oceanography bathymetry and syndepositional tectonics of a precambrian intracratonic basin integrating sediments storms earthquakes and tsunamis in the belt supergroup helena formation ca 1 45 ga western north america abstract the carbonate dominated helena formation of the mesoproterozoic belt supergroup of western north america provides an instructive example of how a range of regional depositional and environmental characteristics of an ancient sea can be deduced on the basis of micron to metre scale features particularly revealing is the window opened by the presence of abundant molar tooth structure onto the paleoceanography paleobathymetry paleoclimate and tectonic regime of this intracratonic precambrian basin the facies hosting molar tooth structure is composed dominantly of lime mud with substantial subangular quartz and feldspar silt and clay derived from the western and southwestern side of the basin these are low energy tempestites deposited on a remarkably flat sea bottom at the limit of storm wave base at about 50 m sporadic domical stromatolite patch reefs confirm that the sea bottom was normally within the photic zone the ubiquity of molar tooth structure suggests frequent near field seismic activity during subsidence which generated ground motion sufficient to liquefy granular lime mud and terrigenous silt sporadic tsunamis from major submarine faults far to the west pounded the shallow water platform to the east tsunami off surge swept ooids and rounded coarse grained feldspathic quartz sand westward into deeper water and created strongly erosive currents that left gutter casts composed of lags of preferentially cemented molar tooth structure in otherwise relatively low energy facies mineralogical and geochemical evidence confirms that the belt basin was marine organic matter was essentially fully oxidized in the water column original high mg composition and cementation of lime mud in molar tooth structure indicate that calcite precipitated above the thermocline in supersaturated seawater under tropical conditions scattered bimineralic ooids in allochthonous grainstones indicate that shoals on the platform to the east were intermittently above a shallow aragonite compensation depth in warm water rare vestiges of halite and gypsum demonstrate occasional temporary salinity stratification for periods of up to a few years unaltered feldspar and mica grains point to a generally arid climate but the large volume of clay is suggestive of protracted weathering and transport

reply to comments by kelletat 2008 comments to dawson a g and stewart i 2007 tsunami deposits in the geological record sedimentary geology 200 166 183

reading of the trigger records of sedimentary events a problem for future studies abstract sedimentary records of various three to four dimensional patterns of nature have recently attracted the special attention of many geologists the dramatic development of rhythmic sequence stratigraphy is a conspicuous recent instance the significance of event sedimentation in geohistory however has not been examined sufficiently the problem to how to read the records of triggers or thresholds of sedimentary events has been ignored the difficulty of solving this problem concerns the difference in patterns between natural sedimentary processes and their resulting sediments terminological confusion of event deposits is another cause which hinders our taking note of this problem a few examples of endeavours to discriminate the threshold of sedimentary events are shown by the analysis of two kinds of turbidites of the mid guatemala trench the investigation of slump derived turbidites of the suruga trough and the study of the seismoturbidites in lake biwa well focused investigations of features of trigger known sediments is the best strategy to find the criteria for discriminating the triggers or thresholds based on sedimentary records of various sedimentary events from this point of view more detailed studies of the santorini eruption induced homogenites the k t boundary impact induced tsunamiites together with the lake biwa seismic turbidites are very important further efforts should be made to find and investigate as many trigger identifiable sediments as possible

numerical modelling of tsunami propagation with implications for sedimentation in ancient epicontinental seas the lower jurassic laurasian seaway abstract tsunamis are frequent events in modern marine environments but evidence of their passing in subtidal ancient epicontinental sea deposits is elusive it has been suggested that this is due to mis identification or poor preservation potential herein a numerical modelling approach is used to show that tsunami propagation in one large ancient epicontinental sea was hindered by the damping effect of shallow bathymetries and reflection refraction and diffraction from emergent landmasses the imperial college ocean model icom is used for this study and is first validated against data from the sumatra andaman tsunami of december 2004 a palaeobathymetric dataset is then presented for the hettangian lower jurassic laurasian seaway with idealised tsunami sources situated on the continental shelf and within the adjacent oceanic basin results show that tsunamis forced from within ocean basins adjacent to the epicontinental sea are rapidly attenuated over the continental slope and fail to propagate great distances onto the shelf similarly the sedimentological effect of tsunamis forced from within the epicontinental sea is also restricted it is concluded that tsunami deposits in ancient epicontinental seas are most likely to occur in relative proximity to the source region and this must contribute to their scarcity in the geological record

