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Bedforms and associated facies of an Early Permian grounding-line fan: Exploring flow regime transitions of glacigenic density flows during the Late Paleozoic Ice Age

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Abstract

Grounding-line fans and their associated supercritical-flow bedforms are still poorly recognized in the upper Paleozoic glacial record. In order to fill this gap and to explore the products of the transition between upper and lower flow regimes, we examined a gravel-rich succession composed of conglomerates, sandstones, and diamictites deposited during the Late Paleozoic Ice Age (LPIA) in southeastern Paraná Basin, Brazil. To investigate this succession, we described the main bedforms and associated facies in laterally persistent outcrops and measured several vertical logs that were subsequently merged into a composite log. From base to top, the succession is composed of a muddy glaciotectonized substrate, and a subglacial to ice-marginal facies association including a basal, boulder-rich, subglacial lag of local extent, and submarine deposits belonging to a grounding-line fan. In the grounding-line fan association, we identified bedforms related to concentrated flows that represent the complete spectrum of flow regime transition from supercritical to subcritical in a waning wall jet model. The proximal zone were dominated by highly turbulent supercritical flows, resulting in highly scoured conglomerates and deposition of unstable and stable antidunes. A transitional zone under transcritical flow conditions resulted in deposits represented by humpback dunes. In the distal zone, the flow behaved as subcritical, resulting in the deposition of dunes and climbing ripples. Debris flows were also active. These cohesive flows underwent dilution by water entrainment and erosion of the head, leading to the generation of turbulent portions in the head of the debris flows. This bipartite behavior of the flows resulted in thin sandstones capping diamictites, which compose the fabric of the glaciogenic debrites observed in the study area. The described facies and bedforms point to the existence of a grounded ice margin at the final stages of the LPIA in the SE portion of the Paraná Basin.

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... Such deposits testify to sustained delivery of sandy material to the basin, possibly in response to a major flood event such as a glacial lake outburst flood (Ghienne et al. 2010). Similar facies are recognized throughout the glacial sedimentary record, from the Ordovician of northern Africa to the Quaternary of northern Germany (Lang et al. 2021), and from the LPIA deglacial record of the SE Paraná Basin (Brazil) where a similar assemblage has recently been recognized (de Souza et al. 2023). In this context, the separation of bedsets by gravelly intervals may suggest somewhat flashy, pulsed or repeated meltwater delivery events indicating oscillating flow velocities. ...
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Black shales represent the transgressive systems tract and HST were not deposited or preserved in the area. High sediment supply associated with lowstand tracts could explain the occasional (Vidal Ramos) to common occurrence of slope failures (slumps and diamictites) involving thin bedded turbidites and delta front sandstones. This situation is quite logical in terms of deglacial periods, and resulting high sediment supply, within a long-term icehouse context, with prevalence of lowstand to transgressive settings. RESUMO: Turbiditos têm sido foco de estudos ao longo das últimas décadas devido a sua importância como reservatórios de óleo e gás. No entanto, a relação destes depósitos com deltas é um tema que ainda merece atenção. Nesse sentido, esse artigo discute um exemplo de turbiditos e sistemas deltaicos deglaciais a pós-glaciais expostos na região de Vidal Ramos (Santa Catarina), na porção sul da Bacia do Paraná. Nessa área, uma sucessão com cerca de 360 metros de espessura associada à porção superior da Formação Mafra e à Formação Rio do Sul recobre o em-basamento Proterozóico (Complexo Metamórfico Brusque). Da base para o topo, esse intervalo inclui folhelhos negros, depósitos de transporte de massa e turbiditos arenosos da Formação Mafra recobertos por turbiditos finamente acamadados, in-cluindo um intervalo de folhelhos negros e turbiditos arenosos, e arenitos de frente deltaica proximal (Formação Rio do Sul). Esse intervalo estratigráfico registra duas espessas sucessões deltaicas superpostas com espessura individual de mais do que 150 m. Cada sucessão comporta turbiditos arenosos na base (prodelta) seguidos por