Drowned shelf-edge deltas, barrier islands and related features along the outer continental shelf north of the head of De Soto Canyon, NE Gulf of Mexico

Four drowned shelf-edge delta complexes, two drowned shelf deltas, three drowned barrier islands, large areas of “hardground”, and fields of bedforms on the mid and outer continental shelf and uppermost slope north of the head of De Soto Canyon, NE Gulf of Mexico were mapped with high-resolution multibeam echosounder. Deltas formed not during the last eustatic low stand, but during one or more interstadials when eustatic sea levels were only 60 to 80 m below present sea level. The barrier islands and deltas must have been cemented prior to rapid falls of eustatic sea level that occurred during global glaciations. Cementation is necessary to have preserved the barrier islands from erosion and subsequent destruction by the rapid sea-level rise during the last deglaciation. The preservation of the relict bathymetry is so good that features that superficially resemble trough blowouts are found in association with one of the relict barrier islands. Asymmetric bedforms on the midshelf in water depths of 50 to 60 m indicate transport directions to the SW but asymmetric bedforms in water depths of the upper slope between 100 and 120 m on the S and SE flanks of the drowned shelf-edge deltas indicate a different current direction, a separate flow that is a continuation of a SW-flowing current that was previously found on the upper slope off NW Florida.     

PUNCTUATED DRAINAGE OF AN ICE-DAMMED QUATERNARY LAKE IN SOUTHERN SOUTH AMERICA

A chain of east‐west elongated lakes fringes the eastern flank of the Andes Mountains between 39 and 52°S in southern Chile and Argentina. These deep lakes fill valleys left by the retreat of outlet glaciers of the Patagonian Icefield about 16000 years ago. Two of the lakes, Lago General Carrera/Buenos Aires and Lago Cochrane, represent the shrunken remnants of a much larger lake (called here the predecessor lake) that developed in a moat alongside the icefield. A series of seven stepped lacustrine braid deltas on the southeastern shore of Lago General Carrera/Buenos Aires mark the progressive, punctuated drainage of the predecessor lake. The deltas were formed by the build‐out of fans of sand and gravel into the lake. They have the form of Gilbert deltas, with virtually flat subaerial topset beds and steeply inclined subaqueous foreset beds. The exposed delta fronts are marked by a series of small terraces produced by wave erosion during falling lake levels. On either side of the fans are embayments with beaches formed by deposition in the lee of the pro‐grading fans. These embayments are filled with ridges of well‐sorted gravel deposited by wave‐driven long‐shore drift. About 13 000 years ago the regional drainage underwent a remarkable change. Rivers flowing eastwards into the Atlantic reversed their direction to flow westwards through the Andes Mountains and into the Pacific Ocean. The watershed moved some 200 km to the east. The cause of this change, and the falling lake levels, was melting of the Patagonian Icefield. As the ice retreated the icefield split into north and south components. This broke the ice dam that impounded the western end of the predecessor lake. The stepped, progressive fall of the lake level suggests that as the ice melted a series of lower overflow sills of hard rock were exposed. Each of the major episodes of sudden drop in lake level was followed by a long period of up to thousands of years when lake level remained constant. Terraces on the delta fronts indicate that each major episode of fall in the lake level was punctuated by up to ten minor episodes possibly representing major storm events or annual cycles of freezing and melting, blocking the overflow sills.     

