an insight into crack density saturation rate and porosity model of the 2001 bhuj earthquake in the stable continental region of western india abstract the 2001 bhuj earthquake mw 7 6 source zone is examined in the light of crack density e saturation rate and porosity parameter using new data set derived from a large aftershock sequence recorded by the gujarat seismic network gsnet during november 2006 december 2009 processes of rupture initiations of the mainshock and its aftershock sequence are better understood by synthesizing the dynamic snapshots of the source zone using the new dataset pattern of crustal heterogeneities associated with high e high and high anomalies at depths varying from 20 km to 25 km is similar to those of earlier study by mishra and zhao 2003 the anomalous zone is found extended distinctly by 50 60 km in the lateral direction indicating the reinforcement of cracks and fractured volume of rock matrix due to long aftershock sequence since 2001 bhuj earthquake in the source area it is inferred that the presence of a fluid filled fractured rock matrix with super saturation may have affected the structural and seismogenic strengths of the source zone and is still contributing significantly to the geneses of earthquakes in and around the source zone anomalous pattern of high e with wider distribution of high indicates the existence of micro cracks in the lower crust while high suggests the cementation of cracks through permeation of residual magma metamorphic fluids into the hypocenter zone the results suggest that the existence of residual fluids in the fractured rock matrix in the mid to lower crust might have played a key role in triggering the 2001 mainshock and is still responsible for its continued long aftershock sequences

holocene evolution of a rock bounded barrier lagoon system cies islands northwest iberia coastal geomorphology results from the combined effects of contemporary dynamics sea level rise and the inherited geological framework yet the relative importance of these driving mechanisms may change throughout the evolutionary history of coastal deposits in this contribution we analyse the depositional history of the cies islands barrier lagoon system based on lithofacies radiocarbon ages and pollen analysis our results reveal a sedimentary sequence that provides evidence for striking changes in the dynamical functioning of this complex since the mid holocene the sedimentary sequence commenced about 7700 cal years bp by fresh water ponding of an upland depression located about 4 m below present mean sea level fresh water ponds were infilled by aeolian sediments following a gradual lowering of the water table 4000 cal years bp post 3600 cal years bp sea level rise allowed water oscillations to reach the elevation of the bedrock causing the inundation of fresh water ponds and subsequent lagoonal and marine sedimentation subsequently landward and upward migration of a sand barrier led to overwash and deposition of sand in the newly formed lagoon the resultant sedimentary sequence suggests that climatic conditions played an important role controlling the sedimentation regime during the entire history of the basin changing water table levels during early stages of evolution and increasing storminess during more recent times in addition background sea level rise related to the holocene transgression was a key factor in controlling the evolution of the system yet its influence depended to an extent on the relative elevation of the bedrock topography copyright 2009 john wiley sons ltd

lithology based sequence stratigraphic framework of a mixed carbonate siliciclastic succession lower cretaceous atlantic coastal plain this study presents a lithology based sequence stratigraphic framework and depositional model for lower cretaceous mixed siliciclastic carbonate sediments of the mid atlantic coastal plain eastern united states lithologic data from cores and cuttings were integrated with wireline logs and two dimensional seismic data to document lithofacies variability and stacking patterns across the albemarle basin of eastern north carolina ten facies associations are defined which are variably present within siliciclastic and carbonate dominated depositional profiles interpreted to extend from onshore lowland coastal plain to deep shelf depositional environments three depositional sequences 0 1 2 were identified each with component upward shoaling parasequences seismic reflectors typically coincided with key sequence stratigraphic surfaces which guided correlations between wells parasequences are grouped into parasequence sets with progressive progradational or retrogradational highstand and transgressive systems tracts respectively stacking patterns transgressive parasequences are thinner uniform in thickness and tend to be more dominated by molluskan carbonate facies highstand parasequences have more variable thickness are siliciclastic dominated and tend to be progradational on seismic data late highstand deposits of sequence 1 are dominated by restricted carbonate facies that likely reflect increased aridity lowstand deposits were not recognized from onshore well and seismic data the sequence stratigraphic framework developed documents the complex spatial and temporal facies relationships within a wave dominated mixed carbonate siliciclastic passive margin succession the strata studied document the complex interplay of lithofacies within a transition zone between near shore carbonate dominated strata to the south southeast georgia embayment and siliciclastic dominated marginal marine successions to the north baltimore canyon trough it also provides a useful stratigraphic calibration set for coeval offshore sediments that have been identified as potential areas for hydrocarbon exploration

deglacial sea level record from tahiti corals and the timing of global meltwater discharge the timing of the last deglaciation is important to our understanding of the dynamics of large ice sheets1 and their effects on the earth s surface2 3 moreover the disappearance of the glacial ice sheets was responsible for dramatic increases in freshwater fluxes to the oceans which probably disturbed the ocean s thermohaline circulation and hence global climate4 7 sea level increases bear witness to the melting of continental ice sheets but only two such records from barbados8 9 and new guinea10 11 corals have been accurately dated but these corals overlie active subduction zones where tectonic movements are large and often discontinuous especially in new guinea so the apparent sea level records may be contaminated by a complex tectonic component here we date fossil corals from tahiti which is far from plate boundaries and thus is likely to be tectonically relatively stable and remote from the locations of large former ice sheets the resulting record indicates a large sea level jump shortly before 13 800 calendar years bp which corresponds to meltwater pulse 1a in the barbados coral records8 9 the timing of this event is more accurately constrained in the tahiti record revealing that the meltwater pulse coincides with a short and intense climate cooling event12 15 that followed the initiation of the bolling allerod warm period12 16 but preceded the younger dryas cold event by about 1 000 years

internal anatomy of a mixed siliciclastic carbonate platform the late cenomanian mid turonian at the southern margin of the spanish central system the late cenomanian mid turonian succession in central spain is composed of siliciclastic and carbonate rocks deposited in a variety of coastal and marine shelf environments alluvial plain estuarine lagoon shoreface offshore hemipelagic and carbonate ramp three depositional sequences third order are recognized the atienza patones and el molar sequences the patones sequence contains five fourth order parasequence sets while a single parasequence set is recognized in the atienza and el molar sequences systems tracts can be recognized both in the sequences and parasequence sets the lowstand systems tracts only recognized for atienza and patones sequences are related to erosion and sequence boundary formation transgressive systems tracts are related to marine transgression and shoreface retreat the highstand systems tracts are related to shoreface extension and progradation and to carbonate production and ramp progradation sequences are bounded by erosion or emergence surfaces whose locations are supported by mineralogical analyses and suggest source area reactivation probably due to a fall in relative sea level transgressive surfaces are subordinate erosion and or omission surfaces with a landward facies shift interpreted as parasequence set boundaries the co existence of siliciclastic and carbonate sediments and environments occurred as facies mixing or as distinct facies belts along the basin mixed facies of coastal areas are composed of detrital quartz and clays derived from the hinterland and dolomite probably derived from bioclastic material siliciclastic flux to coastal areas is highly variable the maximum flux postdates relative sea level falls carbonate production in these areas may be constant but the final content is a function of changing inputs in terrigenous sediments and carbonate content diminishes through a dilution effect carbonate ramps were detached from the coastal system and separated by a fringe of offshore fine grained muds and silts as distinct facies belts the growth of carbonate ramp deposits was related to the highstand systems tracts of the fourth order parasequence sets during the growth of these ramps some sediment starvation occurred basinwards progradation and retrogradation of the different belts occur simultaneously suggesting a sea level control on sedimentation in the study area the co existence of carbonate and siliciclastic facies belts depended on the superimposition of different orders of relative sea level cycles and occurred mainly when the second order third order and fourth order cycles showed highstand conditions

