sand ridges off sarasota florida a complex facies boundary on a low energy inner shelf environment abstract the innermost shelf off sarasota florida was mapped using sidescan sonar imagery seismic reflection profiles surface sediment samples and short cores to define the transition between an onshore siliciclastic sand province and an offshore carbonate province and to identify the processes controlling the distribution of these distinctive facies the transition between these facies is abrupt and closely tied to the morphology of the inner shelf a series of low relief nearly shore normal ridges characterize the inner shelf stratigraphically the ridges are separated from the underlying pleistocene and tertiary carbonate strata by the holocene ravinement surface while surficial sediment is fine to very fine siliciclastic sand on the southeastern sides of the ridges and shell hash covers their northwestern sides the cores of these holocene deposits are a mixture of both of these facies along the southeastern edges of the ridges the facies boundary coincides with the discontinuity that separates the ridge deposits from the underlying strata the transition from siliciclastic to carbonate sediment on the northwestern sides of the ridges is equally abrupt but it falls along the crests of the ridges rather than at their edges here the facies transition lies within the holocene deposit and appears to be the result of sediment reworking by modern processes this facies distribution primarily appears to result from south flowing currents generated during winter storms that winnow the fine siliciclastic sediment from the troughs and steeper northwestern sides of the ridges a coarse shell lag is left armoring the steeper northwestern sides of the ridges and the fine sediment is deposited on the gentler southeastern sides of the ridges this pronounced partitioning of the surficial sediment appears to be the result of the siliciclastic sand being winnowed and transported by these currents while the carbonate shell hash falls below the threshold of sediment movement and is left as a lag the resulting facies boundaries on this low energy sediment starved inner continental shelf are of two origins which both are tied to the remarkably subtle ridge morphology along the southeastern sides of the ridges the facies boundary coincides with a stratigraphic discontinuity that separates holocene from the older deposits while the transition along the northwestern sides of the ridges is within the holocene deposit and is the result of sediment redistribution by modern processes

stratigraphy and genesis of a modern shoreface attached sand ridge peahala ridge new jersey abstract we investigated the genesis and evolution of peahala ridge a modern shoreface attached sand ridge 1 km 6 km in size and 4 7 m in bathymetric relief through an integrated sedimentologic and stratigraphic study involving vibracoring box coring grab samples high resolution seismic paleontology radiocarbon dating and oceanographic measurements near surface strata of peahala ridge include six important stratigraphic units modern shoreface upper ridge sand lower ridge sand swale inlet fill middle holocene back barrier and late pleistocene strandplain radiocarbon dating and determination of the stratigraphic relationships indicates that peahala ridge formed initially from an ebb tidal delta associated with a tidal inlet so thwestward migration of the inlet channel a vector resultant of landward coastal retreat and southerly longshore drift cut and then filled the swale separating peahala ridge from long beach island following inlet closure peahala ridge developed its present form as a shoreface attached shoreline oblique bathymetric feature hydrodynamic processes have played a major role in evolution of the ridge including considerable growth and accretion this combination of long term eustatic and short term hydrodynamic factors is the best explanation for the present morphology and internal stratigraphy of peahala ridge

mixed carbonate siliciclastic infilling of a neogene carbonate shelf valley system tampa bay west central florida abstract the shelf valley system underlying tampa bay florida s largest estuary is situated in the middle of the neogene carbonate florida platform compared to well studied fluvially incised coastal plain valley systems this shelf valley system is unique in its karstic origin and its alternating carbonate siliciclastic infill a complex record of sea level changes paleo fluvial variability and marine processes have controlled the timing and mechanisms of this compound shelf valley infill a dense grid of high resolution single channel seismic data were collected at the mouth of tampa bay in an attempt to define this stratigraphy determine the controls on deposition and define the underlying structure of this shelf valley system the seismic data were correlated with nearby wells and boreholes for lithologic and age control sequence stratigraphic methods were incorporated in order to develop an integrated chronostratigraphy for the depositional infilling of the shelf valley system five seismic sequences were identified sequence boundaries generally show erosional truncation and karstification with downlap of overlying sequences structure contour and isopach maps indicate that the tampa bay shelf valley system has remained in essentially the same location since its formation in the early miocene although the provenance of sedimentary infill has changed this change is due to increasing amounts of siliciclastic material during the neogene seismic facies interpretations indicate lower energy northward prograding deposition dominated by predominantly carbonate sediments within the lowest sequence a higher energy siliciclastic fluvio deltaic deposition within sequences b and c originates to the east and northeast of the shelf valley system related to a pliocene pulse of sedimentation onto the florida platform finally marine processes longshore transport ebb tidal delta formation dominate the upper two sequences d and e reworking these siliciclastic sediments into a spatially mixed carbonate siliciclastic depositional setting

