Ice-rafted Dropstones Research Articles

Late Cretaceous glaciations in a hyper-arid plateau desert of the South China Coastal Mountains

Increasing evidence indicates the existence of a cryosphere during the Cretaceous supergreenhouse. However, current understanding of a potential link between lithosphere dynamics and cryospheric processes in the Cretaceous plateau desert successions of China remains limited. We report the occurrence of ice-rafted dropstones and diamictites from the Upper Cretaceous Chishan Formation of the Subei Basin at the East Asian continental margin. Our provenance results indicate that fluvial deposits of the Lower Chishan Formation were mainly derived from the Sulu Orogen to the north and the Zhangbaling Uplift to the west, whereas aeolian deposits of the Upper Chishan Formation were largely recycled from the two sources with an additionally notable contribution from the post-Cretaceous basement of the Yangtze Block. Combined with previous evidence, provenance analysis indicates that Late Cretaceous collision between the Okhotomorsk Block and the East Asian continent led to the growth of the South China Coastal Mountains via crustal thickening, which generated an arid, high-altitude basin region that experienced desertification and paleohydrological variability, and that was supplied with additional clastic sediment sources from the basement of the South China Block. Our results provide evidence of Late Cretaceous cryospheric processes in a continental mid-latitude plateau desert linked to the northwestward subduction and collision of the paleo-Pacific realm. Global cooling from the late Turonian to Maastrichtian drove the establishment of glaciers in high-altitude mountains leading to the development of ice-related deposits in the plateau deserts, as recorded in the Subei desert basin of the South China Coastal Mountains. The record of ice-rafted debris and the provenance signature reveal an active Cretaceous plateau cryosphere linked to lithosphere dynamics.

Newly discovered diamictite and cap carbonate couplet in the southern Alxa Block, northwestern China: Implications for stratigraphic correlation and Marinoan glaciation

Neoproterozoic glaciogenic diamictite and cap carbonate couplets have played a pivotal role in understanding glacial-interglacial cycles and establishing regional stratigraphic correlation. The Alxa Block in northwestern China preserves a sequence of Neoproterozoic diamictites along its southern margin, but the age and origin of the succession remain debatable due to the lack of discovery of cap carbonate. We report newly discovered cap carbonates that overlie the diamictites of the Shaohuotonggou Formation in the Longshoushan region in the southern Alxa Block. Based on detailed geological investigations, we identified massive diamictites, stratified diamictites, and cap carbonates in the lower part of the formation. The presence of ice-rafted dropstones, bullet-shaped and facetted clasts, glacial striations, and relatively low chemical index of alteration values of sedimentary matrix support a glaciogenic origin of the diamictites. The 2- to 2.6-m-thick cap carbonates are mainly composed of thinly laminated microcrystalline dolomites and show sheet cracks, cemented breccias, and tepee-like structures at the basal part of the unit. These features and their consistently negative δ13C values (−5.2‰ to −2.2‰) are characteristic of Marinoan-age cap carbonates (ca. 635 Ma). The quasi-continuous deposition of the massive diamictites, stratified diamictites, and cap carbonates suggests that the formation of this couplet was closely related to the Marinoan glaciation and subsequent deglaciation. We propose a three-stage depositional model for the glaciogenic succession and recommend that the diamictite and cap carbonate couplet in the Alxa Block provides a credible mark of the Cryogenian−Ediacaran boundary for further stratigraphic correlation and investigation.