tsunami deposits beneath tidal marshes on northwestern vancouver island british columbia two sand sheets underlying tidal marshes at fair harbour neroutsos inlet and koprino harbour on the northwestern coast of vancouver island british columbia were probably deposited by tsunamis the sand sheets become thinner and finer grained landward drape former land surfaces contain marine microfossils are locally graded or internally stratified and can be correlated with earthquakes that generated tsunamis in the region 137cs dating and historical accounts indicate that the upper sand sheet was deposited by the tsunami from the great alaska earthquake in 1964 radiocarbon ages on plant fossils within and on top of the lower sand sheet show that it was deposited sometime after about a d 1660 we attribute the lower sand sheet to a tsunami from the most recent plate boundary earthquake on the cascadia subduction zone about 300 yr ago extending the documented effects of this earthquake north of the nootka fault zone the 1964 tsunami deposits differ little in thickness and continuity among the three marshes in contrast the lower sand sheet becomes thinner and less continuous to the north implying a tsunami source south of the study area

comments to dawson a g and stewart i 2007 tsunami deposits in the geological record sedimentary geology 200 166 183

balancing salt dome uplift and withdrawal basin subsidence in cross section abstract idealized geometric models are presented for cases of cross sections drawn perpendicular to elongate salt domes where a two dimensional model is adequate and cross sections drawn through the center of hemispherical salt domes where a three dimensional model is needed primary attention is devoted to domes during the salt pillow stage of uplift however a simple model is provided for the piercement stage as well mathematical relationships are derived between the cross sectional area of the dome and the cross sectional area of the associated withdrawal basin relationships are also derived relating non dimensional dome uplift to non dimensional basin subsidence these relationships must be taken into account when constructing and restoring cross sections of salt domes failure to do so can result in significant errors in estimating the amount of salt lost to dissolution in addition the relationships in this paper imply that there is a change in either salt dome height or withdrawal basin depth near the ends of an elongate dome and predict that restored cross sections of mini basins developed on thick layers of salt typically will contain more salt than the present day section

syneresis cracks subaqueous shrinkage in argillaceous sediments caused by earthquake induced dewatering abstract syneresis cracks often confused with subaerial desiccation phenomena are traditionally ascribed to subaqueous shrinkage whereby salinity changes caused deflocculation of clay this and other previously proposed mechanisms fail to account for their occurrence in low energy typically non evaporitic facies stratigraphically sporadic distribution intrastratal formation under shallow burial depths variation in morphology degree of contraction generation of sedimentary dikelets as crack fills and deformation of dikelets and enclosing layers instead it is suggested that ground motion from strong synsedimentary earthquakes caused argillaceous sediments to dewater interbedded sands and silts to be almost simultaneously liquefied and injected into the resulting fissures and then these dikelets to be distorted comparative rarity of syneresis cracks in phanerozoic versus precambrian marine strata is considered to be primarily an evolutionary consequence of theological changes caused by increased organic binding of clay flocs in the water column greater input of organic matter into the sediment and the diversification of sediment dwelling bacteria and meiofauna in phanerozoic deposits

eustatically controlled sedimentation in the hettangian sinemurian early jurassic of poland and sweden in earliest jurassic times terrigenous continental and marginal marine deposition occurred in a large epeiric basin along the tornquist line in europe detailed sedimentological studies allow recognition of palaeoenvironmental fluctuations in space and time the main earliest jurassic transgressions occurred in the early hettangian early sinemurian mid sinemurian and latest sinemurian and formed bounding discontinuities transgressive surfaces of considerable correlative significance there is a step wise trend of increasing marine extension and influence during the early hettangian early sinemurian mid sinemurian and latest sinemurian earliest pliensbachian transgressions four sequences four transgressive systems tracts three highstand systems tracts and three levels regarded as equivalents of maximum flooding surfaces are distinguished in the case of type 2 sequences when incised valley fill deposits are not developed and regional erosion is less common it may be rather difficult to define the sequence boundaries which are often concealed within the amalgamated fluvial deposits occurring in the neighbouring parts of two adjacent sequences fluvial deltaic sediments terminate the highstand systems tracts and in this setting the transgressive systems tracts start with continental deposits prior to the transgressive surfaces generally an exact correlation can be achieved between the sequence stratigraphy of the northeast and northwest european lower jurassic and the eustatic curve proposed by epr assuming some changes proposed by a hallam the establishment of this correlation hopefully will stimulate future studies of the sequence stratigraphy of poorly dated siliciclastic deposits of marginal basins in this setting even minor changes in sea level may cause major changes in facies development over large areas

