rhythmic shelf storm beds proterozoic kaimur formation india abstract the shallow marine turbidite like storm beds of the proterozoic kaimur formation exhibit intrabed rhythmic vertical variations in sedimentary structure and texture attributable to different patterns of velocity pulsations within an event of storm current deposition the alternate repetition of parallel laminated s b and ripple cross laminated s c divisions s bcbc within individual storm beds has been interpreted to represent periodic fluctuation in flow intensity beyond the stability limits of ripple and upper flat bed during sedimentation sandstone beds characterised by successively occurring thin internal mud partings indicate pulsing discontinuous flows with alternate periods of slack water within an event of storm current deposition successive mud drapes occurring within the ripple cross laminated divisions s c of some of the storm beds also represent pulsing discontinuous flows palaeoccurrent analyses of these storm beds of the kaimur formation reveal a northwesterly current mode against an e w trending shoreline these storm beds thus may be interpreted to have deposited from geostrophic storm currents that were pulsating in nature with a periodicity of the order of tens of hours similar to that observed on present day storm dominated shelves

shelf construction in a foreland basin storm beds shelf sandbodies and shelf slope depositional sequences in the upper cretaceous mesaverde group book cliffs utah the upper cretaceous campanian kenilworth member of the blackhawk formation mesaverde group is part of a series of strand plain sandstones that intertongue with and overstep the shelfal shales of the western interior basin of north america analysis of this section at a combination of small sedimentological and large stratigraphical scales reveals the dynamics of progradation of a shelf slope sequence into a subsiding foreland basin four major lithofacies are present in the upper mancos and kenilworth beds of the book cliffs a lag sandstone and channel fill shale lithofacies constitutes the thin basal transgressive sequence which rests on a marine erosion surface it was deposited in an outer shelf environment shale interbedded sandstone and shale and amalgamated sandstone lithofacies were deposited over the transgressive lag sandstone lithofacies as a wave dominated delta and its flanking strand plains prograded seaward analysis of grain size and primary structures in kenilworth beds indicates that there are four basic strata types which combine to build the observed lithofacies the fine to very fine grained graded strata of the interbedded facies are tempestites deposited out of suspension by alongshelf storm flows geostrophic flows there is no need to call on cross shelf turbidity currents density underflows to explain their presence very fine to fine grained hummocky strata are likewise suspension deposits created by waning storm flows but were deposited under conditions of more intense wave agitation on the middle shoreface cross strata sets in this region are bed load deposits that accumulated on the upper shore face in the surf zone lag strata are multi event bed load deposits that are the product of prolonged storm winnowing they occur on transgressive surfaces while the graded beds are tempestites in the strict sense all four classes of strata are storm deposits the distribution of strata types and their palaeocurrent orientations suggests a model of the kenilworth transport system driven by downwelling coastal storm flows and probably by a northeasterly alongshore pressure gradient the stratification patterns shift systematically from upper shoreface to lower shoreface and inner shelf lithofacies partly because of a reduction in fluid power expenditure with increasing water depth but also because of progressive sorting which resulted in a decrease in grain size in the sediment load delivered to successive downstream environments the kenilworth member and an isolated outlier the hatch mesa lentil constitute a delta prodelta shelf depositional system their rhythmically bedded lenticular sandstone and shale successions are a prodelta shelf facies and may be prodelta plume deposits major upper cretaceous sandstone tongues in the book cliffs are underlain by erosional surfaces like that beneath the blackhawk formation which extend for many tens of kilometres into the mancos shale these surfaces are the boundaries of upper cretaceous depositional sequences the sequences are large scale genetic stratigraphic units they result from the arranging of facies into depositional systems the depositional systems are in turn stacked in repeating arrays which constitute the depositional sequences the anatomy of these foreland basin sequences differs

pelagic sedimentation on an inactive gullied slope northwest providence channel bahamas abstract the southern flank of little bahama bank is a deep water carbonate slope into which a series of closely spaced canyons 10 200 m deep have been cut it is similar in morphology to many other slopes in the bahamas and these slopes are generally considered to be created by the erosion of downslope sediment gravity flows however little evidence for downslope sediment movement or associated erosion were found in the holocene or latest pleistocene sediments of the slope reported on here the sediments from water depths below 260 m consist of a uniform pelagic drape covering both the gullies and inter gully ridge tops high resolution seismic reflectors are not truncated along individual gully walls and the fine scale stratigraphy of the sediments recorded in glacial interglacial changes in the aragonite calcite ratio of the sediments is preserved in sediments from gullies and ridge tops it appears that for the last two to three glacial cycles there has been only minor downslope transport of sediment despite the gullied morphology erosion is not presently occurring on the slope

