valanginian isotope variation in glendonites and belemnites from arctic svalbard transient glacial temperatures during the cretaceous greenhouse oxygen and carbon isotope data from cretaceous valanginian glendonites and belemnites from arctic svalbard are presented oxygen isotope data from well preserved glendonites in conjunction with the ikaite to glendonite pseudomorph transition temperature are used to provide an estimate of the oxygen isotope composition of ambient seawater calculation of such a factor is essential for robust paleotemperature estimates using this methodology our paleotemperatures calculated from the oxygen isotope compositions of coexisting belemnites yield cool temperatures 4 7 c consistent with transient glacial polar conditions during the cretaceous greenhouse cool polar temperatures during the cretaceous help reconcile geologic data with the simulations of general circulation models nevertheless beyond this postulated and transient cool event within the valanginian the remainder of the isotope data are interpretable in terms of warm polar conditions during the cretaceous greenhouse

sedimentary challenge to snowball earth evidence from the magnetic field fossilized in sedimentary rocks suggests that more than 600 million years ago ice occupied tropical latitudes a popular explanation for these findings the snowball earth concept envisages a fully frozen earth for millions of years caused by a runaway ice albedo feedback a rapid catastrophic meltback at very high levels of atmospheric carbon dioxide is thought to have ended this extreme climatic state however sedimentary rocks deposited during these cold intervals indicate that dynamic glaciers and ice streams continued to deliver large amounts of sediment to open oceans throughout the glacial cycle the sedimentary evidence therefore indicates that despite the severity of glaciation some oceans must have remained ice free significant areas of open ocean have important implications for the survival and diversification of life and for the workings of the global carbon cycle the snowball earth concept envisages a fully frozen earth for millions of years several times during the neoproterzoic era between 1 000 and 542 million years ago however the sedimentary evidence suggests that despite the severity of glaciation some oceans must have remained ice free

inverse relation between ice extent and the late paleozoic glacial record of gondwana the late carboniferous earliest permian age estimate 300 280 ma for maximum ice volume during late paleozoic glaciation of gondwana is challenged past estimates assume a direct relation between extent of depositional glacial record and former ice cover this assumption cannot be sustained given that the glacial record is composed predominantly of glacially influenced marine strata that accumulated on the margins of ice covered areas we argue that gondwana ice cover expanded in the early carboniferous in response to polar position availability of moisture from a mediterranean sea and epeirogenic uplift of the gondwana interior in namurian time ca 325 ma gondwana ice cover attained a maximum extent of about 21 10 6 km 2 nearly the area of maximum pleistocene ice cover despite extensive ice cover the depositional record is meager continental glacial deposits are poorly preserved on a regional unconformity thereafter extensional subsidence of intracratonic basins promoted marine flooding fragmentation of the ice cover and the accumulation of thick glacially influenced marine deposits in stephanian asselian time ca 285 ma ice cover had decreased to about 15 10 6 km 2 but glacial marine strata were being deposited and preserved across a very large area of the gondwana supercontinent

dropstones their origin and significance abstract dropstones are clasts which pose a hydrodynamic paradox or which have an exotic or extra basinal lithology with uncertain provenance recently dropstones have been consistently interpreted as the product of ice rafting and have been used to substantiate the presence of cool climatic phases in the geological record this review illustrates that not all dropstones are reliable as palaeoclimatic indicators dropstones are the product of two distinct entrainment and transport mechanisms by a rafting agent or as a projectile four main processes of transport and formation are identified biological rafting ice rafting floatation and projectiles biological rafting agents include vertebrate gastroliths and other stomach contents vegetational rafting and anthropomorphic activity ice rafting can be the product of ice bergs or seasonal sea river or lake ice floatation of clasts in calm waters has been recorded for stones up to 25 mm in length projectiles are most commonly of volcanic origin given the range of transport and depositional mechanisms reviewed we argue that great care is required in the interpretation of the origin of dropstones

regional geology and petroleum systems of the illizi berkine area of the algerian saharan platform an overview abstract the berkine and illizi basins are palaeozoic mesozoic intraplate depressions that preserve an over 7000 m thick sedimentary rock record and contain world class petroleum systems with over 39 bboe eur hydrocarbon reserves regional seismic transects and a wealth of well data are used to review the tectonostratigraphic evolution of the area the gondwana palaeozoic and tethys mesozoic supercycles make up the bulk of the phanerozoic succession these 1st order cycles record extensional basin formation followed by a protracted period of overall net subsidence and conclude with basin inversion and regional uplift during the hercynian and alpine orogenies respectively structural styles were dictated by transpressional or transtensional reactivation of mostly nne oriented basement structural grain of late proterozoic ea cambrian pan african age and vertical movement of long wavelength regional highs and lows local tectonism is considered to be the far field effect of plate tectonic processes that affected the north african plate boundaries such as the caledonian and hercynian orogenies the opening of the tethys and atlantic oceans and the alpine orogeny these were accompanied by localised thermal mantle processes regional unconformities subdivide the gondwana and tethys supercycles into eight and seven megasequences respectively these regional units often record 2nd order transgressive regressive cycles that constrain the mapping of reservoir seal pairs and of main source rock intervals the lower palaeozoic megasequences are characterised by sand rich laterally extensive coarse grained fluvial and shallow marine reservoirs interbedded with thin and widespread marine shales they were mostly deposited in shorelines of shallow epeiric seas and continental braided river systems or during the late ordovician in a short lived continental glacial system third order sequences display ramp style geometries with largely parallel to low angle sigmoidal stratal patterns early mesozoic sedimentation followed the late palaeozoic uplift and peneplanation in the form of hinterland sequences containing fluvial and alluvial sandstones and playa lake mudstones and evaporites interbedded with thin lacustrine or shallow marine shales and dolomites the prolific petroleum systems that evolved within these series are associated with the early silurian and late devonian world class source rocks and numerous paleozoic and triassic reservoirs this regional review provides a renewed comprehensive insight into the geological evolution of the area that can help guide local exploration efforts but also serves as a model for exploration in other intracratonic basins of the world

overdeepened quaternary valleys in the eastern danish north sea morphology and origin abstract a system of 100 350 m deep overdeepened and buried valleys has been mapped in the eastern danish north sea individual valleys can be traced for several tens of kilometres and valleys generally begin and terminate abruptly a correlation to buried valleys onshore southwest denmark is possible the valleys post date glaciotectonic structures and are interpreted to be elsterian and possibly saalian in age the origin of valleys in the eastern danish north sea is explained by repeated cycles of sub glacial down cutting of meltwater channels and channel closure due to ice creep eventually producing deeply incised valleys glacial erosion may have been significant in cases where valleys are broad and shallow the origin of overdeepened quaternary valleys in general can be attributed to a combination of three end member processes i steady state drainage of meltwater ii catastrophic outbursts of meltwater and iii glacial erosion the main requirement for sub glacial valley formation is the presence of a melting ice sheet overlying a poorly consolidated substrate permeability variations of the substrate may produce subregional variations in the pattern of valley incision while pre glacial rivers faults or salt structures are of local importance only

the origin of deep buried channels of elsterian age in northwest germany buried channels cut to a depth of over 100m into the underlying substratum of north germany are almost exclusively of elsterian age evaluation of geophysical well logs in the hamburg area for the first time allows the construction of detailed longitudinal and transverse profiles it can be shown that an apparently chaotic infill known from borehole records in some cases is actually a regular sedimentary sequence fining from proximal to distal channel formation seems to have been the result of catastrophic meltwater release in the marginal parts of the ice sheet the channel infill reflects these episodic outbursts of meltwater during deglaciation the channel remnants became infilled with fine sediments the channels ceased to exist as morphologically detectable forms at the end of the holsteinian stage

a morphometric analysis of tunnel valleys in the eastern north sea based on 3d seismic data a 1250 km2 3d seismic volume is used to provide a detailed spatial and geometrical analysis of fifteen pleistocene tunnel valleys in the danish north sea all the valleys are buried they are up to 39 km long 3 4 km wide and up to 350 m deep the valleys are part of a vast tunnel valley province covering an area of some 0 5 million km2 of the formerly glaciated lowland areas of north west europe the valleys consist of non branching non anastomosing troughs they exhibit strongly undulating bottom profiles with numerous sub basins and thresholds and are characterised by adverse end slopes cross cutting relationships and theoretical considerations suggest the occurrence of seven major episodes of valley incision attributed to ice marginal oscillations within a few glacials calculations considering the valley end gradients and theoretical ice surface profiles suggest that the valleys were formed by pressurised subglacial meltwater erosion given a range of theoretical ice surface profiles the adverse end slopes are well beyond the supercooling threshold which suggests that the water was not in thermal equilibrium with the basal ice and that flow was concentrated in substantial conduits with sufficient mass and flux to maintain water temperature well above the freezing point copyright 2007 john wiley sons ltd

