2021-06-03   网络出版日期: 2021-07-29

基金资助: 国家自然科学基金项目“积石峡至龙羊峡段黄河的形成与地貌演化研究”.  41871001
国家自然科学基金专项项目“黄河流域地质地表过程与重大灾害效应”.  42041006

Received: 2021-05-10   Revised: 2021-06-03   Online: 2021-07-29

作者简介 About authors

潘保田,教授,主要从事地貌与环境演变研究.E-mail:panbt@lzu.edu.cn , E-mail:panbt@lzu.edu.cn

摘要

揭示河流系统响应气候变化和地表抬升的机制是理解流域地貌演化以及水系发育过程的基础,其核心难题是如何充分认识它们在阶地发育中扮演的角色。以往的研究倾向于分开讨论气候变化和地表抬升在河流阶地发育中的作用,认为河流堆积/侧蚀和下切行为分别与冰期和间冰期气候对应,或者将阶地作为地表抬升的直接证据。首先,从上下游河段对比的视角初步解释了黄河中游响应气候变化和地表相对汾渭盆地抬升发育阶地的过程。1.2 Ma以来黄河下蚀鄂尔多斯地块和峨眉台地分别形成了7级阶梯状阶地和6级堆积阶地序列。黄土地层分析结合年代学研究揭示这些阶地面都直接上覆一层古土壤,指示它们形成于气候由冰期向间冰期的过渡阶段,即使在沉降的盆地依然如此。然而,黄河中游并没有在1.2 Ma以来的每一次冰期向间冰期转换都发育阶地,说明气候虽能通过控制河流堆积-侧蚀与下切行为的转换决定阶地的形成时代,但其本身并不是阶地形成的唯一控制因素。在峨眉台地沉降的背景下,黄河无法形成正常的阶梯状阶地序列,取而代之的是堆叠的阶地序列(阶地越年轻拔河高度越大);而当鄂尔多斯地块相对汾渭盆地抬升缓慢时,黄河仅能在极为干旱的冰期向间冰期过渡阶段形成阶地;相比之下,它们相对汾渭盆地抬升速率都足够快速时,驱动黄河近乎对每一次的冰期向间冰期转换都能做出响应而发育阶地。以上黄河中游阶地与气候和地表抬升的对比模式揭示出,快速地表抬升也是阶梯状阶地序列发育不可或缺的因素,能驱使河流在冰期向间冰期过渡阶段显著下切,拉大相邻阶地面垂直距离从而利于后期保存。因此,研究认为黄河中游发育的系列阶地是响应气候变化和地表相对汾渭盆地抬升的结果。

关键词: 黄河中游 ; 阶地发育 ; 鄂尔多斯地块 ; 汾渭盆地 ; 峨眉台地

Abstract

The challenge to understanding fluvial response to climate change and surface uplift is a thorough distinguishing between their roles within terrace formation, which can provide an excellent insight to the landform and drainage evolution in catchment scale. Previous studies tend to correlate terrace development with climate change or surface uplift separately, attributing deposition-lateral erosion and incision to glacial-interglacial climate, or regarding fluvial terrace even as a direct evidence of surface uplift. A preliminary explanation for the terrace development of the middle reaches of Yellow River is proposed by linking its upper and lower reaches. In comparison with the past work, river terrace here is considered as a combination result of fluvial response to climate change and relative surface uplift. Intermittent downcutting by the middle reaches of Yellow River on the uplifted Ordos Block and Emei Platform relative to the subsiding Fenwei Basin has created 7 terrace staircases and a sequence of 6 aggradational terraces, respectively,during the past 1.2 Ma. The analysis of loess stratigraphy combined with geochronology revealed that these terraces were overlaid directly by a basal paleosol layer on the fluvial sediments, indicating that the abandonment of these treads due to incision by the Yellow River occurred at the transition from glacial to interglacial climates. This suggests that the glacial-interglacial climate cycle probably has a temporal control on the terrace generation by determining the fluvial behavior alternation between deposition-lateral erosion and incision, even though the Yellow River develops in the subsiding Fenwei Basin. Remarkably, terrace records however are absent at some transitions from glacial to interglacial climate, indicating that climate cycle may not be the only decisive factor for terrace occurrence. The terrace generation may be sporadic and in the form of unusual stacked pattern before the Emei Platform was uplifted within the eastern Fenwei Basin. And when the Ordos block were uplifted slowly relative to the Fenwei basin, terraces seem to only be created by the middle reaches of Yellow River at these transitions from extremely dry glacial to interglacial. In contrast, the terrace staircases almost occurred in synchrony with glacial-interglacial climate cycles as they were uplifted fast enough relative to the Fenwei Basin. This contrastive pattern indicates that surface uplift may be necessary in large terrace staircase genesis. It can force river to downcut deeply enough during climatic transitions to separate terrace levels adequately, favoring the generation and subsequent preservation of the terrace tread. Based on above analysis, the terrace sequences created by the middle reaches of Yellow River can therefore be regarded as a combined archive of climate change and relative surface uplift.

