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1.
刘尚仁 《热带地理》2007,27(1):6-10
依据广东西部超过18条河流、39处河流阶地、至少35个14C、热释光的冲积层测龄数据等情况,可知粤西最多有4级河流阶地(不包括湛江组和北海组阶地);多数河流最高阶地靠近现代主河床分布,深切曲流中有河流阶地,反映近数十万年来河床改道不大;西江在封开有广东高度最高(76 m)的河流阶地,而广东高度最高的地下河阶地是111 m,大致显示出地面与地下剥蚀强度的差异;通常山区河流长度越大,河漫滩和河流阶地的高度越大,河流阶地的级数和级别也常增加;在河流上游的下段和中游的上段,河流阶地的高度最大且级数也最多;在晚更新世之前与后,河谷中下游地区的新构造运动趋势由上升变为稳定或沉降;连滩盆地是广东全新世构造沉降最典型的山间盆地。  相似文献   

2.
《Basin Research》2018,30(Z1):269-288
A number of major controversies exist in the South China Sea, including the timing and pattern of seafloor spreading, the anomalous alternating strike‐slip movement on the Red River Fault, the existence of anomalous post‐rift subsidence and how major submarine canyons have developed. The Qiongdongnan Basin is located in the intersection of the northern South China Sea margin and the strike‐slip Red River fault zone. Analysing the subsidence of the Qiongdongnan Basin is critical in understanding these controversies. The basin‐wide unloaded tectonic subsidence is computed through 1D backstripping constrained by the reconstruction of palaeo‐water depths and the interpretation of dense seismic profiles and wells. Results show that discrete subsidence sags began to form in the central depression during the middle and late Eocene (45–31.5 Ma). Subsequently in the Oligocene (31.5–23 Ma), more faults with intense activity formed, leading to rapid extension with high subsidence (40–90 m Myr−1). This extension is also inferred to be affected by the sinistral movement of the offshore Red River Fault as new subsidence sags progressively formed adjacent to this structure. Evidence from faults, subsidence, magmatic intrusions and strata erosion suggests that the breakup unconformity formed at ca. 23 Ma, coeval with the initial seafloor spreading in the southwestern subbasin of the South China Sea, demonstrating that the breakup unconformity in the Qiongdongnan Basin is younger than that observed in the Pearl River Mouth Basin (ca. 32–28 Ma) and Taiwan region (ca. 39–33 Ma), which implies that the seafloor spreading in the South China Sea began diachronously from east to west. The post‐rift subsidence was extremely slow during the early and middle Miocene (16 m Myr−1, 23–11.6 Ma), probably caused by the transient dynamic support induced by mantle convection during seafloor spreading. Subsequently, rapid post‐rift subsidence occurred during the late Miocene (144 m Myr−1, 11.6–5.5 Ma) possibly as the dynamic support disappeared. The post‐rift subsidence slowed again from the Pliocene to the Quaternary (24 m Myr−1, 5.5–0 Ma), but a subsidence centre formed in the west with the maximum subsidence of ca. 450 m, which coincided with a basin with the sediment thickness exceeding 5500 m and is inferred to be caused by sediment‐induced ductile crust flow. Anomalous post‐rift subsidence in the Qiongdongnan Basin increased from ca. 300 m in the northwest to ca. 1200 m in the southeast, and the post‐rift vertical movement of the basement was probably the most important factor to facilitate the development of the central submarine canyon.  相似文献   