lithology mapping of a mixed siliciclastic carbonate evaporite system using 3d seismic and well data lower triassic jialingjiang formation sichuan basin southwestern china abstract high quality three dimensional 3d seismic data acquired in the central sichuan basin southwestern china offer an opportunity to map complex lithologies in a mixed siliciclastic carbonate evaporite system in the lower triassic jialingjiang t1j formation the formation consists of siliciclastics limestone dolostone anhydrite and salt the lithologies consist several source reservoir cap assemblages in the area lithologies in the t1j formation change rapidly in the vertical direction forming different interbed patterns in thin layers in the mean time the lateral extend of each lithology is complex this vertical and lateral distribution makes it difficult to predict lithology by single seismic attribute therefore principle component analysis pca was applied to tens of seismic attributes to extract useful information the first three components contain most 83 02 99 85 of the lithology information preserved in seismic attributes which were used to correlate with lithology content calculated by core calibrated wireline logs correlation coefficients of the three seismic components with lithologies are significantly higher 0 37 0 79 than those of individual seismic attributes near zero to 0 58 different assemblies of end member lithologies were selected from anhydrite siliciclastics tight dolostone limestone and salt to perform pca in different sequences lithologic content distribution of individual end members was shown by color blending method to map the lithology mixture sedimentary history in the t1j formation was reconstructed based on lithology mixture maps and regional geology background in the study area eight 4th order sequences were interpreted in the t1j formation only the second sequence was influenced by siliciclastic input forming a complete mixed siliciclastic carbonate evaporite system open platform and or restricted evaporative environment dominated the other seven sequences forming a mixed carbonate evaporite system

differentiation of the significant late valanginian early cretaceous transgressive event in the spiti himalaya india abstract nearly continuous sedimentary succession of early cretaceous age has been long known in the tethyan himalayan belt without differentiation of the individual stages we discuss here our just realised differentiation of a relatively thin well bedded sandstone interval with significant presence of the characteristic late valanginian early hauterivian ammonoid genus olcostephanus it lies within a dominantly sandy succession with shale interbeds of giumal sandstone formation near gate in spiti himalaya this is the first stratigraphically precise record of olcostephanus from the cretaceous of india it is here circumstantially age constrained to the standard tethyan verrucosum subzone olcostephanus expansion event to the high himalayan belt

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

late holocene sea level rise in tampa bay integrated reconstruction using biomarkers pollen organic walled dinoflagellate cysts and diatoms a suite of organic geochemical micropaleontological and palynological proxies was applied to sediments from southwest florida to study the holocene environmental changes associated with sea level rise sediments were recovered from hillsborough bay part of tampa bay and studied using biomarkers pollen organic walled dinoflagellate cysts and diatoms analyses show that the site flooded around 7 5 ka as a consequence of holocene transgression progressively turning a fresh brackish marl marsh into a shallow restricted marine environment immediately after the marine transgression started limited water circulation and high amounts of runoff caused stratification of the water column a shift in dinocysts and diatom assemblages to more marine species increasing concentrations of marine biomarkers and a shift in the diol index indicate increasing salinity between 7 5 ka and the present which is likely a consequence of progressing sea level rise reconstructed sea surface temperatures for the past 4 kyrs are between 25 and 26 c and indicate stable temperatures during the late holocene a sharp increase in sedimentation rate in the top 50 cm of the core is attributed to human impact the results are in agreement with parallel studies from the area but this study further refines the environmental reconstructions having the advantage of simultaneously investigating changes in the terrestrial and marine environment 2009 elsevier ltd

ordovician carboniferous tectono sedimentary evolution of the north nuratau region uzbekistan westernmost tien shan abstract the tien shan is a c 2500 km long orogenic belt of which the nuratau region of eastern uzbekistan forms the western part petrographical and field analysis of the ordovician carboniferous succession in the north nuratau region provided the basis for a reconstruction of the depositional settings along part of the northern margin of the alai continent and their evolution during the period of closure of the turkestan ocean which separated the alai and the kazakh kyrgyz continents initial sedimentation ordovician was broadly carbonate dominated although by mid late ordovician times siliciclastic input predominated in some areas these variations between clastic and carbonate dominated regions may have been related to tectonic activity within the alai continent carbonate sedimentation was reestablished in the wenlock with broad shelf systems forming along the continental margin volcanic activity in the early devonian records a period of tectonic instability and this was followed by the reestablishment of the carbonate mosaic albeit with a greater degree of instability as indicated by stratigraphic gaps than in the silurian this pattern extended up into the carboniferous culminating in backarc related magmatic activity final closure of the turkestan ocean involved significant folding and thrusting as well as a major change from compressional to strike slip movement

the west central florida inner shelf and coastal system a geologic conceptual overview and introduction to the special issue abstract this paper provides an overview for this special publication on the geologic framework of the inner shelf and coastal zone of west central florida this is a significant geologic setting in that it lies at the center of an ancient carbonate platform facing an enormous ramp that has exerted large scale control on coastal geomorphology the availability of sediments and the level of wave energy in order to understand the holocene geologic history of this depositional system a regional study defined by natural boundaries north end of a barrier island to the apex of a headland was undertaken by a group of government and university coastal geologists using a wide variety of laboratory and field techniques it is the purpose of this introductory paper to define the character of this coastal inner shelf system provide a historical geologic perspective and background of environmental information define the overall database present the collective objectives of this regional study and very briefly present the main aspects of each contribution specific conclusions are presented at the end of each paper composing this volume