early silurian glaciation along the western margin of gondwana peru bolivia and northern argentina palaeogeographic and geodynamic setting the record of the ordovician silurian transition within the southern peru bolivia palaeozoic basin extending across peru bolivia and northern argentina is characterized by a diamictite bearing unit which overlies several different ordovician units and underlies mid silurian shales we review the lithostratigraphy sedimentology and biostratigraphy of this unit revising stratigraphic relationships and several ideas and interpretations previously proposed analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and environments involved during its deposition turbidite and shale interbeds within the diamictites indicate a deep marine environment with interbedded mud flows debris flows slumps and large displaced slabs providing evidence for sediment instability and resedimentation chitinozoan biostratigraphy indicates a llandovery age in contrast with previous proposals of a hirnantian age and suggests the need for more detailed sedimentologic and biostratigraphic studies and a reassessment of this diamictite unit in relation to adjacent regions in south america the resedimented character of the deposit explains some of the ordovician fauna previously described which must be considered as recycled from underlying units in situ fauna needs to be reassessed as it may be endemic diachronic and or indicative of migrations within gondwana glacially faceted and striated clasts as well as large granitoid boulders within the resedimented materials provide evidence for glaciation of the source area and are interpreted as recycled from former glacigenic deposits the evidence found in the central andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late llandovery telychian the precise age of glaciation in this part of gondwana cannot be confirmed do to a lack of true tillites tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western gondwana during the late ordovician and early silurian the new data imply that the ordovician silurian boundary was not preserved in some areas of the basin and that an erosional hiatus disconformity with variable regional significance is present between different ordovician units and the diamictite unit late ordovician late caradoc early ashgill euxinic black shales locally present in the central altiplano and eastern cordillera of bolivia may represent the youngest ordovician unit in the region and more research should be focused on them paleogeographic reconstruction of the early silurian peru bolivia basin depicts the extension of the diamictite unit southwards into northern argentina and western paraguay connecting with the parana basin through the asuncion arch to the north it continued into northern peru and ecuador connecting with the silurian record in colombia and venezuela and to the northeast into brazil solimoes and amazonas basins the sedimentary record of glaciation in the peru bolivia basin was due to the development of local ice fields to the west of the basin related with tectonism and resulting reliefs along the active margin of gondwana and probably did not coincide with the development of the north african late ashgillian ice cap

seven glacial cycles in the middle late pleistocene of northwest europe geomorphic evidence from buried tunnel valleys the deep ocean marine isotope record and the antarctic and greenland ice caps record numerous glacial and interglacial cycles since the middle pleistocene and yet evidence for similar numbers of ice sheet advances over the continent and shallow shelves of northwest europe is absent here we document seven generations of regionally correlatable subglacial tunnel valleys that record the geomorphic imprint of ice sheets traversing the north sea basin between ca 500 and 40 ka consistent with that predicted by proxy records of glacial and interglacial climate change over 180 subglacial tunnel valleys that incise into pleistocene sediments in the north sea basin were mapped over 60 000 km 2 of three dimensional seismic reflection data using a subset of these data we identified seven separate episodes of subglacial erosion that can be correlated regionally in the uk sector of the central north sea the characteristics of the valley morphologies orientations and infill stratigraphy indicate that each set of tunnel valleys formed during a separate ice sheet advance and retreat cycle stratigraphic data suggest that the tunnel valleys formed significantly later than the brunhes matuyama reversal event at 780 ka and before the last glacial maximum marine isotope stage mis 2 21 ka these results imply a more complicated glacial history for northwest europe and more glaciations than the three glaciation model traditionally interpreted from the terrestrial record for the past 500 k y our data provide the most complete documentary evidence for repeated advance and retreat of the northwest european ice sheets since the middle pleistocene and for the first time indicate that terrestrial ice sheet advances in the north sea can be matched in number with the cold events recorded in oceanic and or ice core proxies of climate change over the past 500 k y

the pleistocene of schoningen germany a complex tunnel valley fill revealed from 3d subsurface modelling and shear wave seismics abstract the pleistocene deposits of schoningen represent an outstanding geological and archaeological archive where an up to 45 m thick middle to late pleistocene succession has been preserved and unique artefacts from the lower palaeolithic have been discovered the preservation of such a thick and complete glacial interglacial succession is very rare in the geological record and requires a specific depositional setting we will present a new depositional model for the pleistocene succession of schoningen integrating outcrop data borehole data and high resolution shear wave seismics a total of four outcrop sections and 744 borehole logs were examined to document the complex facies architecture all collected sedimentological and geophysical data sets were integrated into a high resolution 3d geological model gocad for reconstructing the spatial distribution of facies associations and the large scale depositional architecture the spatial distribution of the artefacts will be discussed with respect to the depositional environment the elsterian and holsteinian deposits are restricted to a nnw sse trending elongated trough which is deeply incised into unconsolidated lignite bearing palaeogene deposits the geometry of this erosional structure points to a tunnel valley origin that was incised below the elsterian ice sheet the basal tunnel valley fill consists of cross stratified pebbly sand and gravel overlain by till after deglaciation the tunnel valley remained underfilled and acted as a depocentre for interglacial deposition during the subsequent holsteinian interglacial mis 9 a lake formed within this depocentre and lacustrine sediments accumulated this interglacial succession consists of peat organic rich silt and fine grained sand interpreted as lake bottom and deltaic sediments fed by surface run off shed from the elm ridge the lacustrine deposition was controlled by repeated lake level fluctuations in the range of 1 6 m leading to the formation of laterally stacked delta systems these lake level changes were probably triggered by climate causing variations of precipitation and surface run off during the late saalian glaciation the remnant tunnel valley was completely filled with meltwater deposits the sedimentary facies and depositional architecture point to a shallow water delta subsequently the meltwater deposits were overlain by till the deposition of the middle pleistocene sediments within an elsterian tunnel valley explains the unique preservation of the sedimentary succession of schoningen the long lived interglacial lake provided an attractive site for animals and early humans ambushing them artefacts mainly became embedded on the delta plain which rapidly was transgressed during lake level rise and artefacts were thus preserved

seismic investigations of buried tunnel valleys on and offshore denmark tunnel valleys on and offshore denmark have been investigated based on a database of 1000 km two dimensional 2d onshore seismic data 5600 km 2d offshore seismic data and 1200 km2 three dimensional 3d offshore seismic data from the 2d data we identified 216 onshore and 674 offshore seismic tunnel valley intersections and 55 individual valleys were identified from three 3d surveys the majority of the valleys have depths ranging from 50 to 200 m and widths between 500 and 1500 m up to seven generations of tunnel valleys were identified indicating repeated erosion and deposition within the study area the valleys were most likely formed by subglacial meltwater erosion during the last three glaciations statistical analyses conducted on the data show that there are no significant differences between the onshore and offshore valleys with respect to their depth and shape they share morphological and structural characteristics the onshore seismic data have been analysed in conjunction with lithological information from boreholes the analyses show that tunnel valley bottoms terminate equally commonly in substrates dominated by clay and sand and that the valley shapes are similar for the two substrates the geological society of london 2012

deglacial and postglacial sedimentary architecture in a deeply incised paleovalley paleofjord the pennsylvanian late carboniferous jejenes formation san juan argentina quebrada de las lajas near san juan argentina preserves an early pennsylvanian deglacial postglacial succession in a highly confined paleofjord setting the sedimentary succession records four distinct stages in the evolution of the valley fill stage 1 is represented by the deposits of subglacial diamictites ice contact deltas and related deep water lacustrine environments including several subaqueous channels stage 2 records a glacio eustatic marine transgression and a slow down of the clastic supply into the paleofjord stage 3 records a sandy confined turbidite environment stage 4 consists of a coarse grained delta which represents a significant rejuvenation of the sedimentary system in the paleofjord the transition from stage 1 to stage 2 was abrupt and basinwide and has proven to be a good regional correlation marker in this paleovalley the glacio eustatic rise probably caused floating and consequent rapid melting of the valley glacier this resulted in a paleovalley wide turbidite event that is up to 5 m thick and indicates an overall waning character the rapid sediment emplacement in this event and resulting loading of the paleovalley sediments may have helped cause widespread mass transport events at the stage 1 2 boundary all stages show evidence of mass transport related deposits but stage 1 records the most widespread mass transport with a large spectrum of processes represented including coherently slumped material rafted blocks and completely mixed debris flow deposits these mass transport deposits range from a few meters to over 50 m thick and up to hundreds of meters wide and long most of the large mass transport deposits in the lower part of the fjord fill occurred at the transition to stage 2 implicating a rise in relative sea level as a possible trigger for the slope failures additionally stage 3 deposits were affected by several thrust sense dislocations that have large offsets relative to the scale of the sedimentary succession tens to over 50 m vertically and hundreds of meters laterally strata associated with the dislocations exhibit growth indicating very early movement and they are interpreted as the frontal ramp zones of mass transport deposits the abundance and size of mass transport deposits in stages 1 and 3 helped control sediment pathways throughout the paleofjord