3.
The Basin of Ubaté–Chichinquirá (5°28′N, 73°45′ W, c. 2580 m altitude) includes the Fúquene Valley and is located in the central part of the Eastern Cordillera of Colombia. Rocks and sediments were folded and faulted during the Miocene, uplifted during the (late) Pliocene, and affected by glaciers during the Pleistocene. Successive glacial and interglacial periods left significant marks in the landscape which were used to reconstruct six stages in the development of the landscape along a relative chronology. During early Pleistocene episode 1 glaciers formed U-shape valleys. Evidence of the impact of ice sheets has been found as far downslope as ca. 2900 m elevation. During episode 2 moraines developed which were cut by the present San José River. During episode 3 abundant sediment was produced by glacial erosion. It accentuated the sculpturing of hard rock and deepening of the drainage basin. The ancestral Ubaté–Suarez River constituted a dynamic erosive system that gave rise to deep V-shaped valleys and progressively formed a set of intricate valleys with a high sediment production. Finally, intense glacial and fluvio-glacial erosion led to a geomorphological system with high energy levels and intensive sediment transport leading to wide valleys. During episode 4 the Ubaté–Suarez River eroded and deepened its valley until it captured the old El Hato–San José Valley. It caused intense erosion of the moraine and the fluvio-glacial gravels. Deep V-shaped valleys stabilized in the high areas of the main drainage system and these valleys form the present-day fluvial sub-basins. During episode 5 the deep valley in the northern part of the Basin of Ubaté–Chichinquirá developed. During middle Pleistocene episode 6 colluvial sediments formed the Saboya dam and a lake was formed in the river valley of which the present Lake Fúquene is only a small remnant. Lithological changes indicate fluctuating water levels and Lake Fúquene must have expanded periodically up to an area 5 to 10 times the present-day surface.  相似文献   

4.
The karstification of the tertiary gypsum formations in the Calatayud Graben has given rise to synsedimentary and postsedimentary subsidence phenomena in both the Neogene sediments of the endorheic graben fill and in the Quaternary alluvial sediments deposited under exorheic conditions. In the so-called Maluenda and Perejiles areas, covering 4.4 and 12 km2 respectively, the Neogene sedimentary units stratigraphically above the gypsum have subsided due to the karstification of the underlying evaporites. The maximum subsidence in both areas has reached at least 200 m. The Jalón-Jiloca-Perejiles alluvial system has been affected by karstic subsidence during its Quaternary evolution. Ten stepped alluvial levels (pediment-terrace) have been identified. As a consequence of synsedimentary karstic subsidence the deposit of an alluvial level can be locally thickened, changing in a short distance from less than 10 m to more than 100 m in thickness. Thus, the deposit of an individual terrace can be locally superimposed on the deposits of older terraces. The palaeokarst recorded in the sediments of the different alluvial levels shows how the karstic subsidence has controlled the dynamics of the sedimentary system. The alluvial karstic subsidence is currently active in the flood plains affecting fluvial channels, buildings, communication ways, irrigation ditches and the Calatayud gypsum scarp.  相似文献   

5.
Abstract The uniform stretching model has been applied to seismic reflection profiles and well-log information from the Pearl River Mouth Basin on the northern flank of the South China Sea. Stretching factors were calculated from subsidence curves determined from the stratigraphy by using the backstripping technique to remove the effects of compaction and sediment loading. Variations in rift topography, palaeobathymetry and global sea-level v/ere taken into account. We argue that the Pearl River Mouth Basin formed by lithospheric extension by a factor of about 1.8, lasting from Late Cretaceous to late Oligocene times. Stretching factors calculated from subsidence agree with those determined from the geometry of normal faulting and from crustal thinning. Thus there is no indication of a significant discrepancy between the different estimates of stretching. The geometry of faulting suggests that considerable amounts of local footwall uplift occurred during the rifting period. Small differences between the observed and calculated subsidence curves (∽ 400 m in the middle Miocene) are best explained by minor amounts of extension ( β ∽ 1.1). The time-temperature history of sediments within the basin has also been calculated so that expected vitrinite reflectance and oil abundance could be determined. The results are consistent with each other and are in reasonable agreement with observations from wells.  相似文献   

6.
On the coast of Tosa Bay, Southwestern Japan, there is a wide coastal terrace which was formed in the Last Interglacial and has been uplifted to altitudes of 150–200 m above the present sea level by crustal movement. Many valleys dissect the terrace surface with upwardly convex profiles in downstream reaches. Numerical simulations were conducted to investigate the development of longitudinal profiles of the valleys under the influence of baselevel change and marine erosion. The basic equation used for the calculation had been derived theoretically from the hydraulic mechanism of river erosion. The independent variables are the gradient and curvature of the valley floor and the distance from the divide (drainage area per unit basin width). The parameter in the sediment transport equation is determined based on the results of laboratory experiments. A time sequence of seashore migration which has been caused by marine erosion and relative sea level change due to crustal uplift and glacial eustasy since the Last Interglacial was given as a boundary condition. Numerical calculations were conducted using a finite difference method, starting from 125000 years ago when the shoreline was located at the upper end of the terrace surface, and progressive changes in the valley profiles over time were simulated. The present profiles obtained from the calculation fit quite closely to actual valley profiles, which demonstrates the validity of the equation, and at the same time suggests a formation process for wide coastal terraces.  相似文献   