reservoir characterization and hydrocarbon accumulation in late cenozoic lacustrine mixed carbonate siliciclastic fine grained deposits of the northwestern qaidam basin nw china abstract late cenozoic lacustrine mixed carbonate siliciclastic sequences in the northwestern qaidam basin nw china are mainly composed of high porosity and low permeability fine grained rocks but have big potentials of oil gas commercial production however how these fine grained rocks generate high quality reservoirs and accumulate hydrocarbons remains poorly known this study focuses on shallow buried shang youshashan formation middle late miocene n22 and shizigou formation late miocene pliocene n23 reservoirs and hydrocarbons in xiaoliangshan area of the northwestern qaidam basin we conduct mineralogy petrography and porosity and permeability analysis for reservoir rocks and biomarker analysis for hydrocarbons and potential source rocks the results indicate that the n22 high quality reservoirs are dominated by fractured marlstones and lime mudstones and constitute structural traps whereas the n23 hydrocarbon reservoirs are mainly composed of siliciclastic dominantly rocks and form stratigraphic lithologic traps similar biomarker ratios between crude oil samples and paleogene organic matter rich rocks reveal that hydrocarbons in the shallow system were likely derived from deep buried source rocks rather than late cenozoic lacustrine fine grained rocks themselves in this case large scale vertical fractures created by kink band structures in the northwestern qaidam basin are supposed to facilitate hydrocarbon migration and accumulation preliminary production data and exploration well oil gas test results demonstrate highest yields and thus large hydrocarbon potentials in the n22 marlstone and lime mudstone reservoirs collectively we suggest that relatively high permeability fractured and carbonate rich reservoirs on structural highs are favorable targets for future oil gas exploration in the northwestern qaidam basin this petroleum system provides a typical and significant example for hydrocarbon exploration and development in mixed carbonate siliciclastic fine grained rocks of nonmarine petroliferous basins

cretaceous vertebrate fauna of the cauvery basin southern india palaeodiversity and palaeobiogeographic implications abstract the vertebrate fauna from the late albian to maastrichtian succession of the cauvery basin south india has been known since 1845 but it received only scant attention as compared to the vertebrates already known from the deccan volcanic province of peninsular india recent fossil discoveries appear to support the emergence of significantly diverse marine and non marine vertebrates comprising fishes frogs reptiles ichthyosaurs plesiosaurs turtles crocodiles and dinosaurs and a mammal from the cauvery basin have revived the interest in the fauna of the basin as it has significant implications for understanding the palaeobiogeography of india the latest albian to turonian marine vertebrates such as sharks ichthyosaurs and plesiosaurs show a wide geographic distribution and marine territory while sharks are typically cooler water fauna of high palaeolatitudes the latest maastrichtian non marine vertebrates especially turtles crocodiles dinosaurs and a mammal are considered to show mixed gondwanan and laurasian affinities thus providing new lines of evidence in favour of a latest cretaceous biotic links between india and the neighbouring continents an overview of the vertebrate faunal diversity of the cretaceous sequences of the cauvery basin and its palaeobiogeographic considerations are presented

facies architecture of the mixed carbonate siliciclastic inner continental shelf of west central florida implications for holocene barrier development abstract sediment vibracores and surface samples were collected from the mixed carbonate siliciclastic inner shelf of west central florida in an effort to determine the three dimensional facies architecture and holocene geologic development of the coastal barrier island and adjacent shallow marine environments the unconsolidated sediment veneer is thin generally

stratigraphic framework of sediment starved sand ridges on a mixed siliciclastic carbonate inner shelf west central florida abstract seismic reflection profiles and vibracores have revealed that an inner shelf sand ridge field has developed over the past few thousand years situated on an elevated broad bedrock terrace this terrace extends seaward of a major headland associated with the modern barrier island coastline of west central florida the overall geologic setting is a low energy sediment starved mixed siliciclastic carbonate inner continental shelf supporting a thin sedimentary veneer this veneer is arranged in a series of subparallel shore oblique and to a minor extent shore parallel sand ridges seven major facies are present beneath the ridges including a basal neogene limestone gravel facies and a blue green clay facies indicative of dominantly authigenic sedimentation a major sequence boundary separates these older units from holocene age organic rich mud facies marsh which grades upward into a muddy sand facies lagoon or shallow open shelf seagrass meadows cores reveal that the muddy shelf facies is either in sharp contact or grades upward into a shelly sand facies ravinement or sudden termination of seagrass meadows the shelly sand facies grades upward to a mixed siliciclastic carbonate facies which forms the sand ridges themselves this mixed siliciclastic carbonate facies differs from the sediment on the beach and shoreface suggesting insignificant sediment exchange between the offshore ridges and the modern coastline additionally the lack of early holocene pre ridge facies in the troughs between the ridges suggests that the ridges themselves do not migrate laterally extensively radiocarbon dating has indicated that these sand ridges can form relatively quickly 1 3 ka on relatively low energy inner shelves once open marine conditions are available and that frequent high energy storm dominated conditions are not necessarily required we suggest that the two inner shelf depositional models presented open shelf vs migrating barrier island may have co existed spatially and or temporally to explain the distribution of facies and vertical facies contacts

coeval versus reciprocal mixed carbonate siliciclastic deposition middle devonian baltic basin eastern europe implications from the regional tectonic development the middle devonian narva succession in the baltic basin represents a significant turnaround in the history of the basin the detailed study of core and outcrop sections and the three dimensional correlations across the baltic basin reveal a carbonate dominated mixed retrogressive succession overlain by a siliciclastic dominated progradational succession the palaeogeographic reconstructions show how the shallow tide influenced basin expanded from south west to north east and later during the transgression also to the north south and east the transgressive portion of the basin fill is dominated by carbonate rich sabkha and supratidal to intertidal deposits on the basin margins and subtidal carbonates in the basin centre siliciclastic material was derived by tidal currents and storm waves from the south west through a tidal inlet and flood tidal delta complex this initial transgressive phase is characterized by the lack of subsidence or even episodic uplifts in the northern north western part of the basin margin shown by convergence of timelines and the thin 30 m transgressive succession in contrast on the southern margin the facies associations stack vertically into a 70 to 80 m thick succession indicating significantly higher subsidence rates the upper part of the transgressive phase indicates subsidence across the whole basin the upper progradational portion of the basin fill is dominated by coarse siliciclastic tide influenced deltaic deposits that rapidly prograded from north west to south east this detailed study on the narva succession shows that siliciclastic and carbonate deposition was coeval and that mixing occurred at different temporal and spatial scales the mixing was controlled by grain size volume and location of siliciclastic input rather than relative sea level changes as suggested in widely used reciprocal mixing models it is suggested that the forebulge of the scandinavian caledonian fold and thrust belt migrated to the north western margin of the baltic basin during the earliest eifelian as indicated by the lack of subsidence and probable uplift in the northern north western margin during the early transgressive phase the forebulge migration ceased although the forebulge had already started to subside during the later stages of the transgressive phase the deltaic progradation is interpreted to be associated with the orogenic collapse and uplift in the scandinavian caledonides that caused the erosion of the foreland basin fill and the coarse sediment transport into the baltic basin

