Glaciogenic Deposits Research Articles

Paleozoic basins in West Africa and the Mauritanide thrust belt

The evolution of the Paleozoic basins of West Africa is strongly depending on the structuration of the different belts which are surrounding the West African Craton. We distinguish the “Taoudeni Basin” located in the center of the craton from the basins located on the West African Craton margin (Tindouf, Tamale and several troughs limiting the western side of the Taoudeni Basin). Other basins are located on top of the Pan-African or Hercynian belts (Bové, Kandi, Ouallen in Semmen and Diourbel basins) or on top of the Proterozoic shields (The Ghana basins). Some are evidenced underneath the Mesozoic–Cenozoic coastal basins (Bové and Ghana basins). The sedimentation started with the Marinoan glacial event (620–580 Ma) and ended with the carbonates of the Early Carboniferous. The main tectonic or climatic events that occurred during this period are registered by the sediments. Among them are, the “Série pourprée glaciogenic deposits, the Pan-African II tectonic event (550–500 Ma) which affects the southwestern part of the Taoudeni Basin, the Late Ordovician glaciogenic event, the Early Silurian marine transgression, the Early Devonian marine regression and the Hercynian tectonic event (330–270 Ma) which affects the Paleozoic basins located on the western and northern parts of the West African Craton. The second part of this paper is devoted to a synthetic review of the Mauritanides Belt which is extending from Southern Senegal to the Moroccan High Atlas. This belt includes both old Pan-African belts and Paleozoic sediments (belonging to the western part of the Bové, Taoudeni and Tindouf basins) tectonised and metamorphosed during the Hercynian orogen. The third part points out the close relationships between the Paleozoic basins and the main tectonic event for the main periods of the West African Craton evolution.

A complex subglacial clastic dyke swarm, Sólheimajökull, southern Iceland

A complex network of clastic dykes dissects loosely consolidated Holocene sediments along the east flank of the proglacial braid plain of Sólheimajökull, southern Iceland. The dykes comprise downward-bifurcating intrusions up to 0.5 m thick and several metres in length and are intruded into glaciogenic deposits (sandy gravel, gravelly sand, interlaminated silt and sand, and diamicton). The dykes were sourced from a clast-poor sandy diamicton, interpreted as a subglacial till, and were intruded downwards beneath Sólheimajökull glacier during a previous phase of advance. As the glacier advanced southwards, it loaded the sediment column resulting in the intrusion of dykes with a consistent south–southwest dip (with rare northward-dipping examples). The dyke fills are characterised by laminated sediment, with laminae oriented parallel to the dyke margins and comprise interlaminated clay, silty clay, silt, sand, sandy gravel and diamicton. In some dykes, high concentrations of pebble- to boulder-sized clasts occur in association with rotated pods of the laminated sediment. The laminae are thought to have evolved by a slow, long-lived intrusion process that involved the repeated fracture and expansion of the host sediments followed by viscous smearing-on of subglacial material onto the dyke walls, rather than rapid injection of fluidised sediment.

Glacial meltwater impounding: Evidence from the late Quaternary glaciogenic sediments in the Sangla valley, district Kinnaur, Himachal Pradesh, India

Sangla valley is situated at an altitude of ~ 3500 m above mean sea level and lies in the Kinnaur district of Himachal Pradesh. It is fed by river Baspa, a tributary of river Sutlej, that entrenches through the Quaternary glaciogenic deposits before emerging out of the valley and joining the river Sutlej at Karcham. The unstratified to stratified glaciogenic deposits consist of large boulders to fine silt and are classified into four major depositional facies on the basis of sedimentary texture and depositional environment. The facies — basal conglomerates, debris flow, water/sheet flow and laminites — represents the change in the environment of deposition from glaciofluvial to lacustrine and also the extent of the glacier to the valley floor during late Quaternary.