7.
We present a chronological model of erosion surface development in the Iberian and Cantabrian Ranges of north-central Spain. We map four erosion surfaces and interpret these to be related to Duero basin continental sediments and tectonic activity from Upper Oligocene to Plio-Pleistocene. The oldest erosion surface, SE1, formed across Upper Oligocene–Lower Miocene synorogenic deposits; while surface SE2 was contemporaneous with the Middle Miocene alluvial systems, ending with an uplift stage in the Astaracian. The two most recent erosion surfaces, SE3 and SE4, developed during extensional tectonic episodes and are associated with the deposition of Upper Páramo sedimentary units at the end of the Miocene (Upper Turolian) and alluvial fan deposits, known as rañas (Plio-Pleistocene). With the exception of SE1, which seems to be associated with a relatively wet climate, the surfaces formed during periods of marked aridity and generally warm temperatures. Through geostatistical reconstruction of the best preserved surface (SE2), applying ordinary kriging method to the topography (DEM) of the erosion surface and its correlating sedimentary plains, we identify the deformation processes which occurred on this surface after its formation.  相似文献   

8.
The Seine and the Somme are the two main rivers flowing from northwestern France into the Channel. During the Pleistocene cold stages both rivers were tributaries of the River Manche which was exporting sediments into the central deeps of the Channel. The River Seine has a very well developed terrace system recording incision that began at around 1 Ma. The same age is proposed for the beginning of the main incision in the Somme Valley on the basis of morphostratigraphy, pedostratigraphy, palaeontology, palaeomagnetism and ESR datings. The uplift rate deduced from analysis of the Seine and Somme terrace systems is of 55 to 60 m/Ma since the end of the Lower Pleistocene. The response of the two rivers to climatic variations, uplift and sea-level changes is complex and variable in the different parts of the river courses. For example, the evolution of the lower Seine system is influenced by uplift and climate changes but dominated by sea-level changes. In the middle Seine the system is beyond the impact of sea-level variations and shows a very detailed response to climatic variations during the Middle and Upper Pleistocene in a context of uplift. The Somme Valley response appears to be more homogeneous, especially in the middle valley, where the terrace system shows a regular pattern in which incision occurs at the beginning of each glacial period against a general background of uplift. Nevertheless, the lower Somme Valley and the Palaeo-Somme in the Channel area indicate some strong differences compared with the middle valley: influence of sea-level variations and probably differences in rates of tectonic uplift between the Channel and the present continent. The differences in the responses of the two river valleys during the Pleistocene are related to differences in the size of the fluvial basins, to the local tectonic characteristics, to the geometry of the platform connected to the lower parts of the valleys and to the hydrodynamic characteristics of each river. Finally, it is shown from these examples that the multidisciplinary study of Pleistocene rivers is a very efficient tool for the investigation of neotectonic activity.  相似文献   

9.
对贵州清水江上游马寨、翁东、三江、施洞沿江4个剖面的阶地特征、年代学结果进行了综合分析。发现以凯里断层为界,上游地区的马寨和翁东2个剖面的T2阶地形成时代约为51~57 ka B.P.,T1阶地的形成时代约为25 ka B.P.,下游地区的三江和施洞2个剖面的T2阶地形成时代约为122~102 ka B.P.,T1阶地的形成时代约为78 ka B.P.。选取各剖面的T2阶地的基座高度来计算了河流下切速率,发现上游地区2个剖面（马寨、翁东）的河流下切速率较接近,约为0.41~0.34 m/ka,明显高于下游地区的2个剖面（三江、施洞）的0.16~0.20 m/ka,表现为上游下切速率高,越往下游方向下切速率逐渐降低。这表明自晚更新世以来,清水江上游区域受到构造作用的影响而发生差异抬升,具体表现为西部构造抬升幅度大,阶地下切速率快;东部构造抬升幅度小,阶地下切速率慢。  相似文献   

