ichnological characteristics of late cretaceous hemipelagic and pelagic sediments in a submarine high around the oae 2 event a case from the rybie section polish carpathians abstract the trace fossils alcyonidiopsis isp chondrites isp small and large palaeophycus isp planolites isp taenidium isp thalassinoides isp zoophycos isp and unnamed small spreite burrow structures have been found in the cenomanian turonian sediments in the rybie section they show significant stratigraphic variations in respect to the bonarelli level in the cenomanian from the base of the section up to just below the bonarelli level the fucoid marl in local lithostratigraphy a decrease in diversity and abundance of trace fossils and benthic foraminifers is present and related mainly to decreasing benthic food levels oxygenation changes have comparatively minor incidence but at least three thin anoxic layers occur about 2 m below the bonarelli level the bonarelli level shows the at least episodically anoxic sedimentation in the turonian part only small chondrites isp and planolites isp rarely occur the burial of organic matter and probably benthic food content was extremely low as revealed by red marls a gradual recovery of environmental conditions is marked by an increase in the diversity and abundance of trace fossils and benthic foraminifers by the turonian when grey marls egocina marl accumulated above the red marls the trends in ichnological record are confirmed by data on benthic foraminifers

deep channels in the cenomanian danian chalk group of the german north sea sector evidence of strong constructional and erosional bottom currents and effect on reservoir quality distribution reflection seismic lines from the schillgrund high in the german north sea sector show that the sea floor deposition of the upper cretaceous danian chalk group was extremely irregular the cenomanian campanian part of the group is up to 600 m 1968 ft thick and is dominated by pelagic relatively horizontally deposited chalk it was tilted toward the southwest close to the campanian maastrichtian boundary time and was overlain by up to 800 m 2625 ft of onlapping maastrichtian danian chalk that was eroded and reworked by powerful synsedimentary bottom currents a north south to north northwest south southeast trending slope parallel multistorey channel system branching toward the northwest into smaller laterally directed arms was developed in maastrichtian danian times it was more than 200 km 124 mi long up to 15 km wide 9 mi and 200 m 656 ft deep the main channel system was eroded by powerful contour parallel bottom currents interpreted as having flowed toward the southeast based on the depositional geometry of the channel fill successive channel axes were systematically shifted downslope leaving a series of slightly offset nested slope parallel channels the channels were filled laterally obliquely or longitudinally by chalk drifts sediment gravity flows were a factor locally but were not integral parts of the depositional system erosion winnowing sorting and redeposition by the bottom currents are interpreted to have resulted in channel parallel porosity permeability zones the seismic data indicate that the chalk adjacent to the main channels retained high porosity calculated from logs in an exploration well drilled in 2005 the chalk is of excellent reservoir quality with effective porosity averaging 37 reaching 50 and some levels are completely unconsolidated the influence of powerful syndepositional bottom currents on the anisotropy of reservoir properties of chalk is only now beginning to be realized chalk having traditionally been considered a monotonous and uniform rock type

laminated bioturbated cycles in maastrichtian chalk of the north sea oxygenation fluctuations within the milankovitch frequency band the maastrichtian chalk of the southern central graben danish north sea is a homogeneous pure white coccolithic chalk mudstone deposited in a deep epeiric shelf sea which covered large parts of northern europe the sediment displays a pronounced cyclicity marked by decimetre thick bioturbated beds alternating with slightly thinner non bioturbated mainly laminated beds the laminated half cycles consist of alternating millimetre thick graded high porosity laminae and non graded low porosity laminae the cyclicity has been interpreted previously as caused by periods of slow background sedimentation and bioturbation interrupted by periods of rapid deposition of laminated beds with the latter reflecting random and local resedimentation processes based on textural and structural analysis the millimetre scale non graded laminae are interpreted as having been deposited directly from pelagic rain of pelleted coccoliths representing the primary production the graded laminae were deposited from small volume low density turbidity currents and suspension clouds the sedimentation rates of the cyclical chalk are similar to those known elsewhere and the lamination is interpreted as having been preserved from destruction through bioturbation by anoxic conditions at the seafloor bioturbated laminated cycles are thus formed by slow sedimentation during alternating seafloor redox conditions probably on a milankovitch scale a direct implication of this interpretation is that the cycles are areally widespread probably extending throughout the southern central graben area and may be useful for correlation and high resolution cyclostratigraphy in the chalk fields of the danish north sea if the laminated half cycles represent a few rapid resedimentation events with a high sedimentation rate as suggested by most workers then the sediment would not be truly cyclic but would represent event sedimentation within a pelagic background represented by the bioturbated beds in this case the cycles would have very limited potential for correlation

