试论东北地区多年冻土形成的气候条件及其演化

东北地区多年冻土主要分布在大小兴安岭一带,属欧亚大陆多年冻土区的一部分,可称高纬度多年冻土。但在现代多年冻土南界(48°N)以南的长白山与大兴安岭南端的黄岗梁地区,因海拔超过2000米,有山地多年冻土分布。至于古多年冻土的遗迹,如古冰楔、冰卷泥、不对称谷、冻融蠕流沉积等则一直分布在北纬42°的松辽分水岭一带,甚至更南。     

吉林浑江石人北山组时代的探讨

北山组是1956年建立的。当时,将石人北山村到红石砬一带不整合在奥陶纪马家沟灰岩之上的砾岩称作红石砬砾岩层,整合沉积在红石砬砾岩层上的煤系地层叫北山含煤层。在大石沟里所见的红石砬砾岩和北山含煤层,在岩性、岩相上有明显区别的一套下部为砾岩的含煤地层,称为冷家沟砾岩和冷家沟含煤层,统归中侏罗统。前者成岩深度比后者略高,不含火山岩砾石及火山喷发的物质成分;后者含上述成分。以后,吉林省105队、102队,多次     

庐山地区第四纪泥砾沉积物的成因探讨

庐山第四纪泥砾沉积物的砾石组构特征表明, 砾石岩性与山体基岩基本一致, 砾石的等体积球径较小, 加权平均砾径较中值砾径大, 分选系数大于1, 分选性差, φ值相对较小, 平均为0.66, F值相对较大, 在2左右, F/φ比值介于2.75～4.75, ρ值集中且较小, 为22.75%～37.75%, 砾石以棱角、次棱角状为主, 磨圆较差; a轴和ab面都倾向沟谷下游, a轴倾角集中且较小, 为7～8°, ab面倾角为7～21°, a轴较ab面具有更为优势的组构倾向; 沉积物粒度频率曲线在4～5?范围内表现为峰; 石英砂颗粒为次棱角状, 贝壳状断口发育, 具平行擦痕、上翻解理片、V形坑和硅质沉淀。上述沉积学特征表明庐山第四纪泥砾沉积物系冰碛冰水沉积, 而非泥石流或河流堆积。      

吉林省加里东运动的新证据

依据伊通景家台大黑山晚奥陶世陡山头子组(原误比用石缝组)之顶界不整合面上的大黑山砾岩+火山岩(原误划属桃山组下段),于火山岩中获锆石U-Pb SHRIMP年龄为(413.4±6.6)Ma~(420.3±6.6)Ma,相邻的段家店同一地层为(415.0±6.9)Ma~(421.4±6.6)Ma;永吉张家屯生物化石定年为晚—末志留世的张家屯组底砾岩沉积覆盖的英云闪长岩获锆石U-Pb LA-ICPMS年龄(443±5)Ma;共同为吉林省加里东运动提供了有力的新证据。     

山东诸城晚白垩世王氏群恐龙化石层碎屑锆石定年和物源示踪

山东东部诸城晚白垩世王氏群大面积、高密度集群埋藏的恐龙化石是近年研究的热点,但其沉积物源与精确沉积时限目前尚不明确。本文通过对山东诸城王氏群恐龙化石层中6个砂岩样品的碎屑锆石U-Pb定年表明,样品090414-24-D中最年轻的单颗粒锆石年龄(YSG)为77.3 Ma,应代表王氏群恐龙化石层的最大沉积时限(下限),结合前人对王氏群红土崖组的年代学研究表明,其沉积时限应老于73.5 Ma(上限),也说明赋存恐龙化石层的王氏群上部红土崖组为Campanian期沉积;碎屑锆石定年揭示物源的锆石年龄组成分布在100～130 Ma之间,6个样品的年龄主峰值多在110 Ma或120 Ma左右,与下伏的早白垩世青山群火山岩年龄主峰较为吻合。王氏群恐龙化石层砾岩的砾石成分和古水流研究显示,其中3组砾石统计表明砾岩层砾石成分主要由火山岩或火山碎屑岩组成,3组古水流数据表明,诸城地区王氏群红土崖组砾岩层的古流向以南和南东方向为主,尚有1组显示古流向为向北,表明王氏群物源主要来自盆地北侧莱阳和北西侧沂沭断裂带的青山群火山岩,其次为盆地南缘。本研究为诸城王氏群化石层沉积时限的限定和沉积物源的判断,以及胶莱盆地王氏群时期古地理格局的恢复等提供了重要的依据。     

