sea level earth s dominant elevation implications for duration and magnitudes of sea level variations abstracthigh resolution digital elevation models of earth s solid fluid interface reveal that earth s bimodal elevation distribution is characterized by a dominant mode at or very close to present day sea level it is hypothesized that sea level is earth s dominant modal elevation because it is the only elevation where two opposing first order surface processes erosion and deposition act to increase the area of a single elevation the observation that sea level is the dominant elevation implies that 1 the time needed for the modal elevation to adjust is very short and or 2 long term 106 107 yr mean sea level has not varied significantly from its present height the mass of material that defines the sea level modal elevation is quite small whether integrated over 1 or 100 m of thickness and could be transported in 104 yr this would suggest that the shoreline at no matter what height relative to the present 0 m elevation was probably always earth s dominant elevation however eurasia north am

oxygen isotopes and sea level from the time that detailed oxygen isotope records derived from foraminifera living in the constant temperature environment of the abyssal ocean became available there has been a discrepancy between the ice volume record that these records imply and that derived from the altitude of dated coral terraces around the world here we re examine the data and conclude that the temperature of the abyssal ocean has been an actively varying component of the climate system

disentangling the control of tectonics eustasy trophic conditions and climate on shallow marine carbonate production during the aalenian oxfordian interval from the western france platform to the western tethyan domain the objective of this work is to improve our understanding of the processes controlling changes in the architecture and facies of intracontinental carbonate platforms we examined the facies and sequence stratigraphy of aalenian to oxfordian limestones of western france seventy seven outcrop sections were studied and thirty one sedimentary facies identified in five depositional environments ranging from lower offshore to backshore platform evolution was reconstructed along a 500 km cross section twenty two depositional sequences were identified on the entire western france platform and correlated with european third order sequences at the biozone level demonstrating that eustasy was the major factor controlling the cyclic trend of accommodation the tectonic subsidence rate was computed from accommodation measurements from the aalenian to the oxfordian in key localities tectonism controlled the sedimentation rate and platform architecture at a longer time scale tectonic subsidence triggered the demise of carbonate production at the bathonian callovian boundary while the uplift made possible the recovery of carbonate platform from caen to le mans during the mid oxfordian topography of the paleozoic basement mainly controlled lateral variations of paleodepth within the western france platform until the mid bathonian a synthesis of carbonate production in the western tethyan domain at that time was conducted stages of high carbonate production during the bajocian bathonian and the middle to late oxfordian are synchronous with low 13c high eccentricity intervals and rather dry climate promoting 1 evaporation and carbonate supersaturation and 2 oligotrophic conditions periods of low carbonate production during the aalenian and from the middle callovian to early oxfordian correlate with high 13c and low eccentricity intervals characterized by wet climate and less oligotrophic conditions such conditions tend to diminish growth potential of carbonate platforms this work highlights the importance of climate control on carbonate growth and demise at large scale in western tethyan epicontinental seas

analysis of geological hiatus surfaces across africa in the cenozoic and implications for the timescales of convectively maintained topography geological maps are a powerful but underutilized tool for constraining geodynamic processes and models unraveling the cenozoic elevation history of africa and distinguishing between competing upli

depositional and pedogenic influences on the environmental geology of holocene mississippi river floodplain deposits near ferriday louisiana abstract core descriptions and the mineralogy of holocene mississippi river floodplain deposits in louisiana provide insights on fluid migration pathways and the origin of iron rich ground waters in the mississippi river alluvial aquifer mraa vertical changes in the sedimentologic and pedologic characteristics of floodplain deposits near ferriday louisiana provide evidence for two stages of floodplain development and suggest that depositional processes and drainage conditions changed substantially during the holocene depositional and pedogenic processes produced complex fluid migration pathways in the mraa confining unit and also contributed to the formation of iron rich ground waters lower holocene deposits in the study area are older than 5000 yrs bp and show evidence of crevassing lacustrine delta building and multi channel stream deposition these processes deposited thin and narrow sheet sands which represent fluid migration pathways in the mraa confining unit poor drainage conditions during this initial stage of floodplain development also favored the precipitation of authigenic siderite and pyrite in poorly drained swamps and shallow lakes the pyrite and siderite probably represent the source of iron rich ground waters in the mraa upper holocene floodplain deposits are younger than 5000 yrs bp and represent the transition to the present day meandering regime of the mississippi river near ferriday this second stage of floodplain development was accompanied by pedogenesis which produced slickensides in clayey backswamp soils the abundance of slickensides and the presence of the thin and narrow sheet sands indicates that fluid migration in the mraa confining unit near ferriday is greater than generally recognized seasonal water table fluctuations and the mixing of oxygenated meteoric and reduced ground waters cause iron oxide reduction and pyrite oxidation in backswamp settings which releases iron into solution the presence of high up to 16 mg l dissolved iron concentrations in water wells that are screened beneath pyrite and siderite rich muddy backswamp deposits near ferriday suggests that the distribution of fine grained alluvium is a primary control on the presence of iron rich waters in the mraa in contrast water wells that are screened in sandy meander belt deposits which lack abundant iron bearing minerals have low less than 1 mg l concentrations of total dissolved iron studies of large floodplains such as the mississippi river highlight the importance of floodplain histories for evaluating geologic influences on water quality developing proper floodplain land use strategies and for improving our understanding of the environmental geology of floodplain systems

