antarctic cryogenic sediments biotic and inorganic facies of ice shelf and marine based ice sheet environments abstract cryogenic sediments defined as marine deposits that form in association with ice icebergs sea ice ice shelves or marine based ice sheets are widespread in high latitude regions existing models for cryogenic depositional processes are of limited utility because of 1 lack of direct observational data 2 reliance on older land exposed sections from which inferential conclusions have been drawn 3 employment of inadequate glaciologic and oceanographic concepts and 4 failure to incorporate adequately information on the biotic content of these deposits as a first step toward developing a synthetic model for cryogenic deposition we relate glaciologic sedimentologic and biotic processes of the modern ice shelf and marine based ice sheet environments using data from antarctic continental shelves the principal deposit beneath grounded marine based ice sheets is lodgement till which often contains reworked and fragmented fossil material this till is identical in all important sedimentologic respects with terrestrial lodgement deposits marine lodgement tills commonly form geographically extensive sheets which are restricted to continental shelf sites and which may be differentiated from other cryogenic deposits by their high degree of compaction and reworked fossils waterlain till may form beneath grounded ice in areas where debris melting out of the glacier sole falls through a thin water layer up to a few m between the ice and its bed waterlain till usually lacks fossil material unless the source material for the subglacial debris is fossiliferous and is less compact than lodgement till deposition beneath large polar ice shelves e g ross filchner ronne is largely restricted to a zone probably km in width near the grounding line in this zone a complex of primary and secondary cryogenic deposits including waterlain till ice shelf rafted detritus isrd flow till slumps etc is likely to interfinger processes controlling this deposition include local basal conditions of the ice sheet and ice shelf freezing or melting presence or absence of subglacial outflow marine currents and tidal action sediment supply long and short term grounding line movement sea floor topography and water depth deposition of isrd is limited elsewhere beneath these large ice shelves because high surface accumulation rates cause downward particle paths for ice and debris thus most debris is released soon after passing the grounding line biotic activity beneath ice shelves declines rapidly in diversity and abundance between the calving margin and grounding line deposition rates seaward of the calving margin are much higher than beneath the large ice shelves here icebergs derived from outlet glaciers and ice streams that terminate directly in the sea may carry significant loads of subglacial englacial or superglacial debris which is released by a variety of mechanisms as the icebergs melt this ice rafted detritus mixes with biotic material from planktonic organisms as it falls through the water column to form compound ibrd if currents are present in the water column or at the seafloor sorting may occur to form residual ibrd on shallow banks grounded icebergs carrying subglacial debris may melt in situ forming iceberg till and iceberg ploughing of sediment may form iceberg turbate both these deposits may be difficult to distinguish from lodgement till but both are restricted to areas with modern water depths less than 450 m

sequence stratigraphy of source rocks applied to the study of the kim meridgian tithonian in the north west european shelf dorset uk yorkshire uk and boulonnais france abstract in dorset and yorkshire england the sedimentation of kimmeridgianfrithonian age mutabilis to pallasioides zones shows ordered organic sequences the distribution of the organic matter content per cent total organic carbon petroleum potential hydrogen indices is not random but occurs as widespread primary organic rich cycles nested within each other these organic rich cycles with thicknesses of about 0 5 m show regular variations in organic carbon contents reaching a maximum value midway through the cycle they formed in response to widespread phenomena and can be correlated over long distances the cycles were deposited in marine environments mainly below the storm wave base they show little sedimentological evidence of sea level fluctuation either transgression or regression in the boulonnais france a more proximal setting the comparable strata show a greater lithological contrast and depositional environments corresponding to bathymetric changes are more easily determined a comparison of the distribution of organic matter through the sequence dated by ammonites in the three areas studied shows a good correlation between the distal dorset and yorkshire and the proximal boulonnais areas the main result of the biostratigraphic revision in the boulonnais france concerns the argiles de ch tillon which can be subdivided into two members these belong to the autissiodorensis zone autissiodorensis subzone and the gigas elegans zone respectively thus the kim meridgian tithonian boundary is located within this formation transgression does not appear to be the main factor determining the accumulation of autochthonous organic matter type ii not every major transgressive facies change seen in the kimmeridgian tithonian of the boulonnais is characterized by an enrichment in organic matter in the basinal area good source rocks with high petroleum potentials result from a conjunction between the physiography the transgression and the climate this last causes the variations in primary productivity which produce the organic rich cycles transgressions act mostly to reveal the climatic variations they are a necessary condition for the accumulation of the autochthonous organic matter but they are not sufficient by themselves as demonstrated by the transgressions in the autissiodorensis and elegans zones in the first instance the strata are depleted in organic matter in both the distal and proximal areas in the second instance type ii organic matter accumulated in both distal and proximal environments the vertical fluctuations in the total organic carbon contents the types of organic matter and the petroleum potentials of the organic rich cycles determine the oil potentials of transgressive sequences in sedimentary basins

field and seismic images of sharp based shoreface deposits implications for sequence stratigraphic analysis abstract sharp based shoreface sandstones are of considerable interest because of their potential as hydrocarbon reservoirs and because they play an important role in the stratigraphic analysis of basin fills the sharp based shoreface sandstones studied herein are upper jurassic kimmeridgian tithonian and exposed along the coastal cliffs of the dover strait in northwestern france these series consist of tens of meter thick alternations of sandstones bodies and organic rich shales that can be correlated for over 30 km along coastal cliff exposures and tied to high resolution ca 1 m of sediment marine seismic profiles obtained several hundred meters offshore the units described here comprise two sharp based sandstone bodies each is composed of a basal progradational set of shoreface parasequences overlain by a progradational aggradational shoreface succession each sharp based sand body lies on a marine regressive surface of erosion and is truncated by a marine transgressive surface of erosion which in turn is overlain by a thin retrogradational ravinement lag or coarse grained planar laminated bedset the two progradational packages are separated by a third surface a subaerial exposure surface that is interpreted as a sequence boundary two distinct types of seismic units referred to as type a and type b have been identified in the study area seismic unit a has conformable upper and lower boundaries and parallel aggradational configurations seismic unit b is characterized by downlap and toplap boundaries and simple or compound sigmoid and oblique tangential progradational configurations a single sharp based shoreface sandstone body makes up the type b unit and typically consists of two compound superimposed progradational sets b1 b2 the lowermost set b1 corresponds in the field to the progradational set sensu stricto whereas b2 corresponds to a progradational aggradational set b1 and b2 are separated in outcrop by a sequence boundary these observations led us to reevaluate the sequence stratigraphic interpretation of sharp based shoreface sandstones it is proposed here that complete single sharp based shoreface sandstones bodies can be separated into two different systems tracts 1 a progradational set b1 seismic body at the base which corresponds to the forced regressive wedge systems tracts frwst of hunt and tucker 1992 and 2 a progradational aggradational set b2 at the top above the sequence boundary which corresponds to the lowstand systems tract lst of posamentier et al 1992 a complete sharp based shoreface sandstone body is bounded below the frwst by a regressive surface of marine erosion caused by the downward shift of wave base and by a transgressive surface of marine erosion or ravinement surface at the top of the lst

facies architecture seismic stratigraphy and development of a high latitude basin the powell basin antarctica abstract ten seismic facies assemblages organized into architectural elements have been identified on the basis of multichannel seismic profiles in the powell basin these facies include sheet drapes with parallel high amplitude reflectors sheet drapes with cyclic reflectors sheet drapes with chaotic and moderate amplitude reflectors broad lenses with central channel slope front fills with hummocky reflectors sheet drapes with wavy reflectors sigmoid wedges irregular bodies with contorted reflectors lenses with chaotic reflections and trough fills with divergent reflectors the vertical and horizontal distribution of these architectural elements define the syn rift syn drift and three post drift sequences whose boundaries are characterized by unconformities indicating significant changes in the factors controlling sedimentation on the margins and basin plain the allogenic factors that appear to be more influential on the margin growth patterns are the evolution of subsidence mechanisms the tectonic regime on each of the margins and the distribution of bottom currents related to the weddell sea gyre subsidence controls the evolution of the accommodation space thus causing a trend in the sequence external geometry from wedges during the syn rift phase to sigmoids during the drifting followed by flat downward convex lenses at the beginning of the post drift phase and finally sheet like geometries tectonics has a considerable influence on the extension of the source areas and margin geometry and also modifies the depositional cycles associated to climatic changes processes of ice sheet advance and retreat sea level fluctuations and isostatic adjustments bottom currents influence the distribution of depositional bodies controlling the development of sediment drifts that constitute an important proportion of the sequences on the northern margin and in the basin plain the evolution of the depositional sequences of the powell basin clearly depicts the influence of global processes linked to glacio eustatic changes and the tectonics controlling the different phases in the development of a rifted basin this high latitude basin may be considered as a good example of a semi isolated basin which serves as a model for the stratigraphic reconstruction of margins and basins of high latitudes

the origin and nature of seismic reflections of sharp based shoreface deposits upper jurassic siliciclastics northern france in order to advance understanding of the relationship between geological properties and their physical expression in reflection images this study has focused expertise in reflection geophysics petrophysics and sedimentology on the same geological object in this case a succession of upper jurassic sharp based shoreface deposits embedded in offshore marine shales in northern france this integrated approach to determine firstly the origin and nature of seismic reflections calibration and secondly to provide a means of extracting geological information from seismic imagery inverse calibration was built on the following analytical steps firstly detailed and extensive petrophysical analyses of outcrop plug samples continuous core and sonic well logs in combination with a quantification of mineralogical and textural properties allowed a direct conversion of acoustic properties impedance into sedimentological properties resulting in a quantitative physical sequence stratigraphic model secondly the integration of scale dependent acoustic measurements ranging from 0 01 m and 320 khz on cores up to the wavelength of field seismic data was established using an averaging algorithm an effective medium theory type as an upscaling approach this alternative to a vsp or check shot allows an optimized depth time conversion and hence determination of the origin of the seismic reflections with previously unattainable accuracy finally the shape and scale dependence of impedance contrasts were integrated into so called singularity parameters that directly link depositional changes with information from seismic reflections depositional changes in the shallow water domain are generally characterized by step functions whereas those in more distal depositional environments are represented by spiky functions this approach allows the recognition of the associated reflection events and vice versa it provides a unique opportunity to extract the character of impedance changes and thus changes in depositional environment from seismic reflection records in general