contourite drifts moats and channels in the upper cretaceous chalk of the danish basin during the late cretaceous high global sea level meant that most of the nw european craton was flooded by the deep epeiric chalk sea the classical paradigm for chalk deposition envisages a quiet rain of minute skeletal debris of coccolithophorid algae and other pelagic organisms deposited as horizontal flat lying pelagic oozes with local redeposition by slumps slides and debris flows along faults and other structural features seismic data from the danish basin and elsewhere necessitate a revision of this paradigm these demonstrate that the chalk sea floor had a considerable relief commonly of more than a hundred metres amplitude comprising moats drifts mounds and channels seismic sections from the kattegat sea illustrate the development in the maastrichtian of a deep moat adjacent to a topographic ridge formed over the inverted nw se trending sorgenfrei tornquist zone the moat was up to 120 m deeper than its sw flank which was formed by an internally complex elongate drift up to 20 km wide with an estimated length of ca 200 km smaller mound like features channels and clinoform beds are superimposed on the large scale relief the sea floor relief is interpreted to have formed in response to persistent bottom currents flowing parallel to bathymetric contours the initial build up of the broad gently convex up sheeted drift was controlled by relatively low velocity bottom currents the region of highest current velocity was gradually shifted ne wards towards the inversion zone ridge resulting in the formation of the deep moat flanked by the elongate drift the current is interpreted to have flowed from the se towards nw on the basis of the internal architecture of the elongate drift and the nw ward branching and decrease in moat relief the architecture and morphology of the moat drift and other features of the chalk sea floor are in all aspects similar to contourite systems of modern continental margins it is accordingly proposed that the fundamental physical oceanographic concept contour currents and their resulting contourite drifts is extended to include the deep epeiric seas which covered nw europe during the late cretaceous

benthic palaeoecology of danian deep shelf bryozoan mounds in the danish basin abstract early danian cool water bryozoan mounds exposed in the coastal cliff stevns klint in denmark were formed shortly after the cretaceous tertiary mass extinction they represent a relatively deep water highly diverse benthic ecosystem within the epeiric seaway that covered the danish basin the mounds are 50 110 m long and reached a height of about 5 10 m above the seafloor they are asymmetrical with a steep southern and a gentle northern flank and were dominated by small suspension feeders the benthic elements generally occur as fragments set in a carbonate mud matrix the main skeletal contributors are delicate branching bryozoans with minor contributions of bryozoan sheets and nodular arborescent bryozoans locally abundant octocorals occur on the mound crests and upper parts of the steep flanks echinoids are present in minor amounts but are locally abundant serpulids crinoids asteroids brachiopods bivalves massive calcareous sponges and benthic foraminifers are generally minor contributors to the benthic mound fauna influx of planktonic foraminifers coccoliths and other planktonic organisms was high and was probably a major source of nutrient supply to the mainly suspension feeding benthic fauna the faunal association reflects a relatively low energy environment with a high possibly seasonal influx of particulate nutrients the best growth conditions with respect to nutrient influx were on the mound crest and upper steep flank reflected by the diverse and relatively largest benthic faunal elements periodic reworking and winnowing occurred across the entire mound structure but most prominent on the gentle northern flanks limiting the benthic growth and notably the colony density and size of delicate branching bryozoans vagile benthic faunas were also adapted to different areas on the mound irregular echinoids preferred the intermound areas within fine grained wackestone packstone facies where they ploughed through the sediment whereas regular echinoids were epifaunal and preferred the upper parts of the mounds possibly feeding mainly on bryozoans skeletons of both groups became concentrated at the toe of the steep flanks and in the intermound areas by physical reworking during major storms changes in faunal composition on the mound crests occurred rhythmically on both small and large scale during mound growth rhythmically recurring faunal assemblages reflect alternating hydrodynamic conditions on the seafloor with respect to nutrient influx and energy which probably were linked to short term seasonal and long term climatic variations the long term alternation may be within the milankovitch frequency band blooming events of bryozoan sheets resulted from relatively short periods with large amounts of available food and suitable substrate successful colonisation by octocorals on the other hand reflected longer term favourable conditions on the mounds possibly associated with overall higher energy levels a possible pleistocene analogue to the bryozoan dominated danian mounds occurs at the shelf slope break of the great australian bight both of these cool water mound systems deviate from most other biogenic mounds known from the fossil record in their non cemented nature regular geometry and a lack of core and flank facies

the tectonic evolution of the fennoscandian border zone in denmark abstract the fennoscandian border zone is a complex tectonic zone defining the border between the stable fennoscandian baltic shield and the metastable danish basin and polish trough it forms the northwestern part of the tornquist teisseyre lineament one of the major european tectonic sutures intricate tectonic movements took place within the fennoscandian border zone from early paleozoic to recent time it is postulated that its evolution was dominated during the early palaeozoic by tensional tectonics and during the late paleozoic by dextral wrench movements the latter led to the development of curvilinear faults and the subsidence of pull apart basins transtensional stresses governing the triassic to early cretaceous evolution of the fennoscandian border zone induced the differential subsidence of rift basins during the late cretaceous and early cenozoic dextral transpressional stresses induced the partial inversion of mesozoic basins associated with the fennoscandian border zone this is reflected by the uplift of a major anticlinorium over which cretaceous strata became progressively truncated whilst adjacent areas developed into late cretaceous early tertiary depocentres basin inversion diminishes to the northwest along the fennoscandian border zone and disappears in the skagerrak area