origin evolution and distribution of shoreface sand ridges atlantic inner shelf u s a abstract a computer mapping system was employed to document the location of 259 shoreface attached and detached sand ridges in water although shoreface attached sand ridges appear to have several different modes of formation a two step process for the development of most shoreface attached and detached sand ridges along u s atlantic barrier island and cape coastlines is proposed 1 sand is deposited as ebb tidal deltas or river deltas along the lower shoreface and or inner continental shelf prior to or during transgression followed by 2 further transgression which reworks the deltaic sand bodies into linear sand ridges at the base of the shoreface by shelf processes the best developed shoreface sand ridge fields along the u s atlantic shelf lie adjacent to shorelines characterized by all of the following 1 transgression 2 mixed energy wave dominated barrier islands and 3 laterally migrating tidal inlet systems tidal inlet systems are natural sediment sinks that capture sand carried by longshore sediment transport ebb tidal delta deposits associated with these migrating tidal inlets provide the initial sand body for the development of shoreface attached sand ridges the oblique orientation and linear form of shoreface attached sand ridges appear to be a function of shoreline transgression lateral inlet migration and wave reworking of ebb tidal delta deposits concentrated along an ebb tidal delta retreat path shelf processes act as modifying agents in the evolution of sand ridges during and after ebb tidal delta deposition in general shoreface attached sand ridges are poorly developed or absent along eroding headlands spits and barrier island shorelines characterized by naturally stabilized tidal inlets these latter inlets and shoreline types are dominated or influenced by one or all of the following 1 antecedent topography 2 higher tidal ranges 3 larger tidal prisms 4 lower wave energies or 5 finer grain sizes the coupling of shoreline and shallow marine sedimentary processes during a transgression is critical to the origin evolution and distribution of shoreface sand ridges in the study area modern shoreface attached sand ridges are also known to occur in different coastal and shelf settings where large amounts of sediment were supplied to the shoreface and inner shelf during or immediately before transgression commonly observed vertical and lateral interrelationships of holocene shoreface attached sand ridges and tidal inlets or distributary channels have important ramifications for the development of shelf sandstone facies models in addition the geometric relationship documented in this study provides predictive petroleum and hard mineral exploration models of the spatial and temporal distribution of shoreface and shelf sand ridges

impact of slope gradient on soil surface features and infiltration on steep slopes in northern laos abstract it was recently demonstrated that infiltration into mountain tilled soils with highly stable microaggregates increases with increasing slope gradient in this work we investigate the processes that underpin this phenomenon by means of field experiments and modelling the study area is located in northern laos rainfall simulations were conducted in two 1 m 2 plots using a portable field simulator the drop size distribution and kinetic energy were similar to that occurring on the occasion of tropical downpours soils exhibited a clay loam texture and very similar organic matter contents across experimental plots but differed greatly in slope gradient 30 and 75 runoff water samples were collected at intervals ranging from 1 to 3 min depending on the runoff intensity plots microtopography was measured before and after rainfall simulations using an automatic surface roughness meter on a 1 cm grid high resolution bulk density images were obtained from soil slices using a standard x ray generator final infiltration rates of 6 and 21 mm h 1 soil detachment of 667 and 310 g m 2 surface lowering due to soil loss of 0 82 and 0 38 mm surface lowering due to compaction of 1 21 and 0 90 mm percentage area with sieving crust of 36 and 90 percentage area with erosion crust of 63 and 0 were obtained for the 30 and 75 slopes respectively three main conclusions can be drawn from this work 1 high intensity rainfall can rapidly transform soil surface features of steep bare soil 2 on steeper slopes the micro relief tends to form micro terraces much more pervious and less erodible than the ripple like roughness that formed on gentler slopes and 3 there was a more pronounced lowering of the soil surface due to compaction and denser microlayers on gentler slopes the latter conclusion confirms the hypothesis that higher effective rainfall intensity is responsible for the formation of less permeable erosion crusts under 30 slope gradients while more permeable structural crusts develop under 75 slope gradients the runoff results were modelled with the green and ampt model which accounts for time evolution of soil hydraulic conductivity this modelling shows that soil is undoubtedly non homogeneous evolves with time and that infiltration kinetics is slower and soil permeability greater for the 75 slope