Paleoclimatic and paleogeographic significance of the early Santonian ice-rafted dropstones in the Songliao Basin, NE China

Two ice-rafted dropstones were discovered in the Yaojia Formation (early Santonian) of wells Ao14 and F247 in the Songliao Basin, northeastern China. The dropstone from well Ao14 was further studied using computed tomography (CT) scanning, sporopollen, carbon and oxygen isotope, and zircon U–Pb isotope analyses. The results show that the dropstone is composed of quartz monzonite and has an age of 178.0 ± 3.3 Ma. Based on the bedrock age analysis around the Songliao Basin, it is suggested that the dropstone in well Ao14 may have originated from the Zhangguangcai Range. The palynoflora assemblage indicates a mixed survival of cold and warm palynomorphs. The carbon and oxygen isotope compositions of the synsedimentary calcareous clasts surrounding the dropstone exhibit obvious negative biases, suggesting that glacier meltwater flowed into the paleo-lake, probably due to the significant topographic height difference between the Songliao Basin and adjacent areas during the early Santonian. Additionally, it can be inferred that coastal mountains formed in the Zhangguangcai Range area, which is located southeast of the Songliao Basin, and the dropstones derived from this region. During the cold season, the dropstones may be transported to the edge of Songliao Lake by avalanche and mixed with carbonate debris. As the seasons changed, the lake ice broke into pieces and carrying the dropstones and carbonate debris into the deep lake area. The present research provides direct evidence of a brief global cold event and the presence of high mountains in the southeastern Songliao Basin during the early Santonian in the Cretaceous greenhouse world.

Ice-rafted dropstones at midlatitudes in the Cretaceous of continental Iberia: COMMENT

Ice-rafted dropstones at midlatitudes in the Cretaceous of continental Iberia: COMMENT

Ice-rafted dropstones at midlatitudes in the Cretaceous of continental, Iberia: REPLY

Ice-rafted dropstones at midlatitudes in the Cretaceous of continental, Iberia: REPLY

Multimethod dating of ice-rafted dropstones reveals hidden localized glacial erosion in Wilkes Subglacial Basin, Antarctica

Abstract The Antarctic ice sheet blankets >99% of the continent and limits our ability to study how subglacial geology and topography have evolved through time. Ice-rafted dropstones derived from the Antarctic subglacial continental interior at different times during the late Cenozoic provide valuable thermal history proxies to understand this geologic history. We applied multiple thermochronometers covering a range of closure temperatures (60–800 °C) to 10 dropstones collected during Integrated Ocean Drilling Program (IODP) Expedition 318 in order to explore the subglacial geology and thermal and exhumation history of the Wilkes Subglacial Basin. The Wilkes Subglacial Basin is a key target for study because ice-sheet models show it was an area of ice-sheet retreat that significantly contributed to sea-level rise during past warm periods. Depositional ages of dropstones range from early Oligocene to late Pleistocene and have zircon U-Pb or 40Ar/39Ar ages indicating sources from the Mertz shear zone, Adélie craton, Ferrar large igneous province, and Millen schist belt. Dropstones from the Mertz shear zone and Adélie craton experienced three cooling periods (1700–1500 Ma; 500–280 Ma; 34–0 Ma) and two periods of extremely slow cooling rates (1500–500 Ma; 280–34 Ma). Low-temperature thermochronometers from seven of the dropstones record cooling during the Paleozoic, potentially recording the Ross or Pan-African orogenies, and during the Mesozoic, potentially recording late Paleozoic to Mesozoic rifting. These dropstones then resided within ~500 m of the surface since the late Paleozoic and early Mesozoic. In contrast, two dropstones deposited during the mid-Pliocene, one from the Mertz shear zone and one from Adélie craton, show evidence for localized post-Eocene glacial erosion of ≥2 km.

Ice-rafted dropstones at midlatitudes in the Cretaceous of continental Iberia

Abstract The Cretaceous is widely considered to have been a period subjected to super-greenhouse conditions. Here, we provide multiscale sedimentologic evidence of glaciers developing at mid-paleolatitudes (~45°N) in continental Iberia during the Hauterivian cold snap. Striated and faceted ice-rafted glacial dropstones (cobble to boulder size) and striated and grooved silt- to sand-sized grains (ice-rafted debris [IRD]) occur in a lacustrine sequence of the Enciso Group in the eastern Cameros Basin, Spain. The ice-rafted materials constitute the first evidence reported for a Cretaceous continental cryospheric record in Europe, and they are attributed to calving of glacier snouts, releasing icebergs into an ice-contact lake. The sedimentary succession resembles glacial-deglacial records in lakes overridden by the late Pleistocene Laurentide Ice Sheet in eastern Arctic Canada. The Iberian glacial succession was coeval with plateau permafrost in Asia and IRD records in the Arctic and Australia, revealing a stronger than previously thought cryosphere during the global Hauterivian cold snap.