turbiditos finamente acamadados (talude deltaico) que gradam para arenitos de frente deltaica, ambos os tipos parcialmente colapsados. Os dois pacotes de turbiditos arenosos da base de cada sucessão deltaica recobrem de forma abrupta intervalos de folhelhos negros vinculados a inundações máximas, caracterizando assim duas conformidades relativas. Uma seção estratigráfica elaborada a partir da correlação de quatro perfis levantados na escala 1/100 próximo ao topo do intervalo de estudo sugere que a frente deltaica distal inclui turbiditos finamente acamadados e aren-itos retrabalhados por ondas enquanto que a frente deltaica proximal é dominada por estratos vinculáveis a fluxos hiperpicnais de longa duração. A sucessão como um todo sugere que os turbiditos mais expressivos, que caracterizam a base dos sistemas deltaicos, foram gerados por quedas relativas do nível de base (trato de mar baixo precoce), enquanto que os turbiditos finamente acamadados representam cunhas de mar baixo formadas durante as fases mais tardias dos estágios de mar baixo. Os fol-helhos negros, por sua vez, foram correlacionados a tratos transgressivos enquanto depósitos de mar alto não foram depositados ou preservados. Alto suprimento sed-imentar associado a um nível de mar baixo poderia explicar a ocasional (Vidal Ramos) a frequente ocorrência de feições de remobilização de taludes deposicionais (escorregamentos e fluxos de detritos) que envolviam a remobilização de turbiditos finamente acamadados e areias de frente deltaica. Tal panorama é compatível com episódios de deglaciação, e resultante elevado suprimento sedimentar, vinculados as fases finais de um longo período de glaciação (icehouse) que controlava o domínio de tratos de sistemas de mar baixo e transgressivo.
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A small and poorly diversified bivalve fauna from Taciba Formation, Itararé Group, Paraná Basin (State of Santa Catarina, Mafra Municipality), is described in this paper for the first time, based on new findings. The fauna is recorded in a 30 cm thick interval of fine sandstone locally at the top of Taciba Formation, in the Butiá quarry. The studied fossil-bearing sandstone bed is a marine intercalation recording a brief eustatic rise in sea-level, probably following glacier retreat and climate amelioration at the end of a broad glacial scenario. The fauna is mainly dominated by productid brachiopods, which are not described here, and rare mollusk shells (bivalves and gastropods). Two bivalve species were identified: Myonia argentinensis (Harrington, 1955), and Aviculopecten multiscalptus (Thomas, 1928). The presence of Myonia argentinensis is noteworthy since this species is also present in the Baitaca assemblage found in marine siltstones (Baitaca assemblage) of the Rio do Sul Formation, cropping out at the Teixeira Soares region, Paraná State. This species is also recorded in the bivalve fauna from the Bonete Formation, Pillahinco Group, Sauce Grande Basin, Buenos Aires Province, in Argentina. Hence, the marine bivalves of the Taciba Formation are associated with the transgressive event that characterizes the Eurydesma fauna, indicating a Late Asselian-Sakmarian age for the bivalve fauna. Presence of the Myonia argentinensis megadesmid species reinforces the Gondwanic nature of the studied fauna.
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Soft-sediment glacially striated surfaces are common structures in ancient glacial successions, such as the Gondwanic Paleozoic record, and document ice erosion on non lithified beds. In most cases, they have been interpreted as the product of subglacial erosion beneath advancing marine or terrestrial glaciers, although direct evidence for glacial sedimentation in associated strata is infrequent. In this paper we document soft-sediment striated surfaces of Carboniferous age occurring in cross-laminated sandstones of the lower Itararé Group, southern Paraná Basin, Brazil, and interpret them as scour marks generated by keels of floating ice. Striated surfaces occur in closely spaced, multiple stratigraphic horizons in the lower half of a deglacial sequence, whose facies characteristics indicate sedimentation in subaqueous outwash and deltaic settings. Surfaces are laterally discontinuous, commonly bordered by marginal berms, and in some cases capped by mudstones with dropstones; they have orientations that deviate from the regional paleo-ice flow indicated by glacial grooves carved on the pre-glacial basement. The characteristics observed are consistent with modern and Pleistocene ice-keel scour marks. In addition, soft-sediment striated surfaces from other Gondwanic localities have similar attributes, suggesting that ice-keel scour marks may be more common in the geological record than previously considered and that vertically repeated striated surfaces do not necessarily indicate multiple ice-sheet advances. Therefore, paleo-ice flow reconstructions based on soft-sediment striated surfaces need to be revaluated considering that the movement of free-floating ice masses is controlled by phenomena other than glacier advance. (http://jsedres.sepmonline.org/content/84/1/26.abstract)