late holocene estuarine inner shelf interactions is there evidence of an estuarine retreat path for tampa bay florida abstract the purpose of this study was to determine if and how a large modern estuarine system situated in the middle of an ancient carbonate platform has affected its adjacent inner shelf both in the past during the last post glacial sea level rise and during the present an additional purpose was to determine if and how this inner shelf seaward of a major estuary differed from the inner shelves located just to the north and south but seaward of barrier island shorelines through side scan sonar mosaicking bathymetric studies and ground truthing using surface grab samples as well as diver observations two large submarine sand plains were mapped one being the modern ebb tidal delta and the other interpreted to be a relict ebb tidal delta formed earlier in the holocene the most seaward portion of the inner shelf studied consists of a field of lobate bathymetrically elevated fine sand accumulations which were interpreted to be sediment starved 3d dunes surrounded by small 2d dunes composed of coarse molluscan shell gravel additionally exposed limestone hardbottoms supporting living benthic communities were found as well this modern shelf sedimentary environment is situated on a large buried shelf valley which extends eastward beneath the modern tampa bay estuary these observations plus the absence of an incised shelf valley having surficial bathymetric expression and the absence of sand bodies normally associated with back tracking estuarine systems indicate that there was no cross shelf estuarine retreat path formed during the last rise in sea level instead the modern tampa bay formed within a mid platform low relief depression which was flooded by rising marine waters late in the holocene with continued sea level rise in the late holocene this early embayment was translated eastward or landward to its present position whereby a larger ebb tidal delta prograded out onto the inner shelf extensive linear sand ridges common to the inner shelves to the north and south did not form in this shelf province because it was a low energy open embayment lacking the wave climate and nearshore zone necessary to create such sand bodies the distribution of bedforms on the inner shelf and the absence of seaward oriented 2d dunes on the modern ebb tidal delta indicate that the modern estuarine system has had little effect on its adjacent inner shelf

topographic and hydrodynamic controls on barrier retreat and preservation an example from dogger bank north sea abstract barrier retreat can occur due to in place drowning overstepping or rollover depending on the interplay of controls such as sea level rise sediment supply coastal hydrodynamic regime and topography offshore sedimentary archives of barriers active during rapid holocene sea level rise provide important records of marine transgression which are vital analogues to support appropriate mitigation strategies for future coastal realignment under projected relative sea level rise scenarios this study analyses the sedimentary archive at dogger bank which is a formerly glaciated area in the north sea dogger bank experienced marine transgression due to early holocene rapid relative sea level rise an integrated dataset of vibrocores and high resolution seismic reflection data permits a stratigraphic framework to be established which reveals the buried coastal geomorphology of the southern dogger bank for the first time a transgressive stratigraphy was identified comprising a topographically complicated basal glacial and terrestrial succession overlain by two phases of barrier and tidal mudflat deposition prior to shallow marine sedimentation barrier phase a was a recurved barrier drowned in place and discontinuously overstepped to barrier phase b which experienced continuous overstepping by linking barrier elevations to relative sea level curves the timing of each barrier phase was established both barrier phases retreated during periods of rapid sea level rise with abundant sediment supply coastal hydrodynamics increasing wave energy and antecedent topography with spatially variable accommodation are suggested to be the main reason for differing retreat mechanisms rather than the rate of sea level rise antecedent coastal geomorphology plays a critical role in erosional and depositional patterns during transgression and therefore on the timing rate and location of marine inundation which needs to be included in models that aim to forecast hazards in coastal areas

trace fossils in the mesozoic sediments of kachchh western india abstract the mesozoic succession of kachchh western india is a classic example of the value of trace fossils in the interpretaion of depositional sequences because this sequence has traditionally been referred to as gondwana and because the previous emphasis has been on body fossils a misconception has been established concerning this fascinating nearshore sequence preliminary investigations of generally well preserved physical and biogenic structures in the mesozoic rocks of kachchh reveal that the entire succession represents deposition in a shallow marine coastal setting i e lagoons embayments tidal flats estuarine channels shoals and shallow shelf the patcham formation shows poor preservation of trace fossils due to lack of appropriate lithologies the chari formation contains a rich ichnofauna of special interest are horizons of bioconglomerates showing reworked fillings of thalassinoides burrows and pholad borings the katrol formation exhibits a diversified and well preserved assemblage of burrows including an unusual feeding burrow here recorded for the first time the umia formation though devoid of body fossils contains a rich and varied assemblage of ichnofossils the uppermost part of this unit the bhuj member contains several thick horizons of densely rhizocorallium bioturbated sand which supports a marine setting contrary to the general belief of its fluvial origin

benthic foraminifera of the holocene transgressive west central florida inner shelf paleoenvironmental implications the sedimentology stratigraphic position and benthic foraminiferal biostratigraphy of early to mid holocene deposits from the west central florida shelf suggest that barrier islands developed along this coast as early as 8 3 ka in an environment that was more arid than today predominant foraminifera of three paralic sedimentary facies deposited between 5 3 and 8 3 ka include miliolids elphidium spp and ammonia spp all of which are common in back barrier environments foraminiferal assemblages also suggest that early back barrier sediments were deposited in a hypersaline environment similar to that of the arid laguna madre of the western gulf of mexico modern back barrier foraminifera in the tampa bay region are indicative of the humid subtropical climate of today thus the climate of west central florida at approximately 8 ka was more arid than today which is consistent with recent studies showing that climate in the gulf of mexico was dryer and cooler during this time period

seismites in continental sand sea deposits of the late cretaceous caiua desert bauru basin brazil abstract two large scale sediment deformation structures minor fold occurrences in cross bedded sand dune deposits and complex convolute folds are observed in red sandstones in a zone about 1 5 km long in floodway cuts at the sergio motta porto primavera dam sao paulo state brazil the most important structures are confined to planar zones up to 10 m thick in undeformed dune foreset strata were they can be traced laterally for about 50 60 m the sandstones are part of the rio parana formation caiua group which accumulated in a great sand sea of about 100 000 km2 the caiua desert developed during the late cretaceous in the southern part of the bauru basin an intracontinental subsiding area in the central southern part of the south american platform the basin was filled by a sandy sequence about 300 m thick the sand sea deposits correspond to the caiua group and comprise a deposits of dry sand sheets santo anastacio formation b deposits of medium sized dunes and humid interdunes of the sand sea peripheral zones goio ere formation and c deposits of large sized complex aeolian dunes and draas that correspond to the central part of the inland sand sea rio parana formation the deformations in the sediments are attributed to the effects of fluidization liquefaction and shear stress which are interpreted as being earthquake induced structures mainly because 1 the deformed horizons are confined between undeformed cross bedded strata 2 the complex convolute folds sometimes include nappe like structures that overlie foreset facies 3 during the bauru basin infilling there was tectonic activity associated with alkaline volcanism on the borders of the basin and related silicification in the central southern part the main silicification zones are aligned to regional lineaments that cross the area near the large scale sedimentary deformation structures