Reconstruction of glacial Lake Hind in southwestern Manitoba, Canada

Glacial Lake Hind was a 4000 km² ice-marginal lake which formed in southwestern Manitoba during the last deglaciation. It received meltwater from western Manitoba, Saskatchewan, and North Dakota via at least 10 channels, and discharged into glacial Lake Agassiz through the Pembina Spillway. During the early stage of deglaciation in southwestern Manitoba, part of the glacial Lake Hind basin was occupied by glacial Lake Souris which extended into the area from North Dakota. Sediments in the Lake Hind basin consist of deltaic gravels, lacustrine sand, and clayey silt. Much of the uppermost lacustrine sand in the central part of the basin has been reworked into aeolian dunes. No beaches have been recognized in the basin. Around the margins, clayey silt occurs up to a modern elevation of 457 m, and fluvio-deltaic gravels occur at 434–462 m. There are a total of 12 deltas, which can be divided into 3 groups based on elevation of their surfaces: (1) above 450 m along the eastern edge of the basin and in the narrow southern end; (2) between 450 and 442 m at the western edge of the basin; and (3) below 442 m. The earliest stage of glacial Lake Hind began shortly after 12 ka, as a small lake formed between the Souris and Red River lobes in southwestern Manitoba. Two deltas at an elevation of 450 were formed in this lake. At the same time, the Souris Lobe retreated far enough to allow glacial Lake Souris to expand farther north along the western side of the basin from North Dakota into what was to become glacial Lake Hind. Three deltas were built at an elevation above 460 m in the Canadian part of this proglacial lake. Continued ice retreat allowed the merger of glacial Lake Souris with the interlobate glacial Lake Hind to the east. Subsequent erosion of the outlet to the Pembina Spillway allowed waters in the glacial Lake Hind basin to become isolated from glacial Lake Souris, and a new level of glacial Lake Hind was established at 442 m, with 5 deltas built at this level by meltwater runoff from the west. Next, a catastrophic flood from the Moose Mountain uplands in southeastern Saskatchewan flowed through the Souris River valley to glacial Lake Souris, spilling into Lake Hind and depositing another delta. This resulted in further incision of the outlet (Pembina Spillway). A second flood through the Souris Spillway from glacial Lake Regina further eroded the outlet; most of glacial Lake Hind was drained at this time except for the deeper northern part. Coarse gravel was deposited by this flood, which differs from previous flood gravel because it is massive and contains less shale.     

新月形沙丘迎风坡气流加速模拟

迎风坡气流加速是决定新月形沙丘发育和形态的动力因子，研究这一加速过程的定量关系有利于认识沙丘的发育、沙丘的形态、沙丘的运动等。通过大量的野外实测数据和前人的理论分析，得出沙丘迎风坡气流加速量主要和沙丘坡度、迎风坡前来流风速以及沙丘高度有关，并建立了新月形沙丘迎风坡气流加速和沙丘形态特征的定量关系。根据这一关系得了在风力较强的风沙环境中，沙丘坡度较小；在风力较弱的风沙环境中，沙丘坡度较陡。地表粗糙度越大，迎风坡坡度越陡。沙丘地貌发育高度存在一上限，这一上限与边界层厚度有关。     

库姆塔格沙漠形成演化与区域新构造运动关系研究

库姆塔格沙漠位于塔里木板块东部的阿尔金山北麓地带,受青藏高原新构造运动强烈隆升和贯穿本区的阿尔金断裂系发生左旋走滑变动的影响,构筑了南高北低的地质构造变为盆山格局。通过现今的断裂左旋走滑、多风向吹扬及南侧高山洪水及泥石流侵蚀地形共同作用表现出帚状弧形展布。风沙地貌特征表现为由东北向西南分异明显、沙丘类型复杂、形态独特,主要以羽毛状沙丘、金字塔沙丘著称。     

Intraplate uplift: new constraints on the Hoggar dome from the Illizi basin (Algeria)

Zones of anomalously high topography within continental interiors, distant from active plate boundaries, are interpreted as being either dynamically supported by viscous flow in the underlying mantle or influenced by plate tectonics. Constraining the models of their genesis requires accurate data on the timing and dimensions of such features. New apatite fission‐track and thermal maturity data from the Illizi Basin in Algeria quantify the magnitude and timing of kilometre–scale uplift and exhumation of the northern flank of the Hoggar swell in North Africa. The findings of this study, integrated with previously published thermochronological data, confirm that long‐wavelength regional uplift occurred during the Cenozoic extending over a distance in excess of 1500 km from north to south. The uplift, centred on the Hoggar Massif, significantly impacted the flanking Illizi and Tim Mersoï basins. The combination of thermal history modelling and regional stratigraphic observations indicates that the onset of exhumation of the Illizi Basin likely occurred during the Eocene, broadly coincident with magmatism on the Hoggar Massif to the south and the onset of tectonic shortening in the Atlasic belt to the north.     