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DEPOSITIONAL SEQUENCES AND SYSTEM TRACTS IN THE CARBONIFEROUS-PERMIAN GLACIAL ITARARÉ GROUP, SOUTHERN BRAZIL A representative vertical stratigraphic section about 800m thick was measured in outcrops of the Carboniferous-Permian Itararé Group in northern Paraná State. Five depositional sequences bounded by disconformities were recognized and their facies associations described. The outcrop section shows good correlation with well logs and the sequences may be traceable over 400 kilometers across the basin in a EW depositional strike section. The five sequences have the same basic stratigraphic signature, which represents the glaciomarine sedimentary response to advance and retreat of glaciers. Ideally, three successive facies associations would compose each sequence, but the lower and upper ones may be absent. The lower facies association occurs only in the two lowermost sequences and includes subglacial facies as tillites and boulder conglomerates (glacial maximum system tract). The subglacial facies are covered by conglomerates and sandstones, which in turn are overlain by diamictites, turbidites and finely laminated facies with dropstones, composing a typical retrogradational facies stacking resulting from deglaciation (deglaciation system tract). Lying directly over the disconformities when subglacial facies do not occur, the deglaciation facies association is present in all sequences and represents the bulk of the stratigraphic record of the Itararé Group. Fine-grained laminated facies represent the record of the maximum glacial retreat during interglacial periods and contain the maximum seawater-flooding surface of each sequence. Coarsening upward facies association may be present in the upper part of the sequences, interpreted as the sedimentary response to the beginning of new glacial advances (glacial advance system tract). This facies association could represent progradation in highstand system tracts, but coastal and continental facies were not observed in the studied area.
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During the Late Palaeozoic, the Gondwana supercontinent formed an extensive Southern Hemisphere landmass that was affected by multiple glacial episodes, known collectively as the Late Palaeozoic Ice Age (LPIA). This resulted in the deposition of glacial, periglacial and deglacial sediments over much of the supercontinent. The Mississippian to early Pennsylvanian phase of glaciation is widely represented along the western margin of Gondwana. This constitutes one of the largest glaciations of the Phanerozoic in terms of its recorded extent and the widespread erosional hiatus it produced in the stratigraphic record. It was this mid Carboniferous glaciation, recorded in the Paganzo Basin of NW Argentina, that carved most of the paleovalleys and paleofjords present there. We report new U-Pb zircon ages from a single glacial succession (Guandacol Formation) of 326 ± 3 Ma and 320 ± 5 Ma, that in comparison with neighbouring dated sequences allow for the first time a reliable estimate of the timing and duration of glacial cycles. Palynological studies of these glacial-deglacial events yielded palynoassemblages of the MQ (Late Visean) and DMa (Sepukhovian-Bashkirian) Palynozones. The Carboniferous glacially-related strata and glacial cycles of the Paganzo Basin are compared here with equivalent units of the Paraná Basin of SE Brazil, suggesting a similar climate record for most of western Gondwana. We propose a new correlation between these two basins. The new U-Pb zircon ages reported here indicate that a regional glacial peak occurred almost coincident with the Mississippian-Pennsylvanian boundary, suggesting that the ensuing postglacial transgression is the best regional marker to differentiate the Mississippian and Pennsylvanian as it forms an identifiable interbasinal horizon. The biostratigraphic and chronostratigraphic comparison with other LPIA successions from South America reinforces that the First Appearance Datum of monosaccate pollen grains occurs in the late Serpukhovian. Their correlation confirms that similar climate conditions prevailed across most of western Gondwana during this phase of the LPIA.