oligocene storm induced lacustrine deposits in the yaxi area of the jiuxi basin northeastern margin of the tibetan plateau abstract storm induced deposits in the sedimentary record can now be easily identified on account of a better understanding of processes of unidirectional and oscillatory flow thin bedded sandstone siltstone and interlayered mudstone deposits of the oligocene jianquanzi member in the yaxi area of the jiuxi basin along the northeastern margin of the tibetan plateau were formed by shallow lacustrine storm generated flows petrologic analysis reveals that the sandstones exhibit moderate to high compositional maturity but are texturally sub mature to immature based on investigation of drill cores six storm deposit lithofacies are identified including massive or graded sandstone parallel and trough cross bedded sandstones hummocky cross stratified sandstone wave rippled sandstone and siltstone massive or horizontal bedded mudstone and structurally deformed sandstone and mudstone in a vertical succession these lithofacies occur as three fining upward lithofacies associations each of which represents different architectural elements and depositional processes of proximal to distal storm deposits the relatively coarse proximal architectural element is formed by strong density flows and unidirectional dominated combined flows the transitional architectural element is dominated by unidirectional and oscillatory dominated combined flows to pure oscillatory flows resembling tb e divisions of the classic bouma sequence the relatively fine distal architectural element is dominated by unidirectional dominated combined flows with a very weak oscillatory flow component the origin of storms responsible for these deposits is attributed to asian winter monsoons associated with the paleoclimatic and paleogeographic setting

ichnofabrics and geometric configurations of ophiomorpha within a sequence stratigraphic framework an example from the upper cretaceous us western interior ichnofabric in the upper cretaceous sego sandstone and anchor mine tongue of east central utah can be differentiated into two components 1 discrete ophiomorpha and 2 mottling and other trace fossils the ichnofabric index method was employed to evaluate spatial variability of the ichnofabrics within depositional sequences and component systems tracts indices were logged for amount of bioturbation caused solely by ophiomorpha oii and that represented by all other biogenic features bii values of oii 1 are more pervasive in lowstand systems tracts compared to transgressive systems tracts this is consistent with the predominance of marginal and nearshore marine sand dominated settings that are characteristic of lowstands which are favourable habitats for colonization by ophiomorpha producers ichnofabric index values vary both vertically and laterally within any given systems tract reflecting differences in physical and biological parameters operating in the palaeoenvironment these parameters include the total number and behaviours of organisms occupying the substrate as well as substrate texture and grain size and rates of sedimentation the architectural style of ophiomorpha was examined within five depositional facies shelf storm deposit lower shoreface shoreface and estuarine inclinations of individual burrow elements were approximated relative to bedding planes categorized as either vertical inclined or horizontal and then plotted on ternary diagrams based on the types of facies present these results suggest that variations in the geometric configurations of ophiomorpha are controlled primarily by physical energy levels and the rate and nature of sedimentation results of this study have broad implications for understanding the physical factors affecting facies variability within sequences and systems tracts when coupled with sedimentologic data recognition of variations in the distribution of ichnofabrics and architectural style of ophiomorpha can provide additional information useful for characterizing depositional environments and therefore could be integrated with other basin analysis techniques to test and refine sequence stratigraphic interpretations

allogenic controls on the evolution of storm to tidal shelf sequences in the early proterozoic uncompahgre group southwest colorado usa dominantly coarse grained shallow marine metasedimentary rocks of the early proterozoic uncompahgre group ug record periods of shoaling and drowning on different temporal scales that are attributed to episodic long term oscillations in relative sea level with superimposed shorter duration excursions in relative sea level long term events are probably tectonic whereas short term events are eustatic the 2 5 km thick uncompahgre group consists of 250 600 m thick dominantly coarse grained quartzite units q1 q4 and 200 300 m thick mudstone pelite units p1 p5 five depositional systems comprise the uncompahgre group the outer shelf system oss is composed of bouma type beds and intercalated mudstones that are transitional vertically to parallel laminated to wave rippled sandstones and hummocky cross stratified sandstones of the inner shelf system iss trough cross stratified sandstones comprise the shoreface system shs the tidal inner shelf shoreface system tis shs consists of a complex interlayering of cross bedded sandstones thin bedded conglomerates mudstones and rippled sandstones trough cross bedded pebbly sandstones and thin to thick bedded conglomerates represent the alluvial system alls depositional systems in the ug are associated in transgressive and highstand systems tracts that make up four sequences 1 to 4 sequence boundaries do not correspond with lithostratigraphic boundaries but are defined by subtle unconformities the basal q1 p1 unit lower sequence 1 consists of alls to tis shs to iss comprising a transgressive systems tract a maximum marine incursion is reflected by deposition of oss facies in stratigraphic units p1 p2 shoaling in the transition from p2 to the uppermedial portion of q2 oss iss shs to a thick tis shs alls records the highstand systems tract of upper sequence 1 a subtle disconformity paraconformity delineates a type 2 sequence boundary at the top of the highstand systems tract the drowning to shoaling pattern is replicated in sequence 2 upper q2 to p3 to upper medial q3 sequence 3 upper q3 to p4 to upper medial q4 and an incomplete sequence 4 upper q4 through p5 thinner shoaling intervals of oss iss shs in p3 and in lower q2 q3 and q4 represent parasequences sequences of 107 years duration are attributed to periods of increasing and decreasing subsidence rates due to tectonism marginal to the sedimentary basin parasequences record shorter duration temporal controls of c 104 to 105 years related to eustatic oscillations as a consequence of shoaling and aggradation progradation in the highstand systems tract tis shs and alls overlie and are temporally separated from oss to iss to shs this transition records filling of the basin to sea level leading to a shelf geometry that was conducive to tidal amplification a composite relative sea level curve integrating long term pulsatory subsidence and short term eustasy best explains the stratigraphic evolution of the uncompahgre group

