late ordovician glacial record of the anti atlas morocco abstract late ordovician glaciogenic deposits are exposed intermittently along an 800 km long outcrop belt in the anti atlas mountains of southern morocco these deposits are of economic significance as potential oil bearing sandstones in the tindouf and boudenib basins and thus are here re examined as analogues to subsurface hydrocarbon reservoirs glaciogenic deposits of the upper second bani formation rest unconformably upon underlying shallow marine clastic deposits the unconformity is characterised by a series of palaeovalleys some 0 5 1 0 km wide and up to 100 m deep which may have been cut under elevated hydrostatic pressures as tunnel valleys beneath a late ordovician ice sheet the valleys and intervalley areas are filled with glaciogene sediments categorized into five facies associations namely 1 a tabular sandstone association shallow marine shoreface deposits 2 a massive sandstone and conglomerate ice contact debrites 3 meandriform sandstone deposits ice proximal sandur 4 stratified diamictites ice rafted debris and 5 sigmoidally bedded sandstones intertidal sandstones deformation in these sediments is ubiquitous and includes soft sediment striated pavements metre scale duplex systems thrust and fold belts of deformation affecting some tens of metres of sediment and pervasive lineations these features are interpreted to record the complex nature of deformation processes operating beneath a late ordovician ice sheet including sliding at the ice bed interface folding and deformation within the sediment column and a series of complex ramps detachments and shear zones within an unconsolidated pile of sediment beneath the ice sheet

on the dependence of boundary layer ventilation on frontal type 1 case study simulations with idealized tracers have been used to determine the relationship between the dynamics and conceptual representations of different midlatitude frontal systems and the amount distribution and time scale of boundary layer ventilation by these systems the key features of ventilation by a kata and ana cold frontal system are found to be quantitatively and also often qualitatively similar to the main ventilation pathways which are the conveyor belts cloud head and other convection the conveyor belts and cloud head occur within cloud implying that they can be identified using satellite imagery differences in the transport by the two systems can be related to their conceptual representations and include a sensitive dependence on the diurnal cycle for the kata but not the ana cold frontal case

a palaeolimnological record of diatom community dynamics and late holocene climatic changes from prescott island nunavut central canadian arctic palaeolimnological analyses from lake pw03 a small dilute lake on crystalline bedrock on prescott island nunavut canada 73 07 n 96 41 w 243 m a s l provide a sensitive record of the impacts of holocene climatic change on diatom communities a high resolution sequence from 3500 cal bp to present indicates that a significant change in diatom assemblages takes place at 2300 cal bp when alkaliphilous fragilarioids and planktonic taxa are replaced by benthic epiphytic and acidophilous diatoms these changes coincide with the onset of neoglacial cooling and likely reflect more persistent ice cover and associated acidification of lake water maximum diatom species richness occurs during this period of transition dense bryophyte communities in the littoral zone provide habitats to sustain diatom production even under the cooler conditions following 2300 cal bp the concentration of diatom valves declines at 500 cal bp coincident with cooling at the time of the little ice age and minor chang

440 ma ice stream in north africa landsat images aster digital elevation models aerial photographs and field investigations in the western murzuq basin libya and the adjacent tassili n ajjers algeria provide paleogeomorphological evidence for the existence of a late ordovician ice stream at least 200 km long and 80 km wide this includes mega scale glacial lineations an associated subglacial meltwater drainage system and ice front features this first comprehensive description of a pre cenozoic ice stream may help to identify other examples in the proterozoic to paleozoic glacial record reconstruction of the extent and behavior of former ice sheets and reservoir prospect analysis in glacially related successions have to take into account the potential occurrence of ice streams