禹门口南黄河东岸沙丘初步研究

在禹门口南山西省河津、万荣县境内黄河阶地上发育了一些固定、半固定沙丘,注入黄河的汾河将这些沙丘分成了南北两部分。北部的沙丘多在海拔400m左右,高出现代黄河约35m,位于黄河的第二级阶地上,由两条南北向的沙垄和两垄之间分布的抛物线形沙丘构成特殊的景观。这些抛物线形沙丘一般高度约7~8m,背风面突出,迎风坡坡度一般为15°~20°,背风坡坡度一般大于40°。风沙堆积下伏的河流堆积物的¹⁴C年龄为26.40±0.8kaBP,表明沙丘形成于距今26ka以来。南部沙丘海拔高度近500m,高出现代黄河水面150m,覆盖于黄河第三级阶地之上。一个天然剖面揭示,第三阶地的底部为黄色中粗砂与灰绿色粉砂粘土互层,中部为灰黄色黄土,顶部为黄色风沙层。在黄土地层中可以识别出S₁古土壤和位于风沙堆积下面的可能为L₁SS₁的弱成土层。由此推断风沙堆积于距今3万a以来。黄河阶地上的沙丘是在特殊的地区和特定的时期;强大的风力、丰富的沙源和气候变干条件下形成的。虽然东岸沙丘已是固定、半固定沙丘,但沙化仍然是该地区的一个问题。由于3~4a前开始在丘前洼地栽种苹果树,破坏了下风向沙丘迎风面的植被,造成了原来已固定沙丘的活化,在沙丘顶部生成了新的小抛物线形沙丘,高度大于1 m。沙丘上裸露的树根指示表土已被侵蚀0.3 m。南部沙丘区的风蚀作用也很强,例如,万荣县裴庄乡西范村北一片柿树林的树根由于风蚀已经裸露1.2 m。     

中国西南纵向岭谷区孢粉空间组合对阻隔作用的指示意义

35 surface soil samples and 28 profile soil samples were collected in Longitudinal Range-Gorge Region, Southwest China. The distribution of pollen and spores and their relationship with the environment are analyzed. The results show that there are significant differences in pollen and spores taxa, amounts, dominant taxa and florae between the eastern and western flanks of the Ailao Mountain. There are more pollen and spores taxa, a greater amount and complexity on the western flank of the mountain than those on the eastern flank, while the dominant taxa are more centralized and prominent on the eastern flank of the Ailao Mountain. As for the plant florae distribution, there are significant differences in the number of taxa and composition between the eastern and the western flanks. The analysis of profile pollen and spores assemblage shows that its trends of variation are basically in accordance with the surface pollen and spores, which are also indicators of the environmental differences between the two flanks of the mountain. These significant spatial differences of pollen and spores are very close to the regional topography and climate difference. The mountain running perpendicular to the moisture-laden air flow serves as a barrier and the monsoon circulations the east and the west of the mountain result in different energy-moisture characteristics. We conclude that the main patterns of the pollen and spores can be attributed to the environmental differences.     

North-south vegetation transition in the eastern Qinling-Daba Mountains

The Qinling-Daba Mountains are the main body of China's North-South Transitional Zone.Analysis of the north-south gradual variation of vegetation components is significant for understanding the structural diversity and complexity of this transitional zone.In this study,based on survey data of plant communities,the eastern Qinling-Daba Mountains is divided into four geographic units:the north flank of eastern Qinling Mts.,south flank of eastern Qinling Mts.,north flank of eastern Daba Mts.and south flank of eastern Daba Mts.We also explore division of regional climate according to areal differentiation of plant-species,com-munity structure and species-richness,respectively.The results show that,(1)at plant-species level,there are mainly northern plants in north flank of eastern Qinling Mts.with evergreen species and fewer northern plants in south flank of eastern Qinling Mts.;there are mainly southern plants in eastern Daba Mts.(2)At community structure level,there are 4 formations(3 northern formations and 1 widespread formation)in north flank of eastern Qinling,6 formations(3 northern formations,1 southern formation,and 2 widespread forma-tions)in south flank of eastern Qinling,4 formations(2 southern formations and 2 widespread formations)in north flank of eastern Daba Mts.,and 3 formations(3 southern formations)in south flank of eastern Daba Mts.In terms of the numbers and properties of formations,there is a mixture of northern and southern formations only in the south flank of eastern Qinling Mts.(3)At species-richness level,the diversity of families,genera and species decreased with increasing latitude,but the mixing of northern plants and the southern plants began to occur in south flank of eastern Qinling Mts.This means that the south flank of the eastern Qinling Mts.serves more suitably as the dividing line between China's warm temperate and sub-tropical zones.     

