prerift and synrift sedimentation during early fault segmentation of a tertiary carbonate platform indonesia abstract eocene carbonate deposits of the barru area sulawesi indonesia provide a rare insight into sedimentation prior to and during propagation of normal faults to the surface three main successions late prerift latest prerift earliest synrift and synrift are characterised by distinctive facies associations and sequence development shallow water foraminiferal shoals and intervening lower energy depositional environments occurred during the late prerift in areas which latter formed footwall highs and hangingwall depocentres respectively during the latest prerift earliest synrift shallow water shelves deepened laterally into slope environments in developing hangingwall depocentres in both these sequences sections in developing hangingwall areas are thickest deepen up section and thin laterally towards growing footwall highs active faulting resulted in rapid drowning of hangingwall depocentres and massive reworking of material derived from collapse of the platform margin and adjacent shallow water emergent footwall highs differential subsidence controlling water depths and accommodation space types of carbonate producers and active faulting were the main factors affecting depositional environments and facies distributions carbonate producers are extremely sensitive indicators of depositional water depth and energy hence rapid lateral and vertical facies variations in the barru area provide quantifiable insight into environmental changes prior to and during active faulting

seismic and geologic characteristics of drowning events on carbonate platforms carbonate platform drowning events are recorded in upper jurassic lower cretaceous carbonates of the baltimore canyon area offshore united states east coast and lower miocene carbonates of the pearl river mouth basin offshore people s republic of china cursory examination of seismic data from both areas would indicate that the platforms fit the classic definition of a drowning unconformity however detailed lithologic and paleontologic data indicate that the depositional erosional hiatuses vary widely in each area from 0 25 m y in the baltimore canyon area to a condensed section with no hiatus in the pearl river mouth basin the sedimentary sequence produced during drowning the drowning sequence in both areas is gradational with underlying shallow platform carbo ates and in some places overlying deeper marine limestones and shales the generalized facies models proposed for drowning events suggest that they may appear as instantaneous or gradual changes in geologic and seismic data however high resolution seismic data across the baltimore canyon and pearl river mouth basins show that some drowning sequences may be masked on conventionally processed seismic sections because of this resolution problem a distinction should be made between seismically and geologically defined unconformity surfaces the geologic characteristics of drowning events on carbonate platforms include a gradational lower and sometimes upper contact chemical sedimentation glauconite phosphate open marine shelf sediments and a variable hiatus at the upper boundary late growth shelf margin reefs also can be a diagnostic feature of drowned platforms seismic characteristics include horizontal to sub horizontal basinal marine onlap basin parallel reflector continuity above the carbonate sequence boundary and downlap of later highstand units evidence from platforms adjacent to continental margins baltimore canyon indicates that drowning occurs in a step wise fashion advancing more or less symmetrically across the platform from shelf to nearshore environments data from isolated open ocean atolls pearl river mouth basin indicate that they respond differently to drowning showing an asymmetric decrease in shallow platform environments as drowning occurs proper recognition by a geoscientist of the seismic and geologic characteristics of drowning events can lead to better reservoir and seal estimates and the correct reconstruction of the depositional tectonic and eustatic histories of an area

compositional diversity in late cenozoic mafic lavas in the rio grande rift and basin and range province southern new mexico this study reports major and trace element compositions of late cenozoic 5 ma and younger mafic volcanic rocks from southern new mexico stratigraphic sampling is reported for two of the volcanic fields the potrillo and the jornado del muerto volcanic fields reconnaissance sampling for an additional seven fields these are among the first chemical analyses for some of these volcanic fields and they provide insight into the dynamics of melt generation in this continental rift zone the lavas fall into two groups 1 an alkaline suite of basanite alkali basalt and trachybasalt with low sio2 and high abundances of incompatible elements tio2 nb rb and sr 2 a subalkaline suite of subalkali basalts with high sio2 and low abundances of the incompatible elements compositions of volcanic rocks from the geronimo volcanic field the taos plateau volcanic field and the transition zone in central new mexico are similar to those reported in this study confirming that these are general characteristics of lavas throughout the region for many of the large fields the volcanic rocks belong exclusively to one compositional group or the other furthermore there appear to be no patterns to the distribution of these compositional groups for example the potrillo volcanic field which is composed exclusively of alkaline lavas and the jornado del muerto volcanic field which is dominantly subalkaline both occur in the axis of the rio grande rift we see no evidence for evolution in melt chemistry during the past 5 m y in the potrillo volcanic field for which we have the best documentation of stratigraphic changes in composition incompatible elements increase upsection and are correlated with decreasing mg number suggesting that magmas underwent differentiation before their final ascent phenocrystic olivine and plagioclase which are liquidus phases at low pressure imply a shallow depth for this crystallization event processes responsible for chemical divergence between the alkaline and subalkaline groups occurred at an early stage of magmatic evolution correlation between incompatible element enrichment and silica undersaturation suggests that variable degrees of partial melting played a significant role in the generation of the two lava types