geochemical correlation in deltaic successions a reality check whole rock geochemical analysis can aid the stratigraphic correlation of faunally barren successions compared to petrographic techniques or heavy mineral separations such analysis is quick and objective yet rarely has the reliability of the method been tested against unequivocal correlations we have applied it to the kenilworth member of the upper cretaceous blackhawk formation a well exposed and easily correlated 100 m thick unit in the deltaic successions of the book cliffs utah united states this is the first evaluation of geochemical analysis as an aid to sequence stratigraphic interpretation at the parasequence scale from the geochemistry alone it is not possible with statistical confidence to separate individual parasequences or systems tracts nor the fluvial incised valley from the marine shoreface units the magnitude of geochemical variation in a single sandstone bed over a 30 m lateral traverse is as great as vertically through an entire parasequence y nb rb and ti abundances are inversely correlated with grain size thus geochemical signatures follow sedimentological trends and distal parts of a parasequence appear distinct from proximal parts y ratios compensate for this grain size effect but introduce confusing distortions for elements such as zr that are not correlated with grain size contrary to some predictions zr is as common in the finest grained units as the coarsest and sr is not correlated with ca in some sections sr is also inversely correlated to grain size and cannot be used as an indicator for cement abundance

sedimentology and fractal based analysis of permeability data john henry member straight cliffs formation upper cretaceous utah u s a interpretation of depositional environments combined with field measurement of permeability for a portion of the upper cretaceous straight cliffs formation near escalante utah provides new results for understanding and modeling facies dependent perme ability variations offshore transition lower shoreface upper shore face and foreshore environments are interpreted for part of the john henry member on the basis of outcrop investigation using a newly designed drillhole minipermeameter probe permeability was measured for two of the facies within this unit lower shoreface bioturbated sand stone and upper shoreface cross bedded sandstone approximately 500 permeability measurements at a sample spacing of 15 cm were made along four vertical profiles and three horizontal transects o na6m 3 21 m outcrop permeability ranges from 41 to 1 675 millidarcies md in the bio turbated sandstone facies which is massive bedded moderately to well sorted and very fine to fine grained in contrast permeability ranges from 336 to 5 531 md in the moderately to moderately well sorted fine to medium grained cross bedded sandstone facies a high degree of variability in permeability of the cross bedded facies is caused by small scale variations in grain size and structure related to depositional processes the geometric mean permeability in the bio turbated sandstone is 253 md compared with 1 395 md in the cross bedded facies while the facies dependent differences in permeability k are ap parent and related to depositional and biological processes fractal based statistical analysis of the horizontal ln k increments yields near ly identical results for the bioturbated facies and the cross bedded fa cies possibly suggesting an underlying statistical commonality in the formation of both facies increment distributions from both facies ap pear similar with peaking around the mean ln k increments from the smaller vertical data set appear similar also but with a variance approximately 3 7 times larger than the horizontal value variance scaling analyses of horizontal and vertical data both yield a hurst co efficient near 0 26 which is characteristic of negative spatial correla tion of the increments the methodology developed herein offers a po tential high resolution alternative to existing methods for understand ing and characterizing subsurface properties

morphology and dynamic sedimentology of the eastern nile delta shelf coleman j m roberts h h murray s p and salama m 1981 morphology and dynamic sedimentology of the eastern nile delta shelf mar geol 42 301 326 some 13 000 km of bathymetry and overlapping side scan sonar data were collected over a 4450 km2 area of the eastern nile river shelf these data coupled with more detailed surveys of selected areas including bottom sampling and current measurements contribute to the understanding of shelf morphology lithofacies relationships and sediment transport processes four morphologic zones were defined zone 1 smooth bottomed flocculated clays and silty clays extending from the shore line 40 km seaward to the 25 m contour zone 2 a sand belt 5 20 km wide extending eastward from the damietta mouth and curving toward the coast at the boundary of the study area dominant bottom features were migratory bedforms of various dimensions superimposed on linear sand ridges zone 3 smooth clay to silty clay bottom topography punctuated with distinct algal mounds the mounds up to 10 m relief are composed of living coralline algae which contribute coarse debris to the surrounding sediments zone 4 mud diapirs and shelf edge slumps characterize this distal part of the shelf and the upper slope sand on the inner shelf is actively migrating and probably does not represent a relict deposit the damietta distributary causes a large scale perturbation in the mean easterly drift along the egyptian coast unusually strong currents capable of transporting and reworking large volumes of sand are associated with an eddy trapped behind this feature the directionality of these currents and the curvature of the eddy axis correspond well to the distribution pattern of sand on the inner shelf

tidal and storm influenced sedimentation from a cambrian transgressive passive margin sequence the basal cambrian clastic sediments in northwest scotland record a transgressive sequence of barrier island to tidal shelf quartz arenites eriboll sandstone lower member and pipe rock overlain by a storm dominated interval of mixed clastic carbonate sediments fucoid beds two shallowing events interrupt this sequence the lower near the middle of the pipe rock and the upper at the top of the fucoid beds which led to the deposition of the quartz arenites of the salterella grit variations in the proportion of storm deposited facies and in the thickness of tidally generated structures suggests that tidal currents progressively increased during deposition of the quartz arenites and then decreased until deposition of the mixed clastic carbonate deposits these features suggest amplification of tides as the shelf approached a resonant width during transgression further transgression produced a wide non resonant shelf where tides were frictionally damped and storm processes were dominant similar sequences of the same age in western newfoundland eastern greenland and spitsbergen may also have been produced by the shelf bordering the north american craton responding to tidal amplification followed by frictional damping

