meteoric like fabrics forming in marine waters implications for the use of petrography to identify diagenetic environments petrographic fabrics have long been used to identify meteoric diagenesis of carbonate sediments however on the basis of oxygen isotopic data we document similar fabrics forming in marine pore fluids in the shallow subsurface of great bahama bank therefore petrographic fabrics alone are not reliable indicators of diagenetic environments even for shallow water sediments in our study skeletal grainstones show two distinct diagenetic assemblages either dissolution of aragonitic grains and minimal cementation high permeability intervals or abundant blocky spar cement and neomorphism of aragonitic skeletal grains low permeability intervals these marine burial fabrics are present as shallow as 110 m below sea level well above the aragonite compensation depth a feature that must be considered for models of diagenesis in ancient carbonate sediments marine burial diagenesis may be important in ancient carbonate sediments deposited in moderate water depths or in shallow water during rising sea level where meteoric diagenesis is suppressed

burial dominated cementation in non tropical carbonates of the oligocene te kuiti group new zealand abstract cementation of bryozoan echinoid benthic foraminiferal temperate shelf carbonates of the oligocene te kuiti group north island new zealand occurred mainly during subsurface burial the calcite cements in the limestones are dominated by equant and syntaxial rim spar which typically becomes ferroan given an iron supply and compared to the skeletal material with normal marine 18o values from 2 to 1 more depleted in 18o with depth of burial the 18o composition of bulk cement samples ranging from 1 to 7 these trends reflect the establishment in pore waters during sediment burial of reducing conditions and gradually increasing temperatures 20 50 c respectively the 13c values 0 to 3 of the cements remain the same as the host marine shells suggesting the source of carbon in the cements was simply redistributed marine carbonate derived from shell dissolution two gradational burial diagenetic environments influenced by marine derived porewaters are arbitrarily distinguished shallow burial phase and moderate burial phase during the shallow burial phase down to 500 600 m sub bottom depth the carbonates lost at least 25 of their original porosity by mechanical compaction and were selectively cemented by non ferroan or usually ferroan variably luminescent slightly 18o depleted sparry calcite cement 18o 2 to 4 mainly as syntaxial rims about echinoid grains these shallow burial cements form less than about 10 of total cement in the majority of the limestones and their source was probably mainly mild intergranular dissolution of calcitic skeletal fragments accompanying the onset of chemical compaction during the moderate burial phase between about 600 and 1100 m sub bottom depth porosity loss continued typically to about 70 of its original value as a result of pressure solution of calcitic bioclasts associated with more advanced stages of chemical compaction this involved development of a wide variety of non sutured and microstylolitic solution seams including both single and composite wispy or continuous bedding parallel types and non parallel reticulate forms the released carbonate was precipitated as ferroan or non ferroan where iron supply was negligible dull luminescent strongly 18o depleted 18o 4 to 7 mainly equant calcite spar cement occluding available pore space in the limestones

transformation of aragonite dominated lime muds to microcrystalline limestones scanning electron microscopy of textures in several well lithified pleistocene micrites mixed micrite and microspar from south florida and the bahamas shows significant diagenetic alteration of the lime mud precursors the dominance of aragonite in those precursor muds is supported by the surprising presence of abundant aragonite relics in the neomorphic calcite crystals confirmed by feigl stain x ray and electron diffraction polished very lightly etched surfaces of neomorphic calcite crystals even in the microspar show aragonite relics and pits both absent in coexisting void fill cements similarities between micrite and microspar crystals indicate their common origin through a one step neomorphic process of calcitization that is the mud did not first calcitize to micrite and then alter to microspar by aggrading neomorphism this conclusion has more general implications for the interpretation of the origins of other older microsparitic carbonates both early calcitization and concurrent early meteoric cementation were significant in producing the florida bahamas micrites studied which have low porosities similar to those found in most ancient fine grained limestones these low porosities despite the absence of significant overburden indicate that major porosity reduction without compaction can occur in calcitized aragonite dominated muds

