新元古代冰期及其年代

新元古代在全球范围内出现了几期冰期事件,称之为“雪球地球”事件。这种剧烈的环境变化带来此后地球上生命演化的一次飞跃。“雪球地球”事件的核心是全球冰期的同时性,需要同位素地质年代学的证据。新元古代末期两次主要的冰期事件是Marinoan冰期和Sturtian冰期,其中Marinoan冰期结束于635Ma;Sturtian冰期可能发生在710～720Ma,已发表的年龄数据限定它在670Ma之前结束。Marinoan冰期后的Gaskiers冰期发生在580～590Ma。对华南的古城、铁丝坳、长安组、江口组等进行进一步精确定年,将对限定Sturtian冰期持续时间和Cryogenian、南华系的下限年龄具有重要意义。     

Structural and stratigraphic evolution of Abu Dhabi in the context of Arabia

The Emirate of Abu Dhabi is famed for its coastal carbonate, sabkhas and sand dunes; it is located in the NE part of the Arabian Plate, which formed during the Late Neoproterozoic (~820–750 Ma) by the accretion of island arcs and microcontinents to early Gondwana. Most of Arabia seems to have spent its existence within the Southern Hemisphere until it crossed the Equator during the Mesozoic; parts were involved in four glaciations, two in the Proterozoic (~750–630 Ma—Iceball or Slushball Earth?), and two more in the Palaeozoic (Late Ordovician and Permo-Carboniferous transition). In the early Palaeozoic the Arabian Plate was oriented about 90° counter clockwise relative to today’s poles. Gondwana later skirted the South Pole, migrating to the other side of the planet, eventually emerging the ‘right-way up’ with the Arabian Plate oriented to the poles more or less as seen today. Cold and temperate climate conditions ensured that for much of its early existence, Arabia was the site of mainly quartz-rich deposits. Later in the Neoproterozoic, however, extensive stromatolitic carbonate deposition took the lead, culminating around the Cambro-Precambrian boundary with deposition of the extensive Ara and Hormuz evaporites. Since south Arabia’s Permo-Carboniferous glaciation, the Arabian plate has been drifting northward, crossing temperate climatic zones conducive to fluvial and aeolian sandstone deposition and, from the later Permian, to tropical shallow-marine carbonates and evaporites In parallel with the above, the rifting of Gondwana opened an oceanic trough in the Late Permian off the NE flank of Arabia. Slope carbonates and deepwater Hawasina turbidites with a clear flow to the NE were deposited until they were obducted (together with associated ophiolites) in the Late Cretaceous on the edge of the Arabian plate in Oman and Iran. The deposition of widespread Early Silurian hydrocarbon source rocks in east-central Arabia was followed in the later Permian by extensive reservoir rocks with more during the mid-Late Mesozoic, giving rise to major oilfields both on- and off-shore, including Abu Dhabi. Arabia and Africa began to separate late in the Miocene with the opening of the Red Sea and Gulf of Aden. SSW–NNE compressive stresses caused uplift and volcanic activity in west Saudi Arabia and Yemen. Some products of erosion flowed eastward into Abu Dhabi. At the NE margin of Arabia, the Tethys Ocean narrowed, the NE flank of the newly forming Zagros Mountains of Iran is being subducted beneath southern Asia. To the SE, roughly coeval crustal compression adjacent to the Gulf of Oman led to uplift of the Oman Mountains and deposition of erosional products flanking the mountains mainly to the W and SW. The Oman Mountains are currently rising at about 2 mm/a, while northern Musandam is subsiding into the Strait of Hormuz at some 6 mm/a in association with subduction of the Arabian plate margin below the Eurasian plate. Alternations between polar glaciations and interglacials over the past few 100 ka resulted in considerable climatic changes over Arabia; slow glacial build-ups lasting some 80 to 120 ka led, somewhat erratically, to a fall in sea level of up to 130 m, to strong winds and the building of systems of extensive sand dunes such as the Rub’ al Khali. The joint Tigris–Euphrates river system flowed through a desert landscape, reaching the ocean only SE of the Strait of Hormuz. The peak of the last glaciation about 21 ka was followed by its rapid collapse and flooding of the Arabian Gulf to its present level between about 12 or 10 and 6 ka, a horizontal marine advance of some 200–300 m/a. Abu Dhabi is now the site of shallow-marine carbonates offshore and classical sabkhas and carbonate-rich sand dunes onshore.     

Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea

Passchier, S., Laban, C., Mesdag, C.S. & Rijsdijk, K.F. 2010: Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea. Boreas, Vol. 39, pp. 633–647. 10.1111/j.1502‐3885.2009.00138.x. ISSN 0300‐9483. Changes in subglacial bed conditions through multiple glaciations and their effect on ice dynamics are addressed through an analysis of glacigenic sequences in the Upper Pleistocene stratigraphy of the southern North Sea basin. During Elsterian (MIS 12) ice growth, till deposition was subdued when ice became stagnant over a permeable substrate of fluvial sediments, and meltwater infiltrated into the bed. Headward erosion during glacial retreat produced a dense network of glacial valleys up to several hundreds of metres deep. A Saalian (MIS 6) glacial advance phase resulted in the deposition of a sheet of stiff sandy tills and terminal moraines. Meltwater was at least partially evacuated through the till layer, resulting in the development of a rigid bed. During the later part of the Saalian glaciation, ice‐stream inception can be related to the development of a glacial lake to the north and west of the study area. The presence of meltwater channels incised into the floors of glacial troughs is indicative of high subglacial water pressures, which may have played a role in the onset of ice streaming. We speculate that streaming ice flow in the later part of the Saalian glaciation caused the relatively early deglaciation, as recorded in the Amsterdam Terminal borehole. These results suggest that changing subglacial bed conditions through glacial cycles could have a strong impact on ice dynamics and require consideration in ice‐sheet reconstructions.     

成冰纪全球冰期事件的环境效应研究进展

成冰纪全球冰期是地球历史上最极端的冰室气候事件,冰川作用波及赤道区域,全球可能都遭受了冰封,海洋广泛缺氧,生物演化进程迟滞。然而,冰期结束之后,大气氧浓度迅速升高,海洋发生逐步氧化,大型带刺疑源类和真核多细胞藻类在埃迪卡拉纪开始繁盛,出现最早的动物,地表生物圈发生了翻天覆地的变化。显然,成冰纪全球冰期事件是地球系统演化的重要转折。认识冰期的环境效应是认识埃迪卡拉纪生物演化的关键,也是打开地表宜居环境演化的钥匙。本文总结了近年来成冰纪全球冰期的气候假说、冰期沉积特征、海洋氧化还原条件及冰期后的大气与海洋环境剧变等方面的研究进展,简要分析了全球冰期研究中存在的问题,并对该领域未来研究提出了展望与建议。     

A Weichselian deglaciation model applied to the Early Permian glaciation in the northeast Karoo Basin,South Africa

It generally is assumed that the Early Permian Gondwana deglaciation in South Africa started with a collapse of the marine ice‐sheet. The northeast part of the Karoo Basin became ice‐free as a result of this collapse. The deglaciation here probably took place under temperate glacial conditions. Three glacial phases have been identified. Phase 1: the marine ice retreat of 400 km over the northeast Karoo Basin, which may have been completed over a few thousand years. The glaciers grounded in the shallower areas around the shore of the basin. Phase 2: the smaller, now mainly continental ice‐sheet here re‐stabilised and remained more or less stationary for several tens of thousand years. During this phase, between 50 and 200 m of massive glaciomarine mud with dropstones accumulated in the open, marine basin that became ice‐free during Phase 1. Isostatic uplift, as a response to the first rapid deglaciation phase, can be traced in the inland part of the region. Phase 3: the final deglaciation may have taken 10 to 20 kyr. After this time no new ice sheet was built up over southern Africa. The entire Early Permian deglaciation of the northeast Karoo Basin was completed within thousands rather than millions of years. Phases 1 and 3 had lengths similar to typical Quaternary deglaciations, whereas Phase 2 was a long, stable phase, more similar to a full Quaternary glaciation. Copyright © 2001 John Wiley & Sons, Ltd.     

