late neogene sequence stratigraphic evolution of the foz do amazonas basin brazil the margin of the foz do amazonas basin saw a shift from predominantly carbonate to siliciclastic sedimentation in the early late miocene by this time the amazon shelf had also been incised by a canyon that allowed direct influx of sediment to the basin floor thus confirming that the palaeo amazon fan had already initiated by that time 9 5 8 3 ma above this interval during a prolonged lowstand messinian third order sequences are preserved only in the incised valley fills of the canyon with no equivalent strata on the shelf third and fourth order sequences younger than messinian are preserved on the shelf after sea level rise above the shelf by the early pliocene sequences younger than 3 8 ma often show fourth order cyclicity with an average duration of 400 ka larger scale eccentricity cycles often preserved in high sedimentation rate areas of river deltas mass wasting and transportation of slope sediments to the basin began to play an important role in sediment dispersal at least as far back as the mid pliocene after rapid progradation had produced steeper slopes more prone to failure

the belize margin revisited 1 holocene marine facies belize is best known for its 260 km long barrier reef and associated lagoon reef shoals but also exhibits a complete transition from near shore siliciclastics to pure carbonate deposits across a narrow shelf lagoon and in addition has three of the very few occurrences of caribbean atolls published holocene facies relationships on the belize shelf have been semi quantitative in that they are based on quantitative point count thin section data in the northern shelf lagoon and on qualitative estimates of thin section composition in the southern shelf lagoon this contrast has been rectified by point counting southern shelf lagoon thin sections and incorporating the result with published point count data from the northern shelf supplemented by additional information to produce a modified factor analysis facies distribution for the entire shelf the resulting nine facies are extended seaward to include previously published analysis of sediment on the three belize off shelf atolls resulting in a total of 11 facies in addition the carbonate mud distribution has been mapped over both the shelf and atolls with the not unexpected result that in places the distribution pattern clearly indicates a barrier platform source of lagoon deposited carbonate mud dunham s limestone classification terminology has been used as a basis for description to make these relationships more useful to those reconstructing ancient limestone depositional environments

banktop responses to quaternary fluctuations in sea level recorded in periplatform sediments periplatform sediment from a 12 m core recovered from northwest providence channel bahamas contains a geochemical and paleontological record of sea surface temperatures ice volumes and the response of banktops to highstands of sea level a comparison of these data suggests that fluctuations of carbonate mineralogy in periplatform sediments result from fluctuations of sea level and from patterns of banktop sedimentation highstands of sea level that flood carbonate platforms are recorded in periplatfonn sediments as abrupt increases of exported sr rich aragonite derived from banktop orgiuiisms superposed on a background of sr poor aragonite pteropods and calcite foraminifera and coccoliths derived from planktonic sources the pulses of banktop sediment coincide with increases of water temperature determined from foraminiferal assemblages and with decreases in ratios of oxygen isotopes indicating decreased ice volume and rising sea level following these abrupt changes is a more gradual decline in bank derived sediment although paleotemperatures from foraminiferal assemblages and oxygen isotopic data clearly show that warm conditions and a highstand of sea level persist we suggest that this decrease of offbank transport is part of autocyclic sedimentation patterns of shallow water carbonate environments offbank transport is restricted as reefs sand shoals and islands reach sea level also green algal production may decline because progradation of tidal flats decreases lagoon area and the hydrologic and ecologic conditions change as lagoons are filled

observations of stratified turbulent mixing in an estuary neap to spring variations during high river flow abstract observations of turbulent mixing stratification and currents were made with a free falling microstructure profiler and a 600 khz adcp in the hudson river off manhattan in may 1994 the ship was anchored in a relatively straight and uniform channel about 15 m deep over 4 days of intense observations from 19 to 24 may 1994 the progression from neap tides to spring tides caused a severe reduction in stratification from an initial top to bottom salinity difference of 18 to 3 6 while low gradient richardson numbers ri occurred only in the weakly stratified bottom layer on flood during neap tides low ri spanned the water column on the second half of ebb during spring tides turbulent dissipation rates varied roughly inversely to ri during neap conditions turbulent mixing was intense only in the bottom layer on flood where eddy diffusivities k reached more than 10 2 m 2 s 1 while mixing was weak in the centre of the halocline near 5 m during spring tides strong mixing occurred throughout the water column during the latter part of ebb with typical k 5 10 3 m 2 s 1 and typical vertical salt flux of 0 0015 kg m 2 s 1 if it existed in isolation this flux would eliminate the haline stratification on a time scale of 4 h ebb time bands of low ri and intense mixing showed increasing phase lag with increasing distance from the bottom the spring tide ebb time internal turbulent stress was relatively uniform with depth with a typical magnitude of 0 4 pa 4 dyne cm 2 turbulent overturning thorpe scales ranged from below 1 cm in the neap tide halocline to a few metres in the weakly stratified bottom layer on flood the strong spring tide ebb time mixing had thorpe scales of typically 0 5 m throughout the water column ozmidov scales tracked thorpe scales except in the weakly stratified layer above the bottom

deepwater carbonate deposition in response to re flooding of carbonate bank and atoll tops at glacial terminations the late quaternary has experienced large glacial interglacial climatic variations and related 10 s to 100 meters high amplitude sea level fluctuations at milankovich frequencies from 10 s to 100 kyr during which carbonate platform tops have been exposed and re flooded in many occasions this study focuses on the accumulation of calci turbidites the aragonite onset sharp increase in fine sediments and their timing in deep basins adjacent to carbonate platforms a particular emphasis is developed on the occurrence of the first gravity flow event and aragonite onset sharp increase and their linkage to the initial re flooding of the platform tops during deglaciations three basins adjacent to isolated platforms in the bahamas the northern nicaragua rise and the gulf of papua were selected to represent pure carbonate versus mixed systems in quiescent versus tectonically active settings and various carbonate bank top morphologies ranging from atoll to relatively deeply and narrowly flooded flat top banks in spite of these differences each record illustrates a clear relationship between the timing of platform top re flooding and initiation of significant carbonate export by gravity flows and low density plumes into the surrounding basins the concept of re flooding window is introduced to characterize the prolific period of time during which bank and atoll tops are flooded enough to produce large export of bank derived aragonite and of calci turbidites in adjacent basins according to our datasets the main re flooding windows have occurred mainly on the last part of the sea level rise at each glacial termination t those periods being marked by some of the highest rates of sea level rise the analysis of a long piston core from the earthquakes prone walton basin northern nicaragua rise demonstrates that sea level not seismic activities played a major role as trigger mechanism for the initiation of gravity flows since the last four glacial interglacial transitions and supports the existence of an extra glacial termination during the early portion of the transition from mis 7 4 to mis 7 3 tiiia