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

growth demise and dolomitization of miocene carbonate platforms on the marion plateau offshore ne australia abstract strontium isotope stratigraphy has been used to examine the timing of depositional events and dolomitization in two miocene carbonate platforms cored by ocean drilling program odp leg 194 just seaward of the great barrier reef the results provide firm constraints for correlating surfaces and depositional stages between the two platforms and thereby relating seismic sequences previously defined in the off platform sediments to the lithostratigraphic units described from cores in the seismically transparent platform top sites oyster bearing beds at the base of both platform successions yield early oligocene ages 29 31 ma thus dating initial transgression of the marion plateau s volcanic basement there followed a period of slow accumulation of shallow water grainstones rich in quartz and phosphate grains in late oligocene time 29 23 ma seismic megasequence a the main growth of the carbonate platforms took place in early to late miocene time 23 7 ma comprising five depositional sequences the first four of these seismic megasequence b are common to both platforms and terminated with a possible karst surface at 10 7 ma different sedimentologic expression of this megasequence in the two platforms reflects contrasting progradational versus aggradational geometries in the locations studied the final growth stage seismic megasequence c occurred only in the southern platform and terminated at 6 9 ma both platform demise events 10 7 and 6 9 ma approximately coincide with falls in global sea level combined with longer term trends of decreasing water temperature sr isotope ages of dolostones increase with depositional age and older dolostones in the southern platform have more coarsely crystalline and fabric destructive textures than overlying younger dolostones these relationships are consistent with dolomitization by normal seawater shortly after deposition and overprinting of multiple times of dolomite recrystallization and cementation in the deeper strata

carbonate lithofacies as paleolatitude indicators problems and limitations abstract detailed study of miocene carbonates in the mediterranean region and their analogues on modern carbonate shelves in the mediterranean sea brazil and other areas reveals at least three major types of carbonate platform lithofacies in addition to the classic tropical coral reef chlorozoan lithofacies a chloralgal lithofacies similar to the chlorozoan but without hermatypic corals b rhodalgal lithofacies characterized by abundant encrusting coralline algae and c molechfor lithofacies consisting of benthic foraminifers molluscs echinoids bryozoans and barnacles these carbonate lithofacies present complex distribution patterns seemingly related primarily to latitude and depth that control water temperature although other factors e g water circulation river discharge suspended sediment controlling water salinity and temperature nutrient content light penetration etc also play important roles chloralgal and rhodalgal lithofacies can be considered two transitional terms between the two end members the chlorozoan lithofacies which characterizes shallow tropical shelves and the molechfor lithofacies which characterizes colder and or deeper areas detailed textural and sequential analysis are required for satisfactory interpretation of these lithofacies in ancient rocks

carbonate systems along nutrient and temperature gradients some sedimentological and geochemical constraints research over the past several decades has clearly demonstrated that changes in the ocean environment have had major impacts on carbonate systems changes in climate ocean circulation and seafloor spreading rates have influenced temperature and seawater chemistry including carbonate saturation state and nutrient availability and thereby have determined boundary conditions for the biota that form carbonate platforms in turn the biota determine accumulation rates and facies zonations thus controlling platform geometry and facies dynamics in the first section of this paper we examine how nutrient availability influences carbonate facies associations we first discuss the role of temperature and nutrient gradients in the modern ocean and their influence on biotic associations then we discuss how carbonate sedimentation can be characterized along nutrient gradients in the second section we review proxies currently used to reconstruct paleoproductivity in open ocean environments and discuss their applicability to neritic carbonate systems we highlight the variety of existing proxies and their limitations and suggest that multiple lines of evidence are needed for valid interpretations our short review discusses sedimentological biogenic and geochemical proxies that can be used to reconstruct past nutrient fluxes and to constrain paleoceanographic controls over the distribution of carbonate associations however it also reveals that more data and case studies are needed that integrate shallow and deep water carbonate sequences and elucidate the links between temperature vs nutrient supplies changes and facies in ancient carbonate sequences

numerical models of fluid flow in carbonate platforms implications for dolomitization abstract the role of fluid flow in dolomitization is evaluated through hydrologic simulations designed to capture and test numerically the essential elements of some common dolomitization models a two dimensional finite difference code bethke 1985 was used to simulate single phase flow of variable salinity fluids driven by compaction convection and topography in carbonate platforms and adjacent basinal sediments a simple hydrostratigraphic model consisting of a platform reef complex surrounded by basin filling shale was constructed to examine the magnitude and direction of compaction flow during the initial phase of subsidence most pore fluids in the shale migrate to the surface at velocities less than 0 1 mm yr and basin derived fluids are focused into the reef from only a limited distance of 10 km relatively porous and permeable basinal limestones and sandstones may act as conduits for updip fluid flow into the platform reef complex magnesium mass balance constraints coupled with hydrologic simulations suggest that burial compaction is not an efficient mechanism for regional dolomitization two layer hydrostratigraphic models involving variable salinity fluids show that evaporation of seawater results in downward and seaward fluid migration the magnitude of flow is a function of fluid density even fluids at salinities marginally elevated above normal seawater values are capable of penetration several hundreds of meters into the underlying sediments albeit at rates of less than 1 cm yr paleohydrologic simulations coupled with thermodynamic arguments support that reflux of surface derived waters evaporated past gypsum saturation is a viable dolomitization mechanism calculations utilizing rates of mg mass flux show that dolomitization of large carbonate platforms 10 km3 by fluids below halite saturation requires millions of years of reflux driven flow p thermal convection of seawater through carbonate platforms has been simulated by varying the basal heat flow the thermal pump produces a convection cell in which lateral and vertical flow achieve rates of 75 0 and 13 0 mm yr respectively the potential for dolomitization is high because a large reservoir of mg seawater is tapped elevated temperatures drive fluids further into the stability field for dolomite and kinetic inhibition of dolomite nucleation and growth is decreased simulations of high frequency glacio eustatic sea level oscillations suggest that a dynamic hydrologic regime may be established a sea level fall of 2 m lasting for 10 000 yr can drive lateral flow at rates up to 0 15 m yr dolomitization is possible because marine waters migrate through the carbonate platform in response to gradients of hydraulic head created by eustatic sea level fluctuations