maintenance of the shoreface by wave orbital currents and mean flow observations from the long island coast waves and currents were observed for a total of 36 days in august 1976 and 1977 calculations of wave transport of sediment using a bagnold equation for transport and second order stokes theory show that wave orbital current asymmetry results in landward sediment transport in 10 m or less water depth but does not significantly effect transport at greater depths calculations of sand transport using the madsen grant formulation show that during a 36 day observation period sediment in less than 10 m of water generally moved landward into the breaker and then along coast with the wave driven littoral flow the calculations show that below 10 m the generally upwelling coast parallel mean flow did not move sediment however during two days of a mild summer northeaster storm a downwelling coastal jet developed with 60 cm sec sup 1 maximum current the calculations show that if this flow had been combined with the intense wave regime of the succeeding several days sediment would have been entrained across the lower as well as the upper shoreface movement would have been offshore and alongshore such storm transport is believed to be typical of the more intense winter storms and to be responsible for the erosionalmore shoreface retreat that has caused up to a kilometer of shoreline retreat on the long island coast in the last century less

the origin and preservation of a major hurricane event bed in the northern gulf of mexico hurricane camille 1969 cores collected from mississippi sound in the gulf of mexico were studied using 210 pbxs and 137 cs geochronology x radiography granulometry and a multi sensor core logger our results indicate the presence of a widespread sandy event layer that we attribute to hurricane camille 1969 an initial thickness of more than 10 cm is estimated from the cores which is large compared to the time averaged apparent accumulation rate of 0 29 0 47 cm y 31 physical and biological post depositional processes have reworked the sandy layer producing a regional discontinuity and localized truncation and resulting in an imperfect and biased record of sedimentary processes during the storm the oceanographic and sedimentological processes that would have produced an event bed during hurricane camille are simulated using a series of numerical models i e 1 a parametric cyclone wind model 2 the swan third generation wave model 3 the adcirc 2d finite element hydrodynamic model and 4 a wave current bottom boundary layer model that is coupled to transport and bed conservation equations trans98 the simulated bed ranges from 5 cm to over 100 cm within a tidal channel near the barrier islands seaward of the islands the bed is more than 10 cm in thickness with local variability the magnitude and local variability of the storm bed thickness are consistent with the observed stratigraphy and geochronology on both the landward and seaward sides of the barriers 2002 elsevier science b v all rights reserved

transgressive regressive cycles and the occurrence of coal in some upper cretaceous strata of utah cenomanian santonian strata contain about 60 of the known cretaceous coal resources of utah the areal and temporal distribution of these resources can be related to the transgressive regressive history of the western shoreline of the interior cretaceous seaway two orders of transgressive regressive cycles the larger of which corresponds to third order cycles of vail et al are deemed important in analyzing the distribution of coal in utah the largest coal fields are associated with extensive vertical stacking of the deposits of what are here termed fourth order cycles at the transgressive and regressive maxima of third order cycles deposits of transgressive and regressive phases of third order cycles contain only minor amounts of coal this genetic relationship may prove useful in predicting areas of important cretaceous coal resources in deeper parts of structural basins in the rocky mountain region

beach and nearshore facies southeast australia abstract the morphology texture and facies sequence on seven sand beaches located in low moderate and high wave energy microtidal environments in southern australia were investigated using box coring and scuba observations systematic variation in facies occur both within and between the beaches low energy reflective beaches are limited in lateral and vertical extent and in facies to beach laminations separated by coarse step deposits from finer nearshore cross lamination facies moderate energy intermediate beaches characterised by rip circulation possess increasingly wider surfzones with ridge and runnel and bar trough facies separating the beach and step facies from the more extensive nearshore sequence high energy dissipative beaches may have 500 m wide surfzones containing multiple bar trough topography fine beach laminations with backwash structures grade into 4 5 m thick bar trough sequences then the extensive nearshore facies as wave energy increases from low h b h b 2 5 m the vertical extent of the beach to nearshore sequence increases from

sea level related architectural trends in coarse grained delta complexes distinct stratigraphical styles of delta development can be related to basically different patterns of relative sea level base level change 1 an overall rise with periods of standstill and without significant fall rs 2 an overall rise with superimposed falls rf 3 an overall fall with superimposed rises fr and 4 an overall fall with periods of standstill and no significant rises fs the related stratigraphical models contain either preserved elements of prototype deltas or show characteristic trends in development from one prototype delta into another each stratigraphical model is characterized by lateral and vertical lithofacies features which can be reasonably well established from outcrop in coarse clastic basin margins where age control is poor sedimentological and stratigraphical delta models as presented here may help refining reconstructions of basin fill patterns and basin subsidence kinematics