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Mass-transport deposits (MTDs) may show a variety of deformation aspects, from poorly-deformed layers to highly deformed and homogenized deposits, as a result of the different kinds of mass-transport processes involved in their evolution. Previous studies indicate that the downslope flow transformation of MTDs is characterized by increasing disaggregation, mixing and homogenization of a non-to poorly lithified protolith, which may ultimately generate a homogenite. We here explore the role of deformation processes in the disaggregation and mixing of sediments that tend to lead to sediment homogenization and so may also control the evolution of MTDs. Using the Itararé Group of the Paraná Basin, southern Brazil as our case study, we examine several types of structures in MTDs with varying degrees of mass disaggregation and discuss the role of deformation process in the continuum homogenization of the sediment. Three deformational facies (DF-1, DF-2 and DF-3) were identified and interpreted as recording varying stages of homogenization of sediments (disaggregation and mixing) within the incipient, mature and evolved, flows respectively. These different stages show transitional aspects that indicate a continuum homogenization. The comparison between structural assemblages and the amount of matrix in the three different deformational facies indicates a close relationship between styles of deformation and the homogenization expected for mass flows. The processes that promoted disaggregation include boudinage, faulting, layer-parallel shearing and marginal deformation of intrabasinal clasts, while mixing processes that generate matrix include shearing and liquefaction/fluidization of sediments.
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Cyclic steps are a type of upper flow-regime bedform consisting of trains of upstream- and upslope-migrating bed undulations. The overriding flow is characterised by a series of hydraulic jumps occurring in the troughs of the undulations. Cyclic steps form in open-channel flows such as rivers and comprise a common bedform in subaqueous density flows in oceans, lakes and reservoirs. Cyclic steps are associated with alternating Froude-subcritical and Froude-supercritical flow on respectively the stoss side and lee side of individual bedforms. The transition between these flow states is embodied by the hydraulic jump in the trough of the bedform, leading to the permanent or quasi-permanent morphology of cyclic steps. Over the past decade, numerous studies affirmed the dominant role of cyclic steps in generating bed undulations in modern and ancient glacial outwash, fluvial, delta and turbidite environments as reviewed here. Cyclic steps were previously discriminated as net-depositional (climbing), transportational and net-erosional (falling) in different parts of sedimentary systems. The following cyclic step descriptors, with distinct depositional signatures, are proposed: fully depositional, partially depositional, transportational, partially erosional and fully erosional. Partially depositional cyclic steps are most common. They are associated with backset-bedded sets (high aggradation rate) and nested scours filled with massive-to-backset-bedded deposits (low aggradation rate). The new classification can be used as a predictive tool in the reconstruction of modern and ancient sedimentary successions using repeat bathymetry, seismic reflection or outcrop data. It is applied in three turbidite case studies: classical deep-sea system, small-scale delta slope and line-sourced carbonate slope.
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This paper describes facies relationships and stratigraphic architecture of early Permian deglacial deepwater to fluvio-deltaic deposits exposed in the eastern border of the Paraná Basin, southern Brazil. The studied succession is located in the Rio do Sul depocenter and comprises an upward transition between the topmost interval of the Itararé Group (Rio do Sul Formation) and the lowermost unit of the Rio Bonito Formation (Triunfo Member). Geological mapping and facies analysis allowed the recognition of the following facies associations, vertically stacked: thin and thick-bedded turbidites, hyperconcentrated and concentrated density-flow deposits (hyperpycnites), mass-transport deposits, fine-grained heterolithic beds of prodelta, delta-front, shoreface and delta-plain deposits, as well as fluvial beds resting on incised valleys. The stratigraphic analysis through the interpretation of the facies associations, stacking pattern and paleocurrent dispersal trends were enough to define the clinoform stratigraphic architecture, composing a progradational-aggradational pattern. The paleoflows for all facies associations show a main trend towards the south-southwest. In this context, hyperpycnal beds can represent a glacial source from the northeast, because of striated and faceted clasts immersed in the conglomeratic facies, whereas turbidites have been understood as the products of their dilution. Likewise, mass-transport deposits are related to the collapse of the shelf-margin delta, represented by allochthonous blocks composed of deltaic beds within the mass-flow diamictites. Thus, these mass-transport deposits do not evolve downslope to turbidity currents. This configuration was likely for building the relief of the slope system, probably a shelf-edge delta susceptible to mass failure. Paleo-ice flows towards SW recorded in the Rio do Sul Formation, genetic related to fluvio-deltaic deposits of Rio Bonito Formation, highlight the scenario of final stages of the Late Paleozoic Ice Age in the SW of Gondwana and support the previous researches which point to the presence of diachronous ice-spreading centers from paleohighs.