tsunami sedimentary facies deposited by the storegga tsunami in shallow marine basins and coastal lakes western norway sedimentary successions in small coastal lakes situated from 0 to 11 m above the 7000 year bp shoreline along the western coast of norway contain a distinctive deposit very different from the sediments above and below the deposit is interpreted to be the result of a tsunami inundating the coastal lakes an erosional unconformity underlies the tsunami facies and is traced throughout the basins with most erosion found at the seaward portion of the lakes the lowermost tsunami facies is a graded or massive sand that locally contains marine fossils the sand thins and decreases in grain size in a landward direction above follows coarse organic detritus with rip up clasts here termed organic conglomerate and finer organic detritus the tsunami unit generally fines and thins upwards the higher basins 6 11 m above the 7000 year shoreline show one sand bed whereas basins closer to the sea level 7000 years ago may show several sand beds separated by organic detritus these alternations in the lower basins may reflect repeated waves of sea water entering the lakes in basins that were some few metres below sea level at 7000 years bp the tsunami deposit is more minerogenic and commonly present as graded sand beds but also in some of these shallow marine basins organic rich facies occur between the sand beds the total thickness of the tsunami deposit is 20 100 cm in most studied sites an erosional and depositional model of the tsunami facies is developed

paleolatitudinal sampling bias phanerozoic species diversity and the end permian extinction species diversity varies with latitude almost all marine sedimentary rock in the paleozoic of europe and north america was deposited in the tropics whereas only 24 was deposited in these latitudes during mesozoic and tertiary time a new plot of species diversity through the phanerozoic takes into account the paleolatitudinal distribution of marine sedimentary rock this plot indicates that the permian witnessed the acme of species diversity prior to the cretaceous and that the triassic recovery following the end permian extinction was much less pronounced than suggested by previous studies

interpretations and common challenges of aeolian records from north american dune fields abstract geomorphic and chronologic data for dune fields are evaluated for three contrasting areas of north america 1 the prairie parkland and boreal ecozones of the northern great plains in canada 2 the central great plains of the usa and 3 the deserts of southwestern usa and northern mexico luminescence and radiocarbon ages for periods of dune accumulation and stability are compared with palaeoenvironment proxies to provide an assessment of the boundary conditions of dune system response to changes in sediment supply availability and mobility dune fields in the northern great plains were formed from sediment originating from glaciofluvial or glaciolacustrine sediments deposited during deglaciation 16 11 ka subsequent aeolian deposition occurred in parkland and prairie dune fields as a result of mid holocene 8 5 ka and late holocene despite spatial and temporal gaps in chronometric data as a result of sampling biases the record from north american dune fields indicates the strong influence of sediment supply on dune construction with changes in sediment availability as a result of drought episodes resulting in dune field reactivation and reworking of pre existing sediment

modelling tidal current induced bed shear stress and palaeocirculation in an epicontinental seaway the bohemian cretaceous basin central europe lower to middle turonian deposits within the bohemian cretaceous basin central europe consist of coarse grained deltaic sandstones passing distally into fine grained offshore sediments dune scale cross beds superimposed on delta front clinoforms indicate a vigorous basinal palaeocirculation capable of transporting coarse grained sand across the entire depth range of the clinoforms ca 35 m bi directional alongshore oriented trough cross set axes silt drapes and reactivation surfaces indicate tidal activity however the bohemian cretaceous basin at this time was over a thousand kilometres from the shelf break and separated from the open ocean by a series of small islands the presence of tidally influenced deposits in a setting where co oscillating tides are likely to have been damped down by seabed friction and blocked by emergent land masses is problematic the imperial college ocean model a fully hydrodynamic unstructured mesh finite element model is used to test the hypothesis that tidal circulation in this isolated region was capable of generating the observed grain size distributions bedform types and palaeocurrent orientations the model is first validated for the prediction of bed shear stress magnitudes and sediment transport pathways against the present day north european shelf seas that surround the british isles the model predicts a microtidal to mesotidal regime for the bohemian cretaceous basin across a range of sensitivity tests with elevated tidal ranges in local embayments funnelling associated with straits increases tidal current velocities generating bed shear stresses that were capable of forming the sedimentary structures observed in the field the model also predicts instantaneous bi directional currents with orientations comparable with those measured in the field overall the imperial college ocean model predicts a vigorous tide driven palaeocirculation within the bohemian cretaceous basin that would indisputably have influenced sediment dispersal and facies distributions palaeocurrent vectors and sediment transport pathways however vary markedly in the different sensitivity tests accurate modelling of these parameters in this instance requires greater palaeogeographic certainty than can be extracted from the available rock record