中国北方埃迪卡拉纪冰碛岩及冰川作用

中国北方从东到西绵延3000 km范围上的华北克拉通、塔里木克拉通及其中间微地块上都保存有埃迪卡拉纪冰川沉积记录,但有关它的年代学、冰川规模、古地理重建和大地构造背景等存有争论。本文基于中国北方埃迪卡拉纪冰碛岩空间分布、地层与沉积层序,沉积环境与沉积相等,结合以往国内外文献,系统分析了埃迪卡拉纪冰川上述有关问题。研究揭示,埃迪卡拉纪时期,国内外应存在年轻于(Gaskiers)580 Ma的冰期;中国北方埃迪卡拉纪冰川时限约562.5～551 Ma,堆积了冰下、冰缘和冰前沉积相(物),构成垂向上(由下至上)从冰下至冰前与海相冰碛物沉积层序,符合大陆冰川(盖)沉积响应样式;此外,国内外埃迪卡拉纪冰碛岩及冰川剥蚀地貌均十分发育;部分冰碛岩之上还可见盖帽白云岩,并呈现与成冰纪盖帽白云岩类似的沉积构造,但彼此碳同位素剖面却不尽相同;本研究推测,埃迪卡拉纪时期,原特提斯洋及周缘大陆(群)可能普遍存在至少是洲际性大陆冰盖,甚至是全球性的冰期。研究认为,埃迪卡拉纪时期的亚洲陆块群应与冈瓦纳大陆缺乏构造亲缘性的若干重要证据。本文研究结果在埃迪卡拉纪大陆及其古地理重建和大地构造背景恢复方面具有重要科学意义...     

新元古代晚期盖帽碳酸盐岩的成因与“雪球地球”的终结机制

新元古代晚期约635 Ma的地球发育了到达赤道附近的冰川作用，地质记录上表现为代表寒冷气候的冰期沉积杂砾岩，直接被代表温暖环境的碳酸盐岩层（常称盖帽碳酸盐岩）覆盖。由于盖帽碳酸盐岩奇特的岩石学和地球化学特征，引起了对其成因认识的巨大争论，提出了“雪球地球”和“甲烷渗漏”等假说。“雪球地球”假设可以解释一些令人困惑的地学现象，如低纬度和低海拔冰川沉积、盖帽碳酸盐岩、碳酸盐δ13C负漂移和条带状铁矿层等，但许多科学家对此提出了质疑。最近对盖帽碳酸盐岩的δ13C分析结果（最低达-41‰）、盖帽碳酸盐岩发育的类似现代冷泉碳酸盐岩沉积组构等似乎支持“甲烷渗漏”假说。     

Proterozoic (pre-Ediacaran) glaciation and the high obliquity,low-latitude ice,strong seasonality (HOLIST) hypothesis: Principles and tests

Sedimentological observations and palaeomagnetic data for Cryogenian glacial deposits present the climatic paradox of grounded glaciers and in situ cold climate near sea-level, glaciomarine deposition, and accompanying large (up to 40 °C) seasonal changes of temperature, all in low to near-equatorial (< 10°) palaeolatitudes (equated with geographic latitudes). Neither the “snowball Earth” nor the “slushball Earth” hypothesis can account for such strong seasonality near the palaeoequator, which together with findings from sedimentology, chemostratigraphy, biogeochemistry, micropalaeontology, geochronology and climate modelling argue against those scenarios. An alternative explanation of glaciation and strong seasonality in low palaeolatitudes is offered by a high (> 54°) obliquity of the ecliptic, which would render the equator cooler than the poles, on average, and amplify global seasonality. A high obliquity per se would not have been a primary trigger for glaciation, but would have strongly influenced the latitudinal distribution of glaciers. The principle of low-latitude glaciation on a terrestrial planet with high obliquity is validated by theoretical studies and observations of Mars. A high obliquity for the early Earth is a likely outcome of a single giant impact at 4.5 Ga, the widely favoured mechanism for lunar origin. This implies that a high obliquity could have prevailed during most of the Precambrian, controlling the low palaeolatitude of glaciations in the early and late Palaeoproterozoic and Cryogenian. It is postulated that the obliquity changed to < 54° between the termination of the last Cryogenian low-palaeolatitude glaciation at ≤ 635 Ma and the initiation of Late Ordovician–Early Silurian circum-polar glaciation at 445 Ma.The High Obliquity, Low-latitude Ice, STrong seasonality (HOLIST) hypothesis for pre-Ediacaran glaciation emerges favourably from numerous glacial and non-glacial tests. The hypothesis is in accord with such established or implied features of Cryogenian glaciogenic successions as extensive and long-lived open seas, an active hydrological cycle, aridity and palaeowesterly (reversed zonal) winds in low palaeolatitudes, and the apparent diachronism or non-correlation of some low-palaeolatitude glaciations. A pre-Ediacaran high obliquity also offers a viable solution of the faint young Sun paradox of a warm Archaean Earth. Furthermore, reduction of obliquity during the Ediacaran–early Palaeozoic would have yielded a more habitable globe with much reduced seasonal stresses and may have been an important factor influencing the unique evolutionary events of the Ediacaran and Cambrian. The palaeolatitudinal distribution of evaporites cannot discriminate unambiguously between high- and low-obliquity states for the pre-Ediacaran Earth. Intervals of true polar wander such as postulated by others for the Ediacaran and Early Cambrian imply major mass-redistributions within the Earth at those times, which may provide a potential mechanism for reducing the obliquity during the Ediacaran–early Palaeozoic.     