structure of upper cretaceous chalk benthic communities southwestern arkansas abstract paleoecological investigation of chalk facies in the saratoga and annona formations includes paleoenvironmental analysis as well as examination of benthic macro invertebrate faunas the saratoga sandy chalk facies was deposited in a middle shelf environment 25 50 m depth while the annona chalk marly chalk facies was deposited in a outer shelf environment 75 125 m depth each facies is characterized by a distinct community of benthic macro invertebrate fossils dominated in occurence by epifaunal suspension feeding bivalves and deposit feeding echinoids however sandy chalk also contains moderately common infaunal suspension feeding bivalves and gastropods and rare infaunal deposit feeding bivalves while chalk markly contains only rare infaunal bivalves and no reported gastropods differences in composition within and between the two communities apparently reflect original distribution of shelled taxa trace fossil preservation indicates that substrates in both facies were thixotropic functional analysis demonstrates that many taxa show adaptations for living on soft substrates differences in diversity within each facies may be owing to non uniformity of substrate softness because many suspension feeders were unable to cope with the softest substrates and are therefore sparse in rocks apparently representing these substrates variations in substrate softness may have been caused by differing rates of bioturbation presence of infaunal suspension feeding bivalves in the saratoga but relative lack in the annona may be primarily a manifestation of softer annona substrates caused by generally finer grain size of annona sediments several studies of chalk faunas have concluded that sparseness of originally aragonitic fossils was caused by secondary sea floor dissolution or diagenetic processes results of this study demonstrate that primary organism sediment interactions could have also played a significant role in causing the general lack of originally aragonitic fossils in upper cretaceous chalks

early diagenetic celestite replacement of demosponges in upper cretaceous campanian maastrichtian chalk stevns denmark celestite nodules with subordinate amounts of chalcedony are common in upper cretaceous chalk in two cores drilled at stevns denmark the nodules occur throughout the cores indicating that the environmental conditions that led to their formation were an integral part of the biogeochemical cycle in the chalk sea petrographic evidence indicates that the celestite nodules were formed by replacement of lithistid demosponges in localized reduced environments prior to sediment compaction and chert formation the 87 sr 86 sr values of the celestite nodules indicate that the sr is mainly sourced from seawater and the 34 s values indicate activity of sulfate reducing bacteria this unusual occurrence of celestite is interpreted as a result of rapid burial of the sponges and following decomposition of the organic tissue by sulfate reducing and sulfide oxidizing bacteria forming an anoxic microenvironment the decomposition produced hydrogen sulfide which was oxidized to sulfate by sulfide oxidizing bacteria resulting in decreasing ph strontium precipitated as celestite in dysoxic microenvironments and at the anoxic dysoxic interface within the sponge the crystallization of celestite apparently created an sr gradient that drove sr into the site of crystallization this geochemical environment in dead demosponges represents an unknown celestite forming process

a relative water depth model for the normandy chalk cenomanian middle coniacian paris basin france based on facies patterns of metre scale cycles abstract a relative water depth model for the chalk of the paris basin is proposed based on the lateral variations of the high frequency metre scale cycles which are characteristic features easily identified in the field the studied outcrops are the cenomanian middle coniacian cliffs of normandy the main result of this study is to highlight the importance of storm activity in the deposition of the chalk the relative water depth model is based on storm induced shell concentrations observed within the two components of the metre thick cycles the depositional interval itself and the top hiatal surface six types of shell concentrations are defined along with seven types of depositional facies making up the depositional units as well as eight types of hiatal surface three cycle associations differing in their thickness and the amount and type of non carbonate constituents can be identified in the lower to upper cenomanian the upper cenomanian to lower turonian and the middle turonian to middle coniacian a relative water depth profile model for all these cycles is based on the shell concentrations and a water depth equivalence is proposed between the three cycle associations lateral facies substitution diagram this model is tested using palaeocological data irregular echinoids and by correlating field sections in terms of stacking patterns most of the studied deposits accumulated above the storm wave base upper offshore zone or mid ramp

channelling versus inversion origin of condensed upper cretaceous chalks eastern isle of wight uk evidence from regional stratigraphical patterns in santonian campanian chalk is used to infer the presence of a very broad channel system 5 km across with a depth of at least 50 m running nnw sse across the eastern isle of wight only the western part of the channel wall and fill is exposed within this channel were smaller erosional structures

