Neoproterozoic Successions Research Articles

A new lithostratigraphic subdivision and geodynamic model for the Pan-African western Saldania Belt, South Africa

A provenance study involving U–Pb and Lu–Hf isotope analyses of detrital zircon grains as well as whole rock geochemical, Rb–Sr, Sm–Nd and Pb isotope analyses of siliciclastic metasedimentary rocks from the principal tectonostratigraphic units of the Pan-African western branch of the Saldania Belt in South Africa revealed age relationships and source domains that are not compatible with current stratigraphic schemes and geodynamic models for the evolution of this belt. The previous subdivision into three terranes cannot be upheld. The sediments in both the former Tygerberg and Swartland terranes seemingly have the same provenance but differ from those in the former Boland Terrane. The latter is considered a para-autochthonous zone on top of basement rocks of the late Mesoproterozoic Namaqua-Natal Belt. On the basis of our new data it is recommended to unify the former Tygerberg and Swartland terranes into a single terrane for which the name Malmesbury Terrane is proposed. The Colenso Fault, previously regarded as a terrane boundary, is reinterpreted as reactivated old structure inherited from the basement within the Malmesbury Terrane, whereas the major terrane-bounding fault is the Piketberg–Wellington Fault, which can be linked most likely with the Schakalsberge Thrust further north in the Gariep Belt.The age of sedimentation is constrained within a narrow time window between 557 and 552Ma in the Malmesbury Terrane and between 609 and 532Ma in the Boland Zone. No evidence of older, Cryogenian sedimentary rocks could be found, except for the volcanic-dominated Bridgetown Formation, which is presumably a tectonic sliver. The Cambrian Klipheuwel Group is undoubtedly younger than, and unrelated with regard to its provenance to, the other Saldanian sediments.All geochemical evidence points to an active continental margin as source for the sediments in the Malmesbury Terrane and Boland Zone. While the ages of the detritus in the former are strongly dominated by Neoproterozoic, especially Cryogenian, ages, those in the Boland Zone are mainly early Tonian and late Mesoproterozoic. The most likely source of the Cryogenian detritus is the Dionisio Cuchilla-Pelotas magmatic arc in southeastern Brazil and Uruguay, which formed within a splinter of Kalahari Craton, i.e. Arachania. A back-arc position relative to that arc is inferred for the depo-basin in which the redefined Malmesbury Group, making up the Neoproterozoic succession of the Malmesbury Terrane, was deposited. Consequently, our new data support the hypothesis that the Neoproterozoic successions of the western Saldania Belt and their northern equivalents in the western Gariep Belt do not reflect a full Wilson Cycle but only the waning stages of an Ediacaran back-arc basin. The main suture between the Kalahari and Rio de la Plata cratons must be located, therefore, to the west of the Dionisio Cuchilla-Pelotas magmatic arc, probably along the Major Gercino-Sierra Balena Lineament within southeastern South America. This implies that the opening of the modern South Atlantic followed to a large extent the axis of the postulated former Ediacaran back-arc basin.

Geochronology and geochemistry of the Hekou Group in Sichuan Province, SW China

The Hekou Group, an outcrop at the west margin of the Yangtze Block, consists of volcanic and sedimentary rocks of sodic lava, sodic pyroclastic rocks, coarse to fine-grained siliciclastic rocks and carbonate rocks. These rocks have endured lower to upper greenschist-facies metamorphism. The Hekou Group represents the basement of the late Mesoproterozoic to early Neoproterozoic successions and hosts many Fe‐Cu deposits. We conducted zircon U‐Pb and Lu‐Hf isotope analyses and whole-rock geochemistry analyses on igneous rocks from the Hekou Group to constrain the formation age of the Hekou Group and to understand the genesis of the Fe‐Cu deposits. A zircon 207 Pb/ 206 Pb age of 1669 ± 6 Ma was obtained from the tuffaceous schist of the Hekou Group and is considered to represent the deposition age of the group. Most rocks show negative high field strength element (HFSE, e.g., Ta, Nb, P, Zr, Hf and Ti) anomalies, light rare earth element (LREE) enrichment ((La/Yb)n = 12.10‐109.01) and positive zircon eHf(t) values (+1.5 to +4.5). The geochemical and isotopic compositions indicate that the Hekou Group was formed in a back-arc basin at 1.7 Ga. Our study also implies that Paleoproterozoic tectonic evolution of the western Yangtze Block is correlated with the assembly and fragmentation of the supercontinent Columbia.