push moraines and glacier contact fans in marine and terrestrial environments the local climatic regime and the mass balance state are important determinants of the dynamics of terrestrial and marine glacier fronts which in turn determine the sediments and landforms produced at the glacier front many modern glaciers undergoing overall retreat in areas of maritime climate produce winter push moraines during a late winter readvance followed by a summer retreat whilst in more continental regions no significant winter readvance occurs and annual push moraines are absent the frontal dynamics which lead to these changes are analysed and the form structure sequence and field relations of both terrestrial and marine push moraines are described from iceland spitsbergen and baffin island long term changes in mass balance leading to major glacier advances or readvances also generate large push moraines in terrestrial environments push moraine formation is accompanied by uplift rejuvenation and down cutting of outwash systems whose sediments become closely associated with glaciotectonic structures which permit pre syn and post tectonic sequences to be identified the development of ice marginal fan moraine complexes is modelled as a function of the relative magnitude of two parameters the velocity of ice movement and the calving rate a high ice velocity just exceeded by the calving rate gives closely spaced push moraines and confluent ice marginal fans a high velocity far exceeded by the calving rate produces closely spaced moraines but separate ice marginal fans a low ice velocity in combination with a high calving rate results in well separated and feebly developed push moraines while a low ice velocity and a low calving rate produces feeble push moraines and coalescent fans

diatom community responses to late holocene climatic variability baffin island canada a comparison of numerical approaches a high resolution diatom stratigraphy from the late holocene sediments of a small arctic lake on bafe n island nunavut canada has been analysed by several numerical methods and considered in relation to independent palaeoclimatic proxies to enable an objective assessment of the response of freshwater diatom communities to climatic change diatom relative frequencies and absolute abundances were subjected to several manipulations including application of a weighted averaging transfer function for summer water temperature conversion of valve concentrations to cell biovolumes biostratigraphic rate of change and taxonomic richness estimated by rarefaction analysis the major directions of variability within the derived diatom data were explored using principal components analysis diatom inferred lakewater temperatures and indices of diatom palaeoproductivity declined during late holocene cooling and they faithfully record post little ice age warm ing between 2000 cal bp and the little ice age which includes the coldest intervals of the neoglacial diatom assemblages diversie ed and destabilized ree ecting a state of intermediate disturbance together these results examine different methods of interpreting freshwater diatom stratigraphic data with the objective of gaining palaeoclimatic insights from fossil assemblages

diatom community response to multiple scales of holocene climate variability in a small lake on victoria island nwt canada abstract a lake sediment core spanning 9900 years collected from a small lake on western victoria island nwt canada provides a high resolution record of diatom community dynamics over the holocene ten radiocarbon dates and 210 pb dating provided the core chronology loss on ignition loi magnetic susceptibility biogenic silica content and diatom concentrations provided information on changes in the sedimentary environment loi gradually increased over the holocene whereas magnetic susceptibility showed an inverse trend biogenic silica content showed three distinct peaks spaced approximately 3000 years apart major shifts in diatom assemblages occurred at 8100 8000 5800 5700 and 3800 3500 cal yr bp there is evidence of diatom community response to centennial scale variations such as the medieval warm period 1000 700 cal yr bp little ice age 800 150 cal yr bp and recent warming although recent changes in diatom community composition productivity and species richness are apparent they were surpassed at other periods throughout the holocene variations of the taxa within the genera staurosira pseudostaurosira fragilaria and staurosirella usually combined into one genus in arctic lake sediment studies suggest these taxa provide useful insight into paleonvironmental questions

maximum extent of ice sheets in morocco during the late ordovician glaciation new field data demonstrate that during the late ordovician hirnantian glaciation an ice sheet expanding northwestwards over the anti atlas range reached into the southern meseta of northern morocco its growth to a glacial maximum position resulted in extensive subglacial erosion and deformation including the development of soft sediment striated surfaces and streamlined subglacial bedforms preserved between the high atlas of marrakech and rehamna these features imply that this ice mass extended 200 km further than previously thought and increase its size by at least ca 190 000 km2 comparable in area to the uk correlation between a measured section in the high atlas of marrakech and that of the southern meseta identifies sedimentary evolution within an ice contact system common to both these findings imply that the west african craton and northern morocco were in full glaciological communication during the latest ordovician palaeogeographic reconstruction shows that beyond the ice sheet south and southeastward palaeoslopes persisted on the shelf a palaeohigh beyond the main ice sheet was a major source for sand feeding delta systems that grew along the shelf as far as the shelf break this palaeohigh probably formed as a result of rift shoulder uplift and supported a satellite ice mass in the eastern meseta a thick 350 m underflow dominated deep marine fan was fed both from this shelf delta system and from glaciogenic debris derived from the main ice sheet the occurrence of this unexpected deep marine area in northern morocco implies that continued northward advance of the ice sheet was hampered by a dramatic break in bathymetry two depositional units are recognised across the meseta containing four distinct sedimentary cycles each recognised as a glacioeustatic response to the waxing and waning of ice masses elsewhere in west gondwana