glacial versus interglacial sedimentation rates and turbidite frequency in the bahamas the southern tongue of the ocean is a 1300 m deep flat floored basin in the bahamas that receives large amounts of sediment from the carbonate platforms surrounding it on three sides we have examined five 8 13 m long piston cores and determined bulk sedimentation rates turbidite frequency and turbidite accumulation rates for the past two glacial and interglacial periods the mean of bulk sedimentation rates is four to six times higher in interglacial periods average accumulation rates of recognizable turbidites are higher by a factor of 21 to 45 and interglacial turbidite frequency is higher by a factor of 6 to 14 sediment composition indicates that increased interglacial rates are due to higher accumulation of platform derived material additional data from other bahamian basins as well as published material from the caribbean strongly suggest that highstand shedding is a general trend in pure carbonate depositional systems carbonate platforms without a siliciclastic component export more material during highstands of sea level when the platform tops are flooded and produce sediment the response of carbonate platforms to quaternary sea level cycles is opposed to that of siliciclastic ocean margins where sediment is stored on the inner shelf during highstands and passed on to continental rises and abyssal plains during lowstands of sea level

mixed carbonates and siliciclastics in the quaternary of southern belize pleistocene turning points in reef development controlled by sea level change the belize barrier and atoll reefs represent one of the largest reef structures in the atlantic ocean the southern shelf of belize is a classic location of a modern mixed carbonate siliciclastic system whereas knowledge of the holocene deposits in the area is extensive data on the pleistocene system are fragmentary open questions include i the nature of the reef foundations carbonate versus siliciclastics ii the ages of the deposits including the initiation of the barrier reef and iii the response of the mixed system to sealevel fluctuations the results of a study of borings on the southern belize shelf are presented here six up to 105 m long borings were made to better understand the history of this important mixed system uranium series dating in the pleistocene was not possible because of diagenetic alteration however lithostratigraphy strontium isotopes and calcareous nannofossil biostratigraphy were used to constrain stratigraphic ages results support the contention that the quaternary development in belize was quite similar to that of other major barrier reefs such as the florida reef tract and further afield the great barrier and the new caledonian barrier reefs all of these barrier reefs are mixed carbonate siliciclastic systems and significant reef growth only began after the onset of high amplitude eccentricity controlled sea level changes and as late as during the exceptionally long and warm marine isotope stage 11 some 400 ka in belize early pleistocene sections at bases of borings include mollusc rich wackestones rare coral packstones and marls which were deposited under low to moderate energy conditions in a ramp setting before ca 900 ka during the high sea levels of marine isotope stage 25 and possibly earlier marine isotope stage 31 or 37 the belize shelf was subaerially exposed for most of the mid pleistocene and was dominated by siliciclastic sedimentation possibly during marine isotope stages 24 to 12 when highstands were comparatively low continuous reefs at the shelf margin were developing during highstands in the late pleistocene beginning with the long and high highstand of marine isotope stage 11 some 400 ka the southern shelf was flooded entirely and carbonates started to dominate once more reefs developed on top of siliciclastic deposits on the shelf a continuous barrier reef came into existence and largely developed on top of carbonates at the shelf margin during late pleistocene lowstands siliciclastics presumably no longer reached the shelf margin because of the topographic high of the barrier reef platform the quaternary belize example may serve as a model for reconstructing ancient mixed systems in icehouse worlds however any extrapolations are limited by the fact that fast growing scleractinian reef builders had not yet evolved in the palaeozoic

turbulence measurements in stratified and well mixed estuarine flows from mean velocities measured in estuarine flows it has been found that the velocity distributions are log linear in stratified flows and logarithmic in well mixed flows the results of salinity measurements reveal that the mean salinity profiles are geometrically similar and expressible as a power law the buoyancy parameters such as the monin obukhov length scale the gradient and the flux richardson numbers are independent of the flow state the gradient and the flux richardson numbers are almost equal indicating the existence of a local equilibrium layer the non dimensional parameter describing dissipation rates of turbulent kinetic energy is a constant of 0 2 and 0 3 for stratified and well mixed flows respectively in well mixed flow the drag coefficient varies with time approaching a constant of about 3 2 10 3 when the flow is stratified the shape of the turbulent energy spectra are generally flatter and broader in stratified as compared with those of well mixed flows

the paradox of drowned reefs and carbonate platforms shallow water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation and benthonic carbonate production ceases drowned platforms are common in the geologic record but they present a paradox if one considers rates of processes involved during the early holocene when sea level rose at rates of 6 000 to 10 000 m yr mm 1 000 yr most reefs and platforms were outpaced by the rising sea during the late holocene with sea level rising 500 to 3 000 m yr in the atlantic caribbean area reefs and platforms started to recover built to sea level and prograded seaward 1 000 m yr is thus a conservative estimate of the average growth potential of modern reefs and platforms independently accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year the growth potential of 1 000 m yr exceeds any relative rise of sea level caused by long term processes in the geologic record newly formed ocean crust subsides at a maximum of 250 m yr basin subsidence averages 10 to 100 m yr and sea level rises due to increased sea floor spreading amount to less than 10 m yr rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning the geologic record shows examples of both of these processes global mass extinctions of reefs and platforms occurred in the middle cretaceous eustatic rise due to submarine volcanism or desiccation of a small ocean basin and the late devonian global crisis of ocean environment extraterrestrial cause drowning controlled by regional tectonics prevailed in the jurassic and early cretaceous of the tethyan realm and the drowning of mesozoic platforms in the western north atlantic seems to have been dictated by plate tectonic drift to higher latitudes

storm wave reactivation of a submarine landslide submarine landslides move large amounts of sediment and are often hazardous to offshore installations marine surveying techniques effectively define landslide locations and geometries for example refs 1 4 in various underwater geological settings including deltas fjords and earthquake prone areas1 5 movement rates and mechanics are generally inferred from indirect evidence such as cable breaks and damage to sea floor structures for example refs 6 8 but actual measurements of landslide behaviour are sparse using bottom deployed pressure sensors tiltmeters and accelerometers we have collected data on sea floor landslide reactivation involving sediment collapse and remoulding on the submarine slopes of the huanghe delta china the sensors were emplaced into sea floor clayey silts on 4 october 1987 and recovered on 16 october after the passage of three severe storms

mixing caused by lateral internal seiching within a partially mixed estuary abstract the salinity fluctuations at an std near the halocline in southampton water have been spectrally analysed mixing is shown to commence at a layer richardson number of about 20 the predominant energy inputs have periods of 7 9 and 3 5 4 5 min these motions arise from lateral internal seiching and it is considered that they are produced by interaction of the surface seiche with the shallow side of the estuary the wavelengths of the standing wave are calculated to be about 120 and 60 m respectively and these compare well with echo sounding surveys of the elevation of the halocline