diagenesis of flint and porcellanite in the maastrichtian chalk at stevns klint denmark abstract flint is common in many successions but since the nodules can be the result of a series of replacement and void filling processes their origin and broader paleo oceanographic significance can remain enigmatic this study addresses the origin of flint in the campanian maastrichtian chalk of denmark using the 450 m long stevns 1 core as a natural laboratory four types of siliceous nodules occur in the core porcellanite white flint white flint with a core of dark flint and dark flint all nodules consist mainly of microquartz with subordinate lutecite spherulitic chalcedony megaquartz and dolomite the lutecite replaces macrofossil fragments mainly bryozoans and shells microquartz replaces coccoliths and the chalcedony and megaquartz are pore filling the preservation of the external shape of bryozoans shells and coccoliths suggest that siliceous nodules were formed early by a one by one volume replacement of chalk values of 18osmow of microquartz and lutecite around 33 indicate temperatures of 15 17 c at the time of precipitation in contrast the 18osmow values of megaquartz indicate precipitation at temperatures up to 48 c the occurrence of chalcedony together with more stable and ordered megaquartz in irregular voids indicates that silica gel and or opal ct transformed to the quartz phases microquartz lutecite chalcedony and megaquartz through the dissolution precipitation process ostwald ripening these results motivate a conceptual model in which the replacement process was initiated by microbial decomposition of organic matter and took place during periods of low rate of or even stopped sedimentation which fixed the redox boundary and the microbial metabolic zones at a specific depth below the sea floor biogenic opal a incorporated in the sediment column below the sea floor was dissolved and precipitated at the redox boundary as silica gel and or opal ct possibly aided by sulfide oxidizing bacteria during burial the dissolution precipitation process transformed the silica gel and opal ct to the quartz phases the results of this study indicate that precipitation of the flint precursor can be controlled by a combination of an abundant biosiliceous source and high microbial activity in the sediments immediately below the seafloor flint is therefore likely to reflect deposition in a narrow interval at relatively shallow depths and therefore might be useful in studies of shallow water carbonates

interaction between bottom currents and slope failure in the late cretaceous of the southern danish central graben north sea the nw european chalk group was deposited in a deep epicontinental sea traditionally conceived as a quiet depositional setting that was only affected by redeposition along structural highs the chalk sea was however locally affected by powerful bottom currents during some periods three dimensional seismic reflection data from the southern central graben show abundant intra chalk morphological elements concentrated at two stratigraphic levels within the turonian campanian sequence both levels are topped by extensive unconformities the geometry of the elements produced by alongslope processes is interpreted as caused by se directed bottom currents flowing between the bo jens and adda ridges the highest current intensities led to the formation of the two unconformities judged by the occurrence of the largest current formed channels and drifts at these two stratigraphic levels this sea floor topography was locally modified by large scale downslope mass transport stratigraphical coincidence of the largest slumps and bottom current erosion suggests a coupling between downslope and alongslope processes current erosion impinged upon the slopes during periods of highest current speeds destabilizing the slope sediments and triggering slumping the smoothing of the bathymetry following local relaxation of the tectonic inversion accounted for the coeval deactivation of alongslope and downslope processes during the succeeding maastrichtian deposition

benthic faunal response to late maastrichtian chalk marl cyclicity at rordal denmark abstract chalk marl cyclicity is a common and characteristic feature of the upper cretaceous chalk group of nw europe the mainly small sized benthic fauna from an upper maastrichtian cyclic chalk marl succession at rordal denmark is analysed in order to interpret the composition and structure of the benthic fauna and to test if faunal density diversity composition and mode of life substrate were controlled by changes in lithology the material comprises 21 bulk samples weighing 5 kg each collected from all chalk and marl half cycles the samples were washed and sieved and yielded a total of 25 974 specimens these are referred to at least 75 species and 15 bryozoan morphotypes and form the basis for the establishment of 10 guilds the lithological cyclicity is not reflected by the distribution of individual species or by changes in average adult size but there is generally a higher species diversity and density in chalk compared to marl the chalk thus represents times of more favourable living conditions higher environmental stability and probably higher nutrient influx no changes in guilds between marl and chalk are found the variation in substrate character between the two lithologies was thus too small to have exerted any significant influence on the mode of life of the benthic fauna in spite of a difference in clay content averaging 10 two different fossil associations are recognized each covering one to two chalk marl cycles and characterized by a distinct fauna with similar ecological requirements if the chalk marl couplets represent precession cycles within the milankovitch band then the change from one association to the other and back again may represent the 100 000 years eccentricity cycle based on a calculated sedimentation rate of 6 cm ka 1

trace fossil model for reconstructing oxygenation histories of ancient marine bottom waters application to upper cretaceous niobrara formation colorado abstract trends of decreasing trace fossil diversity burrow size and penetration depth with diminishing oxygen availability in contemporary marine settings have been incorporated with the concept of trace fossil tiering to develop a model for reconstructing the oxygenation history of ancient basin bottom waters more sensitive than traditionally employed criteria the model permits detailed evaluation of relative degree of oxygenation through the recognition of oxygen related ichnocoenosis ori units or units of strata deposited under similar levels of bottom water oxygenation when employed in detailed vertical sequence analyses the model may also be used to construct interpreted oxygenation curves that reflect rates and magnitudes of temporal change in redox conditions the utility of the model is demonstrated by its application to rhythmically bedded calcareous strata of the niobrara formation upper cretaceous exposed in colorado non laminated strata of the niobrara are characterized by four recurring trace fossil suites referred to as the 1 chondrites 2 zoophycos teichichnus 3 planolites and 4 thalassinoides assemblages on the basis of trends in trace fossil diversity burrow diameter and penetration depth these assemblages are recognized as oxygen related ichnocoenoses interpreted oxygenation curves for the basal fort hays member reflect high magnitude redox cycles characterized by rapid deoxygenation events followed by gradual reoxygenation phases curves for the overlying smoky hill member reflect an overall lower degree of oxygenation with more irregular lower magnitude redox fluctuations significant correlation between interpreted levels of paleooxygenation and organic carbon content illustrate the model s potential for evaluating the distribution of organic rich fine grained strata co interpretation of these variables with carbonate content supports the premise that deposition was controlled by paleoceanographic perturbations modulated by milankovitch like climatic cycles comparison with traditionally emphasized macroinvertebrate body fossil criteria clearly demonstrates the greater utility of the trace fossil approach for evaluating paleo oxygenation

