Colluvial sedimentation in a hyperarid setting (Atacama Desert,northern Chile): Geomorphic controls and stratigraphic facies variability

Research on colluvial depositional systems has recently emphasized periglacial and high‐altitude settings, and the relations between Quaternary slope stratigraphy and climate change. This article examines the role of variable slope morphology, surface hydrology and microclimate in controlling colluvial sedimentation along a coastal tract of the hyperarid Atacama Desert in northern Chile. Direct accessibility of active surfaces is accompanied by uninterrupted stratigraphic exposures along the base of slopes, allowing direct comparisons between surface processes and the resulting sedimentary record. Four slope sectors are identified, based on differences in morphology and processes over active surfaces. Colluvial sedimentation is controlled by complex interactions of slope gradients and profiles, exposure to dominant winds, and potential runoff pathways, which vary considerably between different sectors. Major differences are evident between these hyperarid deposits and slope sedimentation in periglacial and temperate settings, including the complete absence of pedogenic activity and clay minerals; the volume of aeolian deposits and their role in controlling processes which redistribute sediment downslope, extending colluvial aprons; and the occurrence of runoff processes only where favoured by particular topographic configurations. Depositional surfaces range from steep talus cones, to debris‐flow‐dominated and aeolian‐dominated colluvial aprons, to an aeolian ramp subject to reworking by mass flows and flash floods. Consequently, facies associations and architectures at outcrop are highly variable and highlight the importance of spatial variations in slope morphology and processes in producing distinct, coeval colluvial stratigraphies within a single environmental context. Discrepancies between active processes and the corresponding stratigraphic signatures are also evident in some sectors; for example, preservation of alluvial and aeolian facies in stratigraphic sections does not always reflect the dominant processes over active slopes. Together with the spatial variability in processes and deposits along these slopes, this suggests that caution is required when extracting palaeoenvironmental information from analyses of colluvial successions.     

Late Holocene sea-surface temperature and precipitation variability in northern Patagonia, Chile (Jacaf Fjord, 44°S)

A high-resolution multi-proxy study including the elemental and isotopic composition of bulk organic matter, land plant-derived biomarkers, and alkenone-based sea-surface temperature (SST) from a marine sedimentary record obtained from the Jacaf Fjord in northern Chilean Patagonia (44°20′S) provided a detailed reconstruction of continental runoff, precipitation, and summer SST spanning the last 1750 yr. We observed two different regimes of climate variability in our record: a relatively dry/warm period before 900 cal yr BP (lower runoff and average SST 1°C warmer than present day) and a wet/cold period after 750 cal yr BP (higher runoff and average SST 1°C colder than present day). Relatively colder SSTs were found during 750–600 and 450–250 cal yr BP, where the latter period roughly corresponds to the interval defined for the Little Ice Age (LIA). Similar climatic swings have been observed previously in continental and marine archives of the last two millennia from central and southern Chile, suggesting a strong latitudinal sensitivity to changes in the Southern Westerly Winds, the main source of precipitation in southern Chile, and validating the regional nature of the LIA. Our results reveal the importance of the Chilean fjord system for recording climate changes of regional and global significance.     

Cross-strata and bedforms of probable transitional dune to upper-stage plane-bed origin from a Late Precambrian fluvial sandstone, northern Norway.

A Late Precambrian fluvial sandstone sequence in northern Norway is dominated by large-scale cross-sets that show either lenticular or tabular geometries in the streamwise sections. The lenticular sets interdigitate and in places show nearly symmetrical formsets. The tabular sets are in places solitary, but are mainly grouped in cosets. In both cross-set types, the cross-strata range from concave-up to sigmoidal in shape, with the latter variety comprising subhorizontal to gently inclined topset strata (with parting lineation) that merge uninterruptedly downflow into the steeper (10–2°) foresets. Within the cross-sets the geometry and dip azimuths of the foresets are conspicuously consistent, although the concave-up and sigmoidal strata commonly alternate downcurrent. The cross-strata characteristics suggest flood stage deposition from relatively high velocity steady currents heavily laden with suspended sand. Both cross-set types are interpreted as representing bedforms generated by flow in the dune to upper-stage plane-bed transition. The lenticular cross-sets probably represent periodic dunes, but it is far less clear whether the long bedforms represented by the tabular sets should be classified as dunes, or rather as solitary to quasi-periodic bars.     