evolution of the holocene mississippi river floodplain ferriday louisiana insights on the origin of fine grained floodplains abstract the alluvial architecture and soil characteristics of holo cene mississippi river floodplain deposits in the southern lower mississippi valley provide evidence for significant changes in floodplain development in response to sea level rise floodplain cores acquired near ferriday louisiana show that holocene deposits consist of 15 30 m ave 20 m of sands silts and clays which overlie late wisconsin sands and gravels on the basis of differences in sediment grain size sediment body geometry and the abundance of soil features the holocene deposits are subdivided into lower and upper holocene units lower holocene deposits 5000 yr b p consist of lacustrine and poorly drained backswamp muds that contain authigenic siderite pyrite and vivianite and show little evidence of soil formation muds encase crevasse splay and floodplain channel sand bodies 1 km wide and collectively these deposits represent a mosaic of shallow lakes poorly drained backswamps and multichannel streams similar to modern examples in the atchafalaya basin 100 km south of ferriday upper holocene deposits 5000 yr b p are represented by large mississippi river meander belt sand bodies that are up to 15 km wide and 30 m thick natural levee silts and sands and well drained backswamp muds are present between meander belt sands upper holocene deposits contain abundant soil features and sandy and silty soils are entisols inceptisols and alfisols whereas clayey soils are vertisols the presence of isolated sand bodies surrounded by mud and the scarcity of soil features suggest that lower holocene sediments reflect a period of rapid floodplain aggradation during which crevassing lacustrine sedimentation and avulsion dominated floodplain construction no evidence of large meandering mississippi river channels represented by buried thick tabular sands exists near ferriday and discharge in lower mississippi valley flow was probably conveyed by a network of small multichannel floodplain streams upper holocene sediments record a dramatic change ca 5000 yr b p from rapid to slower floodplain aggradation which was accompanied by extensive lateral channel migration overbank deposition and soil formation on the basis of differences in meander belt dimensions and numbers of abandoned channels upper holocene meander belts are subdivided into simple and complex forms relative age relationships suggest that the smaller and older simple meander belts represent periods of divided mississippi river flow and early attempts to establish a large single channel meandering regime this type of meandering regime is represented by the larger and younger complex meander belts and includes the modern meander belt similarities in the timing of changes in floodplain processes and fluvial style and decreasing rates of holocene sediment accumulation in the southern lower mississippi valley strongly suggest that decelerating holocene sea level rise in the gulf of mexico affected floodplain development at least 300 km inland from the present day coast the alluvial architecture of the lower holocene deposits and the absence of large meandering mississippi river channel deposits older than 5000 yr b p near ferriday indicates that most of the floodplain muds were deposited by avulsion related crevassing and lacustrine sedimentation rather than by overbank flooding of large mississippi river channels similarities between the floodplain history of the mississippi river and those of modern and ancient rivers elsewhere further suggest that avulsion rather than simple overbank deposition contributes to the construction of fine grained floodplains to a greater degree than generally recognized

late pleistocene evolution of the lower mississippi river valley southern missouri to arkansas the mississippi river served as the principal conduit for meltwaters discharging from the southern margins of north american ice sheets the northern half of the mississippi river alluvial valley in missouri and arkansas can be subdivided into two distinct channel courses the western and eastern lowlands which are separated by crowleys ridge the eastern lowlands contain two entry points the bell city oran and thebes gaps the latter is a narrow bedrock gorge that contains the modern river this paper summarizes a model for evolution of channel courses in the western and eastern lowlands during the illinoian and wisconsin glacial stages for the western lowlands stratigraphic relations between loess units and subjacent fluvial deposits show that major continuous terrace surfaces or valley trains are illinoian or late wisconsin in age investigations of tributary valley fills further show the western lowlands were deeply incised during the early to middle wisconsin stage then aggraded during the late wisconsin stage to such an extent that tributary valleys were backfilled with slackwater deposits moreover analysis of loess thinning and fining trends shows that early to middle wisconsin roxana silt and the late wisconsin peoria loess covers on western lowlands valley trains were derived from deflation of valley trains in the eastern lowlands we conclude that the western lowlands were the primary mississippi river course during illinoian deglaciation but were abandoned shortly thereafter then episodically reoccupied during the late wisconsin stage for the eastern lowlands course valley trains of early to middle wisconsin age are not preserved with the exception of a small remnant where the mississippi river broke through a gap in the southern margins of crowleys ridge and reentered the western lowlands all valley trains in the eastern lowlands are late wisconsin age a series of discontinuous terrace surfaces in the bell city oran gap represent dissected remnants of the primary late wisconsin full glacial and early late glacial course of the mississippi river or combined mississippi and ohio rivers whereas younger continuous braided stream surfaces in the bell city oran and thebes gap courses represent valley trains of latest wisconsin and earliest holocene age hence the bell city oran gap was the primary course during the entire wisconsin stage and the thebes gap was permanently occupied in earliest holocene time as much as 5 m difference in elevation between successive late wisconsin terrace surfaces of the eastern lowlands indicates that valley trains were rapidly constructed incised and abandoned in response to fluctuations in the delivery of meltwater and outwash from the former ice margin moreover a variety of data indicates that deglacial meltwater floods were commonly of sufficient magnitude to overflow from the primary course in the bell city oran gap throughout the late wisconsin substage floodwaters overflowed into the western lowlands constructed a series of secondary valley trains and filled tributary valleys with slackwater deposits during the latest wisconsin stage perhaps in association with the drainage of glacial lake agassiz floodwaters overflowed into the western lowlands and broke through the thebes gap the charleston fan represents the initial deposits of the thebes gap course following deposition of a large crevasse splay at the western lowlands entrance that course was no longer active but flow was routed through the thebes gap during periods of high magnitude flooding until the bell city oran gap was abandoned and the thebes gap course was permanently occupied during earliest holocene time

a contemporary appraisal of some key fiskian concepts with emphasis on holocene meander belt formation and morphology abstract looking back five decades and reflecting on the contributions of harold fisk to lower mississippi valley lmv geological knowledge it is apparent that he was highly successful in making engineers aware of the critical role of glacial advance and retreat sea level changes valley slope and river gradient and time in affecting geomorphic processes sedimentary patterns and river regimes perhaps his greatest contribution to river engineering was in recognizing and elaborating on the effects of bed and bank materials on stream meandering fisk however could only work within the prevailing geological theory and method of his time with regard to glacial chronology and stratigraphy and such matters as global structure and tectonics major advances in these areas with new concepts and techniques have shown his lmv glacial response model to be oversimplified and invalid in some respects more cycles of valley filling and cutting are now known to be represented in the alluvial sequence and his concept of deep valley entrenchment as a direct response to sea level changes is untenable in light of substantial new data the presence of a regional rectilinear fracture pattern with a corresponding network of major fault zones has not been substantiated by subsurface data the most widely known aspect of fisk s work is his detailed interpretation and colorful depiction of the complex pattern of overlapping cross cutting and discontinuous holocene meander belts and their hundreds of abandoned channels however it is now recognized that fisk s elaborate chronological reconstruction is largely invalid both because of some incorrect basic assumptions and his lack of radiocarbon and other numerical dating techniques for precise control a newly emerging model of meander belt formation is more generalized and relies heavily on archaeological evidence it places emphasis on episodes of multiple active river courses partial flow channels frequent formation of distributary channels and the significant influence of mississippi river tributaries and minor basin drainage interesting regional variations over time in meander belt configurations and morphology may reflect climate changes but at present broad scale variations in the lithology of the quaternary alluvial sequence appear to be the dominant control