Depositional environments and sequence stratigraphy of paralic glacial, paraglacial and postglacial Upper Ordovician siliciclastic deposits in the Murzuq Basin, SW Libya

The application of sequence stratigraphy methods to glaciogenic deposits is more problematic than in normal paralic deposits because changes in the relative sea level are strongly influenced by an interplay between glacial advance and retreat, and concomitant isostatic loading and rebound of the continental shelf. The study of outcrop and subsurface sections of the Upper Ordovician glaciogenic Melaz Shuqran and the Mamuniyat formations, Murzuq Basin SW Libya, yielded important information on understanding changes in the relative sea level related to glacier movements. Three depositional sequences were recognized. Depositional sequence one (DS-1) corresponds to the entire Melaz Shuqran Formation that was bounded below by subglacial erosion surface (i.e. sequence boundary, SB) formed during glacier advance into shallow water areas. This surface correlates with a transgressive surface (TS) in deep water areas and records an initial rise in the relative sea level owing to glacier advance and loading of the continental shelf. Transgression occurred due to the slower rate of eustatic sea-level fall than the isostatic loading. The transgressive systems tract (TST) comprises shoreface sandstones and offshore mudstones/diamictites with ice-rafted debris. Further rise in relative sea level was associated with glacial retreat, which resulted in sediment starvation in deep water areas and the formation of a condensed section. These deposits represent glacial depositional systems. The highstand systems tract (HST) comprises prograding deltaic deposits, which was formed when the rate of sediment supply exceeds the rate of relative sea level rise. The HST deposits represent paraglacial depositional systems. Depositional sequence two (DS-2) corresponds to the lower and middle part of the Mamuniyat Formation, and is bounded below by deep erosional surface (SB) that cuts into DS-1 as an incised valley, which was formed during glacial advance across the continental shelf. Incision occurred due to the faster rate of eustatic sea-level fall than the rate of isostatic loading. Glacier retreat and the concomitant rise in relative sea level resulted in the deposition of braided fluvial sediments in a lowstand systems tract (LST) and tide-dominated estuarine sediments in a TST. These deposits represent glacial depositional systems. The HST sediments represent prograding foreshore to shoreface deposits, which were formed when the rate of sediment supply exceeded the rate of relative sea-level rise. These deposits represent paraglacial depositional systems. Depositional sequence three (DS-3) corresponds to the upper part of the Mamuniyat Formation and is bounded below by an erosional surface (SB) that was formed as a result of isostatic rebound, associated with relative sea-level fall. DS-3 LST comprises Gilbert-type, deltaic conglomeratic sandstones grading upwards into sandstones. Subsequent relative sea-level rise during the late Ordovician resulted in deposition of shoreface TST sandstone. Further rise in the relative sea level resulted in sediment starvation in deep water areas and the formation of thin hardground layer (i.e. condensed section) that marks the upper boundary of late Ordovician deposits. This study provides predictive model for the spatial and temporal distribution of ancient glaciogenic depositional facies that may have important implications in hydrocarbon explorations.

Chemostratigraphy of Neoproterozoic cap carbonates from the Volta Basin, West Africa

The Neoproterozoic Era includes some of the most extreme ice ages in Earth history. The exact number of glaciations is unknown, although there were at least two events of global reach and possibly an equal number of lesser advances. Neoproterozoic glacial deposits in West Africa have proven particularly difficult to correlate with better constrained successions elsewhere. In most West African successions, only a single glaciogenic deposit is present, generally within a stratigraphic ‘Triad’of tillite, carbonate, and bedded chert; the age of these deposits and their synchroneity across the craton remain controversial. We report isotopic data from carbonates that cap tillites in the Volta Basin (Sud-Banboli Group). In the three sections measured, ∂13C (‰ PDB) begins at ∼−2 to −3 and drops to −5 to −8, correlated with a drop in both ∂18O and Mg/Ca. This pattern—both the drop in ∂13C and its correlation with ∂18O and lithology—has been observed in Neoproterozoic cap carbonates on other cratons, suggesting that it reflects primary depositional variations. In addition, the strong positive correlation among lithology, ∂13C, and ∂18O suggests that lithological variation may have exerted a principal control on isotopic variation, implying that the dolomite formed in isotopic equilibrium with seawater, as a primary precipitate or very early diagenetic replacement. This would require low levels of sulfate (estimated to be ∼1 mM or less), consistent with available S-isotopic data, the presence of barite crusts in association with some cap carbonates, and the conditions of seawater chemistry hypothesized in the aftermath of a ‘snowball Earth.’