stratal geometries facies and sea floor erosion in upper cretaceous chalk normandy france unusual lenticular stratal geometries and facies of the upper cretaceous chalk of coastal haute normandie france are described and interpreted thirteen facies within these chalks are described and illustrated on the basis of field thin section and sem investigations nannofossil mudstones nannofossil hardgrounds echinoderm wackestones and packstones echinoderm hardgrounds bryozoan mudstones and wackestones bryozoan hardgrounds bryozoan packstones and wackestones inoceramid wackestones inoceramid hardgrounds sponge hardgrounds marly chalks conglomeratic chalks and debris flow chalks these facies occur within lenticular bedded structures with both concave up and concave down geometries which have been previously interpreted as megaripples mud mounds or tectonic structures detailed examination of the structures and the associated facies indicates that the concave up geometries were formed from submarine erosion and redeposition in nw se longitudinal channels the concave down geometries developed between adjacent channels assessment of the regional and temporal setting indicates that the erosion occurred in the armorican cornubian straits of the anglo paris basin during sea level lowstands within these straits channelling is preferentially developed on the positive south western block to the lillebonne fecamp cotentin fault

formation and diagenesis of bedding cycles in uppermost cretaceous chalks of the dan field danish north sea metre scale lithologic cycles visible in core and on logs from maastrichtian chalks of the dan field were examined to determine their mechanisms of deposition and relation to hydrocarbon production the lower parts of cycles consist of porous cream coloured largely non stylolitic commonly laminated chalk with limited bioturbation mainly escape burrows cycles are capped by thinner intervals of white to grey hard stylolitic chalk with concentrations of bioclastic material intense burrowing and few preserved primary sedimentary structures the cycle caps contain nearly twice as much mg as compared to the more porous parts of cycles and also have slightly larger 18o values 4 1 for the caps 4 4 for porous zones there is a significant reduction of average cycle thickness as well as total thickness of the maastrichtian chalk section from sw to ne across the dan field the cycle thinning largely results from a reduced thickness of porous chalks from the lower parts of cycles and thus is reflected in lower average porosity and permeability on the ne side of the field these data indicate that episodic winnowing removed fine grained constituents from highstanding northeastern areas porous cycle bases were deposited at relatively high rates that precluded complete bioturbation preserved laminae coupled with escape burrows reflect episodic sediment influx in areas that flank the seafloor highs cycle tops apparently accumulated more slowly throughout the region but especially on seafloor highs perhaps because of reduced productivity of planktic organisms slower sedimentation allowed more complete bioturbation and destruction of sedimentary structures and also led to incipient high magnesium calcite seafloor cementation sufficient to yield firmer sediment and enhanced burrow preservation but not to form true hardgrounds thus the elevated magnesium contents and reduced porosity of the cycle caps reflect very early diagenetic processes that were only partially modified by burial diagenesis rates of chalk deposition as inferred from physical and geochemical evidence appear to be a significant control on reservoir characteristics in north sea chalks the highest average porosities and permeabilities are found in areas with the highest sediment accumulation rates where seafloor diagenesis is minimized topographic depressions at the time of sedimentation can thus be expected to have the best production characteristics and synsedimentary topographic highs should have the thinnest sections and the poorest petrophysical properties

origin of channel systems in the upper cretaceous chalk group of the paris basin abstract the upper cretaceous chalk group of the anglo paris basin is known to show wedging beds and channel like features which disrupt the quietly deposited pelagic chalk that covered most of nw europe in the late cretaceous two d reflection seismic data from the brie region se of paris show the presence of at least two distinct intra chalk discordant reflections a top santonian and a mid campanian reflection these reflections are in places associated with up to 120 m deep channel like structures trending preferentially n s and nw se the mid campanian reflection is also sporadically associated with a massive secondary dolomite layer the thicknesses of which may reach 110 m diagenesis does not seem to account for the formation of the discordant reflections as there is neither a one to one relationship between the dolomite and discordant reflections nor are there signs of systematic collapse of the cenozoic succession over the channel like features as a result of intra chalk dissolution both reflections correlate with indurated chalk layers and hardgrounds and represent real unconformities the mid campanian reflection is furthermore associated with a stratigraphic hiatus a submarine origin is suggested due to the uninterrupted deep marine chalk facies below and above both unconformities and the unrealistically large sea level drop of more than 200 m which would be necessary for subaerial exposure of the central paris basin during the campanian the channels are oriented parallel to the margins of the basin and important bathymetric elements which could induce erosion by slope failure are not observed the channels are thus interpreted as having formed by strong mainly slope parallel bottom currents major channeling events are common in the chalk group throughout nw europe and represent palaeoceanographic re organization of bottom currents probably driven by changes in sea level and water temperature