Biogeochemical changes across the Ediacaran–Cambrian transition in South China

The late Neoproterozoic successions of South China comprise some of the most complete and well-preserved records of the Ediacaran–Cambrian transition. The articles in this special issue provide a snapshot of ongoing research that is helping us to understand early stages in the development of the modern Earth system. These studies illustrate the transitional character of the Ediacaran–Cambrian interval from the evolution and early extinctions of the first bilaterian metazoans during the Ediacaran Period to their dominance of pelagic and benthic realms by the end of the early Cambrian. Geochemical data reveal a noisy transition of increasing baseline δ13C values punctuated by extreme isotopic oscillations. It is evident that highly negative δ13C values and large amplitude δ13C swings ceased once modern marine ecosystems had become firmly established by ∼520Ma. We postulate that sub-surface bioturbation helped to strengthen the relationship between benthic oxidation state and P-retention, thus tightening an important negative feedback that helps to stabilize productivity, climate and oxygen levels in the modern Earth system. The disappearance of negative excursions after ∼520Ma might also suggest a key role for methanogenesis which would have declined following the rise of in-sediment sulphur cycling due to bioturbation. Further insight into this fascinating interval is needed before we can disentangle the complex interactions between biological evolution and biogeochemical cycling during the emergence of the modern Earth system.

Timing of Precambrian basement from east segment of Tiekelike tectonic belt, Southwestern Tarim, China: Constrains from zircon U-Pb and Hf isotopic

This study focuses on the determination of the age and provenance of a Precambrian unit on the southwestern margin of the Tarim craton, collectively known as the Ailiankate (埃连卡特) Group and Sailajiazitage (塞拉加兹塔格) Group, which were previously referred to as Paleoproterozoic-Mesoproterozoic. Zircon U-Pb LA-ICP-MS dating of chlorite quartz schist and tuff that form the Ailiankate Group and Sailajiazitage Group have yielded age peaks at 780 and 787 Ma, respectively. These data indicate that the Ailiankate and Sailajiazitage groups are Neoproterozoic succession deposited at ca. 740–790 Ma. Those strata are temporally equivalent and are likely parts of a single wide spread succession and reflect a rifting process related to the break-up of the Rodinia supercontinent. In situ zircon Hf isotopic data from Sailajiazitage Group show that older zircons (∼1 994 Ma) have crustal model ages of 2 272–2 784 Ma, suggesting an ancient crustal growth and reworking. The ∼787 Ma zircons have ∼2 000 Ma crustal model ages, suggesting derivations from recycled Paleoproterozoic material.

Mechanisms of volcaniclastic aggradation in fluvial systems influenced by explosive volcanism: An example from Neoproterozoic Hammamat Group, Wadi Queih area, Central Eastern Desert, Egypt

Wadi Queih basin records different continental settings that interacted with explosive volcanism. This paper discusses the contrasting aggradational mechanisms in fluvial systems strongly influenced by explosive volcanism which took place during sedimentation of the Queih basin. Six main facies associations composed of 12 lithofacies have been recognized in the Neoproterozoic succession filling the Queih basin: (1) lava flows and pedogenically modified pyroclastics facies association, (2) debris-flow-dominated alluvial fan facies association, (3) sheetflood-dominated alluvial fan facies association, (4) crevasse-splay facies association, (5) lacustrine facies association, and (6) loess facies association. These facies assemblages are typical of low-sinuosity rivers flowing through unsymmetrical half graben basin. Petrographical and geochemical data of the Queih sediments indicate a predominantly continental volcanic block provenance and stable craton to fault-bounded basement uplift. Low values of Chemical and Plagioclase Index of Alteration (CIA and PIA) are consistent with low intensity source rock weatherning under prevailing semi-arid to arid climate.During deposition of the lower member of the Queih basin, common fall and flow tuff events occurred, indicating syn-eruptive conditions. In contrast, deposition of the upper member occurred in a fluvial–aeolian setting without input of primary volcaniclastic detritus, indicating inter-eruptive conditions. The change in depositional mode from the lower to upper is considered to be due to a change in the balance between the sedimentation rate and the rate of lake-level rise. Lithofacies stacking and rapid lateral changes of lithological units in conjunction with interformational unconformities and basin margin faults suggest tectonically induced sedimentation. Volcanism can also influence basin evolution and the delicate balance between erosion, sedimentation, and prevalent transport processes is affected the Queih basin by volcanic input. Thus, the Queih basin records the response of fluvial system to large, volcanism-induced sediment loads.