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

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

high resolution seismic and resistivity profiling of a buried quaternary subglacial valley northern alberta canada high resolution seismic and resistivity profiles were acquired to assist in the delineation of a buried quaternary subglacial valley the existence of which was first discovered by drilling one aspect of this valley is the glacial sediments that host substantial methane at shallow depths 50 m or less geophysical well logs indicate that the glacial sediments in this area generally have lower compressional wave speeds v p 10 m than adjacent undisturbed cretaceous age rocks which have v p 2200 m s and r 300 m valley the reflection profile reveals a complex series of events consistent with heterogeneous deposition one major reflection dips eastward across the valley the reflection character changes abruptly on either side of this event with dipping clinoform reflections beneath and nearly horizontal reflections above these may be related to changes in deposition processes beneath a retreating ice sheet as has been suggested by earlier workers the buried valley s basal reflections are complex and possibly reflect glaciotectonic deformation or block plucking the near surface methane gas is within but at the edge of the valley the methane gas saturated zone displays anomalously high resistivities both in the well logs and in resistivity tomography

slope failure and volcanic spreading along the submarine south flank of kilauea volcano hawaii 1 new multichannel reflection data and high resolution bathymetry over the submarine slopes of kilauea volcano provide evidence for current and prior landsliding suggesting a dynamic interplay among slope failure regrowth and volcanic spreading disrupted strata along the upper reaches of kilauea s flank denote a coherent slump correlated with the active hilina slump the slump comprises mostly slope sediments underlain by a detachment 3 5 km deep extension and subsidence along the upper flank is compensated by uplift and folding of the slump toe which surfaces about midway down the submarine flank uplift of strata forming papa u seamount and offset of surface features along the western boundary of kilauea indicate that the slump has been displaced 3 km in a south southeast direction this trajectory matches coseismic and continuous ground displacements for the hilina slump block on land and contrasts with the southeast vergence of the rest of the creeping south flank to the northeast slope sediments are thinned and disrupted within a recessed region of the central flank demonstrating catastrophic slope failure in the recent past debris from the collapsed flank was shed into the moat in front of kilauea building an extensive apron seaward sliding of kilauea s flank offscraped these deposits to build an extensive frontal bench a broad basin formed behind the bench and above the embayed flank uplift and back tilting of young basin fill indicate recent and possibly ongoing bench growth the hilina slump now impinges upon the frontal bench this buttress may tend to reduce the likelihood of future catastrophic detachment

large scale development of the mid norwegian shelf over the last three million years and potential for hydrocarbon reservoirs in glacial sediments about 3 million years ago major ice sheets developed over scandinavia and began to deliverlargevolumesofsedimenttothemid norwegianshelf theshelfwasbuilt outina prograd ing pattern towards the west and more than 1000 m of sedimentswas deposited over large areas on the middle outer shelf the dominating large scale depositional pattern is a series of prograding wedges and flat lying sheet like units mainly of glacial origin on top of these units are flat lying till units deposited during the last few glaciations commonly separated from the underlying units by one or several erosional unconformities the lithology of these layers is generally fine grained mainly clay and silt but with sporadic clasts up to boulder size based on regional and detailed bathymetry the ice flow pattern from the last glaciation has been reconstructed on the shelf this involved a very dynamic ice sheet with fast flowing ice streams in the transverse cross shelf troughs separated by more passive ice domes on the intermediate shallow banks theicestreamsappeartofollowthecross shelftroughsfromglaciationtoglaciation butoccasion ally they switched to new flow paths the thick quaternary sediments on the outer part of the mid norwegian shelf represent a challenge for hydrocarbon exploration several large slides have occurred from the shelf break and down the continental slope during the last few years there has been an increased focus on investigating the quaternary succession in order to search for hydrocarbon prospects the potential for finding reservoirs in these sediments is discussed and a few examples of gas discoveries are shown the peon gas field c 250 km 2 is located in the gla cially eroded norwegian channel above the upper regional unconformity uru separating flat lying glaciogenic sediments from dipping sedimentary units the reservoir is developed in glacio fluvial sands a few hundred thousand years old and sealed by flat lying glaciomarine sediments and till units

evidence for rapid sedimentation in a tunnel channel oak ridges moraine southern ontario canada abstract in south central ontario a late wisconsinan regional unconformity consisting of tunnel channels and drumlinized till crops out north of lake ontario the tunnel channels are locally infilled and the unconformity buried by sediment of the oak ridges moraine based on seismic reflection profiling and drillcore the tunnel channels are known to continue beneath the moraine detailed sedimentological analysis of 300 m of continuous core from two drillholes located 7 km apart in subparallel tunnel channels identified three facies the gravel facies is 17 m thick and occurs in only one of the cores it directly overlies the unconformity and may include a number of upward fining units seismic reflection data suggests the gravel forms stacked gravel mesoforms or quasi horizontal gravel layers bed load sheets deposited from fluidal flows along the deep channel axis the graded massive sand facies up to 37 m thick is the most common facies and consists of silty medium sand with minor coarse sand strata are reverse graded normal graded or massive with isolated silt intraclasts and evidence locally for dewatering this facies is interpreted to have been deposited from hyperconcentrated dispersions downflow of a hydraulic jump or major channel confluence the third facies consists of medium scale and small scale cross stratified sand medium scale cross stratification occurs predominantly in the lower 37 m of the core and is interbedded with the graded massive sand facies small scale cross stratified sand is progressively more common upward in core medium and small scale cross stratified sands was deposited respectively by subaqueous dunes and ripples formed in dilute fluid flows the complete succession is interpreted to have been deposited very rapidly within a subglacial tunnel channel before discharge ceased along the channel deposition followed closely and in part coincided with rapid expansion of the channel by erosion in a hydraulic jump or at a major channel confluence along the grounding line of a subglacial lake scour into unconsolidated sediment contributed to the sediment flux and quickly overloaded the flow with suspended sediment which in turn resulted in extremely high rates of sedimentation immediately downflow such depositional conditions support the notion that tunnel channels in the study area formed and or served as conduits for subglacial jokulhlaup discharges and that the laurentide ice sheet most probably did not generally drain by steady state processes but instead by short lived catastrophic events

toward reconciliation of late ordovician 440 ma glaciation with very high co2 levels although phanerozoic glaciations usually coincided with times of estimated low atmospheric co2 the late ordovician 440 ma glaciation is a significant exception co2 levels during that time may have been as much as 10 times greater than present in an earlier paper we suggested that the unique geographic configuration of gondwanaland may explain such a response as the edge of the supercontinent was essentially tangent to the south pole and the moderating effect of the nearby ocean may have suppressed the magnitude of summer warming on the landmass thereby allowing glacial inception one limitation to the earlier study was that it used a linear energy balance model ebm in this paper we further test the above hypothesis in a suite of experiments with a nonlinear ebm that allows for snow albedo feedback we also utilize updated estimates for co2 levels decreases in solar luminosity and variations in orbital forcing baseline experiments with no changes in luminosity or co2 resulted in an ice covered area of 6 3 106 km2 53 of the estimated area covered by the ordovician ice sheet additional experiments for different combinations of orbital forcing 7x 13x co2 and 3 5 5 0 luminosity yielded 0 35 of the estimated ice area in the late ordovician a crude estimate of possible topographic influences increased these numbers to7 47 of total estimated ice area additional factors related to ice sheet growth should increase these values somewhat these results provide additional support for the high co2 glaciation explanation with the caveat that even the partial success occurs only when parameters are at the extreme end of their permissible range the estimated duration of ordovician glaciation is also consistent with the migration of gondwanaland across the south pole with a centrally located pole yielding ice free conditions in the summer thus identical levels of external forcing yield either glaciated or ice free conditions with the solution dependent on location of the landmass although more work is required on this topic our experiments suggest that there may be a relatively parsimonious explanation for this perplexing paleoclimate paradox the results lend further support to the proposition that paleogeography significantly modifies the role of co2 in the long term evolution of climate