neogene evolution of the mixed carbonate siliciclastic system in the gulf of papua papua new guinea this paper outlines the evolution of the late cenozoic mixed carbonate siliciclastic depositional system in the gulf of papua gop using seismic gravity multibeam bathymetry well data sets and landsat imagery the deposition of the mixed sedimentary sequences was influenced by dynamic interplay of tectonics eustasy in situ carbonate production and siliciclastic sediment supply the roles of these major factors are estimated during different periods of the gop margin evolution the cenozoic mixed system in the gop formed in distinct phases the first phase late cretaceous paleocene was mostly driven by tectonics rifting created grabens and uplifted structural blocks which served later as pedestals for carbonate edifices active neritic carbonate accumulation characterized the second phase eocene middle miocene during this phase mostly eustatic fluctuations controlled the large scale sedimentary geometries of the carbonate system the third phase late miocene early pliocene was characterized by extensive demise of the carbonate platforms in the central part of the study area which can be triggered by one or combination of several factors such as eustatic sea level fluctuations increased tectonic subsidence uplift sudden influx of siliciclastics or dramatic changes in environmental conditions and climate the fourth phase late pliocene holocene was dominated by siliciclastics which resulted in the burial of drowned and or active carbonate platforms although some platforms still remain alive until present day

turbulent perturbations of velocity in the conwy estuary abstract field measurements of the vertical and horizontal components of fluid velocity 0 43 m above the bed have been made with an electromagnetic flowmeter in a partially mixed reach of the conwy estuary for parts of a flood and an ebb tide the turbulent mean velocity and density fields showed different effects for flood and ebb tides caused by the interaction of shear and the longitudinal density gradient the turbulent velocity parameters were generally dominated by bed generated turbulence effects but significant longer period contributions attributed to the shear density interactions were detected the shear stress measurements made by two independent methods showed good agreement the results generally show good agreement with previous less complete data the use of the richardson number to quantify vertical stability needs further consideration for partially mixed estuarine flows

relationship between late pleistocene sea level variations carbonate platform morphology and aragonite production maldives indian ocean a piston core from the maldives carbonate platform was investigated for carbonate mineralogy grain size distributions calcium carbonate content and organic carbon the sedimentary record was linked to late pleistocene sea level variations using an age model based on oxygen isotopes obtained from planktonic foramanifera nannofossil biostratigraphy and 14c age determinations the correlation between the sedimentary record and late pleistocene sea level showed that variations in aragonite and mud during the past 150 000 years were clearly related to flooding and sea floor exposure of the main lagoons of the atolls of the maldives carbonate platform platform flooding events were characterized by strongly increased deposition of aragonite and mud within the inner sea of the maldives exposure events in contrast can be recognized by rapid decreases in the values of both proxy records the results show that sediments on the maldives carbonate platform contain a continuous record of pleistocene sea level variations these sediments may therefore contribute to a better understanding of regional and even global sea level changes and yield new insights into the interplay between ocean currents and carbonate platform morphology

turbulent momentum and salt transport in the mixing zone of the elbe estuary measurements in the mixing zone of the elbe estuary were performed during three consecutive tidal cycles with three types of instruments a moored tripod with velocity and temperature conductivity light attenuation sensors a profiling sonde with similar sensors lowered from an anchored vessel and instrumented moorings acoustic travel time sensors were used for velocity measurements spectral analysis of 12 8 min pieces of the obtained time series gives results that are consistent with isotropic turbulence for part of the frequency space temporal changes of turbulent kinetic energy are correlated with tidal current velocity a retardation is found between changes in tidal current and turbulent energy not all shear stress terms are in similar phase with tidal flow mean gradients reynolds stress terms and turbulent salt flux terms are combined to determine eddy viscosity and eddy diffusion coefficients

late quaternary siliciclastic carbonate sedimentation model for the capricorn channel southern great barrier reef province australia abstract a model is presented for hemipelagic siliciclastic and carbonate sedimentation during the last glacial interglacial cycle in the capricorn channel southern great barrier reef gbr stable isotope ratios grainsize carbonate content and mineralogy were analysed for seven cores in a depth transect from 166 to 2892 m below sea level mbsl results show variations in the flux of terrigenous neritic and pelagic sediments to the continental slope over the last sea level cycle during the glacial lowstand terrigenous sediment influenced all the cores down to 2000 mbsl the percentages of quartz and feldspar in the cores decreased with water depth while the percentage of clay increased x ray diffraction analysis of the glacial lowstand clay mineralogy suggests that the siliciclastic sediment was primarily sourced from the fitzroy river which debouched directly into the northwest sector of the capricorn channel at this time the cores also show a decrease in pelagic calcite and an increase in aragonite and high magnesium calcite hmc during the glacial the influx of hmc and aragonite is most likely from reworking of coral reefs exposed on the continental shelf during the glacial and also from hmc ooids precipitated at the head of the capricorn channel at this time mass accumulation rates mars are high 13 5 g cm 3 kyr 1 during the glacial and peak at 20 g cm 3 kyr 1 in the early transgression 16 14 ka bp mars then decline with further sea level rise as the fitzroy river mouth retreats from the edge of the continental shelf after 13 5 ka bp mars remain low 4 cm 3 kyr 1 throughout the holocene highstand data for the holocene highstand indicate there is a reduction in siliciclastic influx to the capricorn channel with little quartz and feldspar below 350 mbsl however fine grained fluvial sediments presumably from the fitzroy river were still accumulating on the mid slope down to 2000 mbsl the proportion of pelagic calcite in the core tops increases with water depth while hmc decreases and is present only in trace amounts in cores below 1500 mbsl the difference in the percentage of hmc in the deeper cores between the glacial and holocene may reflect differences in supply or deepening of the hmc lysocline during the glacial sediment accumulation rates also vary between cores in the capricorn channel and do not show the expected exponential decrease with depth this may be due to intermediate or deep water currents reworking the sediments it is also possible that present bathymetry data are too sparse to detect the potential role that submarine channels may play in the distribution and accumulation of sediments comparison of the capricorn channel mars with those for other mixed carbonate siliciclastic provinces from the northeast margin of australia indicates that peak mars in the early transgression in the capricorn channel precede those from the central gbr and south of fraser island the difference in the timing of the carbonate and siliciclastic mar peaks along the northeast margin is primarily related to differences in the physiography and climate of the provinces the only common trend in the mars from the northeast margin of australia is the near synchronicity of the carbonate and siliciclastic mar peaks in individual sediment cores which supports a coeval sedimentation model

highstand versus lowstand deposition on carbonate platform margins insight from quaternary foreslopes in the bahamas abstract examples of both highstand and lowstand deposition have been identified on the upper platform slopes of the tongue of the ocean bahamas highstand deposits consist of a thick wedge of fine grained skeletal and non skeletal sands and carbonate muds derived predominantly from the platform and deposited on slopes of 25 28 these sediments were deposited subsequent to platform flooding and may be more than 30 m thick along open leeward margins in contrast lowstand deposits consists of coarse skeletal sands gravels and boulders deposited on primary depositional slopes of 35 45 these sediments were derived from fringing reefs along the marginal escarpment and deposited during the initial rise of sea level following the last lowstand these variations in depositional character result in a distinctive signature of highstand versus lowstand deposition for steep sided platforms and further elucidate the response of carbonate platforms to high magnitude 5th order scale 20 000 yrs fluctuations in sea level