Soil stratigraphy and plant–soil interactions on a Late Glacial–Holocene fluvial terrace sequence,Sierra Nevada National Park,northern Venezuelan Andes

Analysis of a flight of alluvial terraces in the Sierra Nevada National Park near Pico Mucuñuque in the Eastern Mérida Andes has yielded information on geomorphic, pedogenic, and vegetational changes from Late Glacial time to the present. The terraces formed in large part due to stream incision/migration triggered by neotectonic uplift (>7000 yr BP) of a Late Glacial/Early Holocene glaciolacustrine lithosequence and, with the exception of the oldest/highest terrace, exhibit near-uniform lithology/parent materials. Soils developed in the terrace materials range from thin, weakly developed profiles (O/C/Cu horizons) to Entisols with O/Ah/Cox/Cu horizons and similar buried counterparts representing former short periods of floodplain stability or slow aggradation. The buried soils provide organic-rich material that yields radiocarbon ages, which provide time constraints on individual pedons and the geomorphic development of the site. Iron and aluminum extracts of soil matrix material provide information on the formation and accumulation of goethite and hematite, the relative accumulation of ferrihydrite (gain/loss), and the downward translocation of organically complexed Al as a function of soil development and age. SEM analysis of heavy mineral grains indicates varying material sources and degrees of weathering in the soil chronosequence. A qualitative study of plant functional types across the terrace sequence shows that older surfaces support greater plant diversity. The study also suggests ways in which the plant communities influence soil development at the site through varying organic matter inputs and varying soil moisture use by specific species (e.g., ferns on the oldest terrace), which may explain the absence of B horizons in the Late Pleistocene/Early Holocene soils.     

Late Wisconsinan deglaciation and proglacial lakes development in the Charlevoix region,southeastern Québec,Canada

The Charlevoix region, in southeastern Québec, is characterized by a dramatic landscape formed by the junction of the Laurentian Highlands, the Charlevoix Astrobleme and the St Lawrence Estuary. At the Last Glacial Maximum (LGM), the region was completely covered by the Laurentide Ice Sheet (LIS). The complex topography of the region was the stage of many of the major deglacial events of southern Quebec (e.g. Goldthwait Sea Invasion, St Lawrence Ice‐Stream, Saint‐Narcisse Episode). We present a detailed reconstruction of the pattern of retreat of the LIS in the Charlevoix region based on the interpretation of ice‐marginal features (e.g. moraines, fans) and glaciolacustrine landforms and deposits, two extensive field campaigns, and the interpretation of high‐resolution 3D digital aerial photographs and LiDAR data. Our results indicate five moraine complexes in the region: the Rochette, the Brûlée, the Sainte‐Anne, the Saint‐Narcisse and the Mars‐Batiscan complexes. Deltas, fans, fine‐grained sediments, littoral deposits, drainage breaches and deposits were used to identify 91 palaeo‐proglacial lakes. The identification of these lakes and their relation to moraine complexes enabled the reconstruction of six stages of lake development during the Charlevoix deglaciation. The development of proglacial lakes occurred in all types of terrain (highlands, lowlands, transitory levels above marine limit). We conclude that local topography had a decisive effect on promoting both moraine deposition and lake development. We suggest that similar topographical regions (hilly‐mountainous) that were affected by major ice‐margin stabilizations during glacial retreat should have experience small lakes dominating valleys and topographical lows.     

The nature and origin of off-fault damage surrounding strike-slip fault zones with a wide range of displacements: A field study from the Atacama fault system, northern Chile

Damage surrounding the core of faults is represented by deformation on a range of scales from microfracturing of the rock matrix to macroscopic fracture networks. The spatial distribution and geometric characterization of damage at various scales can help to predict fault growth processes, subsequent mechanics, bulk hydraulic and seismological properties of a fault zone. Within the excellently exposed Atacama fault system, northern Chile, micro- and macroscale fracture densities and orientation surrounding strike-slip faults with well-constrained displacements ranging over nearly 5 orders of magnitude (0.12 m–5000 m) have been analyzed. Faults have been studied that cut granodiorite and have been passively exhumed from 6 to 10 km depth. This allows direct comparison of the damage surrounding faults of different displacements. The faults consist of a fault core and associated damage zone. Macrofractures in the damage zone are predominantly shear fractures orientated at high angles to the faults studied. They have a reasonably well-defined exponential decrease with distance from the fault core. Microfractures are a combination of open, healed, partially healed and fluid inclusion planes (FIPs). FIPs are the earliest set of fractures and show an exponential decrease in fracture density with perpendicular distance from the fault core. Later microfractures do not show a clear relationship of microfracture density with perpendicular distance from the fault core. Damage zone widths defined by the density of FIPs scale with fault displacement but appear to reach a maximum at a few km displacement. One fault, where damage was characterized on both sides of the fault core shows no damage asymmetry. All faults appear to have a critical microfracture density at the fault core/damage zone boundary that is independent of displacement. An empirical relationship for microfracture density distribution with displacement is presented. Preferred FIP orientations have a high angle to the fault close to the fault core and become more diffuse with distance. Models that predict off-fault damage such as a migrating process zone during fault formation, wear from geometrical irregularities and dynamic rupture are all consistent with our data. We conclude it is very difficult to distinguish between them on the basis of field data alone, at least within the limits of this study.     