Seafloor precipitates and C-isotope stratigraphy from the Neoproterozoic Scout Mountain Member of the Pocatello Formation, southeast Idaho: implications for Neoproterozoic earth system behavior

Abstract Macroscopic seafloor-precipitated crystal fans are rare in post-Paleoproterozoic strata, but they are more common in the unusual carbonates that cap certain Neoproterozoic glacial strata. The uppermost carbonate and marble unit of the Scout Mountain Member of the Neoproterozoic Pocatello Formation (younger than 667±5 Ma) contains seafloor fans, records negative δ 13 C values, and lies within a transgressive sequence, all features consistent with such post-glacial carbonate units. It does not, however, rest directly upon a known glaciogenic deposit or sequence boundary. Instead, it occurs within a cyclic upward-fining succession that overlies an erosional contact on the pink dolostone post-glacial carbonate atop the late Sturtian Scout Mountain Member diamictites. It is possible that the carbonate unit in question is a cap carbonate and thus requires recognition of an additional glaciation recorded in the Pocatello Formation. Alternatively, it may imply that means independent of glaciation, such as overturn of an anoxic ocean, foster the deposition of cap-like carbonates independent of post-glacial processes. Thus, Neoproterozoic carbonate units with unusual lithofacies and negative carbon isotope values need not indicate the aftermath of low-latitude glaciation. Additionally, the radiometrically calibrated δ 13 C record of the Pocatello Formation demonstrates that the interval from 700 to 580 Ma has more variation than previous compilations would suggest.

The Neoproterozoic Quruqtagh Group in eastern Chinese Tianshan: evidence for a post-Marinoan glaciation

The Neoproterozoic Quruqtagh Group in eastern Chinese Tianshan consists of, in ascending order, the Bayisi, Zhaobis- han, Altungol, Tereeken, Zhamoketi, Yukkengol, Shuiquan, and Hankalchough Formations. Diamictite units occur in the Bayisi-Altungol, Tereeken, and Hankalchough Formations. Our field observations confirm the glacial origin of the Tereeken and Hankalchough diamictites, but the glacial origin of diamictites in the Bayisi and Altungol Formations remain ambiguous. While siliciclastic strata dominate the succession, carbonate units are recognized between the diamictites and immediately atop the Tereeken and Hankalchough diamictites. Carbonates in the Altungol Formation are characterized by highly positive 13 C values (up to +10.4‰ versus PDB), comparable to those of pre-Marinoan but post-Sturtian carbonates. High-resolution sam- pling of the pinkish cap dolostone (ca. 10 m-thick) overlying the Tereeken diamictite shows a consistent 13 C chemostratigraphic pattern (ca. −5‰ at the base and slightly decreasing upsection) similar to that of Marinoan cap carbonates. This cap dolostone is also characterized by macropeloids similar to those in the Marinoan-age Raventhroat cap carbonate, Mackenzie Mountains. Chemostratigraphic and sedimentary features are suggestive of a Marinoan age for the Tereeken glaciation. Thus, the younger Hankalchough diamictite may represent a post-Marinoan glaciation. This interpretation is consistent with the occurrence of vendotaenid fossils in the Shuiquan Formation. The alternative interpretation—the Tereeken and Hankalchough diamictites are, respectively, Sturtian and Marinoan in age—is not consistent with the above evidence or the chemostratigraphic features of the Shuiquan Formation and the Hankalchough cap carbonate. A post-Marinoan age for the Hankalchough and other glaciogenic deposits indicates that the disappearance of Doushantuo-type acritarchs and the subsequent radiation of macroscopic animals are closely linked, in geochronological terms, with a Neoproterozoic glaciation. © 2003 Elsevier B.V. All rights reserved.

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.

Sources, winning, properties

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.