vertical turbulent mixing processes on ebb tides in partially mixed estuaries abstract measurements of turbulent fluctuations of the horizontal and vertical components of velocity and of salinity in the region 1 1 25 m above the bed have been made in the teign estuary during parts of three ebb tides of different tidal ranges the turbulent mean flow field determined from vertical profiles of velocity and salinity varied from well mixed to partially mixed depending on the relative importance of the vertical gradient promoting effect of shear and longitudinal density gradient and of the mixing effect of bed generated turbulence the turbulence parameters showed regular temporal trends the relative intensity values for the new and existing data for the velocity components are strongly dependent on relative depth but not generally affected by richardson number the intensity of the salinity fluctuations increased with the salinity gradients the vertical transport of momentum and the vertical reynolds flux correlation coefficients decreased with richardson number the horizontal reynolds flux correlation coefficient increased with richardson number these effects are tentatively explained by the concept of turbulent fluctuations being replaced by wavelike fluctuations in stable conditions results for momentum and solute mixing lengths help to substantiate previously published values

a late quaternary marine palynological record oxygen isotope stages 1 to 7 for the humid tropics of northeastern australia based on odp site 820 a late quaternary marine palynological record from the ocean drilling program odp site 820 adjacent to the humid tropics region of northeastern australia has demonstrated marked variation in orbital scale cyclicity and also trends associated with both climate and human impact however some uncertainties in interpretation have resulted from concerns about the records chronology and continuity here we present for the first time the complete palynological data from detailed analysis of the top 67 m of sediment and examine it in relation to the marine isotope sequence from the core it is proposed that the record is relatively continuous through the last 250 000 years although the latter part of oxygen isotope stage ois 5 as well ois 4 may be missing despite the variation on orbital scales most palynological changes are not in phase with those from the marine isotope record suggesting a lack of direct milankovitch forcing on vegetation this lack of correspondence combined with major trends towards more open and sclerophyllous vegetation in association with increased burning supports a previous proposal that major control is being exercised by el nino southern oscillation variability whose influence may have been initiated by changes in oceanic circulation in the region within the mid pleistocene the lack of impact on the distribution of complex rainforest suggests that increased climate variability did not involve an overall decrease in total precipitation c 2007 elsevier b v all rights reserved

late quaternary cycles of mangrove development and decline on the north australian continental shelf mangrove communities in the australian tropics presently occur as narrow belts of vegetation in estuaries and on sheltered muddy coasts palynological data from continental shelf and deep sea cores indicate a long term cyclical component of mangrove development and decline at a regional scale which can be linked to specific phases of late quaternary sealevel change extensive mangrove development relative to today occurs during periods of marine transgression whereas very diminished mangrove occurs during marine regressions and during rarer periods of relative sea level stability episodes of flourishing mangrove cannot be linked to phases of humid climate as has been suggested in studies elsewhere rather the cycle of expansion and decline of mangrove communities on a grand scale is explained in terms of contrasting physiographic settings characteristic of continental shelf coasts during transgressive and regressive phases in particular by the existence or lack of well developed tidal estuaries copyright c 1999 john wiley sons ltd

eustatic control of calciclastic turbidites abstract global changes in sea level can be recognized in seismic sections by establishing the relationship between sea level changes and the timing of potential deep sea carbonate reservoir facies of turbidity current and debris flow origin their occurrence may be predicted prior to drilling compilation of published data suggests that throughout the phanerozoic most calciclastic turbidites tend to correspond to lowering episodes of sea level calciclastic turbidites therefore should occur primarily in association with major uncorformities caused by lowstands of sea level the data also suggest a possible lowstand of sea level during the late cretaceous campanian maastrichtian

eustatic and tectonic controls on deposition of hybrid siliciclastic carbonate basinal cycles discussion with examples changes in sea level represent one of the dominant controls on basinal sedimentation adjacent to mixed carbonate siliciclastic sediment source areas sedimentary responses to sea level change of the siliciclastic and carbonate components of these hybrid systems commonly result in deposition of alternating siliciclastic and shelf derived carbonate basinal deposits such deposition is particularly pronounced adjacent to carbonate platforms where alternate immersion exposure of the flat platform top results in carbonate highstand deposition and siliciclastic lowstand deposition in contrast small to moderate amplitude sea level drops in distally steepened ramp settings may expose only the inner part of the gently sloping shelf resulting in lowstand deposition of mixed sil ciclastic allodapic footnote 3 carbonate sediments along active margins tectonic uplift and subsidence of sediment source areas act as additional major controls on deposition of siliciclastic carbonate basinal cycles two of the most important tectonic effects are 1 uplift of siliciclastic sediment source areas resulting in increased erosion and swamping of carbonate shelf sources and 2 exposure or immersion of shelf carbonate sediment source areas due to vertical tectonic movements most published seismic stratigraphic studies of sea level control on deep sea sedimentation focus on lowstand deposition of siliciclastic sediment and commonly neglect the contribution of allodapic carbonate sediment deposited during sea level highstands however the examples discussed in this paper indicate that highstand carbonate deposits may make up a significant proportion of many hybrid carbonate siliciclastic basinal sequences

high rates of sea level rise during the last interglacial period sea level during the last interglacial stood at least 4 m higher than at present with evidence of short term fluctuations of up to 10 m a new continuous sea level record from the red sea and coral ages suggest that during these fluctuations sea level changes were on the order of 1 6 m per century

continuous 150 k y monsoon record from lake eyre australia insolation forcing implications and unexpected holocene failure our reconstructed history of lake eyre provides the first continuous continental proxy record of australian monsoon intensity over the past 150 k y this continental record s broad correspondence to the marine isotope record demonstrates that this very large catchment with its hydrology dependent on a planetary scale climate element responds to milankovitch scale climate forcing abrupt transitions from dry phases to wet phases ca 125 and 12 ka coincide with northern hemisphere winter insolation minima rather than southern hemisphere summer insolation maxima indicating that northern hemi sphere insolation exerts a dominant control over the intensity of the australian monsoon stratigraphic and dating uncertainties of other wet phases preclude conclusive correlation to specific insolation signals but within the uncertainties are consistent with northern hemisphere forcing regardless of the hemispheric forcing the low intensity of the early holocene australian monsoon by comparison with the last interglacial and particularly the last high level lacustrine event at 65 60 ka when all forcing elements were modest is an enigma that can be explained by a change in boundary conditions within australia