textural and geochemical alternations in late cenozoic bahamian dolomites dolomitized intervals of a core from san salvador island bahamas exhibit variations of two texturally and geochemically distinct end members in the pliocene section of the dolomitized interval the two end members alternate in a pattern that may reflect originally and or diagenetically modified depositional facies formerly mud free intervals locally capped by exposure surfaces are massive crystalline mimetic dolomites cm muddier sediments are replaced by friable microsucrosic dolomites ms cm and ms dolomites also differ in porosity 30 permeability 100 md mol mgco3 44 9 vs 47 7 mol oxygen isotopic composition 1 7 vs 2 7 and strontium content 241 vs 106 ppm these data indicate that depositional and diagenetic fabric are the principal controls governing the distribution of dolomite types differences in texture and geochemistry are suggested as arising through differential rates of crystallization produced as a result of variations in permeability and reactivity of the precursor sediments and rocks

reef exploration in the east sengkang basin sulawesi indonesia abstract reconnaissance seismic shot in 1971 72 showed a number of well defined seismic anomalies within the east sengkang basin which were interpreted as buried reefs subsequent fieldwork revealed that upper miocene reefs outcropped along the southern margin of the basin a drilling programme in 1975 and 1976 proved the presence of shallow gas bearing upper miocene reefs in the northern part of the basin seismic acquisition and drilling during 1981 confirmed the economic significance of these discoveries with four separate accumulations containing about 750 10 9 cubic feet of dry gas in place at an average depth of 700 m kampung baru is the largest field and contains over half the total both reservoir quality and gas deliverability are excellent deposition in the east sengkang basin probably started during the early miocene a sequence of lower miocene mudstones and limestones unconformably overlies acoustic basement which consists of eocene volcanics during the tectonically active middle miocene deposition was interrupted by two periods of deformation and erosion carbonate deposition became established in the late miocene with widespread development of platform limestones throughout the east sengkang basin thick pinnacle reef complexes developed in the areas where reef growth could keep pace with the relative rise in sea level most reef growth ceased at the end of the miocene and subsequent renewed clastic sedimentation covered the irregular limestone surface late pliocene regression culminated in the holocene with erosion the walanae fault zone part of a major regional sinistral strike slip system separates the east and west sengkang basins both normal and reverse faulting are inferred from seismic data and post late pliocene reverse faulting is seen in outcrop

δ18o and marion plateau backstripping combining two approaches to constrain late middle miocene eustatic amplitude d 18 obenthic values from leg 194 ocean drilling program sites 1192 and 1195 drilled on the marion plateau were combined with deep sea values to reconstruct the magnitude range of the late middle miocene sea level fall 13 6 11 4 ma in parallel an estimate for the late middle miocene sea level fall was calculated from the stratigraphic relationship identified during leg 194 and the structural relief of carbonate platforms that form the marion plateau corrections for thermal subsidence induced by late cretaceous rifting flexural sediment loading and sediment compaction were taken into account the response of the lithosphere to sediment loading was considered for a range of effective elastic thicknesses 10 te 40 km by overlapping the sea level range of both the deep sea isotopes and the results from the backstripping analysis we demonstrate that the amplitude of the late middle miocene sea level fall was 45 68 m 56 5 6 11 5 m including an estimate for sea level variation using the d 18 obenthic results from the subtropical marion plateau the range of sea level fall is tightly constrained between 45 and 55 m 50 0 6 5 0 m this result is the first precise quantitative estimate for the amplitude of the late middle miocene eustatic fall that sidesteps the errors inherent in using benthic foraminifera assemblages to predict paleo water depth the estimate also includes an error analysis for the flexural response of the lithosphere to both water and sediment loads our result implies that the extent of ice buildup in the miocene was larger than previously estimated and conversely that the amount of cooling associated with this event was less important