Keywords: the middle reaches of Yellow River ; terrace ; Ordos block ; Fenwei Basin ; Emei Platform

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本文引用格式

潘保田, 胡振波. 黄河中游响应气候变化和地表相对抬升发育阶地研究[J]. 冰川冻土, 2021, 43(3): 853-863 doi:10.7522/j.issn.1000-0240.2021.0026

PAN Baotian, HU Zhenbo. The study on the terrace development by the middle reaches of Yellow River as a response to the climate change and relative surface uplift[J]. Journal of Glaciology and Geocryology, 2021, 43(3): 853-863 doi:10.7522/j.issn.1000-0240.2021.0026

0 引言

河流阶地作为一类重要的层状地貌面被普遍视为地貌演化的直接证据1,另外气候变化也能够很好地记录在其沉积物中2-4,因此它被广泛地应用于探讨河流系统如何响应地表抬升和气候变化的机制研究中,受到国内外地貌学家的广泛重视5-10。它的形成本质是河流堆积/侧蚀与下切行为转换的结果11,其中的驱动因素可分为河流系统外部和内部因素两类。其中,控制河流堆积/侧蚀与下切行为转换的外部因素主要有气候变化、地面升降和侵蚀基准面变化12,而内部因素主要涉及河流系统复杂响应、河道比降以及粗糙度变化等13-14。近年来数值模拟进一步揭示河流系统响应这些因素变化的结果很可能会最终打破河道泥沙与搬运动力之间的竞争平衡关系15,导致河流堆积/侧蚀与下切行为的转换316。有关河流阶地形成机制的研究虽然取得一定进展,但仍然比较薄弱17。先前的工作把阶地序列与气候记录对比,主要探讨河流堆积/侧蚀与下切分别发生在冰期还是间冰期气候17。相比之下,近年来的研究却普遍认为河流下切可能主要发生在气候转冷或转暖的短暂过渡阶段218-21。尽管气候变化控制河流堆积/侧蚀与下切行为转换的观点似乎越来越趋于共识2,但其本身可能还不足以决定阶地的形成与否22-23。已有研究指示,地表抬升控制的河流下蚀量对阶地形成和后期保存有重要影响1824。诸多来自构造活跃的山区地貌证据也揭示,河流阶地的形成和保存受气候变化和地面抬升的共同控制2225-26。然而要通过阶地发育揭示河流系统响应气候变化和地表抬升的差异就要明确它们在阶地形成中的具体作用16。解决这一问题的关键是构建阶地序列相对精确的年代框架。

黄河作为世界巨型河流之一,穿过我国北方诸多盆地、山脉和断裂系统,其流域范围还跨越不同气候区,是探索地表抬升和气候变化驱动河流阶地发育机制的理想区域22。更重要的是,沿黄河分布的大多数阶地面都直接被黄土覆盖,为准确限定其形成年代创造了极佳条件2527-28。这些优势为最终区分地表抬升和气候变化在黄河阶地发育过程中的作用奠定了基础。相关研究对理解河流阶地发育机制有重要意义,其核心工作是要从多河段对比的角度分析黄河响应地表抬升和气候变化发育阶地的过程。然而综合目前已有的研究进展,这方面的工作依然薄弱,仍需加强。