numerical modelling of thrust structures in unconsolidated sediments implications for glaciotectonic deformation abstract the mechanics of thrust faulting induced by a differential vertical load has been studied by application of a dynamic finite element model the modelled scenario is analogous to the glaciostatic load imposed onto an unconsolidated sedimentary substrate by a stationary ice sheet the objective is to evaluate whether large scale glaciotectonic deformation may be induced by gravity spreading in front of a stationary glacier or if a push from the rear is required the model is capable of handling large strains and rotations combined with visco elastic plastic rheologies model parameters are derived from a pleistocene large scale glaciotectonic thrust complex in the eastern danish north sea the plastic behaviour of the unconsolidated sediments is described by a time and pressure dependent drucker prager yield criterion combined with a non associated flow law the modelling results show that gravity spreading in front of a stationary ice sheet can form strain localisations and possibly cause thrust faults to nucleate in unconsolidated sediments several kilometres beyond the snout of the ice sheet provided a decollement surface is situated at a suitable depth relative to the effective load of the ice sheet hence it is concluded that large scale glaciotectonic thrusting may be formed by gravity spreading alone and a push from the rear is not generally required to explain the observed deformation

sea level change glacial rebound and mantle viscosity fornorthern europe northwestern europe remains a key region for testing models of glacial isostasy because of the good geological record of crustal response to the glacial unloading since the time of the last glacial maximum models for this rebound and associated sea level change require a detailed knowledge of the ice sheet geometry including the ice thickness through time existing ice sheet reconstructions are strongly model dependent and inversions of sea level data for the mantle response may be a function of the model assumptions thus inverse solutions for the sea level data are sought that include both ice and earth model parameters as unknowns sea level data from fennoscandia the north sea the british isles and the atlantic and english channel coasts have been evaluated and incorporated into the solutions the starting ice sheet for fennoscandia is based on a reconstruction of a model by denton hughes 1981 that is characterized by quasi parabolic cross sections and symmetry about the load centre both global northwestern europe as a whole and regional subsets of the data solutions have been made for earth model parameters and ice height scaling parameters the key results are as follows 1 the response of the upper mantle to the changing ice and water loads is spatially relatively homogenous across scandinavia the north sea and the british isles 2 this response can be adequately modelled by an effective elastic lithosphere of thickness 65 85 km and by an effective upper mantle viscosity from the base of the lithosphere to the 670 km depth seismic discontinuity of about 3 4 1020 pa s the effective lower mantle viscosity is at least an order of magnitude greater 3 the ice thickness over scandinavia at the time of maximum glaciation was only about 2000 m much less than the 3400 m assumed in the denton hughes model 4 the ice profiles are asymmetric about the centre of the ice sheet with those over the western part being consistent with quasi parabolic functions whereas the ice heights over the eastern and southern regions increase much more slowly with distance inwards from the ice margin

40ar 39ar dating of penninic front tectonic displacement w alps during the lower oligocene 31 34 ma direct absolute dating of the penninic frontal thrust tectonic motion is achieved using the 40 ar 39 ar technique in the pelvoux crystalline massif western alps the dated phengites were formed syn kinematically in shear zones they underline the brittle ductile stretching lineation pressure shadow fibres and slickensides consistent with underthrusting of the european continental slab below the propagating penninic thrust chlorite phengite thermobarometry yields 10 15 km and t 280 c while 40 ar 39 ar phengite ages mainly range between 34 and 30 ma with one younger age at 27 ma this early oligocene age range matches a major tectonic rearrangement of the alpine chain preservation of prograde 40 ar 39 ar ages is ascribed to passive exhumation of the pelvoux shear zone network sandwiched between more external thrusts and the penninic front reactivated as an e dipping detachment fault partial resetting in the low temperature part of argon spectra below 24 ma is ascribed to brittle deformation and alteration of phengites

time transgressive tunnel valley infill revealed by a three dimensional sedimentary model hamburg north west germany deep elongated incisions often referred to as tunnel valleys are among the most characteristic landforms of formerly glaciated terrains it is commonly thought that tunnel valleys were formed by meltwater flowing underneath large ice sheets the sedimentary infill of these features is often highly intricate and therefore difficult to predict this study intends to improve the comprehension of the sedimentology and to establish a conceptual model of tunnel valley infill which can be used as a predictive tool to this end the densely sampled pleistocene tunnel valleys in hamburg north west germany were investigated using a dataset of 1057 deep wells containing lithological and geophysical data the stratigraphic correlations and the resulting three dimensional lithological model were used to assess the spatial lithological distributions and sedimentary architecture the sedimentary succession filling the hamburg area tunnel valleys can be subdivided into three distinct units which are distinguished by their inferred depositional proximity to the ice margin the overall trend of the succession shows a progressive decrease in transport energy and glacial influence through time the rate of glacial recession appears to have been an important control on the sedimentary architecture of the tunnel valley fill during periods of stagnation thick ice proximal deposits accumulated at the ice margin while during rapid recession only a thin veneer of such coarse grained sediments was deposited ice distal and non glaciogenic deposits i e lacustrine marine and terrestrial fill the remaining part of the incision the infill architecture suggests formation and subsequent infill of the tunnel valleys at the outer margin of the elsterian ice sheet during its punctuated northwards recession the proposed model shows how the history of ice sheet recession determines the position of coarse grained depocentres while the post glacial history controls the deposition of fines through a progressive infill of remnant depressions

the end ordovician glaciation and the hirnantian stage a global review and questions about late ordovician event stratigraphy abstract this paper proposes a global review of hirnantian event stratigraphy the hirnantian gssp in south china is tentatively correlated with latest ordovician strata from the peri gondwanan glacial regions problems of biostratigraphical correlation are highlighted at a worldwide scale the major biostratigraphically useful fossil groups graptolites chitinozoans brachiopods conodonts acritarchs are analysed and their limits for global correlation of the uppermost ordovician are discussed palaeobiogeographical disparities are invoked as the primary cause of the difficulty in establishing an effective late ordovician global biostratigraphical scheme as an alternative correlative tool the hice hirnantian isotopic curve excursion event is often put forward in the literature however carbon isotope chemostratigraphy shows like biostratigraphy some limits to the present state of knowledge no good independent biostratigraphical control of the hice exists in both shallow carbonate deposits and deeper shaly ones recent studies have also demonstrated inconsistencies between carbon isotopic signals obtained from organic 13corg and inorganic 13ccarb carbon species further complicating the use of the hice as an isochronous benchmark all of these difficulties for hirnantian event stratigraphy are discussed in detail in order to enable them to be overcome in the future precise late ordovician and early silurian event stratigraphies are essential for the understanding of the mechanisms linked to the first of the big five extinctions

isotopic evidence for glaciation during the cretaceous supergreenhouse the turonian 93 5 to 89 3 million years ago was one of the warmest periods of the phanerozoic eon with tropical sea surface temperatures over 35 c high amplitude sea level changes and positive 18 o excursions in marine limestones suggest that glaciation events may have punctuated this episode of extreme warmth new 18 o data from the tropical atlantic show synchronous shifts 91 2 million years ago for both the surface and deep ocean that are consistent with an approximately 200 000 year period of glaciation with ice sheets of about half the size of the modern antarctic ice cap even the prevailing supergreenhouse climate was not a barrier to the formation of large ice sheets calling into question the common assumption that the poles were always ice free during past periods of intense global warming