onlap offlap and the origin of unconformity bounded depositional sequences christie blick n 1991 onlap ofttap and the origin of unconformity bounded depositional sequences mar geol 97 35 56 unconformity bounded depositional sequences represent the fundamental building blocks of sedimentary successions they are typically characterized by onlap at the base and by offlap at the top and they tend to be markedly asymmetrical with onlap accounting for a larger part of any cycle of sedimentation than offlap offlap cannot be attributed solely to erosional truncation but instead implies that sequence boundaries develop over a finite interval of time depositional sequences are commonly associated with a cyclic arrangement of facies but transgressive regressive cycles are out of phase with respect to sequence boundaries which in down dip locations are both overlain and underlain by progradational deposits and hence form during times of regression of the shoreline these observations are used to develop some ideas about the origin of unconformity bounded sequences with reference to the inter related roles of changes in depositional base level and sediment supply in particular it is shown than onlap and offlap are due to lateral migration of a line of critical bypassing defined so as to incorporate the effects of sediment loading and compaction as well as the rate of change of elevation with respect to sea level downward shifts in onlap may be achieved by either an increase in the rate of eustatic fall or a decrease in the rate of tectonic subsidence and it is premature to assume that eustatic and tectonic controls on sea level may be distinguished solely on the basis of the frequency of depositional cyclicity small shifts in the position of onlap can also be produced by changes in sediment supply and more attention needs to be paid to the influence of sediment supply in the development of minor boundaries unconformities related to eustatic fluctuations are thought to correspond approximately to times of relatively rapid sea level fall inflection points but questions remain about the existence of possible leads and lags of up to cycle and hence about the degree to which sequence boundaries of eustatic origin may vary in age both within a given basin and from one basin to another

isolated highstand shelf sandstone body of turbiditic origin lower kenilworth member cretaceous western interior book cliffs utah usa abstract an isolated highstand shelf sandstone body of turbiditic origin has been identified in the lower kenilworth member hatch mesa area book cliffs utah relative to most other marine mudstone encased isolated sandstone bodies the hatch mesa succession has an abundance of bouma like turbidite beds a greater proportion of mudstone and siltstone is thinner 6 20 m and is weakly coarsening upward these attributes in combination with locally abundant hummocky cross stratified sandstones a paucity of wave ripples in the background fair weather deposits carbonaceous matter in all facies a lack of clinoforms and a low diversity abundance trace fossil suite suggest deposition in a prodelta environment between fair weather and storm wave base a three component shoreface to shelf model consisting of delta front subaqueous channel and prodelta turbidite deposits is proposed to explain the depositional environment and setting of the hatch mesa succession regional correlation has demonstrated that the hatch mesa succession is time equivalent with the second parasequence in the kenilworth kps2 which is characterized by inner shelf turbidite bodies at a number of other localities paleogeographic reconstructions reveal that the hatch mesa succession was deposited at least 16 km basinward of the kps2 lower shoreface in approximately 20 m of water the kps2 hatch mesa sandstone body package is part of the early highstand systems tract indicating that isolated or stray sandstone bodies are not solely restricted to the falling stage or lowstand systems tracts

baffin island snow extent sensitivity insights from a regional climate model 1 recent modeling efforts suggest that the little ice age lia onset could be explained by a series of four large decadally spaced volcanic eruptions at that time glaciers on baffin island advanced and did not retreat until the past century perhaps due to arctic and north atlantic ocean sea ice feedbacks to try to determine what parameters sustain snow cover we investigate the relative impacts of changes in radiation and advection on minimum summer snow extent over baffin island we used the weather research and forecasting wrf model to run eight 6 month long april september 10 km resolution simulations in which we varied boundary condition temperatures solar radiation and sea ice cover although the control run underestimated cloud cover and thus produced an exaggerated diurnal 2 m temperature cycle the relative changes of snow extent show that wrf accurately simulates snow expansion into the same regions as during the lia with an average temperature decrease from current temperatures by 3 9 1 1 k it only requires one season for the model to lower the snowline by comparable elevation changes seen during the descent into the lia wrf s maximum snow line sensitivity is 7 k km within the range of the typically assumed lapse rate of 5 7 k km in the canadian arctic thus if a shift in the arctic climate greatly expanded sea ice coverage following large volcanic eruptions this would have been enough to perpetuate an ice sheet on baffin island throughout the lia

cretaceous rhythmic bedding sequences a plausible link between orbital variations and climate abstract simple oscillation cycles have been recognized in cretaceous pelagic sequences as a marked periodicity in carbonate and or organic carbon content the estimated periodicities of these cycles resemble milankovitch type variations in orbital elements however during warm geologic periods the nature of the link between orbital variations and the sedimentary record has been problematic climate model studies show that the intensity of continental margin precipitation in specific low latitude regions is a function of land sea thermal contrast which is sensitive to orbital variations cretaceous climate model simulations are characterized by regions of intense precipitation the locations of which are controlled by geographic configuration in particular in the model simulation the northern margin of the tethys ocean has all the climatic characteristics necessary to exhibit a sensitivity to orbital variations detailed studies of the biota sedimentology and geochemistry of the bedding rhythms in greenhorn cyclothem in the western interior seaway of north america are best interpreted in terms of variations in precipitation runoff and stable stratification of the seaway combined the geologic and model studies support a link between orbital periodicities variations of intense precipitation associated with specific paleogeographic situations and cretaceous bedding patterns this hypothesis has important implications for the sensitivity of the atmosphere hydrosphere lithosphere system to external forcing

