petrographic and geochemical analysis of caliche profiles in a bahamian pleistocene dune two caliche profiles from a pleistocene carbonate dune on san salvador island bahamas were examined by petrographic and geochemical analysis profile a is an immature buried caliche profile characterized by caliche pisolites a friable crust and abundant cerion profile b is a more well developed caliche profile at the top of the dune which contains abundant pisolites rhizomorphs laminated calcrete a breccia and abundant cerion geochemical changes in caliche profiles relative to the host rock are an increase in al2o3 fe2o3 and total organic carbon a decrease in mg and sr and a decrease in 13ccarb 18ocarb and 13corg the magnitude of these changes is probably a function of the duration of subaerial exposure and resultant colonization by dune plants and associated microflora abundance of calcified filaments and needle fibre crystals in profile a attests to the importance of microbial processes in the early development of caliche profiles biogenic structures are largely destroyed in profile b due to recrystallization but indirect evidence of biological activity is retained in the form of carbon isotope values

heterogeneity in geochemical expression of subaerial exposure in limestones and its implications for sampling to detect exposure surfaces abstract replicate sampling along stratigraphic horizons in one outcrop of ordovician limestones of the nashville dome reveals considerable variation in 13c 18o and sr concentration among seemingly identical samples ten vertical transects were sampled each consisting of samples taken at 10 cm intervals from 2 m below to 1 m above the m4 m5 sequence boundary of holland and patzkowsky 1997 1998 the 10 transects were each 2 m apart the resulting 10 replicate and thus seemingly interchangeable samples from each horizon have ranges of 13c as great as 2 4 and ranges of 18o as great as 2 8 and sr concentrations that vary by as much as a factor of four variance is great both just below and just above surfaces of subaerial exposure as a result maxima in variance in 13c as well as or in some cases instead of minima in mean 13c can be used to test such surfaces for exposure maxima in variance and minima in mean 13c may be found either below a surface of subaerial exposure in the meteorically altered sediment or above that surface in the transgressive lag where reworked pre exposure sediment is mixed with unaltered post exposure sediment the large variances and ranges found in this study and the dissimilarity of geochemical profiles from the 10 vertical transects indicate that single samples of stratigraphic horizons and thus single vertical transects can yield misleading characterizations of stratigraphic variation of geochemical parameters instead replicate sampling along horizons better characterizes isotopic and elemental variation through stratigraphic sections than does existing single sample strategies and it will be more useful in future testing of surfaces of subaerial exposure for example the replicate geochemical data used in this project confirm the presence of three surfaces of subaerial exposure whereas only two had been recognized as exposure surfaces before data from replicate samples also make unnecessary the use of arbitrary criteria for inferring exposure such as the predetermined numerical cutoffs used by railsback et al 2003

aeolianite calcrete microbialite and karst in southwestern australia as indicators of middle to late quaternary palaeoclimates abstract the mid late pleistocene and holocene aeolianites of the tamala limestone on the northern swan coastal plain in southwestern western australia consist of six members that show cyclic deposition of coastal aeolianite overlain by calcrete microbialite karstified surface and palaeosol field work combined with mineralogical chemical stable isotope analysis and uranium thorium u th and optically stimulated luminescence osl dating provides an insight into the repetitive glacial and interglacial climatic periods over the past 500 kyr deposition of the carbonate aeolianites occurred during interglacial episodes marine isotope stage mis 1 5 7 9 11 and possibly 13 due to migration of coastal dunes under the influence of strong southerly to southwesterly winds rainfall was insufficient to support vegetation cover on the dunes and so was probably limited or seasonal the transition from interglacial to glacial climates was characterised by higher effective rainfall accompanied by aeolianite dissolution and karstification during the colder climates and less effective rainfall of the glacial periods there was no carbonate sand deposition instead laminated microbialite and or laminar calcrete formed followed by palaeosol formation the oxygen isotope composition of the microbialites indicates average temperatures during glacial periods 4 c 8 c lower than today and 13 c values demonstrate a higher proportion of c4 plants and therefore a drier or more seasonal climate data from individual members of the tamala limestone show that the wettest interglacial period was mis 5 when extensive karstification and pinnacle development occurred and rainfall was probably higher than at any other time in the past 500 kyr of the glacial periods mis 10 was relatively wet or less seasonal whereas mis 8 and the last glacial maximum lgm were relatively dry and windy the low rainfall during the peak of the glacial periods was probably intensified by colder water offshore due to weakening of the leeuwin current and its replacement by the cold north flowing west australian current