a mechanism for the formation and maintenance of shore oblique sand ridges on storm dominated shelves a remarkable set of shore oblique sand ridges occupies the inner portion of the storm dominated middle atlantic and south atlantic bights off the east coast of the united states of america a mechanism for present day formation and growth of these distinctive geological features follows from a linearized analysis of the stability of alongshore periodic topographical perturbations based on a depth averaged fluid dynamical model with a simple representation of sediment transport the analysis indicates exponential growth of shore oblique features as a result of offshore deflection of storm driven alongshore flows at ridge crests which leads to a convergence of sediment flux because the effective carrying capacity decreases with increasing distance offshore the analysis indicates a set of conditions required for ridge growth model solutions for topography under conditions favorable for ridge growth are strikingly similar to the observed sand ridges

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

suwannee channel of the paleogene coastal plain support for the carbonate suppression model of basin formation a recently proposed model postulates that a basin separating a carbonate from an adjacent terrigenous clastic depocenter is a consequence of the neighboring depocenters rather than a precondition for their coexistence a test of the model is provided by comparison with the well studied carbonate terrigenous clastic transition of the united states coastal plain in general the model is substantiated although deviations caused by changes in sea level terrigenous clastic input and tectonism are noted

sediment production on sediment starved continental margins the interrelationship between hardbottoms sedimentological and benthic community processes and storm dynamics abstract modern sediment starved continental shelves represent developing condensed sections analogous to those considered key stratigraphic markers in many stratigraphic models condensed sections and their associated hardbottoms on the modern high energy north carolina continental margin provide important benthic habitats that are modified on time scales of days to centuries by interrelated sedimentological biological and physical processes outcropping upper cenozoic strata of varying lithologies form distinct hardbottom morphologies that through differential bioerosion contribute significant volumes of new sediment to the surficial sand regime of the continental shelf 1 vertical and sloped mudstone to muddy sandstone hardbottoms of the miocene pungo river formation are dominated by the endolithic fauna jouanettia quillingi bivalve and upogebia sp shrimp 2 vertical and sloped hardbottoms consisting of harder miocene and pleistocene limestone are dominated by the endolithic bivalves lithophaga bisulcata gastrochaena stimpsoni and g ovata 3 the highly lithified flat hardbottoms of plio pleistocene limestone are dominated by the epifloral macroalgal species dictyopteris hoytii zonaria tournefortii and sargassum filipendula these three groups of bioeroders physically and or chemically degrade their respective lithologies develop relief on hardbottom surfaces produce large scale morphological features on the shelf and recycle ancient sediment into the modern surficial sediment system the rate of sediment production resulting from bioerosion varies from 5 5 kg m2 yr on the vertical and sloped miocene mudstone hardbottoms to 0 4 kg m2 yr on vertical and sloped pleistocene limestone to 0 03 kg m2 yr on the flat highly lithified plio pleistocene limestone hardbottoms depending on lithology and associated bioerosional processes bioeroders excavate exposed hardbottom surfaces and develop relief ranging from millimeters to meters whereas differential rates of bioerosion between different lithologic units results in relief ranging from meters to tens of meters recession rates measured on miocene mudstones at the chapel site range from 2 to 4 cm per year for the outcrop exposure which is 132 m long this would produce a ten meter overhang of the overlying pleistocene limestone in 250 500 years by removing 13 400 metric tons of eroded sediment 25 fine sand that would be contributed to the surficial sediments the overhang would ultimately break off during a storm to produce the next row of the limestone rubble blocks that form a ramp in front of the receding mudstone scarp these rates of sediment production are rapid enough to bury the hardbottoms producing the sediment however surface sediment is generally not accumulating on the shelf it is present only as thin 0 1 m highly variable and ephemeral sand bodies major storms modify the abundance and distribution of surface sediment on hardbottom habitats and routinely export large volumes of these sediments from the shelf system depositing them as fine sand clinoforms off the prograding shelf edge exposed hardbottom habitats free of sand are dominated by highly diverse communities of endolithic fauna and epilithic fauna and flora those habitats with 2 6 cm of sand are generally dominated by scattered epilithic fauna with small growths of epilithic flora irregularly distributed on topographic highs and those habitats with 6 cm of sand are generally dominated by softbottom benthic communities storms modify the distribution of bottom sediments which either exposes or buries additional hardbottom surfaces and controls the expansion or contraction of hardbottom benthic communities thus the intensity frequency and character of individual storms and the seasonal storm pattern determine the amount and location of sand accumulation which controls the benthic community structure in turn the benthic community determines the type rate and volume of hardbottom bioerosion and resulting sediment production