Chapter
Glacitectonics can be defined as "the deformation caused as a glacier or ice sheet pushes into and overrides a pre-existing sequence of sediments and/or bedrock." This deformation typically involves folding and thrusting, and therefore results in a similar range of structures to those found in orogenic mountain belts formed due to crustal shortening at convergent tectonic plate boundaries; only at a much smaller scale and at significantly lower pressures and temperatures. This similarity has invariably led to the application of a thin-skinned thrust tectonic model to deformed glacigenic sequences. However, whilst fold and thrust belts in crustal collision zones take millions of years to evolve, the timescales required for the formation of even the largest glacitectonic moraines is in the order of tens to hundreds of years. In fact, recent studies of contemporary glacial environments suggest that even relatively large thrust moraines may form within only a few years, or even days during the final stages of a glacier surge.
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This paper examines a glacial diamictite-bearing succession from the upper Itararé Group (Taciba Formation) in eastern Paraná Basin, Brazil. The object of study provides the opportunity to investigate in detail the late stages of glacial sedimentation during the Late Paleozoic Ice Age (LPIA) in this sector of SW Gondwana, with implications for glacial cyclicity and regional paleo-ice flow reconstructions. Sedimentology, geological mapping and palynological analysis allowed the recognition of four facies associations, comprising subaqueous outwash, mass-transport, tide-influenced delta-front, and tide-influenced delta plain deposits. The succession records at least two episodes of ice-margin advance and retreat into a marine-influenced environment and can be placed in the earliest Permian based on the palynomorph assemblage. Cross stratification in outwash facies and deltaic deposits indicate sediment transport to the SW, the same revealed by deformational structures in mass-transport diamictites derived from downslope resedimentation of glaciomarine sediments during deglaciation. A glacial source to NE is therefore indicated and is in agreement with paleo-ice flow directions obtained from previously studied localities to the north. This north-derived early Permian glaciation contrasts with glacial sources to the SE recognized in the lower and middle intervals of the Itararé Group. The scenario suggests a geometric reconfiguration of the Paraná Basin during the LPIA and the migration of ice centers with time during the Late Paleozoic Ice Age in SW Gondwana.
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Most theories of glacier movement and subglacial erosion have assumed that glaciers rest on rigid bedrock surfaces. Whilst this is probably correct for much of the bed area of most modern glaciers, deformable sediments do occur beneath them and formed a substantial area of the beds of large ice sheets during glacial periods. Observations and theories are presented and reviewed about the processes of glacier erosion of rock and unlithified sediment beds both when they are frozen and unfrozen. Erosional bedrock landforms, such as roches moutonnées, indicate two principal subglacial erosional processes, plucking and abrasion. Where supraglacially derived debris is unimportant, plucking provides the tools which abrade the bed, and must be a quantitatively more important process than abrasion, though more localized. Where plucking is suppressed, erosion rates must be slow. Subglacial measurements of abrasion rates beneath a temperate glacier are used to test an earlier abrasional theory (Boulton, [C1974]). The form of the predicted abrasion-rate curve for changing ice velocity and pressure is verified. This theory successfully simulates two-dimensional erosional bedforms. Subglacial observations demonstrate how flow basal ice around the flanks of bedrock obstacles causes streaming of debris to occur. It is suggested that this streaming process is primarily responsible for the longitudinally lineated form of large-scale surfaces typical of glacially eroded bedrock. Plucking and abrasion also occur beneath cold ice, though at slow rates, and are probably restricted to places where the ice thickness is small. Where the glacier bed is composed of unlithified sediment, subglacial measurements show that deformation can produce very large discharges of subglacial material, which makes this a potential agent of very rapid subglacial landform production. The heterogeneity of subglacial sediment leads to spatially variable rates of deformation, and it is suggested that relatively stronger parts of the sediment body may form the nuclei for drumlin and mega-flute formation. Whereas unlithified unfrozen sediment deforms beneath the glacier rather than being incorporated within it, ice-cemented subglacial sediments can behave like bedrock, because of their relative rigidity, and are readily plucked and incorporated englacially. They may also deform beneath the glacier.