江西抚崇盆地上白垩统河口组砾石统计特征及其地质意义

华东南晚白垩世陆相断陷盆地以发育红色粗碎屑岩为特征。通过对赣杭构造带西段的江西抚崇盆地上白垩统河口组19个砾岩露头测点的砾组分析,对河口组砾岩的砾性、粒度、砾态和砾向等进行了详细研究。总体上砾石成分复杂,分选性较差,磨圆程度中等,呈次棱角－次圆状,风化程度较低。结合红盆周边基岩地层特点,对红盆不同区域砾岩的物质来源、成因和形成条件等进行分析和讨论。研究结果表明,河口组砾岩为晚白垩世半干旱－干旱气候条件下的一套冲积扇和辫状河相粗碎屑沉积,具有多物源和快速堆积的特点。盆地不同地区砾石成分组合特征具有明显差别,反映了盆地周围复杂的物源供应条件。根据盆地基岩地层岩性特点和扁平状砾石倾向玫瑰花图,认为河口组沉积时期,红盆碎屑沉积物主要来自盆地的北东、北、北西和南东部。     

云南省南涧地区下白垩统南新组砾岩层 砾组特征及其物源意义

该文通过对哀牢山构造带西侧云南省南涧地区下白垩统南新组砾岩层的砾性、砾度、砾态和砾向等的详细研究与分 析,结合盆地基岩地层特点和周边构造带的岩性组合特征,发现南新组砾岩总体上具有砾石成分复杂、分选性较差、磨圆程 度中等-较好（次圆状-圆状）、风化程度较低等特征;探讨了砾岩层本身的性质、物质来源,以及砾石沉积时盆地的水动力条 件、气候环境、古地貌特征和构造条件。认为南新组砾岩层为半干旱-干旱古气候条件下的一套冲积扇-辫状河相粗碎屑沉 积,具有快速堆积的特点;砾岩层沉积前砾石遭受了较强的物理化学作用。根据盆地基岩及周边地层岩性特点和砾石扁平面 倾向统计,砾石的主要来源是盆地内部的基底地层及其东缘的哀牢山构造带。     

易门、峨山的花岗岩是古生代的侵入体

易门、峨山一带出露的几个花岗岩株,一直被人们看做是燕山期的产物。最近,我们在易门赵普发现这里岩株上复的禄丰层底部,有以花岗岩物质为砾石材料的底砾岩。禄丰层以北70°东-北75°西走向,向南10°-25°倾斜,并且极为稳定而平整地     

四川盆地西缘晚新生代大邑砾岩的物源及其成因：来自重矿物和孢粉的证据

新生代早期印度与欧亚板块的俯冲碰撞造就了巍峨高耸的青藏高原.然而,在其相邻的四川盆地,伴随青藏高原强烈隆升所堆积的碎屑沉积物,最老仅能追索到晚新生代的大邑砾岩.因此,解读广泛发育于四川盆地西缘的大邑砾岩成因对获取青藏高原及其周缘古环境信息及理解相关的构造演化过程具有重要意义.本文报告了对三个典型大邑砾岩剖面样品的重矿物和孢粉组合研究结果,试图从大邑砾岩的物源区和沉积环境来解读大邑砾岩的成因.大邑砾岩中重矿物种类繁多,表明相应物源区具有多种岩石类型.除极不稳定矿物外,大邑砾岩新鲜剖面样品的重矿物组合与现代岷江沉积极其相似,表明大邑砾岩是经由岷江通道输送到四川盆地的;富含石榴子石等远源矿物表明当时的古岷江已切穿龙门山,深入到松潘-甘孜褶皱带内.孢粉分析显示孢粉含量极低,为0.2粒/克,孢粉类型以高山黯针叶林成分为主,混有少量落叶阔叶林和草本植物花粉,显示了高寒的环境.结合已有的年代学结果,本文认为大邑砾岩是形成于2.0Ma的冰水沉积物,其形成反映了青藏高原第四纪早期的寒冷环境.     

库车坳陷中部新近系一第四系砾岩层的沉积特征及分布规律

为了解决库车坳陷新近系一第四系砾岩层引起的速度变化和工程决策问题,利用砾岩层野外露头和钻井岩屑等资料,对砾岩层的沉积特征及分布规律进行了系统研究.分别对野外砾岩层和钻井岩屑的厚度、岩性、结构构造及砾石成分进行统计分析,在区域上对各层系的砾岩层进行划分和对比.认为库车坳陷新近系一第四系砾岩层可以划分为四期,具有冲积扇、扇三角洲及冲积平原三种沉积成因,它们的规模、水动力条件及沉积构造等具有较大差异.以冲积扇砾岩层规模大,发育四大物源体系,各物源体系的砾石成分分区明显,砾石粒径随相带分异.库车坳陷砾岩层整体具有纵向上多期次连续发育、相带差异明显,横向上相变快、集中分布的特点,新生代构造演化、构造样式及南北向断裂是控制砾岩层发育、沉积特点及分布规律的主要因素.     