10.
The Quaternary evolution and the morpho-sedimentary features of some of the most important rivers in Spain (Ebro and Tagus rivers among others) have been controlled by subsidence due to alluvial karstification of the evaporitic bedrock. The subsidence mechanism may range from catastrophic collapse to slow sagging of the alluvium by passive bending. In the Ebro Basin, the mechanisms and processes involved in karstic subsidence were studied through the analysis of present-day closed depressions as well as through old subsidence depressions (palaeocollapses and solution-induced basins) and associated deformations recorded in the Quaternary alluvial sediments. The Gállego–Ebro river system is presented as a case study of channel adjustments and geomorphic and sedimentary evolution of fluvial systems in dissolution-induced subsidence areas. In this fluvial system, evaporite dissolution during particular Quaternary time intervals (namely early and middle Pleistocene) have lead to the development of a solution-induced basin, approximately 30 km-long by 8 km-wide, filled by Quaternary deposits with a total thickness in excess of 190 m. The main river response to balance the subsidence in the alluvial plain was aggradation in the central reach of the subsiding area, and degradation both in the upstream reach and in the valley sides where alluvial fans and covered pediments may prograde over the fluvial sediments. The main sinking areas are recognized in the sedimentary record by anomalous thickenings in the alluvial deposits and fine-grained sediments deposited in backswamp and ponded areas.  相似文献   

11.
This study focuses on the upper part of the Muskegon River system in north-central Lower Michigan and is the first to reconstruct the post-glacial history of fluvial landform development in the core of North America's Great Lakes region. Results indicate that the upper Muskegon River valley contains four alluvial terraces and numerous paleomeanders. Radiocarbon dating of peats within these old channels provides a good chronology for stream behavior and landform development. The T-4 terrace is a paired Pleistocene outwash/lacustrine surface that probably formed about 12,500 years ago. The T-3 terrace is a fill-strath surface that was cut between about 12,000 and perhaps 9500 years ago. The geometry of macromeanders on this surface suggests that stream discharge was  8 times greater than during the Holocene.The Pleistocene/Holocene transition is marked by a major period of downcutting that likely began as the climate warmed/dried and sediment yield diminished. This period of downcutting potentially lasted through the drier middle Holocene, creating a 6-m-high escarpment in the valley. The Muskegon River then began to aggrade when the climate became wetter. Subsequently the river again incised, creating the paired T-2 terrace, about 3400 years ago when the climate became still wetter. T-2 paleomeanders indicate that stream discharge at this time was consistent with the modern river. In the past 2500 years, the stream has constructed a poorly defined complex of T-1 terraces. These surfaces likely formed due to complex response associated with more variable climate. This study demonstrates that the upper Muskegon River has a similar post-glacial history as streams on deglacial and periglacial landscapes elsewhere in the world.  相似文献   

12.
Staircases of strath terraces and strongly incised valleys are the most typical landscape features of Portuguese rivers. This paper examines the incision achieved during the late Cenozoic in an area crossed by the Tejo river between the border with Spain and the small town of Gavião. In the more upstream reach of this area, the Tejo crosses the Ródão tectonic depression, where four levels of terraces are distinguished. During the late Cenozoic fluvial incision stage, the Ródão depression underwent less uplift than the adjacent areas along the river. This is reflected by the greater thicknesses and spatial extent of the terraces; terrace genesis was promoted by impoundment of alluvium behind a quartzitic ridge and the local presence of a soft substratum. Outside this tectonic depression, the Tejo has a narrow valley incised in the Hercynian basement, with some straight reaches that probably correspond to NE–SW and NNW–SSE faults, the terraces being nearly absent. Geomorphological evidence of tectonic displacements affecting the Ródão dissected terrace remnants is described. Geochronological dating of the two younger and lower terrace levels of this depression suggests a time-averaged incision rate for the Tejo in the Ródão area, of ca. 1.0 m/ka over the last 60 thousand years. A clear discrepancy exists between this rate and the 0.1 m/ka estimated for the longer period since the end of the Pliocene. Although episodes of valley incision may be conditioned by climate and base-level changes, they may also have been controlled by local factors such as movement of small fault-bounded blocks, lithology and structure. Regional crustal uplift is considered to be the main control of the episodes of valley incision identified for this large, long-lived river. A model is proposed in which successive regional uplift events—tectonic phases—essentially determined the long periods of rapid river downcutting that were punctuated by short periods of lateral erosion and later by some aggradation, producing strath terraces.  相似文献   