hardbottom development and significance to the sediment starved west central florida inner continental shelf abstract hardbottoms are sequence boundaries and condensed sections that offer clues for the interpretation of the incomplete record of tertiary continental shelf evolution seaward of 5 km 50 of the inner west central florida shelf seafloor is flat hardbottom these lithified surfaces are punctuated by shorefacing scarped hardbottoms that trend shore parallel 330 0 and vary in relief up to 4 m scarped hardbottoms are the only natural relief on the inner shelf and support a diverse benthic community the activities of which erode the outcrops producing undercuts in excess of 1 m outcropping hardbottom strata are comprised of distinct phosphate rich mixed carbonate siliciclastic lithofacies that range in age from miocene to quaternary miocene units are dolomite rich and mark the upper surface of the inner shelf bedrock hawthorn group dolomite within these beds silt sized cloudy centered rhombs fall into two age groups correlating with highstands at 15 and 5 ma this lithofacies is consistent with models that indicate an increased flux of organic matter resulting from topographically induced upwelling promoting dolomitization during early burial diagenesis in the sulfate reduction zone quaternary units are calcite rich and perched atop the shelf bedrock samples of these units record a complex diagenetic history and multiple sea level fluctuations based on evidence of primary marine cementation they are interpreted to be hardground non deposition surfaces forming as a function of sediment starvation and minimal sediment movement decreased highstand magnitude or duration may have resulted in the absence of a significant organic component to quaternary hardbottoms which in turn may prevent subsequent dolomitization these outcrops are a potential source for sediments to the inner shelf not only as habitat for biological sediment production but also through their destruction the undercut shorefacing scarped hardbottom morphology displayed by west central florida hardbottoms is indicative of bio erosion preliminary studies indicate a potential mass of 0 04 kg m 2 yr 1 of siliciclastic sediment is released to the inner shelf

physico chemical tuning of palaeogeographic shifts bhuj formation kutch india abstract the long standing dilemma between two extremes in palaeogeographic interpretation is resolved for the basal 115 m of the eastern exposures of the lower cretaceous bhuj formation at the top of the hydrocarbon prospective mesozoics of kutch india an explicit record of a mega scale event of relative sea level rise is found sandwiched between two fluvial stratigraphic intervals that amalgamate only at the eastern extremity of the exposure of the formation a localized estuary in the middle had its axis roughly parallel to the pre and post inundation river channels although a tributary made a significant detour variation in the fine sandstone mudstone lithology documents decline in depositional energy in the mid estuary amenable to coal formation sediment within the estuary had been routed through both the ends palaeocurrent turning shore parallel only at the estuary mouth exception arises in case of some isolated lenses of very poorly sorted and coarse grained deposits depicting flash floods in the tributary burrow characteristics change delicately in response to the inferred changes in the depositional scenario along the estuary resting on a coarsening upward ramp the initially fining and then coarsening upward estuarine succession is prograding though not steadily it was wave dominated but tide left imprint behind the estuary mouth bar gaining in intensity because of valley constriction macroscale changes in relative sea level perhaps eliciting local tectonics are recorded in the transient hysteresis in the stratigraphic trend with or without palaeocurrent diversions the fluvial stratigraphic intervals encasing the marine interval have in terms of architectural elements many commonalities but also some significant disparities the latter indicates channel meandering in the pre inundation period and braiding after inundation unlike the marine sandstones bar channel sandstones of the riverine deposits are distinctly less sorted bimodal even polymodal in grain size distribution mineralogical composition identifies the sandstones as craton derived but their geochemical distinctions point to variation in parent rock composition under the maze of palaeogeographic overprints

post miocene stratigraphy and depositional environments of valley fill sequences at the mouth of tampa bay florida abstract post miocene sea level low stands allowed rivers and karst processes to incise the exposed carbonate platform along the gulf coast of florida few miocene to mid pleistocene deposits survived erosion along the present coast except within incised valleys since their formation these valleys have been filled and incised multiple times in response to sea level changes the thick sedimentary sequences underlying the mouth of tampa bay have been recorded as a range of depositional environments and multiple sea level incursions and excursions during pre holocene time and subsequent to the accumulation of the miocene carbonate sequences sediment analysis of cores collected from a north south transect across the mouth of tampa bay has enabled the identification of lithofacies ranging from well sorted quartz sand to dense fossiliferous phosphatic grainstone these facies were deposited in freshwater estuarine and shallow open marine environments as a result of channel development and migration within the paleovalley and cut and fill associated with individual transgressions and regressions correlation of the lithofacies does not extend across the entire transect fining upward sequences truncated by tidal ravinement surfaces that extend throughout the paleovalley can however be identified age determinations based on 14 c analysis amino acid racemization and strontium isotope analysis dating of numerous samples yield ages of miocene pliocene early pleistocene and late pleistocene as well as holocene for sequences that accumulated and were preserved in this valley fill complex numerous inconsistencies in the stratigraphic organization of the age determinations indicate that there are bad dates considerable reworking of shells that were dated or both for this reason as well as the lack of detailed correlation among the three relatively complete cores it is not possible to place these strata in a sequence stratigraphic framework

calculating shoreline erosion potential using nearshore stratigraphy and sediment volume outer banks north carolina 1 despite the acknowledged influence of coastal geological framework on the behavior of beaches and barrier islands and a wealth of geological and bathymetric observations from the inner shelf quantitatively connecting those observations to shoreline behavior has been difficult nearshore geologic and morphologic variability described by recent research is not well represented by conventional geologic parameters such as mean grain size and shoreface slope used in most shoreline change models we propose that total nearshore sediment volume as calculated to a continuous seismic reflection surface provides a flexible and robust metric for use in the prediction of shoreline change this method of determining the volume of sediment in the nearshore accounts for three dimensional sandbar morphologies and heterogeneous seafloor sediments the decadal scale shoreline change rate for northeastern north carolina is significantly correlated to the volume of sediment in the nearshore when a geologically defined base is used in volume determinations suggesting that the shallow stratigraphic framework of transgressive coasts is an important influence on decadal shoreline behavior nearshore sediment volume was overestimated when an arbitrary depth constant baseline was used and was not correlated to decadal shoreline change this implies that a volume metric which accounts for both framework geology and variable seafloor morphology better represents the geologic character of the shoreface and may help to improve existing models of shoreline change an empirical model of regional shoreline erosion potential demonstrates the importance of incorporating nearshore sediment volume shallow framework geology and surface morphology when predicting seasonal to decadal shoreline evolution