Cryogenian glaciations on the southern tropical paleomargin of Laurentia (NE Svalbard and East Greenland), and a primary origin for the upper Russøya (Islay) carbon isotope excursion

Neoproterozoic successions in NE Svalbard and East Greenland host a pair of glacigenic formations, the younger of which are correlated with the terminal Cryogenian (Marinoan) glaciation based on their lithologically and isotopically diagnostic cap dolostones. A deep negative carbon isotope excursion (CIE) occurs stratigraphically beneath the older glacigenic formations in both areas, but no CIE is preserved beneath the younger glacial horizon in either area. This led to uncertainty over the age of the CIE and the paleoenvironmental significance of the units separating the glacigenic formations. 87Sr/86Sr ratios in Sr-rich limestone associated with the CIEs are ≤0.7064 in East Greenland and ≤0.7068 in NE Svalbard, consistent with early Cryogenian values globally and inconsistent with late Cryogenian ratios, which are exclusively ≥0.7071. The CIEs are tentatively correlated with the Islay excursion in the Scottish and Irish Caledonides, and potentially correlative subglacial CIEs in northwestern Canada, northwestern Tasmania and the southwestern United States. In NE Svalbard, newly-acquired δ13Corg data covary with δ13Ccarb across the CIE, but the organic excursion is roughly half the amplitude of the inorganic excursion. δ13Ccarb and δ18Ocarb do not covary, nor does δ13Corg vary as a function of total organic carbon content. A primary origin for the CIE is supported and the accompanying proliferation of stromatolites suggests rising carbonate saturation (falling pCO2) prior to glaciation. New data suggest that the hiatus responsible for the missing Trezona CIE occurs at the top, not the bottom, of the Bråvika member sandstone in NE Svalbard. A dramatic decline in regional average thickness of older Cryogenian glacial deposits from the paleomagnetically-determined subtropics (British Isles) to the deep tropics (Svalbard) is consistent with meteoric net accumulation and sublimation zones on a snowball Earth, and inconsistent with the Jormungand climate zonation.

The Amadeus Basin, central Australia

The Amadeus Basin of central Australia has a depositional history spanning the Neoproterozoic to the Devonian/Carboniferous. It was initiated as part of the Neoproterozoic Centralian Superbasin, which formed in an intracratonic setting related to the break up of Rodinia. Sedimentation continued until the 580-540 Ma Petermann Orogeny, coinciding with the assembly of Gondwana, which resulted in the fragmentation of the superbasin into separate intracratonic basins. The Petermann Orogeny was focused in the Musgrave Province and the southern part of the Amadeus Basin, and involved significant N-directed shortening on large-scale structures that involved both the basement and the overlying Neoproterozoic sedimentary rocks. It significantly transformed the basin architecture, with the development of major basin features that controlled subsequent sedimentation. Deposition of Paleozoic successions was largely concentrated in sub-basins and troughs in the N of the basin, where up to 14 km is preserved. Minor events or uplifts punctuated this depositional history and account for local disconformities and absent sections. The 450-300 Ma Alice Springs Orogeny was a multi-phase, intracratonic event concentrated in the Arunta Region and the northern part of the Amadeus Basin. Like the earlier Petermann Orogeny, the Alice Springs Orogeny involved both basement and basin sedimentary rocks, but with overall S-directed movement. Synorogenic sedimentation accompanied Mid-Late Devonian uplift, with Late Devonian-Carboniferous basin inversion terminating sedimentation, and folding the youngest successions. The Amadeus Basin has known reserves of U, minor historic and recent Au production, and is prospective for base metals, especially Cu, and phosphate. The Ordovician succession supports commercial gas production, and the Neoproterozoic succession is considered prospective for oil and gas.

