Stratigraphic and tectonic settings of Proterozoic glaciogenic rocks and banded iron-formations: relevance to the snowball Earth debate

Among Palaeoproterozoic glacial deposits on four continents, the best preserved and documented are in the Huronian on the north shore of Lake Huron, Ontario, where three glaciogenic formations have been recognized. The youngest is the Gowganda Formation. The glacial deposits of the Gowganda Formation were deposited on a newly formed passive margin. To the west, on the south side of Lake Superior, the oldest Palaeoproterozoic succession (Chocolay Group) begins with glaciogenic diamictites that have been correlated with the Gowganda Formation. The >2.2 Ga passive margin succession (Chocolay Group=upper Huronian) is overlain, with profound unconformity, by a >1.88 Ga succession that includes the superior-type banded iron-formations (BIFs). The iron-formations are therefore not genetically associated with Palaeoproterozoic glaciation but were deposited 300 Ma later in a basin that formed as a result of closure of the “Huronian” ocean. In Western Australia, Palaeoproterozoic glaciogenic deposits of the Meteorite Bore Member appear to have formed part of a similar basin fill. The glaciogenic rocks are, however, separated from underlying BIF by a thick siliciclastic succession. In both North America and Western Australia, BIF-deposition took place in compressional (possibly foreland basin) settings but the iron-formations are of greatly different age, suggesting that the most significant control on their formation was not oxygenation of the Earth’s atmosphere but rather, emplacement of Fe-rich waters (uplifted as a result of ocean floor destruction?) in a siliciclastic-starved environment where oxidation (biogenic?) could take place. Some of the Australian BIFs appear to predate the appearance of red beds in North American Palaeoproterozoic successions and are therefore unlikely to be related to oxygenation of the atmosphere.Neoproterozoic glaciogenic deposits are widespread on the world’s continents. Some are associated with iron-formations. Two theories have emerged to explain these enigmatic BIFs. According to the snowball Earth hypothesis (SEH), ice-covered oceans would have permitted buildup of dissolved Fe. Precipitation of Fe-rich sediments would have taken place following reoxygenation of the hydrosphere as the ice cover disappeared. A second theory involves glaciation of Red Sea rift-type basins. Fe-charged brines in such basins would have precipitated on being mixed with “normal” seawater as a result of glacially driven thermal overturn. Both theories provide an explanation of the hydrothermal imprint on the geochemistry of Neoproterozoic BIF but the restricted development of BIF (relative to glacial deposits), evidence of rift activity such as significant facies and thickness changes, and association with volcanic rocks, all favour deposition in a rift environment.Cap carbonates are one of the cornerstones of the SEH. Escape from the snowball condition is said to have resulted from buildup of atmospheric CO₂ while the weathering cycle was stopped. Under such conditions, the first siliciclastic deposits following glaciation, should be extremely weathered, and should be overlain by sedimentary rocks that show a gradual return to more “normal” compositions. Using a chemical index of alteration (CIA) it can be shown that, in the case of the Gowganda Formation, the CIA shows a gradual upward increase, opposite to that predicted by the SEH. The Earth underwent severe climatic perturbations both near the beginning and end of the Proterozoic Eon but whether it attained a totally frozen surface condition (as postulated under the SEH) remains speculative.     

Petrological and isotopic implications of some contrasting Late Precambrian carbonates, NE Spitsbergen

The dominantly shallow-marine Vendian succession of NE Spitsbergen contains distinctive types of carbonate rock. Limestones deposited before Vendian glaciation resemble those described from other Upper Proterozoic successions, being high in Sr and inferred to have been originally aragonitic, including the distinctive 5–10 Jim equant polygonal calcite of cemented shrinkage cracks. In contrast, manganoan stromatolitic limestones within marginal-marine glacial-outwash deposits, and consisting of micrite, microspar and fascicular-optic calcite are interpreted as originally calcitic. The restriction of primary marine calcite to cold seawater is comparable with Recent and Permian carbonates, although the Precambrian example formed in a sea diluted with meltwater. There is good textural preservation of relatively ¹⁸O-rich oolitic dolostones which were cemented in a supratidal environment by artesian fluids. Nevertheless, early diagenetic replacement is inferred, immediately prior to a glacial episode. Post-glacial dolostones are either replacive marine, or evaporative lacustrine, but share rather more negative δ¹⁸O values, closer to the mean of Late Precambrian dolostones. The heaviest oxygen isotope values constrain seawater δ¹⁸O to no more negative than — 2 to — 4_SMOW. The main reason for the pronounced oxygen isotopic depletion of most Late Precambrian carbonates is their initial metastable mineralogy. The possibility of determining palaeolatitudes of the enigmatic widespread Late Proterozoic glaciations by isotopic analysis of freshwater periglacial calcareous precipitates is raised. Significant carbon isotope variations reflect changes in depositional water chemistry: some of these could be global in extent.     