depositional response to seagrass mortality along a low energy barrier island coast west central florida abstract analysis of aerial photographs and nearshore surficial sediment samples from the northern islands of the west central barrier system of florida indicates that 1 seagrass beds in the nearshore zone have controlled onshore longshore sand transport and 2 resulting sedimentary accumulations within nearshore seagrass beds make differentiation between nearshore and backbarrier facies difficult between 1957 and 1967 an extensive seagrass community occupying the nearshore zone off anclote key disappeared thus allowing the sudden and rapid onshore and longshore transport of sand the 1 000 year old barrier island lengthened 30 percent by recurved spit growth in a 15 year period from 1967 82 although there are no direct observations four possible causes of seagrass mortality have been postulated which are as follows 1 physical destruction by storms 2 infection by pathogens 3 decline of water quality due to human development and 4 overgrazing by sea urchins because of the ability of seagrasses to trap fine grained sediments contribute organic matter and provide for low energy sheltered molluscan biocoenose strong similarities remain between surficial sediments of recently devegetated active nearshore zones and backbarrier lagoonal facies the stable carbon isotopic ratios and the molluscan assemblages within the ubiquitous fine quartz sands in particular are similar within these two normally disparate environments this study indicates that the development and destruction of benthic floral communities should be considered as a process that generates episodic cyclic depositional events in the sedimentary record additionally such changes in seagrass communities should be expected to present a blurred distinction between nearshore and backbarrier sedimentary facies

patterns and controls of surface sediment distribution west central florida inner shelf abstract the west central florida inner shelf represents a transition between the quartz dominated barrier island system and the carbonate dominated mid outer shelf surface sediments exhibit a complex distribution pattern that can be attributed to multiple sediment sources and the ineffectiveness of physical processes for large scale sediment redistribution the west florida shelf is the submerged extension of the florida carbonate platform consisting of a limestone karst surface veneered with a thin unconsolidated sediment cover a total of 498 surface sediment samples were collected on the inner shelf and analyzed for texture and composition results show that sediment consists of a combination of fine quartz sand and coarse biogenic carbonate sand and gravel with variable but subordinate amounts of black phosphorite rich sand the carbonate component consists primarily of molluskan fragments the distribution is patchy and discontinuous with no discernible pattern and the transition between sediment types is generally abrupt quartz rich sediment dominates the inner 15 km north of the entrance into tampa bay but south of the bay is common only along the inner 3 km elsewhere carbonate rich sediment is the predominate sediment type except where there is little sediment cover in which cases black phosphorite rich sand dominates sediment sources are likely within or around the periphery of the basin fine quartz sand is likely reworked from coastal units deposited during pleistocene sea level high stands carbonate sand and gravel is produced by marine organisms within the depositional basin the black phosphorite rich sand likely originates from the bioerosion and reworking of the underlying strata that irregularly crop out within the study area the distribution pattern contains elements of both storm and tide dominated siliciclastic shelves but it is dictated primarily by the sediment source similar to some carbonate systems other systems with similar sediment attributes include cool water carbonate sediment starved and mixed carbonate siliciclastic systems this study suggests a possible genetic link among the three systems

characteristics and formation mechanism of multi source mixed sedimentary rocks in a saline lake a case study of the permian lucaogou formation in the jimusaer sag northwest china abstract the permian lucaogou formation in the jimusaer sag of the southeastern junggar basin in china is an uncommon set of mixed sedimentary rocks the mixed sediment content of the lucaogou formation is unusual due to the large amounts of tuffaceous material which makes the system more complex than the commonly mixed siliciclastic carbonate rocks in this study the volcanic material acts as a third end member component in a multi source mixed sedimentary system therefore a new type of mixed siliciclastic carbonate tuffaceous rocks has been introduced methods including core observations thin section analysis grain size measurements and x ray diffraction are used to investigate the characteristics of mineral components sedimentary structures and lithological assemblages in upper and lower sections of the formation these analyses are then used to determine the lateral distribution area of mixed sedimentation sedimentary mixing processes originally described for siliciclastic carbonate systems are used and expanded here for the multi source mixed rocks in a saline lacustrine basin controlling factors on mixed sedimentation included climate change lake level fluctuation tectonic activity and sediment supply rate mentioned in previous studies as well as variations of energy levels in different parts of the lake mixed depositional models for the lower and upper sections of the lucaogou formation in saline lacustrine basin have been established respectively

synsedimentary seismic activity in an immature passive margin basin lower member of the katrol formation upper jurassic kutch india abstract seismites are difficult to identify in a sedimentary sequence that has been exposed to a number of other synsedimentary disturbances the paper identifies some possible signatures of earthquakes in a lower submarine fan complex that formed in an extensional basin at the northwestern margin of the indian plate dying growth faults preferential basal brecciation of shale beds frequent local intraformational unconformities reverse grading attributed to mechanical sieving and intrabasinal mass flows through sand volcanoes bear the distinctive records of seismicity abundant synsedimentary grabens and horsts and profuse signs of liquifaction are suggestive in this association the thinning and fining up channel fill sequences with evidence of gradually declining mass flow regimes also corroborate

pore structure evaluation of tight reservoirs in the mixed siliciclastic carbonate sediments using fractal analysis of nmr experiments and logs abstract the study on mixed depositional reservoirs is widely attended in recent years however the mixed depositional reservoirs are generally characterized by complex pore structure which is caused by strong heterogeneity complex mineral compositions and varying pore sizes presenting a major challenge to describe the difference in pore structure between different samples to address this challenge the pore structure of mixed depositional reservoirs was evaluated by the fractal dimensions and the influences of various mineral compositions on pore structure were analyzed the fractal dimensions derived from nmr measurements or nmr logging data can be divided into two segments at t 2cutoff values representing different fractal values and characteristics of pore structure for the e 3 2 formation in yingxi field the fractal dimension of movable fluid pores ranges from 2 628 to 2 917 average 2 765 whereas the bound fluid pore systems are not following the fractal theory meanwhile the fractal dimensions show a negative relationship with movable fluid porosity and permeability the effects of various mineral compositions on fractal laws of the pore networks are different fractal dimension increases with the increases of clay mineral contents and the decrease of dolomite content while the effect of calcite content on fractal laws of the pore system is uncertain and quartz from different genetic type represents different effect on fractal dimensions this study is contributed for petroleum exploitation in mixed depositional reservoirs and shows that the fractal dimension can comprehensively indicate the complexity of the pore structure in mixed depositional reservoirs

implications of new data on mesozoic rocks of kachchh western india a lack of detailed facies studies has resulted in a false impression of the mesozoic depositional setting in kachchh western india previously kachchh has been termed kutch or cutch in addition designation of part of this sequence as gondawana implies by tradition a predominantly non marine origin recently we examined these jurassic cretaceous units and found numerous wave built sedimentary structures that have been previously overlooked or unreported abundant marine trace fossils and highly bioturbated and glauconite rich beds thus we now propose a marine origin for the entire kachchh succession this interpretation has important implications regarding similar sequences around the indian plate margin