Paleomagnetism of the Sierra de Las Animas Complex, southern Uruguay: its implications in the assembly of western Gondwana

Global paleogeographies in the Neoproterozoic are poorly known. In particular, the assembly of Gondwana at the end of the Proterozoic is controversial. In order to constrain the kinematic evolution of the Rio de la Plata craton, a paleomagnetic study was carried out in the Vendian to Cambrian Sierra de Las Animas magmatic Complex, exposed in southeastern Uruguay (34.7°S, 55.3°W). One hundred and forty-seven samples from 23 sites were studied. These included two sites on the Vendian glaciogenic sediments of the Playa Hermosa Formation. After stepwise AF and thermal cleaning a characteristic remanence was isolated at several sites. According to the available isotopic data for the complex and the correlation between remanence directions and lithologic type and age, two paleomagnetic poles were computed for the Sierra de Las Animas complex. SA1 (338.1°E, 5.9°N, dp=19.6°, dm=26.7°, N=7 sites, mean declination: 44.5°, mean inclination: 58.0°, α 95: 18.1°), with a likely age around 520 Ma, shows a distribution of VGPs that suggests a remanence acquisition that spanned a long time during a period of fast apparent polar wander for Gondwana. SA2 (250.9°E, 16.9°S, dp=15.9°, dm=21.5°, N=6 sites, mean declination: 95.3°, mean inclination: −58.5°, α 95: 14.5°) corresponds to a likely age around 550 Ma and falls close to a pole of similar age from the Congo craton. This suggests that the Rio de la Plata craton was at that time part of Gondwana and confirms previous postulates that a single APWP can be defined for the supercontinent since 550 Ma. A preliminary mean geomagnetic pole obtained from the Playa Hermosa Formation (PH, 198.4°E, 43.0°S, dp=8.6°, dm=16.0°, n=6 samples, mean declination: 226.0°, mean inclination: 24.2°, α 95: 15.0°) is consistent with a previous pole of 595 Ma from the Campo Alegre (CA) lavas and may suggest, if confirmed, that the glaciogenic deposits were produced at low to intermediate latitudes. Despite high uncertainties, a new speculative model on the process of assembly of Gondwana is proposed.

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 CO 2 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.

Late Devensian glaciation, ice-dammed lake and river diversion, Stiffkey, north Norfolk, England

SUMMARY A sequence of disturbed and undisturbed samples from a borehole drilled at Warham, near Stiffkey in north Norfolk, is described and sub-divided into three lithofacies on the basis of sedimentary structures, particle size distribution, calcium carbonate content, selected heavy mineral composition and indicator lithologies. These properties are used to determine the environment of deposition and establish correlation with glaciogenic deposits elsewhere. The basal sands and gravels (Lithofacies A) overlie the Chalk bedrock and are interpreted as having formed by a braided River Stiffkey that flowed northward through the site in pre-Late Devensian periglacial conditions. Lithofacies B consists of &gt;8 m of laminated sands, silts and clays that were formed in a proglacial lake created by the obstruction of the River Stiffkey when the Late Devensian (Dimlington Stadial) glaciation reached the north Norfolk coast. These sediments are covered by gravelly sands (Lithofacies C), which were deposited as an ice-marginal river deposit as part of the Late Devensian end moraine-ridge complex. Finally, the lake drained and the sediments were dissected as the diverted River Stiffkey adopted a new course to the north-east and the lower, incised section of the present Stiffkey valley was formed.

Geochemical investigation of a Neoproterozoic glacial unit: The Mineral Fork Formation in the Wasatch Range, Utah

The Mineral Fork Formation is an isolated unit of Neoproterozoic glaciogenic rocks in the Wasatch Range in northwestern Utah. Geochemical analyses of diamictite and mudstone samples were carried out to document its major and trace element geochemistry. Comparison of the lower and upper parts of the formation, which have been ascribed to different tectonic and sedimentological settings, revealed no striking differences, but the analyzed samples show a gradual upward increase in CaO, MgO, Fe 2 O 3 , MnO, and P 2 O 5 and a decrease in SiO 2 . TiO 2 :Al 2 O 3 ratios that suggest a provenance throughout the formation with an average composition slightly more mafic than average upper crust. The mudstones are more K rich than the diamictites. Calculation of a chemical index of alteration (CIA) is complicated by the presence of carbonate minerals. After making corrections for carbonate content, CIA values range from 65 to 70. These values are higher than those from Paleoproterozoic glaciogenic deposits. Some mudstones in the upper part of the formation are Fe rich (∼15% Fe 2 O 3 ), approaching the composition of iron formations. The ferruginous nature supports previous correlation with Fe-rich glaciogenic rocks of the Rapitan Group in the northern Canadian Cordillera. Trace elements, including rare earth elements, suggest that the materials in the Mineral Fork Formation were largely derived from a post-Archean source. Most of the widespread glaciogenic rocks of the Neoproterozoic were derived from older sedimentary materials, whereas the deposits of the earlier great glacial episode in the Paleoproterozoic contain a much higher proportion of crystalline materials.