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

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

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

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

a neogene mixed siliciclastic and carbonate foundation for the quaternary carbonate shelf florida keys abstract new core holes and marine seismic profiles show the distribution and paleoenvironments of a siliciclastic foundation sandwiched between the much studied quaternary carbonates of the middle and upper florida keys and oligocene to miocene carbonates of the arcadia formation a distance of some 160 km throughout this area the quaternary carbonates are generally no more than 30 m thick and are underlain by up to 145 m of plio pleistocene mixed limestones and sands and marine siliciclastics peace river formation quartz sands days and minor reworked phosphatic grains of the peace river are the distal deposits of an almost peninsula wide blanket that interrupted prolonged tertiary carbonate deposition in southernmost florida in the lower keys the peace river siliciclastics change fac es to very fine to fine grained carbonates composed of bioclastic planktonic foraminiferal grainstones and packstones the position of the present southern florida shelf margin with its discontinuous coral reefs is determined by the seaward paleoslope of siliciclastics and or carbonates of the plio pleistocene section defined in the florida keys lithostratigraphy of five continuous cores on the keys and southeastern mainland each about 300 m long combined with numerous well logs and sample descriptions from the keys and mainland establish the southern limit of peace river siliciclastics to at least the middle keys the well data show intervals of quartz of coarse sand to granule size localized as a north south pathway of maximum paleocurrents that extend from the central part of the peninsula approximately 350 km to the middle keys this pathway of maximum paleocurrents about 100 km wide is interpreted as the palimpsest record of unusually strong southward moving shoreline and channeled deposition or a giant prograding spit the ultimate source of these siliciclastics is from the appalachians approximately 1000 km to t e north we propose that the plio pleistocene marine siliciclastics and carbonates including the peace river formation recovered in wells on the keys extend seaward beneath the present southern florida outer shelf and are present in seismic profiles acquired across the shelf margin slope a series of profiles forming an offshore traverse from the lower to upper keys show seaward dipping clinothems that locally dip obliquely to the arcuate strike of the florida keys these clinothems downlap on a prominent reflector that can be traced to an exposed submarine surface of upper tertiary carbonates the pourtales and miami terraces some reflectors show configurations similar to contourites and others to channels we contend that southward entry of the siliciclastics into southernmost florida w s deflected eastward by west to east channeled flow of an ancestral florida current