danian cool water bryozoan mounds at stevns klint denmark a new class of non cemented skeletal mounds abstract a laterally extensive danian early paleogene bryozoan mound complex is well exposed in 14 km of nearly continuous outcrop in the coastal cliff stevns klint in eastern denmark a nw se oriented seaway covered the danish basin in danian time it widened towards the nw where it passed into the deeper north sea basin and narrowed towards the se where it was connected with the polish trough the exposed mound succession is up to 20 m thick individual mounds are oval in plan view internally asymmetrical 5 11 m high 45 110 m long in nne ssw oriented dip sections parallel to the direction of mound migration and 50 300 m in strike sections the southern flanks are steep and dip 15 30 mainly towards the ssw whereas northern flanks have gentle dips of 0 15 towards the nne four facies associations are distinguished 1 the initial mound association consisting mainly of bryozoan packstone rudstone 2 the steep flank mound crest association consisting of bryozoan rudstone with subsidiary octocoral and encrusting bryozoan rudstone and bryozoan grainstone 3 the gentle flank association characterized by bryozoan packstone and incipient to fully developed hardgrounds and 4 the intermound association consisting of echinoid rudstone packstone and minor occurrences of bioclastic grainstone and argillaceous wackestone packstone beds and laminae facies analysis and mesoscale mapping indicate that the bryozoan mound complexes were formed in relatively deep cool water below the photic zone and swell wave base deposition was influenced by along slope currents with prominent seasonal and long term variations mound growth was governed by primarily in place benthic carbonate production of mainly delicate branching bryozoans which baffled and trapped pelagic and detrital benthic carbonate mud gentle currents rich in particulate nutrients flowing towards the wnw deflected by the coriolis force and possibly a tidally or wind induced upslope component towards the nne resulted in mound growth mainly towards the ssw high production in the surface waters combined with an effective tidally induced mixing of the water column was important for transporting particulate nutrients to the bottom waters periodic intensification of the mean wnw ward flowing bottom current system combined with a storm associated downslope component towards the ssw resulted in winnowing and erosion of the northern mound flanks long term periodic waning or cessation of bottom currents and possibly stratification of the water column resulted in very limited benthic carbonate production and mound growth associated with formation of extensive thalassinoides burrow galleries mirrored by the rhythmic occurrences of flint nodule bands mound development thus reflects the delicate balance between biogenic growth towards particulate nutrient carrying currents current winnowing and erosion the lack of framework buildup and the poorly cemented nature of the mounds is remarkable in this respect they deviate from most other mound types known from the geological record and may possibly together with the sub recent analogues on the great australian bight represent a new class of noncemented skeletal mounds

buried pockmarks on the top chalk surface of the danish north sea and their potential significance for interpreting palaeocirculation patterns three dimensional 3d seismic data from the southern danish north sea were used to analyse the morphology and spatial distribution of depressions in the danian chalk deposit previously these depressions were either interpreted as karst structures or pockmarks the observed depressions occur in an interval from 25 ms below to 12 ms above the top chalk surface three types of depressions were differentiated based on their plan view geometry and their degree of symmetry type 1 comprising sub circular and symmetrical depressions is the dominant group ca 70 type 2 elongated and symmetrical depressions represents only a small fraction ca 5 the elongated and asymmetrical depressions of type 3 compose ca 25 in cross section each depression type can be either characterised by a v or a u shape the maximum size of the depressions ranges from 50 to 580 m with an average internal depth of 10 m we interpret the depressions as pockmarks formed by the expulsion of biogenic or thermogenic fluids at the danian seafloor likely the initial form of the pockmarks has been circular type 1 and was subsequently modified for types 2 and 3 to an elongated form by currents the long axis of the pockmarks is interpreted to represent the effective current direction the inferred direction is sub parallel to the palaeobathymetric contours the 3d seismic interpretation of pockmarks presented in this paper contributes to the understanding of fluid migration and palaeocirculation patterns during the sedimentation of the terminal chalk group in the southern danish north sea

large bipolar lightning discharge events in winter thunderstorms in japan using a low frequency lightning location system comprising nine stations we have observed and analyzed 374 large and bipolar electric field change waveforms that occurred during the winter of 2012 2013 since the waveforms are different from those produced by any well studied lightning discharge processes we refer to these source discharge events using a new name large bipolar events lbes lbes can be characterized by the following features 1 all have the same polarity as negative return stroke 2 all exhibit a single bipolar pulse with a pulse width around 15 s and similar positive and negative cycles 3 all are located on the land along the japan sea coast indicating they are probably associated with high grounded objects 4 most lbes produce very large electric field changes that are even larger than that of positive and negative return strokes 5 most lbes are temporally isolated within several milliseconds but are frequently followed by intracloud discharges after tens of milliseconds 6 most lbes produce a single well distinguished ionospheric reflection pulse and the time difference between lbe pulse and the corresponding reflection pulse can be used to calculate ionospheric reflection height it is speculated that lbe is a type of powerful and transient lightning discharge event produced within a compact region of strong electric field formed when the negative charge layer in thunderclouds is very close to the top of a tall grounded object