contrast comparison of differential diagenetic pathways of lower tithonian carbonate materials from the betic cordillera s spain evidence for physico chemical paleo seawater properties abstract the reconstruction of paleoenvironmental conditions is based on the study of ancient rock materials for this purpose skeletal materials are often assumed to have higher preservation potential sometimes favored over the use of matrix micrite however factors such as ecology depositional setting mineralogy and diagenetic processes may obscure original geochemical signals and care must be taken when reading such an intricate record this work provides a thorough comparison between geochemical signatures of different carbonate materials interpreted in the light of their paleoenvironmental significance burial and diagenetic processes are taken into account contextualized by ammonite and belemnite ecology two sections deposited in epioceanic tethyan areas and corresponding to the same lower tithonian ammonite biozone are under scope a condensed pelagic and cephalopod rich limestone the alamedilla sector intercalated in thick mafic rocks piles and a typical ammonitico rosso facies the cardador section in contrast to depositional and taphonomic histories typical for a tethyan top swell site i e rather calcareous ammonitico rosso events of carbonate mud deposition rapid burial and early lithification synsedimentary sliding and over imposition of firmground horizons characterize pelagic cephalopod rich limestones differential taphonomy of carbonate skeletals agrees with differential preservation favoring inner cast preservation in ammonitico rosso and shell preservation neomorphic calcite in pelagic cephalopod rich limestones stable carbon and oxygen isotope data together with trace element data sr mg fe mn are presented for a wide range of carbonate materials matrix micrite neomorphic ammonite shells belemnite rostra and carbonate cements optical inspection including cathodoluminescence imaging and geochemical data suggest a fair preservation of the above mentioned materials except for diagenetic marine burial carbonate cements carbon isotope values fall within the typical tethyan late jurassic range ca 2 except data from belemnite rostra which are depleted around 1 which is interpreted as resulting from metabolic fractionation oxygen isotope values are more variable but the clustering of different carbonate materials within a narrow range of values between 1 and 1 is consistent with conservative diagenesis warmer burial fluids explain lower values in later diagenetic carbonate cements no evidence of pervasive diagenetic imprint is revealed by elemental composition the increased abundance of mg fe and especially mn up to 1500 ppm is recognized as hydrothermal contamination proximity to hydrothermal sources the mid subbetic volcanic ridge accords with higher mn concentration at the alamedilla site interestingly the timing of the mineralogical stabilization of each carbonate material reveals a potential record of a post sedimentary geochemical event the obtained information demonstrates the complexity of ancient carbonate records and encourages future research focused on the preservation potential of geochemical proxies throughout time the studied case hopes to be a representative example to approach the interpretation of potential past and present rare analogs

role of climate in partial drowning of the queensland plateau carbonate platform northeastern australia abstract ocean drilling program leg 133 core data samples and geophysical data logs and seismics were analyzed to document late miocene early pliocene partial drowning of the queensland plateau carbonate platform off northeast australia the modern plateau consists of a mosaic of pinnacle reefs and larger 10 50 km reefs representing relicts of early to middle miocene buildups late miocene early pliocene floatstones packstones and mudstones rich in the larger benthic foraminifers lepidocyclina and cycloclypeus drilled in a transect of sites across the drowned margin of a middle miocene buildup show that the late pliocene partial drowning of the platform was preceded by 4 myr of neritic carbonate deposition without any reefs the carbonate factory was unable to aggrade to sea level during this period as indicated by the lack of any shallowing trend in the succession monitoring of the miocene to recent neritic shedding pattern on the windward and leeward side of a pinnacle reef on the queensland plateau supports this interpretation shedding on the leeward side of the reef which records periods of active reef growth played a minor role during the late miocene increased during the pliocene and reached very high values during the late pliocene and pleistocene shedding on the windward side of the reef which was active during lowstands of relative sea level occurred during the late middle early late miocene and during the pliocene quaternary the data presented here when combined with paleoceanographic data points to low surface water temperatures 17 19 c as a major factor which suppressed reef growth during the late miocene early pliocene period