identification of a 15 m wisconsin shoreline on the texas inner continental shelf abstract the maximum sea level position during the middle wisconsin oxygen isotope stage 3 is controversial it being interpreted as anywhere from at or above present day levels to 85 m high resolution seismic lithologic paleontologic and radiocarbon data from the east texas continental shelf were used to map a stage 3 shoreline the shoreline was placed in a regional context with a study of the texas shelf based on seismic profiles core and platform boring descriptions that identifies the stage 3 maximum flooding surface and the stage 2 sequence boundary these surfaces were then traced up dip on the east texas shelf where a shore parallel escarpment was recognized landward of the escarpment the stage 3 maximum flooding surface is amalgamated with the stage 2 sequence boundary therefore the location of the escarpment marks the up dip limit of stage 3 deposits and a stage 3 shoreline lagoonal sediments with in situ brackish water fossils on lap the escarpment seaward of the escarpment remnants of barrier island facies have been preserved as a bank freeport rocks bathymetric high located in 18 m of water the shoreline adjusted for subsidence is located at 15 2 m which is shallower than what oxygen isotope and coral records indicate

joint seismic geodynamic mineral physical modelling of african geodynamics a reconciliation of deep mantle convection with surface geophysical constraints recent progress in seismic tomography provides the first complete 3 d images of the combined thermal and chemical anomalies that characterise the unique deep mantle structure below the african continent we present a tomography based model of mantle convection that provides an excellent match to fundamental surface geodynamic constraints on internal density heterogeneity that includes both compositional and thermal contributions where the latter are constrained by mineral physics the application of this thermochemical convection model to the problem of african mantle dynamics yields a reconciliation of both surface gravity and topography anomalies to deep seated mantle flow under the african plate over a wider range of wavelengths than has been possible to date on the basis of these results we predict flow in the african asthenosphere characterised by a clear pattern of focussed upwellings below the major centres of late cenozoic volcanism including the kenya domes hoggar massif cameroon volcanicline cape verde and canary islands the flow predictions also reveal a deep seated large scale active hot upwelling below the western margin of africa under the cape verde islands that extends down to the core mantle boundary the scale and dynamical intensity of this west african superplume is comparable to the south african superplume that has long been assumed to dominate the large scale flow dynamics in the deep mantle under africa we evaluate the plausibility of the predicted asthenospheric flow patterns through a comparison with seismic azimuthal anisotropy derived from independent analyses of african shear wave splitting data

sea level variations global sedimentation rates and the hypsographic curve sea level changes during the neozoic have been estimated by two different methods the first involves measuring the amount of present day land area which was flooded during the past and using the present day hypsographic curve to estimate the amount of sea level rise necessary to produce this flooding the second involves the estimation of the changing volume of mid oceanic ridges through time and estimating sea level changes after having allowed for isostatic adjustment a difference in sea level of 170 m is obtained from the two methods for the cretaceous 80 100 m y b p this is equivalent to a difference in continental flooding of 24 mm 2 using the present day hypsographic curve we attempt to explain this difference firstly by allowing for the fact that the present day ocean basins have more sediment in them than did the cretaceous ocean basins this produces a sea level change in the opposite direction to that produced by the reduction in mid ocean ridge volume since the cretaceous secondly we suggest another large factor in producing the difference is that the present day hypsographic curve is not the correct one to use when studying sea level stands in the cretaceous present day average continental heights are closely related to continental areas accepting this principle if continents are joined together in the past their average height must be greater and so their hypsographic curve must be steeper a given sea level rise would produce less continental flooding during times of continental aggregation than it would today

palaeosols in westphalian coal bearing and red bed sequences central and northern england westphalian a c coal bearing strata and red beds in the pennine basin of central and northern england contain a wide variety of palaeosols in coal bearing facies associations alluvial gley semi gley organic and rare well drained palaeosols reflect the predominance of poor drainage conditions during deposition in red beds more evolved ferruginous and ferallitic palaeosols testify to free drainage conditions during deposition catenary relationships can be inferred between most of the palaeosols in both facies associations providing further information on the palaeogeomorphology of the depositional systems and assisting in the understanding of the environments and conditions of coal seam formation

terrace stratigraphy in the tunica hills of louisiana abstract the age and number of important fossil producing late pleistocene terraces in the tunica hills have recently become quite controversial one hypothesis holds that only a single loess mantled farmdalian terrace flanks the streams in this area the other maintains that there are two terraces the youngest is considered essentially holocene in age while the older is sangamon in age radiometric and stratigraphic evidence collected for this study indicates that there are two terraces the youngest is late woodfordian to holocene in age while the other is farmdalian