13.
贵州高原北部发育平缓丘丛和深切峰丛2种喀斯特地貌组合,保存于喀斯特山间盆地的河流阶地对区域地貌演化具有指示意义。本文根据阶地发育特征和光释光（OSL）测年,分析阶地形成的时代和动力,结合区域地质背景,探讨构造抬升和河流侵蚀对黔北喀斯特地貌演化的驱动作用。结果显示,绥阳盆地T1阶地时代18.8~8.2 ka,T2时代144.4~104.1 ka;旺草盆地T1年龄为5.5 ka,T2年龄为45.1 ka。绥阳盆地阶地以漫滩相沉积物为主,旺草盆地阶地则多切割了白云岩基岩。分析认为,气候条件影响了阶地的沉积过程,但差异性构造抬升应为区域河流阶地差异发育的主要因素。阶地测年显示,旺草盆地的河流平均下切速率明显高于绥阳盆地,表明芙蓉江流域构造抬升和河流下切强度明显高于洋川河。在差异性构造抬升和河流侵蚀综合作用下,北部大娄山区形成了深切的喀斯特峰丛-峡谷地貌,南部乌江中游流域则发育以平坦盆地和宽缓丘丛为主的地貌组合。  相似文献   

14.
The Dien Bien Phu fault zone (DBP), orientated NNE to N, is one of the most seismically active zones in Indochina. In NW Vietnam, this zone is 160 km long and 6–10 km wide, cutting sedimentary and metamorphic rocks of the Late Proterozoic, Palaeozoic and Mesozoic age, as well as Palaeozoic and Late Triassic granitoids. Along the DBP relatively small, narrow pull-apart basins occur, the three largest of which (Chan Nua, Lai Chau and Dien Bien Phu) have been studied in detail. All of them are bounded by sinistral and sinistral-normal faults, responsible for offset and deflected drainage, presence of numerous shutter ridges and displaced terraces and alluvial fans. The normal component of motion is testified to by well-preserved triangular facets on fault scarps, highly elevated straths in river watergaps, overhanging tributary valleys, as well as high and uneven river-bed gradients.Our observations indicate a minimum recent sinistral offset ranging from 6–8 to 150 m for Holocene valleys to 1.2–9.75 km for middle–late Pleistocene valleys in different fault segments. The thickness of Quaternary sediments varies from 5–25 m in the Lai Chau area to some 130 m in the Dien Bien Phu Basin. In the Lai Chau Basin, the middle terrace (23 m) alluvia of Nam Na River at Muong Te bridge have been optically stimulated luminescence/single aliquot regenerative dose technique (OSL-SAR) dated at 23–40 to 13 ka. These sediments were normal-faulted by some 11 m after 13 ka, and mantled by vari-coloured slope loams, 8–12 m thick, containing colluvial wedges composed of angular debris. These wedges were probably formed due to at least three palaeoseismic events postdating 6 ka. In the Dien Bien Phu Basin, in turn, alluvium of the upper Holocene terraces has been OSL-SAR dated to 6.5–7 and 1.7–1.0 ka, whereas the younger (sub-recent) terrace sediments give ages of 0.5–0.2 ka.Displaced terraces and alluvial fans allow us to suppose that the sinistral and sinistral-normal faults bounding narrow pull-apart basins in the southern portion of the DBP fault reveal minimum rates of left-lateral strike-slip ranging from 0.6 to 2 mm/year in Holocene and 0.5–3.8 mm/year in Pleistocene times, whereas rates of Holocene uplift tend to attain 1 mm/year north of Lai Chau and 0.4–0.6 mm/year west of Dien Bien Phu. More precise estimations, however, are difficult to obtain due to poor age control of the displaced drainage. Rates of Quaternary strike-slip are comparable with those of the Red River fault; the sense of movement being, however, opposite. Taking into account the presence of two phases of Late Cenozoic strike-slip of contrasting sense of motion, as well as the geometry of the two fault zones, we hypothesize that the Red River and Dien Bien Phu faults are conjugate faults capable of generating relatively strong earthquakes in the future.  相似文献   