large scale stratigraphic architecture eustatic variation and unsteady tectonism a theoretical evaluation we incorporate a process based depositional model with basin subsidence models to predict stratigraphic records this allows us to investigate the importance of subsidence geometry on coastal stratigraphy and thus to characterize and compare the stratigraphic architecture of two categories of tectonic basins the models demonstrate that the correlation of stratigraphic sequences to eustatic cycles is not the same in passive margin basins as in foreland basins and that in a foreland basin the record of episodic tectonism is distinct from that of eustatic sea level change in the model sediment transport is approximated by slope controlled diffusion nonmarine transport is treated as more efficient diffusion than marine transport three different subsidence and sediment supply models are examined a simple passive margin basin a simple foreland basin and then a foreland basin for which vertical motions are driven by thrust shortening that is compensated flexurally and for which sediment supply is related to relief predicted passive margin stratigraphies for cases of varying eustatic sea level are similar to those of natural basins and include progradational packages and subaerial unconformities which are used to define sequence boundaries that form during sea level fall we explore the timing relationships between stratigraphic features and a sinusoidal sea level history showing that the phase relationship depends on subsidence sediment flux efficiency of sediment transport and period and amplitude of sea level when the basic geometry of the basin is inverted placing the sediment supply on the side with maximum subsidence as is the case in foreland basins the sequence character changes markedly subaerial erosion does not generate unconformities in the models of a dynamic foreland basin sediment supply and subsidence are linked to the structure of the flanking thrust belt and are not necessarily constant for steady thrusting and variable sea level unconformities that define sequence boundaries form only on the distal or forebulge side of the basin and the ages of the sequence boundaries correlate to limes of rising sea level in cases of constant sea level but variable thrusting subaerial unconformities are cut locally on both the proximal margin of the basin and the distal margin of the basin yet the ages of the proximal margin and distal margin unconformities are out of phase in the tectonic cycle erosion is most pronounced during quiescence on the proximal side and during thrusting on the distal side

barrier island evolution middle atlantic shelf u s a part i shoreface dynamics abstract in part i of this paper near bottom fluid velocity and sediment concentration measurements from the shoreface of a middle atlantic barrier long island coast are analyzed to provide insight into the dynamics of erosional shoreface retreat in part ii these data are combined with observations of coastal stratigraphy in order to construct a model for the evolution and behavior of middle atlantic barriers wave motions on the long island shoreface tend to drive sediment onshore calculations of the onshore sediment flux driven by asymmetrical shoaling waves show that for the wave states commonly encountered on the middle atlantic shelf values become strongly positive landward of approximately 10 m water depth upper shoreface but drop to very low values shortly seaward of that isobath on the lower shoreface and inner shelf floor fair weather wave current interaction tends to cause a landward creep of sediment even though the wave orbital component of the near bottom velocity field is nearly symmetrical this movement occurs because the wave approach direction generally lies within 90 of the direction up coast or down coast of the alongshore wind driven current therefore fluid shear stress acting on the sediments tends to be reinforced during the shoreward stroke of the wave orbital motion and partially cancelled during the seaward stroke during major storms the rate of sediment transport increases by at least an order of magnitude and the role of wind driven currents becomes important much more sediment is resuspended by the wave orbital current component because storm waves are more powerful than fair weather swells but the wind driven flow component is also much stronger and is now available for transporting the resuspended sediment storm flows over the shoreface occur in distinct dynamic zones surf zone friction dominated zone transition zone geostrophic zone the zones are defined by dynamical considerations and expand or contract with the intensity of the causative wind stress during peak flow events the characteristic length scales of the friction dominated and geostrophic zones tend to correspond with the morphologic zones of the shoreface and inner shelf the storm intensified wind driven alongshore flows of the lower shoreface are frequently jet like in nature and may be upwelling or downwelling wind driven coastal flows with a strong downwelling component occur during most storms on the long island coast and are particularly important to the coastal sand budget during these events sand is entrained by the storm intensified upper shoreface circulation system of wave driven alongshore currents and rip currents and is fed into the main wind driven coastal current because of the offshore component of bottom flow sand is swept down the lower shoreface and onto the adjacent inner shelf fair weather processes may be unable to return storm deposited sand to the beach from such an offshore position the shoreface transport regime of the long island barrier coast thus consists of long periods of time months during which sand moves slowly toward the beach punctuated by short intense periods hours or days during which sand is transferred from the shoreface to the adjacent inner shelf the long term sense of movement of the shoreface whether retreating the long island case or prograding must depend on the loss or gain of sand by shoreface with respect to the inner shelf the data shows that the coastal sand budget is controlled not only by the upper shoreface cycle withdrawl of sand from the beach prism storage in the breakpoint bar and its subsequent return to the beach but by a cycle of larger spatial and temporal scale in which sand is exchanged between the shoreface as a whole and the adjacent inner shelf

shallow marine sedimentation within an active margin basin james ross island antarctica abstract the santa marta formation exposed on northern james ross island antarctica represents shallow marine shelf sedimentation within an active margin basin the formation is approximately 1000 m thick is of uppermost santonian to campanian age and represents part of a 5000 6000 m thick sedimentary sequence forming the larsen basin the larsen basin represents either a back arc basin or a half graben basin developed on the extending margin of the weddell sea twelve sedimentary facies have been recognised which can be subdivided into two groups those which are controlled by shelf processes and those which are related to active arc volcanism background shelf processes include fair weather suspension sedimentation storm sands and rare tidal current reworking active arc processes include direct settling of airfall detritus and rapid resedimentation of volcaniclastic detritus by sandy debris flows and both high and low concentration turbidity currents in addition sheet conglomerates represent debris flows that evolved from syn sedimentary slumps two facies associations representing a mid to outer shelf and an inner shelf depositional setting respectively can be distinguished with an apparent regression in the mid to late campanian