displacement field in contraction driven faults we investigate the distribution of strain and deformation in the host sediment that arises once a contraction driven shear fault has localized and propagated under a zero lateral strain condition numerical modeling of displacement distributions compares well with those measured using 3d seismic data the parameters that determine the displacement field for a single normal fault embedded in sediments are fault height overburden effective stress stiffness and residual friction angle or post peak strength proximity to the free boundary biases the displacement pattern which becomes asymmetric although the measured displacements and numerical predictions are similar the measured magnitude requires pronounced low stiffness of the sediment as well as low post peak shear strength this requirement suggests that sediments hosting contraction driven shear faults most likely have high porosity and high clay fraction and have undergone diagenetic reactions involving significant mineral dissolution the diagenetic evolution of the sediment and its current composition may explain the global scaling relationship between the measured displacement and fault height for polygonal fault systems

the cretaceous palaeogene boundary at stevns klint denmark inversion tectonics or sea floor topography the cretaceous palaeogene boundary interval is exposed over 12 km in the coastal cliff stevns klint denmark an important lowermost danian hardground has been interpreted as an originally horizontal marine abrasion surface its present elevation varies from a few metres below to about 35 m above sea level this relief has traditionally been considered as resulting from late or post danian laramide folding new seismic profiles offshore stevns klint show however that the base chalk reflector is not folded is remarkably planar and has a gentle northward dip thus the folding hypothesis cannot be upheld seismic stratigraphic analysis of the chalk group necessitates a fundamental revision of general ideas of chalk deposition a highly irregular sea floor topography was formed at many levels and includes broad valleys ridges channels drifts and mounds a system of major wnw ese oriented valleys and ridges can be traced into the succession exposed in stevns klint and further inland where it corresponds to the relief of the cretaceous palaeogene boundary the marked topographic elements of the chalk sea floor are elongate with a wnw orientation roughly parallel to the axis of the danish basin and to the sorgenfrei tornquist zone forming the ne border of the basin the sea floor relief undoubtedly reflects the influence of strong contour parallel bottom currents the cretaceous palaeogene boundary succession at stevns klint was thus developed on an underlying sea floor topographic relief of about 40 m recognition of the highly irregular current influenced topography of the late cretaceous sea floor stands in marked contrast to the conventional picture of quiet pelagic deposition of the chalk

the development and seismic expression of synsedimentary features within the chalk of southern england seismic reflection data acquired onshore to the north of bournemouth southern england image clearly a series of prominent concave up troughs or depressions and concave down mounded structures within the white chalk subgroup the chalk lithostratigraphy in the area of study is established from borehole geophysical log correlations these correlations then provide calibration of the seismic reflection data indicating that the structures are concentrated at levels between the lewes nodular chalk formation and tarrant chalk member and towards the base of the portsdown chalk formation onlap downlap and truncation of reflectors are observed with the most dramatic features forming a stacked series directly overlying faulting of jurassic strata similar stratal geometries have been described from comparable levels in the chalk of the north sea and in outcrop in britain and france to many they represent channels formed by submarine erosion and redeposition of the chalk during relative sea level falls linked to tectonics we also interpret a number of apparently incisive structures and slumped infill sequences described here as being of primary syndepositional origin the slumping and the close association of these features with underlying faults and proximity to nearby major crustal faults may be indicative of submarine erosion of the chalk caused by sea level changes linked to syndepositional fault movements however evidence of toplap downlap pairs is also suggestive of more aggradational mounding or laterally migrating surfaces or facies belts the presence of any such structures implies rapid lateral variations in the chalk that would have implications not only for aquifer management and future storage schemes but also during the processing of seismic reflection data and general studies and correlations of the chalk across southern britain

late maastrichtian chalk mounds stevns klint denmark combined physical and biogenic structures abstract upper maastrichtian chalk exposed at the sigerslev quarry stevns klint denmark is characterized by wavy and mound like bedding geometries outlined by bands of black flint nodules four morphological elements are recognized although bedding geometries are highly variable southward migrating mounds eastward migrating mounds chalk waves and evenly bedded chalk the mounds are interpreted as having been formed by currents carrying fine grained suspended sediment which was primarily deposited on the up current mound flanks bryozoans were prolific on the up current flanks and mound summits which stabilized the mounds increased bed roughness and the overall accumulation rate however accumulation thicknesses do not correlate consistently with bryozoan density the bryozoans were therefore important for the formation of the mounds but the distribution of bryozoans did not solely determine depositional thickness across a mound and thus mound growth pattern relatively long wavelength wavy bedded chalk show gentle convex up geometries and would probably be described as sediment waves if recognized in seismic sections the chalk waves were deposited under weaker current velocities than those active during mound formation the exposed succession is topped by more evenly bedded chalk which was deposited by quiet pelagic fall out of fine grained material the whole succession was deposited on the upper part of the northern flank of a large wnw ese trending 3 km wide depositional ridge with an amplitude of 35 40 m formed by contour parallel wnw ward flowing bottom currents the mounds may have been deposited by regional bottom currents or by spill over currents from the valley south of the large ridge the succession was deposited during varying bottom current intensities and the depositional architecture indicates a complex and dynamic environment the depositional style seems to be controlled by the interplay and relative importance of two end member processes those that were purely physical and related to the interaction between available grain sizes and hydrodynamics and those that were largely biological and associated with benthic growth and sediment trapping of mainly bryozoans examples of end member conditions are known from haute normandy france and the lower danian of stevns klint the structures described here adds to the considerable complexity of depositional styles in chalk