near surface mapping using sh wave and p wave seismic land streamer data acquisition in illinois u s human activities have considerable impact on the upper 300 m of sediments of the earth conflicting activities in highly populated areas involving groundwater exploitation aggregate extraction and hazardous chemical related production and storage induce stress on the environment nearly half the population living around the great lakes depends on shallow groundwater aquifers a coalition of four great lakes states illinois indiana michigan and ohio in conjunction with the u s geological survey has initiated a program to map this sensitive upper 300 m layer as part of this effort the illinois state geological survey isgs has undertaken the task of preparing 3d quadrangle maps of the upper 100 m of the entire state at a scale of 1 24 000 although large scale maps indicating potential aquifer contamination already exist figure 1 very little is known about the groundwater pathways through the upper sedimentary column which mainly consists of deposits from the quaternary continental glacier

anatomy of a subaqueous ice contact fan and delta complex middle pleistocene north west germany this paper presents a detailed analysis of the high resolution facies architecture of the middle pleistocene porta subaqueous ice contact fan and delta complex deposited on the northern margin of glacial lake weser northwest germany a total of 10 sand and gravel pits and more than 100 wells were examined to document the complex facies architecture the field study was supplemented with a ground penetrating radar survey and a shear wave seismic survey all collected sedimentological and geophysical data were integrated into a high resolution three dimensional geological model for reconstructing the spatial distribution of facies associations the porta subaqueous fan and delta complex consist of three fan bodies deposited on a flat lake bottom surface at the margin of a retreating ice lobe the northernmost fan complex is up to 55 m thick 6ae2 km wide and 6ae5 km long the incipient fan deposition is characterized by high energy flows of a plane wall jet very coarse grained highly scoured jet efflux deposits with an elongate plan shape indicate a high froude number probably 5 these jet efflux sediments are deposited in front of a large 3ae2 km long up to 1ae2 km wide and up to 25 m deep flute like scour indicating the most proximal erosion and bypass area of the jet that widens and deepens with distance downstream to the region of maximum turbulence approximately five times the conduit diameter evidence for subsequent flow splitting is given by the presence of two marginal gravel fan lobes deposited in front of 1ae 3t o 2ae5 km long flute like scours that are 0ae8 to 1 km wide and 7 to 20 m deep in response to continued aggradation small jets developed at the periphery of these bar like deposits and filled in the low areas adjacent to the original superelevated regions locally raising the depositional surface and characterized by large scale trough cross stratified sand and pebbly sand the incision of an up to 1ae2 km wide and up to 35 m deep channel into the evolving fan is attributed to a catastrophic drainage event probably related to a lake outburst and lake level fall in the range of 40 to 60 m at the mouth of this channel highly scoured jet efflux deposits formed under hydraulic jump conditions during flow expansion subsequently gilbert type deltas formed on the truncated fan margin recording a second lake level drop in the range of 30 to 40 m these catastrophic lake level falls were probably caused by rapid ice lobe retreat controlled by the convex up bottom topography of the ice valley

lower silurian hot shales in north africa and arabia regional distribution and depositional model abstract lowermost silurian organic rich hot shales are the origin of 80 90 of palaeozoic sourced hydrocarbons in north africa and also played a major role in petroleum generation on the arabian peninsula in most cases the shales were deposited directly above upper ordovician peri glacial sandstones during the initial early silurian transgression that was a result of the melting of the late ordovician icecap deposition of the main organic rich shale unit in the north african arabian region was restricted to the earliest silurian rhuddanian stage acuminatus atavus and probably early cyphus graptolite biozones during this short period 1 2 m y a favourable combination of factors existed which led to the development of exceptionally strong oxygen deficiency in the area in most countries of the study area the post rhuddanian silurian shales are organically lean and have not contributed to petroleum generation the distribution and thickness of the basal silurian hot shales have been mapped in detail for the whole north african region using logs from some 300 exploration wells in libya tunisia algeria and morocco in addition all relevant accessible published and unpublished surface and subsurface data of the lower silurian shales in north africa and arabia have been reviewed including sedimentological biostratigraphic and organic geochemical data the lowermost silurian hot shales of northern gondwana are laterally discontinuous and their distribution and thickness were controlled by the early silurian palaeorelief which was shaped mainly by glacial processes of the late ordovician ice age and by pan african and infracambrian compressional and extensional tectonism the thickest and areally most extensive basal silurian organic rich shales in north africa occur in algeria tunisia and western libya while on the arabian peninsula they are most prolific in saudi arabia oman jordan and iraq the hot shales were not deposited in egypt which was a large palaeohigh at that time the depositional model presented may help in better understanding the source potential of the basal silurian shales in less explored regions of north africa and arabia including morocco northern niger and the kufra basin in southeast libya

the subsurface record of the late palaeozoic glaciation in the chaco basin bolivia the late palaeozoic glaciation was the longest of the phanerozoic era it is recorded in numerous gondwanan basins with some such as the chaco basin having a high petroleum potential in this basin the quality of the available seismic well and outcrop data permits us to characterize the late palaeozoic glacial record palaeovalleys that are c 500 m deep and c 7 km wide have been analysed here focusing on the glaciogenic carboniferous deposits seismic data with well ties and outcrop analogues provide new sedimentological insights the palaeovalley infill is imaged as a chaotic seismic facies overlain by an aggrading prograding prism interpreted as tillites covered by a fluvio deltaic system tillites form both under the ice and during rapid ice recession whereas fluvio deltaic systems can only originate from a stable ice margin and last until the ice sheets withdraw inland these two depositional modes are repeated several times generating a progressive burial of the carboniferous palaeovalleys this succession of erosions and fills records major glacial stages including a series of glacial and interglacial phases from the late devonian to the early permian depicting the late palaeozoic glacial history of the chaco basin seems crucial for the localization of potential good reservoirs

cenozoic global sea level sequences and the new jersey transect results from coastal plain and continental slope drilling the new jersey sea level transect was designed to evaluate the relationships among global sea level eustatic change unconformity bounded sequences and variations in subsidence sediment supply and climate on a passive continental margin by sampling and dating cenozoic strata from coastal plain and continental slope locations we show that sequence boundaries correlate within 0 5 myr regionally onshore offshore and interregionally new jersey alabama bahamas implicating a global cause sequence boundaries correlate with 18o increases for at least the past 42 myr consistent with an ice volume glacioeustatic control although a causal relationship is not required because of uncertainties in ages and correlations evidence for a causal connection is provided by preliminary miocene data from slope site 904 that directly link 18o increases with sequence boundaries we conclude that variation in the size of ice sheets has been a primary control on the formation of sequence boundaries since 42 ma we speculate that prior to this the growth and decay of small ice sheets caused small amplitude sea level changes 20 m in this supposedly ice free world because eocene sequence boundaries also appear to correlate with minor 18o increases subsidence estimates backstripping indicate amplitudes of short term million year scale lowerings that are consistent with estimates derived from 18o studies 25 50 m in the oligocene middle miocene and 10 20 m in the eocene and a long term lowering of 150 200 m over the past 65 myr consistent with estimates derived from volume changes on mid ocean ridges although our results are consistent with the general number and timing of paleocene to middle miocene sequences published by workers at exxon production research company our estimates of sea level amplitudes are substantially lower than theirs lithofacies patterns within sequences follow repetitive predictable patterns 1 coastal plain sequences consist of basal transgressive sands overlain by regressive highstand silts and quartz sands and 2 although slope lithofacies variations are subdued reworked sediments constitute lowstand deposits causing the strongest most extensive seismic reflections despite a primary eustatic control on sequence boundaries new jersey sequences were also influenced by changes in tectonics sediment supply and climate during the early to middle eocene low siliciclastic and high pelagic input associated with warm climates resulted in widespread carbonate deposition and thin sequences late middle eocene and earliest oligocene cooling events curtailed carbonate deposition in the coastal plain and slope respectively resulting in a switch to siliciclastic sedimentation in onshore areas oligocene sequences are thin owing to low siliciclastic and pelagic input and their distribution is patchy reflecting migration or progradation of depocenters in contrast miocene onshore sequences are thicker reflecting increased sediment supply and they are more complete downdip owing to simple tectonics we conclude that the new jersey margin provides a natural laboratory for unraveling complex interactions of eustasy tectonics changes in sediment supply and climate change