A newly discovered Neoproterozoic diamictite-cap carbonate couplet from the Western Himalaya: The expansion of the Marinoan snowball Earth glaciation to the northwestern margin of the Indian Plate in North Pakistan

One of the central tenets of the snowball Earth hypothesis is the global recognition of the glacial diamictite-cap carbonate couplets in the Cryogenian sedimentary record. However, the scarcity of such peculiar stratigraphic elements in the Neoproterozoic successions of the Himalaya brings into question the severity and global extent of the snowball Earth event and likewise hampered glacio-stratigraphic correlation within the Himalaya and elsewhere. This study provides the first convincing evidence of the glacial diamictite-cap carbonate couplet from the Tanakki Member of the basal Kakul Formation, previously unknown from the Western Himalaya in North Pakistan. Detailed sedimentological analysis of the diamictite from the Tanakki Member reveals deposition in glacially-influenced proximal to distal subaqueous debris apron. The presence of glaciogenic clasts (striated, facetted and bullet-shaped) together with evidence of the ice-rafted dropstones in pervasive facies association provides credence to the glaciogenic affinity. The thin cap carbonate (herein referred to as Tanakki-cap dolomite; TCD) overlying the glacial diamictite record deposition in a deeper shelf (offshore) setting. The lithological, depositional and persistent negative C-isotope characteristics (ca. −3.2 to −5.8‰) combined with regional stratigraphic and available geochronological data allow us to interpret TCD as a ‘Marinoan’ cap carbonate and the underlying diamictite as an expansion of the terminal-Cryogenian (Marinoan) glaciation in the Western Himalaya. Moreover, the analyses of the tectonic and depositional history of the Tanakki Member coupled with the Neoproterozoic paleogeographic evolution of the northern margin of the Indian Plate argue against a previous interpretation of the culminating foreland basin orogeny and instead support deposition in an extensional fault-controlled rift basin. Finally, this study permits us to revise the Neoproterozoic stratigraphic framework of the Western Himalaya by describing the Cryogenian-Ediacaran boundary interval in the region that ultimately helps to overcome the previous glacio-stratigraphic discrepancies in the Neoproterozoic record of the Himalaya.

A tale of two rift shoulders, and two ice masses: the Cryogenian glaciated margin of Death Valley, California

Abstract The Death Valley area of California, USA, exposes an outstanding record of a Neoproterozoic (Cryogenian) glaciated margin: the Kingston Peak Formation. Despite the quality of the exposure, however, the outcrops of glaciogenic strata are fragmentary, forming isolated, laterally offset outcrop belts at the western extremity of the Basin and Range province. Excellent evidence for glacially modulated sedimentation includes (1) ice-rafted dropstones in most ranges, (2) thick diamictites bearing a variety of exotic (extrabasinal) clasts, (3) striated clasts and (4) local occurrences of glacitectonic deformation structures at the basin margins. In tandem with this, there is a distinct signature of slope collapse processes in many ranges, including (1) up to kilometre-scale olistoliths, (2) extensional growth fault arrays, (3) dramatic proximal-distal thickness changes and (4) basalt occurrences. New sedimentological observations reinforce long-held views of rifting superimposed on glaciation (or vice versa), with both processes contributing to a complex record whereby rift and glacial processes vie for stratigraphic supremacy. We consider that a mechanism of diamictite accumulation in a series of rift-shoulder minibasins produced greatly contrasting successions across the Death Valley area, under the incontrovertible influence of hinterland ice sheets.