黄河中游深切鄂尔多斯地块和太行山南麓的崤山,沟通了河套盆地、汾渭盆地和华北平原,形成了峡谷与盆地相间的地貌格局。其中沿晋陕峡谷和汾渭盆地,黄河发育了比较连续且保存较完整的河流阶地序列,其上还普遍堆积厚层黄土,为研究阶地形成机制创造了条件。因此本文的主要工作将从上游峡谷与下游盆地对比的角度探讨黄河响应地表抬升和气候变化发育阶地的过程。著名地理学家、地貌学家李吉均院士在长江、黄河等水系形成演化与河流阶地发育方面开展了大量研究工作,例如长江三峡贯通时代29、黄河上游形成时代和演化过程30、黄河阶地发育与地文期31等等,提出了一系列创新性的认识,极大地推动了学科的发展。2021年7月21日是先生仙逝一周年祭日,谨以此文缅怀先生高尚的人格风范和卓越的学术贡献。

1 区域概况

黄河作为世界上巨型河流之一,起源于青藏高原东北部,向东流经鄂尔多斯与黄土高原后经华北平原注入渤海,河道总长达到5 464 km(图1)。它的中游自内蒙古托克托径直向南沿吕梁山西麓切开鄂尔多斯地块,形成了长达700 km、深度超过200 m的晋陕峡谷(图2)。进一步向南,黄河在禹门口出峡谷后进入汾渭盆地,经潼关转向东流并切开太行山南麓的崤山,形成三门峡峡谷后进入地势相对平坦的华北平原。由于贯通了不同构造背景的地貌单元,整个黄河中游河谷形态也因此表现为深切峡谷与宽阔谷地相间的特征。

图1

图1   鄂尔多斯地块及周缘盆地区域概况图

注:黄河中游深切鄂尔多斯地块,在托克托与禹门口之间形成了晋陕峡谷,然后它进一步向南切开峨眉台地后在潼关附近转向东流,最后贯通三门峡进入华北平原。在晋陕峡谷中部吴堡,黄河发育了7级阶梯状阶地序列,而在峨眉台地南赵村附近形成了6级堆积阶地序列。二者的横断面图参见图3(a)和3(b)。右上角插入图指示鄂尔多斯地块及周缘盆地相对青藏高原位置以及宏观构造格局

Fig.1   Regional setting across the Ordos Block and surrounding basins

Note:The middle reaches of Yellow River excavates the Ordos Block,creating the Jinshaan Gorge between Tuoketuo and Yumenkou,and then it further flows toward south to cut through the uplifted Emei Platform relative to the Fenwei Basin. Finally,the middle reaches of Yellow River turns eastward at Tongguan to go across the Sanmen Gorge and debouch into North China Plain. Seven terrace staircases have been created by the middle reaches of Yellow River at Wubao located in the middle part of the Jinshaan Gorge,while six aggradational terraces occurred at Nanzhao by its downcutting into the Emei Platform. Their cross sections are coincident with the Fig.3(a) and Fig.3(b). The inset map displays the location and tectonic framework of the Ordos Block and surrounding basins relative to the Tibetan Plateau


图2

图2   黄河中游托克托至潼关间的河道纵剖面图

注:河道纵剖面图提取于ASTER-GDEM 30米分辨率的DEM,跨过鄂尔多斯地块、汾渭盆地以及峨眉台地。黄河在吴堡和南赵形成的阶地序列被标记在河道纵剖面的对应位置上。插入图指示这些阶地面的拔河高度

Fig.2   The longitudinal channel profile of the middle reaches of Yellow River between Tuoketuo and Tongguan

Note:This profile is extracted from the DEM data of ASTER-GDEM with 30 m resolution across the Ordos Block,Fenwei Basin,and Emei Platform. These terrace sequences created by the Yellow River at Wubao and Nanzhao are marked on the corresponding positions with respect to this profile


鄂尔多斯块体为稳定的克拉通,中生代表现为巨大的盆地,堆积了较厚的陆源碎屑沉积32。进入新生代,随着太平洋板块、印度板块与欧亚板块持续汇聚33,在青藏高原与鄂尔多斯地块间形成了一系列大型弧形逆冲和走滑断裂34。它们将印度与欧亚板块间的地壳缩短进一步向东传递,导致鄂尔多斯地块周缘拉张,形成了中卫-中宁盆地、宁夏盆地、河套盆地和汾渭地堑(盆地)35。尤其在喜马拉雅构造运动的驱使下,原鄂尔多斯盆地也在早中新世开始抬升并接受侵蚀夷平作用32,形成了占据黄河中游主体的唐县期夷平面3236。总体而言,相对稳定的鄂尔多斯地块与周围的地势高差自晚新生代以来逐渐加大,形成了现今被系列拉张盆地围限的地貌格局(图1)。