late quaternary sedimentation on the great barrier reef continental shelf and slope east of townsville australia cores collected in a shelf to basin transect from northeastern australia document the stratigraphic development of late quaternary deposits low sea level to 120 m slope sedimentation is dominated by prograding fluvial deposits with wave dominated deltas forming on the shelf edge and upper slope the onset of sea level transgression into the excavated shelf river channels on the outer shelf resulted in estuarine sedimentation along an indented coastline 15 000 11 000 yrs b p reducing terrigenous sediment supplies to the slope on the outer shelf halimeda muddy sandy gravels were deposited over a pleistocene limestone erosional surface from about 10 500 yrs b p transgression submerged the limestone outcrops and estuaries of the middle shelf where coral reef complexes formed the outer and middle parts of the shelf thus became dominated by carbonate sedimentation while the inner shelf received fluvial input mixed with locally derived carbonates the sequence is bounded by an unconformity clearly marked by erosional down cutting the results of the study are combined with those of previous workers to develop transgressive and continued high sea level system tract models the latter continued high sea level model is used to extrapolate present high sea level sedimentation patterns into the future providing a model for comparison with ancient sequences which may also have formed in such a depositional environment

salt intrusion and secondary flow in a shallow well mixed estuary observations of an axial convergence front formed within the conwy estuary north wales u k during the flood tide were indicative of density driven secondary circulations it has been proposed that these were the result of lateral density gradients established by the interaction of the lateral shear of the longitudinal currents with the axial salinity gradients through a differential advection mechanism in the present study the evolution of axial salinity gradients during a tidal cycle along the length of a shallow well mixed estuary was examined using six recording instruments deployed over a 25 day period the evolution of channel geometry and of the lateral shear of axial current speed was examined using simple models in conjunction with the observations of near bed pressure and current speed it is hypothesized that the complex interaction between these three parameters channel geometry lateral shear of axial current and axial salinity gradient results in the observed evolution of secondary flow throughout the tide which itself causes a redistribution of the density field

sea level rise depth dependent carbonate sedimentation and the paradox of drowned platforms a mathematical model of carbonate platform evolution is presented in which depth dependent carbonate growth rates determine platform top accumulation patterns in response to rising relative sea level this model predicts that carbonate platform evolution is controlled primarily by the water depth and sediment accumulation rate conditions at the onset of relative sea level rise the long standing paradox of a drowned platform arose from the observation that maximum growth rate potentials of healthy platforms are faster than those of relative sea level rise the model presented here demonstrates that a carbonate platform could be drowned during a constant relative sea level rise whose rate remains less than the maximum carbonate production potential this scenario does not require environmental changes such as increases in nutrient supply or siliciclastic sedimentation to have taken place a rate of relative sea level rise that is higher than the carbonate accumulation rate at the initial water depth is the only necessary condition to cause continuous negative feedbacks to the sediment accumulation rates under these conditions the top of the carbonate platform gradually deepens until it is below the active photic zone and drowns despite the strong maximum growth potential of the carbonate production factory this result effectively resolves the paradox of a drowned carbonate platform test modelling runs conducted with 2 5 m and 15 m initial sea water depths at bracketed rates of relative sea level rise have determined how fast the system catches up and maintains the keep up phase this is the measure of time necessary for the basin to respond fully to external forcing mechanisms the duration of the catch up phase of platform response termed carbonate response time scales with the initial sea water depth and the platform top aggradation rate the catch up duration can be significantly elongated with an increase in the rate of relative sea level rise the transition from the catch up to the keep up phases can also be delayed by a time interval associated with ecological re establishment after platform flooding the carbonate model here employs a logistical equation to model the colonization of carbonate producing marine organisms and captures the initial time interval for full ecological re establishment this mechanism prevents the full extent of carbonate production to be achieved at the incipient stage of relative sea level rise the increase in delay time due to the carbonate response time and self organized processes associated with biological colonization increase the chances for platform drowning due to deepening of water depth ca 10 m furthermore this implies a greater likelihood for an autogenic origin for high frequency cyclic strata than has been estimated previously

late pleistocene history of turbidite sedimentation in a submarine canyon off the northern great barrier reef australia cores from slopes east of the great barrier reef gbr challenge traditional models for sedimentation on tropical mixed siliciclastic carbonate margins however satisfactory explanations of sediment accumulation on this archetypal margin that include both hemipelagic and turbidite sedimentation remain elusive as submarine canyons and their role in delivering coarse grained turbidite deposits are poorly understood towards addressing this problem we investigated the shelf and canyon system bordering the northern ribbon reefs and reconstructed the history of turbidite deposition since the late pleistocene high resolution bathymetric and seismic data show a large paleo channel system that crosses the shelf before connecting with the canyons via the inter reef passages between the ribbon reefs high resolution bathymetry of the canyon axis reveals a complex and active system of channels sand waves and local submarine landslides multi proxy examination of three cores from down the axis of the canyon system reveals 18 turbidites and debrites interlayered with hemipelagic muds that are derived from a mix of shallow and deep sources twenty radiocarbon ages indicate that siliciclastic dominated and mixed turbidites only occur prior to 31 ka during marine isotope stage mis 3 while carbonate dominated turbidites are well established by 11 ka in mis1 until as recently as 1 2 ka the apparent lack of siliciclastic dominated turbidites and presence of only a few carbonate dominated turbidites during the mis2 lowstand are not consistent with generic models of margin sedimentation but might also reflect a gap in the turbidite record these data suggest that turbidite sedimentation in the ribbon reef canyons probably reflects the complex relationship between the prolonged period 25 ka of mis3 millennial sea level changes and local factors such as the shelf inter reef passage depth canyon morphology and different sediment sources on this basis we predict that the spatial and temporal patterns of turbidite sedimentation could vary considerably along the length of the gbr margin

filling the gap a 60 ky record of mixed carbonate siliciclastic turbidite deposition from the great barrier reef abstract late pleistocene to holocene margin sedimentation on the great barrier reef a mixed carbonate siliciclastic margin has been explained by a transgressive shedding model this model has challenged widely accepted sequence stratigraphic models in terms of the timing and type of sediment i e carbonate vs siliciclastic deposited during sea level oscillations however this model documents only hemipelagic sedimentation and the contribution of coarse grained turbidite deposition and the role of submarine canyons in this process remain elusive on this archetypal margin here we present a new model of turbidite deposition for the last 60 ky in the north eastern australia margin using high resolution bathymetry 58 new and existing radiometric ages and the composition of 81 turbidites from 15 piston cores we found that the spatial and temporal variation of turbidites is controlled by the relationship between sea level change and the variable physiography along the margin siliciclastic and mixed carbonate siliciclastic turbidites were linked to canyons indenting the shelf break and the well developed shelf edge reef barriers that stored sediment behind them turbidite deposition was sustained while the sea level position allowed the connection and sediment bypassing through the inter reef passages and canyons carbonate turbidites dominated in regions with more open conditions at the outer shelf and where slope confined canyons dominated or where canyons are generally less abundant the turn on and maintenance of carbonate production during sea level fluctuations also influenced the timing of carbonate turbidite deposition we show that a fundamental understanding of the variable physiography inherent to mixed carbonate siliciclastic margins is essential to accurately interpret deep water coarse grained deposition within a sequence stratigraphic context

