晚更新世共和盆地风沙沉积及其对地貌演化的指示意义

Late Pleistocene aeolian sedimentation in the Gonghe Basin:Implications for landform evolutions

共和盆地位于青藏高原东北缘,为一断陷盆地。早-中更新世以沉降为主,发育了广泛的河湖相沉积;中更新世末期的共和运动使得盆地抬升。盆地内广泛发育的河湖相沉积和风成沉积地层,记录了共和盆地的演化历史。然而,由于沉积地层年代数据相对匮乏,目前针对共和盆地晚更新世风沙活动及环境变化的研究尚未深入。此外,共和盆地由沉降转为抬升并导致黄河下切等地貌演化的关键时限也有待进一步认识。本研究选取共和盆地6个沉积剖面,利用光释光测年以及剖面岩性变化、粒度和元素等古环境代用指标,重建了共和盆地晚更新世风沙活动历史,分析了风沙活动可能的驱动因素。考虑到干旱-半干旱地区河湖环境不但为风沙沉积提供了丰富的物源,而且可能埋藏风沙沉积使其得以保存等事实,依据风成砂沉积时代探讨了晚更新世以来盆地地貌演化过程。结果表明,共和盆地风沙活动主要发生在3个时期,分别为203±12 ka前后、176.2±21.1~143.3±27.8 ka和108±7~86±5 ka。203±12 ka前后,自周围山体的流水携带松散碎屑物由边缘向盆地中心充填,在风力作用下盆地边缘部分地点沉积了风成砂;176.2±21.1~143.3±27.8 ka时期,盆地仍处于充填阶段,盆地内部应以曲流发育为主的河流泛滥平原、浅湖等交错分布,推测河岸与湖泊周边滩地等地貌部位堆积了典型风沙沉积。由此可见,共和运动(或高原面下切)的时限大体应晚于150 ka;108±7~86±5 ka期间,黄河从高原面下切至二塔拉高度(约3000 m a.s.l.),持续至86±5 ka,这一时期大量河湖相物质暴露于地表,为区域风沙沉积提供了丰富的物源,同时形成的层状地貌面也为风沙沉积提供了有利的堆积场所。共和盆地3个时期的风沙活动与盆地水系演化有关,主要受控于风力以及流水作用产生的沙物质供给等的变化,在滨岸或湖泊滩涂等地点堆积风成砂。

The Gonghe Basin(35°27'~36°56'N,98°46'~101°22'E)is a large intermontane fault basin,located in the northeastern margin of Qinghai-Tibetan Plateau.It started to form at the end of the Paleogene and subsided until the early-to mid-Pleistocene.Accordingly,extensive fluvio-lacustrine sediments were deposited in the basin.Since the late mid-Pleistocene,the basin has been tectonically uplifted due to the Gonghe Movement.The widely outcropped fluvio-lacustrine and aeolian sediments in the basin can be used to reconstruct the history of the Gonghe Basin.However,researches on Late Pleistocene aeolian activity and environmental change in Gonghe Basin is still superficial due to the scarcity of numerical ages from sedimentary strata.In addition,the transition timing of the Gonghe Basin from subsidence to uplift,i.e.,the Yellow River incision,is also necessary to be further understood.In this study,we chose six outcrop aeolian sections MGTC(35°41'48.84″N,100°33'35.41″E;3184 m a.s.l.),SZY(36°19'18.30″N,99°59'46.07″E;2949 m a.s.l.),GGHA(36°10'13.53″N,100°06'15.05″E;2875 m a.s.l.),YDE(36°20'49.24″N,100°09'27.83″E;3097 m a.s.l.),YDH(36°12'57.52″N,100°02'51.66″E;3006 m a.s.l.),and HK(35°46'17.50″N,100°11'53.78″E;3191 m a.s.l.)in the Gonghe Basin and analyzed luminescence dating,stratigraphic variability,grain size distribution,and elemental composition of different strata.This research is intended to(1)reconstruct the history of aeolian activity,(2)analyze the possible driving factors affecting aeolian activity,and(3)discuss the geomorphological evolution processes of the basin since the late Pleistocene,assuming that a fluvio-lacustrine environment in arid and semi-arid areas not only provides abundant sand materials for aeolian deposition,but also may have been able to rapidly conceal aeolian deposits and preserve them well.The results indicate that aeolian activities occurred mainly at around 203±12 ka,176±21~143±27 ka and 108±7~86±5 ka.At 203±12 ka,the ancient lake developed in the Gonghe Basin,and loose debris were carried by flowing water from surrounding mountains to fill the basin from periphery to center during this period,whereas aeolian sand was deposited on the margin of the basin by wind.During 176±21~143±27 ka,aeolian sand deposited in the center of the western Gonghe Basin,where a depression was developed and the ancient aeolian sand deposition was exposed subsequently.According to the analysis of sedimentary characteristics,geomorphic elevations and luminescence dating,it is inferred that the basin was continuously filled during this period.The floodplain and shallow lakes may have been alternatively distributed in the basin.Under the influence of wind power and seasonal water-level fluctuations of rivers or lakes,aeolian sand may have been deposited on either riverbanks and;or surrounding beaches of lakes.It thus can be seen that the occurrence of Gonghe Movement is later than this period,probably being later than 150 ka ago.Between 108±7 ka and 86±5 ka,the aeolian sand was deposited on different landforms in the basin.The aeolian sand was overlaid by fluvial sediments,indicating that rivers were still flowing on the"plateau surface"and even the second Tara terrace during this period.The Yellow River may not have incised from the"plateau surface"to the height of the second Tara terrace(approximately 3000 m a.s.l.)until about 86±5 ka ago.As a result,the previously deposited fluvio-lacustrine sediments were exposed to provide abundant fine-grained material for deflation and the stratified landforms that formed during this period providing favorable places for sand deposition.Overall,aeolian activity during the Late Pleistocene were highly related to the evolution of the basin water system,mainly controlled by changes in sand supply and wind regime;aeolian sand was developed and preserved along ancient riverbanks or lake beaches.