SERRA DO ESPINHAÇO MERIDIONAL: UM ORÓGENO DE COLISÃO DO MESOPROTEROZÓICO

The Serra do Espinhaco Meridional (SdEM) is built mostly by Paleo- and Mesoproterozoic rocks of the homonymous supergroup. This supergroup includes sedimentary and volcanic rocks of the Guinda Group deposited between 1750 and 1700 Ma in a continental environment. The rift breakup generated a passive continental margin recorded by sedimentary and volcanic rocks (including banded iron formations) of the Serro Group.Tec tonic inversion occurred between 1.5 and 1.3 Ga. Syntectonic crustal stacking resulted in the formation of sedimentary throughs in fr ont of the orogenic belt, initially as a foredeep basin which is represented by deposits of the Conselheiro Mata Group, and later as foreland basins along the margins of the belt. During Late Mesoproterozoic times (about 1050 Ma) this orogenic belt underwent a mountain glaciation, recorded by the glaciogenic deposits of the Macaubas Group along the borders of the SdEM. After an episode of intracontinental magmatism (Pedro Lessa Suite) the SdEM together with a large region of the adjoining Sao Francisco craton, was covered by an epicontinental sea and, by that time, carbonate and pelitic rocks of the Bambui Group were deposited. The SdEM underwent uplift during the Late Neoproterozoic, causing local gravitional collapse of units of the Bambui Group at the western border of this orogenic belt.

Chemostratigraphic correlation of carbonate successions in the Gariep and Saldania Belts, Namibia and South Africa

ABSTRACT Inter‐ and intrabasinal correlation of Neoproterozoic carbonate successions and associated glaciogenic deposits from the Pan‐African Gariep Belt and the Kango inlier in the Saldania Belt (Namibia and South Africa) is proposed on the basis of new δ13CCarb and 87Sr/86SrI data. Highly positive δ13CCarb values (as much as + 8.65‰) and low 87Sr/86SrI ratios (0.7071–0.7077) were obtained on carbonate successions of the Hilda Subgroup between an older and a younger diamictite in the Port Nolloth Group (Gariep Belt). These results are in agreement with data elsewhere that suggest deposition between the global ∼ 750 Ma Sturtian and ∼ 580 Ma Marinoan glaciations. Considerably lower, positive δ13CCarb values (up to + 1.01‰) and higher 87Sr/86SrI ratios (0.7082–0.7085) mark the carbonates (Bloeddrif Member, Holgat Formation) on top of the Numees Formation diamictite and support a correlation of this diamictite with the Marinoan glaciation.In the southern extension of the Gariep Belt, correlation of the Widouw Formation limestone (Gifberg Group) with the Bloeddrif Member carbonates is proposed based on similar isotopic characteristics. In the Kango inlier of the Saldania Belt, two carbonate‐bearing members exist in the Matjies River Formation. The lower one of these two (Nooitgedacht Member) compares well with the Hilda Subgroup, whereas the upper one (Kombuis Member) shows strong similarities to the Bloeddrif Member carbonates. This implies that all younger stratigraphic units of the Kango inlier are not correlatives of the Port Nolloth Group, as previously assumed, but syn‐ to post‐orogenic with respect to the 540–580 Ma Pan‐African orogeny.