the role of sea surface temperature as a control on carbonate platform development in the western coral sea the derivation of a detailed sea surface paleotemperature curve for the middle miocene holocene 10 0 ma from odp site 811 on the queensland plateau northeast australia has clarified the role of sea surface temperature fluctuations as a control on the initiation and development of the extensive carbonate platforms of this region this curve was derived from isotopic analyses of the planktonic foraminifer globigerinoides ruber and converted to temperature using the surface water paleotemperature equation accounting for variations in global ice volume the accuracy of these data were confirmed by derivation of paleotemperatures using the water column isotopic gradient 13o corrected for salinity and variations in seafloor water mass temperature results indicate that during this period surface water temperatures were on average greater than the minimum required for tropical reef growth 20 c veron 1986 with the exception of the late miocene and earliest early pliocene 10 4 9 ma when there were repeated intervals of temperatures between 18 20 c tropical reef growth on the queensland plateau was extensive from the early to early middle miocene 21 13 ma after which reef development began to decline a lowstand near 11 ma probably exposed shallower portions of the plateau after re immersion near 7 ma the areal extent of reef development was greatly reduced 50 paleotemperature data from site 811 indicate that decreased sea surface temperatures were likely to have been instrumental in reducing the area of active reef growth on the queensland plateau reduced reefal growth rates continued until the late pliocene or quaternary despite the increase of average sea surface paleotemperatures to 22 23 c studies on modern corals show that when sea surface temperatures are below 24 c as they were from the late miocene to the pleistocene off northeast australia corals are stressed and growth rates are greatly reduced consequently when temperatures are in this range corals have difficulty keeping pace with subsidence and changing environmental factors in the late pliocene sedimentation rates increased due to increases in non reefal carbonate production and falling sea levels it was not until the mid quaternary 0 6 0 7 ma that sea surface paleotemperatures increased above 24 c as a result of the formation of a western coral sea warm water pool because of age discrepancies it is unclear exactly when an effective barrier developed on the central great barrier reef the formation of the warm water pool was likely to have either assisted the formation of this barrier and or permitted increased coral growth rates fluctuations in sea surface temperature can account for much of the observed spatial and temporal variations of reef growth and carbonate platform distribution off northeast australia and therefore we conclude that paleotemperature variations are a critical control on the development of carbonate platforms and must be considered an important cause of ancient platform drowning

the gomantong limestone of eastern borneo a sedimentological comparison with the near contemporaneous luconia province abstract the luconia province lying to the north of borneo is an important hydrocarbon bearing province faulting in the oligo miocene led to the uplift of a series of blocks upon which at least 240 carbonate build ups were developed in the late lower to middle miocene as part of the search for appropriate analogues an extensive field study was undertaken on the slightly older to contemporaneous gomantong limestone which outcrops in eastern sabah some 800 km to the east of the luconia province in eastern sabah southeast of the crocker range eocene to miocene clastic and subordinate carbonate sedimentary rocks were deposited in a large east west trending foreland basin the gomantong limestone outcrops in the eastern third of this basin the central sabah basin and is interpreted as having been deposited in shallower shoaling portions of the basin exposure is limited to isolated outcrops of deformed often overturned or steeply dipping carbonates the studied localities form a linear trend extending some 50 km in a ssw nne orientation the bulk of the limestone consists of coral dominated framestones and boundstones and associated fore reef and open marine facies coral morphology foraminiferal coralline red algal content and detailed sedimentological observations enabled an interpretation of the depositional settings of these carbonates to be made the varied carbonate deposits were then interpreted in terms of palaeobathymetry a very rich fauna is present throughout the formation with some intervals dominated by large benthic foraminifera particularly lepidocyclina these indicate a chattian to burdigalian age interpreted depositional environments range from open marine shelfal marls to a variety of reef associated deposits these have been subdivided on the basis of the dominant component corals coralline algae foraminifera or siliciclastic components limited burial of these carbonates has resulted in excellent preservation of these sediments with very little diagenetic alteration the gomantong limestone is interpreted to have been deposited in an open shelf setting adjacent to an east west trending shoreline this shoreline bordered a palaeohigh within the foreland basin and the basin is interpreted to have deepened to the north this is in extreme contrast to the isolated carbonate build ups in the luconia province to the west similarities in primary facies can be observed and both the gomantong limestone and luconia province exhibit evidence of drowning events but the facies are affected by very different diagenetic histories which have left the two provinces with completely different reservoir characteristics the disparity in secondary alteration shows the tremendous importance of diagenesis on the development of porosity in carbonates

strontium isotope stratigraphy potential resolution and event correlation abstract a new generation 87sr 86sr isotope curve for the phanerozoic seawater based on a compilation of 3635 stratigraphically well defined and well preserved skeletal components results in a substantially narrower band with some discrepancies if compared to the curve of burke et al 1982 which is based mostly on whole rock samples the results obtained from low mg calcite shells foraminifers belemnites brachiopods and oysters are mostly superior to those from phosphatic skeletal components such as conodonts even if the latter are exceptionally well preserved cai 1 5 at this stage the phanerozoic data set based on fossils is patchy for the cambrian upper ordovician lower devonian and upper carboniferous triassic intervals the major practical constraint that precludes delineation of an unequivocal sr isotope curve for the phanerozoic seawater is the uncertainty in the assignment of absolute ages to the samples as a result the curve is in reality a band this is a reflection of the limitations of biostratigraphy and geochronology that are inherited by any derivative correlation and dating technique including isotope stratigraphy it is therefore unlikely that in the near future the sr isotope stratigraphy will surpass the resolution capability of biostratigraphy as a global correlation tool on the other hand if complemented by lithostratigraphy it is potentially a valuable tool for correlation of sequences on intrabasinal and regional scales taking into account the above limitations careful consideration must be given to assignment of higher order isotope peaks to specific events within a biozone and to the use of such events as a global correlation tool