subtropical to temperate facies from a transition zone mixed carbonate siliciclastic system palaeogene north carolina usa palaeogene passive margin sediments on the us mid atlantic coastal plain provide valuable insight into facies interaction and distribution on mixed carbonate siliciclastic shelves this study utilizes well cuttings outcrop core and seismic data to document temporal and spatial variations in admixed bryozoan rich skeletal carbonates and sandy siliciclastic units that were deposited on a humid passive margin located in the vicinity of a major marine transition zone this zone was situated between north flowing warm waters of the ancestral gulf stream carbonate dominated settings and south flowing cold waters of the ancestral labrador current siliciclastic dominated settings some degree of mixing of carbonates and siliciclastics occurs in all facies however siliciclastic prone sediments predominate in nearshore settings while carbonate prone sediments are more common in more open marine settings of the inner shelf break and deep shelf a distinctive dual break shelf depositional profile originated following a major late cretaceous to palaeocene transgression that drowned the earlier shallow platform this profile was characterized by prominent mid shelf break dividing the shallow shelf from the deep shelf and a major continental shelf slope break incomplete filling of available accommodation space during successive buildup of the shallow shelf preserved the topographic break on this passive margin storm wave base also contributed to the preservation of the dual break shelf geometry by beveling shallow shelf sediments and transporting them onto and seaward of the mid shelf break sediment fines in deep shelf facies were produced in place transported downdip from the shallow shelf by storm ebb currents and boundary currents and reworked from adjacent areas of the deep shelf by strike parallel boundary currents regional climate and boundary currents controlled whether carbonate or siliciclastic material was deposited on the shelf with warmer waters and more humid climates favouring carbonate deposition and cooler more arid conditions favouring glaucony and siliciclastic dominated deposition continuous wave and current sweeping of the shallow shelf favoured deposition of mud lean facies across much of the shallow shelf skeletal components in much of the carbonate rich strata formed in warm nutrient rich subtropical waters as indicated by widespread occurrences of larger benthic foraminifera and molluscan assemblages these indicators of warm water deposition within the bryozoan mollusk rich carbonate assemblage on this shelf provide an example of a warm water bryomol assemblage such facies generally are associated with cooler water depositional settings

recent geological history and modern sedimentary processes along an incipient low energy epicontinental sea coastline northwest florida abstract a major portion of florida s gulf of mexico coastline is a siliciclastic sand starved low wave energy system dominated by marshes that face the open sea because of its location at the center of an enormous 350 000 km2 flooded broad flat ancient carbonate platform this coastline can be viewed as the margin of an incipient epicontinental sea within a 65 km sector along this 300 km long open marine marsh coast four distinctly different morphological sectors have been identified 1 berm ridge marsh shoreline 2 marsh peninsula shoreline 3 marsh archipelago shoreline and 4 shelf embayment shoreline the underlying paleogene limestone bedrock topography results from karstification and dissolution processes this antecedent topography and the distribution of actively discharging freshwater springs affect sedimentary processes facies and stratigraphic units the morphology and processes within the four coastal sectors vary as a function of a northerly decrease in the thickness of relict pleistocene quartz sand cover and an increase in the complexity and relief of the underlying bedrock topography each of the four coastal sectors has a unique distribution of sedimentary products in addition each has responded differently to rising sea level a process that continues to control the spatial distribution of organisms and sediments however the sedimentary lithosomes in all four coastal sectors share a similar fate during landward shoreline translation within the inner continental shelf very little of the marsh stratigraphy or large oyster bioherms is preserved only those sediments that have accumulated within bedrock depressions or sinkholes have the best chance for long term retention thus a condensed stratigraphic section is produced since only thin and discontinuous sediment accumulations are preserved the relief and texture of the bedrock surface due to chemical etching and bioerosion must also be used as clues to the recognition of similar coastal systems in ancient settings

barrier island stratigraphy and holocene history of west central florida abstract although the morphology of the barrier inlet system along the west central florida coast is quite complicated the stratigraphy of these barriers is rather simple the basal holocene unit in most cores is an organic rich muddy sand that represents a vegetated paralic marine coastal environment similar to that which is north and south of the present barrier system above that unit is a muddy bioturbated sand that displays a marine fauna at most locations but also contains crassostrea virginica in a few places these sediments accumulated in a low energy marine setting that may or may not have been protected by a barrier island much of this facies also represents sediment that was delivered as washover deposits in an intertidal or subtidal setting and was subsequently bioturbated the facies that can be attributed to a barrier island with some certainty are no more than 3000 years old and on most islands are much younger these are the shelly sand and sorted sand facies the shelly strata represent deposition in nearshore beach supratidal washover or intertidal spillover environments and tidal inlet and tidal delta channels whereas the sorted sand is typical of eolian deposition in dunes or the backbeach and some tidal delta elements the presence of holocene oyster beds offshore of a present barrier suggests that some of these islands formed significantly offshore and moved to their present position through washover it is likely that most of these barriers initially formed through upward shoaling by waves although there is significant morphologic difference between the wave dominated and mixed energy drumstick barrier islands their stratigraphy is quite similar the only significant difference is the presence of extensive progradation on at least part of the drumstick islands and a relatively high amount of former washover deposits on the wave dominated type

variations in the strontium isotopic composition of seawater during the neogene the authors report 261 strontium isotopic analyses of well preserved planktonic foraminifers from three deep sea drilling project sites 519 588 and 607 these samples cover the period from 24 ma to present with an average of approximately one sample per 100 ka the combination of high sample density and uniformity of analytical procedures has produced a well defined record of changes in the sup 87 sr sup 86 sr of seawater during the neogene the record can be viewed as a series of essentially linear segments with slopes ranging from as high as 6 times 10 sup minus 5 m y to as low as 0 m y the times associated with major inflections in the curve do not appear to correspond to simple geologic phenomena such as eustatic cycles but are probably controlled by a combination of tectonic and climatic factors that influenced the abundance and isotopic composition of terrestrial strontium input to the oceans the strontium isotopic data are consistent with a progressive increase in the chemical weathering rates of the continents during the neogene probably related to repeated glaciations increased exposure of continents by lowered sea level and increased continental relief resulting from high rates of tectonic uplift

regional stratigraphic framework linking continental shelf and coastal sedimentary deposits of west central florida abstract a regional study of the holocene sequence onlapping the west central florida platform was undertaken to merge our understanding of the barrier island system with that of the depositional history of the adjacent inner continental shelf key objectives were to better understand the sedimentary processes sediment accumulation patterns and the history of coastal evolution during the post glacial sea level rise in the subsurface deformed limestone bedrock is attributed to mid cenozoic karstic processes this stratigraphic interval is truncated by an erosional surface commonly exposed that regionally forms the base of the holocene section the holocene section is thin and discontinuous and north or south of the tampa bay area is dominated by low relief sand ridge morphologies depositional geometries tend to be more sheet like nearshore and mounded or ridge like offshore sand ridges exhibit 0 5 4 m of relief with ridge widths on the order of 1 km and ridge spacing of a few kilometers the central portion of the study area is dominated nearshore by a contiguous sand sheet associated with the tampa bay ebb tidal delta sedimentary facies in this system consist mostly of redistributed siliciclastics local carbonate production and residual sediments derived from erosion of older strata hardground exposures are common throughout the study area regional trends in holocene sediment thickness patterns are strongly correlated to antecedent topographic control both the present barrier island system and thicker sediment accumulations offshore correlate with steeper slope gradients of the basal holocene transgressive surface proposed models for coastal evolution during the holocene transgression suggest a spatial and temporal combination of back stepping barrier island systems combined with open marine low energy coastal environments the present distribution of sand resources reflects the reworking of these earlier deposits by the late holocene inner shelf hydraulic regime