Relic permafrost structures in the Gobi of Mongolia: age and significance

Relict permafrost structures (ice-wedge casts and cryoturbation structures) are present in the Gobi of southern Mongolia. Luminescence dates of sediments are presented to constrain the age of formation of permafrost structures. These data show that there was a phase of permafrost development during the latter part of the Last Glacial (after about 22 to 15 ka) that resulted in cryoturbated sediments and ice-wedge casts. Furthermore, permafrost degradation occurred during late Pleistocene times (13–10 ka) and was absent during the early Holocene. These permafrost structures mark the southernmost evidence of permafrost in northern Asia during late Quaternary times and indicate that the mean annual air temperature was below approximately −6°C during their formation. © 1998 John Wiley & Sons, Ltd.     

Ice wedges: Growth,thaw transformation,and palaeoenvironmental significance

Frost-cracking and ice-wedge growth are fundamental processes within the permafrost environment. Extensive areas of contemporary permafrost terrain are characterised by frost-fissure polygons, formed by repeated thermal contraction-cracking of the ground. The incremental growth of ice veins and wedges along the axes of contraction-cracks contributes significantly to the volume of ground ice in near-surface permafrost. In areas beyond the present limit of permafrost, the recognition of ice-wedge pseudomorphs provides one of the few unambiguous indications of the former existence of permafrost conditions. An understanding of the processes of ice-wedge growth and thaw transformation is essential if contemporary ice wedges are to be used as analogues for Pleistocene frost-fissure structures, in palaeoenvironmental reconstructions.     

黄河源区冰楔假型群的发育及其古气候意义*

在黄河源区两岸的第二级阶地砂砾层和基座中发现了两种不同形态的冰楔假型群。一种是发育在阶地砂砾石层的冰楔假型,其特点宽而浅,底部边界呈圆滑锅状,深约0.5~0.9m,宽0.8~1.4m;另一种是发育在第二级阶地基座的基岩中,以窄深倒三角状为特点,其底部尖锐,深约0.7~2.0m,宽为0.3~1.0m。前者形成于全新世中期(5.69±0.43kaB.P.,5.43±0.41kaB.P.),后者形成于末次冰期的冰消期(13.49±1.43kaB.P.)。另外,还在洪积的砂砾石层中发现了规模较大的冻融褶皱(宽3~4m),其时代晚于39.83±3.84kaB.P.,也是末次冰期的产物。根据冰楔假型的对比研究,在全新世的中期(约5.5kaB.P.前后)和冰消期,黄河源区的降温幅度达6~7℃。尤其值得注意的是全新世中期的冰楔假型形成,表明了大暖期气温的不稳定性。     

青海湖北岸原生砂楔的发现及其古气候意义

青海湖北岸刚察县城东与泉吉乡西发现原生砂楔群, 楔内充填物均为中细粒砂, 楔体底部中细粒砂ESR测年数据表明, 刚察县城东3个原生砂楔发育时代分别为(774±70) ka、(773±70) ka和(229±20) ka, 泉吉乡西1个原生砂楔形成时代为(197±18) ka, 反映该区至少发育两期原生砂楔, 其形成时代分别对应昆仑冰期和倒数第二次冰期. 基于原生砂楔的特殊形成环境, 估算青海湖盆地在昆仑冰期早期的年均温不高于-7.5~-10 ℃, 比现今至少低9.2~11.7 ℃, 中更新世早期青海湖盆地发育多年冻土, 青藏高原东北部可能已经进入冰冻圈; 倒数第二次冰期的年均温不高于-9.5~-10 ℃, 比现今低11 ℃以上.     

Periglacial wedges and the late Pleistocene environment of Wyoming's intermontane basins

Nonsorted polygons in the uppermost 2 to 3 m beneath Pleistocene surfaces indicate permafrost at 1340 m and higher elevations in the intermontane and piedmont plains of Wyoming during the Wisconsin, and perhaps earlier, glacial maxima. The polygons, as much as 10 m in diameter, are delineated by wedges that vary in depths, range from narrow to moderately flared forms, and deform host materials. The wedges have silty fine-to-medium sand matrices (largely eolian) with pebbles or clasts from hosts of gravel or bedrock. Some wedges may reflect seasonal cracking in a periglacial active zone, but most are either permafrost sand-wedge relics or, less commonly, ice-wedge casts. Alternative explanations are rejected largely because similar features are apparently lacking in the lower and warmer plains from eastern Colorado southward. The wedges suggest an arid, windy, periglacial environment whose mean-annual temperatures are conservatively estimated as some 10° to 13°C colder than those at present. Although late Wisconsin-early Holocene floral and faunal evidence indicates lowered montane biotic zones, the eolian and periglacial features indicate a lack of extensive forest cover on the basin floors. In conjunction with vertebrate-fossil associations of grazing and tundra animals, the wedges may provide a parallel line of evidence for a former periglacial steppe, or “steppe-tundra”, in the Wyoming basins.     