Late Quaternary palaeoenvironment of northern Guatemala: evidence from deep drill cores and seismic stratigraphy of Lake Petén Itzá

Long sediment cores were collected in spring 2006 from Lake Petén Itzá, northern Guatemala, in water depths ranging from 30 to 150 m, as part of an International Continental Scientific Drilling Program project. The sediment records from deep water consist mainly of alternating clay, gypsum and carbonate units and, in at least two drill sites, extend back >200 kyr. Most of the lithostratigraphic units are traceable throughout the basin along seismic reflections that serve as seismic stratigraphic boundaries and suggest that the lithostratigraphy can be used to infer regional palaeoenvironmental changes. A revised seismic stratigraphy was established on the basis of integrated lithological and seismic reflection data from the basin. From ca 200 to ca 85 ka, sediments are dominated by carbonate‐clay silt, often interbedded with sandy turbidites, indicating a sediment regime dominated by detrital sedimentation in a relatively humid climate. At ca 85 ka, an exposure horizon consisting of gravels, coarse sand and terrestrial gastropods marks a lake lowstand or partial basin desiccation, indicating dry climate conditions. From ca 85 to ca 48 ka, transgressive carbonate‐clay sediments, overlain by deep‐water clays, suggest a lake level rise and subsequent stabilization at high stage. From ca 48 ka to present, the lithology is characterized by alternating clay and gypsum units. Gypsum deposition correlates with Heinrich Events (i.e. dry climate), whereas clay units coincide with more humid interstadials.     

Late Quaternary vegetational and climatic change in the Popayán region,southern Colombian Andes

Late Pleistocene and Holocene vegetational and climatic change have been studied palynologically at a site at 1750 m elevation in the subandean vegetation belt near Popayán, in the southern Colombian Andes. Time control on the pollen record is based on six AMS ¹⁴C ages, ranging from possibly Middle Pleniglacial time (around 50000 yr BP) to 1092 ± 44 yr BP. Because of the presence of two hiatuses only the Middle Pleniglacial and Late Holocene periods (the last 2300 yr BP) are represented. Pollen data indicate the presence of closed subandean forest during glacial time. Changes in the contribution of pollen originating from the uppermost and lowermost subandean forest belts, changes in the contribution of a number of other subandean forest taxa, and changes in species composition between the three pollen zones, suggest that the climate during the Middle Pleniglacial was markedly colder, and perhaps also wetter, than during the Late Holocene. Pollen assemblages from the Late Holocene indicate that the landscape has been affected by deforestation and agriculture since at least 2300 yr BP, but that human impact decreased in the last 780 yr BP. © 1998 John Wiley & Sons, Ltd.     

Forced regressive deposits of a deglaciation sequence: Example from the Late Quaternary succession in the Lake Saint‐Jean basin (Québec,Canada)