a simple autocorrelation algorithm for determining grain size from digital images of sediment abstract autocorrelation between pixels in digital images of sediment can be used to measure average grain size of sediment on the bed grain size distribution of bed sediment and vertical profiles in grain size in a cross sectional image through a bed the technique is less sensitive than traditional laboratory analyses to tails of a grain size distribution but it offers substantial other advantages it is 100 times as fast it is ideal for sampling surficial sediment the part that interacts with a flow it can determine vertical profiles in grain size on a scale finer than can be sampled physically and it can be used in the field to provide almost real time grain size analysis the technique can be applied to digital images obtained using any source with sufficient resolution including digital cameras digital video or underwater digital microscopes for real time grain size mapping of the bed

foraminifera of petroleum platforms louisiana shelf gulf of mexico abstract species of benthic foraminifera presumably colonized the bacteria and diatom rich biofilms on the metallic surfaces of louisiana shelf petroleum platforms soon after their emplacement at present however most underwater platform legs are covered by a thick growth of macrofoulers predominantly barnacles and bivalves scraped samples from five such platforms water depth 16 61 m yielded 104 species of benthic foraminifera 44 of them with attached individuals among the obligate sessile species dyocibicides biserialis and planorbulina mediterranensis were especially widespread this group of epibionts also includes calcituba polymorpha carpenteria candei cornuspiramia antillarum lepidodeuterammina ochracea nubeculinita inhaerens planorbulina acervalis planorbulinoides retinaculatus sagenina divaricans webbina bonairensis and webbina rugosa the platforms are habitat islands the surrounding soft clastic substrate does not support the growth of most sessile species the putative source areas of these colonists of louisiana platforms are carbonate hardgrounds to the west banks or reefs formed over salt diapirs and other platforms the foraminiferal species diversity on the platforms is most likely controlled by their distances from these sources and the dimensions of the settlement surfaces the eastward transport of propagules in summer by the louisiana texas coastal current facilitates the species migration

deposition and diagenesis of miocene limestones senkang basin sulawesi indonesia abstract miocene knoll reef limestones form gas reservoirs in the east senkang basin of southwest sulawesi they are composed predominantly of packstones and wackestones but grainstones and boundstones also occur uplift and exposure of the reef occurred during deposition this together with burial resulted in dissolution of bioclasts precipitation of meniscus and blocky spar cements extensive fissuring of the rock and filling of the fissures by internal sediment predominantly carbonate silts the meniscus cements are unusual in that they occur mostly within biomouldic pores particularly after corals the meniscus form of the cement may be a result of air bubbles trapped within the complex pore system formed from the dissolution of corals the fissures which are probably a product of fracturing associated with local fault movements form complex networks linking biomouldic pores carbonate internal sediments mostly crystal silt but also pelleted which form thin graded layers in places completely fill the fissures and adjacent biomouldic pores these features in association with bioclasts dissolution and meniscus cements suggest a vadose origin this interpretation is supported by the variation in the 18 0 and 13 0 stable isotope values of the internal sediment compared with the depositional matrix equant sparry calcite cements are locally extensive on the basis of only moderately depleted 8 0 values and elevated strontium values they are tentatively interpreted to be of marine phreatic origin

well log and seismic character of tertiary terumbu carbonate south china sea indonesia the esso exploration and production inc natuna d alpha block lies approximately 125 mi 200 km northeast of natuna island in the indonesian portion of the south china sea the block contains a large miocene platform carbonate complex called the l structure the terumbu formation l structure situated in front of and isolated from a much larger carbonate shelf is similar to the carbonate atolls developed in front of the barrier reef complex offshore of belize the terumbu carbonate on the shelf and in the l structure has been divided into upper and lower units on the basis of a prominent seismic reflector and the l structure carbonate has been divided into three main facies on the basis of wireline logs and seismic character the facies are the deep water detrital carbonate the upper terumbu platform carbonate and the lower terumbu platform carbonate the lower platform and upper platform have a similar log character they have a 46 sec ft matrix interval transit time and a chaotic dipmeter response the rocks are boundstones grainstones and packstones that have been homogenized by organic activity they are separated by a hiatus overlying a thick tight deep water limestone that causes a prominent seismic reflector porosity in the platform is controlled by depth of burial sea level fluctuations and depositional texture the detrital facies is composed of fore reef talus pelagic and hemipelagic carbonates the facies has a matrix interval transit time of 49 sec ft and good dipmeter response and it is composed of 25 to 75 ft 8 to 23 m depositional pods that have a sigmoidal cross section when stacked in sufficient thickness they have a similar cross section on seismic records and fringe the platform in a downlapping pattern by mapping the terumbu carbonate facies the growth history of the l structure is revealed