南部的汾渭地堑在海原、六盘山和秦岭断裂的控制下被拉张成巨大的新月形沉降盆地,周围被诸多正断层和走滑断层限定,面积达2万多平方公里35。持续的拉张使盆地内部沉降速度出现差异,发育系列正断层37,导致峨眉台地和中条山相对抬升。另外,汾渭盆地堆积了巨厚的新生代沉积,地层总体水平分布,指示有规律的持续沉降作用34。在第四纪期间地震和裂缝依然沿盆地内部和周缘断层频发,形成的断层崖连绵延续几百公里,说明扩展和沉降作用至今仍然十分活跃32

黄河中游从北向南纵贯鄂尔多斯地块和汾渭盆地,形成了深切的晋陕峡谷和宽阔的汾渭段河谷。根据野外调查,黄河沿程发育多级河流阶地。它们在河谷两岸非对称分布。在晋陕峡谷中部的吴堡和汾渭盆地的南赵地区,黄河阶地序列较完整,其上还堆积连续的黄土(图3)。因此,它们能典型地代表稳定的鄂尔多斯地块和沉降的汾渭盆地两类不同构造背景下的阶地发育过程,为进一步从上下河段对比分析的角度出发探讨黄河中游响应地表相对抬升和气候变化发育阶地的机制奠定了基础。

图3

图3   黄河阶地序列及上覆黄土横断面图

注:黄河在吴堡形成的阶地序列横断面图(a)。在该区唐县期夷平面限定了晋陕峡谷的谷肩,之下分布7级黄河阶地,除T2和T1为堆积阶地外其余均为基座阶地。每级阶地面上覆厚层黄土,其底部都为一层古土壤直接覆盖在下伏阶地的沙砾石层上。对唐县期夷平面和黄河阶地上覆红黏土与黄土地层划分和测年结果参见文献[25,35];黄河在南赵发育的阶地序列横断面图(b)。黄河在该区深切峨眉台地,形成6级堆积阶地,其中T6~T4为堆叠的阶地序列(阶地越年轻拔河越高)而T4~T1为正常的阶梯状阶地序列。每级阶地上覆不同厚度的黄土,其黄土-古土壤地层划分和测年结果参见文献[39]。横断面图3(a)与3(b)在黄河中游的位置参见图1和图2

Fig.3   Schematic cross section of the Yellow River terraces and covering loess deposits

Note:The cross section of the fluvial terraces created by the Yellow River at Wubao (a). The Yellow River excavates the Tangxian Planation Surface here leading to the formation of the Jinshaan Gorge. A total of 7 fluvial terraces has been generated by the Yellow River along this gorge, which are all strath terraces except the two accumulation terraces of T2 and T1. Their treads are covered by thick loess depositions whose basal paleosols all overlie directly the fluvial sediments. Pedostratigraphic identification and geochronological results of these Red Clay and loess covers have been given in the reference[25,35];The cross section of the Yellow River terraces occurring at Nanzhao (b). The Yellow River downcuts the Emei Platform here generating a sequence of 6 aggradational terraces which is characterized by the switch from the unusual stacked sequence of terrace T6~T4 to the terrace staircase of T4~T1. The basal paleosols of the loess covers, accumulated on these terrace treads, also superposes directly the underlying fluvial sediments. Their pedostratigraphic identification and geochronological results have been given in the reference[39]. See Fig.1 and Fig.2 for the locations of the two cross sections[Fig.3(a),3(b)] with respect to the middle reaches of Yellow River


2 黄河中游阶地序列与年代

2.1 晋陕峡谷中部吴堡黄河阶地序列与年代

现今鄂尔多斯地块主体以及黄河中游分水岭被唐县期夷平面占据,海拔高度在1 500 m左右38。它横切下伏不同时代的基岩和构造变形的地层,形成地势高差较小的平缓地貌面,并总体上呈现出由西北向东南略微倾斜的趋势。另外,唐县期夷平面还限定了晋陕峡谷的谷肩,其上普遍接受了较连续的红黏土和黄土堆积。其中在吴堡地区,该级夷平面上覆16 m厚的红黏土和140 m厚的黄土。这些风成沉积物的古地磁定年显示,唐县期夷平面形成于370万年左右27。在它之下,黄河深切峡谷形成了7级河流阶地序列,除最低的两级阶地T2和T1为堆积阶地外,其余均为基座阶地[图3(A)]。相比于晋陕峡谷其他地点,该区的7级黄河阶地面均覆盖较厚且连续的黄土。野外根据黄土标志地层古土壤S5以及地层横向对比详细划分了堆积在每级阶地面上的黄土-古土壤地层,同时采用差分GPS系统测量了阶地砾石层、河漫滩和上覆黄土的厚度,详细数据如表1所示。