recurrent neomorphic and cement microtextures from different diagenetic environments quaternary to late neogene carbonates great bahama bank abstract the cores clino and unda taken near the western margin of great bahama bank contain a diverse suite of quaternary to late neogene carbonates that have a variety of neomorphic and cement microtextures the microtextures of each of the different neomorphosed aragonitic grain types such as corals mollusks and plates of the calcareous green algae halimeda are very similar wherever the neomorphosed grains are present in the cores limestone samples that contain neomorphosed aragonitic fossils are cemented predominantly with blocky calcite despite the similar neomorphic and cement microtextures light stable isotope data reveal that these calcites precipitated in different diagenetic environments in different parts of the cores the neomorphic calcites and associated calcite cements in the upper parts of clino and unda precipitated in meteoric influenced pore waters as indicated by 18o values ranging from 1 9 to 3 7 whereas in the lower part of clino the calcites precipitated in marine pore waters as indicated by 18o values ranging from 0 1 to 2 4 essentially identical diagenetic microtextures were thus produced in different diagenetic environments revealing a significant limitation in the use of microtextural data to determine the diagenetic histories of carbonate rocks

an introductory perspective on non tropical shelf carbonates abstract this brief account introduces the topic of non tropical shelf carbonate deposits both modern and ancient specific aspects of which are described and discussed in the 22 accompanying papers in this special issue of sedimentary geology non tropical shelves are loosely defined as lying poleward of the present limit of development of hermatypic coral reefs or beyond the mean annual surface water isotherm of about 20 c typically near 30 latitude most geoscientists have traditionally regarded shallow water carbonate formation as a low latitude phenomenon however wherever the rate and extent of terrigenous sediment dilution is small the potential exists for extensive deposits of skeletal carbonate to accumulate on shelves beyond the sub tropics the facies attributes of the modern non tropical carbonates are distinctive and contrast significantly with the structural lithological faunal floral mineralogical and diagenetic character of their tropical counterparts in the rock record non tropical limestones probably occur throughout the phanerozoic but generally remain to be identified and or interpreted as such they are presently described mainly from the ordovician permian and tertiary quaternary periods of major continental glaciations and accentuated latitudinal thermal gradients a uniformitarian approach to the recognition and interpretation of ancient shallow marine limestones requires acceptance of both tropical and non tropical carbonate facies models

climate change and population history in the pacific lowlands of southern mesoamerica core man015 from pacific coastal guatemala contains sediments accumulated in a mangrove setting over the past 6500 yr chemical pollen and phytolith data which indicate conditions of estuarine deposition and terrigenous inputs from adjacent dry land document holocene climate variability that parallels the maya lowlands and other new world tropical locations human population history in this region may be driven partly by climate variation sedentary human populations spread rapidly through the estuarine zone of the lower coast during a dry and variable 4th millennium b p population growth and cultural florescence during a long relatively moist period 2800 1200 b p ended around 1200 b p a drying event that coincided with the classic maya collapse

non tropical carbonate deposits on the modern new zealand shelf abstract extensive ca 50 000 km 2 shallow marine platforms 70 caco 3 despite proximity to a tectonically active plate margin in these regions the terrigenous sediment supply is presently low and growth of epibenthos is fostered by firm substrates rock gravels shells seaweeds and the generally energetic nature and high nutrient levels of open shelf waters rapid transition into adjacent terrigenous dominated facies is characteristic irrespective of water depth the carbonates are coarse grained and fragmental carbonate mud is rare calcite dominates over aragonite high mg calcite widespread off northern new zealand is rare in the south skeletal material is dominated by bryozoans and bivalve molluscs with significant local contributions from foraminifers barnacles calcareous red algae and echinoderms the name bryomol is suggested for this distinctive temperate region skeletal carbonate facies which can be usefully subdivided based on dominant zoarial growth forms of the bryozoan component known to be habitat related bioerosion is an important mechanism of skeletal fragmentation and degradation many grains especially aragontic bivalves are infested by endolithic borers and have low preservation potential ages of skeletal material in the surficial deposits range from more than 20 000 years b p to modern which is consistent with both low rates of carbonate production and sediment accumulation and the wide range in preservation state of grains some data suggest that the skeletal carbonates are dispersed and mixed mainly during infrequent movement of sand ribbons sand waves and sand sheets driven by storm assisted tidal flows tracts of modern palimpsest and relict carbonates can occur in juxtaposition the facies characteristics of the new zealand shelf carbonate deposits contrast significantly with those of the classical bahaman type carbonate model however they are similar to those reported from many other mid to high latitude carbonate shelves and afford good analogues for most onland occurrences of new zealand cenozoic limestones