the role of early lithification in development of chalky porosity in calcitic micrites upper cretaceous chalks egypt abstract petrological and geochemical examination of upper cretaceous chalks from the abu roash area and bahariya oasis egypt provide insights into factors which control porosity development in fine grained calcitic carbonates petrological studies indicate that primary skeletal fabrics of coccoliths and foraminifera in micrites of abu roash are well preserved in contrast primary skeletal fabrics of the bahariya oasis chalks are pervasively altered geochemical analysis of these chalks reveals a striking contrast in composition the high porosity chalks at bahariya oasis have depleted isotopic values 13 c 5 0 18 o 8 9 pdb and low concentrations of trace elements sr and na in contrast the low porosity chalks at abu roash are less depleted in terms of their isotopic values 13 c 1 0 18 o 4 0 and possess elevated concentrations of sr na fe and zn importantly fracture and collapse veins present only at abu roash are filled with calcite whose 18o value is coincident with that of altered micrites at bahariya oasis the lithic fragments cemented by this calcite however retain compositions identical to other lithified limestones at abu roash differences in fabric and chemistry of the pelagic marine micrites observed between abu roash and bahariya oasis reflect the degree of early lithification and compaction prior to exposure and meteoric diagenetic alteration associated with regional late cretaceous pre eocene subaerial unconformity

structure and tectonic evolution of the tornquist zone and adjacent sedimentary basins in scania and the southern baltic sea area abstract southernmost sweden bornholm and the surrounding baltic sea region are located on a large scale releasing bend in the dextral strike slip system of the tornquist zone with its resulting pull apart basins the well constrained geology of scania and bornholm has been combined with detailed on and offshore borehole data and three proprietary marine seismic surveys this in conjunction with supplementary babel deep seismic reflection findings allows a combined 3d interpretation of sediment structure interactions as a result a regional interpretation has emerged which gives a new understanding of the interplay between structural movement on a complex strike slip fault system tornquist zone and its intrazonal depressions vomb trough and colonus shale trough as well as the sedimentation history of associated areas of sediment accumulation ronne and arnager grabens hollviken halfgraben hano bay basin and skurup platform detailed sequential litho and seismo stratigraphic descriptions have been possible by combination of the various data sets this resulted in the clarification or recognition of previously unknown structural limits to sub basins and highs in the study area a 3d chronological 4d model for the development of the region is proposed this model takes into account the long lived structural history combining elements of strike slip extension and inversion tectonics the deep seated faulting controlling these structures is integrated with the deep structure as revealed by the babel line in this area

integrated seismic analysis of the chalk group in eastern denmark implications for estimates of maximum palaeo burial in southwest scandinavia abstract the origin of the topography of southwest scandinavia is subject to discussion analysis of borehole seismic velocity has formed the basis for interpretation of several hundred metres of neogene uplift in parts of denmark here refraction seismic data constrain a 7 5 km long p wave velocity model of the chalk group below the stevns peninsula eastern part of the danish basin the model contains four layers in the 860 m thick chalk group with mean velocities of 2 2 km s 2 4 km s 3 1 km s and 3 9 4 3 km s sonic and gamma wireline log data from two cored boreholes represent the upper 450 m of the chalk group the sonic velocities are consistent with the overall seismic layering although they show additional fine scale layering integration of gamma and sonic log with porosity data shows that seismic velocity is sensitive to clay content in intervals near boundaries of the refraction model moderate increases in clay content correlate with reduction of porosity and increase in velocity higher clay contents do not further increase velocity the reduction of porosity and increase in velocity are interpreted as clay causing increased pressure dissolution and cementation the interpreted velocities are systematically higher than values of a chalk velocity curve determined in previous studies and it is estimated that a significant part of the velocity anomaly may be explained by the presence of clay the remaining velocity anomaly can be explained by 450 500 m palaeo burial of the chalk group the burial anomaly will be over estimated by 150 200 m if the analysis is based on the average chalk group velocity and clay content is disregarded burial anomaly values of 450 600 m result if the strongest velocity contrast at 600 650 m depth is interpreted to be a result of diagenetic effects consistent with the clay corrected estimates within uncertainty