表1   吴堡黄河阶地特征

Table 1  Characteristics of the Yellow River terraces in the Wubao area

阶地序列拔河高度/m砾石层 厚度/m上覆黄土 厚度/m最底部 黄土地层
T112.0>120.2S0
T227.0>95.0Sm
T345.12~315.0S1
T478.11~233.5S2
T5111.9329.0S4
T6129.01~262.0S7
T7175.1>1110.0S14

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吴堡地区至少在晚上新世已开始稳定地堆积风成沉积物39。那么黄河下切导致原河床或河漫滩一旦废弃,脱离一般洪水水位的影响,其上可能即刻积累风成沉积物。这样每级黄河阶地上覆黄土的底界年龄就大致限定了原河床或河漫滩被废弃的时代,即阶地形成年代2240。已有的研究通过古地磁、黄土-古土壤地层和OSL定年,获得了吴堡地区7级黄河阶地上覆黄土的底界年龄,从而限定了下伏阶地T7~T1分别形成于1.2 Ma、0.8 Ma、0.4 Ma、0.24 Ma、0.13 Ma、60 ka和10 ka38

2.2 汾渭盆地南赵黄河阶地序列与年代

在汾渭盆地的南赵地区,黄河切开相对抬升的峨眉台地形成了6级堆积阶地[图3(b)],其河流沉积物主要由沙和粉沙互层构成,偶尔夹杂砾石透镜体,砾石的粒径一般不超过3 cm。每级阶地面都十分平坦,上覆不同厚度的黄土,其中最高级阶地T6的阶地面最为宽广,达30 km。野外同样根据黄土的标志地层以及地层横向对比详细划分了每级阶地面上覆黄土-古土壤序列,同时采用差分GPS系统测量了每级阶地高度和上覆黄土厚度,详细数据如表2所示。测量结果显示,该阶地序列与黄河在晋陕峡谷吴堡地区形成的阶梯状阶地序列有很大区别。其中阶地T6~T4上覆黄土依次减薄,但阶地面高度却表现出相反的增高趋势,而阶地T4~T1则呈现出高级阶地面高度依次高于低级阶地面的正常阶梯状序列41。两组阶地在序列上截然相反的转变可能与峨眉台地相对汾渭盆地抬升有关。

表2   南赵黄河阶地特征

Table 2  Characteristics of the Yellow River terraces in the Nanzhao area

阶地序列拔河高度/m砂层厚度/m上覆黄土 厚度/m最底部 黄土地层
T12.5>2.5<2S0
T24.5>4.55~10Sm
T346.6>46.610~15S1
T491.2>91.220~25S2
T588.4>88.430~35S3
T678.8>78.880~100S14

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野外调查显示,南赵地区的黄河阶地面上黄土堆积较厚,而且没有侵蚀间断发生,这为下伏地貌面定年创造了极好的条件。然而黄土底界年龄是否能够充分限定下伏阶地面的形成年代主要取决于二者之间是否存在长时间的间断。根据南赵周边的黄土剖面记录,典型黄土堆积的历史至少追溯至第四纪初期39。因此,这6级阶地上覆黄土的底界年龄就大致代表了下伏阶地面的形成时代。已有的研究工作通过古地磁、黄土-古土壤地层和OSL交叉测年成功获得了每级阶地面上覆黄土的底界年龄,从而限定南赵黄河阶地T6~T1分别形成于1.2 Ma、0.33 Ma、0.24 Ma、0.13 Ma、60 ka和10 ka41