depositional events and their records an introduction abstract event deposits of some lateral extent as defined here occur almost in all types of sedimentary basins the reflect either local intra basinal processes or they are associated with regional or global mechanisms some types of event beds such as sediment gravity flows sandy and muddy turbidites are common and usually well preserved they make up large proportions of basin fills whereas others e g tsunami deposits and in situ earthquake structures appear to be less frequent or rare in the geological record volume frequency and facies of event deposits are controlled by several processes pre event sediment accumulation triggering and transport mechanisms and mode of final deposition

sedimentary features of seismites seismo turbidites and tsunamiites an introduction abstract catastrophic events such as deposits from sediment flows generated by seismic events or major tempests have not been intensively studied sedimentological procedures for deducing the relationships of seismic foci intensity period and amplitude of the seismic waves have been initiated only recently this special volume arose from a session entitled sedimentary features of seismites seismo turbidites and tsumaniites held at the 15th international sedimentological congress in alicante spain in april 1998 the papers in this volume cover a somewhat broader range of subjects our goal is to stimulate further research on the sedimentary features of seismites seismoturbidites and tsunamiites

plate tectonic evolution of the southern margin of eurasia in the mesozoic and cenozoic abstract thirteen time interval maps were constructed which depict the triassic to neogene plate tectonic configuration paleogeography and general lithofacies of the southern margin of eurasia the aim of this paper is to provide an outline of the geodynamic evolution and position of the major tectonic elements of the area within a global framework the hercynian orogeny was completed by the collision of gondwana and laurussia whereas the tethys ocean formed the embayment between the eurasian and gondwanian branches of pangea during late triassic early jurassic times several microplates were sutured to the eurasian margin closing the paleotethys ocean a jurassic cretaceous north dipping subduction boundary was developed along this new continental margin south of the pontides transcaucasus and iranian plates the subduction zone trench pulling effect caused rifting creating the back arc basin of the greater caucasus proto south caspian sea which achieved its maximum width during the late cretaceous in the western tethys separation of eurasia from gondwana resulted in the formation of the ligurian penninic pieniny magura ocean alpine tethys as an extension of middle atlantic system and a part of the pangean breakup tectonic system during late jurassic early cretaceous times the outer carpathian rift developed the opening of the western black sea occurred by rifting and drifting of the western central pontides away from the moesian and scythian platforms of eurasia during the early cretaceous cenomanian the latest cretaceous paleogene was the time of the closure of the ligurian pieniny ocean adria alcapa terranes continued their northward movement during eocene early miocene times their oblique collision with the north european plate led to the development of the accretionary wedge of the outer carpathians and its foreland basin the formation of the west carpathian thrusts was completed by the miocene the thrust front was still propagating eastwards in the eastern carpathians during the late cretaceous the lesser caucasus sanandaj sirjan and makran plates were sutured to the iranian afghanistan plates in the caucasus caspian sea area a north dipping subduction zone jumped during paleogene to the scythian turan platform the shatski terrane moved northward closing the greater caucasus basin and opening the eastern black sea the south caspian underwent reorganization during oligocene neogene times the southwestern part of the south caspian basin was reopened while the northwestern part was gradually reduced in size the collision of india and the lut plate with eurasia caused the deformation of central asia and created a system of nw se wrench faults the remnants of jurassic cretaceous back arc systems oceanic and attenuated crust as well as tertiary oceanic and attenuated crust were locked between adjacent continental plates and orogenic systems

distinguishing tsunami and storm deposits an example from martinhal sw portugal abstract geological identification of past tsunamis is important for risk assessment studies especially in areas where the historical record is limited or absent the main problem when using the geological evidence is to distinguish between tsunami and storm deposits both are high energy events that may leave marine traces in coastal stratigraphic sequences at martinhal sw portugal both storm surge and tsunami deposits are present at the same site within a single stratigraphic sequence which makes it suitable to study the differences between them excluding variations caused by local factors the tsunami associated with the lisbon earthquake of november 1st 1755 ad had a major impact on the geomorphology and sedimentology of martinhal it breached the barrier and laid down an extensive sheet of sand as described in eyewitness reports besides the tsunami deposit the stratigraphy of martinhal also displays evidence for storm surges that have breached and overtopped the barrier flooding the lowland and leaving sand layers both marine derived flood deposits show similar grain size characteristics and distinctive marine foraminifera the most important differences are the rip up clasts and boulders exclusively found in the tsunami deposit and the landward extent of the tsunami deposit that everywhere exceeds that of the storm deposits identification of both depositional units was only possible using a collection of different data and extensive stratigraphical information from cores as well as trenches