ecological control of sedimentary accommodation evolution from a carbonate ramp to rimmed shelf upper miocene balearic islands abstract stratal patterns and facies architecture develop in response to sediment flux and the available space for sediment accumulation accommodation in siliciclastic systems these two parameters are independent in carbonates however they are highly interdependent 1 sediment input production highly depends on biological systems and consequently on intrabasinal conditions nutrients temperature salinity etc 2 sediment dispersal depends on the production loci and on the interaction between the amount and type of sediment being produced and the hydraulic energy additionally biological processes binding baffling framework construction and even cementation frequently modify it 3 accommodation is not an independent factor with respect to sediment input type efficiency and area of the carbonate factory depend on sea level changes and sea floor morphology the base level for sediment to accumulate accommodation depends on the type and locus of sediment being produced the upper miocene carbonate platforms of the balearic islands illustrate how changes in intrabasinal environmental conditions nutrients and or temperature produce changes in stratal patterns and facies architecture if they affect the biological system during early tortonian carbonate production occurred both in the shallow euphotic zone foramol and in the deeper oligophotic zone rhodalgal originating a distally steepened ramp during late tortonian and early messinian carbonate production mainly occurred in the euphotic zone where framework producing biota coral reefs formed a rimmed platform although deposited under similar conditions of high frequency sea level fluctuations these two types of platforms exhibit different internal facies architecture and distribution of heterogeneities because of differences in base level in carbonates base level for sediment accumulation depends on the type and amount of sediment being produced on the production loci and on the processes controlling sediment dispersal base level for the loose foramol rhodalgal sediment associations of the ramp was controlled by wave and storm base and the associated currents whereas base level for the framework dominated reef complex was sea level furthermore the increase on effective accommodation space to allow the reef rimmed shelf to prograde onto the distally steepened ramp both platforms being depositional sequences resulted from an ecological change rather than significant relative sea level change namely by the shift from grain to framework producing biota the euphotic framework producing biota was able to fill the accommodation space existing between wave and storm base level and sea level that was not previously filled by the loose bioclasts

strontium isotope stratigraphy of kita daito jima atoll north philippine sea implications for neogene sea level change and tectonic history abstract a detailed chronology of a 432 m core from kita daito jima an atoll on the philippine sea plate has been obtained by strontium isotope stratigraphy the atoll carbonate extends to late oligocene age the top 100 m plio pleistocene section of the core has been dolomitized and dolomite also extends to middle miocene sediments sr isotopes identify two dolomitization events at 2 and 5 ma atoll growth appears to have been continuous between 18 8 and 24 3 ma and the fit of the growth history to a standard model provides a successful confirmation of darwin s subsidence theory the atoll growth appears to have commenced about 42 ma ago 6 ma after volcanism producing a 2 km column of atoll carbonate in agreement with bathymetry the subsidence model defines an apparent loss of 360 m of the sedimentary record since the early middle miocene boundary the amount in excess of the 100 m associated with sea level fall is attributed to plio pleistocene uplift at rates of 20 30 m ma associated with the forebulge of the philippine sea plate prior to its subduction uplift rates have also been estimated using a the heights of dated last interglacial coral reefs on kita daito jima and nearby minami daito jima 17 m ma and 29 m ma b sr isotope ages of calcites from surface samples on minami daito jima 21 m ma and c contrasts in elevation with okino daito jima 100 km away 22 m ma the locations of hiatuses and ages of dolomitization indicate sea level falls of 80 m at 17 16 ma 30 m at 16 15 ma 125 m at 11 ma and 90 m at 5 ma and at 2 ma the data have been combined with those from enewetak atoll to produce a curve of sea level change for the neogene

environmental factors influencing skeletal grain sediment associations a critical review of miocene examples from the western mediterranean the presence of foramol rhodalgal and bryomol skeletal grain associations in ancient shallow marine limestones is commonly interpreted as evidence for non tropical palaeoclimate despite temperature being only one of several factors influencing skeletal grain associations such interpretations neglect the multitude of factors other than temperature that influence carbonate producing biota these include nutrients water energy water transparency depth of the sea floor salinity oxygen ca2 and co2 concentrations mg ca ratio alkalinity substrate requirements competitive displacement as well as biological and evolutionary trends this uniformitarian approach also disregards the probability that conditions of present day biological systems may not be representative of past conditions of analogous systems here the importance of considering these other factors is illustrated through two examples of carbonate platforms in the western mediterranean these platforms are dominated by foramol rhodalgal and bryomol associations of miocene age in spite of having formed in tropical conditions the platforms discussed are 1 the lower tortonian ramp on menorca balearic islands and 2 the lower middle miocene ramps of the central apennines italy evidence for tropical conditions in the mediterranean during the period of growth of these platforms is provided by species of red algae and larger foraminifera by data from coeval continental basins and by global oxygen isotope data

paleo oceanographic and tectonic settings of early miocene reefs and associated carbonates of offshore southeast asia the late oligocene to early middle miocene was a period of widespread reef and associated carbonate deposition over a wide latitudinal range in the offshore region of southeast asia a generally rising eustatic sea level during this period allowed the accumulation of thick reefal units an expansion of the coral algal regional facies belt north to japan and south to new zealand occurred during the early miocene as the result of a reduced oceanic latitudinal temperature gradient an episode of enhanced organic carbon burial is reflected in a positive dgr13c excursion from 17 5 to 13 5 ma increasing paleo oceanographic equability may have resulted in reduced ocean circulation with the resultant development of an expanded oxygen minimum zone marine oil prone or anic carbon was deposited in silled anoxic basins both bounded by and within island arc systems a combination of tectonic and paleo oceanographic factors therefore created conditions that favored contemporaneous deposition of organic carbon rich source beds and reef reservoirs during the latest early to early middle miocene paleogeographic and facies reconstructions for an 18 m y interval indicate that deposition occurred in three tectonic settings 1 passive continental margins 2 convergent plate boundaries and 3 obliquely convergent plate boundaries in all these settings the vertical motion histories of fault bounded blocks and of larger tectonic elements within which they are contained directly controlled the geographical distribution of carbonate buildups their scal and morphology duration diagenesis and relationship to siliciclastics