3 讨论

3.1 黄河中游下切速率及反映的地表相对抬升

从宏观构造演化来看,鄂尔多斯地块晚新生代比较稳定,而周缘的盆地新生代一直处于拉张沉降状态3537,导致二者区域构造背景显著不同。因此,比较稳定的鄂尔多斯地块相对于沉降的汾渭盆地表现出明显的抬升。黄河中游由北向南深切它们,不但形成了与构造背景对应的两种截然不同的深切峡谷和宽阔河谷的地貌差异,而且沿谷坡形成的阶地类型也表现出显著的差异(图3)。另外,以吴堡和南赵两处黄河阶地序列为代表,通过阶地形成年代和拔河高度可以计算出黄河下切鄂尔多斯地块和峨眉台地的平均速率如图4所示。进一步线性回归分析显示,1.2 Ma以来黄河在两地的下切速率都可以划分成两个时段,其回归系数均达0.9以上。在晋陕峡谷中部的吴堡,黄河在1.2~0.4 Ma间的平均下切速率为0.08 m∙ka-1,而0.4 Ma以来的平均下切速率已提升至0.24 m∙ka-1。相比之下,黄河在南赵地区两个时段的平均下切速率反差更明显,其中1.2~0.24 Ma间的平均下切速率为-0.01 m∙ka-1,而0.24 Ma以来陡然增加至0.41 m∙ka-1。无论在晋陕峡谷还是汾渭盆地上述黄河平均下切速率的每个时段都超过多个冰期-间冰期旋回,因此下切速率急剧加快不可能归因于高震级的地震事件42

图4

图4   黄河中游下切速率与黄土高原气候记录对比图

注:基于南赵和吴堡阶地拔河高度与年代,分别计算获得1.2 Ma以来黄河下切鄂尔多斯地块和峨眉台地的速率。进一步线性回归分析将它们划分成两个时段。黄土高原孢粉ln(NAP/AP)记录来自文献[43],而深海氧同位素δ18O曲线来源于文献[44]。所有黄河阶地形成于气候由冰期向间冰期的过渡阶段

Fig.4   Correlation between the incision rate of the Yellow River and the climate record over the Chinese Loess Plateau

Note:Based on the heights of the fluvial terraces occurring at Wubao and Nanzhao versus their ages,the average incision rates of the Yellow River downcutting the Ordos Block and Emei Platform since 1.2 Ma have been calculated. Further linear regression analysis divided them into two periods respectively. The pollen ln(NAP/AP) record of the Chinese Loess Plateau mentioned here is from the reference[43], while the benthic δ18O curve is obtained from the reference[44]. All the Yellow River terraces have been formed at the transitions from glacial to interglacial climate


新生代伴随青藏高原持续向东挤出,鄂尔多斯地块周缘系列盆地(地堑)开始拉张断陷34-35,不但导致盆地边界断裂活动而且内部也不断沉降形成诸多新的断裂系统35。相对稳定的鄂尔多斯地块与周缘盆地间的地势高差因此不断加大。沿着鄂尔多斯地块与汾渭盆地交界的韩城-罗云山断裂,一系列不同期次的断层三角面极为发育45。不仅如此,阶地宇生核素测年显示,0.4 Ma左右黄河在青铜峡发生了显著下切46,可能与牛首山抬升以及中卫-中宁和银川盆地进一步沉降有关。另外,鄂尔多斯地块以北的河套盆地钻孔岩心记录也显示,约0.4 Ma以来盆地进一步沉降47。这些证据都揭示了鄂尔多斯地块约0.4 Ma以来可能相对周缘盆地表现出加速抬升的趋势。综合目前已有的证据来看,鄂尔多斯地块相对周缘盆地加速抬升的时间与黄河沿晋陕峡谷加速下切发生的时代可以很好地对比25。然而,河流下切与地表抬升之间一般来讲很难表现出简单的线性关系1748,但越来越多来自构造活跃地区的证据显示,时间尺度至少超过一个冰期-间冰期旋回的河流阶地序列可以视为区域地表抬升的直接记录49。这样在其拔河高度和年代限定下的河流下切速率变化情况就可以评估和大致量化地表抬升速率的快慢50-52。黄河在吴堡和南赵表现出的加速下切很可能反映了鄂尔多斯地块和峨眉台地分别于0.4 Ma和0.24 Ma以来相对沉降的汾渭盆地加快抬升(图4)。最新的研究认为,诸多构造相对稳定的板块内部地表抬升可能是响应中更新世气候转型导致侵蚀加剧而发生下地壳均衡反弹的结果53。然而鄂尔多斯地块和峨眉台地开始加快相对抬升的时间明显晚于中更新世气候转型时段,因此更可能对应于构造引起的地表相对抬升而非气候变化,这与汾渭盆地活跃的构造背景一致45。总的来说,鄂尔多斯地块和峨眉台地分别在0.4 Ma与0.24 Ma左右都相对汾渭盆地开始进一步加速构造抬升。