Neoproterozoic successions in Peninsular India and their hydrocarbon prospectivity

Abstract The burgeoning oil and gas consumption in India in recent years has necessitated looking into the Proterozoic basins of India, which are sparsely explored and have a scanty knowledge base. The rationale for hydrocarbon exploration in Indian Proterozoic basins is derived from the fact that they have large basinal areas, wide geographical distribution, varied geotectonic setting and sedimentary fill. The favourable tectonic settings of these basins, pronounced biological activity, known hydrocarbon gas seepages, and subsurface commercially viable oil and gas shows in the Bikaner–Nagaur and Vindhyan basins and analogous basins throughout the world necessitate proactive exploration strategies in these basins. The basins of Bikaner–Nagaur, Vindhyan, Cuddapah and Chhatishgarh include thick Neoproterozoic/basal Lower Palaeozoic (Cambrian) successions, in addition to Palaeoproterozoic and Mesoproterozoic sequences. The Neoproterozic sediments in these basins incorporate thick successions of shale, limestone and sandstone. These successions have rich organic matter of high-quality cyanophycean (stromatolites, acritarchs and filamentous algae) affinity that is proven to be high-quality (type one) source material for hydrocarbon generation and also involved in later structurization. However, the Neoproterozoic sedimentary pack in the Bhima–Kaladgi basins is comparatively less thick, and appears to have less prospectivity. The available geological and source-rock data are reassessed for their hydrocarbon prospectivity in order to help in planning a strategy for exploration in these basins.

Neoproterozoic deglacial sediments and their hydrocarbon source rock potential

Abstract Many Neoproterozoic successions contain viable hydrocarbon source rocks, even though they were deposited before most extant life forms evolved. Eukaryotic microalgae, bacteria, chlorophyte micoalgae, marine pelagophyte algae and dinoflagellates may have contributed organic matter. Major global-scale glaciations, which are commonly attributed to a ‘snowball’ or ‘slushball’ Earth scenario, or deposited under a ‘zipper rift’ scenario, are believed to have played an important role in the deposition of hydrocarbon source rocks during the mid- Neoproterozoic (Cryogenian). Phases of Cryogenian deglaciation may have culminated in the deposition of high total organic carbon shales and ‘cap carbonates’ in restricted anoxic basins, which may have been carved by ice sheets themselves or, alternatively, formed as restricted extensional half graben as Rodinia began to fragment. One example of these organically enriched deglacial sediments comprises shales and dolostones deposited following the Sturtian glaciation in the Centralian Superbasin of Australia, an amalgam of basins that extends almost continent-wide across Australia. Data from the Marmot MMDD-1 drill core on the Stuart Shelf in the southern part of the Centralian Superbasin, together with previously published data on organic enrichment in the Amadeus Basin in the central part of the Centralian Superbasin, suggest that the deposition of organically enriched shales was widespread during the Sturtian deglaciation.

Does the global stratigraphic reproducibility of δ13C in Neoproterozoic carbonates require a marine origin? A Pliocene–Pleistocene comparison

The large and systematic negative shifts in the δ 13 C values (>12‰) of carbonate-dominated rocks that preceded Neoproterozoic glacial successions have been interpreted to record a dramatic series of global environmental and evolutionary events. These values are widely considered to be marine rather than diagenetic in origin because stratigraphic patterns of change are systematic and reproducible from basin to basin, distinct in magnitude, and associated with recognizable stratigraphic markers such as glacial deposits. In contrast, diagenetic systems are commonly considered to have a more local and stochastic influence on δ 13 C values. Cores taken in Quaternary carbonate platform sediments, however, reveal a curious similarity in magnitude, thickness, and core to core reproducibility where diagenetic alteration has occurred in response to sea-level fall. Sea-level changes produced similar δ 13 C and δ 18 O stratigraphic records at globally disparate locations, which are unrelated to the global marine δ 13 C signal and bear no relation to the global carbon cycle. By analogy with the Pliocene–Pleistocene, we propose that spatial reproducibility of δ 13 C in some Neoproterozoic successions might be attributed to causes other than secular variation of the global carbon cycle, including diagenesis. This observation does not negate the stratigraphic utility of the carbon isotopic values, only the origin of the signal.