ice sheet dynamics and ice streaming along the coastal parts of northern norway abstract in this paper regional and high resolution bathymetric data are used to reconstruct the pattern and dynamics of ice sheet flow in the fjord and shelf areas of northern norway interpretation of the morphology of the sea floor reveals a series of glacigenic features which are mainly related to the most recent glaciation of the area the shelf area of troms county comprises four cross shelf troughs with intermediate shallow banks glacial features within the cross shelf troughs mega scale glacial lineations msgl lateral shear margin moraines grounding zone wedges are interpreted to have been formed under fast flowing ice streams on the shallow bank areas there are far fewer ice flow indicators and here relatively passive ice acted as interstream ridges outside the finnmark coast to the north and east a coast parallel depression djuprenna deflected rapidly flowing ice from the fjords in a generally east to west direction and then farther out through ingoydjupet and into the bear island trough the fjords guided most of the ice flow and functioned as tributaries to the major ice streams on the shelf including the vast bear island trough ice stream at the last glacial maximum the landform distribution and their cross cutting relationships indicate that major switches in ice stream flow direction took place during deglaciation additionally grounding zone wedges are found within the cross shelf troughs and these reveal temporal variability in ice stream dynamics in different troughs during deglaciation the distribution of glacigenic features on the ice stream beds also reveals a downstream evolution that has been reported by other workers whereby small bedrock features progressively merge into more elongate forms and msgl this downstream evolution in subglacial geology subglacial bedforms implies that the force balance e g basal shear stress of the ice streams also evolves in the downstream direction and more crucially that the flow mechanism of the ice stream is spatially variable along its length

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

th u total pb geochronology of authigenic monazite in the adelaide rift complex south australia and implications for the age of the type sturtian and marinoan glacial deposits the adelaide rift complex in south australia contains the type sections for sturtian and marinoan glacial deposits the litho and chemo stratigraphy of these deposits play a central role in evaluating global neoproterozoic ice age hypotheses and rodinia supercontinent reconstructions but reliable depositional age constraints have been extremely limited we report results of in situ th u total pb electron microprobe dating of detrital and authigenic monazite in two samples from the umberatana group sturtian holowilena ironstone and pre marinoan enorama shale in the central flinders ranges several texturally and chemically distinct detrital and authigenic populations are recognized detrital dates range from 1600 ma to 760 ma and most relate to well known orogenic or igneous events in surrounding cratonic regions authigenic monazite grew in three or more pulses ranging from 680 ma to 500 ma the date of 680 23 ma 2 for the earliest generation of authigenic monazite in sandstone from the enorama shale 1 provides an estimate for the age of the base of the trezona carbon isotopic anomaly just beneath the marinoan glacial deposits 2 provides an absolute minimum age constraint on the underlying sturtian glacial deposits and 3 supports proposed correlations between type marinoan deposits and precisely dated glacial deposits in namibia and china which bracket the presumed marinoan equivalents between 655 and 635 ma this age is inconsistent with a re os isochron age of 643 2 4 ma 2 on shales near the bottom of the sturtian marinoan interglacial succession stratigraphically 3000 m below the enorama shale sample and militate against the hypothesis that the type marinoan is correlative with the 580 ma gaskiers glaciation monazite growth near 600 ma and again at about 500 ma probably represent hydrothermal fluid flow events the latter of which also corresponds to the well known delamerian orogeny during which the adelaide sediments were folded into their present structural pattern

tunnel valleys concepts and controversies a review abstract although erosion of valleys by subglacial meltwater was first proposed more than 100 years ago the processes that produced these valleys under past ice sheets have remained a topic of vigorous debate the most commonly hypothesized mechanisms have included gradual formation by sediment deformation into the incipient valleys under steady state conditions and subsequent transport by meltwater to the ice margin time transgressive formation along a retreating ice margin by drainage of surface meltwater to the bed or release of impounded meltwater behind a marginal permafrost wedge probably catastrophically and rapid erosion during widespread catastrophic basal sheetflood events recent application of new and enhanced imaging techniques has led to the identification of many new tunnel valleys both on land and offshore throughout the regions glaciated during former ice ages in addition new advances in understanding of subglacial hydrology both from modelling and field instrumentation have helped to constrain the drainage of meltwater from past ice sheet beds this work elucidates the role of subglacial groundwater flow systems in evacuating meltwater from the bed but also shows that groundwater flow alone was not enough to maintain ice bed meltwater pore pressures below the level of ice flotation both modelling and field observations indicate that subglacial conduits oriented parallel to glacial flow lines reorganize groundwater flow systems into basins containing lateral flow toward conduits cut upward into the ice or downward into the bed because of the inability of groundwater flow to account for all the meltwater produced by basal melting other drainage mechanisms involving both non channelized and channelized meltwater flow have been proposed the notion of catastrophic drainage of impounded basal meltwater is still favored by many and the discovery of hundreds of subglacial lakes beneath the antarctic ice sheet and the likelihood of similar lakes beneath pleistocene ice sheets provides a source of water for those advocating catastrophic formation for tunnel valleys proponents of catastrophic drainage consider the valleys to be channels reflecting the width and perhaps depths of flows that formed them tunnel valleys produced by the scandinavian ice sheet have been described and mapped on land as well as on the sea bed valleys and networks of valleys some of which are totally buried have been linked to 3 or more glaciations tunnel valley networks from the last weichselian glaciation in denmark can be related to specific ice margins developed during ice retreat in north america most research has focused on tunnel valleys associated with ice margins formed by the last wisconsin glaciation along the southern margin of the laurentide ice sheet

glaciation and deglaciation of the libyan desert the late ordovician record detailed outcrop studies at the flanks of al kufrah basin libya reveal the nature of glacially related sedimentation and post depositional deformation styles produced in association with the late ordovician glaciation during which ice sheets expanded northward over north africa to deposit the mamuniyat formation at the se basin flank jabal azbah the mamuniyat formation is sand dominated and incises interfingering braidplain and shallow marine deposits of the hawaz formation the glacially related sediments include intercalations of mud chip bearing tabular sandstones and intraformational conglomerates which are interpreted as turbidite and debrite facies respectively these record aggradation of an extensive sediment wedge in front of a stable former ice margin an increase in mudstone content northward is accompanied by the occurrence of more evolved turbidites a widespread surface bearing streamlined nw se striking ridges and grooves punctuates this succession the structures on the surface are interpreted to have formed during a regional north westward ice advance above siltstones bearing arthrophycus burrows and orthocone bearing sandstones beneath tidal bars testify to glaciomarine conditions for deposition of the underflow deposits beneath by contrast the northern basin margin jabal az zalmah is appreciably different in recording shallower water paralic sedimentation styles and major glaciotectonic deformation features although facies analysis also reveals northward deepening here a siltstone wedging from 8 to 50 m toward the north was deposited lower delta plain succeeded by climbing ripple cross laminated sandstones up to 60 m in thickness distal through proximal delta mouth bar deposits with occasional diamictite interbeds these rocks are deformed by thrusts and 50 m amplitude fault propagation folds the deformation locally sealed by a diamictite then overlain by conglomeratic lag during ultimate deglaciation integrating observations from both basin margins a model of fluvial dominated delta systems feeding a pulsed debrite and turbidite fan system in a shallow proglacial shelf is proposed

black shale grey shale fossils and glaciers anatomy of the upper ordovician silurian succession in the tazzeka massif of eastern morocco abstract a 400 m thick clastic succession of late ordovician through silurian age crops out in the tazzeka massif eastern morocco biostratigraphic data chitinozoa acritarchs constrain these rocks to the late katian through hirnantian coeval with glaciation in gondwana with a 8 myr hiatus at the ordovician silurian boundary sedimentological analysis reveals six facies associations including interbedded black and grey shale couplets poorly oxygenated shelf sediments a bioturbated shale with wave rippled sandstone inner shelf deposits several occurrences of diamictite of probable glaciogenic origin interbedded sandstone and mudstone storm and fair weather wave agitated shoreface rippled sandstone storm return flow deposits and amalgamated sandstone deposits delta front debris flows direct evidence for glaciation in the hirnantian deposits is poor third order sequence stratigraphic analysis reveals two transgressive systems tracts late katian and intra hirnantian two highstand systems tracts both intra hirnantian lowstand wedges probably corresponding to two principal glacial lowstands two maximum flooding surfaces late katian and intra hirnantian and a major ravinement surface hirnantian silurian contact remote from the centre of glaciation the tazzeka succession is suggested to be an excellent reference section with which to understand glacially moderated sea level changes in the late ordovician silurian late llandovery or younger shale geochemistry indicates organic enrichment toc 4 5 this organic enrichment greatly increases the known extent of the upper of two silurian organic shales across north africa potential hydrocarbon source rocks across the region