Sedimentology,stable‐isotope geochemistry and palaeoenvironment of dolostones capping late Precambrian glacial sequences in Australia

Thin (<10 m), extensive dolostones conformably capping late Precambrian glaciogenic rocks in the Kimberley region and the Adelaide Geosyncline commonly comprise micritic dolomite apparently of ‘primary’ or early diagenetic origin. Their aphanitic texture, the presence of algal and cryptalgal lamination, tepee structures and intraclastic rocks, and the lack of saline evaporites or their pseudomorphs, suggest deposition mainly in supratidal to lagoonal environments under a seasonally evaporitic/humid climate. Less common oolitic or pelletal rocks and a finely laminated shaly dolostone facies also suggest peritidal to marine influences.

The ‘cap’ dolostones (omitting possible large erratics) have a δ¹⁸O mean of —6.4 ± 1.9% vs. PDB and a δ¹³C mean of —1.9 ± 2.1% vs. PDB. The oxygen mean is comparable to that determined for other late Precambrian ‘primary’ or early diagenetic dolostones not associated with glaciogenic rocks and presumably deposited under warm conditions. The carbon mean, in contrast, is lighter than that determined for other late Precambrian dolostones.

The sedimentological and oxygen‐isotope data are consistent with relatively high formation‐temperatures for the cap dolostones. Abrupt climatic warming at the close of late Precambrian glacial epochs is implied.     

On the early archaean isua iron-formation,West Greenland

In the pre-3.7 Ga old Isua supracrustal belt, West Greenland, a banded iron-formation occurs. The iron-formation can be subdivided into different facies according to composition and mineralogy, and these facies resemble the facies subdivision of younger Archaean and Precambrian iron-formations. The geochemistry of the Isua iron-formation indicates that the secular variation in the contents of phosphorus, calcium and aluminium as well as the Na/K ratios of Precambrian iron-formations can be extended into the Early Archaean. A remarkable feature of the Isua iron-formation is the high chalcopyrite/iron-sulphide ratio. Field relationships and geochemical evidence indicate that the iron-formation is mainly of submarine-exhalative origin from brines of basaltic pedigree.     

Stratigraphy and mineralogy of the manganese ores of Urucum,Mato Grosso,Brazil

Preliminar results of geological mapping and microscopic studies are presented. The mineral deposits of Urucum are clearly different from the rest of the banded iron-formations in Brazil: Urucum is essentially younger, and probably Late Proterozoic, the sequence is unmetamorphic. Two main types of manganese ore exist in Urucum, namely: The Urucum-Santa Cruz type is with finely laminated rhythmic layering, the Mato Grosso type is concretionary, and sometimes nodular. The origin of the Fe- and Mn-minerals is traditionally explained as due to weathering. However, the involvement of volcanic processes cannot be totally excluded. The conglomerates, intercalated in the banded iron-formation, are glacial and possibly belong to the Eocambrian.     

Characteristics of the sinian glaciogenic rocks of the Shennongjia region,Hubei Province,China

Sinian glaciogenic rocks are widespread in Shennongjia, Hubei. They are best preserved in western Shennongjia where the glacial Gucheng and Nantuo Formations are separated by the non-glacial Datanpo Formation, which is chiefly composed of carbonaceous silty shale and siltstone. In eastern Shennongjia most of the Lower Sinian succession is, however, absent and the Nantuo Formation rests with angular unconformity on the pre-Sinian basement.The glacial sedimentary facies mainly comprise three types; namely the lodgement facies, meltwater facies and proglacial subaqueous facies. The lodgement facies is predominant in the eastern area but outwash deposits are more abundant in the western area. Major glacial transport during the Nantuo Ice Age is inferred to have been from the ESE, based on pre-glacial palaeogeography, provenance of clasts and regional variations in glacial sedimentary facies.During the Nantuo Ice Age, the Jingshan Mountain, which is situated to the east of Shennongjia, was a glacial source area. Jianghan glacial erosion and deposition was in the form of an incomplete ring-shape; the Mufu Mountain area, which is situated at the juncture between southeastern Hubei and northwestern Jiangxi provinces, and was far removed from the source areas, received outwash deposits.     