15.
An episode of high lake levels prior to the last maximum glaciation has been identified at many localities in wastern Australia. Similar events have been recognized at playa lakes in central Australia, where gypsum dunes along playa margins formed during one or more episodes of high groundwater discharge, with a large influx of calcium sulphate.At Lake Lewis, exposures at two islands show similar sediment sequences: three pedogenic gypcrete layers interbedded with aeolian quartz and gypsum sand horizons form three units within gypsum dunes up to 7 m high. The lowest unit has cliffed edges buried by the upper units, indicating a significant time break. Four TL dates (coarse-grained quartz) show that this lowest unit was deposited at or before 70–80 ka. The middle unit of mixed gypsum and quartz sand capped by gypcrete represents the major phase of gypsum dune formation, and 6 TL dates range from 33 to 46 ka with overlapping error bars. These are slightly younger but statistically similar to TL dates (from 39 to 59 ka) of the shoreline gypsum dune at Lake Amadeus in the same region. The top unit of the two islands, up to 1 m thick, has not yet been well dated. One date is inconsistent with the well dated middle layer below, possibly because of incomplete bleaching, and has been rejected. The other date (17 ± 5 ka) is much younger which possibly indicates a minor and local reactivation of old gypsum sediments. At the lake margin, there are quartz dunes overlying the gypsum dunes, and a buried aeolian quartz sand layer occurs in a lake-margin terrace. These represent reactivation of the regional quartz dune field after the major gypsum dune formation. Two consistent TL dates (21 ± 4 ka and 23 ± 6 ka) indicate that regional dunes were active at about the time of the Last Glacial Maximum.  相似文献   

16.
青弋江上游泾县段阶地砾石层砾组结构及其沉积环境研究   总被引：1，自引：1，他引：0  
胡春生  吴立  杨立辉 《地理科学》2016,36(6):951-958
通过对青弋江上游泾县段阶地砾石层进行砾组分析,讨论阶地砾石层的沉积环境及其对青弋江发育的启示。结果表明：① 砾径以中砾和粗砾为主,砾石沉积时水动力条件较强,流速基本为2 m/s左右,最大可达到3.5 m/s,特别是T3砾石层形成时期;② 砾向在T3和T2砾石层形成时期分别为南南西（SSW）和南西西（SWW）方向,古流向变化不大,呈自南而北的基本流向;③ 砾态以次圆和圆为主,其总含量超过70%,较高的磨圆度暗示砾石经历了较远距离的搬运;④ 砾性主要有石英砂岩、砂岩、脉石英和石英岩,其总含量达到90%以上,且砾石物源区变化不显著;⑤ T2砾石层和T1砾石层是典型的河流沉积,而T3砾石层可能是河流沉积和泥石流沉积叠加作用的产物,并且T3砾石层的沉积特征对于分析古青弋江的发育有一定的启示作用。  相似文献   