Condensation origin for Neoproterozoic cap carbonates during deglaciation

Neoproterozoic deglacial stratigraphy is commonly characterized by a sharp contact separating glacial sediments from laminated capping carbonates. This stratigraphic relation is generally assumed to have time significance and to reflect an abrupt shift from icehouse to greenhouse conditions. In contrast to this, sequence stratigraphic field studies of an Ediacaran (ca. 635 Ma) glacial to postglacial transition in the Amadeus Basin of central Australia reveal a complex deglacial stratigraphy, in which more than 175 m of conglomerate, sandstone, marl, and carbonate at the basin margin, and portions of four unconformity-bounded sequences, pass basinward into no more than 3 m of laminated dolomicrite of typical cap carbonate facies. The unconformities, which are characterized by as much as several tens of meters of erosional relief (oblique sections of incised valleys), separate intervals of contrasting sediment provenance, and are confidently mapped on the basis of both criteria. Comparable unconformities are absent in the overlying Neoproterozoic succession, which is >2 km thick and encompasses many tens of millions of years. The Amadeus Basin cap carbonate was thus deposited during a protracted interval of multiphase (cyclical) transgression more similar to Phanerozoic cyclical sea-level rise than to the single catastrophic deglaciation and instantaneous precipitation invoked by popular current models to explain the classic cap carbonate. The superposition of carbonate on glacial facies in distal sections evidently records condensation in the absence of siliciclastic sediment rather than abrupt shifts between glacial and tropical conditions. Facies lithologically similar to cap carbonates may be less obvious in Phanerozoic successions because of a secular change in carbonate composition to reefal and deep-sea pelagic deposits.

Chapter 33 Neoproterozoic glaciogenic diamictites of the Tarim Block, NW China

Abstract Neoproterozoic glaciogenic outcrops are mostly limited to the southwestern and northwestern margins of the Tarim basin and the Tianshan Mountains. Only two glaciogenic diamictites are recorded in the Neoproterozoic successions from the Tielikeli and Aksu-Wusi areas of the Tarim basin. The lower diamictite from the Polong Formation in the Tielikeli area is c. 800 m thick. The upper diamictite in the area is very thin (Yutang diamictite, 10 m; Yulmeinak diamictite, 51 m). In contrast to the Tarim basin, the Neoproterozoic successions along the northern margin of the Tarim Block in the Tianshan Mountains record three or four glaciogenic diamictites, which are well developed and crop out in the Qurugtagh area. Although the glaciogenic nature of the oldest Beiyixi diamictite remains questionable, all other diamictites (Altungol, Tereeken and Hankalchough) exhibit distinct glaciogenic sedimentary features. SHRIMP zircon U–Pb dating demonstrate that the Bayisi diamictite was deposited between 740±7 Ma and 725±10 Ma, the Altungol and Tereeken diamictites between 725±10 Ma and 615±6 Ma, respectively, and the Hankalchough diamictite between 615±6 Ma and c. 542 Ma. Carbonates atop the Altungol, Tereeken and Hankalchough diamictites show distinct C-isotopic values that are typical for those recorded in ‘cap carbonates’ on other continents. The striking similarity between the cap carbonate of the Tereeken and Marinoan-age Nantuo diamictites further support a 635 Ma ‘Marinoan’ age for the Tereeken glaciation. Unlike the 582 Ma Gaskiers glaciations of Newfoundland, the Hankalchough diamictite is possibly <551 Ma, as suggested by C-isotope chemostratigraphy and biostratigraphy, indicating that post-Marinoan glaciations on different continents may be diachronous. Supplementary material: Data are available at: http://www.geolsoc.org.uk/SUP18469 .

Chapter 3 Recognition of glacial influence in Neoproterozoic sedimentary successions

Abstract This chapter provides an overview and key references of glacial processes and resulting sedimentary products in subglacial, terrestrial proglacial and glaciomarine or glaciolacustrine settings. These settings are characterized by a wide variety of processes ranging from subglacial lodgement and deformation, ice-push and sediment remobilization, which in turn result in a wide range of products such as diamictite, conglomerate, sandstone, siltstone and mudstone. The sedimentary record of proglacial settings exhibits the most lateral and vertical variability due to the dynamic nature of ice margins and the most direct record of climatic fluctuations. Many Neoproterozoic successions, however, preserve glaciomarine deposits that can provide a more continuous and high-resolution (though indirect) record of change. This chapter will enable the reader to identify features that may be used to infer a glacial influence on the formation of ancient deposits. The chapter also outlines some of the important issues that require consideration when evaluating palaeoclimatic models for Neoproterozoic sedimentary successions. These include the equivocal significance of most commonly used proxies such as occurrence of diamictite, outsized clasts in laminated sediments, clast characteristics, lithostratigraphic trends and sequence boundaries. Careful analysis of multiple lines of sedimentary evidence, together with other proxies of climatic changes, can yield meaningful reconstructions and provide a basis for testing palaeoclimate models for this time period. A summary table outlining the characteristics of diamictite with different depositional origins is also included in order to assist with the interpretation of the Neoproterozoic sedimentary record.