late ordovician glacial pavements revisited a reappraisal of the origin of striated surfaces the recognition of ice related unconformities is of prime importance when dealing with the sequential architecture of pre pleistocene glacial successions late ordovician striated surfaces strikingly preserved in north gondwana were long considered as abrasion surfaces at the sole of a grounded glacier overriding unlithified sediments or having resulted from the scouring action of drifting icebergs field observations from mauritania and libya show that these striated surfaces require an alternative interpretation which can be applied to similar surfaces described elsewhere such as in permo carboniferous or neoproterozoic glacial sequences a new model is proposed that involves superimposed concurrent decollement planes within a subglacial brittle shear zone in unlithified sand beds shear zones in subglacial sediments are well known in quaternary deposits but to date it has not been demonstrated that they may form striated surfaces occurring at continent scale with orientations consistent with palaeo ice flow reconstructions based on larger scale indicators

characterization of pebble fabrics in modern terrestrial glacigenic sediments pebble fabric data are available from several facies of glacigenic sediments deposited by modern glaciers where sedimentary processes can be observed or inferred with relatively little ambiguity over 100 samples from contemporary environments illustrate fabrics characterizing melt out till deformed and undeformed lodgement till sediment flow deposits and ice slope colluvium lodgement till fabric variability is related to the two layer structure of these sediments a structureless friable upper layer with low shear strength and high consolidation coefficient overlying a very compact material of horizontal platy structure fabric strength assessed by eigenvalue analysis is weaker and pebble dip is more dispersed in the upper structureless horizon stronger fabrics in the lower platy horizon may be primary depositional fabrics which are destroyed by subglacial shearing to give weaker fabrics in the upper horizon alternatively upper horizon fabrics may be characteristic of all recently deposited lodgement tills with stronger fabrics developing at depth by dewatering and consolidation there is a general reduction in fabric strength and an increase in particle dip associated with the transition from melt out tills through undeformed and deformed lodgement tills to sediment flow deposits and ice slope colluvium there is however considerable overlap in the fabric strengths characteristic of sediment flow deposits and deformed lodgement tills fabric data from modern glacial sedimentary facies are used to assist in interpreting the mode of deposition of some quaternary glacial sediments relatively strong fabrics characteristic of melt out tills and undeformed lodgement tills are more likely to be diagnostic of genesis than weaker fabrics associated with deformed sediments

the origins of glacially related soft sediment deformation structures in upper ordovician glaciogenic rocks implication for ice sheet dynamics in the upper ordovician glaciogenic rocks of west gondwana many soft sediment deformation structures are attributed to glaciotectonic processes however little is known about the subglacial and ice marginal processes associated with this ice sheet a systematic description and interpretation of these soft sediment deformation structures is supplemented by a comparison with late quaternary and modern glacial settings to provide qualitative constraints on the character of ancient subglacial thermal regimes soft sediment striations were formed by discontinuous shear induced deformation beneath an ice sheet that oscillated between warm and cold based thermal regimes beneath warm based ice coupling across the ice bed interface was poor with sliding and loading occurring at this surface loading of soft waterlogged sediments allowed the development of fluted surfaces which are also associated with the formation of truncated folded zones during times of cold thermal regimes coupling across the ice bed interface was enhanced by freezing which caused changes in the position of the freezing front these discontinuous changes coupled with sustained subglacial shearing resulted in the development of stacked soft sediment striated surfaces in areas where the subglacial substrate had an irregular topography complex shear zones formed that comprised stacked duplex systems interlayered with highly strained units preserving pervasive folded lineations at the ice front large glaciotectonic deformation structures comparable with push moraines developed during sustained periods of re advance that created composite thrust and fold systems deformation was transmitted for up to 1 km from the ice front and reduced in intensity with distance other deformation structures include dome like folds that consist of central anticlines 30 to 70 m in width with tight rim synclines that formed by the load induced diapiric intrusion of muds an ice sheet surging mechanism is inferred for the development of composite thrust and fold systems and the coeval sediment diapirs these surges potentially caused the widespread reorganisation of a glacial load resulting in the pro to subglacial extrusion of soft sediments