Late Proterozoic African glacial era

A series of large-scale glaciations occurring in the second half of the Late Riphean and Vendian are referred to as the African Glacial Era (Glacioera), which was separated from the preceding Huronian Glacioera by an interglacial of 1.5 Gyr. Six large discrete glacial events (glacioperiods) repeating each 30–50 Myr, occurred during the African Glacioera. The following glacioperiods (in geochronological succession) are recognized: Kaigas, Rapitan, Sturtian, Marinoan, Gaskiers, and Baykonur. Most glacioperiods included several discrete glaciation episodes. Glaciations were accompanied by repeated biosphere change and crises. The dynamic coevolution of climate and biosphere at the end of the Proterozoic facilitated an accelerated development of Earth’s biota, which culminated in the appearance and divergence of multicellular and skeletal fauna. The African Glacioera terminated the Proterozoic and was the time when the Phanerozoic climatic system and the biosphere were forming on the Earth.     

Glaciogenic and related sedimentary rocks: Main lithochemical features. Communication 2. The Paleozoic and Cenozoic

Lithochemical features of the Early-Late Paleozoic and Cenozoic glaciogenic and related sedimentary rocks around the world are considered. Comparison of the bulk chemical composition of diamictites with the average Post-Archean Australian Shale (PAAS) revealed that the diamictites do not show any lithochemical characteristics unambiguously indicating their formation under cool climatic conditions. The chemical index of alteration (CIA) often used in the paleoclimatic reconstructions should be considered as additional (though very important) tool, because it is controlled by local factors. Taking into account these facts, there are likely no grounds to expect that the amount of the redeposited sedimentary material in the Late Paleozoic and Cenozoic glaciogenic rocks is more than in their Early Proterozoic and Late Riphean-Vendian analogues.     

A fundamental Precambrian–Phanerozoic shift in earth's glacial style?

It has recently been found that Neoproterozoic glaciogenic sediments were deposited mainly at low paleolatitudes, in marked qualitative contrast to their Pleistocene counterparts. Several competing models vie for explanation of this unusual paleoclimatic record, most notably the high-obliquity hypothesis and varying degrees of the snowball Earth scenario. The present study quantitatively compiles the global distributions of Miocene–Pleistocene glaciogenic deposits and paleomagnetically derived paleolatitudes for Late Devonian–Permian, Ordovician–Silurian, Neoproterozoic, and Paleoproterozoic glaciogenic rocks. Whereas high depositional latitudes dominate all Phanerozoic ice ages, exclusively low paleolatitudes characterize both of the major Precambrian glacial epochs. Transition between these modes occurred within a 100-My interval, precisely coeval with the Neoproterozoic–Cambrian “explosion” of metazoan diversity. Glaciation is much more common since 750 Ma than in the preceding sedimentary record, an observation that cannot be ascribed merely to preservation. These patterns suggest an overall cooling of Earth's longterm climate, superimposed by developing regulatory feedbacks involving an increasingly complex biosphere.     

Aspects of magmatism and plate tectonics in the Precambrian of England and Wales

The geological features of igneous, metamorphic and sedimentary rocks comprising the Precambrian of England and Wales suggest formation in one or more Precambrian orogenic cycles. They are now interpreted in terms of plate tectonics. Evidence for Late Proterozoic plate subduction in the Mona Complex of Anglesey is suggested by the association of pillow lavas, cherts, high P/T metamorphic rocks and by the occurrence of gabbros and serpentinites with similar features to rocks believed to comprise the oceanic crust. Precambrian rocks in England and South Wales include calc-alkaline plutonic complexes (Malvern and Johnston Complexes), calc-alkaline lavas (Uriconian and Charnian) and basic and intermediate intrusions of tholeiitic affinity (dykes in the plutonic complexes and granophyric diorites in Charnwood Forest). The features of these rocks indicate formation in a continental margin setting and this is consistent with features of the Rushton Schist and Primrose Hill “gneiss” which suggest that they predate the Late Proterozoic orogenic activity. This evidence is consistent with plate tectonic models involving oceanic plate subduction below the Mona Complex from an ocean to the northwest, or from a small ocean basin southeast of the complex. The Warren House lavas show some affinities to ocean floor basalts and are problematic with regard to the Precambrian history of the area.     