late jurassic palaeoclimatic change from clay mineralogy and gamma ray spectrometry of the kimmeridge clay dorset uk abstract the late jurassic was a time of increasing aridity in nw europe here a new clay mineral dataset is presented from a 600 m thick composite core through the kimmeridge clay formation southern england clay mineral assemblages comprise mainly illite and kaolinite with minor randomly interstratified illite smectite mixed layer clays sem observations indicate that clay minerals are mainly detrital except in silty strata of late tithonian age which contain abundant pore filling kaolinite aggregates th k ratios determined from gamma ray spectrometry mirror palaeoclimatically significant variations in kaolinite illite ratios with notable exception where diagenetic kaolinite occurs comparison with age equivalent strata identifies regional cyclic variations in clay minerals at the 100 300 m scale small scale 10 20 m variations in clay minerals and th k also occur it is proposed that the region experienced progressively intense humidity through the kimmeridgian followed by a return to more arid conditions during the early tithonian following a mid tithonian peak in aridity the hudlestoni event9 more humid climatic conditions returned prior to the development of latest tithonian intense aridity late jurassic climate was apparently subject to synchronously cyclic changes across a broad area of the laurasian continent such changes could not have resulted from mountain building or continental rotations supplementary material clay data are available at http www geolsoc org uk sup18376

late quaternary changes in antarctic bottom water velocity inferred from sediment grain size in the northern weddell sea abstract newly formed bottom water 0 7 c in the northern weddell sea flows e or ne at speeds up to 10 15 cm s with velocity decreasing towards the centre of the weddell gyre preliminary results from long term current meter moorings upper quaternary sediments from this area contain a fine grained terrigenous component from the nepheloid layer plus biogenic silica mostly diatoms with a small amount of ice rafted debris in cores from between 61 and 66 s and from 3300 m to 4700 m water depth the proportion of biogenic silica increases northwards corresponding to increasing seasonal extent of open water vs sea ice cover and the proportion of silt and well sorted fine sand in surface sediments increases with average current velocity downcore diatom rich and diatom poor sediments alternate on a scale of 1 3 m and intervals with more diatoms contain a higher proportion of silt to clay preliminary stratigraphy suggests the cyclicity in composition and texture is related to glacial interglacial cyclicity during warm periods indirectly correlated with isotope stages 1 3 5 etc biogenic silica production takes place during several months of each year and silt and fine sand are transported by bottom currents during glacial periods with greater sea ice cover than at present biogenic productivity was suppressed and no silica was preserved in the sediments in addition a lower proportion of terrigenous silt implies that bottom currents were weaker at sites with a present day average velocity of 10 cm s a last glacial maximum average velocity of 1 cm s or less is inferred from grain size measurements

interpreting benthic oxygen levels in mudrocks a new approach quantified paleoecology and gamma ray spectrometry have been applied in the analysis of the kimmeridge clay a highly organic rich british jurassic mudrock decreasing benthic oxygen trends are reflected in decreasing species richness and dominance diversity values similarly the degree of fragmentation of the benthos reflects the benthic energy levels and covaries with benthic oxygen the calculation of authigenic uranium values from data gathered by gamma ray spectrometry shows enrichment in more oxygen deficient environments the good correlation between the independently derived paleoecological and authigenic u data indicates the importance of these techniques in environmental analysis of marine petroleum source rocks

comparison between high resolution seismic and sequence stratigraphic approaches applied to the upper jurassic deposits of the dover strait area northern france abstract this study is an attempt to interpret very high resolution seismic data usually devoted to shallow marine surveys in terms of very high resolution seismic and sequence stratigraphy at the scale of the reservoir seismic data are compared with outcrop based observations in order to discuss the geometrical relationships between rock bodies and the nature of the seismic reflections in terms of seismic facies related to type lithologies two high resolution seismic surveys using a sparker were performed in the dover strait area some 100s of metres away from the boulonnais coastal cliffs the aim was to look to the 130 m thick mixed siliciclastic carbonate deposits of upper jurassic age kimmeridgian tithonian cropping out at the sea floor and to compare them to the coastal cliffs exposures the 100 m apart seismic profiles exhibit a 100 m deep penetration and 1 m in vertical resolution upper 30 ms part of the profiles four seismic facies are recognised on the seismic profiles they comprise ten seismic units which can be grouped into three broad families group 1 has a parallel aggradational reflection configuration and conformable boundaries which may correspond to the transgressive and highstand systems tracts the seismic facies exhibit either a high continuity high amplitude corresponding to clayey mudstone lithologies of the mid ramp environment or a high continuity low amplitude corresponding to shaly lithologies of the outer ramp environment group 2 has a sigmoid progradational reflection configuration bounded at the base by a downlap surface and at the top by a toplap surface these boundaries correspond respectively in the field to a marine regressive surface of erosion and a marine transgressive surface of erosion the seismic facies is cross layered and exhibits a moderate continuity low to moderate amplitude and moderate frequency group 2 corresponds to the bioclastic sand prone facies of the inner ramp environment it is interpreted as lowstand sharp based shorefaces group 3 exhibits a progradational complex to chaotic configuration it lies on an erosional unconformity with paleogullies the seismic facies shows discontinuous reflections with moderate amplitude and high to moderate frequency it corresponds in the field to cross bedded sandstones in channel fill arrangements of fluvial origin wealdian deposits marking the top of the section the different systems tracts sequence boundaries and depositional sequences identified onshore can be recognised as well on the seismic profiles but the comparison demonstrates that seismic data complement the onshore sequence stratigraphic analysis by providing the geometrical relationships between systems tracts onlap downlap toplap that cannot be observed onshore the maximum flooding surfaces however present no specific seismic signature on such a low angle ramp profile it can only be identified precisely in the field which also help to relate lithologies to seismic facies this study represents one of the first attempts to interpret very high resolution seismic data at the outcrop scale it will be improved again in the future by the collection of impedance logs and synthetic seismic traces from cored sections on the coastal cliffs for comparison with the seismic data

quartz silt in mudrocks as a key to sequence stratigraphy kimmeridge clay formation late jurassic wessex basin uk sequence stratigraphic interpretation of mudrocks is often carried out using proxy indicators for grain size or by consideration of other aspects of lithology thought to relate to sea level change such as organic matter content an alternative stratigraphic analysis based on direct estimation of quartz silt content was carried out on a major late jurassic mudrock and oil source rock the kimmeridge clay formation of the wessex basin dorset uk the new data generated by backscatter sem x ray and image analysis show decametrescale stratigraphic patterns that are incompatible with many previous sequence stratigraphic interpretations based on gamma ray logs or visual lithofacies and biofacies description correlation with a basin margin section in the boulonnais northern france indicates that silt rich intervals in basinal facies are coeval within the limits of biostratigraphic resolution with shallow water sand rich packages on the margin variation in silt content in the kimmeridge clay therefore appears to be a record of relative sea level change of at least regional extent it is suggested that analysis of silt content offers the most reliable basis for generation of a regional sequence stratigraphy in basinal mudrocks a revised relative sea level curve for the wessex basin kimmeridgian and early tithonian is presented based on this premise

chemostratigraphy of the jurassic system applications limitations and implications for palaeoceanography current chemostratigraphical studies of the jurassic system primarily involve the use of one sedimentary component marine organic carbon one divalent transition metal substituted in carbonate manganese and two isotopic tracers strontium isotope ratios 87 sr 86 sr and carbon isotope ratios 13 c carb and 13 c org in carbonate and in organic matter other parameters such as mg ca and sr ca ratios in calcite oxygen isotope ratios 18 o in carbonate sulphur isotope ratios 34 s in carbonate hosted sulphate nitrogen isotope ratios 15 n org in organic matter osmium isotope ratios 187 os 188 os in black shales and neodymium isotope ratios 143 nd 144 nd in various mineral phases are also useful but at present give poor resolution because the database is incomplete or compromised by various factors stratigraphical patterns in total organic carbon toc can be of either local or regional significance depending on the lateral extent of the former nutrient rich and productive water mass divalent manganese follows a similar pattern being concentrated most probably as a very early diagenetic phase only in oxygen depleted waters that typically underlie zones of elevated organic productivity shifts in mg ca and sr ca ratios on the time scale of ammonite subzones seem largely to reflect temperature changes strontium isotope ratios from pristine skeletal calcite provide a global signal 13 c values from carbonates with minimal diagenetic overprint potentially do the same although small spatial differences in palaeo water mass composition may have been locally significant oxygen isotope determinations on carbonate rocks and fossils generally yield values that are too scattered to be stratigraphically useful because they reflect palaeotemperature the evaporation precipitation balance in sea water and the impact of any diagenesis involving an aqueous phase nitrogen isotope ratios in organic matter reflect the chemistry of ancient water masses as affected by nitrate utilization and denitrification and the stratigraphical pattern of this parameter is more likely to correlate only on a regional basis neodymium isotope ratios in sea water are also water mass dependent and greatly affected by regional sources and oceanic current systems preliminary data on sulphur isotope ratios in carbonates and osmium isotope ratios in organic rich shales both potentially offering global correlation indicate that these tracers may be valuable although the records at present are not sufficiently well established to allow high resolution regional correlation in all cases biostratigraphically well dated reference sections against which the relevant geochemical data have been calibrated are required in the first instance to date studies on the stratigraphical distribution of organic carbon have been principally carried out in both northern boreal and southern tethyan europe carbon isotope stratigraphy has been undertaken primarily but not exclusively on bulk pelagic sediments from the alpine mediterranean or tethyan domain and strontium isotope stratigraphy has been undertaken largely on calcitic skeletal material belemnites and oysters from northern and southern europe in many sections including those containing ammonites multi parameter chemostratigraphy can give resolution that exceeds that attainable by classic biostratigraphical means strontium isotope ratios in skeletal calcite are a particularly powerful tool for illustrating changes in sedimentary rate and revealing gaps in the stratigraphical record