sedimentology and diagenesis of tertiary carbonates on the mangkalihat peninsula borneo implications for subsurface reservoir quality abstract combined field and laboratory studies of surface exposures on the mangkalihat peninsula borneo provide new constraints on the chronostratigraphy sedimentology and diagenesis of tertiary carbonate deposits the reservoir characteristics of carbonate facies are described providing an analogue model for carbonate reservoirs in the subsurface of this region a mixed carbonate siliciclastic shelf with intervening probably fault bounded deeper water areas developed on the eastern part of the mangkalihat peninsula in the late eocene to oligocene during the oligo miocene shallow water platform carbonates accumulated over much of the area of the mangkalihat peninsula this platform had a steep reef rimmed or possibly locally faulted northern margin where high energy conditions prevailed and marine cements were pervasive in contrast the platform interior was a low energy area affected by leaching of aragonitic bioclasts the best reservoir quality on the platform occurs in moderate energy grainstones and packstones tens to hundreds of metres towards the platform interior from the platform margin where a combination of primary intergranular and secondary mouldic porosity results in good porosity and permeability

the chronostratigraphic significance of seismic reflections along the bahamas transect abstract continuous cores drilled during the bahamas drilling project bdp and the ocean drilling program odp leg 166 along a transect from the top of great bahama bank to the basin in the straits of florida provide a unique data set to test the assumption in seismic stratigraphy that seismic reflections are time lines and thus have a chronostratigraphic significance seismic reflections that are identified as seismic sequence boundaries ssbs were dated by means of biostratigraphy in the five odp sites and by a combination of biostratigraphy magnetostratigraphy and sr isotope stratigraphy in the two bdp sites the seismic reflection horizons are carried across a variety of facies belts from shallow water carbonates over slope carbonates to drift deposits in the straits of florida within this system 17 ssbs were identified and dated despite the fact that the seismic reflections cross several facies belts their ages remain remarkably constant the average offset in all sites is 0 38 myr in no cases do the seismic reflections cut across time lines the age differences are the combined result of the biostratigraphic sampling frequency the spacing of marker species that required extrapolation of ages and the resolution of the seismic data in addition uncertainties of age determination in the proximal sites where age diagnostic fauna are rare add to the age differences between sites therefore it can be concluded that the seismic reflections which mark the ssbs along the bahamas transect are time lines and can be used as stratigraphic markers this finding implies that depositional surfaces are preferentially imaged by reflected seismic waves and that an impedance contrast exists across these surfaces facies successions across the sequence boundaries indicate that the sequence boundaries coincide with the change of deposition from times of high to low sea level in the carbonate setting of great bahama bank sea level changes produce changes in sediment composition sedimentation rate and diagenesis from the platform top to the basin the combination of these factors generates differences in sonic velocity and thus in impedance that cause the seismic reflection the impedance contrasts decrease from the proximal to the distal sites which is reflected in the seismic data by a decrease of the seismic amplitude in the basinal area

late tertiary tectonic subsidence on the northeast australian passive margin response to dynamic topography abstract the carbonate platforms and adjacent troughs off northeast australia record a phase of accelerated tectonic subsidence in the late miocene pliocene previous attempts to analyse tectonic subsidence in this area were hampered by poor core recovery and dating uncertainties at some of the odp ocean drilling program leg 133 sites on the queensland plateau in particular at sites 812 and 814 in order to establish a general chronostratigraphy for these sites we have integrated geophysical logs biostratigraphic lithologic and seismic reflection data to identify and correlate miocene hardgrounds on the queensland plateau to third order eustatic sea level lowstands on the haq et al haq b u hardenbol j vail p r 1987 chronology of fluctuating sea levels since the triassic science vol 235 pp 1156 1167 curve this enables us to estimate ages where biostratigraphic information is poor we have used sediment backstripping to compute tectonic subsidence histories for sites on the queensland plateau 811 825 812 and 814 the marion plateau 815 and the queensland trough 823 in order to determine regional subsidence patterns results of these calculations show that although the leg 133 sites are located on a passive margin approximately 1000 km south of the pacific australian plate boundary they record a greater amount of subsidence than can be predicted from simple elastic models post 9 ma subsidence of 1300 200 m in the queensland trough 650 200 m on the western margin of the queensland plateau post 5 ma subsidence of 500 30 m on its southern margin and 660 50 m on the northern margin of the marion plateau these models show that it is difficult to account for observed subsidence either by means of thrust loading in papua new guinea or by a combination of the latter and in plane stresses originating from collision along the australian pacific plate boundary alternatively shear wave tomography van der hilst r d kennett b l n shibutani t 1998 upper mantle structure beneath australia from portable array deployments in brown j dooley j goleby b van der hilst r klootwijk c eds structure and evolution of the australian continent geodynamics series 26 pp 39 57 displays a nnw sse trending band of anomalously high velocities in the upper mantle stretching from the queensland plateau to indonesia these anomalies likely reflect subducted slab material originating from late eocene late oligocene subduction north of papua new guinea thus we suggest that the observed post 9 ma tectonic subsidence of the queensland and marion plateaus and queensland trough is largely caused by dynamic surface topography due to australia s northeastern margin overriding a slab burial ground modulated by flexural deformation resulting from collision tectonics north of australia the observed rates of tectonic subsidence are much slower than those of third order sea level changes and can thus be differentiated from glacial eustasy