barrier island evolution middle atlantic shelf u s a part ii evidence from the shelf floor where the atlantic coastal gradient is low and the substrate is unconsolidated shoreface processes lead to an equilibrium shoreface surface characterized by a straightened plan view and a concave up profile maintenance of this surface during the post glacial sea level rise may lead to barrier formation by mainland beach detachment where the outer oceanic shoreface is maintained by wave and current processes downwelling storm currents sweep sand from the shoreface of the middle atlantic bight transport it downcoast and seaward and deposit it on the adjacent inner shelf some of this sand is returned by the action of asymmetrical wave orbital currents during fair weather however there is a net loss in most areas the loss is not made good by river sand input since on the atlantic coast all river sand is trapped by estuaries as a result the shoreface in these areas is undergoing erosional retreat mean horizontal retreat rates are on the order of 1 3 m yr 1 but most movement occurs in brief episodes with recurrence rates on the order of several events per century erosional shoreface retreat coupled with aeolian overshoot storm washover and the movement of sand through inlets into the lagoons has lead to landward migration of the atlantic barriers through the holocene period of sea level rise the landward retreat of the barrier as a whole is as dependent on inlet formation as it is on erosional shoreface retreat repeated inlet breaching and the downdrift migration of inlets yields coalescing flood tidal deltas within the lagoon the resulting surface forms a platform on which the subaerial barrier deposits can advance under the impetus of storm washover and aeolian action the backbarrier deposits eventually re emerge at the shoreface where their upper beds are eroded and recycled the barrier thus migrates over a pavement of its own flood tidal deltas and washover fans sand is added to the system partly through the erosion of updrift headlands but mainly by means of scour in inlets which reaches down to the underlying pleistocene the modern shelf sand sheet is the debris blanket resulting from the retreat process the leading edge occurs as a thin veneer of rip current fallout on the shoreface this is periodically stripped off by winter storms to expose the underlying backbarrier strata further seaward shelf sands eroded from the shoreface rest disconformably on backbarrier deposits shoreface beach and dune strata are always missing every grain of sand in this shelf sand sheet has occupied a position on the beach or upper shoreface within the recent past however the sheet as a whole has been reconstituted its primary structures are those of the inner shelf floor backbarrier sand and mud deposits characterized by channeling and landward dipping reflectors can be traced for over 100 km seaward across the middle atlantic shelf beneath the modern sand sheet the backbarrier stratum together with the overlying shelf sand layer constitutes the record of barrier retreat across the atlantic shelf surface during holocene time the sheet like nature of this record indicates that viewed at the appropriate time scale barrier migration is a continuous process

stratigraphical architecture at the muddy margin of the cretaceous western interior seaway southern utah the tropic shale and correlative tununk shale member of the mancos shale accumulated during cenomanian turonian time within prodeltaic environments near the western margin of the western interior seaway of north america stratigraphical and sedimentological analysis has revealed a detailed history of relative sea level change in the thick fine grained succession the tropic and tununk shales were deposited during the greenhorn second order sea level cycle over a time span of about 2 5 million years in southern utah six depositional sequences are superposed upon the record of this long term sea level change the sequences developed during third order relative sea level cycles of hundreds of thousands of years duration and are composed of at least 37 parasequences arranged in retrogradational aggradational and progradational parasequence sets the tropic shale and tununk shale member accumulated just basinward of the axis of maximum subsidence of a foreland basin stratal geometries and facies distribution patterns in the succession indicate that in southern utah the greenhorn cycle was tectonically controlled during the greenhorn transgression and highstand rapid rates of tectonic subsidence trapped terrigenous sediment to the west of the study area in the more proximal foreland at this time hemipelagic facies accumulated at relatively slow rates in southern utah and type 2 sequences developed during third order sea level cycles in contrast during the greenhorn regression rates of thrust induced subsidence in the proximal foreland basin evidently slowed and deltaic clinoforms prograded across the study area at least one forced regression occurred in southern utah at this time and type 1 sequences developed the formation of type 1 sequence boundaries in the upper part of the tropic shale and tununk shale member points to episodes of base level fall and indicates that the six third order sea level cycles recorded in the succession were not the result of changes in sediment supply alone the third order cycles may have been a consequence of episodic tectonism the timing of these cycles however suggests that development of sequences and parasequences in the tropic shale and tununk shale member may have been related to orbital forcing in the milankovitch band glacioeustasy or climatically related fluctuations in the amount of groundwater stored on continents may explain these high frequency sea level changes

a stochastic modeling approach to the fine sediment budget of the new york bight a stochastic model of fine sediment transport on the continental shelf is modified to incorporate the effects of storm flows the model based on surface gravity wave resuspension now includes transport by both tides and episodic storm flows effects of gravity wave storm flow correlation as deduced from near bottom current meter measurements are also included sediment dispersion is significantly faster when the effects of storm flows are included the effects of the gravity wave storm flow correlation and the regional net flow pattern are small the model is solved numerically for equilibrium dispersal patterns for sediment entering at estuary inlets bordering the new york bight region additional dispersal patterns are found for upcoast and downcoast sources as well as the dredge spoil dump site combinations of these dispersal patterns are least squares fit to the observed fine sediment distribution in the new york bight to evaluate the magnitudes of the sources of sediment negative source magnitudes are interpreted as sediment sinks the results are consistent with other estimates of sediment sources and sinks in the new york bight