the structural and sedimentological controls on the reoccupation of quaternary incised valleys belize southern lagoon a grid of high resolution single channel seismic data from the belize southern lagoon has documented the development of several generations of stacked late pleistocene 400 ka holocene incised valley fills a feedback mechanism between tectonics reefal buildups and incised valley location is apparent in the data incised valleys are recognized as medium to high amplitude reflections that truncate reflections produced by older strata on the seismic data the holocene valley fills in the belize southern lagoon contain three seismic facies units from bottom to top 1 a basal unit characterized by moderately high amplitude chaotic reflections or progradational patterns that downlap onto the valley floor 2 a middle unit characterized by continuous horizontal near transparent reflections that drape the basal unit and onlap the valley walls and 3 an upper unit that consists predominantly of transparent to low amplitude low angle downlapping reflections the middle and upper units are separated by an erosional surface based on core evidence and seismic facies analysis the basal middle and upper seismic and sedimentary facies units are interpreted as fluvial and carbonate sands estuarine muds and marine sands and muds respectively the erosional surface separating the upper and middle units is interpreted to be a ravinement surface that marks the transition from brackish estuarine conditions to open marine conditions faulting decreases and reefal buildups increase upsection the oldest valleys mimic structural trends and flow through areas currently occupied by extensive carbonate reefs rapid high amplitude late pleistocene sea level rises may have led to wides pread reef development creating a template that subsequent incisions reoccupation valleys followed regardless of structural trends the intimate relationship among tectonics sea level change carbonate reef buildups and incised valley reoccupation serves as a model for understanding ancient tropical mixed carbonate siliciclastic systems accelerator mass spectrometer 14c dates of sediment core samples marine gastropods marine mollusks barnacles worm tubes peat and oysters indicate all the sediment cored 1 4 m in this study is holocene in age between 4740 60 and 11 230 90 14c age this radiocarbon dating consistently links the erosional surface between the upper and middle units with the intersection of the regional sea level curve nonmarine marine boundary

quaternary aragonite cycles and oxygen isotope record in bahamian carbonate ooze carbonate ooze in bahamian troughs displays cyclic variation in the content of aragonite a 10 m core devoid of turbidites shows 3 5 cycles each defined by a sharp increase followed by a gradual decrease of aragonite the aragonite curve matches closely the oxygen isotope curve of planktic foraminifera from the same core aragonite highs correspond to light oxygen isotope values and thus to interglacial stages content of terrigenous material is low during interglacials and higher during glacial periods intermittent flooding and exposure of the bahama banks is most probably not the cause of the cycles because the latest increase in aragonite precedes bank flooding by 8 000 yr and because the cycles are distinctly asymmetric with rapid increase and gradual decrease of aragonitic content carbonate dissolution cycles tied to the glacial rhythm of earth9s climate are a more likely explanation of the variations in aragonite content

carbonate shedding and sedimentary cyclicities of a distally steepened carbonate ramp miocene great bahama bank depositional geometries as imaged in seis mic lines logging data and quantitative petrographic data were used to analyse the slope and toe of slope deposits of the miocene distally steepened carbonate ramp of great bahama bank the shedding pattern along the slope of this ramp is more complex than it is along the slope of the pliocene pleistocene flat topped carbonate platform compositional changes and compositional trends of periplatform sediments correlate with the positions of geophysically defined sequence boundaries two types of depositional sequences occur the first sequence is characterised by laterally traceable sequence internal reflections whereas the second type contains major intrasequen tial incisions erosional incisions and sedimentary infills of these canyons by turbidites formed during sea level lowstands as is indicated by the composition of the turbidites having a mixture of shallow water and pelagic particles highstand turbidites are charac terised by more extensive laterally traceable geome tries and by the occurrence of abundant shallow water particles in contrast to highstand turbidites shed from the pliocene pleistocene flat topped platform shal low water components in highstand turbidites of the miocene ramp are of skeletal origin the stacking pat tern of the periplatform deposits is controlled by sea level fluctuations four orders of sea level changes are distinguished third order cycles are delimited by sequence boundaries and fourth 100 000 a order cycles govern the bundling of turbidites into packages fifth 40 000 a and sixth 23 000 a order cycles are recorded in the background sediments and resolvable with spectral analysis

mixed carbonate siliciclastic sedimentation along the great barrier reef upper slope a challenge to the reciprocal sedimentation model abstract results of studies involving numerous cores and odp holes along the great barrier reef gbr margin and adjacent queensland trough and queensland plateau have challenged the use of a reciprocal sedimentation model to describe the sedimentary response of slope and basin settings to glacioeustatic sea level fluctuations upper slope sedimentation results from the relationships between sea level fluctuations antecedent topography and regional climate that play an important role in the type and amount of sediment deposited on the upper slope during glacial deglacial and interglacial times during the last glacial maximum lgm 20 ka ago upper slope sediments generally lacked siliciclastic material and are characterized by very low accumulation rates whereas early deglacial time termination i ti deposits are dominated by a siliciclastic and neritic carbonate pulse siliciclastic sedimentation was significantly reduced in the holocene while carbonate sedimentation remains elevated a new borehole iodp expedition 325 hole m0058a hole 58a recovered 82 of a 40 4 m hole on the upper slope east of noggin passage on the central gbr margin near cairns australia hole 58a provides a detailed sedimentary record during termination ii tii marine isotope stage 6 5e mis 6 5e deglacial transition and through most of interglacial mis 5 this hole along with two others odp leg 133 holes 820a and 819a from the upper slope east of grafton passage show carbonate siliciclastic cyclicity as the result of glacioeustatic change with the gbr shelf sedimentation at hole 58a is consistent with that of previous studies along the gbr margin focusing on the lgm to present and extends the upper slope sedimentary record back to tii and interglacial mis 5 a siliciclastic pulse similar to the one during ti occurred during the penultimate deglaciation tii however the maximum neritic aragonite export to the upper slope occurred not during peak mis 5e highstand when sea level was a few meters above modern position but subsequently during a time mis 5d to 5a when lowered sea level fluctuated between 30 and 50 m below present sea level siliciclastic sediments were reworked and exported to the upper slope when the lowstand fluvial plain was re flooded whereas neritic carbonate export to the slope reached a maximum when sea level fell and much of the mid to outer shelf re entered the photic zone subsequent to a drowning interval thus this analysis refines the mixed sedimentation models of upper slope sedimentation along the central gbr margin during the penultimate deglacial transgression and subsequent interglacial early and late highstand this study provides further evidence that mixed carbonate siliciclastic margins do not always behave in a predictable manner and that mixed margins both modern and ancient would benefit from detailed study of sediment transport in the context of sea level rise and fall