Co-occurrence patterns of ice-rafted dropstones and brachiopods in the Middle Permian Wandrawandian Siltstone of the southern Sydney Basin (southeastern Australia) and palaeoecological implications

The ecological significance of ice-rafted dropstones in present-day glacial marine benthic environments has received considerable attention from ecologists, but similar studies based on the geological records of dropstones and associated fossils are rare. In this study, we report statistically significant co-occurrences of ice-rafted dropstones with brachiopod shells in multiple stratigraphic horizons at multiple sites from the Middle Permian Wandrawandian Siltstone of the southern Sydney Basin in southeastern Australia. We analysed the distribution patterns of both dropstones and brachiopod-dominated fossil assemblages by using a quadrat-based sampling method and spatial point pattern analysis. It was revealed that the co-occurrences of ice-rafted dropstones and brachiopod shells are not random; rather, they demonstrate statistically significant and stratigraphically recurring associations that are here interpreted to represent dropstone-associated, brachiopod-dominated palaeoecological communities. In these recurring palaeocommunities, the presence of ice-rafted dropstones is considered to have added habitat complexity and heterogeneity to the benthic environment, especially suited to the settlement of brachiopods. In addition, the sinking of dropstones, from floating ice masses through the water column to the silty seafloor, is interpreted to have enriched the nutrient and oxygen supply to the benthic environment, further aiding the flourishing and maintenance of a diverse and stable marine benthic fauna. Supplementary material: Original data collection (Supplementary material 1) and spatial point pattern analysis results of rest quadrats (Supplementary material 2) are available at https://doi.org/10.6084/m9.figshare.c.4101743

Large kelp-rafted rocks as potential dropstones in the Southern Ocean

Cyclonic winds in April 2017 forced oceanic kelp rafts onshore in southeastern New Zealand, providing a ‘snapshot’ of ongoing kelp-rock transport. About 10% of observed southern bull kelp (Durvillaea antarctica) holdfasts contain substantial remnants of their original rocky substrates. The largest rock weighs ~7kg, and was derived from the Mesozoic sedimentary Murihiku Terrane of southern New Zealand, >100km from the beach where it was deposited. Several other rafted rocks were derived from the same general source area, including a conglomerate clast with 10cm igneous cobbles. Genetic sequencing of the kelp specimens confirmed their southern New Zealand derivation, which is distinct from sub-Antarctic or northern New Zealand clades. The floating rocks had been colonised by goose barnacles (Lepas australis), the largest of which were approximately 5mm long, implying that the rocks had been floating for approximately 16days before being blown ashore. The rocks may have travelled up to 200km in that time in a northeasterly directed ocean current that flows at 10–15km/day. If these rock-bearing kelp rafts had not been blown ashore in the cyclonic weather event, they would likely have continued to travel northeast in the Subtropical Front zone where they would likely have been deposited as dropstones. Abundant dropstones noted on the nearby Chatham Rise and Campbell Plateau, have previously been assumed to have been ice-rafted. A component of these dropstones could have been derived by kelp rafting, especially on the Chatham Rise, where the Subtropical Front is deflected east. A thick weathering rind, and remnant organic veneer, if present, along with clast provenance, may be the best distinguishing features of kelp-rafted dropstones compared to ice-rafted dropstones.