loess studies in central united states evolution of concepts abstract few words in the realm of earth science have caused more debate than loess it is a common term that was first used as a name of a silt deposit before it was defined in a scientific sense because this loose deposit is easily distinguished from other more coherent deposits it was recognized as a matter of practical concern and later became the object of much scientific scrutiny loess was first recognized along the rhine valley in germany in the 1830s and was first noted in the united states in 1846 along the lower mississippi river where it later became the center of attention the use of the name eventually spread around the world but its use has not been consistently applied over the years some interpretations and stratigraphy correlations have been validated but others have been hotly contested on conceptual grounds and semantic issues the concept of loess evolved into a complex issue as loess and loess like deposits were discovered in different parts of the us the evolution of concepts in the central us developed in four indefinite stages the eras of 1 discovery and development of hypotheses 2 conditional acceptance of the eolian origin of loess 3 bandwagon popularity of loess research and 4 analytical inquiry on the nature of loess toward the end of the first era around 1900 the popular opinion on the meaning of the term loess shifted from a lithological sense of loose silt to a lithogenetic sense of eolian silt however the dual use of the term fostered a lingering skepticism during the second era that ended in 1944 with an explosion of interest that lasted for more than a decade in 1944 r j russell proposed and h n fisk defended a new non eolian property based concept of loess the eolian advocates reacted with surprise and enthusiasm each side used constrained arguments to show their view of the problem but did not examine the fundamental problem which was not in the proofs of their hypothesis but in the definition of the term between 1944 and about 1950 the debates about loess reached a maximum level of complexity the main semantic problem was submersed in peripheral arguments about physical properties and genetic interpretations the scholarly treatment of the subject by fisk and russell stimulated quality responses from a diversity of earth scientists interested in academic and applied studies particularly geo history pedology soil mechanics and stratigraphy the long lasting popularity of loess studies during the bandwagon era lasted to about 1970 by that time the analytical and technical interests had attracted the mainstream into the fourth era with a focus beyond the old arguments although fisk and russell found themselves defending an unpopular theory they stimulated a scientific interest in the late quaternary history of the mississippi valley that may never be exceeded

topographic growth around the orange river valley southern africa a cenozoic record of crustal deformation and climatic change we reconstruct the history of topographic growth in southern africa on both sides of the orange river valley from an integrated analysis of erosion surfaces crustal deformation and climate change first we propose an inventory of erosion surfaces observed in the study area and classify them according to their most likely formative process i e chemical weathering or mechanical erosion among the various land units observed we define a new class of landform the pedivalley which corresponds to a wide valley with a flat erosional floor in the orange river valley we mapped three low relief erosion surfaces each bevelling a variety of lithologies the oldest and most elevated is 1 a stripped etchplain evolving laterally into 2 a stepped pediplain bearing residual inselbergs 3 a younger pediplain later formed in response to a more recent event of crustal deformation these are all cenozoic landforms the etchplain is associated with a late palaeocene to middle eocene weathering event and the two pediplains are older than the middle miocene alluvial terraces of the orange river landscape evolution was first driven by slow uplift 10 m ma followed by a second interval of uplift involving a cumulative magnitude of at least 200 m this event shaped the transition between the two pediplains and modified the drainage pattern a final phase of uplift magnitude 60 m occurred after the middle miocene and drove the incision of the lower terraces of the orange river climate exerted a major control over the denudation process and involved very humid conditions responsible for lateritic weathering followed by more arid conditions which promoted the formation of pedivalleys collectively these produce pediplains

multiple paleosols of the late albian boulder creek formation british columbia canada fifteen lithified paleosols closely spaced in vertical sequence occur in the top 90 m of the late albian boulder creek formation in the foothills of northeastern british columbia canada the paleosols have well developed profiles 0 5 to 1 5 m thick including a b and c horizons the paleosols are characterized by their grey colour cutans vertical roots peds spherulitic siderite and absence of sedimentary structures the paleosols formed during a period when one or more basin wide unconformities occurred as a result of either eustatic sea level fluctuations or local tectonic events these unconformities represent the terrestrial record of a lowered base level which caused valley incision and decreased rates of sedimentation on the incised flood plain the climate was humid to subhumid overall the environment in which these soils developed was of low relief and subject to little erosion the water table was high for part of the year but there is also evidence of periodic drying and oxidation of organic debris cumulatively the paleosols in this interval may represent 150000 yr of non deposition

dynamic topography and long term sea level variations there is no such thing as a stable continental platform abstract globally uniform i e eustatic sea level trends with time scales approaching 100 myr have been inferred from both seismic and backstripping stratigraphic analysis at a small set of geographic sites that are presumed to lie on stable continental platforms and passive continental margins characterized by simple thermal subsidence histories e g the new jersey margin the western african margin we demonstrate using mantle flow simulations based on high resolution seismic tomography that both the new jersey margin and the conjugate western african margin have been subject to orders of 100 m of dynamic i e flow induced topography change over the last 30 myr we also show that the changing pattern of downwelling mantle flow associated with plate subduction is a significant contributor to the background eustatic sea level trend which is also of order 100 m during the 30 myr time window therefore late cenozoic variations of dynamic topography on these passive margin sites are comparable to the eustatic sea level changes and can partially mask the latter furthermore even if the observed trend could be accurately corrected for local dynamic topography variation the residual eustatic signal does not merely reflect changes in mean spreading rates at mid ocean ridges we conclude that the observed long term sea level variations at so called stable sites cannot be interpreted as eustatic moreover previous analyses that have used long term sea level trends as a proxy for spreading rates and geochemical fluxes must be revisited

post rift stratigraphic evolution of the atlantic margin of namibia and south africa implications for the vertical movements of the margin and the uplift history of the south african plateau abstract the aim of this study is to constrain the post rift deformations of the atlantic passive margin of namibia and south africa using an extensive industrial 2d reflection seismic dataset calibrated by wells and onshore outcrops that have been revaluated in age biostratigraphy in order to discuss the evolution of the south african plateau uplift the first order evolution of the margin is tectonically driven and can be divided into three principal phases the first 131 93 5 ma comprises an overall retrogradational trend that results from a rate of accommodation created by the thermal flexure of the margin that is higher than the sediment supply the second 93 5 66 ma comprises an overall aggradational progradational trend that results from a relative increase in sediment supply due to an uplift and subsequent erosion of the margin and the inland domain the third 66 0 ma is retrogradational again in response to a decrease in sediment supply induced by a relative stability of the margin and climate conditions this study demonstrates that the present day configuration of the margin is acquired during the late cretaceous from 81 to 66 ma with a strike long wavelength and seaward tilt of the margin which could be related to the growth of the onshore main escarpment we also characterize a regional oligocene unconformity marked by a significant downward shift of the shoreline which suggests a moderate uplift of the inland domain and could be associated with a reactivation of the relic late cretaceous relief