depositional environments and sequence stratigraphy of paralic glacial paraglacial and postglacial upper ordovician siliciclastic deposits in the murzuq basin sw libya abstract the application of sequence stratigraphy methods to glaciogenic deposits is more problematic than in normal paralic deposits because changes in the relative sea level are strongly influenced by an interplay between glacial advance and retreat and concomitant isostatic loading and rebound of the continental shelf the study of outcrop and subsurface sections of the upper ordovician glaciogenic melaz shuqran and the mamuniyat formations murzuq basin sw libya yielded important information on understanding changes in the relative sea level related to glacier movements three depositional sequences were recognized depositional sequence one ds 1 corresponds to the entire melaz shuqran formation that was bounded below by subglacial erosion surface i e sequence boundary sb formed during glacier advance into shallow water areas this surface correlates with a transgressive surface ts in deep water areas and records an initial rise in the relative sea level owing to glacier advance and loading of the continental shelf transgression occurred due to the slower rate of eustatic sea level fall than the isostatic loading the transgressive systems tract tst comprises shoreface sandstones and offshore mudstones diamictites with ice rafted debris further rise in relative sea level was associated with glacial retreat which resulted in sediment starvation in deep water areas and the formation of a condensed section these deposits represent glacial depositional systems the highstand systems tract hst comprises prograding deltaic deposits which was formed when the rate of sediment supply exceeds the rate of relative sea level rise the hst deposits represent paraglacial depositional systems depositional sequence two ds 2 corresponds to the lower and middle part of the mamuniyat formation and is bounded below by deep erosional surface sb that cuts into ds 1 as an incised valley which was formed during glacial advance across the continental shelf incision occurred due to the faster rate of eustatic sea level fall than the rate of isostatic loading glacier retreat and the concomitant rise in relative sea level resulted in the deposition of braided fluvial sediments in a lowstand systems tract lst and tide dominated estuarine sediments in a tst these deposits represent glacial depositional systems the hst sediments represent prograding foreshore to shoreface deposits which were formed when the rate of sediment supply exceeded the rate of relative sea level rise these deposits represent paraglacial depositional systems depositional sequence three ds 3 corresponds to the upper part of the mamuniyat formation and is bounded below by an erosional surface sb that was formed as a result of isostatic rebound associated with relative sea level fall ds 3 lst comprises gilbert type deltaic conglomeratic sandstones grading upwards into sandstones subsequent relative sea level rise during the late ordovician resulted in deposition of shoreface tst sandstone further rise in the relative sea level resulted in sediment starvation in deep water areas and the formation of thin hardground layer i e condensed section that marks the upper boundary of late ordovician deposits this study provides predictive model for the spatial and temporal distribution of ancient glaciogenic depositional facies that may have important implications in hydrocarbon explorations

late ordovician glaciogenic reservoir heterogeneity an example from the murzuq basin libya in north africa late ordovician glaciogenic reservoirs contain large volume of recoverable hydrocarbon reserves but their heterogeneity and internal complexity means they are poorly understood to improve this understanding this paper presents the case study of the murzuq basin sw libya and a synthesis of the stratigraphic architecture of upper ordovician glaciogenic reservoirs the mamuniyat formation within it particular attention is paid to regionally extensive stratigraphic boundaries and the geometry of sandstone units of potential reservoir quality four disconformity bound units are recognised the bounding surfaces of which are flat or of high relief unit 1 the oldest and unit 3 are mud dominated and tend towards non reservoir in contrast units 2 and 4 the youngest are sand dominated have poor to excellent reservoir quality but distinctly different sedimentological architectures across the basin unit 2 comprises transitions from glaciofluvial intertidal sandstones to offshore turbidites and formed exclusively during glacial retreat the occurrence of a significant palaeotopography affected sandbody geometry with glaciofluvial intertidal deposits concentrating in the lee of these palaeohighs or infilling the bottom of tunnel valleys unit 4 is a compound stratigraphic unit recording the effects of glacial retreat and isostatic rebound in deep marine settings turbidite fans were deposited during retreat but in shallow water settings structural reactivation and sediment reworking had a more profound effect on sedimentation sedimentological architectures are variable at this level and include prograding coarse clastic wedges away from a palaeohigh or the infill of half graben basins during isostatic rebound