The role of glaciations in the biosphere

Glaciations took place in five long intervals of the geologic history, called glacioeras: Kaapvaal (Late Archean), Huronian (Early Proterozoic), African (Late Proterozoic), Gondwanan (Paleozoic), and unfinished Antarctic (Late Cenozoic). The glacioeras were similar in structure, duration, and dynamics of evolution. They consisted of three to six glacioperiods including several discrete glacio-epochs. The glacioeras lasted ~ 200 Myr. They started with small regional glaciations, which expanded, reached intercontinental sizes, and then quickly degraded. There were serious differences between the Precambrian and Phanerozoic glacioeras. A series of ecologic crises related to numerous glacial events led first to abiotic and then to biotic factors. Glaciations caused extinction and stagnation of the Earth’s biota, the appearance of bionovations and new biota, and acceleration of evolution processes. Thus, the glacioeras were the turning intervals of the biosphere evolution.     

Early Permian climate change in the Falkland Islands

An Early Permian glacial diamictite forms a distinctive unit within the Falkland Islands sedimentary succession and two aspects of its significance have recently been serendipitously enhanced. Fossil discoveries in exotic limestone clasts bear on palaeogeography, whilst a series of mineral‐exploration borehole cores have allowed a detailed study of the sedimentary record of deglaciation that followed deposition of the diamictite. Statistical analysis of reflectance and XRF core‐scanning data has identified likely Milankovitch periodicities and enabled tentative time‐scale modelling. The ‘icehouse to greenhouse’ transition appears to have spanned approximately 1.2 million years, with waning cycles of re‐advance superimposed on overall glacial retreat. The new results play into a long‐debated geological paradox: although the Falkland Islands are now proximal to the South Atlantic coastline of South America, their geology bears an uncanny resemblance to that of the Cape Fold Belt and Karoo Basin in South Africa. This puzzled the geological pioneers, but became readily explicable when first continental drift and then plate tectonics were invoked to reconstruct the break‐up of the Gondwana supercontinent—although the details remain controversial. One of the key stratigraphical correlation levels throughout the major fragments of southern Gondwana—South Africa, South America, Antarctica and Australia—is the glacigenic deposit left behind by the extensive, Late Carboniferous to Early Permian regional glaciation; in the Falkland Islands it is designated the Fitzroy Tillite Formation.     

Regional geochemical and isotopic characteristics of high-grade metamorphics of the Prydz bay area: The extent of proterozoic reworking of Qrchaean continental crust in East Antarctica

Preliminary isotopic data for Late Proterozoic (～ 1100 Ma) granulite-facies metamorphics of the Prydz Bay coast indicate only very minor reworking (i.e., remetamorphism) of Archaean continental crustal rocks. Only two orthopyroxene—quartz—feldspar gneisses from the Rauer Group of islands, immediately adjacent to the Archaean Vestfold Block, show evidence for an Early Archaean origin (～ 3700—3800 Ma), whereas the vast majority of samples have Middle Proterozoic crustal formation ages (～ 1600–1800 Ma). The Prydz Bay rocks consist largely of garnet-bearing felsic gneisses and interlayered aluminous metasediments, although orthopyroxene-bearing gneisses are common in the Rauer Group; in contrast, Vestfold Block gneisses are predominantly orthopyroxene-bearing orthogneisses. The extensive Prydz Bay metasediments may have been derived by erosion of Middle Proterozoic rocks, such as the predominantly orthogneiss terrain of the Rauer Group, and deposited not long before the Late Proterozoic metamorphism. Data from nearby parts of the East Antarctic shield also suggest only limited Proterozoic reworking of the margins of the Archaean cratons.As in the Prydz Bay area, high-grade metamorphies in nearby parts of the East Antarctic shield show a secular increase in the sedimentary component. Archaean terrains like the Vestfold Block consist mainly of granitic orthogneisses derived by partial melting of igneous protoliths (I-type), whereas Late Proterozoic terrains (such as the Prydz Bay coast) include a much higher proportion of rocks derived either directly or by partial melting (S-type granitic orthogneisses) from sedimentary protoliths. Related chemical trends include increases in K₂O₂, Rb, Pb, and Th, and decreases in CaO, Na₂O₂ and Sr with decreasing age, essentially reflecting changes in the proportions of plagioclase and K-feldspar.     