ocean temperature induced change in lithospheric thermal structure a mechanism for long term eustatic sea level change eustatic sea level change can be produced by a change in ocean bottom water temperature through an alteration of the upper thermal boundary condition of oceanic lithosphere the consequent evolution of lithospheric thermal structure can cause thermal expansion or contraction of the lithosphere if continental lithosphere remains unaffected by the change in ocean temperatures eustatic sea level will change as measured in the continental frame of reference a two dimensional finite difference model has been formulated and shows that if ocean bottom water temperature were to change rapidly by 14 c as it may have during the tertiary sea level would change by 8 m in 8 m y and 17 m in 40 m y this result is in agreement with the one dimensional error function solution for a heating or cooling halfspace however the changes in lateral heat flow between ocean water and the continental slope and between oceanic and continental lithosphere can produce epeirogenic motions of the continental margins and th

the sequence stratigraphy of fluvial depositional systems the role of floodplain sediment storage abstract the currently used model for alluvial sequence stratigraphy has been criticised miall 1991 and an alternative model is offered here in this model alluviation mainly takes place during transgressive and not highstand phases when accommodation space allows sediment storage the high capacity for some river systems to store floodplain sediments is a critical consideration the effects of climatic and intrinsic geomorphic changes during fluvial deposition are likely to greatly complicate simple base level controlled sequence models and recognising higher order fourth and fifth order cycles will be particularly difficult

earth s constant mean elevation implication for long term sea level controlled by oceanic lithosphere dynamics in a pitman world abstracton a spherical earth the mean elevation approximately 2440 m would be everywhere at a mean earth radius from the center this directly links an elevation at the surface to physical dimensions of earth including surface area and volume which are at most very slowly evolving components of the earth system earth s mean elevation thus provides a framework within which to consider changes in height of earth s solid surface as a function of time in this article the focus will be on long term nonglacially controlled sea level long term sea level has long been argued to be largely controlled by changes in ocean basin volume related to changes in the area age distribution of oceanic lithosphere as generally modeled by walter pitman and subsequent workers the age depth relationship of oceanic lithosphere including both the ridge depth and the coefficients describing the age depth relationship are assumed constant this article examines the consequences of adhering to these assumptions when pl

the impact of humans on continental erosion and sedimentation rock uplift and erosional denudation of orogenic belts have long been the most impor tant geologic processes that serve to shape continental surfaces but the rate of geomor phic change resulting from these natural phe nomena has now been outstripped by human activities associated with agriculture con struction and mining although humans are now the most important geomorphic agent on the planet s surface natural and anthro pogenic processes serve to modify quite dif ferent parts of earth s landscape in order to better understand the impact of humans on continental erosion we have examined both long term and short term data on rates of sediment transfer in response to glacio fl u vial and anthropogenic processes phanerozoic rates of subaerial denuda tion inferred from preserved volumes of sedimentary rock require a mean conti nental erosion rate on the order of 16 m per million years m m y resulting in the accumulation of 5 gigatons of sediment per year gt yr erosion irregularly increased over the 542 m y span of phanerozoic time to a pliocene value of 53 m m y 16 gt yr current estimates of large river sediment loads are similar to this late neogene value and require net denudation of ice free land surfaces at a rate of 62 m m y 21 gt yr consideration of the variation in large river sediment loads and the geomorphology of respective river basin catchments suggests that natural erosion is primarily confi ned to drainage headwaters 83 of the global river sediment fl ux is derived from the high est 10 of earth s surface subaerial erosion as a result of human activity primarily through agricultural practices has resulted in a sharp increase in net rates of continental denudation although less well constrained than estimates based on surviving rock volumes or current river loads available data suggest that present farmland denudation is proceeding at a rate of 600 m m y 75 gt yr and is largely confi ned to the lower elevations of earth s land surface primarily along passive continental margins 83 of cropland erosion occurs over the lower 65 of earth s surface the conspicuous disparity between natu ral sediment fl uxes suggested by data on rock volumes and river loads 21 gt yr and anthropogenic fl uxes inferred from measured and modeled cropland soil losses 75 gt yr is readily resolved by data on thicknesses and ages of alluvial sediment that has been deposited immediately downslope from erod ing croplands over the history of human agriculture accumulation of postsettlement alluvium on higher order tributary channels and fl oodplains mean rate 12 600 m m y is the most important geomorphic process in terms of the erosion and deposition of sedi ment that is currently shaping the landscape of earth it far exceeds even the impact of pleistocene continental glaciers or the cur rent impact of alpine erosion by glacial and or fl uvial processes conversely available data suggest that since 1961 global cropland area has increased by 11 while the global population has approximately doubled the net effect of both changes is that per capita cropland area has decreased by 44 over this same time interval 1 per year this is 25 times the rate of soil area loss antici pated from human denudation of cropland surfaces in a context of per capita food pro duction soil loss through cropland erosion is largely insignifi cant when compared to the impact of population growth

a pedotype approach to latest cretaceous and earliest tertiary paleosols in eastern montana a pedotype approach to the study of paleosols emphasizes individual profiles it is an alternative approach to the study of geosols which are laterally extensive suites of paleosols or pedofacies which are pedogenically distinctive sedimentary facies this is the first pedotype study of paleosols across the cretaceous tertiary boundary in eastern montana pedotypes allow assessment of sedimentation and fossilization the sequence of paleosols across the cretaceous tertiary boundary at bug creek is one of high temporal resolution because most available geological time can be accounted for by differing degree of development inferred for each pedotype paleosols at bug creek include pedotypes that preserve plant fossils well but were unfavorable for preservation of fossil vertebrates pedotypes also allow reassessment of ecosystem change despite indications of catastrophe at the cretaceous tertiary boundary the array of paleocene and cretaceous pedotypes are not strikingly different the upper cretaceous hell creek formation includes pedotypes interpreted as gleyed alfisols inceptisols and entisols probably formed under seasonally water logged forest and mean annual rainfall of the order of 900 1200 mm most paleosols of the lower paleocene tullock formation were histosols but some can be interpreted as gleyed inceptisols and entisols probably formed under bald cypress swamps in a humid climate with 1200 mm mean annual rainfall broadly comparable pedotypes were present before and after the cretaceous tertiary boundary although paleocene flood plain forest lived in paleosols chemically a little more oligotrophic than cretaceous paleosols this modest difference supports the idea that change to coaly facies in the earliest paleocene was a local shift in sedimentary environment such local changes do not begin to account for profound disruption in specific composition of plant and animal communities at the cretaceous tertiary boundary in eastern montana