subsurface geophysics of the phlegrean fields new insights from downhole measurements 1 the volcanic complex of phlegrean fields located northwest of naples italy has been the site of deep geothermal exploration in the 1980s several wells were drilled by an agip enel joint venture with downhole continuous physical properties acquired in each well the main purpose of this study is to map and describe the spatial variations of the volcanic deposits in terms of their nature thickness and in situ physical properties we apply fuzzy k means classification of the well logging data available in the wells cored in the san vito area fuzzy set theory provides an approach that quantitatively assigns individuals to physically continuous classes the optimal number of classes is found by minimization of three mathematical functions thus reducing subjectivity the resulting classifications are found 1 to reflect the main lithologies in the san vito plain 2 to provide more detail on the geological stratigraphy allowing a more precise volcanic history and 3 to differentiate between transition zones and interbedded well defined deposits in addition to discriminating between lithologies that have different physical properties this study gives information on the degree of homogeneity of each lithologic unit and the range of variation for the measured properties finally using the physical classification of the deposits we are able to propose a detailed two dimensional compressional acoustic velocity structure of the studied area

environmental cycles in the late jurassic northwest european epeiric basin interaction with atmospheric and hydrospheric circulations abstract despite the large size and the heterogeneity of the northwest european late jurassic kimmeridgian lower tithonian epeiric basin the sedimentary environment was rather uniform in general deposition of mudstones and the microlaminated organic rich sediments took place under low oxic to anoxic conditions detailed environmental analysis clearly points to a seasonal alternation of aerobic with anaerobic conditions in substrate and bottom water resulting in one year life cycles in the benthic environment the widespread uniformity of facies and the seasonal alternation in the oxygen budget of the benthic environments are best explained by a summer to winter change in the atmospheric circulation of the northern hemisphere this change strongly influenced the oceanographic situation in the northwestern european epeiric basin superposed on this one year cyclicity three more hierarchical levels of cycles are stratigraphically recorded they reflect a complex history of orbital forced and non orbital forced climatic changes and or eustatic sea level fluctuations in the late jurassic

glacially influenced sedimentation in the later proterozoic of the paraguay belt mato grosso brazil abstract late proterozoic probably early vendian 610 590 ma glaciogenic and turbiditic sediments and metasediments are present along the transitional zone between the amazonian craton to the northwest and the paraguay belt to the southeast in the cuiaba region mato grosso brazil a few exposures of diamictites are associated with conglomerates and graded pebbly sandstones on the cratonic fringe and pass southeastwards into fine grained sediments deposited in the deeper part of the probably marine basin in the paraguay belt these rocks have been affected by the brasiliano orogeny possibly around 570 ma they show increasing deformation and metamorphism from the craton towards the fold belt the sedimentation scheme proposed for the western border of the paraguay belt and the eastern edge of the amazonian craton involves three probably marine main depositional systems to the west on the craton the platform domain can be subdivided into a western inner shelf and an eastern outer shelf the deposits on the inner shelf subdomain show an alternation of dominant massive diamictites sandstones and fine grained sediments with few dropstones this diamictite association is tentatively interpreted as glaciomarine in origin in the outer shelf subdomain the massive diamictites are progressively replaced southeastwards by an association of massive diamictites stratified diamictites and fine grained sediments this diamictite association is interpreted as a whole as resedimented glacial debris deposited by subaqueous debris flows the slope domain located at the transition between craton and fold belt is characterized by strong reworking of the glaciomarine sediments by gravity flows responsible for a type of submarine fan deposition the coarse clastic facies which result from a reworking by rapid downslope transport of cohesive debris flows and gravity flows with high viscosity fill feeder channels in the upper fan eastward inside the belt a progressive sorting in the deeper portions of the fan is shown by the transition from diamictites massive gravels sandstone intercalations with occasional inverse and or normal grading debris flow turbidity current to normal graded finer sediments turbidity current sandstone and siltstone intercalations represent interchannel deposits formed by turbidity currents east of the slope well bedded fine grained sediments were generated by low density turbidity currents direct glacial influence is reduced to the presence of a few isolated clasts or dropstones two distinct upper proterozoic glaciogenic units are known in brazil the older and more widespread one dated at about 950 ma is located on the sao francisco craton and in the adjacent brasiliano fold belts the younger one 600 ma is restricted to the paraguay belt and the adjacent border of the amazonian craton in southern western gondwana late proterozoic glaciation is of mountain type and developed at latitudes of 60 on highs probably generated by pan african brasiliano rifting at the same time in its northern part a widespread ice sheet covered the west african craton

milankovitch theory of ice ages hypothesis of ice sheet linkage between regional insolation and global climate abstract j d hays j imbrie and n j shackleton 1976 science 194 1121 1132 showed that the astronomical theory explained many features of late quaternary ice age climates but they did not specify the physical mechanisms involved here it is proposed that interlocked variations of ice sheet heat sinks in both polar hemispheres amplified and transmitted milankovitch summer half year insolation changes a version of the astronomical theory between 45 and 75 n into the globally synchronous climate changes recorded in geologic records it is suggested that late quaternary ice sheets had terrestrial components grounded above sea level melting margins fluctuations controlled by climate and marine components grounded below sea level drained largely by ice streams limited melting margins fluctuations controlled primarily by sea level and secondarily by climate interior surface elevations coupled to downdraw through ice streams northern hemisphere ice sheets were largely marine with minor melting margins in the arctic and terrestrial with major melting margins in the midlatitudes west antarctic and peripheral east antarctic ice was marine based and lacked melting margins because of their geographic array these terrestrial and marine components formed an ice sheet system whose variations were coupled on a global scale milankovitch summer isolation changes near midlatitude northern hemisphere melting margins controlled most variations of this system because advance or retreat of melting margins initiated concurrent eustatic sea level change such sea level change afforded the critical interlocking mechanism between terrestrial and marine components because it forced simultaneous expansion or contraction of marine margins in both polar hemispheres this initiated an amplifying feedback loop among all marine components and influenced interior downdraw through ice streams arctic summer insolation change was less important because northern melting margins were relatively minor its greatest influence was on surface ablation of ice streams that controlled interior downdraw this affected eustatic sea level and activated global linkage of marine sectors by analogy with present day antarctica late quaternary ice sheets were enormous planetary heat sinks due to their reflective and radiative surface characteristics it is suggested that the effectiveness of these ice sheet heat sinks varied with their areal extent and interior surface elevation thus it is postulated that concurrent growth or decay of these interlocked ice sheet heat sinks in both polar hemispheres served as the global amplifier of regional milankovitch summer insolation

evidence for a late wisconsin glaciation of the weddell sea the huge ice shelves in west antarctica the ross and filch ner ronne ice shelves have probably extended out on the continental shelf during the late wisconsin1 previous discussions which have focused on the ross sea have suggested 1 that the ice extended across the whole continental shelf2 3 or 2 that there was only a minor expansion4 here we present sedimentological data from the weddell sea fig 1 which suggest that a late wisconsin grounded ice sheet extended to the shelf edge the evidence includes a recent thicker ice in ellsworth mountains at the head of the filchner ronne ice shelf5 this thickening would lead to an expansion of the inland ice sheet over the continental shelf filling up the weddell sea embayment

sedimentation beneath ice shelves the view from ice stream b abstract ice shelf development is favored by rapid flow of cold ice from outlet glaciers or ice streams into protected embayments with localized high spots basal melting of ice shelves is rapid near the ice front and may occur near the grounding line ice from outlet glaciers may contain significant englacial debris that is deposited as a dropstone diamicton in regions of basal melting englacial debris is sparse or absent in ice streams evidence from ice stream b draining into the ross ice shelf of west antarctica suggests that the rapid ice velocity arises from deformation of a several meter thick water saturated basal till layer that is eroding an unconformity on sediments beneath and that has deposited a till delta tens of meters thick and tens of kilometers long at the grounding line sea level fall would cause conveyor belt recycling of this till delta and grounding line advance across the ross sea to the edge of the continental shelf forming an ice sheet with a low ice stream profile resting on a several meter thick deforming till layer eroding an unconformity the modern ross sea is characterized by a regional unconformity overlain by a diamicton of probable latest pliocene pleistocene age measuring several meters to tens of meters thick we hypothesize that this diamicton is a deformed glacial till and that the ross sea sediments record one or more expansions of the till lubricated west antarctic ice sheet to the edge of the continental shelf

late quaternary glacial history of george vi sound area west antarctica during the last glacial maximum in west antarctica separate ice caps developed on alexander island and on palmer land became confluent in george vi sound and discharged northward from latitude 72 s radiocarbon 32 000 yr and amino acid approximately 120 000 yr age determinations on shell fragments hiatella solida found in basal till suggest a wisconsin age for the glaciation that incorporated them the pattern of ice flow differed from that deduced for this area in the climap reconstruction following the maximum stage there was a stadial event when outlet valley glaciers flowed from smaller ice caps into george vi sound more widespread recession permitted the george vi ice shelf to deposit palmer land erratics on eastern alexander island before isostatic recovery raised them to final elevations of about 82 m the ice shelf may have been absent at about 6500 yr b p when large barnacles bathylasma corolliforme were living in the sound small glaciers readvanced to form at least two terminal moraines before the ice shelf re formed and incorporated the barnacle shells into its moraine on alexander island the shells gave a 14c age corrected for antarctic conditions of about 6500 yr b p and an amino acid ratio consistent with a holocene age valley glaciers readvanced over the ice shelf moraine before oscillations of both valley glaciers and the ice shelf led to the formation of the present sequence of contiguous ice cored moraines probably during the little ice age such oscillations may represent a climatic control not yet observed in the dry valleys of victoria land the only other part of antarctica studied in detail for glacier fluctuations

late holocene cyclic glaciomarine sedimentation in a subpolar fjord of the south shetland islands antarctica and its paleoceanographic significance sedimentological geochemical and paleontological evidence the glaciomarine sedimentary record of the fjord head collins harbor in maxwell bay south shetland islands west antarctica a large marine calving embayment contains repeating couplets of organic rich massive diamicton and organic poor stratified diamicton the massive diamicton is characterized by high total organic carbon toc content and carbon to nitrogen c n ratios and was deposited in a cold climate regime by iceberg rafted sedimentation from coastal fast ice in which algal plants as well as gravels were entrained the stratified diamicton is characterized by low toc content and c n ratios and was formed in a warmer climate regime when the flux of icebergs was suppressed but turbid meltwater discharge continued to produce lamination when the meltwater discharge decreased in cold climatic conditions and resultant phytoplankton productivity was reduced due to the increased sea ice coverage ice rafting from shorefast sea ice might have played a major role in entraining benthic algae as well as loads of sand and gravel along the coastal area resulting in an increased c n ratio and gravel content in the massive diamicton accelerator mass spectrometry ams radiocarbon analyses conducted on well preserved calcite shells were used to construct a chronology for the past 3000 years fluctuations in toc are recorded approximately four cycles over this time period with the average duration of a cooling cycle being 500 years these cycles may be correlative with the high frequency 550 yr variability in reduced circumpolar deep water cdw on the west antarctic peninsula shelf because a decrease in cdw may be related to reduced deep water production in the north atlantic during colder periods as demonstrated for glacial intervals throughout the pleistocene