Late-glacial and post-glacial deposition in a large, low relief, epicontinental basin: the northern Baltic Sea

This paper presents a model of late‐glacial and post‐glacial deposition for the late‐Neogene sedimentary succession of the Archipelago Sea in the northern Baltic Sea. Four genetically related facies associations are described: (i) an ice‐proximal, acoustically stratified draped unit of glaciolacustrine rhythmites; (ii) an onlapping basin‐fill unit of rotated rhythmite clasts in an acoustically transparent to chaotic matrix interpreted as debris‐flow deposits; (iii) an ice‐distal, acoustically stratified to transparent, draped unit of post‐glacial lacustrine, weakly laminated to homogeneous deposits; and (iv) an acoustically stratified to transparent unit of brackish‐water, organic‐rich sediment drifts. The debris‐flow deposits of the unit 2 pass laterally into slide scars that truncate the unit 1; they are interpreted to result from a time interval of intense seismic activity due to bedrock stress release shortly after deglaciation of the area. Ice‐berg scouring and gravitational failure of oversteepened depositional slopes may also have contributed to the debris‐flow deposition. Comparisons to other late‐Neogene glaciated basins, such as the Hudson Bay or glacial lakes formed along the Laurentide ice sheet, suggest that the Archipelago Sea succession may record development typical for the deglaciation phase of large, low relief, epicontinental basins. The Carboniferous–Permian glacigenic Dwyka Formation in South Africa may provide an ancient analogue for the studied succession. Chronological control for the studied sediments is provided by the independent palaeomagnetic and AMS‐¹⁴C dating methods. In order to facilitate dating of the organic‐poor early post‐glacial deposits of the northern Baltic Sea, the 10 000 year long Lake Nautajärvi palaeomagnetic reference chronology ( Ojala & Saarinen, 2002 ) is extended by 1200 years.     

塔里木盆地西南缘叶城地区新元古代冰期事件

新元古代冰期事件记录了“雪球地球”事件重要的地质信息。塔里木盆地周缘新元古代冰碛岩地层露头发育,是研究新元古代冰期事件的理想基地。由于发育多套新元古代火山岩,盆地东北缘库鲁克塔格地区新元古代冰碛岩地层时代已获得较多年代学数据约束;但盆地周缘其他地区新元古代冰碛岩地层公开报道年代学数据较少,不能准确限定其沉积时代,导致冰期事件对比存在争论。为此,本文选择塔里木盆地研究程度较低的西南缘叶城地区新元古代冰碛岩地层,开展岩石学、同位素年代学、岩石地球化学等研究,明确其冰期沉积特征,约束其沉积时代,开展冰期事件对比,讨论古气候风化条件等。南华系波龙组和雨塘组冰碛岩地层具有较低的化学蚀变指数(CIA),分别代表新元古代2次寒冷的冰川气候记录。冰川沉积及其相邻层位的碎屑锆石U-Pb年代学数据显示,波龙冰期的起始年龄晚于(710±13) Ma,与全球Sturtian冰期对应;雨塘冰期的起始年龄不会早于(656±18) Ma,其结束年龄可被南华系顶界年龄635 Ma或上覆震旦系库尔卡克组碎屑锆石年龄(634±9) Ma限定,与全球Marinoan冰期对应。     

One,two or no record of late neoproterozoic glaciation in South-East Cameroon?