Differentiating between forced regressive deposits from deglacial periods in high latitude domains and forced regressive deposits from the onset of glacial periods in low latitude domains is fundamental for the accurate interpretation of glacial cycles within the geological record and then for the reconstruction of palaeogeography and palaeo‐climate. A forced regressive deglacial sequence is documented from the Lake Saint‐Jean basin (Québec, Canada). In this area, the Late Pleistocene to Holocene sediments have recorded the Laurentide ice sheet retreat accompanied by the invasion of marine waters (Laflamme Gulf) from ca 12·9 cal kyr bp . Subsequently, fluvio‐deltaic and coastal prograding wedges were deposited; they followed the base‐level fall due to glacio‐isostatic rebound. This succession, representing a transition from glacial to post‐glacial periods within a previously glaciated area, was investigated through recent mapping, preserved landforms, facies analysis, and new optical stimulated luminescence and radiocarbon dates. Three basin‐scale geological sections share a common lower part made of isolated ice‐contact fan deposits overlying bedrock. Throughout the entire basin, ice‐contact fans are capped by glacimarine muds. Above, fluvial and coastal prograding systems were deposited and evolved through four steps: (i) deltaic systems progressively increased in width; (ii) coastal influence on sedimentation increased; (iii) hydrographic drainage systems became more organised; and (iv) deltas graded from steep (Gilbert delta) to low‐angle foresets (mouth‐bar delta). Deposited during the base‐level fall from glacio‐isostatic rebound, the complete succession has been designated as a single falling stage system tract referred to as a deglacial falling stage system tract. It is representative of a deglaciation sequence in areas previously covered by ice during glacial periods (i.e. medium to high latitude domains). Diagnostic criteria are provided to identify such a deglacial falling stage system tract in the geological record, which may aid identification of previously unknown glacial cycles.     

Late Quaternary vegetation and fire history in the northernmost Nothofagus forest region: Mallín Vaca Lauquen,Neuquén Province,Argentina

The last 16 000 cal. a of vegetation, fire and limnological history are described from the steppe‐forest ecotone in the northernmost Nothofagus forest region east of the Andes (Mallín Vaca Lauquen, Neuquén Province, Argentina, latitude 36° 51.336′ S, longitude 71° 02.538′ W). Between 16 000 and 14 800 cal. a BP, scrub steppe with substantial open ground expanded in formerly glaciated valleys, whereas Nothofagus–Prumnopitys andina woodland covered mountain slopes. The site was a relatively deep and unproductive small lake at this time. After 14 800 cal. a BP, both steppe and woodland vegetation became denser, indicating increased moisture and temperatures, although not to present levels. The lake was still relatively deep and dystrophic, but became more alkaline by 10 000 cal. a BP. Between 8900 and 5500 cal. a BP, conditions were markedly drier than before; a Cyperaceae marsh developed and disturbance taxa increased. After 5500 cal. a BP, moisture increased but varied greatly, as evidenced by fluctuating water levels and high fire activity from 5500 to 4400 cal. a BP and from 2300 to 1000 cal. a BP. Human activity, in terms of forest clearance and livestock grazing, is documented in the uppermost levels. The evidence of high environmental variability in the middle and late Holocene is consistent with the onset or strengthening of the El Niño–Southern Oscillation, but differences in the timing of fire activity among sites on the west and east sides of the Andes suggest that fuel conditions were important in determining the local occurrence of fire. Copyright © 2008 John Wiley & Sons, Ltd.     

Late Holocene vegetation and fire dynamics from a savanna-forest ecotone in Roraima state,northern Brazilian Amazon

Two sediment cores from Mauritia flexuosa palm swamps have been studied by pollen and charcoal analysis. The cores Fazenda Cigana (FC) and Terra Indígena Aningal (TIA) were taken from a savanna-forest ecotone area in the Roraima State, northern Brazilian Amazon. Based on 5 radiocarbon dates, these records allow the reconstruction of the vegetation fire and climate dynamics during the past 1550 years. At the FC site was recorded a higher proportion of forest cover, suggesting local wetter climatic conditions favorable for forest expansion, especially by gallery forests, between 1550 and 1400 cal yr BP. Stands of M. flexuosa started to establish on the site indicating sufficient soil moisture. From 1400 to 1050 cal yr BP, forest cover retreated while savanna, and the Mauritia palm swamp expanded considerably. The FC site was marked by savanna and Mauritia cover with a slight increase of forest between ca. 1050 and 900 cal yr BP. From 900 to 300 cal yr BP the savanna and palm swamp taxa became dominant and the forest area decreased. At the TIA site the savanna cover was dominant between 1200 and 1000 cal yr BP. From 1000 to 700 forest expanded while savanna and Mauritia palm swamp reduced. Between 700 and 300 cal yr BP savanna and Mauritia palm swamp increased and forest area decreased. The high amount of charred particles found in the sediments, indicate fires with a marked increase between 1400 to 1000 cal yr BP (FC site) and 700 to 300 cal yr BP (TIA site), and probably caused the retreat of forest cover during these two time intervals. The relatively lower fire activity after 300 cal yr BP until present-day favored the increase of forested area at both TIA and FC sites. The arrival of the European settler and the subsequent introduction of cattle, is suggested as the main reason for the decrease of fire in the study region. The results point the fire caused by indigenous people as the principal controlling factor for forest and savanna dynamics during the past 1550 years.     