Abyssal foraminifera attached to polymetallic nodules from the eastern Clarion Clipperton Fracture Zone: a preliminary description and comparison with North Atlantic dropstone assemblages

We examined encrusting organisms on seven polymetallic nodules from the eastern Clarion Clipperton Fracture Zone (CCFZ, ∼4070 m water depth, eastern equatorial Pacific). Apart from occasional sponges and a single bryozoan, all the organisms were foraminifera or foraminifera-like protists. A total of 75 morphotypes (presumed to be morphospecies) was recognised, with between 9 and 19 being present on individual nodules. Additional species were observed during shipboard examination of the nodules, bringing the total number of species to 86. The assemblage was dominated by a variety of mat-like formations, clusters of patches, isolated domes, broad trails, anastomosing networks and branched or unbranched tubular structures that either lay flat against the nodule surface or projected away from the surface. These forms were interpreted as monothalamous foraminifera (monothalamids). Most have mainly agglutinated walls but a few are predominately organic. Some can be assigned to the Komokiacea (notably the genus Chondrodapis) or families such as the Hemisphaeramminidae ('domes'), while others (e.g. many of the mats and patch-like forms) are difficult to place into existing monothalamid groupings. Some of the branching and anastomosing tubes resemble the genus Rhizammina. The most easily recognisable morphotypes include Telammina, in which tiny chambers are linked by extremely thin tubes to form a network, and sinuous orange tubes that incorporate sponge spicules and can be assigned to the genus Saccorhiza based on the occasional presence of a proloculus. Polythalamous foraminifera are also fairly common. They include various calcareous species (mainly Cibicides spp.), as well as agglutinated forms such as Hormosina, ?Placopsilina and trochamminaceans. Similar assemblages, including some morphospecies that are clearly identical to ours, have been described previously from somewhat deeper sites (4500–5000 m) in the CCFZ. In order to explore distributions at a global scale, we compared our Pacific nodule assemblages to foraminifera attached to ice-rafted dropstones from several deep seamounts in the abyssal northeast Atlantic Ocean, mainly on the Porcupine Abyssal Plain (4630–4680 m depth) with additional material from the BIOTRANS area (3796–4351 m). These hosted superficially similar assemblages of mats, tubular forms, komokiaceans and polythalamous calcareous and agglutinated foraminifera. However, apart from the sometimes extensive development of Telammina networks, there were no morphospecies in common between the Pacific and Atlantic assemblages. These preliminary observations, based on limited material, suggest that most of the foraminiferal morphospecies encrusting hard substrates are widely distributed at regional scales in the abyssal Pacific but not necessarily at global scales. The study of abyssal encrusting assemblages poses considerable challenges. Priorities for the future include the development of reliable methods for distinguishing living and dead individuals, and molecular approaches to clarifying the taxonomic affinities of novel morphotypes.

Chapter 36 Neoproterozoic glacial record in the Mackenzie Mountains, northern Canadian Cordillera