a diatom based holocene palaeoenvironmental record from a mid arctic lake on boothia peninsula nunavut canada a 485 cm sediment core from a lake unoffcially called jr01 boothia peninsula nunavut canada yielded a high resolution diatom record documenting environmental change in the mid arctic five radi

large scale channel fill structures in late ordovician glacial deposits in mauritania western sahara abstract late ordovician siliciclastic glacial and related deposits on the north gondwana continental shelf comprise striking incised features referred to as large scale channel fill structures these are described from exposures and aerial photographs in two areas adrar and hodh in mauritania the channel fill structures are up to several kilometres in length and several hundred metres wide they are slightly sinuous narrow sandstone bodies deeply incised into the late ordovician glacial drift and the cambro ordovician bedrock palaeogeographical reconstructions indicate that these structures were located in the ice marginal zone they are preferentially oriented parallel to palaeo ice flow directions sedimentary facies analysis reveals three vertically stacked architectural units infilling a u shaped erosional basal unconformity unit 1 is thin and made up of conglomeratic sandstones of various origins comprising debris flows deltaic like progradational foresets and trough cross stratified sands in adrar architectural unit 2 shows sheet like evenly laminated fine to medium grained well sorted sandstones characteristic of a high energy marine environment in contrast in the hodh area unit 2 displays vertically stacked successions of coarse grained sandstones that indicate high discharge sediment laden flows these are interpreted as meltwater sediments in both adrar and hodh architectural unit 3 is made up of gravelly coarse grained trough cross stratified sandstone emplaced in a braided low sinuosity fluvial environment the three stage infilling history and the palaeogeographic location combined with size and shape criteria make it possible to compare the channel fill structures with pleistocene tunnel valleys found in areas of low relief in cool temperate and low arctic zones such features are incised by high pressure subglacial meltwater and later infilled by proglacial to postglacial deposits based on this comparison architectural unit 1 represents subglacial or proglacial outwash high sediment discharge in unit 2 took place in a proglacial environment in the hodh area while an isostatically downwarped shelf was responsible for a marine incursion in the adrar channels and deposits in unit 3 are fluvial postglacial sediments identification of preserved channel fill structures may be useful in reconstructing late ordovician ice sheet dynamics

multi proxy record of postglacial environmental change south central melville island northwest territories canada abstract a sediment core from lake bc01 75 10 945 n 111 55 181 w 225 m asl on south central melville island nwt canada provides the first continuous postglacial environmental record for the region fossil pollen results indicate that the postglacial landscape was dominated by poaceae and salix typical of a high arctic plant community whereas the arctic herb oxyria underwent a gradual increase during the late holocene pollen based climate reconstructions suggests the presence of a cold and dry period 12 000 cal yr bp possibly representing the younger dryas followed by warmer and wetter conditions from 11 000 to 5000 cal yr bp likely reflective of the holocene thermal maximum the climate then underwent a gradual cooling and drying from 5000 cal yr bp to the present suggesting a late holocene neoglacial cooling diatom preservation was poor prior to 5000 cal yr bp when conditions were warmest suggesting that diatom dissolution may in part be climatically controlled diatom concentrations were highest 4500 cal yr bp but then decreased substantially by 3500 cal yr bp and remained low before recovering slightly in the 20th century an abrupt warming occurred during the past 70 yr at the site although the magnitude of this warming did not exceed that of the early holocene

holocene climatic records from agassiz ice cap ellesmere island nwt canada four ice cores from the top of the agassiz ice cap and down a flow line have been variously analysed for 8 18o ecm solid conductivity and ice melt layer stratigraphy stratigraphic correlation