Relationship between volcanism and marine sedimentation in northern Austral (Aisén) Basin, central Patagonia: Stratigraphic, U–Pb SHRIMP and paleontologic evidence

The northernmost part of the oil-producing Austral Basin, known as Aisén Basin or Río Mayo Embayment (in central Patagonian Cordillera; 43–46°S), is a special area within the basin where the interplay between volcanism and the initial stages of its development can be established. Stratigraphic, paleontologic and five new U–Pb SHRIMP age determinations presented here indicate that the Aisén Basin was synchronous with the later phases of volcanism of the Ibáñez Formation for at least 11 m.yr. during the Tithonian to early Hauterivian. In this basin marine sedimentary rocks of the basal units of the Coihaique Group accumulated overlying and interfingering with the Ibáñez Formation, which represents the youngest episode of volcanism of a mainly Jurassic acid large igneous province (Chon Aike Province). Five new U–Pb SHRIMP magmatic ages ranging between 140.3 ± 1.0 and 136.1 ± 1.6 Ma (early Valanginian to early Hauterivian) were obtained from the Ibáñez Formation whilst ammonites from the overlying and interfingering Toqui Formation, the basal unit of the Coihaique Group, indicate Tithonian, early Berriasian and late Berriasian ages. The latter was a synvolcanic shallow marine facies accumulated in an intra-arc setting, subsequently developed into a retro-arc basin.     

Growth and linkage of a segmented normal fault zone; the Late Jurassic Murchison–Statfjord North Fault, northern North Sea

The structure of the 25 km long northeastern portion of the Murchison–Statfjord North Fault Zone and adjacent syn-rift stratigraphy are integrated to reconstruct the temporal and spatial evolution during c. 30.5 myr of Late Jurassic, North Sea rifting. Based on a structural analysis (D–d data) alone, approximately 14 precursor fault strands are identified. Incorporation of stratigraphic data shows that only six of these strands were important in controlling stratal architecture and distribution. Three main stages in the evolution of the fault zone are recorded in the syn-rift stratigraphy and are biostratigraphically constrained. These are: (1) following initiation of rifting, six isolated fault strands developed (each <4 km long) and controlled the stratigraphy for c. 13 myr; (2) the isolated strands linked along-strike forming two >9 km long fault segments separated by a 2 km wide relay ramp, that controlled the stratigraphy for at least the following c. 10.5 myr; and (3) the two fault segments hard-linked forming a single, continuous fault trace during the final c. 7 myr of rifting. The results of this study reveal the necessity to adopt an integrated structural and stratigraphic approach when reconstructing the evolution of normal fault zones. The results may also help to further constrain models of fault evolution.     

Landscape and climatic changes during the end of the Late Prehistory in the Amblés Valley (Ávila, central Spain), from 1200 to 400 cal BC

Correlations between ecological and cultural changes occurred during a short period between the end of Subboreal and the beginning of Subatlantic in the Amblés Valley (Ávila, central Spain) are analyzed, taking into account palaeopalynological and archaeological data. Plant dynamics from pollen analyses, both from archaeological sites and peat bogs, have been interpreted in relation to human settlements and the transformation of economic practices. These provided a comprehensive hypothesis on human/climate interactions at the beginning of the 1st millennium cal BC. There was an ecological crisis in the region that lasted for a century (ca. 850–760 cal BC). This was especially sharp due to both the geographical constraints of the area and its previous agrarian history. This event implied a sudden and abrupt climatic change from xeric and warm conditions (Subboreal) to more humid and cooler ones (Subatlantic). Environmental stress derived from climatic crisis could be an important factor in the explanation of the historical process, whose main consequences were both the origin of the Iron Age peasant villages and the end of the ‘dehesa’ type landscape.     