Sedimentary facies of a glacially influenced continental succession in the Pennsylvanian Jericho Formation, Galilee Basin, Australia

Recent work on the Late Palaeozoic Ice Age in eastern Australia has shown the Joe Joe Group in the eastern Galilee Basin, Queensland, to be of critical importance as it is one of few records of Pennsylvanian glacial activity outside South America. This paper presents detailed sedimentological data, from which the Late Palaeozoic environment of the region is reconstructed and which, consequently, allows for robust comment on the broader Gondwanan glaciation. The Jericho Formation, in the lower Joe Joe Group, was deposited in an active extensional basin in lacustrine to fluvial environments, during the mid‐Namurian to early Stephanian. The region experienced a cool climate during this time, and polythermal mountain or valley‐type glaciers periodically advanced into the area from highlands to the north‐east. The Jericho Formation preserves a suite of proglacial to terminal glacial facies that is characterized by massive and stratified diamictites deposited from debris flows, massive and horizontally laminated conglomerates and sandstones deposited from hyperconcentrated density flows, laminated siltstones with outsized clasts and interlaminated siltstone/conglomerate deposited through ice‐rafting into lakes, and sedimentary dykes and breccias deposited through overpressurization of groundwater beneath permafrost. Non‐glacial facies are dominated by fluvial sandstones and lacustrine/overbank siltstones. The glacigenic rocks of the Jericho Formation are confined to discrete packages, recording three separate glacial advances during the latest Namurian to late Westphalian. This arrangement is consistent with the temporal distribution of glacigenic rocks from around the remainder of Australia and Gondwana, which supports the theory that glacial deposits occurred in discrete intervals. The Joe Joe Group is a key succession in the world in this context as, at this time, eastern Australia provides the only unequivocal evidence of a Namurian/Westphalian glaciation outside South America. The continuous record of sedimentation through the Pennsylvanian and Early Permian is indicative of significant warming between glacial intervals, which is difficult to reconcile with the development of long‐lived, cold‐based ice sheets across the supercontinent.     

Rare earth elements in Huronian (Lower Proterozoic) sedimentary rocks: Composition and evolution of the post-Kenoran upper crust

The Huronian sequence (Lower Proterozoicl. north of Lake Huron, contains tillites and abundant fine-grained sedimentary rocks. Analyses of rare earth elements (REE) in the matrix of tillite samples from the Gowganda Formation (~ 2.3 Gal is considered to be a reasonable estimate of upper crustal REE abundances for the region north of Lake Huron at the time of Gowganda deposition. The average is characterized by a moderately steep pattern (σLREEσHREE = 9.1) and a slight negative europium anomaly ($\text{Eu}\text{Eu1}\text{= 0.89}$). This pattern is similar to estimates of the composition of the surface of the Canadian Shield and is intermediate between estimates of typical Archean and post-Archean sedimentary rocks. REE patterns for framework granitoid clasts from the tillite suggest that K-rich granites, which were apparently unimportant in the formation of Archean sedimentary rocks, were abundant in the source regions of the Gowganda Formation. This may explain the intermediate nature of the Gowganda pattern.Comparison of the tillites and associated Gowganda mudstones suggests that previous estimates of upper crustal REE abundances, which were based on the analyses of fine-grained sedimentary rocks, may be systematically high. Relative distributions, however, are the same.Analyses of mudstones from the McKim. Pecors. Serpent Gowganda Lorrain and Gordon Lake Formations suggest rapid evolution in the composition of the exposed upper crust at the close of the Kenoran orogeny. REE patterns at the base of the Huronian are similar to typical Archean sedimentary rocks. REE characteristics change up section: patterns at the top resemble typical post-Archean sedimentary rocks.It is inferred that an essentially episodic change from an early exposed upper crust dominated by a tonalite-greenstone suite to one approximating granodioritic composition is recorded in Huronian sedimentary rocks. A deviation from the evolutionary trend of the Huronian, documented in the Gowganda Formation, may be related to the glacial origin of the Gowganda.     