Abstract In the Mackenzie Mountains, an arcuate foreland thrust-fold belt of Late Cretaceous–Paleocene age in the northern Canadian Cordillera, two discrete glacial–periglacial sequences of Cryogenian age (the Rapitan Group and the Stelfox Member of the Ice Brook Fm.) are separated by c. 1.0 km of non-glacial strata. The older Rapitan diamictite occurs in an amagmatic rift basin; the younger Stelfox diamictite occurs on a passive-margin continental slope. The Rapitan Group consists of three formations. The lower Mount Berg Fm. is a complex of diamictites and conglomerates of limited extent. The middle Sayunei Fm. is a thick sequence of maroon-coloured mudrocks hosting innumerable graded layers of silt- and fine-grained sandstone. It lacks wave- or traction current-generated bedforms, and is lightly sprinkled with granule aggregates (‘till pellets’) and lonestones of dolostone and rare extrabasinal granitoids. It is capped by a hematitic Fe-formation that was reworked into the disconformably overlying Shezal diamictite. The Shezal Fm. is a complex of olive-green coloured boulder diamictites with subordinate, dark-grey shales, siltstones and parallel-sided sandstones. Some of the boulders are faceted and striated, and include dolostone, quartzite, siltstone and gabbro in declining order of abundance. Diamictite terminates abruptly at the top of the Shezal Fm., which is sharply overlain by dark shales or by <52 m of fetid, dark-grey, 13 C-depleted limestone with graded bedding. The Stelfox Member is dominated by non-stratified, carbonate-clast diamictite with faceted and striated clasts, locally associated with subordinate, well-laminated shales containing till pellets and ice-rafted dropstones. It is thin or absent on the palaeocontinental shelf, but thickens seaward (southwestward) on the palaeocontinental slope. A thin clay drape separates it from a laterally continuous post-glacial ‘cap’ dolostone, which is a very pale coloured, micro- to macropeloidal dolostone with low-angle cross-laminae, giant wave ripples and local bioherms of corrugated stomatolites. In the NW, the dolostone is followed by reddish and greenish marls, followed by black shale of the Sheepbed Fm. In the SE, the dolostone is overlain by pink or grey limestones with well-developed sea-floor cements pseudomorphic after aragonite. In this area, the top of the dolostone is ferruginous and contains digitate rosettes of sea-floor barite cement, variably calcitized. The dolostone–limestone contact is perfectly conformable, and synclinal structures previously intepreted as karst features are tectonic in origin. The grand mean palaeomagnetic pole for the well-studied Franklin Large Igneous Province ( c. 718 Ma) of Arctic Laurentia, coeval with the basal Rapitan Group in the Mount Harper area, Yukon Territory, places the Mackenzie Mountains firmly in the tropics, at 18±3°N palaeolatitude, at the onset of the Rapitan glaciation. Carbon (δ 13 C), oxygen (δ 18 O) and strontium ( 87 Sr/ 86 Sr) isotopes have been measured in carbonates bracketing the Rapitan and Stelfox diamictites. Sulphur isotope data (δ 34 S) have been obtained from carbonate-associated sulphate and barite above the younger diamictite, and calcium isotope data (δ 44 Ca) from the younger carbonate itself. The results are broadly consistent with data from other areas. Iron isotope (δ 57 Fe) and cerium anomaly (Ce/Ce*) values increase systematically upwards through the Sayunei Fe-formation, supporting an interpretation that deposition occurred within a redox chemocline through which the basin floor descended as a consequence of isostatic loading by the advancing Shezal ice sheet. Supplementary material: Data are available at http://www.geolsoc.org.uk/SUP18470 .

Chapter 43 The Squantum Member of the Boston Basin, Massachusetts, USA

Abstract The Neoproterozoic diamictite-bearing Squantum Member is located in the Boston Basin in eastern Massachusetts, USA. The Boston Basin forms part of the Avalonia island arc terrane ( c. 650 Ma), and appears to have originated as a rift-type basin in an extensional setting along the northern margin of Gondwana, although its exact position is debated. Inferred palaeoenvironmental reconstructions of the Boston Basin have alternated between a fluvial basin where ice played a major role in transporting much of the coarse material and an evolving marine basin dominated by non-glacial subaqueous mass flow, submarine fans and turbidity-current deposition. The age of the Squantum is bracketed between c. 595 and 570 Ma, and is correlated by some to the glaciogenic diamictite succession of the Gaskiers Formation (eastern Newfoundland) as part of the putative global Gaskiers Glaciation c. 582–585 Ma. However, the Squantum Member consists of diamictite, graded sandstone and siltstone units, and fine-grained laminated argillite/mudstone units typical of debris flow and turbidite facies that accumulate in a submarine setting. A glacial influence is not readily identified and revolves around early interpretations of the diamictite as being ‘till-like’, the presence of laminated horizons that resemble glaciolacustrine ‘varvites’ and the disputed recognition of ice-rafted dropstones. There are no associated carbonates and, consequently, no geochemical data are available in connection with the Squantum Member.