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

chronologic model and transgressive regressive signatures in the late neogene siliciclastic foundation long key formation of the florida keys abstract recent drilling of continuous cores in southernmost florida has documented a thick unit of upper neogene siliciclastics subjacent to surficial shallow water quaternary carbonates exposed on islands of the florida keys the siliciclastics comprise the long key formation and were identified in two cores collected from the middle and upper florida keys a chronologic model based on new planktic foraminiferal biochronology and strontium isotope chronology suggests the timing of siliciclastic deposition and provides a basis for regional correlation the chronologic model supplemented by vertical trends in quartz grain size pattern of planktic menardiiform coiling direction and paleoenvironmental interpretations of benthic foraminiferal assemblages shows that the long key formation contains three intervals i iii of varying thickness grain size composition and paleo water depth interval i is uppermost miocene the quartz grains in interval i fine upward from basal very coarse sand to fine and very fine sand benthic foraminifera indicate an upward shift from an outer shelf to inner shelf depositional environment interval ii deposited during the late early to early late pliocene contains reworked upper miocene siliciclastics and faunas in the upper keys quartz grains in interval ii range from very coarse sand that fines upward to very fine sand and then coarsens to very coarse and medium sand in situ benthic faunas indicate an upward shift from outer shelf to inner shelf deposition in the middle keys interval ii is different with the quartz grains ranging primarily from medium to very fine sand in situ benthic taxa indicate deposition on an inner shelf in both the middle and upper keys the upper pliocene siliciclastics of interval iii contain quartz grains ranging from very coarse to very fine sands that were deposited on an inner shelf a sequence boundary between interval i and interval ii is suggested by an abrupt shift in the strontium isotope chemostratigraphy coarsening in quartz grain size above the boundary an abrupt landward shift in depositional facies in the upper keys core and a distinct variation in the predominant coiling direction of the menardiiform planktic foraminifera from fluctuating dextral sinistral to dextral in the upper keys core successive siliciclastic infilling likely associated with eustatic sea level change and current redeposition formed a foundation for subsequent carbonate deposition deep sea biostratigraphic techniques integrated with ages derived from strontium isotope chemostratigraphy can be successfully applied to coastal margin sequences even though a depauperate suite of faunal markers is common

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

interplay of late cenozoic siliciclastic supply and carbonate response on the southeast florida platform abstract high resolution seismic reflection data collected along the length of the caloosahatchee river in southwestern florida have been correlated to nannofossil biostratigraphy and strontium isotope chemostratigraphy at six continuously cored boreholes these data are interpreted to show a major late miocene to early pliocene fluvial deltaic depositional system that prograded southward across the carbonate florida platform interrupting nearly continuous carbonate deposition since early in the cretaceous connection of the platform top to a continental source of siliciclastics and significant paleotopography combined to focus accumulation of an immense supply of siliciclastics on the southeastern part of the florida platform the remarkably thick 100 m sand rich depositional system which is characterized by clinoformal progradation filled in deep accommodation while antecedent paleotopography directed deltaic progradation southward within the middle of the present day florida peninsula the deltaic depositional system may have prograded about 200 km southward to the middle and upper florida keys where late miocene to pliocene siliciclastics form the foundation of the quaternary carbonate shelf and shelf margin of the florida keys these far traveled siliciclastic deposits filled accommodation on the southeastern part of the florida platform so that paleobathymetry was sufficiently shallow to allow quaternary recovery of carbonate sedimentation in the area of southern peninsular florida and the florida keys

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

bedrock controls on barrier island development west central florida coast abstract fifty vibracores and 130 km of high resolution seismic profiles were taken in the lagoons and nearshore area of northern pinellas county florida to determine the role of antecedent topography upon barrier island location origin and development east west seismic profiles show a terrace with 1 1 5 m of relief located along the seaward trend of the barrier islands between 4 and 6 m msl this antecedent topography is composed of a relatively erosion resistant miocene limestone and the terrace is usually present as an increase in slope a contour map of the pre holocene surface shows that this subsurface feature is continuous along the entire study area and is mimicked by the opposing orientations of honeymoon and caladesi islands the correspondence of the modern barrier system to the antecedent topography indicates that 1 the islands formed on the terrace or that 2 landward migration of the barriers was stopped by grounding on this feature stratigraphic analyses of lagoonal and nearshore environments were conducted in order to establish the overall response of the northern pinellas coastal system to the late holocene sea level rise this analysis indicates a basically transgressive holocene sequence with lagoonal sediments occurring seaward of the islands underlying the lagoonal deposits in all areas is a 1 m thick relict pleistocene unit that has been flooded reworked and vegetated by intertidal to subtidal organisms a model of development for these barriers is proposed which incorporates an early transgressive history followed by a recent progradational phase the progradational phase was probably initiated by the late holocene slowdown in sea level rise and an increased sediment supply as the barrier islands migrated over a relict sediment source the antecedent topography thus served as a stabilization point for landward barrier migration and seaward progradation the antecedent topographic control of barrier island location described by this model is dependent upon the coincident decline in the rate of sea level rise and an increased sediment supply