青海湖西岸风成沙丘特征及成因

沙漠化是青海湖周围生态环境恶化中一个非常突出的问题,尤其湖西岸沙丘是近十多年来才形成的风沙堆积,并逐年扩大,已成为青海湖区第二大风沙堆积区。通过对青海湖西岸沙丘的分布、形态、物源及粒度特征的分析,对其形成进行了研究,并对布哈河三角洲的形成发育与沙漠化之间的关系进行了初步探讨。认为河流的自然摆动和截直造成了该三角洲东北缘的停止发育,加之气候的持续干旱和严重的鼠害,是导致研究区土地沙化、沙漠面积不断扩大的一个主要因素。     

腾格里沙漠东南缘格状沙丘表面气流及其地貌学意义

对腾格里沙漠东南缘沙丘形态与区域气流的分析表明,格状沙丘发育在双向低风能环境。沙丘表面气流及沉积过程的观测结果,主梁长期处在横向气流环境,其形态主要受背风坡分离气流控制,并以迎风坡侵蚀、背风坡净堆积形式顺主风向迁移。副梁处在双向-斜向气流环境,背风坡的附体偏向气流控制其形态和动力学过程,而且其强度是原始风入射角余弦的对数函数。在斜向风作用下,副梁迎风坡侵蚀的同时,背风坡中下部也发生沙粒的纵向输移且堆积于主梁背风坡,从而导致副梁沿年合成输沙方向延伸。由此认为,格状沙丘的主梁形成于主风-西北风的作用,而副梁是在主梁基础上并在主风和次风-东北风的交替作用下形成的;其形态动力学类型应属纵向沙丘置于横向沙丘之上一种复杂型沙丘。     

Role of fault interactions in controlling synrift sediment dispersal patterns: Miocene, Abu Alaqa Group, Suez Rift, Sinai, Egypt

Although fault growth is an important control on drainage development in modern rifts, such links are difficult to establish in ancient basins. To understand how the growth and interaction of normal fault segments controls stratigraphic patterns, we investigate the response of a coarse-grained delta system to evolution of a fault array in a Miocene half-graben basin, Suez rift. The early Miocene Alaqa delta complex comprises a vertically stacked set of footwall-sourced Gilbert deltas located in the immediate hangingwall of the rift border fault, adjacent to a major intrabasinal relay zone. Sedimentological and stratigraphic studies, in combination with structural analysis of the basin-bounding fault system, permit reconstruction of the architecture, dispersal patterns and evolution of proximal Gilbert delta systems in relation to the growth and interaction of normal fault segments. Structural geometries demonstrate that fault-related folds developed along the basin margin above upward and laterally propagating normal faults during the early stages of extension. Palaeocurrent data indicate that the delta complex formed a point-sourced depositional system developed at the intersection of two normal fault segments. Gilbert deltas prograded transverse into the basin and laterally parallel to faults. Development of the transverse delta complex is proposed to be a function of its location adjacent to an evolving zone of fault overlap, together with focusing of dispersal between adjacent fault segments growing towards each other. Growth strata onlap and converge onto the monoclinal fold limbs indicating that these structures formed evolving structural topography. During fold growth, Gilbert deltas prograded across the deforming fold surface, became progressively rotated and incorporated into fold limbs. Spatial variability of facies architecture is linked to along-strike variation in the style of fault/fold growth, and in particular variation in rates of crestal uplift and fold limb rotation. Our results clearly show that the growth and linkage of fault segments during fault array evolution has a fundamental control on patterns of sediment dispersal in rift basins.     

Development and setting of the Alcudia Bay beach–dune system (Mallorca, Spain)

The beach–dune system of Alcudia Bay is located in the north of the island of Mallorca. The system includes both simple and compound parabolic dunes formed by N to NNE winds and is made up of two dune areas with different dynamics: 1) the northern area, where, despite the fact that the source of beach sediment is from the south, dunes are formed by northerly winds and develop a narrow and linear barrier which separates a marsh from the bay; and 2) the southern area, where complex parabolic dunes formed by north-northeasterly winds, prograde south-southwest landward extending several kilometers inland. The broad pattern of the dunefield size is limited to the southeast by a mountain range and to the west by the effects of a topographic corridor, oriented north–south, which channels the prevailing northerly wind, causing a southward dune progression and limiting the westward extension of the dunefield.     