tropical view of quaternary sequence stratigraphy siliciclastic accumulation on slopes east of the great barrier reef since the last glacial maximum generic models of continental margin evolution predict that siliciclastic fluxes to slopes should be maximal and minimal during major sea level lowstands and transgressions respectively here we document the opposite for the northeast australian margin the largest extant mixed siliciclastic carbonate depositional system cores from slopes of this margin consistently contain siliciclastic rich intervals 0 3 1 m thick in the upper few meters radiocarbon dates of planktonic foraminifera show that this interval was deposited between 12 and 7 ka and represents greatly increased siliciclastic fluxes during late transgression this massive terrigenous discharge to slopes occurred along at least 450 km of the margin irrespective of modern variations in bathymetry or climate although we cannot dismiss a significantly different early holocene climate with greatly enhanced sediment discharge available data instead suggest that rivers aggraded on the shelf during lowstand because of an extensive subaerially exposed reef system this phenomenon may occur on other margins rimmed by reefs requiring a major revision of concepts used to interpret mixed siliciclastic carbonate systems

high production and highstand shedding from deeply submerged carbonate banks northern nicaragua rise abstract because the shallow isolated carbonate banks on the northern nicaragua rise the nicaragua honduras and southern jamaica carbonate shelves and many other modern carbonate banks worldwide are covered by an average of 20 to 30 m of water and by a thin blanket of coarse carbonate sediments other carbonate sedimentologists have considered these banks good examples of incipiently drowned or even drowned carbonate banks however based on recent research on the northern nicaragua rise we can demonstrate that these banks currently are healthy producers of large volumes of periplatform sediments fine aragonite and magnesian calcite which are exported almost entirely to the deep surrounding slopes these sediments which were deposited during the past 9000 10 000 years form peripl tform wedges on the middle and upper slopes which are seen clearly on 3 5 khz seismic profiles radiocarbon ages of the wedge surface sediment range between 230 and 610 ybp and provide clear evidence for the contemporaneous production of sediments on the shallow bank and shelf and their instantaneous export to the upper slopes for the last 5000 years sedimentation rates ranged from 2000 mm ky off pedro bank to 1300 mm ky off jamaica these rates are somewhat lower than but of the same order of magnitude as sedimentation rates on the western leeward slopes of great bahama bank deposition of the metastable carbonate sediments discussed in this paper occurred as soon as sea level rose to flood the bank and shelf which flank walton basin following the last glacial lowstand they com rise a wedge shaped package of transgressive and highstand sediments on the upper and middle slopes of pedro bank and the southern jamaica shelf

development of a bathymetric grid for the gulf of papua and adjacent areas a note describing its development 1 a new bathymetric grid for the gulf of papua and northern australia was produced for the area 140 150 e 6 14 s with a 3 6 110 m cell size new multibeam sonar surveys have added much needed detail to a region of the seabed where previously little was known in shallow australian waters bathymetry derived from landsat satellite imagery was used to supplement traditionally acquired bathymetric data for onshore areas shuttle radar topography mission data were used for topographic control the final grid revealed numerous features not observed in previous compilations of bathymetric data for the region bathymetric surveys on the continental shelf revealed an incised shelf with highly variable valley morphology prograding clinoforms are infilling valleys on the continental shelf and mark the seaward extension of the fly river delta a linear relict shelf edge barrier marks the shelf break in the northern ashmore trough region elsewhere the shelf break is scalloped and incised by canyons large mass transport deposits are widespread on the continental slope and indicate regions where mass wasting is common

timing and distribution of calciturbidites around a deeply submerged carbonate platform in a seismically active setting pedro bank northern nicaragua rise caribbean sea the sedimentological study of thirteen sediment cores from the periplatform setting surrounding pedro bank northern nicaragua rise caribbean sea shows that during the last 300 ka turbidite deposition is controlled by at least four factors 1 late quaternary sea level fluctuations 2 prolific fine grained sediment production and export resulting in oversteepening of the upper slope environment 3 the proximity to the bank margin and 4 local slope and seafloor morphology the most intriguing finding of this study is the paucity of turbidites with only 101 turbidites in 13 cores in this tectonically active setting near the caribbean plate boundary throughout the last 300 ka the frequency of turbidite input during interglacial stages is three times higher than during glacial stages also it is obvious that changes in sea level influence the timing of turbidite deposition this is especially evident during the transgressions resulting in rapid renewed bank top flooding subsequent neritic sediment overproduction and offbank export the flooding event during each transgression is usually recorded by the onset of turbidite deposition at various sites along several platform to basin transects in down and upcurrent slope settings overall however more turbidites are deposited during the regressive rather than the transgressive phases in sea level probably as a result of sediment reorganisation on the slope resulting in slope failure five cores show highstand bundling of calciturbidites i e higher number of turbidites during highstands than during lowstands in sea level

massive siliciclastic discharge to slopes of the great barrier reef platform during sea level transgression constraints from sediment cores between 15 s and 16 s latitude and possible explanations abstract the northeast australian continental margin including the great barrier reef gbr platform provides an outstanding modern analogue for understanding sedimentation in tropical mixed siliciclastic carbonate depositional systems previous studies of cores from this margin have suggested that in contrast to widely accepted models maximum siliciclastic fluxes to slopes over the last 30 ky occurred when sea level transgressed the shelf ca 11 7 ka rather than when sea level was at a lowstand before 18 ka however climate and bathymetry can affect regional sediment deposition significantly and the cores examined to date have come from the southern queensland trough where the margin has a wet tropical climate and a broad shelf 50 km with gradual slopes in this study we examine the abundance and accumulation of siliciclastic material in four cores from lower slopes of the northern queensland trough where rivers drain a drier region and debouche onto a narrow shelf with steep incised slopes similar to previously examined cores all cores contain a dark greyish brown horizon within the upper few meters this horizon represents a low in carbonate content and corresponding high in siliciclastic material primarily clay and oxygen isotope stratigraphy shows that it was deposited during late transgression nominally between 11 and 7 ka moreover mass accumulation rate calculations demonstrate that this horizon reflects a significant rise in siliciclastic fluxes when combined with the previous work to the south it appears that slopes all along a large portion of the northeast australian continental margin received greatly enhanced inputs of siliciclastic material during late transgression independent of proximal climate and bathymetry two general explanations are offered for this observation 1 climate change induced highly variable riverine discharge over the past 30 ky with a prominent maximum coincident with late transgression or 2 the shelf stores large quantities of siliciclastic sediment during lowstand and releases this material to the slope during late transgression in either case siliciclastic accumulation on the northeast australia continental margin behaves completely opposite to predictions from conventional models