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Erosion and deposition by supercritical density flows can strongly impact the facies distribution and architecture of submarine fans. Field examples from coarse-grained channel-levée complexes from the Sandino Forearc Basin (southern Central America) show that cyclic-step and antidune deposits represent common sedimentary facies of these depositional systems and relate to the different stages of avulsion, bypass, levée construction and channel backfilling. During channel avulsion, large-scale scours (18 to 29 m deep, 18 to 25 m wide, 60 to > 120 m long) were incised by supercritical density flows. The multi-storey infill of the large-scale scours comprises amalgamated massive, normally coarse-tail graded or widely spaced subhorizontally stratified conglomerates and pebbly sandstones, interpreted as deposits of the hydraulic-jump zone of cyclic steps. The large-scale scour fills can be distinguished from small-scale channel fills based on the preservation of a steep upper margin and a coarse-grained infill comprising mainly amalgamated hydraulic-jump zone deposits. Channel fills include repeated successions deposited by cyclic steps with superimposed antidunes. The deposits of the hydraulic-jump zone of cyclic steps comprise regularly spaced scours (0.2 to 2.6 m deep, 0.8 to 23 m long) infilled by intraclast-rich conglomerates or pebbly sandstones, displaying normal coarse-tail grading or backsets. These deposits are laterally and vertically associated with subhorizontally stratified, low-angle cross-stratified or sinusoidally stratified sandstones and pebbly sandstones, which were deposited by antidunes on the stoss side of the cyclic steps during flow re-acceleration. The field examples indicate that so-called spaced stratified deposits may commonly represent antidune deposits with varying stratification styles controlled by the aggradation rate, grain-size distribution and amalgamation. The deposits of small-scale cyclic steps with superimposed antidunes form fining-upwards successions with decreasing antidune wavelengths, indicating waning flows. Such cyclic step-antidune successions form the characteristic basal infill of mid-fan channels, and are inferred to be related to successive supercritical high-density turbidity flows triggered by retrogressive slope failures.
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During the Late Paleozoic, the Gondwana supercontinent was affected by multiple glacial and deglacial episodes known as “The Late Paleozoic Ice Age” (LPIA). In Brazil, the evidence of this episode is recorded mainly by widespread glacial deposits preserved in the Paraná Basin that contain the most extensive record of glaciation (Itararé Group) in Gondwana. The Pennsylvanian to early Permian glaciogenic deposits of the Itararé Group (Paraná Basin) are widely known and cover an extensive area in southern Brazil. In the Doutor Pedrinho area (Santa Catarina state, southern Brazil), three glacial cycles of glacier advance and retreat were described. The focus of this article is to detail the base of the second glacial episodes or Sequence II. The entire sequence records a deglacial system tract that is represented by a proximal glacial grounding-line system covered by marine mudstones and shales associated with a rapid flooding of the proglacial area. This study deals with the ice proximal grounding-line systems herein interpreted according to lab model named plane-wall jet with jump. Detailed facies analysis allowed the identification of several facies ranging from boulder-rich conglomerates to fine-grained sandstones. No fine-grained deposits such as siltstone or shale were recorded. According to this model, the deposits are a product of a supercritical plane-wall outflow jet that changes to a subcritical jet downflow from a hydraulic jump. The hydraulic jump forms an important energy boundary that is indicated by an abrupt change in grain size and cut-and-fill structures that occur at the middle-fan. The sedimentary facies and facies associations show a downflow trend that can be subdivided into three distinct stages of flow development: (1) a zone of flow establishment (ZFE), (2) a zone of transition (ZFT), and (3) an established zone (ZEF). The proximal discharge is characterized by hyperconcentrated-to-concentrated flow due to the high energy and sediment-laden nature of the flows. At the transitional zone, a hydraulic jump produces a rapid shift of conglomeratic to sandy facies with associated scour features. Towards the distal zones, the jet detaches to originate a vertical turbulent jet characterized by more diluted flows. Discussion of fan facies and architecture within a framework of jet-efflux dynamics provides an improved understanding of grounding-line fans systems that produce coarse-grained strata commonly enclosed by fine-grained rocks. The results have clear implication in terms of prediction of facies tract and geometry of oil and gas reservoirs deposited under similar conditions. And also can be useful to identifying the position of a glacial terminus through time.