Clastic wedges and patterned ground in the Late Ordovician—Early Silurian tillites of South Africa

The widespread Late Ordovician to Early Silurian glaciations are represented in South Africa by glaciogenic sediments of the Pakhuis Formation. Previously unrecorded clastic dykes within the basal tillite of this formation on Table Mountain, Cape Town, are interpreted as casts of frost-fissures filled directly with clastics (sand-wedges or ground-wedges) rather than filled subsequent to the melting of ice-wedges (ice-wedge casts). This suggests sub-aerial deposition for the tillite of the Sneeukop Member and the presence of a cold, dry climate with steep temperature gradients following deposition of that tillite in the Table Mountain area. A peculiar three-dimensional joint-pattern shown by the Pakhuis Formation tillites is interpreted as a relict of a reticulate ice-vein network that formed in sandy till in permafrost. Such networks have been documented from the modern environment, but so far their relicts have not been recorded in ancient sediments.     

Thaw modification of frost-fissure wedges,Richards Island,Pleistocene Mackenzie Delta,Western Arctic Canada

Thaw modification is the general process whereby frost-fissure wedges are modified during thaw, and by which frost-fissure pseudomorphs may develop. Specific processes of thaw modification are inferred from ice-wedge pseudomorphs, composite-wedge pseudomorphs and deformed sand wedges in the Pleistocene Mackenzie Delta: i.e. thermal erosion, collapse, subsidence, refreezing, loading, buoyancy, spreading, folding and shearing. Thaw modification is believed to result in selective preservation of pseudomorphs and wedges. Sand wedges are more likely to be preserved than are ice-wedge pseudomorphs or compositewedge pseudomorphs, because only those sand wedges that penetrate massive ice or icy sediments are prone to thaw modification. Furthermore, whereas ice wedges preferentially develop in ice-rich, fine-grained sediments (thaw-sensitive), their pseudomorphs appear to be selectively preserved in ice-poor, coarse-grained sediments (thaw-stable).     

青藏高原2.8Ma来的环境演化及其对构造事件响应

本文根据青藏高原中部错鄂湖深钻研究的最新成果,结合东部若尔盖盆地湖泊沉积物记录,探讨了青藏高原2.8Ma以来的环境演化过程和高原构造隆升运动对环境演化的影响。初步研究显示:大约2.8MaBP错鄂湖构造成盆;2.6MaBP左右孢粉组合、粒度特征、岩性变化等均记录了一次强烈的构造隆升运动;2.6Ma~0.8Ma时段,高原可能处于一种整体隆升过程中的相对夷平阶段;若尔盖古湖揭示了0.9MaBP来的3次构造隆升运动,反映了高原环境演变的三个阶段;和黄土底界相当的错鄂湖沉积记录显示并未干旱的特征;黄土旺盛堆积时期(L₁₅、L₉、L₆)高原湖泊记录的气候特征为偏湿气候。      

土楔和冰楔假形及其古气候意义

由于土楔及冰楔所处冻土的含冰状况不同,二者对冻土退化的响应不同。土楔的形状及大小乃至其中的充填物可以完整地保存下来,而冰楔在融化过程中经受强烈变形,其原形状很难保存下来,以至无法辨认。已报道的冰楔假形中,有相当部分可能是原生土脉,甚至是活动层土楔。冰楔在多年冻土环境下生长,其假形可以指示古冻土曾存在过。但它与温度之间没有简单的对应关系,因为楔形构造的形成是地-气综合因素相互作用之结果。土楔可以是原生的,也可以是次生的,它与多年冻土环境并无必然联系。     

中国的多年冻土──过去与现在

中国多年冻土区的总面积约占中国陆地面积的２２．４％，达２１５００００ｋｍ²。多年冻土的分布特征受气候条件在三度空间的变化所制约。自晚更新世以来，其分布情况已有相当的变化。在东次冰期最盛期，东北地区多年冻土南界曾推进到北纬４１—４２°，在全新世暖期，南界向北退缩，但晚更新世形成的冰楔和多年冻土至今仍存在于大兴安岭北部，全新世中期严寒期冻土有所扩展并形成冰楔。随着气候变化，中国西部高山和高原区高海拔冻土的分布下界已上移８００—１０００ｍ，但高山和高原的主要部分仍处于冰缘环境，有的地方在全新世还发育了共生型多年冻土。