3.2 黄河中游响应地表相对抬升和气候变化发育阶地的过程

理论而言,河流阶地的形成既可以归因于河流系统自身动力演化也可以与气候、构造、侵蚀基准面等外部因素变化相联系1012。其中,河流系统内部动力过程的调整自始至终都在发挥作用,可能驱动河道比降、粗糙度等内部要素发生渐变或突变并打破其临界值从而改变河流行为,发生堆积/侧蚀与下切间的转换11,最终在地貌上表现为阶地的形成2。然而这些河流系统内部动力过程调整仅仅发生在局部河段,影响范围有限54-58,形成的阶地在时间上不超过千年尺度而在空间上也不大于百米规模59。相比之下,本文关注的黄河中游阶地在时空尺度上均超过上述范围1~2个数量级(图3)。诸多概念模型研究认为,时间尺度至少大于一个冰期-间冰期旋回的阶地发育是河流系统响应地表抬升、冰期-间冰期气候旋回、侵蚀基准面调整等外部因素变化的结果18。晋陕峡谷和汾渭盆地距黄河入海口至少1千多公里(图1),作为黄河终极侵蚀基准面的海平面变化很难影响到如此遥远的内陆河段48。因此,地表抬升和冰期-间冰期气候旋回可能是导致晋陕峡谷和汾渭盆地黄河阶地发育的潜在因素。

整个黄河中游流域内的河流阶地面普遍被不同厚度的黄土覆盖,其黄土-古土壤序列明确的气候意义对揭示下伏河流阶地的形成原因有重要的指示意义2738。野外黄土地层划分结合磁性地层分析和测年结果显示,吴堡和南赵黄河阶地表现出的显著统一特征是河流相沉积物上都直接发育一层古土壤(图3)。粒度分析揭示这层古土壤起源于黄土的成壤作用而非河漫滩沉积物2260,指示下伏阶地面是由黄河在冰期向间冰期过渡或间冰期时下切形成。河流侵蚀过程的研究结果显示,气候在冰期与间冰期的过渡阶段所表现出的频繁且显著的波动可能驱动河流系统形成高频洪水,因此相比于冰期或间冰期,气候转换阶段更利于河流下切而发育阶地61。另外,野外调查也发现黄河中游每级阶地面上覆古土壤层在厚度上可以与黄土高原典型剖面的相应古土壤层对比,未出现发育间断的现象。因此,这里更倾向于认为黄河中游阶地可能形成于冰期向间冰期的过渡阶段。该结果与上游兰州盆地、支流洮河、大通河,以及西北欧和中亚地区的河流阶地研究结论一致2262-66。总结过去十多年相关研究,大部分的观点也是越来越倾向于将冰期-间冰期气候旋回视为河流阶地形成的重要驱动因素之一221。黄河中游地处我国东部季风区与内陆干旱区交界,流域植被、温度、径流等因素能够对冰期与间冰期间的气候转变做出敏感的响应而引发水文过程显著调整37,导致河道泥沙与搬运能力间失衡,使黄河表现出堆积或侧蚀向下切的转变,最终形成阶地11

来自黄土高原的孢粉以及深海氧同位素记录显示,气候在1.2~0.4 Ma间虽然发生多次冰期向间冰期的转换43,但晋陕峡谷段黄河仅在极为干旱的冰期,向间冰期转换阶段发育了3级阶地,尤其是下游汾渭盆地段黄河也只形成了一级十分宽广的最高级阶地(图4)。不可否认,受后期保存条件和侵蚀的影响,河流阶地年龄越老其记录越不完整。因此,这里对上述黄河在部分冰期向间冰期过渡阶段缺失阶地记录的定义就包括阶地未形成和形成后未保存两种情况。本文认为,在1.2~0.4 Ma间黄河阶地级数相比于冰期-间冰期气候旋回数缺失的原因可能与该时段鄂尔多斯地块相对汾渭盆地抬升缓慢以及峨眉台地依然沉降有关。当然,反复的地表抬升和沉降也可能导致河流堆积/侧蚀与下切频繁转换以至相互抵消,无法在地貌上表现出明显的阶梯状阶地形态22。然而,鄂尔多斯地块与周缘盆地晚新生代的构造特征并不支持地表反复升降,而且目前也没有任何相关记录。黄河下切速率显示,峨眉台地在1.2~0.24 Ma间持续沉降而鄂尔多斯地块在1.2~0.4 Ma间相对汾渭盆地的抬升也可能确实比较缓慢(图4)。这说明气候由冰期向间冰期转换仅能驱动黄河中游从堆积或侧蚀行为转向下切,即控制了阶地的形成时代,但其本身并不是阶地发育的唯一驱动因素。地表抬升对黄河中游阶梯状阶地发育也可能有重要影响。