quantification of vertical movement of low elevation topography combining a new compilation of global sea level curves and scattered marine deposits armorican massif western france a wide range of methods are available to quantify earth s surface vertical movements but most of these methods cannot track low amplitude 5 ma e g cosmogenic isotope studies vertical movements characteristic of plate interiors the difference between the present day elevation of ancient sea level markers deduced from well dated marine deposits corrected from their bathymetry of deposition and a global sea level gsl curve are sometimes used to estimate these intraplate vertical movements here we formalized this method by re assessing the reliability of published gsl curves to build a composite curve that combines the most reliable ones at each stage based on the potential bias and uncertainties inherent to each curve we suggest i that curves which reflect ocean basin volume changes are suitable for the ca 100 to 35 ma greenhouse period ii whereas curves that reflects ocean water volume changes are better suited for the ca 35 to 0 ma icehouse interval and iii that for these respective periods the fit is best when using curves that accounts for both volume changes we used this composite gsl curve to investigate the poorly constrained paleogene to neogene vertical motions of the armorican massif western france it is characterized by a low elevation topography a variscan basement with numerous well dated cenozoic marine deposits scattered upon it using our method we identify low amplitude vertical movements ranging from 66 m of subsidence to 89 m of uplift over that time period their spatial distribution argues for a preferred scale of deformation at medium wavelengths i e order 100 km which we relate to the deformation history of northwestern european lithosphere in three distinct episodes i a phase of no deformation between 38 and 34 ma that has been previously recognized at the scale of northwestern europe ii a phase of low subsidence between 30 and 3 6 ma possibly related to buckling of the lithosphere and iii a phase of more pronounced uplift between 2 6 ma and present which we relate to the acceleration of the africa apulia convergence or to enhanced erosion in the rapidly cooling climate of the pleistocene

paleoweathering of mississippian monteagle limestone preceding development of a lower chesterian transgressive systems tract and sequence boundary middle tennessee and northern alabama the contact between the hartselle sandstone mississippian lower hombergian and the underlying monteagle limestone mississippian upper gasperian in central tennessee and northeastern alabama is a regional disconformity a paleoweathering surface developed in the uppermost monteagle is truncated by this disconformity and only remnant pedogenic features are preserved these paleosol remnants and associated meteoric diagenetic features are evidence for the subaerial exposure and weathering of the monteagle limestone prior to intense erosional scour accompanying deposition of the hartselle sandstone petrographic evidence for subaerial exposure and meteoric diagenesis includes extensive micritization reddening dissolution and calcitization of allochem grains stable carbon and oxygen isotope compositions of whole rock samples exhibit depth patterns consistent with subaerial exposure pedogenesis and meteoric diagenesis stable isotope compositions of nonferroan calcite spar occluding moldic pores are considerably depleted relative to mississippian marine carbonate compositions further evidence for subaerial exposure and meteoric diagenesis the paleoweathering surface was modified after burial when reducing conditions developed resulting in pyritization stabilization of high mg calcite grains and cements after burial provided a source of magnesium for localized replacement by ferroan dolomite this disconformity is interpreted to be a third order vail exxon sequence boundary that marks a dramatic basinward shift in facies belts in the overlying hartselle sand stone the result of a sea level fall

fluvial responses to climate and sea level change a review and look forward summary fluvial landforms and deposits provide one of the most readily studied quaternary continental records and alluvial strata represent an important component in most ancient continental interior and continental margin successions moreover studies of the long term dynamics of fluvial systems and their responses to external or allogenic controls can play important roles in research concerning both global change and sequence stratigraphy as well as in studies of the dynamic interactions between tectonic activity and surface processes these themes were energized in the final decades of the twentieth century and may become increasingly important in the first decades of this millennium this review paper provides a historical perspective on the development of ideas in the fields of geomorphology quaternary geology vs sedimentary geology and then summarizes key processes that operate to produce alluvial stratigraphic records over time scales of 103 106 years of particular interest are changes in discharge regimes sediment supply and sediment storage en route from source terrains to sedimentary basins as well as changes in sea level and the concept of accommodation late quaternary stratigraphic records from the loire france mississippi usa colorado texas usa and rhine meuse the netherlands rivers are used to illustrate the influences of climate change on continental interior rivers as well as the influence of interacting climate and sea level change on continental margin systems the paper concludes with a look forward to a bright future for studies of fluvial response to climate and sea level change at present empirical field based research on fluvial response to climate and sea level change lags behind a the global change community s understanding of the magnitude and frequency of climate and sea level change b the sequence stratigraphic community s desire to interpret climate and especially sea level change as forcing mechanisms and c the modelling community s ability to generate numerical and physical models of surface processes and their stratigraphic results a major challenge for the future is to catch up which will require the development of more detailed and sophisticated quaternary stratigraphic sedimentological and geochronological frameworks in a variety of continental interior and continental margin settings there is a particular need for studies that seek to document fluvial responses to allogenic forcing over both shorter 102 103 years and longer 104 106 years time scales than has commonly been the case to date as well as in larger river systems from source to sink studies of quaternary systems in depositional basin settings are especially critical because they can provide realistic analogues for interpretation of the pre quaternary rock record