Vreeland Diamictites - Neoproterozoic glaciogenic slope deposits, Rocky Mountains, northeast British Columbia

Vreeland Diamictites - Neoproterozoic glaciogenic slope deposits, Rocky Mountains, northeast British Columbia

Reply to Discussion: Vreeland Diamictites - Neoproterozoic glaciogenic slope deposits, Rocky Mountains, northeast British Columbia

Hoffman et al. (1998b) developed a modified ‘snowball’ Earth hypothesis to explain the association of Neoproterozoic low-latitude glaciation with the deposition of “cap carbonate” rocks bearing highly depleted carbon isotopic values. According to this hypothesis, a runaway albedo feedback caused the world’s oceans to freeze for millions of years. During this time interval, the continental ice cover was thought to have become thin and patchy because of the virtual elimination of the hydrological cycle (Kirschvink, 1992; Hoffman et al., 1998b). The Vreeland diamictites record vigorous erosion by an extensive ice sheet at or near the pressure melting point (McMechan, 2000). Their deposition and that of several other thick (>800 m) Neoproterozoic glaciogenic deposits, including the Mineral Fork diamictites in Utah (Christie-Blick, 1983), Rapitan diamictites in northwestern Canada (Yeo, 1981), the Sturtian diamictites in South Australia (Young and Gostin, 1989), the Hangeerqiaoke diamictites in northwest China (Wang et al., 1981), the Port Askaig diamictites in southwest Scotland (Spencer, 1971), and the Chuos diamictites in northwest Namibia (Hoffman et al., 1998a), show that substantial areas of vigorous contential glaciation occurred in the Neoproterozic. The existence of these deposits is incompatible with the hypothesized thin and patchy continental ice cover of the ‘snowball’ Earth model (Christie-Blick et al., 1999; McMechan, 2000). Rather than taking the sedimentary record as evidence of warmer climates and partially open oceans, Hoffman (in Hoffman and Schrag, …

Vreeland Diamictites - Neoproterozoic glaciogenic slope deposits, Rocky Mountains, northeast British Columbia

Research Article| September 01, 2000 Vreeland Diamictites – Neoproterozoic glaciogenic slope deposits, Rocky Mountains, northeast British Columbia* M.E. McMechan M.E. McMechan Geological Survey of Canada, 3303 - 33rd St. NW, Calgary, AB, T2L 2A7 Search for other works by this author on: GSW Google Scholar Bulletin of Canadian Petroleum Geology (2000) 48 (3): 246–261. https://doi.org/10.2113/48.3.246 Article history received: 09 Dec 1999 accepted: 27 Apr 2000 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M.E. McMechan; Vreeland Diamictites – Neoproterozoic glaciogenic slope deposits, Rocky Mountains, northeast British Columbia. Bulletin of Canadian Petroleum Geology 2000;; 48 (3): 246–261. doi: https://doi.org/10.2113/48.3.246 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of Canadian Petroleum Geology Search Advanced Search Abstract Neoproterozoic diamictites of the Vreeland Formation exposed in the Snake Indian and Wapiti Pass thrust sheets of the central Rocky Mountains, northeast British Columbia, form a thick (350 to 2000 m) glaciogenic deposit within the passive margin succession of the Windermere Supergroup. They are the deposits of the younger worldwide episode of Neoproterozoic glaciation. Diamictites were deposited as laterally extensive sheets in a mid slope setting with minimal sediment reworking or transport by bottom currents. In the Wapiti Pass thrust sheet, the absence of dropstones and lone-stones, in facies-equivalent and interbedded fine-grained strata, indicates there was little, if any, ice rafting during sedimentation. Diamictites are the deposits of unconfined debris flows derived from the slumping of glacial deposits near the shelf-slope break. In the Snake Indian thrust sheet, dropstones and lonestones indicate ice rafting. Diamictites formed from both unconfined debris flows and the “rain-out” of ice-rafted and sediment plume material. Deposition of thick diamictites in the Mount Vreeland area was probably localized by syndepositional, west-side-down normal fault(s).Diamictites contain abundant extrabasinal (felsic and basic intrusive, volcanic, quartzite) and intrabasinal (argillite, sandstone, carbonate) clasts. They record a major source of material entering the Windermere depositional basin distinct from the basinal turbidite system of the southern Cordillera. The high volumes of glaciogenic sediment and large extrabasinal clasts indicate the transportation of massive amounts of material to the shelf-slope break by glaciers and glacial processes. Their deposition appears to be incompatible with the snowball Earth model for Neoproterozoic glaciation, with its thin patchy continental ice cover and the virtual elimination of the hydrological cycle.Locally, the diamictites are overlain by a thin deep-water limestone (cap carbonate). Olistoliths of plafformal carbonates in the immediately overlying argillite indicate deposition of postglacial (cap) carbonates on the shelf to the east, continued deposition in a slope environment in the study area, and continued instability and fault-related uplift of the shelf edge. Detailed mapping of the southwest flank of Ice Mountain shows that carbonate olistoliths, rather than a carbonate platform, overlie the diamictites. No evidence was found of penetrative deformation, uplift or erosion of diamictites during the Neoproterozoic to support a previous interpretation of the Neoproterozoic “Vreeland High”. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Stratigraphic, geochronological, and paleomagnetic constraints upon the Neoproterozoic climatic paradox