地表抬升能够增加河道比降从而提升河流的下切速率9。然而峨嵋台地在1.2~0.24 Ma间可能持续沉降,使黄河在南赵的下切速率为负值表现出显著的加积特征。期间黄河在冰期向间冰期转换阶段依然下切,但制造的河谷空间可能不足以容纳后期的堆积量,所以不但无法形成阶梯状阶地序列而且还可能显著高过前期遗弃的阶地面。在此情况下,阶地面上覆黄土越厚对下伏地貌面免受后期河河流积物覆盖的保护作用就可能越强,使黄河在1.2 Ma、0.33 Ma和0.24 Ma左右形成的阶地面有机会保存,构成了三级堆叠式的阶地序列(阶地越老拔河越低)。从目前已有的黄土高原孢粉记录来看,这三个黄河下切时间前的冰期气候确实比较干旱9。相比之下,鄂尔多斯地块在1.2~0.4 Ma间相对汾渭盆地确实表现出地表抬升但明显慢于0.4 Ma以来,导致黄河在该时段内的每次冰期向间冰期转换过程中无法显著下切。在此情况下,黄河只能沿晋陕峡谷先充分堆积或侧蚀以拓宽河谷,然后再以有限下切量形成阶地。最终,黄河只能对极为干旱的冰期,向间冰期转换作出响应9,在吴堡形成了1.2 Ma、0.78 Ma和0.4 Ma左右的三级阶梯状阶地。0.4 Ma以来鄂尔多斯地块相对汾渭盆地开始加速抬升,驱动黄河在每一次冰期向间冰期转换阶段都显著下切,在吴堡几乎形成了与气候旋回同步的阶梯状阶地序列。进一步对比发现,断层活动记录与黄河下切速率都显示峨嵋台地自0.24 Ma以来相对汾渭盆地以更快的速度加速抬升3545图4),驱使黄河敏感响应每一次冰期向间冰期转换而显著下切,在南赵形成了与气候旋回数严格对应的阶梯状阶地序列。

4 结论

黄河中游1.2 Ma以来间歇性下切侵蚀相对汾渭盆地抬升的鄂尔多斯地块和峨眉台地分别形成了7级阶梯状阶地和6级堆积阶地序列。黄土-古土壤地层分析结合年代学研究揭示这些阶地面都直接上覆一层古土壤,指示它们形成于气候由冰期向间冰期的过渡阶段。然而1.2 Ma以来的冰期-间冰期气候旋回数远多于黄河中游的阶地级数,说明气候仅能通过改变河流堆积-侧蚀与下切行为的转换来控制阶地的形成时间,但其本身并不是河流阶地形成的唯一决定因素。在峨眉台地沉降的背景下,黄河在南赵无法形成阶梯状阶地,取而代之的是堆叠的阶地序列。在鄂尔多斯地块相对汾渭盆地抬升不足够快速的情况下,黄河沿晋陕峡谷仅能对极为干旱的冰期向间冰期转换作出响应形成阶梯状阶地。随着鄂尔多斯地块和峨眉台地相对汾渭盆地相继进一步加速抬升,黄河在吴堡和南赵的阶地序列开始与气候旋回表现出时间上的同步特征。上述对比分析揭示,快速地表抬升也是阶梯状阶地发育不可或缺的驱动因素,它能驱使河流在冰期向间冰期过渡阶段显著下切,拉大相邻阶地面垂直距离从而利于后期保存。因此,本文从上下游河段对比的视角分析河流阶地的形成过程,认为黄河中游发育的阶地是河流响应气候变化和地表相对抬升共同影响的结果。

谨以此文,纪念李吉均院士!

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