sediment grains moved by passing tsunami waves tsunami deposits in deep water abstract tsunamis propagating in the open ocean have associated horizontal particle velocities that do not change with depth yet the limiting water depth where a tsunami of given characteristics will initiate sediment motion remains unknown based upon linear wave theory and a parametrization of the shields curve equations are derived and solved using an iterative scheme to address the topic of grain movement by tsunami waves as a function of water depth and wave amplitude the focus is on waves in deep water where tsunami waves behave linearly and on non cohesive sediment grains furthermore the question is addressed of which grain sizes are picked up on a sloping beach as the wave shoals according to the results even the boxing day tsunami in 2004 was incapable of moving fine sand in water deeper than 985 m in the bay of bengal and 335 m in the indian and pacific oceans the results suggest that tectonic tsunamis of size equal to or smaller than the boxing day tsunami cannot initiate motion of deep water cohesionless sediments that can be correlated on an oceanic basin wide scale

storms versus tsunamis dynamic interplay of sedimentary diagenetic and tectonic processes in the cambrian of montana the upper cambrian deadwood formation of north central montana is composed mainly of fine grained calcareous sandstone and intercalated silty shale and contains scattered intraclastic flat pebble conglomerate these rocks typical of many mixed carbonate siliciclastic subtidal shelf successions of proterozoic through early ordovician age are distal storm deposits in which the conglomerates are conventionally thought to record occasional very high energy events that scoured incipiently cemented layers in the deadwood formation most sandstone beds have a linear to crudely reticulate pattern of vertical cracks filled with injected silt and clay the cracks exhibit a consistent northwest southeast trend and are very shallow burial deformation structures caused by the passage of seismic waves possibly originating from strong earthquakes in idaho most conglomerates are composed of angular to subangular sandstone intraclasts the polygonal plan view of which is similar in outline to the areas between cracks in the sandstones the deformation implied by the cracks sporadic distribution of conglomerates high degree of scouring indicated and angularity of the intraclasts argue that rather than storms the conglomerates were generated by occasional tsunamis the sweep of these tsunamis across the shallow intracratonic sea created extraordinarily strong oscillating bottom currents at the deeper reaches of storm wave base tsunamites are therefore identified with confidence virtually for the first time in shallow subtidal shelf deposits

molar tooth structure in proterozoic carbonate rocks origin from synsedimentary earthquakes and implications for the nature and evolution of basins and marine sediment proterozoic argillaceous lime mudstones commonly contain molar tooth structure a curious synsedimentary feature consisting of crumpled generally vertically oriented veins filled with calcite microspar long enigmatic these veins are interpreted here as earthquake induced deformation structures a model is proposed whereby violent ground motion caused shrinkage dewatering and fissuring of the colloidal sea floor sediment of clay and lime mud a slurry of equant particles of lime mud segregated from the clay platelets trapped in the matrix was expelled into these voids the host bed underwent as much as 80 compaction as well as shear causing the lime mud filled fissures to be ptygmatically folded and shingled both vertically and horizontally and in places pulled apart this lime mud lithified rapidly by high mg calcite grain growth such that subsequent earthquakes shattered the folded veins in the belt basin of western north america tsunamis from different events created currents that transported ooids and coarse sand from shoals into deeper water and scoured deeply into the now consolidated sediment leaving behind a lag of vein fragments molar tooth structure is a signature of basin tectonism recorded specifically in argillaceous lime mudstones deposited in low energy settings above the thermocline in supersaturated marine water syneresis cracks are the counterpart in terrigenous facies in that silt and sand were injected into the shrinkage fissures this process points to the importance of seismically induced compaction under negligible burial smectite might have made up a significant proportion of the sediment making it more conducive to shrinkage the absence of molar tooth structure in phanerozoic strata is ascribed mainly to changes in sediment rheology brought about by increased organic binding due to diversification of the microbiota