carboniferous glaciation in gondwana evidence for grounded marine ice and continental glaciation in southwestern argentina abstract upper paleozoic sedimentary rocks in western argentina accumulated in retro arc foreland basins in the gondwana continental margin a glacial component in these rocks was recognized early in this century but reconstructions of the nature of the glaciation differ markedly the dominant proposal sustains that glaciation in this region was of alpine type with glaciers descending from mountain ranges both inland and along the edge of the continent and that the numerous intercalated diamictites were mostly deposited as subaquatic debris flows only indirectly connected with the presence of glacier ice a stratigraphic and sedimentologic study of middle carboniferous glacigenic deposits of the tepuel group in central western patagonia southwestern argentina shows that two major diamictite facies are present a massive diamictite the more abundant forms regionally extensive and thick

sequence analytical approach to the upper kimmeridgian lower tithonian storm dominated ramp deposits of the boulonnais northern france a landward time equivalent to offshore marine source rocks an interdisciplinary approach based on high resolution physical stratigraphy biostratigraphy organic matter analysis and clay mineralogy has been applied to the mixed siliciclastic carbonate kimmeridgian tithonian deposits of the boulonnais north western france these rocks are the age equivalents of some of the offshore marine source rocks of the north sea and were deposited in an overall storm dominated homoclinal ramp setting which may represent one of the most shore proximal sediments of this age cropping out in north western europe comparison with data from the yorkshire dorset lorraine and aquitaine areas allows the discrimination of three major transgressive regressive sediment packages with disconformities of interregional extent 1 from the baylei zone to top eudoxus zone 2 from the autissiodorensis zone to base scitulus zone 3 from the upper scitulus zone to the pectinatus zone the lower two regionally correlative disconformities correspond to the sequence boundaries at the top of the eudoxus zone and the top of the autissiodorensis zone formed by high frequency relative sea level variations during the periods of maximum transgression of the platform the latter disconformity the p1 nodule bed reflects a major tectonically induced reorganization of the north west european intrashelf sedimentary basin characterized by a shift in both the location of the main depocentre and the sources of sediment production

a systematic classification of glacial and periglacial environments facies and deposits abstract several aspects of the deposits formed in the glacial and periglacial environments are presented in order to establish a framework that can be used in various disciplines dealing with sedimentation under glacial and periglacial conditions the almost unlimited number of different deposits are grouped into 37 types that are all described briefly most of them are also illustrated by field examples mainly from poland these 37 types are grouped within 23 facies belonging to 3 continental glacial 3 continental periglacial and 3 marine periglacial subenvironments all environments subenvironments facies and deposits are defined and presented with a hierarchic code that enables unambiguous definition attention is paid to both the general characteristics of the various deposits as they have been described by previous investigators and to specific deposits that have been studied by the present authors the factors influencing peri glacial sedimentation are discussed briefly

cenozoic prograding sequences of the antarctic continental margin a record of glacio eustatic and tectonic events abstract sedimentary sections up to 6 14 km thick lie beneath many areas of the antarctic continental margin the upper parts of the sections contain up to 6 km of cenozoic glacial and possibly non glacial sequences that have prograded the continental shelf up to 85 km we describe the cenozoic sequences using two general categories based on their acoustic geometries type ia sequences which account for most prograding of the antarctic continental shelf have complex sigmoidal geometries and some acoustic characteristics atypical of low latitude margins such as troughs and mounds lying parallel and normal to the shelf edge and high velocities 2 0 2 6 km s for flat layers within 150 m of the seafloor type iia sequences which principally aggrade the paleoshelf lie beneath type ia sequences and have mostly simple geometries and gently dipping reflections the prograding sequences are commonly located near the seaward edges of major mesozoic and older margin structures relatively rapid cenozoic subsidence has occured due to the probable rifting in the ross sea thermal subsidence in the antarctic peninsula and isostatic crustal flexure in wilkes land in prydz bay and the weddell sea prograding sequences cover mesozoic basins that have undergone little apparent cenozoic tectonism grounded ice sheets are viewed by us and others as the principal mechanism for depositing the antarctic prograding sequences during the initial advance of grounded ice the continental shelf is flexurally overdeepened the inner shelf is heavily eroded and gently dipping glacial strata are deposited on the shelf i e type iia sequences the overdeepened shelf profile is preserved a during glacial times by grounded ice sheets episodically crossing the shelf eroding sediments from onshore and inner shelf areas and depositing sediments at the front of the ice sheet as outer shelf topset banks and continental slope foreset aprons i e type ia sequences and b during interglacial times like today by little or no clastic sedimentation on the continental shelf other than beneath retreated ice shelves lying far from the continental sheld edge ice streams carve broad depressions across the shelf and carry abundant basal sediments directly to the continental shelf edge thereby creating troughmouth fans and sheet like prograding sequences i e type ia sequences numerous acoustic unconformities and multiple overcompacted layers within the prograding sequences suggest major fluctuations of the antarctic ice sheet the available drilling and seismic interpretations provide the following history 1 cenozoic ice sheets have existed in places near the continental shelf since middle to late eocene time 2 a grounded antarctic ice sheet first expanded to the continental shelf edge with probable overdeepening of the outer shelf in late eucene to early oligocene time in prydz bay possibly in early miocene time in the ross sea and at least by middle miocene time in the weddell sea 3 the relative amounts of shelf prograding and inferred ice volume variations and related sea level changes have increased since middle to late miocene time in the eastern ross sea prydz bay and possibly weddell sea our analysis is preliminary further acoustic surveys and scientific drilling are needed to resolve the proximal antarctic record of glacio eustatic climatic and tectonic events recorded by the prograding sequences

the vendian succession of northeastern spitsbergen petrogenesis of a dolomite tillite association abstract the sedimentary record of late precambrian time is magnificently displayed in the highland snowfields of northeastern spitsbergen svalbard vendian strata are represented essentially by the polarisbreen group which consists mostly of dolostone and includes two dolomitic glacial units the oldest sediments in the polarisbreen group compose the elbobreen formation c 400 m which is divided into four laterally persistent members the lower carbonate member e1 125 m contains a distinctive basal dark grey limestone with microspar filled synaeresis cracks suggested to be of lagoonal origin and associated with minor dolostone shale and chert higher parts of the member are dominantly dolostone partly stromatolitic with some shale and sandstone shallow subtidal to intertidal deposition is indicated by the dominance of intraclastic lithologies and relics of anhydrite penecontemporaneous dolomite is partially overprinted by microsparry dolomite thought to be of groundwater origin the redefined petrovbreen member e2 consists of diamictite and other detrital dolostone pronounced thickness variations 2 40 m are thought to be original depositional features the member represents the deposits of a short glacial period in which the following depositional processes are inferred lodgement massive diamictite subaqueous meltout massive and bedded diamictite ice rafting lithologies bearing dropstones and possibly also diamictite redeposition by sediment gravity flows some diamictite and conglomerate rhythmite and shale current winnowing thin tabular conglomerate subaerial or subaqueous meltwater action channelled conglomerate and sandstone periglacial shrinkage diamictite wedge fillings the macdonaldryggen member e3 230 m is a monotonous succession of shaly dolostone of lagoonal origin it grades up into the slangen member e4 25 m which consists of subtidal to intertidal dolarenite with anhydrite relics succeeded by fenestral dolostone that was fractured and cemented by saline groundwaters in an emergent environment the wilsonbreen formation 160 m represents a return to glacial deposition but this time longer lasting and with substantial extra basinal material represented the gropbreen member w1 28 107 m and the ormen member w3 44 139 m consist dominantly of dolomitic diamictite with subordinate conglomerate and sandstone and are separated by a middle carbonate member w2 3 30 m which contains distinctive rhythmitic and stromatolitic limestone as well as sandstone the same depositional processes can be recognised as in the petrovbreen member but the wilsonbreen formation is overall of somewhat more continental aspect lower proportion of rhythmite and dropstone structures in addition there are basal breccia and crack fillings formed by frost shattering of the underlying cemented dolostone tabular sandstone thought to be formed by wave reworking of outwash a striated terrestrial cobble pavement supraglacially derived breccia horizons periglacial wedges filled by sand and the w2 assemblage of possible lacustrine origin the dracoisen formation 525 m represents an abrupt return to non glacial conditions an upward deepening wave dominated succession of pure dolostone d1 20 m and impure dolostone d2 105 m is succeeded by offshore black shale d3 150 m and then by a very shallow water succession of evaporite lacustrine aspect with a dolostone containing evaporite relics d5 10 m separating dolomitic sandstone and shale d4 80 m and d6 150 m the contact with the transgressive cambrian sandstones above is poorly exposed deposition of the succession dominantly under marine conditions is inferred but it is difficult to rule out a lacustrine environment at any particular horizon this dolomite tillite association can be explained by penecontemporaneous and minor secondary dolomite formation in marginal environments with warm climatic indicators at some levels being sharply interrupted because of rapid climatic changes by glacial sediments containing abundant detrital dolomite since the latter sediments make up only 17 of the 1080m thick succession glacial conditions only occupied a small proportion of vendian time