cenozoic carbonates in southeast asia implications for equatorial carbonate development cenozoic equatorial carbonate production was at its most extensive and diverse in the seas of southeast asia yet the carbonates of this region remain poorly documented this paper presents the first published review of carbonate deposition during the cenozoic collating sedimentological data for carbonate successions throughout the region current models of warm water carbonate platform evolution sedimentation and facies distribution inspired by studies of the classic sub tropical areas provide inadequate analogues for the evaluation of modern or cenozoic equatorial carbonates in southeast asia the equatorial carbonates differ in being dominated by bioclasts coated grains and associations with evaporites or dolomites are rare the carbonate systems occur in a range of depositional settings which were often affected by coeval tectonism siliciclastic input or volcanism an understanding of the carbonate depositional environments spatial facies distributions and controls on deposition and diagenesis is essential in order to characterize equatorial carbonate development and to evaluate their considerable economic potential

integrated 3 d reservoir modeling based on 3 d seismic the tertiary malampaya and camago buildups offshore palawan philippines three dimensional 3 d seismic interpretation and drilling results indicate complex sedimentary geometries of the malampaya and camago build ups offshore palawan philippines with localized progradation due to unidirectional offbank transport alternating with vertical aggradation successive reduction of size during buildup growth and backstepping of the protected landward margin in response to transgression ultimately appear to have triggered the demise of carbonate production and platform drowning the shallow water platform top sediments repeatedly show signs of subaerial exposure before reflooding a modeling functionality was developed to allow development of multiple scenario 3 d reservoir models in an exploration or appraisal stage the model enables merging of seismic scale observations based on 3 d volume and horizon analyses with subseismic scale information from well data however inherent noise within the seismic data introduced by the complex buildup morphology has resulted in inconsistent attribute distribution and fault dimming these difficulties are compounded by erratic velocity distribution within the limestone nonhyperbolic move out and a narrow relatively low frequency spectrum all of which prevent the use of the 3 d seismic volume as hard data but rather allow its use as a soft constraint for guiding the geological interpretation and ultimately the modeling process seismic data quality in such complex morphologic settings and scarcity of well data hamper greatly the use of geostatistically driven modeling approaches therefore a new functionality was developed within shell s proprietary integrated 3 d modeling suite geocap which allows deterministic model reservoirs using seismic horizon and volume interpretation sequence and cyclo stratigraphic architecture and the concept of reservoir rock type seismic velocity in clean carbonate formations is predominantly a function of porosity distribution to assess time to depth conversion uncertainty the reservoir rock type based models were first produced in the time domain only after differential 3 d depth conversion of these models could the scenarios be reconstructed in the depth domain the depth models subsequently were used to derive permeability and saturation 3 d distortions and thus hydrocarbon volumes for each deterministic scenario the models were then used for simulation purposes

dissolution methods for strontium isotope stratigraphy whole rock analysis the preparation of samples for strontium isotope stratigraphy sis by whole rock dissolution results in contributions of sr from contaminant phases as well as from the target carbonate phase here we highlight and quantify the errors in 87sr 86sr values that might result when calcium carbonate is dissolved using an inappropriate method we report the variation in 87sr 86sr value of sr released from four samples of differing calcite content and degrees of cementation when subject to different methods of dissolution we show that errors of up to 0 000076 in 87sr 86sr which correspond to errors of 3 68 ma in age can be avoided by careful preleaching of the samples

dolomitization of atolls by sea water convection flow test of a hypothesis at niue south pacific the core of the former atoll on niue island south pacific is extensively dolomitized a detailed stable isotope study reveals that the dolomites are uniformly enriched in 18o and 13c 18o 2 8 0 5 13c 2 0 0 3 relative to their carbonate precursor the sr isotope ratios are equally constant with depth yielding 87sr 86sr values between 0 70912 and 0 70916 the observed isotopic compositions point to sea water as the dominant dolomitizing fluid the sr age of the dolomites is younger than the biostratigraphic age mid to late miocene placing the dolomitization event at plio pleistocene time a model of sea water convection is proposed for atoll dolomitization on the basis of a thermal gradient between the atoll and the ambient ocean water sea water is drawn through the atoll margin and transferred upward by convective flow delivering mg to the sites of dolomite precipitation transported with the sea water are volcano derived metals fe cu zn mn as evidenced by the chemical gradient in