triassic latemar cycle tops subaerial exposure of platform carbonates under tropical arid climate the triassic latemar platform in the dolomites italy is the site of several ongoing controversies perhaps the most interesting debate focuses on apparent cyclic deposition within the latemar platform whose nature and duration are still open to debate further disagreement concerns the lack of meteoric diagenesis related isotope shifts at cycle tops that bear circumstantial petrographic evidence for subaerial emergence here an evaluation of the nature of latemar cycle tops is presented combining evidence from previous work and new field petrographic and geochemical data cycle tops are ranked according to increasing exposure duration and spatial extent type i surfaces lacking unequivocal evidence of prolonged supratidal conditions type ii dolomite caps formed in warm evaporitic intertidal lagoonal waters followed by exposure of perhaps intermediate duration type iii clastic rich red calcareous horizons with some showing platform wide extent representing prolonged supratidal conditions and type iv discontinuities in tepee belts genetically related to type ii and iii surfaces but likely representing shorter lived exposure stages petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking this is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that where present are here interpreted as marine vadose in origin the scarcity of carbon and oxygen isotope signatures commonly assigned to subaerial exposure stages is best explained in the context of mid triassic climate the low latitude tropical but arid setting of the latemar situated in the western extension of the tethys ocean its isolation from nearby continental areas and overall short term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops high amounts of aeolian clastic material beneath some cycle tops along with high fe and mn elemental abundances argue for intermittent subaerial conditions this study proposes an enhancement of the classical allan and matthews 1982 isotope model for subaerial exposure under strongly arid climates as the subaerial exposure nature of latemar cycle tops and therefore eustasy as the cause for cyclicity have been previously challenged due to the lack of meteoric induced isotopic signatures the outcome of this study is of significance for the ongoing latemar stratigraphic controversy

kalahari valley calcretes their nature origins and environmental significance abstract calcretes that form in non pedogenic settings have been widely reported in the geomorphological and geological literature yet they are still poorly understood in comparison with pedogenic varieties this may be because there are assorted types of non pedogenic calcretes often loosely referred to as groundwater calcretes but encompassing groundwater phreatic open valley or confined channel calcrete types forming within vadose and phreatic environments in different geomorphological settings relatively few studies have described the detailed petrological characteristics of such calcretes leading wright and tucker calcretes international association of sedimentologists reprint series 1991 vol 2 blackwell scientific publications oxford p 10 to suggest that much more work is needed to define the groundwater calcrete facies and to devise criteria for its recognition so that the various non pedogenic calcretes can be identified within the quaternary as well as further back in the geological record most descriptions of groundwater calcretes are highly generalistic and do not take into account the variability that may occur as a result of calcretes forming in different positions within a landscape and affected by separate controls thus the range of groundwater calcretes that exists now needs to be studied individually to identify patterns within the distinct types as a step towards expanding our knowledge of the variety of non pedogenic calcretes this paper describes the petrology micromorphology and mode of development of one of these types of calcrete namely that which forms within valley locations but not in definite fluvial channels late quaternary to holocene valley calcretes are described from various dry valleys mekgacha across the kalahari region of central botswana with the majority of samples collected from trunk and tributary valleys of the okwa samples have been analysed in thin section and under scanning electron microscope in order to determine the carbonate matrix type and calcrete micromorphology in general most samples consist of grains of quartzose kalahari sand cemented by fine crystalline often glaebular grain coating and pore filling micrite cement types are fairly consistent and not as micromorphologically variable as has been noted for other kalahari calcretes biological inputs are prevalent in many samples and include networks of calcified rootlets these characteristics reflect the fact that the calcretes formed in a relatively near surface environment with relatively high rates of evaporation

characterization and interpretation of discontinuity surfaces in a jurassic ramp setting high atlas morocco discontinuity surfaces are widely recognized but often poorly understood features of epeiric carbonate settings in sedimentary systems these features often represent hiatus surfaces below biostratigraphic resolution and may represent a considerable portion of the time contained in the sediment record from an applied perspective discontinuities may represent horizontal flow barriers and result in reservoir compartmentalization here a total of 80 condensed surfaces s1 firmgrounds s2 and hardgrounds s3 from a jurassic middle and upper bajocian assoul formation ramp setting of the high atlas in morocco are carefully documented with respect to their morphology their secondary impregnation by fe and mn oxides and phosphates and their palaeoecological record a statistical frequency distribution of two surfaces of the s1 type 1 1 surfaces of the s2 type and 0 4 surfaces of the s3 type per 10 section metres is observed along a 220 m long carbonate succession based on two stratigraphically and spatially separated study windows and correlative sections the stratigraphic frequency distribution the lateral extent and the nature of facies change across discontinuities are documented in a quantitative manner specific features of the study site include the considerable stratigraphic thickness of the assoul formation and the conspicuous absence of subaerial exposure related features based on the data presented here firmground and hardground surfaces are best interpreted as maximum regression related features relative sea level lowstand results in a lowered wave base and wave orbitals and currents result in sea floor omission and lithification care must be taken to avoid overly simplistic interpretations as differences in bathymetry and carbonate facies result in marked changes in discontinuity characteristics in proximal distal transects the data shown here are of significance for those concerned with the interpretation of shoal water carbonate environments and are instrumental in the building of more realistic carbonate reservoir flow models