comment on abrupt environmental change in canada s northernmost lake inferred from fossil diatom and pigment stratigraphy by dermot antoniades et al 1 antoniades et al 2007 hereinafter referred to as a07 present an analysis of a 19 cm lake sediment core from ward hunt island from northern canada they conclude that a significant and abrupt change in the aquatic communities occurred in this lake during the past 2 centuries that was unusual with respect to the previous 8ka in the paper and especially in quotes to the press e g http www cbc ca http www sciencedaily com http scitizen com the authors conclude that their results along with those from smol et al 2005 provide evidence for amplification of climates with latitude due to human caused global warming an event unique in the past 8 ka however there are a number of problems in these data and flaws in the author s interpretations that render their results questionable their conclusions do not hold up to scrutiny as they do not concord with other information we have about climate variability in the arctic i have four major comments on the data and several on the interpretation

abrupt environmental change in canada s northernmost lake inferred from fossil diatom and pigment stratigraphy 1 an analysis of diatoms and fossil pigments in a sediment core from perennially ice covered ward hunt lake at latitude 83 n in nunavut canada revealed striking changes in diatom communities and sedimentary pigment concentrations during the last two centuries diatoms were found only in the upper 2 5 cm of the sedimentary record and where present diatom assemblages were composed almost entirely of staurosirella pinnata photosynthetic pigments were present in low concentrations throughout the sedimentary profile consistent with the ultra oligotrophic nutrient status of the lake pigment concentrations varied slightly in the lower sections of the core and began to increase gradually at the 4 cm horizon followed by an increase of two orders of magnitude in the uppermost 2 5 cm the changes observed in the sedimentary record of ward hunt lake had similar trajectories to those observed post 1850 elsewhere in the circumpolar arctic and imply that aquatic communities even in the most extreme northern lakes have been strongly impacted by recent climate warming

paleoenvironments of the canadian high arctic derived from pollen and plant macrofossils problems and potentials abstract analyses of peat sections lake sediments and ice cores provide information about late quaternary arctic environments palynomorphs and plant macrofossils from each of these three types of sediments record different aspects of the environment with particular spatial and temporal scales of resolution in the arctic the limits to the interpretation of past environments are particularly significant problems of low pollen concentrations long distance transport stratigraphic inversions and contamination by fossils from older deposits are more serious in this region due to the biogeographic context that characterizes these high latitudes and influences the particular ecological and geomorphological processes however recent work has shown that past environments can be reconstructed from evidence preserved in high arctic sediments and ice cores if these problems are taken into account before their interpretation

modern and holocene pollen assemblages from some small arctic lakes on somerset island nwt canada abstract modern pollen samples from 15 lakes along a north south transect on western somerset island nwt canada show a decrease in pollen concentrations from the high arctic to the mid arctic zone but there are few differences in the pollen percentages between these sites long distance transport accounts for up to 50 of the pollen in these lake sediments cores from two lakes show few changes in the percentages of important pollen types except for an initial period before 6000 yr b p of increased salix the pollen concentration of lake rs36 from the mid arctic is twice that of lake rs29 from the high arctic and at both sites the concentrations decreased during the past 6000 yr b p this suggests a climatic deterioration during the past 6000 yr which has caused a decrease in the abundance of plants on the landscape

holocene climate and vegetation change on victoria island western canadian arctic abstract a detailed pollen record from victoria island provides the first quantitative holocene climate reconstruction from the western canadian arctic the pollen percentage data indicate that arctic herbs increased over the holocene in response to long term cooling the influx of locally and regionally derived pollen grains varies throughout the core and tracks several major changes observed in the biogenic silica record from arolik lake alaska and the gisp2 ice core suggesting that climate change closely controlled arctic plant productivity using modern analogue and transfer function techniques we generated quantitative reconstructions of mean july temperature and total annual precipitation for the past 10 000 years to place recent climate changes within the context of holocene climate variability the quantitative reconstructions indicate that july temperature cooled by 1 1 5 c during the holocene the pollen based reconstructions record an increase in temperature of 0 5 c over the last 100 years and the pollen percentage and influx data indicate impacts of recent warming on the regional vegetation