山西芮城水峪地区罗圈组海侵沉积物

华北陆块南缘、西南缘广泛分布的罗圈组及其相应地层多被认为是新元古代末期冰川沉积。山西南部芮城水峪剖面位于华北南缘罗圈组出露范围的最北端,罗圈组受下伏蓟县系龙家园组白云岩风化面形态控制,横向岩性变化剧烈,分别由准原地堆积的风化角砾岩、紫红色含砾泥岩及一套特殊的"含砾砂质鲕粒白云岩"组成,均为温暖气候下海侵或海侵改造产物,与冰川作用无关。"含砾砂质鲕粒白云岩"在华北南缘罗圈组中尚属首次发现,具有明显的指相意义。该地区罗圈组海侵沉积物的存在,可能表明华北南缘罗圈组在各地区存在沉积环境乃至地层时代的巨大差异,不能简单以某一冰期的"冰碛岩"笼统看待。     

Geochemistry of the Late Precambrian Sturt Tillite,Flinders Ranges,South Australia

Analyses of major oxides and some trace elements (Ga, Rb, Sr, and B) were carried out on 25 samples of tillite matrix materials of the major Late Precambrian glacial episode in South Australia. The results indicate the dominantly granitic nature of the source rocks and suggest deposition under marine glacial conditions.     

Beobachtungen zum Problem der jung-Präkambrischen glazialen Ablagerungen in Südwestafrika

The Late Precambrian Numees Formation contains besides very thick deposits of tillite-like habit also laminated siltstones resembling glacial varved rocks. These enclose coarser sand grains, pebbles and boulders of all sizes which have dropped from above into the soft sediment. As only floating ice can have transported these large components, the conclusion is inescapable that the laminated siltstones are true glacial varves and that the tillite-like rocks are true tillites.This evidence for Late Precambrien glacial conditions makes it highly probable that other formations of a similar age, which have been described as tillites, are also of glacial origin. The Buschmannsklippe Formation begins locally with a tillite which seems to be more or less contemporaneous with the Numees Formation. A high percentage of facetted and deeply striated pebbles and boulders makes it probable that this tillite originated as a basal moraine.The Nama tillite in the Klein Karas Mountains is stratigraphically not very far removed and in time probably not very much younger than the Numees tillite. Striated and grooved floors are associated with this tillite.The Chuos tillite and the Otavi tillite of the Damara System are contemporaneous deposits of Late Precambrian age. Their age relative to the Numees tillite is not known. The Chuos- and the Otavi tillite are probably glaciomarine drifts. For these a glacial origin cannot be proved with the same degree of conclusiveness as for the above described deposits. However, the existence of glacial conditions in the Late Praecambrian having been proved for an adjoining area, a glacial origin may reasonably be assumed for the tillite-like rocks of the Damara System too.Both the Numees tillite and the Otavi tillite are intimately associated with sedimentary iron ore deposits. Oxygen deficiency in stagnating bottom waters, caused by an ice cover, is thought to be responsible for this peculiar combination of sediments.The excellent evidence for the existence of glacial conditions during parts of the Late Precambrian, found in South West Africa, strongly supports the assumption that the many tillite-like deposits of a similar age, which have been described from Central and West Africa are also of glacial origin.

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Published under Government Printer's Copyright Authority No. 3320 of 30/4/64.     

中国元古宙地质构造格局演化对于沉积作用的控制

一、五台运动对构造格局的影响五台运动表现的地区,主要是在华北的五台山区、太行山区、冀东、鲁西、豫西地区以及东北的辽东地区,其绝对年龄时限大致为22—23亿年。现已证实,经历五台运动后,华北地区的克拉通化更加明显,被认为属于中国境内克拉通化最早与最完备的地区。在太古宙末期,华北古陆块与西部的塔里木古陆块可能相联的局面至早元古宙末期已逐渐解体;并且这