interpretation of late ordovician glaciogenic reservoirs from 3 d seismic data an example from the murzuq basin libya understanding the recessional behaviour of ancient pre cenozoic ice sheets based on seismic reflection studies is generally difficult through scarcity of data in north africa however hydrocarbon exploration has produced high quality seismic reflection datasets that permit an analysis of the morphology and internal sedimentary architecture of incisions of late ordovician age related to the hirnantian glaciation analysis of a high resolution 3 d seismic dataset covering a small area in western libya the n murzuq basin reveals a sharply defined wnw ese oriented palaeo escarpment with a higher cliff forming western margin and a lower basin forming eastern margin the palaeo escarpment defines the western flank of a sub basin extending up to 60 km in width known as the awbari trough the escarpment and the trough are interpreted as the morphological expression of a major unconformity dividing pre glacial sediments below from late ordovician hirnantian glacially related sediments above two hypotheses are considered for the origins of both the escarpment and the awbari trough 1 as a tectonic feature such as a half graben that was active during sedimentation and 2 a glacially related palaeotopography with the latter interpretation preferred owing to the lack of evidence for syn sedimentary fault activity the width of the awbari trough compares to the large scale cross shelf troughs in modern high latitude settings such as the barents shelf produced by ice streams the awbari trough was progressively filled in by gravity flow deposits throughout the course of the glaciation until the initial incision became filled in with sediments during an overall glacial retreat phase and ceased to influence sedimentation patterns glacial re advance across the basin produced a second unconformity observed in seismic data above this unconformity meltwater processes incised a shallow 20 m and wide 5 km subglacial tunnel valley stabilization of the ice front prior to its ultimate retreat resulted in the deposition of a delta complex prior to the early silurian transgression

trace fossils on a late ordovician glacially striated pavement in algeria abstract an exceptional exposure of late ordovician glaciogenic sediments crops out in dider se algeria within the tassili n ajjer region the sediments consist of sandstones and diamictites sandwiched between a mid ordovician fluvial and tidal sandstone in tahouite formation below and early silurian shale oued imirhou formation above stratigraphic discontinuities within the late ordovician glaciogene succession include palaeovalley incisions and glacially striated pavements striation and fluting of a soft bedded sediment beneath an ancient ice sheet is supported by abundant dewatering structures and soft sediment gouges interpreted to have been produced by the action of stone ploughing in dider two types of previously undescribed circular structures sit in negative relief on this glacial pavement namely 1 paired thumb shaped impressions 2 cm in diameter and 3 mm in depth and 2 a 5 cm wide impression with 3 segmented nested cycles a framboid or aggregate origin may be appropriate for the smaller of the features but the larger impression is interpreted as biogenic internal complexity is characteristic discounting concretion moulds and water escape structures a biogenic origin as a coelenterate resting trace is proposed with speculation on conditions of exceptional preservation in an ancient periglacial environment

buried and open tunnel valleys in denmark erosion beneath multiple ice sheets abstract tunnel valleys are large elongate and irregular depressions cut beneath the margin of former ice sheets they are generally believed to play a substantial role for the entire hydraulic system beneath ice sheets and thus also for ice sheet behaviour their origin however remains poorly understood examination of newly collected danish hydrogeophysical and lithological data has revealed numerous systems of buried valleys 50 350 m deep and 0 5 4 km wide these valleys are similar to the open tunnel valleys found in the present day landscape infills comprise a variety of sediments often with subglacial clay till and meltwater deposits the incised substratum consists of limestone heavy paleogene clay sand and silt the tunnel valleys have preferred orientations and can be divided into several generations each ascribed individual ice advances or ice sheets occurring during multiple glaciations the processes forming the valleys appear to prefer pre existing open and buried valleys for the renewed erosion thus old subglacial erosion pathways have been re used several times and some of the present tunnel valleys may have been established in the middle or early quaternary or even earlier this valley re use effect causes apparently anastomosed valley systems to emerge in the data images the valleys were mainly eroded by meltwater supposed to have drained from subglacially stored reservoirs probably behind a frozen margin the water was most likely released in repeated jokulhlaups and flowed in relatively small channels on the floors of the tunnel valleys which gradually became ice filled with the lowering of the bed selective linear glacial erosion is also believed to have contributed to the erosion this however was most pronounced for the widest valleys and is supposed to have played a secondary role

an interglacial on snowball earth dynamic ice behaviour revealed in the chuos formation namibia the sturtian is the oldest ca 716 ma of three pan global glaciations in the cryogenian at omutirapo in northern namibia a 2 km wide 400 m deep palaeovalley is filled by glaciogenic strata of the chuos formation which represents the sturtian glacial record sedimentary logging of an exceptionally high quality exposure permits detailed stratigraphic descriptions and interpretations allowing two glacial cycles to be identified at the base of the exposed succession strong evidence supporting glaciation includes diamictites ice rafted dropstones and intensely sheared zones of interpreted subglacial origin these facies collectively represent ice proximal to ice rafted deposits upsection dropstone free mudstones in the middle of the succession and the absence of diamictites imply sedimentation free from glacial influence however the reappearance of glacial deposits above indicates a phase of sturtian glacial re advance comparison with age equivalent strata in south australia where evidence for sea ice free sedimentation has been established previously suggests that a sturtian interglacial may have been extensive implying global scale waxing and waning of ice sheets during a cryogenian glacial event

drainage beneath ice sheets groundwater channel coupling and the origin of esker systems from former ice sheets abstract the nature of the drainage system beneath ice sheets is crucial to their dynamic behaviour but remains problematic an experimentally based theory of coupling between groundwater and major channel systems is applied to the esker systems in the area occupied the last ice sheet in europe which we regard as a fossil imprint of major longitudinal drainage channels we conclude that the large scale distribution and spacing of major eskers is consistent with the theory of groundwater control in which esker spacing is partly controlled by the transmissivity of the bed it is concluded that esker patterns reflect the large scale organisation of the subglacial drainage pattern in which channel development is coupled to groundwater flow and to the ice sheet s dynamic regime the theory is then used to deduce basal meltwater recharge rates and their spatial variability from esker spacing in an area in which the ice sheet was actively streaming during its final retreat patterns of palaeo groundwater flow and head distribution and the seasonally varying magnitude of discharge from stream tunnels at the retreating ice sheet margin major channel esker systems appear to have been stable at least over several hundred of years during the retreat of the ice sheet although major dynamic events are demonstrably associated with major shifts in the hydraulic regime modelling suggests that glaciation can stimulate deep groundwater circulation cells that are spatially linked to channel locations with groundwater flow predominantly transverse to ice flow that the circulation pattern has the potential to create large scale anomalies in groundwater chemistry and that the spacing of channels will change through the glacial cycle influencing water pressures in stream tunnels subglacial hydraulic gradients and effective pressure if the latter is reduced sufficiently it could trigger enhanced bed deformation thus coupling drainage to ice sheet movement it suggests the possibility of distinctive phases of sediment deformation and drumlin mobilisation during a glacial cycle

a guide to phanerozoic cold polar climates from high latitude ice rafting in the cretaceous the high latitude extent of warm climate indicators at certain times in earth history has been considered as evidence that the globe was ice free for long intervals despite theoretical considerations and results from numerical modelling experiments1 2 indicating that this was unlikely one of the warmest periods the cretaceous displays faunal and floral evidence for cool temperate to sub tropical conditions very near to the poles however our studies of lower cretaceous mudstones of central australia that contain outsized exotic blocks have led to the conclusion that the blocks were emplaced by ice rafting implying that high latitude ice was present at sea level strata of a similar origin of mid jurassic to mid cretaceous age occur on other continents that were positioned between 65 and 78 palaeolatitude indeed there is a record of high latitude ice rafting throughout the phanerozoic suggesting that ice was present on earth for much of its history and that ice free conditions could have been at most only episodic over the past 600 myr