microspar development during early marine burial diagenesis a comparison of pliocene carbonates from the bahamas with silurian limestones from gotland sweden comparison of ultrastructures in pliocene periplatform carbonates from the bahamas with silurian limestones from gotland sweden reveals that despite the differences in primary sediment composition and age they reflect a similar mechanism of lithification in both sequences calcite microspar was formed as a primary cement at an early stage of marine burial diagenesis neither significant compression nor meteoric influence are necessary for the formation of calcite microspar a model is proposed for the process of microsparitic cementation of fine grained aragonite needle muds comprising four stages 1 unconsolidated aragonite dominated carbonate mud 2 precipitation of microspar that engulfs aragonite needles 3 dissolution of aragonite resulting in pitted surfaces of the microspar crystals and 4 slight recrystallization our results contradict the widespread opinion that microspar necessarily is a product of secondary recrystallization of a previously lithified micrite

petrogenesis of cenozoic temperate water calcarenites south australia a model for meteoric shallow burial diagenesis of shallow water calcite sediments abstract the gambler limestone is one of several extensive shallow water shelf carbonates of eocene to miocene age exposed along the southern margin of australia it is a muddy to grainy bryozoan calcarenite with accessory benthonic foraminifers and echinoids the sediments originally composed almost entirely of calcite or mg calcite have been in vadose and phreatic environments for over 10 ma yet are virtually unlithified the only cements of any consequence are epitaxial on echinoids numerous karst features dolines caves speleothems and surface karren attest to prolonged residence in the meteoric zone the gambler limestone is presently one of the best fresh water aquuifers in australia most flow occurs through intergranular pores in sediments with over 30 porosity cementation is by minor intergranular pressure solution which has developed under overburden of less than 100 m the overlying naracoorte limestone miocene a calcarenite of warmer water aspect contains numerous aragonite molds and is well cemented we propose that such cool water limestones are a better model for the meteoric diagenesis of calcite sediments of all ages than are aragonite rich tropical sediments it is probable that many similar early and middle paleozoic calcite limestones may have been in the meteoric zone for prolonged periods yet contain little or no petrographic or geochemical record of such exposure

recrystallization in living porcelaneous foraminifera archaias angulatis textural changes without mineralogic alteration abstract mineralogic stabilization of porcelaneous foraminifera has been cited as an example of diagenesis in which chemical change occurs without any textural alteration in contrast our study of living specimens of a common species of this group archaias angulatis indicates a sequence in which textural alteration of the magnesium calcite skeleton occurs without loss of magnesium from the test in samples of a angulatis from florida bay florida skeletal rods 1 2 m long and 0 1 m wide are extensively recrystallized to dense minimicrite 0 05 0 1 m anhedral equant crystals both rods and minimicrite contain 9 11 5 mole mgco3 micritization increases with age of the skeletons and occurs more rapidly in septal and basal walls than in lateral walls and pillars recrystallization could be due to change in the partial pressure of co2 resulting from changing distributional patterns of respiration and photosynthesis of algal symbionts recrystallization within living foraminifera has important implications for studies of fossil diagenesis and biomineralization