analytical method for estimating palaeowave climate and water depth from wave ripple marks analytical expressions are developed for evaluating palaeodepth and palaeowave conditions from wave ripple marks palaeodepth and palaeowave climate estimates can be extremely powerful predictors in reconstructions of vertical movements related to basin subsidence local and regional ancient wave climate trends and the basin shape the analytical expressions developed herein are applied to wave ripple marks preserved in the lower marine molasse of switzerland western austria and southern germany the results suggest that in contrast to traditional concepts of an eustatic sea level change the lower marine molasse regression was a result of mainly depositional processes the subsidence rate greatly exceeded the rate of sea level fall over a wide geographical area furthermore the widening of the lower marine molasse basin to the ene is well documented by the increase of the ancient wave period and wave power calculated from wave ripple marks found in storm deposits

conditions for the ignition of catastrophically erosive turbidity currents abstract the basic impediment to a clear understanding of eroding and depositing turbidity currents has been the lack of a proper formulation of bed sediment entrainment this problem is addressed herein a detailed analysis indicates that the bagnold criterion is necessary but insufficient for self sustaining turbidity currents the analysis reveals two possible equilibrium states a relatively low velocity ignitive state and a high velocity dense catastrophic state a stability analysis indicates that the ignitive state is unstable flows below it invariably die out and flows above it ignite i e accelerate and entrain sediment to the catastrophic state which is stable the ignitive state thus defines the criterion for a self sustaining turbidity current estimates of the catastrophic state suggest that it is highly erosive and competent to scour out submarine canyons

the role of wave reworking on the architecture of storm sandstone facies bell island group lower ordovician eastern newfoundland stacked shallow marine cycles in the lower ordovician bell island group of bell island newfoundland show upward thickening and upward coarsening sequences which were deposited on a storm affected shelf in the beach formation each cycle has a facies sequence comprised from base to top of dark grey mudstones light grey mudstones tabular sandstones and mudstones lenticular sandstones and mudstones and thick bedded lenticular sandstones reflecting a progressive increase of wave orbital velocities at the sediment surface the mudstones and tabular sandstones reflect an environment in which the sea floor lay in the lower part of the wave orbital velocity field and in which tempestites were deposited as widespread sheets from weak combined flow currents the lenticular sandstones in the succeeding facies are wave reworked sands commonly lying in erosional hollows and having erosional tops and internal hummocky cross stratification planar lamination is relatively uncommon and sole marks are mainly absent in this facies oscillatory currents were dominant and accumulated sand in patches generally 10 30 m in diameter the facies formed on the inner shelf where the oscillatory currents generated by storm waves had powerful erosional effects and also determined the depositional bedforms mud partings and second order set boundaries within sandstone beds are believed to separate the products of individual storms so that many lenticular sandstone beds represent the amalgamation of several event beds this interpretation has important implications for attempts to estimate event frequency by counting sandstone beds within a sequence and for estimates of sand budgets during storm events the thick bedded lenticular facies appears to have been formed by erosion of the mud beds between the lenticular sands leading to nearly complete amalgamation of several lenticular sand bodies except for residual mud partings in the overlying redmans formation the process of amalgamation progressed even further so that nearly all the mud partings were removed resulting in the formation of thick bedded tabular sandstones sequence stratigraphic analysis of the cyclical sequence suggests that the cycles were eustatically controlled the rising limb of the sea level curve produced only the dark grey mudstone part of the cycle while the remainder of the cycle was deposited on the falling limb there is a gradational but rapid facies transition from the tabular to the lenticular sandstone facies which is interpreted as occurring at the inflexion point on the falling limb the thick bedded facies of the beach formation and the thick bedded tabular facies of the redmans formation represent periods of maximum sea level fall the stacked cycles in the beach formation are interpreted as an aggradational high frequency sequence or parasequence set bounded at the top by a sequence boundary and succeeded by the three aggradational parasequences of the redmans formation the recognition of storm facies with sandstone beds of very different bed length has important implications for the reservoir modelling of such facies

modern sediment accumulation and strata formation on the amazon continental shelf abstract formation of fine scale cm strata on the amazon continental shelf was investigated using radiographic and radiochemical e g 210pb techniques three distinct sedimentary environments are found within the inner shelf m water depth mud deposit 1 interbedded mud and sand in a corridor extending from the river mouth across the inner shelf 2 faintly laminated muds on both flanks of the corridor and 3 bioturbated muds comprising the remainder of the deposit high rates of sediment accumulation 2 cm yr are found in the region of interbedded mud and sand and in the regions of faintly laminated muds accumulation rates in the region of bioturbated muds are relatively low 0 1 2 cm yr and an inverse relationship is observed between accumulation rate and extent of sedimentary structure alteration by benthic fauna a first order sediment budget indicates that sediment accumulation on the amazon shelf between the brazil french guiana border and the para river is nearly equal to estimates of sediment discharged by the amazon river 9 10 8 tons yr the model of amazon shelf sedimentation most consistent with the radiographic and radiochemical data suggests that the inner shelf mud deposit is a subaqueous delta prograding seaward over relict basal sands and that a turbulent jet emanates from the river mouth with competence during some flood periods to carry sand as bedload across the inner shelf the presence of cross bedded sands in the corridor of interbedded mud and sand supports the hypothesis of a turbulent jet the faintly laminated muds probably represent suspended sediment rapidly deposited on both sides of the jet