Severe climate changes culminating in at least three major glacial events have been recognized in the Neoproterozoic sedimentary record from many parts of the world. Supportive to the global nature of these climatic shifts, a considerable amount of data have been acquired from deposits exposed in Pan-African orogenic belts in southwestern and western Africa. By comparison, published data from the Pan-African belts in Central Africa are scarce. We report here evidence of possibly two glacial events recorded in the Mintom Formation that is located on the margin of the Pan-African orogenic Yaoundé belt in South-East Cameroon.In the absence of reliable radiometric data, only maximum and minimum age limits of 640 and 580 Ma, respectively, can at present be applied to the Mintom Formation. The formation consists of two lithostratigraphic ensembles, each subdivided in two members (i.e., in ascending stratigraphic order the Kol, Métou, Momibolé, and Atog Adjap Members). The basal ensemble exhibits a typical glacial to post-glacial succession. It includes diamictites comprising cobbles and boulders in a massive argillaceous siltstone matrix, and laminated siltstones followed by, in sharp contact, a 2 m-thick massive dolostone that yielded negative δ¹³C values (<?3‰ V-PDB) similar to those reported for Marinoan cap carbonates elsewhere. However, uncertainty remains regarding the glacial influence on the siliciclastic facies because the diamictite is better explained as a mass-flow deposit, and diagnostic features such as dropstones have not been seen in the overlying siltstones. The Mintom Formation may thus provide an example of an unusual succession of non-glacial diamictite overlain by a truly glacial melt-related cap-carbonate.We also report the recent discovery of ice-striated pavements on the structural surface cut in the Mintom Formation, suggesting that glaciers developed after the latter had been deposited and deformed during the Pan-African orogeny. Striations, which consistently exhibit two principal orientations (N60 and N110), were identified in two different localities, in the west of the study area on siltstones of the Kol Member, and in the east on limestones of the Atog Adjap Member, respectively. N60-oriented striae indicate ice flow towards the WSW. Assigning an age to these features remains problematical because they were not found associated with glaciogenic deposits. Two hypotheses can equally be envisaged, i.e., either the striated surfaces are correlated: (1) to the Gaskiers (or Neoproterozoic post-Gaskiers) glaciation and represent the youngest Ediacaran glacial event documented in the southern Yaoundé belt; or (2) to the Late Ordovician Hirnantian (Saharan) glaciation, thereby providing new data about Hirnantian ice flows in Central Africa.     

Chemostratigraphy and lithological characters of Neoproterozoic cap carbonates from the Kuruktag Mountain, Xinjiang, western China

The Neoproterozoic Era includes some of the most largest ice ages in the geological history. The exact number of glaciations is unknown, though there were at least two events of global glaciation. Neoproterozoic glacial deposits in the Kuruktag Mountain, Xinjiang, western China have proven that there had occurred three discrete Neoproterozoic glaciations. Diamictite units occurred in the Bayisi, Tereeken, and Hankalchough formations, carbonate units were recognized among the diamictites and immediately overlied the Bayisi, Tereeken and Hankalchough diamictites. Carbonates at the top of the Bayisi Formation are characterized by the dolo-sility stones with negative δ13C values ranging from -4.10‰ to -8.17‰ (PDB), comparable to the Sturtian cap carbonates that overlie the Sturtian glacial deposits from other Neoproterozoic sequences. Carbonates overlying the Tereeken Formation are characterized by the pinkish cap dolostones (ca. 10 m thick) with negative δ13C values ranging from -2.58‰ to -4.77‰ (PDB), comparable to the Marinoan cap carbonates. The cap is also characterized by tepee-like structures, barite precipitates and pseudomorphous aragonite crystal fan limestones. Carbonates at the top of the Hankalchough Formation are characterized by subaerial exposure crust (vadose pisolite structure, calcareous crust structure) dolostones with negative δ13C values ranging from -4.56‰ to -11.45‰ (PDB) and the calcareous crust dolostones, implying that the Hankalchough cap carbonates differ from either the Sturtian or Marinoan cap carbonates in sedimentary environment and carbon isotopic composition. In addition, it is suggested the Hankalchough glaciation belongs to a terrestrial glaciation and it is the third largest glaciation during the Neoproterozoic period on the Tarim platform.     