Neoproterozoic glaciation in the mid-oceanic realm: An example from hemi-pelagic mudstones on Llanddwyn Island, Anglesey, UK

We report the first occurrence of ice-rafted dropstones in mid-oceanic sediments belonging to an ocean plate stratigraphy within a Neoproterozoic accretionary complex on Llanddwyn Island, Wales, UK. Dropstones of sandstone, chert, and basalt occur as matrix-supported exotic clasts in a 1 m-thick, hemi-pelagic mafic mudstone; the largest clast is 20 × 25 cm across. These dropstones occur specifically in hemi-pelagic mafic mudstone that is located at the structural top of ocean plate stratigraphy that records a ridge-trench transition; they are supplementary to dropstones associated with extensive tillites reported in shallow marine sequences of continental shelf facies and in back-arc basins.

Cold climate in the eastern Australian mid to late Permian may reflect cold upwelling waters

A suite of ice-rafted dropstones and glendonites throughout the Permian succession of eastern Australia indicates the cold climate associated with the late Palaeozoic ice age persisted longest in this part of Gondwana. Paradoxically, these cold climate indicators are preserved in transgressive and highstand facies and formed at mid to high latitudes at a time when palaeofloral and sedimentological data suggest equable onshore environments during the intervening lowstands and temperate conditions at the pole. These apparent inconsistencies suggest that eastern Australia was anomalously cold in the context of post-Sakmarian Gondwanan climates, and the distribution of sedimentary indicators could indicate localized cooling by oceanographic processes. Modern upwelling of cold abyssal waters produces the specific physiochemical conditions necessary for ikaite formation, and is likely to have contributed to the development of glendonites in the eastern Australian Permian system. Such upwelling would have locally lowered surface water temperatures such that seasonal sea or river ice could form, which rafted debris across the marine shelf, and a cold-water fauna could develop. This hypothesis is supported by coupled atmosphere–ocean models for the Permian, which suggest that wind systems may have driven upwelling along this section of the Gondwanan coast. Colder conditions dissipated with the onset of the Hunter–Bowen Contractional Event and the transformation of the basin system from an open marine shelf to a foreland basin. This new hypothesis reconciles the prolonged deposition of cold-climate indicators in the eastern Australian Permian with an apparent post-Sakmarian warming elsewhere in Gondwana.

Late Vendian-?Cambrian glacially influenced deep-water sedimentation, Macduff Slate Formation (Dalradian), NE Scotland

The deep-water metasedimentary rocks of the late Dalradian Macduff Slate Formation preserve a record of distal glaciomarine sedimentation in the form of ice-rafted dropstones and related bedding-deformation structures, glaciogenic debris-flow diamictites and an allochthonous slump unit. The input of ice-rafted debris was cyclical and probably related to periodic disintegration of the marine margin of an ice sheet. Downslope resedimentation processes, typical of glaciated continental margins, were responsible for the diamictites and slump deposits which were probably derived from the adjacent outer shelf and upper slope, in common with Pleistocene analogues off NW Britain. The uppermost diamictite bed is an integral part of the deep-marine succession, in contrast to the previously proposed lodgement-till hypothesis.