origin of a cool water oligo miocene deep shelf limestone eucla platform southern australia the abrakurrie limestone is an areally extensive bryozoan rich unit within the eucla platform a tertiary carbonate shelf which caps the central part of the southern australian continental margin the onshore portion the topic of this study has been exposed since middle miocene time and lies beneath the nullarbor plain the rocks are fine sand to granule sized calcarenites composed of bryozoans bivalves benthic foraminifera and echinoids with lesser numbers of brachiopods solitary corals and serpulids they conspicuously lack significant numbers of planktonic foraminifera and coralline algae most bryozoan remains are from delicate branching cyclostomes although delicate branching robust branching foliose fenestrate multilaminar encrusting and free living cheilostomes are variably abundant in specific units the poorly lithified sequence is punctuated by well cemented layers with erosional tops which are interpreted as hardgrounds the limestone is interpreted as a cool water deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the eucla platform a model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation the shelf is envisaged as partitioned by the depth of the zone of wave abrasion sediments were produced throughout but accumulated only below this depth when the seafloor was above this depth it was an environment of cementation and erosion the vertical sequence correlates in a general way with the global sea level model for the mid cenozoic while accumulation rates for southern australian carbonates are similar to rates of cool water carbonate deposition elsewhere c 2 5 cm kyr 1 the rate of abrakurrie accumulation is much less suggesting that significant time periods are represented by the hardgrounds

porosity permeability relationships in miocene carbonate platforms and slopes seaward of the great barrier reef australia odp leg 194 marion plateau this paper reports a series of 700 porosity permeability analyses and supporting petrographic and sedimentologic descriptions from early to late miocene carbonate strata cored on the marion plateau offshore from north eastern australia during ocean drilling program odp leg 194 the samples analysed are not only mainly coarse bioclastic limestones and dolomitized equivalents from platform top facies but also include 79 plugs from deeper water slope to hemipelagic drift facies outstanding characteristics of this data set are the wide ranges of porosity and permeability in both limestones and dolostones the large degree of short range heterogeneity typical of these strata and the better porosity permeability correlation of dolostones than limestones the platforms have experienced widely varying calcite cementation dolomitization and dissolution but show little clear evidence of meteoric diagenesis suggesting that subaerial exposure may have played little role in porosity permeability evolution permeability for given porosity is controlled by grain size and calcite cement content in grainstones and by occurrence of larger shelter pores and vugs in mud rich samples dolomitization tends to reduce the variation of permeability for given porosity by recrystallizing mud matrix to form intercrystalline macroporosity that connects vugs and moulds to become integrated with the effective pore system as a result there are no differences in permeability porosity trends for different dolostone textures whether dominated by intercrystalline vuggy or preserved intergranular pore types two platform top sites separated by only 5 km display a major lateral variation in dolostone porosity permeability characteristics within the youngest dolostone units this difference is interpreted as reflecting a relatively windward current facing setting of the site with the overall higher permeability for given porosity site 1199 that led to less muddy depositional facies greater cementation and lesser grain dissolution pore geometry parameters measured by petrographic image analysis confirm that the windward dolostones have pores that are both larger and less intricate than dolostones comprising the more current protected location

diagenetic formation of gypsum and dolomite in a cold water coral mound in the porcupine seabight off ireland authigenic gypsum was found in a gravity core retrieved from the top of mound perseverance a giant cold water coral mound in the porcupine basin off ireland the occurrence of gypsum in such an environment is intriguing because gypsum a classic evaporitic mineral is undersaturated with respect to sea water sedimentological petrographic and isotopic evidence point to diagenetic formation of the gypsum tied to oxidation of sedimentary sulphide minerals i e pyrite this oxidation is attributed to a phase of increased bottom currents which caused erosion and enhanced inflow of oxidizing fluids into the mound sediments the oxidation of pyrite produced acidity causing carbonate dissolution and subsequently leading to pore water oversaturation with respect to gypsum and dolomite calculations based on the isotopic compositions of gypsum and pyrite reveal that between 21 6 and 28 6 of the sulphate incorporated into the gypsum derived from pyrite oxidation the dissolution of carbonate increased the porosity in the affected sediment layer but promoted lithification of the sediments at the sediment water interface thus authigenic gypsum can serve as a signature for diagenetic oxidation events in carbonate rich sediments these observations demonstrate that fluid flow steered by environmental factors has an important effect on the diagenesis of coral mounds