Late Cryogenian glaciation in South Australia:Fluctuating ice margin and no extreme or rapid post-glacial sea-level rise

Postulated extreme sea-level rise of up to 1-1.5 km with the late Cryogenian Ghaub deglaciation in Namibia is contentious,as is the great rapidity(<104 yr)of the sea-level rise.Such extreme glacioeustatic events,if real,would have been global and affected all continents.In South Australia,up to six glacial advances and retreats during the late Cryogenian Elatina glaciation indicate a fluctuating ice margin.The latter stage of the Elatina glaciation and the immediate post-glacial environment are examined here for evidence of extreme and rapid sea-level rise.In the central Adelaide Rift Complex,diamictite with faceted and striated clasts occurs at the top of the Elatina Formation<1-2 m beneath the early Ediacaran Nuccaleena Formation’cap carbonate’.One hundred kilometres to the south,~30 m of siltstone and sandstone followed by^6 m of clast-poor diamictite with clasts 10+cm long occur between tidal rhythmites and the cap carbonate.Three hundred kilo metres further south,~70 m of siltsto ne,dolo mitic siltstone and minor dolomite separate tidal rhythmites and early Ediacaran strata.Hence the rhythmites were deposited during a high stand(interstadial or interglacial),not during post-glacial sea-level rise.Storm-generated erosional surfaces within tidal rhythmites at Warren Gorge indicate intermittent rhythmite deposition,and water depth and other palaeoenvironmental factors are uncertain,casting doubt on a published estimate of rapid sea-level rise during rhythmite deposition.The lack of late Cryogenian deeply incised valleys and thick valley-fill deposits in South Australia and central Australia argues against extreme sea-level variations.A hiatus occurred between Elatina deglaciation and deposition of the Nuccaleena cap carbonate,and three palaeomagnetic polarity chrons identified in the cap carbonate imply slow deposition spanning 10^5-10^6 yr.This is supported by independent evidence from magnetic chronostratigraphy for Ediacaran strata in South Australia and California,and by stratigraphic and sedimentological arguments for condensed deposition of cap carbonates.It is concluded that neither extreme nor rapid sea-level rise was associated with late Cryogenian deglaciation in South Australia.     

The last glaciation of the Arctic volcanic island Jan Mayen

The volcanic island of Jan Mayen, remotely located in the Norwegian-Greenland Sea, was covered by a contiguous ice cap during the Late Weichselian. Until now, it has been disputed whether parts of the island south of the presently glaciated Mount Beerenberg area were ever glaciated. Based on extensive field mapping we demonstrate that an ice cap covered all land areas and likely also extended onto the shallow shelf areas southeast and east of the island. Chronological interpretations are based on K-Ar and ⁴⁰Ar/³⁹Ar dating of volcanic rocks, cosmogenic nuclide (³⁶Cl) surface exposure dating of bedrock and glacial erratics, and radiocarbon dating. We argue that ice growth started after 34 ka and that an initial deglaciation started some 21.5–19.5 ka in the southern and middle parts of the island. In the northern parts, closer to the present glaciers, the deglaciation might have started later, as evidenced by the establishment of vegetation 17–16 cal. ka BP. During full glaciation, the ice cap was likely thickest over the southern part of the island. This may explain a seemingly delayed deglaciation compared with the northern parts despite earlier initial deglaciation. In a broader context, the new knowledge of the Late Weichselian of the island contributes to the understanding of glaciations surrounding the North Atlantic and its climate history.     