late weichselian and holocene sediment flux and sedimentation rates in andfjord and vagsfjord north norway late weichselian and holocene sediment flux and sedimentation rates in a continental shelf trough andfjord and its inshore continuation vagsfjord north norway have been analysed the study is based on sediment cores and high resolution acoustic data andfjord was deglaciated between 14 6 and 13 14c kyr bp 17 5 and 15 6 calibrated cal kyr bp the vagsfjord basin before 12 5 14c kyr bp 14 7 cal kyr bp and the heads of the inner tributary fjords about 9 7 14c kyr bp 11 2 cal kyr bp in andfjord five seismostratigraphical units are correlated to a radiocarbon dated lithostratigraphy three seismostratigraphical units are recognised in vagsfjord a total volume of 23 km3 post glacial glacimarine and marine sediments was mapped in the study area of which 80 are of late weichselian origin sedimentation rates in outer andfjord indicate reduced sediment accumulation with increasing distance from the ice margin the late weichselian sediment flux and sedimentation rates are significantly higher in vagsfjord than andfjord basin morphology the position of the ice front and the timing of deglaciation are assumed to be the reasons for this late weichselian sedimentation rates in andfjord and vagsfjord are comparable to modern subpolar glacimarine environments of greenland baffin island and spitsbergen downwasting of the fennoscandian ice sheet and winnowing of the banks owing to the full introduction of the norwegian current caused very high sedimentation rates in parts of the andfjord trough at the late weichselian holocene boundary holocene sediment flux and sedimentation rates in andfjord are about half the amount found in vagsfjord and about one tenth the amount of late weichselian values a strong bottom current system established at the late weichselian holocene boundary caused erosion of the late weichselian sediments and an asymmetric holocene sediment distribution copyright 2002 john wiley sons ltd

kimmeridge clay sedimentation a new cyclic model abstract the kimmeridgian upper jurassic fine clastic sequences of the boulonnais northern france and dorset southern england represent oxygen controlled environments five facies types with increasingly deteriorating living conditions ranging from normal shelf environments to submarine lifeless desert have been distinguished the widespread occurrence of this facies in the late jurassic north atlantic shelf sea and the palaeogeographic situation require a large scale model to explain the kimmeridge clay sedimentation anoxic conditions are generally explained by one of three models none of which applies directly to the kimmeridge clay a new model derived from the overall palaeogeographic and palaeoclimatic situation is therefore proposed in this model water mass stratification and anoxic bottom conditions refer to a high latitude monsoonal type wind circulation causing a water current and counter current system in the north atlantic shelf sea and the arctic ocean variations in the amount of oxygen in the bottom waters allow recognition of four different cycles with periods ranging from seasonal to 3 107 years they influenced the counter current system and the resulting anoxic conditions

the vaca muerta quintuco system tithonian valanginian in the neuquen basin argentina a view from the outcrops in the chos malal fold and thrust belt the vaca muerta quintuco system uppermost lower tithonian lower valanginian is a thick shallowing upward sedimentary cycle consisting of dark bituminous shales marlstones limestones and sandstones cropping out in the neuquen basin west central argentina this paper analyzes three outcrop sections in chos malal area northem neuquen province detailed facies analysis allows us to differentiate six facies associations representing basinal to proximal outer ramp facies of a homoclinal carbonate ramp system vaca muerta formation and basinal to shoreface facies of a mixed carbonate siliciclastic shelf system quintuco formation prograding westward from the eastern margin of the basin two sequence hierarchies were recognized 5 composite depositional sequences third order and 15 high frequency sequences fourth order fluctuations in organic matter content within the vaca muerta formation suggest a relationship with depositional sequences finding the highest values associated with transgressive systems tract whereas the transition to the quintuco formation shows a strong decrease in total organic carbon the x ray diffraction studies show an increase of clay minerals and quartz in the transgressive systems tract of the vaca muerta formation and an increase in the content of calcite in highstand systems tracts this pattern is reversed in the quintuco formation our sequence stratigraphic approach contributes to the understanding of the relationship between organic matter clay minerals facies stacking pattern and relative sea level changes in this exceptional shale oil and shale gas unconventional reservoir this study may be helpful for a better postulate of petrophysical and geomechanical models for unconventional exploration

glacial marine sedimentation and quaternary glacial history of marguerite bay antarctic peninsula abstract marguerite bay situated between the subpolar glacial regime of the northern antarctic peninsula and the polar glacial regime of west antarctica is ideally located to test various models of glacial and glacial marine sedimentation and specific scenarios of late wisconsin ice sheet expansion piston cores and single channel seismic reflection data were collected during the deep freeze 85 and 86 expeditions to determine the late quaternary history of the area seismic data in the bay show a rugged seafloor with numerous deep troughs and a very thin layer of sediment over crystalline basement or older sediments glacial erosion is important in modifying existing features although the ultimate repository of the eroded material is not known it is not found within the bay the piston cores are topped by diatomaceous muds which are underlain by terrigenous muds and muddy gravels that imply deposition beneath an ice shelf basal tills were penetrated at three sites reflecting deposition by a grounded marine ice sheet a reconstruction of the glacial history of marguerite bay since the last glacial maximum shows grounded ice filling the bay in late wisconsin time rising sea level caused an uncoupling of the ice sheet and slow retreat of an ice shelf throughout the holocene

sedimentological petrophysical and seismic characterization of an upper jurassic shoreface dominated shelf margin the boulonnais northern france abstract the integration of high resolution 25 cm vertical spacing and continuous quantitative data on grain size carbonate content porosity density and p wave velocity from outcrop borehole and core over 1000 samples and a real seismic section allows a calibration of seismic sequence stratigraphic principles this approach has been tested in the boulonnais along the english channel coast in northern france on a succession of upper jurassic sharp based shoreface deposits embedded in offshore marine shales statistical analyses of grain size distributions allow a characterization of hydrodynamic conditions and depositional processes affecting low inclination shelf margins and create a framework for identifying locations of preferential and enhanced carbonate cementation which exerts primary control on reservoir and acoustic properties stratigraphic surfaces and systems tracts are quantified for physical properties and results in a quantitative mechanical sequence stratigraphic model the combination of this model with seismic wavelets and a contrast between this synthetic and a real seismic section shows that key bounding sequence stratigraphic surfaces are in fact discontinuous reflectors because stratal relationships of sharp based shoreface deposits are subtle physical contrasts are generally stratabound and the deposits are typically not very thick the geological recordings of sea level falls and potential reservoirs are easily overlooked or go unseen in seismic sequence stratigraphic analyses by defining key diagnostic seismic criteria for sharp based shoreface deposits this study contributes to the understanding of processes controlling sedimentary fills of epicontinental basins

sequence stratigraphical interpretation of organic facies variations in marine siliciclastic systems general principles and application to the onshore kimmeridge clay formation uk

iron availability as a dominant control on the primary composition and diagenetic overprint of organic matter rich rocks iron is known to stimulate surface ocean productivity as well as intervene with bacterially mediated processes of organic matter remineralization during early diagenesis in this paper we examine the influence of iron supply on the geochemistry trace metals 34s organic matter of sedimentary rocks deposited in a clastic dominated marine ramp environment to this end we studied two late jurassic formations of the boulonnais area north france both formations were deposited under quite similar conditions but they differ in the reactive iron supply they received only one of the two formations was affected by the particulate iron shuttle process our results indicate that 1 the iron shuttle may be recorded through concomitant enrichments in p mo as and sb 2 a limited reactive iron supply will allow the sulfurization of organic matter even in a context of moderate productivity thus sulfurization can be a factor favoring a noticeable accumulation of organic matter iron may thus be an important agent in the c cycle

developing sediment geochronologies for high latitude continental shelf deposits a radiochemical approach during january and february of 1986 47 box cores and 55 piston cores were collected from the bransfield strait gerlache strait and marguerite bay antarctica apparent sediment accumulation rates established for 100 yr time scales with excess 210pb chronologies range from 0 08 to 0 34 cm yr in the bransfield strait 0 15 to 0 51 cm yr in gerlache strait and from 0 10 to 0 27 cm yr in marguerite bay for the antarctic peninsula continental shelf 137cs appears to have a limited usefulness in assessing the effects of deep sediment mixing by benthic fauna on seabed excess 210pb profiles because the observed activities for the regions examined are close to counter detection limits conventional 14c ages of surface sediments from the peninsula region range from 1370 to 2820 yr the spatial variations in the conventional 14c surface ages are thought to be influenced predominantly by inputs of older eroded sediment from lateral transport current winnowing mass wasting and or a variable carbon source on a 1000 yr time scale 14c measurements of five piston cores from the bransfield strait yielded sediment accumulation rates ranging from 0 06 to 0 49 cm yr these data support the assertion by nelson 1988 that the excess 210pb sediment accumulation rates range of 0 08 to 0 34 cm yr for this region represent true rates the two sediment accumulation rates of 0 01 and 0 08 cm yr for the gerlache strait are significantly lower than the observed apparent 210pb accumulation rates mean 0 27 0 12 cm yr for this region likewise two piston cores from marguerite bay yielded 14c sediment accumulation rates of 0 02 and 0 05 cm yr which also are considerably lower than the excess 210pb apparent accumulation rates mean 0 17 0 06 cm yr for this area the one dimensional advection diffusion equation of goldberg and koide geochim cosmochim acta vol 51 1962 was solved simulatneously with 210pb and 14c data for bc18 and bc113 to resolved the effects of sediment mixing and accumulation in bransfield strait and marguerite bay box core samples respectively this model indicates that 210pb and 14c sediment accumulation rates agree within a factor of 2 in the bransfield strait whereas in marguerite bay the apparent 210pb accumulation rates may be as much as an order of magnitude too high in the southern high latitude environments of the antarctic peninsula radiocarbon measurements in the seabed are a better approach than 210pb for establishing accurate rates of sediment accumulation the 14c chronologies when coupled with 210pb data provide more reliable assessments of biological deep mixing and sediment accumulation rates than do excess 210pb data coupled with measurements of bomb produced 137cs the sediment accumulation rates for both 100 yr and 1000 yr time scales tend to decrease southward from the bransfield strait to marguerite bay which is consistent with glacial marine sedimentation processes as modulated by climate for the western antarctic peninsula seabed measurements of 210pb 137cs and 14c also provides valuable insights on water circulation and upwelling atmospheric inputs and sediment depositional processes for the antarctic peninsula region

glacigenic and associated marine sediments in the weddell sea fjords of spitsbergen and the barents sea a review abstract studies of glacigenic and associated marine sediments in the weddell sea fjords of spitsbergen and barents sea show that during periods of limited glacial coverage interstadials large amounts of material may be brought into the sea mainly by meltwater the major load is deposited relatively close to the icefront 1 at times of maximum glacial extent grounded ice extends onto the shelf with the shelf edge as the outer limit and land and inner shelf areas are eroded main glacial depositional sites are then the outer shelf and shelf edge the sedimentary environment outside an ice shelf is characterized by low clastic supply mainly deposited from icebergs due to low turbidity and a supply of nutrient rich water biogenic matter accumulates leading to a glaciomarine sequence rich in bioclastics sea level lowering results in grounding and seaward expansion of the ice shelf subsequent eustatic rise causes the grounded ice to float deglaciation and sea level rise reduces clastic input to the shelves glacial sediments are reworked and carbonates may accumulate forming tillite carbonate sequence e g late precambrian break up of marine ice sheets such as the barents sea ice sheet seems to have taken place stepwise in response to eustatic rise and climatic changes