sediment fluxes to marion plateau southern great barrier reef province over the last 130 ky new constraints on transgressive shedding off northeastern australia generic models for sediment accumulation predict that fluxes of siliciclastic and neritic carbonate material to continental slopes of tropical mixed systems will be out of phase over cycles of sea level change siliciclastic and carbonate fluxes should be highest during lowstands and highstands respectively and condensed sections should form during transgressions recent studies have documented the opposite response for slopes along portions of the largest modern tropical mixed system the northeast australian margin siliciclastic and carbonate fluxes are in phase and highest during transgression an outstanding issue however is the extent of this response because studies to date have mostly focused on sedimentation over the last 40 ky off the central gbr province an area flanked by a narrow shelf and a wet tropical hinterland here we investigate mixed sediment deposition in a core from marion plateau offshore a wide shelf and a dry tropical hinterland in the southern gbr province bulk carbonate content and carbonate mineralogy were determined downcore while foraminiferal oxygen isotopes and radiocarbon ages were used to constrain ages and accumulation carbonate content and mineralogy vary predictably with changes in sea level over the last 130 ky carbonate and aragonite are more abundant during highstands and less abundant during lowstands however component mass accumulation rates reveal that highest fluxes of all material siliciclastic low mg calcite aragonite and high mg calcite occurred during transgressions sedimentation consistently diverges from conventional models in all cores examined to date from slopes of the northeast australian margin regardless of core location and modern differences in climate and physiography the unexpected sediment response could result from changes in climate particularly increased precipitation during deglaciations although this does not explain elevated fluxes of low mg calcite alternatively fluvial sediments and diagenetically altered carbonate aggraded on the shelf during lowstands and were eroded and transported to the slope during transgressions

anatomy and growth of a holocene clinothem in the gulf of papua 1 high resolution seismic profiles and sedimentological data from grab samples and long cores provide an unprecedented picture of the structure sedimentology and late quaternary development of two gulf of papua gop clinothems one probably stage 3 and 4 in age and one holocene in age the older was partially eroded during stage 2 and partially covered by the younger clinothem during stage 1 the younger clinothem consists of three stratigraphic units separated by two surfaces of erosion bypass or correlative surfaces of lap the surfaces were formed by changes in accommodation and sediment supply the underlying physiography of the older clinothem also appears to play an important role in governing the shape of the younger clinothem in the northern gulf oblique clinoforms of the younger clinothem suggest that the rate of sediment supply from the northern rivers outstripped the formation of new accommodation whereas in the south sigmoidal clinoforms indicate that accommodation increased faster than sediment supply the origin of the new accommodation remains uncertain because of limited age constraints on the basis of sediment thickness stratal geometry and acoustic character off shelf transport appears to be the dominant sediment transport direction with preferential accumulation on the promontories and bypass in the valleys presently observed and computed modern flows and complex gyres in shallow water coupled with wave and current supported gravity flows or river floods can explain the form internal clinoform shapes and mineralogy of the younger gulf of papua clinothem

the effects of suspended sediment on turbulence within an estuarine turbidity maximum predictions of cohesive sediment transport are hampered by poor understanding of the effects of sediment suspensions upon the nature of near bed turbulence in this study current velocities associated turbulence and flow characteristics were measured within an estuarine turbidity maximum during two spring ebb tides measurements at two heights both within and above a near bed high concentration layer show the importance of intermittent internal waves the ratio of vertical to horizontal turbulence intensities exceeded 1 0 when waves were present the waves were more significant above the layer than within it comparison between the turbulent kinetic energy and the reynolds stresses showed inactive turbulence occurring at stresses less than about 0 2 n m 2 during these periods negative suspended sediment fluxes occurred indicating that the turbulence could enhance settling of the sediment the effect of the intermittent internal waves on the turbulent bursting structure and their interaction with the concentration profile appears to increase the variability of the reynolds fluxes of suspended sediment and cause downwards reynolds fluxes especially beneath the lutocline

late quaternary shedding of shallow marine carbonate along a tropical mixed siliciclastic carbonate shelf great barrier reef australia the north east australian margin is the largest modern example of a tropical mixed siliciclastic carbonate depositional system with an outer shelf hosting the great barrier reef gbr and an inner shelf dominated by fluvially sourced siliciclastic sediment wedges the long term interplay between these sediment components and sea level is recorded in the queensland trough a 1 2 km deep n s elongate basin situated between the gbr platform and the queensland plateau in this paper 154 samples from 45 surface grabs and six well dated piston cores were analysed for total carbonate content carbonate mineralogy and sr concentration to establish spatial and temporal patterns of carbonate accumulation in the queensland trough over the last 300 kyr surface carbonate contents are lowest on the inner shelf 5 and in the trough axis 60 because of siliciclastic dilution carbonate on the shelf is mostly sr rich aragonite and high mg calcite hmc whereas that in the basin is mostly low mg calcite once normalized to remove the effects of siliciclastic dilution surface sr rich aragonite and hmc abundances decrease linearly to background levels 100 km seaward of the shelf edge core samples show that over time normalized aragonite and sr abundances are greatest during periods of shelf flooding and lowest when sea level drops below the shelf edge this is consistent with changes in the production of coral and calcareous algae and the shedding of their debris from the shelf interestingly normalized hmc concentrations on the slope peak during periods of major transgression perhaps because of maximum off shelf transport from inter reef areas or intermediate water dissolution after accounting for siliciclastic dilution there are strong similarities in both spatial and temporal patterns of carbonate minerals between slopes and basins of the north east australian margin and those of pure carbonate margins such as the bahamas a limited set of basic processes including the formation and breakdown of carbonate on the shelf the transport of carbonate off the shelf and eustatic sea level probably controls carbonate accumulation in slope and basin settings of tropical environments irrespective of proximal siliciclastic sediment sources

systems tracts and depositional sequences in a carbonate setting a study of continuous outcrops from platform to basin at the scale of seismic lines abstract the application of sequence stratigraphy concepts to carbonates is feasible and this paper presents an example of its successful application the continuity and lateral extent of outcropping sedimentary sections observed at the scale of a seismic line from platform to basin in the southern vercors plateau western alps france confirm the validity of sequence stratigraphy concepts when applied to carbonate depositional systems from the stratal patterns displayed in outcrop we determined that carbonate lowstand systems tracts are major components in the basin even in the deep pelagic realm transgressive and highstand systems tracts are very thin a response to starvation in contrast on the shelf transgressive and highstand systems tracts are the major components carbonate lowstand and shelf margin systems tracts pinch out in the vicinity of the offlap break it clearly appears that the stratal patterns and stratal termination patterns of carbonate systems tracts especially of carbonate lowstand systems tracts are basically similar to those of siliciclastic systems tracts this study documents that carbonate platforms may provide the basin with carbonate clastics during periods of relative sea level falls and lowstand this is one of the most controversial aspects of the application of sequence stratigraphy concepts in carbonate environments a second controversial aspect of the application of sequence stratigraphy concepts to carbonate rocks is the distinction between transgressive and regressive facies cycles and depositional sequences and systems tracts this paper demonstrates that the shoreline position for carbonates which indicates the facies transgression or regression is independent of prograding or retrograding shelf breaks the stratal pattern and facies within systems tracts vary depending on the overall evolution of the accommodation space i e on their position within the transgressive and regressive cycles