Paleozoic glaciation in NE Africa: field and remote sensing-based evidence from the South Eastern Desert of Egypt

ABSTRACT

Exhumed Paleozoic glacial deposits and landforms of the North Gondwana are reported here for the first time from the South Eastern Desert (SED) of Egypt. Using field observations and remote sensing datasets (Advanced Land Observing Satellite [ALOS], Phased Array L-band Synthetic Aperture Radar [PALSAR] radar, multispectral Landsat TM datasets, and digital elevation models [DEMs]), we mapped the distribution of Paleozoic glacial features (i.e. deposits and landforms) in the SED. Two main glaciogenic facies were identified in three locations in the SED: (1) massive, poorly sorted, matrix supported, boulder-rich diamictites in Wadi El-Naam and Korbiai, and (2) moderately-sorted, occasionally bedded outwash deposits in Betan area. Inspection of radar, DEMs, and Landsat Operational Land Imager (OLI) images revealed previously unrecognized ENE-WSW trending glacial megalineations (MLs) over the peneplained Neoproterozoic basement rocks in the central sections of the SED, whose trends align along their projected extension with those of glacial features (tunnel valleys and striation trends) reported from Saudi Arabia. The glaciogenic features in the SED are believed to be largely eroded during the uplift associated with the Red Sea opening, except for those preserved as basal units beneath the Nubia Sandstone Formation or as remnant isolated deposits within paleo-depressions within the basement complex. The apparent spatial correlation of the SED glacial features with well-defined Late Ordovician deposits in North Africa and in Saudi Arabia, and the reported thermochronometric analyses and fossil records are consistent with a Late Ordovician age for the SED glaciogenic features and support models that call on the continuation of the Late Ordovician (Hirnantian) ice sheet from the Sahara into Arabia through the SED of Egypt.     

华南扬子陆块成冰纪冰川作用的启动时限及其全球对比*

江口冰期和南沱冰期是华南地区引人注目的2次成冰纪冰川事件,但其确切启动时间及其全球对比关系仍未有定论。为此,对桂北地区成冰系(南华系)长安组底部和南沱组底部冰成杂砾岩开展了碎屑锆石U-Pb年代学研究。长安组碎屑锆石U-Pb年龄集中分布于958—717 Ma,显著峰值为720、753、805及848 Ma,最年轻一组 ²⁰⁶Pb/²³⁸U 年龄的加权平均值为719.6±6.1 Ma,可解释为长安组最大沉积年龄;南沱组碎屑锆石U-Pb 年龄集中分布于987—649 Ma,显著峰值为650、720、753、779、803、823及848 Ma,最年轻一组 ²⁰⁶Pb/²³⁸U 年龄的加权平均值为649.3±6.2 Ma,可解释为南沱组最大沉积年龄。结合已发表的相关年龄数据可知,江口冰期很可能启动于ca.715 Ma,与塔里木、阿拉伯—努比亚、劳伦等陆块的Sturtian冰川作用高度同步;南沱冰期的启动应晚于650 Ma,与西伯利亚、澳大利亚、劳伦等陆块的Marinoan冰川作用基本同步。另外,碎屑锆石U-Pb年龄谱与CL图像显示,长安组和南沱组的物质来源主要为下伏新元古界岩浆—沉积记录,揭示出冰川对下伏地层的强烈刨蚀作用和华南新元古代幕式构造岩浆热事件。扬子陆块成冰纪冰川刨蚀作用可能与Rodinia 超大陆“裂离”有关的强烈伸展活动存在联系,并可能持续至Marinoan 冰期结束。     

Palaeolatitude of glacial deposits and palaeogeography of Neoproterozoic ice ages

At the end of the Neoproterozoic, the Earth may have experienced important environmental changes, with a transition between two supercontinents (from Rodinia to Gondwana), extensive glaciations with ice caps reaching the Equator and the beginning of metazoan diversification. In such a context, the palaeomagnetic record can be used to constrain both the palaeogeography and the palaeoclimate (palaeolatitudinal distribution of glacial deposits). Here we present an up-to-date geochronological and palaeomagnetic database for the Neoproterozoic glacial deposits, including poles recently obtained on ‘cap carbonates’ from China, Oman, and Amazonia. The database comprises ten poles (from eight different cratons), obtained directly on the glacial deposits or on the overlying ‘cap carbonate’, and two other palaeolatitudes derived from reference poles coeval to well-dated glacial units in the same craton. The occurrence of glacial deposition at low latitudes (<30 °) is attested by some good-quality poles, two of them well dated at ∼740 and ∼635 Ma. Based on these poles and on reference poles obtained on igneous rocks, tentative palaeogeographic reconstructions for ∼750, ∼620, and ∼580 Ma (ages for which the database has limited but still sufficient entries) were performed in order to investigate the tectonic context within which the glacial events were produced.     