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

glaciogenic banded phosphorites from permian sedimentary rocks numerous bands of lower permian marine sedimentary phosphorite have been discovered in a cyrogenic diamictite as well as in a late glacial fissile shale sequence in the natal part of the great karoo basin in south africa these thin 1 30 cm bands are extensive range in p 2 o 5 content from 3 to 29 percent and average 12 5 percent the bands are presumed to have formed authigenically by direct precipitation of micron sized francolite carbonate fluorapatite microsphorite that is optically pseudoisotropic and contains an average of 1 1 percent co super 2 3 that replaces po super 3 4 in its structure lithic associates include quartz clay minerals and dropstones of varied composition in both the fissile shale and the diamictite the phosphorite bands formed within 60 degrees to 75 degrees south paleolatitude in a restricted epicontinental basin at the onset of a marine transgression following the melting and recession of continental ice sheets the occurrence of phosphorite within the glacial diamictite as well as in the fissile shale sequence in which scattered dropstones indicate continued glacial influence suggest polar to cold climatic conditions during phosphorite deposition phosphogenesis therefore seems to have taken place in an environment which is characteristically different in its climate and latitude setting from those described for major marine sedimentary phosphorite deposits of special interest is the banded morphology which is not characteristic of post cambrian deposits

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

terrestrial or waterlain glacial diamictites three case studies from the late precambrian and late ordovician glacial drifts in west africa abstract three examples of extensive diamictites within glacial sequences well exposed in the sedimentary cover of the west african cratonic platform are described they belong to the late precambrian and upper ordovician glacial periods and the paleogeography is summarized the late precambrian diamictites of the jbeliat group in adrar of mauritania displays characteristics of tillite deposited as lodgement or melt out till detail studies on structure clast shape nature and fabric texture in thin section clay mineralogy and carbonate content are given and discussed in order to delimitate useful criteria for tillite identification compared to this first example the two other diamictites from the late precambrian bakoye group in western mali and from the upper ordovician tichit group in southeastern mauritania have obviously been deposited in an aquatic environment waterlain glacial diamictites by the action of ice rafting bottom current and flow processes these examples show that for such ancient deposits sedimentological interpretations are partly solved by broad scale stratigraphical relationships however detailed sedimentological analyses remain essential for precise facies identification especially structural and petrographic studies must be encouraged in the lights of modern glaciodynamic processes

glacimarine processes and inductive lithofacies modelling of ice shelf and tidewater glacier sediments based on quaternary examples abstract three variables control the type of glacial debris source and where and when debris is introduced to an ocean 1 type of glacial ice source ice sheet or valley glacier 2 condition of a grounded glacier melting freezing or frozen base and cold or warm internal ice and 3 type of glacier front ice shelf or tidewater using these variables in different combinations eight glacimarine regimes are postulated to exist by integrating information from holocene sediment and pleistocene glacimarine sequences with the eight inductive regimes lithofacies relationships are predicted and modelled the models show a complexity of glacimarine lithofacies the importance of recognizing this complexity lies in realistic interpretations of ancient sequences ice shelf sediment may be limited in the geologic record because of tectonic and climatic constraints required for ice shelf formation alternatively in some tidewater glacier regimes larger volumes of glacimarine sediment are produced than in ice shelf regimes ice proximal sediment is distinctive especially near subglacial meltwater stream effluxes ice shelves and tidewater glaciers may produce similar ice proximal lithofacies e g morainal bank associations and laminites in certain regimes however details of lithofacies may aid in distinguishing ice shelf from tidewater regimes ice shelf and tidewater iceberg zone sediment can be distinguished from each other if basal and high level englacial debris can be distinguished ice shelf zone sediment out from the grounding line has a basal debris source high level englacial debris is deposited in more distal ice shelf zones resulting in sediment by pass in corresponding positions of a tidewater glacier regime iceberg rafted debris comprises mixed basal and high level englacial debris deeper water zones receive large volumes of sediment when a melting freezing base grounding line is at the edge of a continental shelf when the grounding line retreats sediment supply may be drastically reduced because of a back sloping continental shelf and trapping by morainal banks cool water bioclastic carbonates may accumulate once a grounding line has retreated although ice shelf iceberg zones are more extensive than those in tidewater regimes ice shelf icebergs have low debris content and resulting ice rafted debris may be scarce the smallest volume of sediment is produced by a frozen base ice sheet ending as a tidewater front and the largest volume by a melting freezing base valley tidewater glacier glacimarine sediment has a relatively high preservation potential near continental interiors or margins however at continental margins mass flow olistostrome and tectonic melange deposits may be preserved with glacimarine diamictites these deposits may be difficult to distinguish from each other especially if metamorphosed such as at a convergent margin the models presented do not fully represent the glacimarine environment because many other variables are omitted however the principal concepts of generating glacimarine lithofacies associations are considered continued research will verify if each regime exists and will provide data on those regimes that exist today to compare and modify these models

models of glaciomarine sedimentation and their application to the interpretation of ancient glacial sequences abstract this paper argues that glaciomarine environments can be regarded as special glacially influenced types of continental margin environments e g continental shelf slope rise and basin plain knowledge of the stratigraphic architecture and typical sedimentary sequences of non glacial margins is becoming well known but remains limited for those that have been glacially modified the principal influences on sedimentation in these environments relate to the glacial sediment input controlled by relief of basin margin glacier thermal regime and ice flow dynamics and depositional environments influence of traction currents substrate relief and proximity to nearby ice margins typical ranges of sedimentation rates can be established for glacially influenced continental margin environments and these may provide a framework for ancient sequences starvation of sediment supply to glacially influenced continental margins is common the nature of sub ice shelf sedimentation a model that has been applied to many ancient glacial sequences is critically reviewed the significance of such sedimentation in the rock record has probably been exaggerated because of oversimplistic interpretations of diamictite sequences existing process models of glaciomarine sedimentation derived from study of modern environments are sometimes difficult to employ in investigation of ancient sedimentary sequences because simple lithofacies criteria and typical vertical profiles are not available to aid in interpretation in addition compositional data emphasized by many workers for distinguishing glaciomarine from continental glacial diamict ite s frequently fingerprint sediment source and not mode of deposition the importance of facies analysis methods for isolating depositional environments is illustrated by three examples of ancient glaciomarine sequences these are the early proterozoic gowganda formation 2 3 ga of northern ontario canada the late proterozoic port askaig formation 670 ma of scotland and ireland and the late cenozoic yakataga formation 20 ma to recent of the gulf of alaska these examples illustrate the significance of detailed genetic studies of ancient glacial rocks in the interpretation of palaeogeographic and tectonic settings diamictite units in ancient glaciomarine sequences cannot be easily interpreted in terms of climatic or ice advance retreat cycles because of the varied controls on diamict accumulation and diamictite preservation in marine basins

evidence against a late wisconsinan ice shelf in the gulf of maine abstract proposals for the formation of a late wisconsinan ice shelf in the gulf of maine during the retreat of the laurentide ice sheet are considered to be inappropriate an antarctic type ice shelf does not fit the field data that indicate temperate glacial terrestrial and marine climates for the region between 18 ka and 12 ka a temperate ice shelf has no modern analogues and may be physically impossible the preponderance of stratified drift in the gulf of maine region supports temperate climates during late wisconsinan time it also indicates that glacial meltwater rather than ice in either an ice sheet or ice shelf was the primary transport mechanism of glacial sediment and the source for the glaciomarine mud for these reasons we have proposed glacial analogues for the deglaciation of the gulf of maine that consist of temperate or subpolar marine based glaciers characterized by depositional environments dominated by meltwater discharge directly to the sea or the sea by way of subaerial meltwater streams these analogues include alaskan fjord glaciers glaciers on the alaskan continental shelf that discharged meltwater directly into the sea in the not too distant past and austfonna nordaustandet svalbard norway that is presently discharging meltwater in the sea along a grounded ice wall this last example is the best modern day analogue for the depositional environment for most of the glaciomarine mud in the gulf of maine and deglaciation of the gulf

petroleum geochemistry of the kimmeridge clay of onshore southern and eastern england abstract the kimmeridge clay is considered a major oil source rock for the north sea hydrocarbon province the formation is also developed onshore in an organic rich mudstone facies this paper examines the possibility of onshore oil generation from the kimmeridge clay geochemically onshore basin margin sediments contain rich potential source horizons with mainly type l type ll oil prone kerogen but are immature some deeper cleveland basin sediments have reached marginal maturity burial history reconstruction suggests significant formation palaeoburial depths in central areas of the cleveland and wessex basins computed vitrinite isoreflectance contours show the wealden and isle of wight kimmeridge clay to be thermally mature basin modelling suggests an early palaeogene onset of oil generation in parts of the cleveland basin while maximum oil generation could have been reached by the formation base in the isle of wight area during the late cretaceous although basin subsidence ceased in the neogene in the weald and isle of wight where the formation is still deeply buried oil generation probably continued for some time during uplift thus significant quantities of oil could have been generated whether or not this oil is present today however would depend on the correct timing of suitable migration and trap structures

the influence of a sudden climatic change on marine deposition in the kimmeridgian of northwest europe a sudden change from humid style to semi arid style deposition markedly affected the accumulation of the upper kimmeridge clay in southern england many of the changes appear to be related to a change in sedimentation rate at this time thus softground faunas are replaced by firmground faunas diagenetic dolostones formed in the methanogenic zone are replaced by sulphate reduction zone carbonate nodules and depositional gradients recorded by lateral biofacies changes becomes steeper the evidence available is in accord with a decline in offshore sedimentation rates during this interval other changes such as a decline in kaolinite abundance were more directly controlled by the drying out of the hinterland similar changes elsewhere in the marine geological record could be used as climatic indicators the climatic change is part of a wider northern hemisphere dry event which affect a broad area in the late jurassic the kimmeridge clay of southern england was one of the last depositional environments to be influenced by the climatic change at this latitude