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

removal of overburden velocity anomaly effects for depth conversion a method of compensating for the presence of discrete overburden velocity anomalies during depth conversion of time horizons interpreted on conventional post stack time migrated seismic data is presented positive and negative time delays are estimated either from the push down or pull up of reflectors directly beneath the anomalies or from interpreted time thickness of the anomalous body and interval velocities estimated from well data the critical steps are pre stack simulation of seismic acquisition across the velocity anomalies incorporating the effects of a fresnel volume which changes its width as a function of depth and simulation of commonmidpoint cmp stacking using a linear regression of time delay at versus offset squared x 2 the time correction method predicts the time distortion for any target horizon and the distortion is removed as a correction in time depth conversion is then performed using a background velocity function the final average velocity map is calculated from the resulting depth structure and the raw times at the target horizon the method is implemented by manipulating time grids within an industry standard mapping package the final average velocity map shows steep lateral velocity gradients which are constrained by the interpreted boundaries of the velocity anomalies

earth s oldest reported glaciation physical and chemical evidence from the archean mozaan group 2 9 ga of south africa the pongola supergroup is a thick succession of archean 2 9 ga supracrustal rocks in the eastern part of the kaapvaal craton of southern africa its upper part the mozaan group 5000 m thick includes diamictites with a highly varied suite of clasts some of which are striated and faceted clasts in associated stratified siltstones are interpreted as ice rafted debris a glacial origin is also supported by geochemical investigation of diamictite matrix materials and associated mudstones plots of sio2 al2o3 and tio2 al2o3 suggest that these rocks underwent little chemical weathering an interpretation confirmed by calculation of a chemical index of alteration with low values typical of other glacial deposits the analyzed rocks are rich in fe average fe2o3 t is 24 wt like rocks of many other glaciomarine successions the diamictites and associated mudstones of the mozaan group have low th sc ratios suggesting preferential incorporation of mafic materials into the diamictite matrix material

sandstones glaciers burrows and transgressions the lower palaeozoic of jabel az zalmah al kufrah basin libya abstract field studies in the jabel az zalmah region at the northern flank of al kufrah basin provide new insights into the stratigraphy and evolving depositional environments of cambro ordovician through devonian sediments at the base of the succession the hawaz formation is about 300 m thick and is characterised by frequently skolithos burrowed fine to medium grained sandstones with occasional silty intervals and rare conglomerates deposited in a generally high energy tidal shelf setting bioturbation is pervasive and includes spectacular cruziana traces produced by early to mid ordovician trilobites at the bottom and the top of the hawaz formation respectively these strata were incised during the late ordovician hirnantian glaciation ice sheets flowing over jabel az zalmah cut a palaeo topography of up to 100 m relief this relief is overlain by a coarsening upward succession of clastic sedimentary rocks recording re advance of an ice sheet which overrode the uppermost sandstones ice sheet retreat was accompanied by marine transgression and erosion which left a thin transgressive lag conglomerate capped sharply by siltstones of the tannezuft formation progradation resumed after post glacial maximum flooding depositing tide storm and river influenced parasequences of a major delta complex 200 m thick acacus formation fluvial incision and the development of a disconformity preceded the deposition of the tadrart formation 50 m thick a succession of trough cross bedded braided river deposits transgression and the re establishment of shallow or marginal marine systems is represented by the extensively bioturbated binem formation in which zoophycos burrows offer a tentative middle devonian stratigraphic fix an understanding of the lithological character of the lower palaeozoic strata in jabel az zalmah together with the character of unconformities that punctuate the succession will be vital for successful hydrocarbon exploration in this frontier basin which has until now been unsuccessful

mid pleistocene ice drainage pattern in the norwegian channel imaged by 3d seismic abstract several investigations have shown that a huge late weichselian ice stream flowed along the norwegian channel and deposited thick debris flow deposits at the north sea fan the development of the channel is probably mainly a result of several cycles of ice stream activity during the quaternary a merged 3d seismic image shows a lineated relief pattern interpreted as a uniquely well preserved footprint of a moving ice sheet this deep quaternary horizon corresponds to a slightly irregular reflector on top of a parallel layered seismic sequence seismic tie to the troll core 8903 south of the study area shows that the sequence comprises early middle pleistocene marine sediments the pattern of lineations the seismic stratigraphy as well as the chronostratigraphic investigations of the troll core strongly indicate that the image reflects the initial phase of an extensive middle pleistocene glaciation inferred age ca 0 5 ma prior to the development of a massive norwegian channel ice stream the northwesterly oriented pattern seen in the southeastern part of the study area demonstrates that the ice flowed into the channel mainly from the coastal zone north of bergen farther west various sets of fan shaped lineations partly cross each other showing that the 3d image represents a certain short time window in the northern study area the ice flow was dominantly northwards and directed towards the area below the present shallow maloy plateau there are no indications that the glaciers north of sognefjorden affected the marine ice sheet

a new microstructural mapping methodology for the identification analysis and interpretation of polyphase deformation within subglacial sediments although integrated clast macro and microfabric studies have been used to provide evidence of palaeo ice flow directions to discriminate between diamictons deposited in differing sedimentary environments and to model the response of tills to subglacial deformation such studies have typically only examined the bulk fabric developed within the diamicton largely ignoring the potential for several phases of fabric development within these typically polydeformed deposits we present a new graphical method for the identification description and analysis of clast microfabrics in polydeformed tills this method utilises high resolution digital scans of orientated thin sections imported into a standard computer graphics package to measure the orientation of long axes of detrital skeleton grains included within the till these data are then used to identify the main clast microfabrics present within the till allowing the delineation of clast microfabric domains in which elongate grains share a similar preferred orientation this process leads to the identification characterisation and most importantly the interrelationships between the various generations of microfabrics the relationships between these clast microfabrics and other microstructures e g plasmic fabrics turbate structures shears folds faults etc can also be examined providing a detailed microfabric microstructural map of the thin section clast long axis orientation data collected for different parts of a thin section are plotted on a series of rose diagrams allowing the variation in clast microfabric orientation to be displayed graphically a new set of terminology for the description of clast microfabrics in tills which is based upon the system used by metamorphic petrologists is also presented three case studies of the fabrics present within subglacial sediments of pleistocene age as well as the present are used as examples of how to apply this methodology

3d seismic analysis of buried tunnel valleys in the central north sea tunnel valley fill sedimentary architecture fourteen overlapping 3d seismic reflection data sets originally acquired by the oil and gas industry were used to image and investigate complex networks of buried tunnel valleys in the uk sector of the central north sea more than 180 individual tunnel valleys were observed and mapped to horizontal resolutions of 12 50 m and vertical resolutions of 8 10 m the results presented here identify three recurring seismic facies that locally define stratigraphic organization from base to top of the buried tunnel valley fills i high amplitude disrupted units ii a unit containing gently dipping disrupted reflections or transparent facies and iii well layered continuous reflections a comparison to similar works in the region is used to infer tunnel valley infill during changing depositional conditions from high energy subglacial meltwater to quiet glaciolacustrine or glaciomarine environments

heinrich events massive late pleistocene detritus layers of the north atlantic and their global climate imprint 1 millennial climate oscillations of the glacial interval are interrupted by extreme events the so called heinrich events of the north atlantic their near global footprint is a testament to coherent interactions among earth s atmosphere oceans and cryosphere on millennial timescales heinrich detritus appears to have been derived from the region around hudson strait it was deposited over approximately 500 250 years several mechanisms have been proposed for the origin of the layers binge purge cycle of the laurentide ice sheet jokulhlaup activity from a hudson bay lake and an ice shelf buildup collapse fed by hudson strait to determine the origin of the heinrich events i recommend 1 further studies of the timing and duration of the events 2 further sedimentology study near the hudson strait and 3 greater spatial and temporal resolution studies of the layers as well as their precursory intervals studies of previous glacial intervals may also provide important constraints

the pre weichselian glaciations of north west europe abstract in north west europe no evidence has been found for glaciations older than the elsterian the elsterian seems to consist of at least two separate ice advances with a minor interval in between during the elsterian in the area south of the baltic sea over 400 m deep channels were cut into the underlying substratum by a combination of glacial erosion and meltwater activity the channels were active until the late elsterian and were subsequently filled with meltwater deposits and glaciolacustrine silt and clay during the saalian no comparative channels were being formed three different ice advances can be distinguished within the saalian the stratigraphical correlation of which is to be discussed no convincing evidence was found for any interglacial within the saalian sequence there are good reasons to assume that the ice advances were only separated from each other by minor ice free intervals