IX. The St. Vincent Basin

Late Palaeozoic glaciation in Australia, discovered over a century ago, is now known to have covered a large part of the continent. In South Australia, tillite and outwash debris lie upon clearly striated pavements within glacial valleys, and show that ice sheets with valley tongues moved northward from sources now occupied by deep ocean south of the continent. These glaciers reached into the Cooper, Arckaringa, and Pedirka Basins at the end of the Carboniferous and laid down patches of till in the Early Permian, now preserved largely in the subsurface. In Tasmania, an ice sheet waxed in the latest Carboniferous from sources to the west of the island, and deposited till and “drop‐stones” into fossiliferous marine strata until well into the Late Permian. In Victoria, the ice cap laid down till on a striated floor, and here and there sequences of outwash, including boulder pavements. In New South Wales, continental glaciation expanded eastward to the sea early in the Permian, and left a record intercalated with volcanics and coal beds into the Late Permian. Bordering the Tamworth Trough of northern New South Wales, and occurring also in the highlands of New England, alpine glaciers left a record in the form of striated stones and dropstones, in very thick sequences of fluviatile, lacustrine, and marine clastic sediments. The mountains existed in Middle and early Late Carboniferous times, and were largely worn down to gentle relief when continental glaciers expanded northward in the Early Permian. A non‐glacial interval at the end of the Carboniferous therefore probably occurred in New South Wales. In Queensland, alpine glaciers occupied mountains at the western rim of the Bowen Basin at the end of the Carboniferous. Large blocks carried by icebergs from glaciers of unknown locations were dropped into Lower and Upper Permian strata of the Bowen Basin as well. In Western Australia Early Permian ice centres were located on the Yilgarn Block, east of the Perth Basin, on the Pilbara Block southwest of the Canning Basin, and on the Kimberley Block. Evidence for this glaciation consists mostly of ice‐rafted debris and fluvial‐glacial and glacial‐marine strata that reached as far north as the Bonaparte Gulf Basin.

The rapid growth northward of continental glaciers in Australia near the end of the Carboniferous corresponds with a rapid shift of palaeolatitude as judged from Irving's palaeomagnetic studies. The ice sheet grew quickly upon upland areas when Gondwanaland moved to a near polar position and the unfrozen Palaeo‐Pacific lay near at hand to provide an abundant source of moisture.     

Climatic catastrophes in Earth history: two great Proterozoic glacial episodes

Near the beginning and end of the Proterozoic Eon (2.5 Ga–542 Ma) the Earth went through dramatic climatic perturbations. The Palaeoproterozoic (Huronian) glaciations are best known from the Canadian Shield where there is evidence of at least three such episodes. Glacial deposits of comparable age are also known from Fennoscandia, South Africa and Western Australia. In the type area, the Huronian glacial deposits are preserved in an ancient rift system that preceded break‐up of the supercraton, Kenorland, whereas those in the southern hemisphere may have been deposited in a foreland basin setting. Detailed correlations between the two hemispheres must await more geochronological data. Following a long period (~1.5 Ga) with little evidence of glaciation, the climatic upheavals of the Neoproterozoic Era began. The two most widespread glacial events are known as the Sturtian and Marinoan. The Neoproterozoic glaciations also took place on a supercontinent (Rodinia). Some were accompanied by unexpected rock types such as dolomitic cap carbonates and iron formations, both of which show evidence of hydrothermal influence. Major influences on surface temperatures on Earth include solar luminosity (increasing throughout geological history) and the concentration of atmospheric greenhouse gases such as CO₂ (generally diminishing with time). It is suggested that the two great Proterozoic climatic oscillation periods resulted from perturbations of the balance between these two variables, triggered by drawdown of atmospheric CO₂ during intensive weathering of supercontinents. A weathering‐related negative feedback loop resulted in multiple glaciations with intervening warm periods. Climatic stability only returned after the supercontinent broke apart and reduced continental freeboard moderated continental weathering. Copyright © 2012 John Wiley & Sons, Ltd.