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Caddisflies (Trichoptera) are small, cosmopolitan insects closely related to the Lepidoptera (moths and butterflies). Most caddisflies construct protective cases during their larval development. Although the earliest recognisable caddisflies date back to the early Mesozoic (Early and Middle Triassic), being particularly numerous and diverse during the Late Jurassic and Early Cretaceous, the first records of their larval case constructions are known exclusively from much younger, Early to Middle Jurassic non-marine deposits in the northern hemisphere. Here we present fossils from the Early Permian (Asselian-Sakmarian) marine deposits of Brazil which have strong morphological and compositional similarity to larval cases of caddisflies. If they are, which is very probable, these finds not only push back the fossil record of true caddisflies, but also indicate that their larvae constructed cases at the very beginning of their evolution in marine environments. Since modern caddisflies that construct larval cases in marine environments are only known from eastern Australia and New Zealand, we suggest that this marine ecology may have first evolved in western Gondwana during the Early Permian and later spread across southern Pangea.
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This work is a stratigraphic, reservoir, and environmental analysis of the Itarare Group (Permo-Carboniferous) using the well data of the Parana Basin which covers about 1 000 000 km2 in Brazil alone. It is proposed a new stratigraphic subdivision of the Itarare Group in subsurface. Three new formations and four new members are proposed. The lowermost formation is called the Lagoa Azul, which is subdivided into the Cuiaba Paulista Member, and the Tarabai Member. The new middle unit is the Campo Mourao Formation and the new uppermost unit is the Taciba Formation. The Taciba is subdivided into the Rio Segredo Member, the Chapeu do Sol Member and the Rio do Sul Member. Well logs show that the Itarare Group has three major depositional cycles termed lower, middle, and upper, which correspond broadly to the three new formations. It is likely that the cycles are response to climatic and sea level changes. Three major ice lobes seem to have entered the Parana Basin during the Permo-Carboniferous. -from English summary
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Dropstones display bending, penetration, rucking, and complete rupture of stratum occurring beneath them, and onlap above them, with the degree of deformation varying as a function of the size, shape, and axial disposition of the clast and of the sediment type into which they fell. Dump structures are conical mounds of gravel or diamict formed by the break-up and overturning of dirt-laden icebergs and the consequent release of large quantities of debris to the lake floor. Grounding structures are caused by the grounding of icebergs, the down-warping of underlying lake-floor sediment, and the subsequent in situ melt-out of contained debris to form isolated troughs of diamict.-from Authors
Chapter
This chapter provides information on processes operating both beneath modern glaciers and at their margins. The information is applied to explain quite distinct styles of glaciotectonic deformation that can be found in a glacial landscape. Emphasis lies on tectonic style and strain, starting from the assumption that different glacial regimes are reflected in certain characteristic glaciotectonic styles and widely varying amounts of finite strain. Glaciotectonics involves structural deformations of the upper horizon of the lithosphere caused by glacial stresses. Glaciotectonism refers to the processes leading to these deformations. Their detachment from undeformed bedrocks varies from a few centimeters to a few hundred meters. This definition excludes deformations of an entire crust owing to glacioisostatic movements and reactivation of crustal faults under ice loading. Deformations within the ice itself are excluded, as are processes of glacial erosion, such as plucking of bedrock and the particle-by-particle removal and transport of sediment by glacier ice.
Article
Globally, Sturtian (early Cryogenian) glacial deposits are well expressed, and belong to the oldest Neoproterozoic icehouse Earth event. The evidence for glaciation typically includes phenomena such as striated pavements, striated clasts in diamictites, and abundant dropstones. More problematic, and potentially more significant, are intercalated deposits that exhibit no apparent evidence of a glacial influence on deposition. These apparently non-glacially-influenced intervals may represent deposition during interglacial periods, or at times when ice sheets transitioned to cold-based ice masses where sediment advection into basins wassuppressed. Here, using three case studies from South Australia, northern Namibia, and Death Valley (USA), we show that many IRD-free intervals occur at the top of backstepping successions, where they are best interpreted as glacial minima deposits. In other cases, the volume of IRD in a succession shows less distinct increases and decreases upsection. Rhythmic intercalation of IRD-bearing and IRD-free intervals with glaciomarine turbidites can also be observed. These latter examples may be interpreted to record variations in debris content of ice margins, switch on / switch off of ice streams, or simply dynamic oscillation of a hinterland ice margin.