dynamic changes and processes in the mississippi river delta research in the modern delta of the mississippi river has revealed short term changes and processes that are of significant magnitude deltaic lobes each lobe covering an area of 30 000 sq km and having an average thickness of 35 km switch sites of deposition on an average of every 1 500 yr through short periods of geologic time this process results in a relatively thick accumulation of stacked deltaic cycles covering extremely large areas within a single delta lobe and operating on an even higher frequency are bay fills and overbank splays bay fills having areas of 250 sq km and thickness of 15 m require only 150 yr to accumulate four major events have taken place in the modern balize delta since 1838 overbank splays are much smaller covering areas of less than 2 sq km and having thicknesses of 3 m but are associated with high floods on the river at the river mouth continued progradation of the distributary channel can form distributary mouth sand bodies that have dimensions of 17 km long 8 km wide and a thickness of 80 m in a period of only 200 yr differential sedimentary loading at the river mouth results in formation of diapirs that display vertical movements in excess of 100 m in a period of 20 yr on the subaqueous delta platform sediment instabilities operate nearly continuously mass moving large quantities of shallow water deposits to deeper water environments via arcuate rotational slides and mudflow gullies and depositional lobes all of these changes and processes operate at differing spatial and temporal scales but all result in deposition of large volumes of sediment over extremely short periods of time

stratigraphy geochemistry and thermoluminescence ages of lower mississippi valley loess four periods of loess deposition in the lower mississippi valley can be identified on the basis of geochemical and mineralogical criteria radiocarbon dating and thermoluminescence dating these are designated loess units 1 2 3 and 4 in order of increasing age carbonate rich unit 1 loess comprises more than 70 per cent of the thickness of the loess profiles 14c and tl dates indicate this loess was deposited between 9 000 and 20 000 years ago a maximum sedimentation rate of 2 17 mm yr 1 has been recorded near vicksburg just after the last laurentide glacial maximum between 17 190 and 15 580 years ago the unit 2 loess which is thin and partly decalcified was deposited slowly between about 25 000 and 20 000 years ago the unit 3 and unit 4 loess formations which are both highly weathered have yielded tl ages of 76 000 85 000 years and 119 000 132 000 years suggesting they were deposited during the altonian substage of the early wisconsinan and the illinoian glacial stage respectively the four loess units are stratigraphically equivalent to the peoria farmdale roxana and loveland loess formations previously recognized in illinois the source of dust in both areas was glacial outwash in the mississippi valley during interglacials and interstadials when the supply of glacial debris was reduced and the mississippi river changed from a braided to a meandering regime dust sedimentation in southern mississippi virtually ceased allowing weathering and pedogenesis to proceed

alluvial pedogenesis in pleistocene and holocene mississippi river deposits effects of relative sea level change comparison of pleistocene and holocene soils developed in mississippi river meander belt deposits in louisiana illustrates the influence of relative sea level change on pedogenesis holocene and pleistocene meander belts have similar landscape morphologies lithofacies and soil characteristics and probably formed under similar hydrologic regimes the pleistocene avoyelles prairie meander belt is now terraced but it was originally graded to a lower relative sea level position than the holocene flood plain holocene alluvial soils are generally weakly expressed cumulative profiles with abundant redoximorphic features and formed on an aggrading flood plain with a shallow fluctuating water table the soil morphology varies with respect to landscape position and overbank lithofacies pleistocene alluvial paleosols show evidence for two stages of pedogenesis during the first stage of pedogenesis water table fluctuations and flood plain aggradation produced profiles with color mottles iron and carbonate nodules and weak b and c horizons similar to holocene flood plain soils the second stage of pedogenesis occurred after the avoyelles prairie was terraced and the regional water table fell a minimum of 30 m improved soil drainage created oxidizing and acidic conditions removed free carbonates dehydrated iron oxides and transformed clay minerals this stage produced pleistocene profiles with brighter matrix and mottle hues lower ph values larger and harder iron nodules and greater kaolinite contents than comparable holocene flood plain soils the primary influence on regional water table lowering in the southern lower mississippi valley was relative sea level fall mississippi river flood plain lowering adjacent to the avoyelles prairie most likely occurred between the end of oxygen isotope stage 3 and the maximum period of valley entrenchment during oxygen isotope stage 2 water table fall affected pedogenic processes a minimum of 400 km upvalley of the continental shelf margin pleistocene mississippi river paleosols represent excellent analogs for interpreting ancient alluvial paleosols formed during episodes of base level fall and valley incision as fluvial entrenchment terraced the meandering avoyelles prairie a pedogenic threshold was crossed that altered the pathway of chemical weathering the complexity of a multiple step pedogenic history also highlights the fact that interpretation of relative profile development as a function of time can be problematic

sedimentary architecture depositional facies and diagenetic response to intracratonic deformation and climate change inferred from outcrops for a pivotal period jurassic cretaceous boundary paris basin france abstract the aim of this study is to decipher the respective influences of geodynamic and climate disturbances at the jurassic cretaceous boundary on sedimentary facies and carbonate diagenesis in a stable intracratonic basin using isotopic geochemistry and subsidence quantification fourteen lithofacies were deposited in a 1 carbonate platform and 2 a delta plain environment climate change from cool and wet to warm and semi arid conditions during the early tithonian influenced the syn sedimentary dolomitization process within the carbonate platform during the mid tithonian architecture and facies reconstructions well constrained the jurassic cretaceous unconformity jcu which was an important local structural episode marked by 1 an 80 m uplift in the eastern paris basin and by 2 the formation of a nw se low wavelength 15 km wide and 30 km long flexure this first tectonic event tended to maintain brine ponds and supratidal marsh environments in the platform during the late tithonian and early berriasian forming purbeckian facies and associated dolomitic facies a major depositional change occurred between the early and late berriasian from shallow carbonate platform environments to fluviatile deltaic clastic deposits wealden facies this facies change is underlain by a major unconformity corresponding to the ryazanian unconformity it is marked 1 by erosion processes karstification of the carbonate substrate and the development of ferruginous weathering products goethite followed by 2 incision processes in a fluvial deltaic environment this unconformity is consecutive to a 40 m uplift in the eastern paris basin the rifting phase in the bay of biscay in the pyrenean zone and in the artic north atlantic together with the opening of the ligurian sea had a major influence on the northern part of france by causing uplifts 120 m from the tithonian and flexuring geodynamics played a major role in carbonate demise in the paris basin leading to exposure and karstification of the carbonate platform added to the generalized uplift western tethyan cool and humid conditions from the late berriasian caused terrigenous influx into large scale marine domains which was detrimental and unfavorable to carbonate growth