seismic geomorphology and growth architecture of a miocene barrier reef browse basin nw australia abstract the cenozoic succession of browse basin is characterized by a carbonate system that developed from a non tropical ramp in eocene lower miocene times to a tropical rimmed platform in the middle miocene the evolution of the platform was unraveled through the interpretation of the seismic geomorphology and borehole data of the middle miocene tropical reef system the first reef structures developed during the early middle miocene as narrow linear reef belts with an oblique orientation with respect to shelf strike direction subsequently they prograded toward the platform margin to form a barrier reef with a minimum length of 40 km the barrier reef itself comprises three distinct ridges separated by progradational steps the second and third step are separated by a karstified horizon which is interpreted to represent the global sea level fall shortly before the serravallian tortonian boundary the following third ridge formed in a slightly downstepped position during the sea level lowstand and initial transgressive phase further sea level rise during the early tortonian first drowned the barrier reef system and subsequently also the patch reefs and relic atolls that had established in a backstepped position in the platform interior the similar evolution of the browse basin reef system and other contemporaneous carbonate systems indicates a strong impact of eustatic sea level changes relatively large subsidence rates in the study area possibly augmented the eustatic sea level rise in the tortonian and hence contributed to the drowning of the reef system however the initiation and final demise of the reef system was also governed by global and regional climate variations the first seismically defined reefs developed simultaneous to a maximum in the transport capacity of the indonesian throughflow which brings warm low salinity waters to the north west shelf reef drowning followed the restriction of this seaway close to the middle to early miocene boundary this near closure of the indonesian seaway possibly led to a regional amplification of the global middle to late miocene cooling trend and hampered the potential of the reef system to keep up with the rising sea level

diagenesis and compositional partitioning of quaternary cool water carbonate aeolianites southeastern australia abstract middle to late pleistocene stranded carbonate sea beaches adjacent aeolian dunes and pleistocene aeolianites together with holocene marginal marine calcareous sediments across southeastern australia contain the longest record of such deposition in the modern world stretching back to at least 900 ky mis 23 the grains are were derived from a suite of cool water heterozoan carbonate sediments produced on the adjacent offshore shelf most modern shelf carbonate sediments in the region today are aragonite rich but the aragonite is presently slowly being dissolved on or just below the seafloor before the deposits enter the rock record beach dune sediments are derived mainly from the shallow 30 meters deep adjacent seafloor they are mollusk rich but with most bryozoan particles having been destroyed in the surf zone the mollusks are dominantly aragonite these nearshore sediments are swept onshore before any significant seafloor diagenesis can take place and so are compositionally different from the eventual aragonite poor open shelf sediments aragonite skeletal fragments in the dunes progressively dissolve with time under a semi arid climate and produce diagenetic low magnesium calcite meteoric cements a well known process common in tropical aragonitic pleistocene aeolianites the loss of metastable minerals with time is matched by a progressive and well understood change in stable isotopes such alteration in this environment however seems to take longer than in tropical carbonates with lithification herein not beginning until 200 ka and complete cementation being achieved in 450 ky thus neither aeolianites nor offshore marine carbonates record the composition of the original sediment aeolianites lack the important bryozoan constituents whereas neritic deposits lack the prolific mollusks this partitioning revealed for the first time answers the conundrum as to why some cool water neritic shelf and coastal sediments in the rock record are compositionally different