sequence stratigraphy of the late miocene early pliocene peace river formation southwestern florida abstract platform wide carbonate deposition ceased on the south florida platform during the early pliocene for a period of about one m y a sequence of late miocene age shallow marine sediments containing mixed carbonates and siliciclastics was succeeded by an early pliocene age predominantly siliciclastic deltaic sequence within the peace river formation to temporarily end carbonate deposition on the southern interior part of the florida platform the peace river formation was deposited from about 11 to 4 3 ma it is separated from the underlying middle miocene arcadia formation by an unconformity with a hiatus of 2 to 4 m y the unconformity separating the shallow marine sequence late miocene from the overlying deltaic sequence early pliocene has a hiatal gap of 2 to 6 m y depending on geographic location the unconformity between the upper peace river formation and the overlying tamiami formation shows a distinctive change in bedding orientation and lithology but only a minimal hiatus occurs perhaps 0 2 m y the late miocene sequence contains a series of shallow marine shoaling upward parasequences 1 to 9 depending on location that are either transgressive or regressive the early pliocene sequence is a single prograding deltaic deposit that pinches out from north to south in the central part of the platform

origin and development of anclote key west peninsular florida abstract the morphology of barrier system elements on the west coast of peninsular florida is atypical for microtidal coasts because it includes numerous drumstick shaped barriers and large ebb tidal deltas anclote key in pinellas county is the most northern island of the system it was investigated in detail in order to determine its origin and developmental history extensive vibracoring and shallow seismic sub bottom profiling demonstrate that a miocene limestone platform underlies the area and is covered by only a few meters of pleistocene and holocene strata shoreward of anclote key the irregular limestone surface slopes very gradually toward the gulf whereas the gradient on the gulf side is two to three times greater a subtle but well defined limestone ridge with relief of approximately 1 m is present beneath anclote key stratigraphic analysis of 24 vibracores has identified nine quaternary facies above the miocene limestone a pleistocene muddy sand unconformably overlies the limestone and is unconformably overlain by holocene units as sea level rose to within a few meters of its present position beginning about 4200 yrs b p extensive shallow sub tidal grass flats and inter tidal to supratidal mangrove swamp and salt marsh environments were present in this area wave action near the break in slope of the limestone surface caused concentration of sand and shell to be deposited as a shallow subtidal shoal continued wave and tidal action accompanied by slowly rising sea level caused vertical accretion and spillover as the shoal became intertidal and eventually supratidal once the sand body became subaerially exposed as a barrier island processes typical of such environments gave rise to the deposition of well defined beach dune and mangrove covered washover facies the configuration of the shallow limestone platform was instrumental in controlling the location of anclote key as it developed during the late holocene

on the nature of radar backscatter and 250 mhz scintillation linked with an intense daytime es patch 1 in this paper we present and analyze the gadanki radar observations of an unusual event of daytime radar echoes from the e region which spread over a range of 105 150 km and displayed a u shape in the range time snr map the u shape echoing structure was an added feature to the commonly observed lower e region echoes with slowly descending features and 150 km echoes with range migration displaying a forenoon descent and afternoon ascent distinctly different doppler velocities were observed in the two arms of the u shape structure with velocities increasing with height and surprisingly exceeding the doppler velocities of the 150 km echoes the doppler spectra display features very similar to those observed below 100 km indicating turbulence as the underlying process a collocated ionosonde observed unusually strong sporadic e es activity with maximum reflected scattered frequency ftes reaching 16 mhz in close correspondence with the u shape structure during the same duration a collocated 250 6 mhz scintillation receiver revealed scintillation activity not observed before from gadanki the unusual radar observations strong es activity and daytime scintillation are first of its kind from gadanki detailed analysis suggests that the u shape radar echoes extending to 150 km range were due to the sidelobe detection of the e region irregularities with special features that were also responsible for the daytime scintillation the genesis and implication of the irregularities are discussed