Late Precambrian glaciation of central East Greenland

The Vendian succession of the central East Greenland fjord region is among the best preserved and most extensively exposed in the world and is represented approximately by the Tillite Group, comprising five formations, which from bottom to top are: the Ulvesø Formation (formerly Lower Tillite, dominantly diamictite), the Arena Formation (formerly Inter-Tillite beds, dominantly sandstone and shale), the Storeelv Formation (formerly Upper Tillite, dominantly diamictite, sandstone and conglomerate), the Canyon Formation (dominantly shale and dolostone) and the Spiral Creek Formation (dominantly sandstone, shale and dolostone). The group is underlain by Bed group 20 (limestone) and Bed group 19 (dominantly carbonate, shale and breccia) in different places and in the peripheral areas by older Bed groups of the Eleonore Bay Group. Above, the transgressive Cambrian sandstones of the Kløftelv Formation cap the Vendian sequence, though the exact level of the Precarmbrian-Cambrian boundary may be slightly lower. This paper reviews previous work on the succession and presents numerous new stratigraphic sections and an assessment of the sedimentary structures. The stratigraphic and palaeoenvironmental significance of the succession can therefore be assessed. Both main diamictite horizons contain other facies, which together with sedimentary structural and fabric data indicate that the following sediments have been produced: lodgement tillites, waterlain tillites, proximal and distal glaciomarine sediments (with ice-rafted dropstones), glaciolacustrine sediments, fluvioglacial deposits, subaqueous and sub-aerial mass-flows and periglacial phenomena. The other sediments of the Tillite Group were deposited in a dominantly shallow marine or lacustrine environment. Deeper water turbidites are recorded in part of the Arena Formation, whilst the upper part of the group indicates periodic emergence. Stromatolites, desiccation cracks and halite pseudomorphs are particularly distinctive in this part of the succession. A hiatus preceded deposition of the Kløftelv Formation, a transgressive unit of tidally-influenced sandstones. Stratigraphic thicknesses vary considerably, and whereas individual formations can be recognised throughout most of the fjord region, intraformational horizons cannot normally be traced for more than several kilometres. The nature of the bottom contact of the Tillite Group remains enigmatic, and transitional sedimentary, unconformable and thrust contacts are all present. It is possible that Bed Group 20 may be the lateral equivalent of the Ulves0 Formation, or of Bed Group 19. Although local palaeogeography was complex during deposition of the diamictites, these rocks indicate that a low-level ice sheet prevailed at the time. Together with Svalbard and western Scotland they represent the periphery of an ice-sheet which stretched across much of northern Europe to the Urals. However, much of Vendian time was not characterised by glacial conditions, and warm climates prevailed when the upper part of the Tillite Group was deposited.

Recognition of late Precambrian glaciogenic sediments in Liberia

Research Article| November 01, 1986 Recognition of late Precambrian glaciogenic sediments in Liberia A. W. Magee; A. W. Magee 1Department of Geological Sciences, Old Dominion University, Norfolk, Virginia 23508 Search for other works by this author on: GSW Google Scholar S. J. Culver S. J. Culver 1Department of Geological Sciences, Old Dominion University, Norfolk, Virginia 23508 Search for other works by this author on: GSW Google Scholar Geology (1986) 14 (11): 920–922. https://doi.org/10.1130/0091-7613(1986)14<920:ROLPGS>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A. W. Magee, S. J. Culver; Recognition of late Precambrian glaciogenic sediments in Liberia. Geology 1986;; 14 (11): 920–922. doi: https://doi.org/10.1130/0091-7613(1986)14<920:ROLPGS>2.0.CO;2 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 SocietyGeology Search Advanced Search Abstract Late Precambrian glaciation in West Africa is now suggested to have extended as far south as Gibi Mountain in west-central Liberia, 200 km farther south than previously recognized glacial deposits in central Sierra Leone. The Gibi Mountain Formation includes a basal diamictite, interpreted as a probable tillite, and overlying shallow-marine laminites containing isolated, ice-rafted dropstones and dropgrains. These rocks rest on Late Archean age gneisses and are overlain by Late Archean? age quartzite klippen emplaced during the pan-African orogeny (ca. 550 Ma). First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Cretaceous and Tertiary fossils in glacio-marine strata at Cape Melville, Antarctica

Glacio-marine sediments of the Cape Melville Formation (King George Island, South Shetland Islands, West Antarctica) contain a rich and predominantly invertebrate fauna. The occurrence of nannoplankton and belemnites might be used as evidence for dating the glacio-marine Cape Melville Formation as Cretaceous. However, the co-occurrence of the coral genus Flabellum (Eocene–Recent) and the foraminiferal genus Uvigerina (Eocene–Recent) indicate that recycling of the nannoplankton and belemnites has occurred. The sediments of the Cape Melville Formation contain a variety of glacially striated ice-rafted dropstones (up to 2 m) of Antarctic continent provenance. These sediments lie between lavas of the Sherratt Bay Formation and Pliocene Polonez Cove Formation, and may correlate with a late Miocene glaciation of Antarctica.