Chapter 1 The geological record of Neoproterozoic ice ages

The IUGS- and UNESCO-funded International Geoscience Programme Project #512 (Neoproterozoic Ice Ages) was conceived to contribute towards a global synthesis of current geological data on the number, duration, extent, causes and consequences of glacial episodes during the Neoproterozoic Era. IGCP 512 attracted more than 200 scientists from over 30 countries, many of whom provided their regional and specialist expertise on Neoproterozoic successions around the world to the realization of this volume. IGCP 512 focused on integrating various aspects of Neoproterozoic geology: geochronology, geochemistry, sedimentary geology, biostratigraphy, palaeomagnetism and economic geology. At its inaugural meeting on 27 August 2005 during the International Association of Sedimentology conference on glacial processes and products in Aberystwyth, Wales, IGCP 512 members decided to produce a volume that summarized existing data sets in a form similar to Earth's Pre-Pleistocene Glacial Record by Hambrey & Harland (1981). An enormous amount of work has been carried out in the 12 years since the publication of Hoffman et al. 's (1998) paper on the Snowball Earth hypothesis for Neoproterozoic glaciation (Fairchild & Kennedy 2007). The Snowball Earth hypothesis and, more generally, Neoproterozoic climate, have been the topic of numerous special volumes, special sessions, a dedicated conference in Ascona (Switzerland) in 2006 (Shields 2006), and numerous documentaries. Motivated by this intense worldwide interest in the Neoproterozoic glaciations and an exploding body of research into the topic, this volume synthesizes the state-of-the-art in this now highly multidisciplinary research field. It is intended to facilitate the integration of data sets, inspire new research projects, and inform ongoing work into the definition and subdivision of the Neoproterozoic timescale, including selection of the Global Stratotype Section and Point (GSSP) for the base of the Cryogenian Period. Despite such lofty aims, any book such as this cannot claim to be complete, and there are …

Chapter 65 Neoproterozoic glacial deposits of the Kimberly Region and northwestern Northern Territory, Australia

Abstract Neoproterozoic glaciogenic formations are preserved in the Kimberley region and northwestern Northern Territory of northern Australia. They are distributed in the west Kimberley adjacent to the northern margins of the King Leopold Orogen, the Mt Ramsay area at the junction of the King Leopold and Halls Creek orogens, and east Kimberley, adjacent to the eastern margin of the Halls Creek Orogen. Small outlier glaciogenic deposits are preserved in the Litchfield Province, Northern Territory (Uniya Fm.) and Georgina Basin, western Queensland (Little Burke Fm.). Glaciogenic strata comprise diamictite, conglomerate, sandstone and pebbly mudstone and characterize the Walsh, Landrigan and Fargoo/Moonlight Valley formations. Thin units of laminated dolomite sit conformably at the top of the Walsh, Landrigan and Moonlight Valley formations. Glaciogenic units are also interbedded with the carbonate platform deposits of the Egan Fm. and Boonall Dolomite. δ 13 C data are available for all carbonate units. There is no direct chronological constraint on these successions, and dispute over the regional correlation of the Neoproterozoic succession has been largely resolved through biostratigraphic, chemostratigraphic and lithostratigraphic analysis. However, palaeomagnetic results from the Walsh Fm. are inconsistent with sedimentologically based correlations. Two stratigraphically defined glaciations are preserved in northwestern Australia: the ‘Landrigan Glaciation’, characterized by SW-directed continental ice-sheet movement and correlated with late Cryogenian glaciation elsewhere in Australia and the world; and, the ‘Egan Glaciation’, a more localized glaciation of the Ediacaran Period. Future research focus should include chronology, palaeomagnetic constraint and tectonostratigraphic controls on deposition.