characterizing surface geology liquefaction potential and maximum intensity in the kachchh seismic zone western india through microtremor analysis abstract this study presents results of microtremor analysis carried out over diverse geological formations and liquefaction sites in the kachchh seismic zone of western india which is host to the deadliest intraplate earthquake m w 7 7 microtremors were recorded at 36 sites with single stations and 7 sites with an array the microtremor array measurements were made using triangular arrays with sensors kept at 30 90 m radii the rayleigh wave velocities are estimated from the array data using high resolution frequency wavenumber f k analysis and the 1d shear wave velocity structure is determined by inversion we found a good correlation between surface geology resonant frequency and amplitude of the horizontal to vertical spectral ratio hvsr curve and shear wave velocity the ground vulnerability index k g values in the liquefied areas are found to be higher than those in the adjacent areas devoid of liquefaction broad or multiple peaks in the hvsr curves and large variations in the shear velocity models could be attributable to the structural and tectonic complexity in the maximum intensity area further our results show that the average fundamental frequencies are around 0 32 hz for the quaternary 0 80 hz for tertiary 1 8 hz for cretaceous 1 8 hz for jurassic and 2 3 hz for the deccan trap formations the inverted s wave velocity varies from 190 to 1350 m s down to a depth of 200 m for the quaternary 220 to 680 m s down to 100 m for tertiary 236 to 1160 m s for cretaceous 670 to 1780 m s for jurassic and 260 to 2440 m s for the deccan trap sites the results of microtremor analysis corroborate the observations from the available geophysical geological and borehole data

sediment starved sand ridges on a mixed carbonate siliciclastic inner shelf off west central florida high resolution side scan mosaics sediment analyses and physical process data have revealed that the mixed carbonate siliciclastic inner shelf of west central florida supports a highly complex field of active sand ridges mantled by a hierarchy of bedforms the sand ridges mostly oriented obliquely to the shoreline trend extend from 2 km to over 25 km offshore they show many similarities to their well known counterparts situated along the us atlantic margin in that both increase in relief with increasing water depth both are oriented obliquely to the coast and both respond to modern shelf dynamics there are significant differences in that the sand ridges on the west central florida shelf are smaller in all dimensions have a relatively high carbonate content and are separated by exposed rock surfaces they are also shoreface detached and are sediment starved thus stunting their development morphological details are highly distinctive and apparent in side scan imagery due to the high acoustic contrast the seafloor is active and not a relict system as indicated by 1 relatively young ams 14c dates 1600 yr bp from forams in the shallow subsurface 1 6 meters below seafloor 2 apparent shifts in sharply distinctive grayscale boundaries seen in time series side scan mosaics 3 maintenance of these sharp acoustic boundaries and development of small bedforms in an area of constant and extensive bioturbation 4 sediment textural asymmetry indicative of selective transport across bedform topography 5 morphological asymmetry of sand ridges and 2d dunes and 6 current meter data indicating that the critical threshold velocity for sediment transport is frequently exceeded although larger sand ridges are found along other portions of the west central florida inner shelf these smaller sand ridges are best developed seaward of a major coastal headland suggesting some genetic relationship the headland may focus and accelerate the n s reversing currents an elevated rock terrace extending from the headland supports these ridges in a shallower water environment than the surrounding shelf allowing them to be more easily influenced by currents and surface gravity waves tidal currents storm generated flows and seasonally developed flows are shore parallel and oriented obliquely to the nw se trending ridges indicating that they have developed as described by the huthnance model although inner shelf sand ridges have been extensively examined elsewhere this study is the first to describe them in a low energy sediment starved dominantly mixed siliciclastic carbonate sedimentary environment situated on a former limestone platform

the morphology and migration of transverse bars off the west central florida coast abstract a series of migrating shore normal sandbars with wavelengths of 75 120 m and heights up to 2 m have been identified off the northern tip of anna maria island a barrier island on the west central florida coast similar features have been described elsewhere since the 1930s and termed transverse bars the transverse bars identified off anna maria island are found for about 3 km along the coast and extend 4 km offshore well outside the normal surf zone width no cusps or any other associated beach expression is evident despite the fact that the bars come to within about 75 m of the beach sediments on the crests of the bars are a well sorted fine quartz sand whereas sediments in the troughs are a poorly sorted coarse carbonate shell hash historical aerial photographs and repeated high resolution bathymetric surveys provide a means of quantifying the migration of the transverse bars analyses of orthorectified aerial photographs from the early 1940s through the mid 1990s clearly show movement or migration taking place in the bar field in the 40 yr period from 1951 to 1991 the southern edge of the bar field moved 200 350 m to the south with an average long term migration rate of 8 m yr repeated bathymetric surveys over an 8 month period give an average short term migration rate of 21 m yr to the south wave and current measurements suggest that southerly winds associated with the passage of cold fronts drive near bed currents to the south that are strong enough to initiate sediment transport and cause the southerly migration of the transverse bars

refining the link between the holocene development of the mississippi river delta and the geologic evolution of cat island ms implications for delta associated barrier islands abstract the geologic evolution of barrier islands is profoundly influenced by the nature of the deposits underlying them many researchers have speculated on the origin and evolution of cat island in mississippi but uncertainty remains about whether or not the island is underlain completely or in part by deposits associated with the past growth of the mississippi river delta in part this is due to a lack of comprehensive geological information offshore of the island that could augment previous stratigraphic interpretations based on terrestrial borings an extensive survey of cat island and its surrounding waters was conducted including shallow water geophysics e g high resolution chirp seismic side scan sonar and swath and single beam bathymetry and both terrestrial and marine vibracoring high resolution seismic data and vibracores from south and east of the island show two horizontally laminated silt units marine radiocarbon dates indicate that they are st bernard delta complex sbdc deposits furthermore seismic data reveal that the sbdc deposits taper off toward the southern shoreline of cat island and to the west morphology consistent with the distal edge of a delta complex the sedimentology and extent of each unit suggest that the lower unit may have been deposited during an earlier period of continuous river flow while the upper unit may represent reduced or sporadic river flow osl dates from the island platform beneath beach ridge complexes indicate three stages of terrestrial evolution island emergence resulting from relative sea level rise 5400 ybp island aggradation via littoral transport 2500 4000 ybp and island degradation due to delta mediated changes in wave direction present 3600 ybp finally the combination of terrestrial and marine data shows that portions of cat island that are lower in elevation than the central part of the island are younger and are likely underlain by a thin layer of deltaic sediments this underscores the potential for increased future vulnerability of barrier islands that develop adjacent to major river delta complexes