influence of experimental temperature and duration of laboratory confined thermal maturation experiments on the evolution of the porosity of organic rich source rocks abstract to investigate the influence of the duration temperature pairs chosen for laboratory thermal maturations on the evolution of the organic matter om and porosity of the clay rich source rocks with increasing maturity long duration 104 days thermal maturations bulk molecular geochemical om characterization rock eval gc tcd gc ms and nitrogen adsorption porosimetry were applied on six type ii mudstones from the kimmeridge clay formation these results were compared with shorter duration experiments 72 h previously carried out the results show that the increase in the duration of thermal maturation experiments enhanced the degradation of heavy polar om components into saturated and aromatic hydrocarbons leading to the production of similar amounts of extractible om eom enriched in saturated and aromatic hydrocarbons then in preventing the early breaking of low energy bonds the lower temperature used for the longer duration experiments led to a less pronounced early cracking of om resulting in a different timing of gas generation this process did not influence the general evolution of the pore volume which increased during gas generation whatever the duration temperature pair chosen for maturation experiments highly oil prone samples formed lower pore volumes during gas generation increasing the duration of thermal maturation seemed nevertheless to influence the amplitude and the timing of pore alteration events that occur during gas generation leading to slightly different pore size distributions and pore volumes between short and long duration experiments however these differences remain limited suggesting that results from different time duration experiments can be easily compared

identification of log units in clay rock formations based on local and spatial statistics of well log properties application to the opalinus claystone in the benken borehole

glacial geology of outer bjornoyrenna southwestern barents sea the upper cenozoic glacigenic succession in outer bjornoyrenna bear island trough southwestern barents sea is investigated utilizing data from six shallow boreholes 143 m below sea bed and seismic profiling overdeepened troughs in the lowermost part of a large plio pleistocene wedge at the shelf margin are interpreted as glacial erosion forms this suggests that the overlying succession which basinward attains a thickness of several km was deposited during a period with glaciations large scale possible sand ridges in the stratigraphic lower part of this succession are confined to two north south trending zones parallel to the present shelf break they probably formed in a continental shelf coastal environment with exceptionally high sediment supply and fluctuating relative sea level related to glaciations a core from a channel fill in the lower part of the succession consists of possibly glaciofluvial gravelly sand intercalated with till most of the other cores are dominated by middle and upper pleistocene muddy massive partly fissile diamictons interpreted to be mainly tills undrained shear strength of these cores ranges from about 20 kn m 2 to more than 300 kn m 2 and both underconsolidated intervals compared to the present burial depth are found an underconsolidated interval comprising eemian interglacial biourbated marine sediments overlain by laminated weichselian glaciomarine sediments on the outher shelf is covered by 50 100 m of muddy diamicton this diamicton is suggested to be mainly till deposited by a late weichselian ice sheet which was grounded to more than 500 m below present sea level earlier weichselian ice expansions did not reach the outer shelf based on the stratigraphy in the shallow cores and seismic interpretation we suggest that there have been five major expansions of grounded ice to the shelf break since ca 440 ka the present study suggests that glacial erosion was a major process in the late cenozoic denudation of the barents sea and in large areas perhaps as much as 200 250 m of preexisting strata could have been eroded by glaciers since about 440 ka

glaciomarine sediments along the filchner ronne ice shelf southern weddell sea first results of the 1983 84 antarktis ii 4 expedition abstract sediment sampling with box corer and gravity corer was conducted along a profile parallel to the filchner ronne ice shelf from 48 to 61 w twenty two sampling locations were determined after evaluation of 12 and 3 5 khz sub bottom profiling records the sediment retrievals show a wide diversity varying from very well sorted pure sands in the se of the profile to heavily glacially influenced pebbly muds close to the foot of the antarctic peninsula in the middle part of the profile mainly soft sediments of muddy to sandy muds were found which were partially influenced by glacially derived dropstones or accumulations of pebble sized material the striking changes of surface sediments marine to glacial observed along the profile led to an attempt to investigate the concurrence of marine and glacial depositional processes controlling the accumulation of these recent sediments

seawater redox variations during the deposition of the kimmeridge clay formation united kingdom upper jurassic evidence from molybdenum isotopes and trace metal ratios 1 the kimmeridge clay formation kcf and its equivalents worldwide represent one of the most prolonged periods of organic carbon accumulation of the mesozoic in this study we use the molybdenum mo stable isotope system in conjunction with a range of trace metal paleoredox proxies to assess how seawater redox varied both locally and globally during the deposition of the kcf facies with lower organic carbon contents toc 1 7 wt were deposited under mildly reducing suboxic conditions while organic rich facies toc 7 wt accumulated under more strongly reducing anoxic or euxinic local conditions trace metal abundances are closely linked to toc content suggesting that the intensity of reducing conditions varied repeatedly during the deposition of the kcf and may have been related to orbitally controlled climate changes long term variations in 98 95mo are associated with the formation of organic rich intervals and are related to third order fluctuations in relative sea level differences in the mean 98 95mo composition of the organic rich intervals suggest that the global distribution of reducing conditions was more extensive during the deposition of the pectinatites wheatleyensis and lower pectinatites hudlestoni zones than during the deposition of the upper pectinatites hudlestoni and pectinatites pectinatus zones the global extent of reducing conditions during the kimmerigidan was greater than today but was less widespread than during the toarcian early jurassic oceanic anoxic event this study also demonstrates that the mo isotope system in jurassic seawater responded to changes in redox conditions in a manner consistent with its behavior in present day sedimentary environments

recurring patterns of total organic carbon and source rock quality within a sequence stratigraphic framework sequence stratigraphy has been combined with total organic carbon toc analysis from well logs core and cuttings to develop a model of toc accumulation in marine source rocks routine well profiling of toc in marine source rocks has revealed that recurrent patterns in the vertical distribution of toc occur that can be explained with sequence stratigraphic concepts the model assumes that the principle controls on organic carbon accumulation are 1 the presence of low oxygen conditions at the sediment water interface and 2 sedimentation rate under these circumstances toc will vary inversely with terrigenous clastic influx if accommodation is high clastic influx is low in distal locations and the resulting weight percent toc in basinal sediments is increased th maximum toc in a vertical profile through a single marine sequence probably correlates to the maximum flooding surface toc decreases above this surface due to increasing sediment dilution during highstand progradation and decreases below it with the higher sedimentation rate of the older transgressive systems tract commensurate with toc increase towards the maximum flooding surface the organic matter type becomes more marine and therefore increasingly oil prone several marine source rock examples are presented as well as a lacustrine example which also conforms to the marine model

deciphering the history of climate and sea level in the kimmeridgian deposits of bure eastern paris basin abstract an integrated stratigraphic study was conducted on a kimmeridgian succession of 3rd order cycles including marls and limestones in a shelf context at bure paris basin the study was possible due to the exceptional opportunity provided by well boring activities related to the construction of an underground laboratory of the french national agency for radioactive waste management andra studies of macrofossils microfossils sedimentology clay mineralogy isotopic composition c o of shells and organic molecular geochemistry lead to a detailed description of the stratigraphic column which allows us to address the history of sea level and climate changes six transgressive regressive cycles are recognised in the studied kimmeridgian succession in these 3rd order cycles the deepest environments are systematically represented by marls and organic rich sediments whilst the shallowest are represented by limestones these cycles do not correspond to changes in temperatures or carbonate production rates at a regional or global scale on the contrary long term palaeontological sedimentological and geochemical changes during the early kimmeridgian are interpreted as climatically induced these climatic changes are considered as responsible for bringing significant granular carbonate production to an end in contrast to carbonate mud that was deposited in alternation with marls throughout the late kimmeridgian

astrochronology of the late jurassic kimmeridge clay dorset england and implications for earth system processes the late jurassic kimmeridge clay formation kcf is an economically important organic rich source rock of kimmeridgian early tithonian age the main rock types of the kcf in dorset uk include grey to black laminated shale marl coccolithic limestone and dolostone which occur with an obvious cyclicity at astronomical timescales in this study we examine two high resolution borehole records swanworth quarry i and metherhills 1 obtained as part of a rapid global geological events rgge sediment drilling project datasets examined were total organic carbon toc and borehole wall microconductivity by formation microscanner fms our intent is to assess the rhythmicity of the kcf with respect to the astronomical timescale and to discuss the results with respect to other key late jurassic geological processes power spectra of the untuned data reveal a hierarchy of cycles throughout the kcf with similar to 167 m similar to 40 m 9 1 m 3 8 m and 1 6 m wavelengths tuning the similar to 40 m cycles to the 405 kyr eccentricity cycle shows the presence of all the astronomical parameters eccentricity obliquity and precession index in particular similar to 100 kyr and 405 kyr eccentricity cycles are strongly expressed in both records the 405 kyr eccentricity cycle corresponds to relative sea level changes inferred from sequence stratigraphy intervals with elevated toc are associated with strong obliquity forcing the 405 kyr tuned duration of the lower kcf kimmeridgian stage is 3 47 myr and the upper kcf early part of the tithonian stage elegans to fittoni ammonite zones is 3 32 myr two other chronologies test the consistency of this age model by tuning similar to 8 10 m cycles to 100 kyr short eccentricity and similar to 3 5 m cycles to 36 kyr jurassic obliquity the obliquity tuned chronology resolves an accumulation history for the kcf with a variation that strongly resembles that of earth s orbital eccentricity predicted for 147 2 ma to 153 8 ma there is evidence for significant non deposition up to i million years in the lowermost kcf baylei mutabilis zones which would indicate a kimmeridgian oxfordian boundary age of 154 8 ma this absolute calibration allows assignment of precise numerical ages to zonal boundaries sequence surfaces and polarity chrons of the lower m sequence c 2009 elsevier b v all rights reserved