Chapter 48 Neoproterozoic successions of the São Francisco Craton, Brazil: the Bambuí, Una, Vazante and Vaza Barris/Miaba groups and their glaciogenic deposits

Abstract The Neoproterozoic successions of the São Francisco Craton are primarily represented by the Bambuí and Una groups, deposited in cratonic epicontinental basins, and by the Vazante and Vaza Barris/Miaba groups, which accumulated on passive margins on the edges of the craton. The epicontinental basins comprise three megasequences: glaciogenic, carbonate platform (marine) and dominantly continental siliciclastics. Possible correlative sequences are observed in the passive margin deposits. At least two major transgressive–regressive sea-level cycles occurred during the evolution of the carbonate megasequence, which lies above glaciomarine diamictites of probable early Cryogenian (i.e. Sturtian) age. C, O, Sr and S isotope trends from analyses of well-preserved samples, together with lithostratigraphic observations, provide reasonable correlations for most of the Neoproterozoic successions of the São Francisco Craton. The 87 Sr/ 86 Sr record of these successions, ranging from 0.70769 to 0.70780, supports the proposed correlation with the Bambuí, Una and Vaza/Barris successions, and with the basal units of the Vazante Group. In addition, C-isotope positive excursions ranging from +8.7 to +14‰ and negative excursions from –5.7 to –7‰ VPDB in the Bambuí, Una and Vaza-Barris successions provide key markers for correlations. The precise ages of the sedimentation in these successions remains a matter of debate, but organic shales of two units of the Vazante Group have been dated by Re–Os techniques in two different laboratories, both yielding Mesoproterozoic ages. The Neoproterozoic and Mesoproterozoic successions preserve significant glaciogenic deposits.

Chapter 56 Neoproterozoic (Cryogenian–Ediacaran) deposits in East and North-East Greenland

Abstract The Neoproterozoic succession of East and North-East Greenland (over 14 000 m thick) includes the Eleonore Bay Supergroup (?Tonian–Cryogenian) and the Tillite Group (Cryogenian–Ediacaran). The upper units of the Eleonore Bay Supergroup consist of shallow to deeper-water carbonates, succeeded by siliciclastic fine-grained sediments (Bedgroup 19) that characterize the top unit of the supergroup. The Tillite Group includes two diamictite-bearing units (Ulvesø and Storeelv formations) of glaciogenic origin and two upper, upwards-shallowing strata (Canyon and Spiral Creek formations) that were deposited during semiarid conditions and concluded the Neoproterozoic depositional cycle. Diamictite is preserved on the craton and compares with the Storeelv Formation (Fm.) of the Tillite Group. Detailed investigations of the diamictite-bearing units (i.e. Ulvesø and Storeelv formations) demonstrate that the lower of the two formations is mainly of marine origin, whereas the upper one has both marine and terrestrial origins. Chemostratigraphic data include analyses on total carbon (TC), total organic carbon (TOC), total sulphur (TS) and δ 13 C. The data set for δ 13 C shows a substantial and abrupt shift towards negative values of ≥10%, from below Bedgroup 19. Low-diversity acritarch assemblages (Cryogenian) are recorded from the Andrée Land and Tillite groups; a thin cherty dolostone unit present above the Storeelv Fm. suggests that the diamictite units are of late Cryogenian age and the upper part of the Tillite Group is Ediacaran. Bedgroup 19 disconformably overlies older carbonates and the unit is a prelude to the succeeding (upper Cryogenian–lower Ediacaran) diamictite sediments of the Tillite Group. A disconformity separates the Tillite Group from the overlying Lower Palaeozoic sediments. Both disconformities are, according to palaeomagnetic data, related to rift–drift episodes that occurred during the late Neoproterozoic. Alternatively, the isotope data suggest that the diamictites were deposited during the late Cryogenian glaciation and the older disconformity may be interpreted as a significant gap developed by the lowering of sea level during an early Cryogenian glaciation.

About this title ‐ The Geological Record of Neoproterozoic Glaciations

In recent years, interest in Neoproterozoic glaciations has grown as their pivotal role in Earth system evolution has become increasingly clear. One of the main goals of the IGCP Project No. 512 was to produce a synthesis of newly available information on Neoproterozoic successions worldwide similar in format to Hambrey & Harland's (1981) Earth's pre-Pleistocene Glacial Record . This Memoir therefore consists of a series of overview chapters followed by site-specific chapters. The overview chapters cover key topics including the history of research on Neoproterozoic glaciations, identification of glacial deposits, chemostratigraphic techniques and datasets, palaeomagnetism, biostratigraphy, geochronology and climate modelling. The site specific chapters for 60 successions worldwide include reviews of the history of research on these rocks and up-to-date syntheses of the structural framework, tectonic setting, palaeomagnetic and geochronological constraints, physical, biological, and chemical stratigraphy, and descriptions of the glaciogenic and associated strata, including economic deposits.