四川西南峨眉地区晚三叠世瑞替期须家河组地球化学特征及其对物源和构造背景的指示

基于砂岩碎屑组成、古流向恢复、全岩地球化学分析并结合已报道岩相古地理等研究成果的综合分析,探讨川西南晚三叠世瑞替期须家河组的物源体系、沉积记录及其形成的构造背景。砂岩碎屑组成分析结果显示,峨眉川主地区须家河组砂岩成分成熟度较低,组分与已报道的须家河组砂岩组分相似,物质以再旋回造山带来源为主;古流向测试数据统计显示须家河组主要接受来自东至东北部和东南部的物源,少量接受来自西南部和西北部的物源;地球化学主量元素及风化指数显示峨眉川主须家河组形成于构造活动背景下以被动大陆边缘和活动大陆边缘为主的沉积环境,物源成分成熟度较低;微量和稀土元素分析提示须家河组来源于长英质碎屑且含有较多古老沉积组分的加入,其球粒陨石标准化配分模式、原始地幔标准化微量元素模式与上地壳配分形式具有高度相似性,且物源主要与再旋回的扬子西南攀西-滇黔地区太古代-二叠纪变质岩、秦岭造山带前中生界变质岩和沉积岩、江南雪峰造山带西段的新元古代沉积有关,少量来自康滇古陆新元古代-元古代变质岩和龙门山周缘三叠纪沉积岩。结合须家河组已报道的岩相古地理数据,表明川西南峨眉地区晚三叠世瑞替期须家河组物源主要来自东南部的江南雪峰构造带和北部的秦岭造山带,少量来源于西南部的康滇古陆和西北部的龙门山构造带,川西南峨眉地区形成了主要受江南雪峰构造带、秦岭造山带为主控制的须家河组海陆交互相-陆相沉积盆地。     

Architecture and morphodynamics of a 1·6 Ga fluvial sandstone: Ellice Formation of Elu Basin,Arctic Canada

Precambrian fluvial deposits have been traditionally described as architecturally simple, forming shallow and wide braidplains with sheet‐like geometry. The varied architecture and morphodynamics of the 1·6 Ga Ellice Formation of Elu Basin, Nunavut, Canada, are examined from detailed studies of section and planform exposures along coastal platforms and stepped cliffs. The Ellice Formation overlies older Proterozoic sandstones and Archean crystalline rocks, recording sedimentation in fluvial, aeolian, coastal and nearshore‐marine environments. The fluvial deposits display palaeoflow towards the west/north‐west, while overlying shallow‐marine deposits record transgression towards the east/south‐east. The Ellice Formation displays dispersed palaeoflow at its base, and also at higher stratigraphic levels, where fluvial and aeolian deposits are associated. Elsewhere, mainly unimodal palaeoflow points to extensive low‐sinuosity fluvial deposition. Within the terrestrial deposits, fluvial, fluvial–aeolian and coastal architectural elements are recognized. Fluvial elements comprise cross‐bedded sandstone and minor conglomerate, exhibiting an overall fining‐upward trend with associated decrease in preservation, dimension and amalgamation of channel bodies. These motifs are interpreted to portray a shift in depositional environment from proximal trunk rivers to distal alluvial plains. Low‐sinuosity fluvial elements are the most common, and include major channel bodies, elongate side bars and mid‐channel bars with well‐developed scroll topography. High‐sinuosity channel‐bar complexes exhibit upbar‐flow rotation and yield evidence of bar expansion coupled with rotation and translation. Fluvial–aeolian elements are composed of aeolian dunes juxtaposed with isolated channel bodies and bank‐attached bars. Minor mixed fluvial–aeolian sheets record local deposition in unconfined settings (possibly floodbasins) or inter‐distributary highlands. Finally, coastal elements comprise small deltaic complexes composed of sand‐rich distributary‐channel bodies feeding heterolithic mouth bars. Overall, the sedimentary record of the Ellice Formation demonstrates an example from the Precambrian where alluvium was locally characterized by a higher geomorphic variability than previously recognized.     

Late Weichselian glacial history and postglacial emergence of Phippsøya, Sjuøyane, northern Svalbard: a comparison of modelled and empirical estimates of a glacial-rebound hinge line