Of the many models that have been proposed to account for the enigmatically widespread and apparently low-latitude Neoproterozoic glaciogenic deposits, three are widely considered: (1) the Phanerozoic archetype of glaciated polar regions and mid-latitudes only, (2) the "Snowball Earth" model with globally synchronous glaciations, and (3) the high-obliquity hypothesis. These models respectively predict purely high-to-moderate paleolatitudes, all paleolatitudes, and preferentially low paleolatitudes of glacial deposits. To distinguish among these alternatives, I present a thorough compilation of the Neoproterozoic glacial deposits and their current age constraints, avoiding intercontinental correlations in almost all cases. In this conservative view, paleomagnetic data are relevant only if directly measured upon the glaciogenic deposits or conformable units, or if the glaciogenic formations are precisely dated enough for application of equally well dated paleomagnetic poles from the same craton. The primary conclusion to be drawn from this compilation is that very few of the deposits have reliable paleomagnetic constraints. Of that subgroup, however, low latitudes are more common than one would expect if randomly drawn from a uniform distribution on the sphere. Not a single high-paleolatitude (poleward of 60 degrees ) deposit has been documented convincingly. Both the "Snowball Earth" hypothesis and the high-obliquity model are permitted by the present paleomagnetic dataset. The Phanerozoic archetype fails to account for robust determinations of near-Equatorial paleolatitude from several Neoproterozoic glaciogenic deposits. If a non-uniformitarian model such as the high-obliquity hypothesis is correct, then its transition to the Phanerozoic archetype must have occurred rapidly, near the beginning of Cambrian time. Alternatively, if the Snowball Earth model is correct for Precambrian time then the lack of tropical glaciations since 550 Ma may be fortuitous or may indicate secular changes in the boundary conditions or processes governing surficial conditions on planet Earth.

Seismic data from the Northern basin, Ross Sea, record extreme expansions of the East Antarctic Ice Sheet during the late Neogene

Approximately 3700 km of intermediate-resolution single-channel seismic data from the Northern basin were analyzed to investigate the late Neogene history of East Antarctic Ice Sheet (EAIS) grounding events on the shelf, and to evaluate how glacial unconformities on the shelf are manifested on the upper slope. The Northern basin was chosen as the site of this study because ice-sheet reconstructions show that the basin received sediment by ice emanating from East Antarctica . In addition, seismic correlations to DSDP Site 273 suggest that a relatively thick late Neogene section exists on the basin's outer shelf and upper slope. On the Northern basin shelf, glacial unconformities exhibiting broad, low-angle relief, topset truncation, and cross-cutting relationships reveal a dynamic history of expansions and contractions during which the EAIS was larger than present on at least eight occasions during the late Neogene. On the upper slope, the correlative conformities of the glacial unconformities are indistinct reflections within thick trough-mouth fan (TMF) depocenters at the mouths of Drygalsky and Joides basins. The glacial unconformities and correlative conformities define TMF sequences, and each TMF sequence contains several topset-truncated prograding-slope reflectors. We infer that the correlative conformities on the continental slope correspond to the interface between prograding glaciogenic deposits (glacial maximum) and diatomaceous glacial-marine sediments (glacial minimum). The seismic–stratigraphic analysis and regional mapping indicate that the upper slope does not contain a more complete late Neogene section than that which exists on the shelf. We infer that diatomaceous glacial-marine sediments on the slope may be relatively undisturbed, and hence may provide a means of dating the TMF sequences. It is hoped that these results will stimulate efforts to core the late Neogene section in the Northern basin TMFs to investigate how these EAIS expansions and contractions relate to other records of late Neogene climate and eustasy .