SWEAT and the end of SWEAT: the Laurentia–Siberia configuration during Meso-Neoproterozoic times

The majority of reconstructions of the Neoproterozoic continental crust show the blocks comprising ‘East Gondwana’ (Australia, Antarctica, India, and South China) in proximity to the western margin of Laurentia. This is the ‘SWEAT’ (Southwest US–East Antarctica) constraint developed initially from the correlation of Neoproterozoic sedimentary successions and amplified by supposed tectonic extrapolations between these two regions. The failure of palaeomagnetism to support proximity during the tenure of the stratigraphic correlation and any ability to accommodate the widespread evidence for global continental break-up at 0.6–0.55 Ga appear to invalidate this premise and require the testing of alternative models. The Sears–Price (1978, 2000) reconstruction that places the Siberian Shield against the western margin of Laurentia satisfies, with marginal modification, the Meso–Neoproterozoic palaeomagnetic data in the age range 1.2–0.8 Ga, and with somewhat less confidence, the magnetic poles in the age range 1.5–1.2 Ga. The reconstruction employed here uses a clockwise rotation of the Siberian Shield of 107.3° about an Euler pole at 77°S, 91°E, and incorporates a crustal link between Laurentia and West Africa/South America in the Meso-Neoproterozoic supercontinent Palaeopangaea. The geological case is supported by Palaeoproterozoic tectonic trends and dike swarms, continuity of Mesoproterozoic Belt–Purcell and Taimyr sedimentary troughs, and a common history of the late Neoproterozoic–Early Cambrian passive margin development leading to basic magmatism/rift development and thermal subsidence.

Detrital zircon ages of Neoproterozoic sequences of the Moroccan Anti-Atlas belt

Detrital zircon dating from Neoproterozoic successions in the Sirwa inlier of the Anti-Atlas belt in Morocco confirms that the maximum depositional age of the main stratigraphic groups is significantly younger than has been previously proposed in lithostratigraphic correlations. This can probably be extended to the whole Anti-Atlas according to other recent data from the Saghro inlier. Although the relative stratigraphic position of the different units remains valid as published previously, a crucial implication of the new ages is that the sequences believed to be contemporaneous with oceanic crust and island arc formation during the rifting and break-up of the northern margin of the West African Craton (WAC), and believed to be involved in the first phases of the Pan-African orogeny, are actually late to post-orogenic. The age of the main deformation associated with the collision of the oceanic- and arc-derived terranes to the WAC, allegedly affecting the sediments of the Saghro Group, has been estimated at around 663–640 Ma. However, the youngest zircon populations of sediments of the Saghro and Bou Salda Groups, obtained in this study, cluster around 620–610 Ma, constraining the maximum age of deposition. This age of sedimentation is indistinguishable from the age of intrusive high-K calc-alkaline plutons of the Assarag Suite, suggesting a very rapid cycle of magmatism, relief formation, erosion and sedimentation in an active geodynamic scenario. Moreover, the proportion of the 610 Ma detrital zircons becomes less with respect to the Paleoproterozoic zircons at higher stratigraphic levels, suggesting that the source of young zircons was progressively eroded and more extensive cratonic areas, that probably underlie the Neoproterozoic rocks, were exposed. We interpret these data in terms of the development of a ca. 610 Ma magmatic arc, built upon WAC basement, and its progressive dismantling. This arc can be correlated with the voluminous late Neoproterozoic (ca. 640–570 Ma) arc magmatism characteristic of the north Gondwana margin and the peri-Gondwanan terranes. The diamictite beds that appear in the Imghi Formation of the Saghro Group have been correlated with the Sturtian glacial period ca. 700 Ma. However, zircons from one sample of these diamictites indicate that this correlation cannot be longer maintained, and instead they should be correlated with the Marinoan glacial period ca. 630–610 Ma, with a widespread distribution of glaciogenic deposits in West Africa. In addition, around 375 U–Pb concordant analyses obtained from Paleoproterozoic zircons from six samples represent a statistically significant population of this area of the WAC basement, which can be a useful database for comparison with the detrital zircon populations of the peri-Gondwanan terranes of Europe and North America, as the WAC margins were one of the major sediment suppliers for these terranes.