近40ka来海南岛海岸沙地气候与环境演变

选择海南岛东、西海岸具有代表性和高分辨率特征的木堆、棋子湾等地层剖面,在建立年代序列的基础上,对风成沙粒度组成与参数、SC/D值及磁化率等代用指标的研究表明,末次冰期间冰阶时气候相对温暖,滨海平原上河流三角洲前移,湿地、沙丘与交错分布;末次盛冰期时气候干凉,海岸沙地上形成了多道平行延伸的古沙垄(丘),并与大陆架古沙丘形成连续性沉积;末次冰消期时气候快速变化,古沙垄(丘)或加积发育或固定成壤;全新世以来气候回暖并波动变化,次生沙丘经历多次半固定、固定－沙丘加积、活化的演变。     

腾格里沙漠西部和西南部风能环境与风沙地貌

腾格里沙漠西部和西南部的沙漠边缘地区位于石羊河流域的下游地区,该地区生态环境脆弱,成为近年来备受关注的区域之一。利用自动气象站2009年年度风况资料和Google Earth影像,对该地区的风能环境与风沙地貌进行讨论,为评价区域风沙活动强度,风沙地貌形态特征提供依据。研究表明,腾格里沙漠西部和西南部的风况、风能环境呈自北向南逐渐变化的趋势,年平均风速西部最大,中部次之,南部最小;起沙风风向在北部以西北风和东北风为主,中部以西北风和东南风为主,而南部以西北风为主,东南风很少。研究区的北部为高风能环境,中部和南部为低风能环境。研究区的沙丘类型主要为格状沙丘,在沙漠边缘的部分地区为新月形沙丘链,南部为植被线形沙垄,其主要是由地形作用形成的,沙垄之间为新月形沙丘链。风能环境、沙源和植被共同影响沙丘的形态参数,研究区中部的风能比南部大,沙源比南部多,植被比南部少,因此,格状沙丘主梁之间的间距要比南部新月形沙垄的间距大。格状沙丘的走向近似相同,均在205°~225°之间。研究区南部有范围较大的植被线形沙垄,其间距在0.8～2.0 km之间,平均间距为1.37 km;走向为近似南北（164°～176°之间）。     

Significance of pollen and spores distribution to the barrier function of the Longitudinal Range-Gorge Region, Southwest China

35 surface soil samples and 28 profile soil samples were collected in Longitudinal Range-Gorge Region, Southwest China. The distribution of pollen and spores and their relationship with the environment are analyzed. The results show that there are significant differences in pollen and spores taxa, amounts, dominant taxa and florae between the eastern and western flanks of the Ailao Mountain. There are more pollen and spores taxa, a greater amount and complexity on the western flank of the mountain than those on the eastern flank, while the dominant taxa are more centralized and prominent on the eastern flank of the Ailao Mountain. As for the plant florae distribution, there are significant differences in the number of taxa and composition between the eastern and the western flanks. The analysis of profile pollen and spores assemblage shows that its trends of variation are basically in accordance with the surface pollen and spores, which are also indicators of the environmental differences between the two flanks of the mountain. These significant spatial differences of pollen and spores are very close to the regional topography and climate difference. The mountain running perpendicular to the moisture-laden air flow serves as a barrier and the monsoon circulations the east and the west of the mountain result in different energy-moisture characteristics. We conclude that the main patterns of the pollen and spores can be attributed to the environmental differences.     

金字塔沙丘形成的动力条件分析

金字塔沙丘(或星状沙丘)形成的动力条件主要有三个方面:一是在非沙质床面的气流场辐合区,一般具有三个以上近似均布的主风向,各主风向的风力强度、出现频率及持续时间不尽相同,但年总输沙能力较为接近,而且月输沙能力具有明显的季节变化;二是不太充裕的沙源和高浓度不饱和风沙流;三是不同尺度地形条件的动力作用,和沙漠与戈壁下垫面的热力影响。在沙质床面,往往由于高浓度饱和或过饱和风沙流的作用,导致新月形沙丘或新月形沙丘链的变态,或重新组合成金字塔状的沙丘形态,但不可能发育为典型的金字塔沙丘。     

青海湖东岸的风沙堆积

风沙堆积和风成沙丘的发育在分布上服从于两条规律, 一条是地带性规律, 即绝大多数沙漠均分布于干旱地区, 或在副热带高压带及其产生的信风带, 或在温带干旱大陆中心, 另一条是非地带性规律, 即在某些非干旱区域如河流、海岸或湖泊的沙质滨岸地带, 只要有一定的风力条件也可形成风沙堆积。     

海岸沙丘成因的讨论

本文以中国和西非二个典型沙丘海岸为例,说明海滨沙丘是海陆相互作用下形成的,特别是在冰期极地气候条件下,低海面时形成广大的岸外沙质浅滩,极地气旋风暴作用,促进海岸沙丘的形成发育。