shallow burial diagenesis of skeletal carbonates selective loss of aragonite shell material miocene to recent queensland plateau and queensland trough ne australia implications for shallow cool water carbonates in burial environments carbonate sediments undergo mineralogical stabilization and increasing lithification with depth as yet however little knowledge exists with respect to the corresponding effects on fossil preservation and taphonomic modification of the original sediment composition countings of particles 63 m in miocene to recent periplatform sediments odp leg 133 ne australia exhibit a clear trend of reduction of skeletal aragonite downcore low and high mg calcite grains occur in a continuous order of magnitude over the studied interval 600 m sub bottom depth original microtextures are retained in high mg calcite biota although converted to low mg calcite thus the conversion to low mg calcite appears to occur without introducing a significant quantitative bias aragonite skeletons pelagic gastropods however exhibit a successive exposure of deeper crystal layers and a chalky preservation with burial depth which we interpret to result from dissolution hints for the originally more numerous existence of aragonite biota exist in soft sediments and chalks by the presence of internal moulds shells replaced by microspar and mouldic porosity in early cemented hardgrounds in deep sections barren of aragonite the number of casts and replaced shells remains unchanged and is insignificant as compared to aragonitic biota probably originally present within the sediment 2 in ooze chalk vs 30 50 of grains in modern periplatform sediments therefore palaeontological information must be significantly biased through selective removal of aragonite rates of preservation and destruction depend on external factors during sediment accretion sedimentation rates clay content total organic carbon content and rates of fluid flow within the sediment these observations are relevant with respect to the diagenetic potential and patterns of fossil preservation in little cemented calcitic cool water carbonates because they may originally have contained more aragonite biota as important constituents of an ecosystem than is commonly suspected and calcite aragonite ratios in ancient carbonate sediments may not necessarily reflect original input signals climate or sea level

horizontal plate motion a key allocyclic factor in the evolution of the great barrier reef the great barrier reef complex of northeastern australia thins dramatically and becomes younger from north to south these variations are a consequence of the cenozoic northward movement of the indian australian plate the temperate climatic conditions that applied off northeast australia during the early tertiary were progressively replaced by tropical conditions the present day south to north facies distribution along the eastern australian continental margin mimics the cenozoic vertical facies sequence through the northern part of the great barrier reef region

mineralogical stabilization of high magnesium calcite geochemical evidence for intracrystal recrystallization within holocene porcellaneous foraminifera mineralogical stabilization of porcellaneous foraminifera is known to consist of chemical change without textural alteration at any scale however the nature of the alteration process has not been fully reconciled porcellaneous foraminifera from the freshwater diagenetic zone of two small islands in the schooner cays bahamas are in the midst of mg loss and delta 18 o change with no textural alteration at any scale these data indicate that the mineralogical stabilization process or recrystallization is a repetitive intracrystal incongruent dissolution precipitation reaction each recrystallization produces a calcite with a slightly lower mg content than its predecessor the rate of stabilization is dependent on time and hydrologic flux older phreatic zone material is the most altered and younger vadose zone material is the least altered numerical modeling of the chemical diagenesis indicates that the molar water rock ratio of a single recrystallization is mg between 0 0001 and 0 0003 these values are two orders of magnitude less than that generally assigned to calcite precipitation from a large fluid reservoir and they suggest that d mg is dependent on the type and scale of reaction though the nature of that dependency is unknown

quaternary crusts on slopes of the mediterranean sea a tentative explanation for their genesis abstract quaternary deep sea crusts i e lithified omission surfaces are widespread on slopes and subsea relief features in the western and especially the eastern mediterranean sea the lithification is induced by the early precipitation of a magnesian calcite cement in the uppermost few centimetres of a calcareous ooze based on their thickness and degree of hardening the collected samples have been divided into three main types each of them corresponding to a different degree of lithification except in rare probably old crusts collected at water depths below 2000 2500 m and in infills of some sponge skeletons or in the case of inferred internal tension recrystallization and generally mineral alterations are very limited the precipitation mechanisms appear to be essentially physicochemically controlled and ions involved in cementation come from the seawater reservoir by means of an inward diffusion of solutions diffusion is enhanced by bottom currents these currents maintaining a low or zero rate of sedimentation moreover lithification also depends on the chemical characteristics of the seawater i e supersaturation with respect to low magnesian calcite and low contents of phosphates and dissolved organic matter which would inhibit crystal growth local factors and primary composition of the original sediment also appear to influence the speed of early lithification when conditions of lithification are favourable a few hundreds of years to a few thousand years at the most seem sufficient for the first two stages of lithification to be reached the submarine lithification was active during the last glacial episode and as exhibited by numerous samples especially towards the end of this period the previous cold stages were probably also episodes of efficient lithification this correlation with quaternary climatic cycles certainly reflects an intensification of thermohaline and bottom current circulations during periods of severe climatic conditions there is also some evidence of holocene and even present day crusts