Field research on Phippsøya, the largest island in the Sjuøyane archipelago, defines the course and timing of postglacial emergence, documents past-glacier movements, and reinterprets deglacial sedimentary sequences. Previously described tills were not identified in sections exposed along the northeast shore of Phippsøya, but instead sublittoral sediments with rock-fall concentrations derived from the adjacent slope. A glacio-isostatically higher sea level >40 ka deposited sublittoral sediment and is possibly correlative to a deglacial event in oxygen isotope stage 4 or 5 identified at other sites on Svalbard. The postglacial marine limit is 22 ± 1 m aht and occurs as an escarpment or washing limit into a stony drift. This drift contains granite and quartzite erratics from Nordaustlandet that indicate coverage by a northward flowing ice sheet during the Late Weichselian. Datable material on the raised-beach sequence was rare and a ¹⁴C age of c. 9.2 ka on an articulated Balanus balanus from 10 m aht provides a minimum constraining age on the marine limit. A mild transgression occurred by 6.2 ka, with sea level falling close to present levels by c. 5.0 ka. The zone of zero emergence (hinge line) lies 10 to 20 km north of Sjuøyane and is approximately coincident with the last glacial maximum limit on the continental shelf. There is an approximately 75 to 100 km offset between observed and modelled zone of zero emergence, indicating a need to refine earth rheology-based ice-sheet models.     

Stratigraphic evolution of a fluvial–eolian succession: The example of the Upper Jurassic—Lower Cretaceous Guará and Botucatu formations, Paraná Basin, Southernmost Brazil

The Guará and Botucatu formations comprise an 80 to 120 m thick continental succession that crops out on the western portion of the Rio Grande do Sul State (Southernmost Brazil). The Guará Formation (Upper Jurassic) displays a well-defined facies shift along its outcrop belt. On its northern portion it is characterised by coarse-grained to conglomeratic sandstones with trough and planar cross-bedding, as well as low-angle lamination, which are interpreted to represent braided river deposits. Southwards these fluvial facies thin out and interfinger with fine- to medium-grained sandstones with large-scale cross-stratification and horizontal lamination, interpreted as eolian dune and eolian sand sheets deposits, respectively. The Botucatu Formation is characterised by large-scale cross-strata formed by successive climbing of eolian dunes, without interdune and/or fluvial accumulation (dry eolian system). The contact between the Guará and the Botucatu formations is delineated by a basin-wide deflation surface (supersurface). The abrupt change in the depositional conditions that took place across this supersurface suggests a major climate change, from semi-arid (Upper Jurassic) to hyper-arid (Lower Cretaceous) conditions. A rearrangement of the Paraná Basin depocenters is contemporaneous to this climate change, which seems to have changed from a more restrict accumulation area in the Guará Formation to a wider sedimentary context in the Botucatu Formation.     

Geology, geochemistry, and U–Pb geochronology of the Archean (2.74 Ga) Serra do Rabo granite stocks, Carajás Metallogenetic Province, northern Brazil

The Carajás region, located in the southeastern part of the Amazon Craton, has been considered one of the most important mineral provinces in the world. The Serra do Rabo Granite (SRG) crops out near the eastern termination of the Carajás fault as two granite stocks, elongated approximately in an E–W direction, concordant with the regional structures. Leucomicrocline granite, hornblende–microcline granite, biotite–hornblende–microcline granite, hornblende syenogranite, and subordinate aplite are identified. The granites are grayish pink and coarse to medium grained and have mainly hypidiomorphic granular texture. Granophyric textures are common. The accessory minerals are ilmenite, apatite, zircon, allanite, and rare pyroxene.The SRG rocks are either massive or foliated, with a slightly anastomosed continuous S₁ foliation (E–W/subvertical) outlined by the preferred orientation of quartz, feldspars, and mafic minerals. Locally, decimeter- to meter-wide mylonite/ultramylonite bands (S_1m) occur along the E–W foliation. The S₁ foliation was developed under higher temperatures than those of the S_1m mylonite foliation. The SRG structural evolution was controlled by progressive deformation under decreasing temperature, indicative of syntectonic emplacement. The SRG also has relatively high SiO₂, K₂O, and Na₂O contents; high FeO*/(FeO*+MgO) ratios; high Zr, Ba, Nb, and Ga; and very high rare-earth element contents. The chemical signature is moderately alkaline and metaluminous, comparable to that of the A-type, A2, and ALK-3 granites. The origin of the SRG magmas may be related to the partial melting of crustal sources, such as previously metamorphosed calc-alkaline granites.The SRG crosscuts supracrustal rocks, promoting low-pressure/high-temperature metamorphism. The interaction between regional compressive stresses and the ballooning effect of the granite stocks promoted slight aureole flattening and rheological changes in the supracrustal rocks. The U–Pb zircon age of 2743±1.6 Ma is interpreted as the age of zircon crystallization, granite stock emplacement, and regional horizontal shortening. Other 2.7 Ga syntectonic alkaline granites (e.g. Estrela, Plaquê, Planalto) have been reported in the region.