Chapter
Mass movements are significant geological processes which have an important impact on human life. They are environmental hazards, both on land and in the sea, and justify intensive attention. Much research has been related to human or industrial loss and welfare, and the importance of further efforts cannot be underestimated. The effects on modern communities of avalanches, major rock falls and mudslides are well known from recent catastrophies (Bolt et al. 1975; Press and Siever 1978; Voight 1978a; Brunsden and Prior 1984), and mass movements can seriously affect the stability of offshore installations (Prior & Coleman 1982). The importance in the context of this book is that mass movements present a very widespread situation in which sediments are subject to deformation.
Article
Grounding-zone wedges (GZWs) are asymmetric sedimentary depocentres which form through the rapid accumulation of glacigenic debris along a line-source at the grounding zone of marine-terminating ice sheets during still-stands in ice-sheet retreat. GZWs form largely through the delivery of deforming subglacial sediments. The presence of GZWs in the geological record indicates an episodic style of ice retreat punctuated by still-stands in grounding-zone position. Moraine ridges and ice-proximal fans may also build up at the grounding zone during still-stands of the ice margin, but these require either considerable vertical accommodation space or sediment derived from point-sourced subglacial meltwater streams. By contrast, GZWs form mainly where floating ice shelves constrain vertical accommodation space immediately beyond the grounding-zone. An inventory of GZWs is compiled from available studies of bathymetric and acoustic data from high-latitude continental margins. The locations and dimensions of GZWs from the Arctic and Antarctic, alongside a synthesis of their key architectural and geomorphic characteristics, is presented. GZWs are only observed within cross-shelf troughs and major fjord systems, which are the former locations of ice streams and fast-flowing outlet glaciers. Typical high-latitude GZWs are less than 15 km in along-flow direction and 15 to 100 m thick. GZWs possess a transparent to chaotic acoustic character, which reflects the delivery of diamictic subglacial debris. Many GZWs contain seaward-dipping reflections, which indicate sediment progradation and wedge-growth through continued delivery of basal sediments. GZW formation is inferred to require high rates of sediment delivery to a fast-flowing ice margin that is relatively stable for probably decades to centuries. Although the long-term stability of the grounding zone is controlled by ice-sheet mass balance, the precise location of any still-stands is influenced strongly by the geometry of the continental shelf. The majority of high-latitude GZWs occur at vertical or lateral pinning points, which encourage grounding-zone stabilisation through increasing basal and lateral drag and reducing mass flow across the grounding zone.
Article
The stratigraphicaldjstribution of the Late Paleozoic marine fauna and megafIora is examined with respect to the available biostratigraphic framework for part of the Upper Paleozoic sequence of the Parana Basin, Brazil (Tubarao Group) possibly spat:lning the Upper Carboniferous-Lower Permian intervalo ' Within this interval the megaflora succession shows a transition from assemblages with inci­ dence of northem forms to others in which the Glossopterids are more frequent and finally predo­ minate. The invertebrate megafauna (mollusks and brachiopods) succession shows a high frequency of cosmopolitan forms associated with Gondwanic forms in the early assemblages, followed later by more typical gondwanic faunal assemblages. The evidence at hand for the correlation of the studied sequence with the Caroniferous-Per­ mian boundary is discussed.
Article
Ordovician proglacial deposits form gas reservoirs in the In Amenas field, Illizi Basin, Algeria. Depositional models were developed to understand the context and disposition of the main reservoirs through an evaluation of core and analogous outcrops from the Tassili N'Ajjer. Tunnel valleys initially accumulated sandstones with tractional structures. Subsequent failures of subaqueous grounding line sediment deposited proglacial debrites comprising poorly sorted argillaceous sandstone with granules. These were interbedded with high-density turbidity sandstones; their fine grain size indicates they were dynamically disconnected from the lithologically varied debrites. A lobate geometry has been defined for one subsurface composite turbidite. Periodic catastrophic outflows, possibly evacuating subglacial lakes, incised the network of subglacial tunnels and in the process delivered sand to the turbidite outwashes. Bedforms indicate high-energy, transcritical to supercritical outflows that were stable for extended periods. During ice retreat, a period of ice margin stability may have occurred due to grounding over the In Amenas granitic palaeohighs. Outwash fan apices were located along this grounding line with feeder channels developing where the substrate was more easily eroded such as between the palaeohighs. Following further ice retreat, deposition evolved to variably sinuous channels and thence to pelagic fines with dropstones.