organic facies variation in the late kimmeridgian of the boulonnais area northernmost france recent studies of the upper kimmeridgian of the boulonnais area northernmost france have provided a sequence stratigraphical framework that is here used to help interpret variations in sedimentary organic matter om content and composition in response to fluctuations in relative sea level the organic facies are characterised using a combination of palynofacies bulk organic geochemistry toc rock eval pyrolysis and sulphur data and the particle size distribution of the total palynological residues the organic facies show a good correlation with the sequence stratigraphy exhibiting peak toc hi total s and sorg values in the lithofacies deposited around the two mfs the palynofacies in these intervals is characterised by a high content of fluorescent orange marine aom there is a strong correlation between the orange aom and the total and organic sulphur contents similar positive correlations between orange aom and organic sulphur have previously been observed in the distal organic rich sediments of the kimmeridge clay formation of dorset lateral time equivalents of the boulonnais facies but there the sulphur contents are lower indicating that organic sulphur content of orange aom is not fixed suggestive of preservation related parallel but not intrinsically related trends the s data can be used to estimate an apparent burial efficiency and hence the proportion of the primary productivity preserved this allows a rough estimate of palaeoproductivity model calculations suggest that for a sulphide retention of 30 70 and uncompacted sedimentation rate estimates in the range 5 10 cm ka the mean palaeoproductivity was in the range 52 175 gc m2 a up to 6 of which was preserved thus if the preservation is high as is the case for mfs the palaeoproductivity does not need to be above average for shelf waters to produce a given toc

enhanced trapping of molybdenum by sulfurized marine organic matter of marine origin in mesozoic limestones and shales there have been many studies devoted to trace metals and their value in assessing the paleoredox conditions of ancient marine deposition among them molybdenum mo is frequently cited as an effective proxy for sediments and sedimentary rocks recently helz et al helz g r miller c v charnock j m mosselmans j l w pattrick r a d garner c d vaughan d j 1996 mechanisms of molybdenum removal from the sea and its concentration in black shales exafs evidences geochim cosmochim acta 60 3631 3642 and adelson et al adelson j m helz g r miller c v 2001 reconstructing the rise of recent coastal anoxia molybdenum in chesapeake bay sediments geochim cosmochim acta 65 237 252 suggested that mo does not behave conservatively in the water column when h2s reaches a threshold concentration above this concentration a switch operates and mo is scavenged by forming bonds with metal rich notably iron particles sulfur rich organic molecules and pyrite in this paper mo trapping by sulfur rich organic matter om in ancient marine deposits is emphasized the following mesozoic geological formations were selected for study because of their relatively high concentration of sulfurized om the akkuyu formation turkey the calcaires d orbagnoux france and kimmeridge clay uk and its timeequivalent in boulonnais france the kashpir oil shales russia and the la luna formation venezuela the sulfur rich om is identified by either measured organic s abundance or kerogen microscope observation our results show that mo is systematically more enriched relative to the other redox sensitive sulfide forming elements studied u v ni cu zn cr and mo enrichment is positively correlated with the amount of sulfurized om but not with pyrite abundance these results illuminate the role played by sulfurized om in geologic scale mo capture and retention but they also underline the role played by reactive iron significant om sulfurization is only possible when reactive iron is limited nevertheless pyrite formation though limited could act as an initial mo trap prior to mo uptake by om that is sulfurized after the pyritization step in future paleoenvironmental reconstructions attention must be paid to this enhanced mo enrichment in the presence of sulfurized organic matter in such cases the use of mo could lead to overestimation of the reducing conditions of the depositional environment

sedimentation on the ross sea continental shelf antarctica abstract glacial marine sedimentation has prevailed throughout most of the early glacial history of the ross sea region however since late miocene to early pliocene time the west antarctic ice sheet has advanced onto the continental shelf perhaps several times glacial troughs of the western ross sea may also have formed at this time these glacial troughs border outlet glaciers of the east antarctic ice sheet a widespread lodgement till facies was deposited during this glacial advance holocene retreat of this expanded ice sheet left no conspicuous meltwater record so decoupling of the ice sheet from the seafloor rather than rapid melting is implied modern glacial sediment is far less prevalent than biogenic sediment and reworked glacial sediment except in coastal areas adjacent to small ice shelves and ice tongues little or no debris is being transported beyond the calving line of the ross ice shelf siliceous mud and ooze occurs in the western ross sea where surface circulation is most vigorous and warm deep waters up well this siliceous sediment along with terrigenous silt and clay blankets the seafloor below approximately 300 m water depth most of the ross sea continental shelf is deeper than this surface sediments on shallow banks and coastal platforms have clearly been reworked by marine currents the most vigorous bottom current activity occurs in those areas where geostrophic currents impinge onto the shelf debris flows and turbidity currents are also important in eroding and redistributing glacial sediments on the shelf terrigenous silt and clay are more abundant in surface sediment than basal till the source of this silt and clay is problematic the most plausible sources being onshore transport of suspended sediment by impinging currents and subglacial meltwater input

linked sequence stratigraphy and tectonics in the sichuan continental foreland basin upper triassic xujiahe formation southwest china abstract intracontinental subduction of the south china block below the north china block in the late triassic resulted in formation of the transpressional sichuan foreland basin on the south china block the upper triassic xujiahe formation was deposited in this basin and consists of an eastward tapering wedge of predominantly continental siliciclastic sedimentary rocks that are up to 3 5 km thick in the western foredeep depocenter and thin onto the forebulge and into backbulge depocenters five facies associations a e make up the xujiahe formation and these are interpreted respectively as alluvial fan transverse and longitudinal braided river meandering river overbank or shallow lacustrine and deltaic deposits this study establishes a sequence stratigraphic framework for the xujiahe formation which is subdivided into four sequences sq1 2 3 and 4 sequence boundaries are recognized on the basis of facies tract dislocations and associated fluvial rejuvenation and incision and systems tracts are identified based on their constituent facies associations and changes in architectural style and sediment body geometries typical sequences consist of early to late transgressive systems tract deposits related to a progressive increase in accommodation and represented by facies associations a b and c that grade upwards into facies association d regionally extensive and vertically stacked coal seams define maximum accommodation and are overlain by early highstand systems tract deposits represented by facies associations d e and c late highstand systems tract deposits are rare because of erosion below sequence boundaries sequence development in the xujiahe formation is attributed to active and quiescent phases of thrust loading events and is closely related to the tectonic evolution of the basin the sichuan basin experienced three periods of thrust loading and lithospheric flexure sq1 lower sq2 and sq3 two periods of stress relaxation and basin widening upper sq 2 and sq3 and one phase of isostatic rebound sq4 paleogeographic reconstruction of the sichuan basin in the late triassic indicates that the longmen mountains to the west consisting of metamorphic sedimentary and pre neoproterozoic basement granitoid rocks was the major source of sediment to the foredeep depocenter subordinate sediment sources were the xuefeng mountains to the east to backbulge depocenters and the micang mountains to the northwest during the late history of the basin this study has demonstrated the viability of sequence stratigraphic analysis in continental successions in a foreland basin and the influence of thrust loading on sequence development

an integrated and quantitative approach to petrophysical heterogeneity exploration in anything but the simplest of reservoirs is commonly more challenging because of the intrinsic variability in rock properties and geological characteristics that occur at all scales of observation and measurement this variability which often leads to a degree of unpredictability is commonly referred to as heterogeneity but rarely is this term defined although it is widely stated that heterogeneities are poorly understood researchers have started to investigate the quantification of various heterogeneities and the concept of heterogeneity as a scale dependent descriptor in reservoir characterization based on a comprehensive literature review we define heterogeneity as the variability of an individual or combination of properties within a specified space and or time and at a specified scale when investigating variability the type of heterogeneity should be defined in terms of grain e pore components and the presence or absence of any dominant features including sedimentological characteristics and fractures hierarchies of geologic heterogeneity can be used alongside an understanding of measurement principles and volumes of investigation to ensure we understand the variability in a dataset basic statistics can be used to characterise variability in a dataset in terms of the amplitude and frequency of variations present a better approach involves heterogeneity measures since these can provide a single value for quantifying the variability and provide the ability to compare this variability between different datasets tools measurements and reservoirs we use synthetic and subsurface datasets to investigate the application of the lorenz coefficient dykstraeparsons coefficient and the coefficient of variation to petrophysical data e testing assumptions and refining classifications of heterogeneity based on these measures 2015 the authors published by elsevier ltd this is an open access article under the cc by license

milankovitch cyclicity in purbeck peritidal limestones of the prebetic berriasian southern spain peritidal carbonate rinks purbeck facies of the uppermost portlandian to berriasian in the type section of the sierra del pozo formation in the prebetic zone southern spain are divisible into 141 shallowing upward cycles averaging 2 m in thickness the subtidal facies in these cycles consist of micritic or marly limestones with dasycladacean algae and lituolids the intertidal facies are micritic limestones containing birdseyes and miliolids the supratidal facies comprise laminated algal limestones the tops of which display desiccation cracks and rhizocretions or more locally palaeosols calcretes or palaeokarst surfaces a statistical study using power spectra of the fast fourier transform demonstrates that the periodicity of these cycles is in the milankovitch frequency band

marine controls on modern sedimentation on the antarctic continental shelf abstract the extreme depth and rugged topography of the antarctic continental shelf its seasonal to perennial sea ice cover and its large slowly calving ice shelves exert major influences on the adjacent ocean and sediments regionally variable exchanges of heat and momentum between the atmosphere and ocean result in net off shelf transport of sea ice and net downward salt flux this buoyancy partitioning drives a vertical convection that is followed by westward and northward flows of ventilated dense water across the continental shelves shelf waters meet mix with and modify circumpolar deep water at a dynamic slope front near the continental shelf break locally or periodically forming antarctic bottom water nutrient rich deep water upwells along this front regionally intruding onto the continental shelf in long thermally attenuated filaments these intrusions are frequently linked to mass balance processes at the base of ice tongues and ice shelves melting freezing and tidal currents beneath glacial ice are related to the larger scale circulation and to the fate of basal debris in the ice sheet the ocean circulation near ice fronts and decadal variability in ice front positions may account for several features of the sediment record time series 1 yr current measurements in the ross and weddell seas have revealed moderate to strong currents that are seasonally variable vertically coherent and dominated by the diurnal tide and longer term events few long term current measurements exist in the shelf and slope bottom boundary layers that are most important to sediment deposition and erosion icebergs may frequent certain routes e g in association with currents and fronts near the edge of the continental shelf and are capable of scouring and resuspending sediments down to shelf break depths sea ice plays a role in shelf sedimentation by rafting eolian and bottom debris away from the nearshore regions in addition generally offshore winds produce numerous leads and polynyas along the coastline and within the sea ice fields higher biological productivity in these polynyas may be linked to higher local sedimentation rates biogenic marine sediments apparently comprise a major percentage of the regionally variable modern shelf deposits despite the impressive glacial setting and polar environment