perspectives on the sequence stratigraphy of continental strata this report is the result of a working group on continental sequence stratigraphy that was set up at the 1991 nuna conference in banff canada to date sequence stratigraphic concepts have been applied mainly to the marine realm but unconformity bounded units have long been recognized in nonmarine strata successful application of sequence stratigraphic concepts to continental strata requires careful consideration of controls on base level and sediment supply as with shallow marine environments relative sea level can be considered as the stratigraphic as well as the geomorphic base level for coastal nonmarine settings farther inland stratigraphic base level which determines accommodation space is more complex and takes various forms such as the graded profile for fluvial strata groundwater tables for some eolian strata and lake level for some intermontane sediments sediment supply is also generally a more complex variable for nonmarine environments than in the marine realm because of the proximity to the source area the influence of climate and tectonism on sediment supply can clearly be seen in many continental sediments the fact that the major controls of climate tectonism and eustasy are somewhat interdependent and that a change in one parameter will most likely be reflected in others is also more readily apparent in continental strata although in its infancy sequence stratigraphic concepts have been applied to a wide variety of continental settings in attempts to explain variations in facies architecture of particular interest are studies that have linked fluvial architecture on coastal plains to variations in relative sea level as indicated by coeval marine strata and studies of lacustrine environments that show marked variation in facies between highstand and lowstand deposits the application of sequence stratigraphy to continental strata will likely result in the development of better correlation techniques and models that better predict the location and nature of fluvial and eolian reservoirs

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

recognition of interfluve sequence boundaries integrating paleopedology and sequence stratigraphy in shallow marine and coastal plain strata sequence boundaries are easily recognized at the base of incised valley fills between valleys however it is more difficult to recognize sequence boundaries where they are expressed as interfluve paleosols we suggest that interfluve paleosols can be most reliably identified using a combination of 1 stratigraphic position 2 field observations such as thickness structure color and degree of rooting and 3 micromorphological features such as bioturbation fabric clay coatings ferruginous features and siderite and barite only micromorphology permits recognition of temporal changes in drainage surface stability and protracted pauses in sedimentation that typify these surfaces this three part approach has been successfully employed to identify interfluve sequence bounding paleosols in the cenomanian dunvegan formation in alberta and british columbia canada

pleistocene stratigraphy in the southern lower mississippi valley abstract scientific inquiry into pleistocene stratigraphy of the lower mississippi valley lmv dates to early writings of european naturalists in the late 19th century by the early 20th century landscape evolution concepts stratigraphic models and regional syntheses had developed for most areas the 1944 monograph of h n fisk marks the advent of a predictive stratigraphic and landscape evolution model that links form and process to a predominantly glacioeustatic mechanism the fiskian model gained widespread acceptance and decades passed before significant alternate models began to emerge revised stratigraphic and geomorphic concepts are presently developing from newly acquired environmental and engineering data present scenarios classify pleistocene outcrop areas into erosional and constructional landscapes and veneers of eolian colluvial fluvial coastal and marine origin can drape both types of surfaces the southern lmv and adjacent gulf coastal plain gcp experienced significant landscape change during the pleistocene late tertiary pliocene to early pleistocene deposition of the upland complex was by streams with a high sand and gravel load relative to its mud load the regional drainage network and fluvial system behavior was probably significantly different from the modern braided stream alluvial fan complexes received sediment from highland source areas adjacent to the lmv and the glaciated mid continent it is plausible that part of upland complex deposition predates initial glacial advances from early to middle pleistocene an erosional landscape formed during a dissection period that chiefly postdates soil formation on stable landscape positions of the upland complex slope evolution truncated a regionally extensive geosol in multiple phases and parts of the erosion surface complex are graded to the oldest preserved constructional alluvial plains in present valleys toe and foot slope positions of the erosion surface complex and its correlative alluvial plains are presently delineated as the intermediate complex constructional landscapes formed at this time are sparsely preserved fisk s montgomery terrace in the lower red river valley lrrv is the best preserved example influences on the development of erosion surfaces in the lmv are not well understood however interactions of relative sea level fall climate change and epirogenic crustal movement are plausible factors from the latter part of middle pleistocene to the holocene there was widespread evolution of modern constructional landscapes constructional alluviation preserved lithofacies of mixed load laterally accreting meandering streams that developed over large areas of the southern lmv to form parts of the prairie complex lateral planation in valleys and stable rates of upland sediment generation were dominant processes during prairie complex deposition pleistocene stratigraphic examples considered important by fisk are still considered relevant to modern stratigraphic investigators presently pleistocene units of the southern lmv the adjacent lrrv and central gcp can be correlated only by relative stratigraphic relationships refined chronostratigraphic and paleoenvironmental models for these areas would help improve the understanding of the geomorphic influences on quaternary landscape evolution in the region

rapid erosion of the southern african plateau as it climbs over a mantle superswell we present new sedimentary flux data confirming that a large pulse of erosion affected the southern african plateau in the late cretaceous and is likely to be related to a major uplift episode of the plateau this short phase of erosion i e less than 30 myr in duration has commonly been difficult to reconcile with a mantle origin for the plateau anomalous uplift given its size the rise of the african superplume is likely to have lasted much longer here we demonstrate by using a simple model for fluvial erosion that tilting of the continent as it rides over a wide dynamic topography high cannot only cause rapid uplift of the plateau but also trigger continent wide drainage reorganization leading to substantial denudation in a relatively short amount of time the amplitude and short duration of the sedimentary pulse are best reproduced by assuming a strong erodibility contrast between the karoo sedimentary and volcanic rocks and the underlying basement we also present a new compilation of paleoclimate indicators that shows a transition from arid to very humid conditions approximately at the onset of the documented erosional pulse suggesting that climate may have also played a role in triggering the denudation the diachronism of the sedimentary flux between the eastern and western margins of the plateau and the temporal and geographic coincidence between the uplift and kimberlite eruptions are however better explained by our tilt hypothesis driven by the migration of the continent over a fixed source of mantle upwelling