17.
萨拉乌苏河地区粘土矿物组合分析与古气候的关系   总被引：6，自引：1，他引：5  
卢小霞 《中国沙漠》1985,5(2):27-35
萨拉乌苏河嘀哨沟湾剖面中晚更新统下部萨拉乌苏组,晚更新统上部的城川组中部,全新统中下部河湖沉积或黑垆土中的粘土矿物组合以伊利石为主,但伴有少量的高岭石类矿物;而晚更新统上部的城川组上下部,全新统上部嘀哨沟湾组以风成沙为主的粘土矿物组合均以伊利石-绿泥石为主,并有少量的蒙脱石。萨拉乌苏河雷家咀子、马达渠以北西北部梁地的第四纪地层中的粘土矿物组合与萨拉乌苏河嘀哨沟湾剖面的结果基本吻合。这表明本区第四纪风成沙与黄土是在相对干冷气候下沉积的,而河湖相、湖沼相及古土壤是在相对暖湿的气候下形成的。由本区第四纪地层中的粘土矿物组合,反映自晚更新世以来,本区气候曾经历暖湿→干冷→温湿→干冷→暖湿→干冷的多次更替,大体与北半球的玉木——里斯间冰期、前玉木冰期、玉木间冰阶、玉木主冰期、冰后转暖期和新冰期气候变化趋势一致。  相似文献   

18.
青藏高原东缘水系的演化历史长期存在着重大争议,鉴于任一水系的形成演化都是通过主要河谷的发育及其不断延展与整合完成的,因此确定河谷发育的起始时代是研究水系演化的关键。本文针对渭河上游三阳川盆地最高级阶地形成时代的研究,发现李家小湾河流阶地砾石层的ESR年代为1.26±0.15 Ma和1.32±0.19 Ma,26Al/10Be埋藏年代为1.45±0.70 Ma和1.04±0.43 Ma,说明该段河谷形成于早更新世晚期。综合青藏高原东缘夷平面、剥蚀面与河流阶地的研究成果,推断该区现代河谷系列主要形成于1.2 Ma以后,河流平均下切速率较高,为0.1~0.32 m/ka,指示了中更新世以来该区快速的地表抬升与河谷发育过程;而其前少数地段的先成河谷下切速率介于0.04~0.29 m/ka之间,说明区域地势总体低平,地表过程以剥蚀夷平为主,即高原东缘的现今水系格局主要是第四纪期间构造和气候共同作用下河流侵蚀的产物。  相似文献   

19.
芙蓉峡是常山"国家地质公园"内一具独特地貌现象的山区河段——河床狭窄、谷坡陡峭、阶地宽广。该文从地貌、沉积、年代和构造四方面研究导致该现象的地貌事件过程、发生时间和发生原因等。结果显示:正是由于芳村溪袭夺金源溪上游及所引发的河流强烈下切,才形成芙蓉峡段地貌景观;这次河流袭夺发生的年代约在52.38-33.6 kaB.P.,可能与区域一NE向断裂的一次强烈活动有关。该研究成果有助于揭示公园的地学科学内涵,挖掘其地学旅游价值,指导当地地学科普旅游的开发。  相似文献   

20.
民国时期广州市建成区主体建筑立体形态的恢复及其特征   总被引：1，自引：0，他引：1  
基于1928年1﹕10 000的地形图,解译土地利用类型,确定建成区边界与建筑用地的空间范围。将建筑用地分为单体建筑、线状道路两侧的骑楼和面状同类建筑区3类。根据史料与现场调查,分别为点、线、面3类建筑的高度赋值,恢复不考虑海拔的建成区主体建筑立体形态。基于所建模型,从平面形态、平均高度、空间格局、宽高比、高度轴、天际线以及形态组合几方面对民国时期建成区立体形态特征进行分析归纳,得出如下结论：１）民国时期广州城市沿珠江发展使得城区轮廓呈狭长的带状;２）城市平均高度仅为6.07 ｍ,建筑物普遍低矮;３）建筑物在平面上为多级的棋盘网组合,而骑楼街构成的宽浅网格,成为当时空间尺度最大的建筑单元组合形态;４）道路宽度与两侧建筑物高度形成的宽高比大,城市具有较好的通透性;５）沿江西路与当时的城市中轴线中山纪念堂―市府合署大楼构成了倒“T”型城市空间高度轴,并在单体建筑高度空间分布与天际线的变化上得到很好的反映;６）城市立体形态主要由盆地、格网、谷地、台地等4种宽浅的单元组成。立体形态深层次上反映了当时生产力技术条件下政府与市场力量的综合作用,以及城市功能区分化在立体形态上的表现,从而也成为该时期华南地区依山傍水城市立体形态的重要标志。  相似文献   

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