diagenesis of quaternary bahamian beachrock petrographic and isotopic evidence abstract carbonate beachrock is a common feature on many beaches of san salvador island bahamas these well indurated slabs of cemented beach sand are jointed normal and parallel to the shoreline and slope gently seaward constituent sediment grains are identical to those of adjacent unlithified beach sands beachrock exposed along the northern western and southern coastline of the island is commonly characterized by aragonite needle cement micritized grains and micritic envelopes which are features of early marine phreatic diagenesis these beachrocks have whole rock 13c values of 4 75 pdb and 18o values of 0 75 pdb which are identical to carbon and oxygen isotopic compositions of isolated aragonite needle cement and unlithified beach sand inclusion of recent artifacts in the rock indicates that these deposits are holocene beachrock from one locality on the western side of the island is covered with a calcrete crust indicating prolonged subaerial exposure and vadose weathering associated with the formation of organic rich soils petrographic features include low mg calcite cement evidence of neomorphism of aragonitic cement and micritic rims to low mg calcite random needle fiber calcite calcified filaments and micritic networks whole rock 13c values of these beachrocks range from 3 66 to 6 85 and 18o from 2 16 to 3 68 the wide range of 13c values in these samples is probably a function of the relative proportions of meteoric vadose to meteoric and or marine phreatic cements in individual samples isolated meteoric phreatic cements are only slightly enriched in the light isotopes relative to marine cements whereas meteoric vadose cements are significantly lighter the diagenetic features suggest the beachrock passed through marine phreatic to meteoric phreatic and meteoric vadose diagenetic environments this diagenetic sequence commonly forms in response to sea level lowering evidence of deposition in a shoreline environment and prolonged residence in meteoric phreatic and meteoric vadose environments suggests that these beachrocks may be late pleistocene

a petrological and geochemical study of the calcareous sandstone of west african marine pleistocene cap des biches senegal abstract in senegal the marine pleistocene is represented by several outcrops of calcareous sandstone slabs which underly the recent sandy shoreline south of dakar these outcrops show dissolution and recrystallization phenomena increasingly evident as one moves south they disappear at toubab dialao i e approximately 40 km from dakar the outcrop at cap des biches has been studied mainly because of the exceptional preservation of some ostrea calcitic tests which allowed isotopic u th measurements this outcrop of metric thickness was subjected to petrographic and mineralogical examination and to several geochemical analyses trace elements stable isotopes amino acids which lead to the conclusion that the base of the deposit has been preserved two u th apparent ages obtained 178 ka and 240 ka are older than the only age of the isotopic stage 5c obtained in the area a post depositional mobility of u cannot be rejected and ages may not be reliable however a minimal age considerably older than finite 14c ages previously published may be proposed eemian interglacial i e isotopic stage 5 in these environments geochemical analyses show high pleistocene sea levels of either warmer and or more diluted waters than the present levels i e a probable northern extension of guinean waters

deposition and diagenesis of marine oncoids implications for development of carbonate porosity abstract microbially mediated carbonates form in numerous environments and produce a range of lithologic fabrics and textures while the environmental and biological factors controlling microbial carbonate precipitation are becoming more established the influence of precursor microbial textures on diagenetic processes is less understood here we describe microbial oncolites from mare new caledonia to assess their formation textures and subsequent diagenetic alteration calcified filaments and sheets are interpreted as microbial mat constituents bacterial sheaths and eps sheets within a constructive micritic cortex of uniform crystal size 5 10 m and shape formed around a nucleus grain thin section analyses show trapped and bound grain inclusions and irregular laminations within the cortex also consistent with a microbial origin the combination of a dense micrite cortex subrounded shape and irregular laminations are consistent with oncoid formation under periodic wave agitation in a back reef setting the mare oncolites also display an unexpected pseudomorphic stabilization of the nucleus grains and cortex likely due to the original mineralogy and low permeability of the cortex micrite to our knowledge this is the first example of microbial coatings leading to pseudomorphic stabilization of aragonite and high mg calcite skeletal grains this textural preservation lies in stark contrast to the extensive dissolution and moldic porosity found outside the oncolite beds and common to many tropical shallow water carbonates these findings have implications for development of meteoric porosity the interpretation of 13c values in marine carbonates and potential controls on fluid flow

sedimentation and diagenesis of upper smackover grainstone jay field area west florida abstract the examination of core and logs from a well 3 mi 5 km southwest of the jay field has given considerable insight into the upper smackover facies distribution diagenesis and the application of recent models for the sedimentation and diagenesis of this jurassic reservoir a 63 ft 19 m thick unit of oolitic and oolite oncolite grainstones is recognized in the upper smackover high angle inclined bedding visible on both core and dipmeter with a consistent 15 to 20 northeast dip demonstrates the presence of oolite bars these bars formed a barrier which affected subsequent deposition and diagenesis in the jay field area a complicated diagenetic history of marine and vadose cementation and pervasive and selective dolomitization have left a unique imprint on the porosity and permeability of these rocks dipmeter results and petrographic analysis of the grainstones indicate that cementation and diagenesis have not been uniform within the large scale cross strata permeable beds are interstratified with tightly cemented or compacted impermeable beds horizontal flow should be greatest along the strike of the inclined units because the flow would remain within the permeable planes of the inclined strata thus dipmeter correlation permits an interpretation of the direction of bedding permeability anisotropy produced by the inclination of the pore system the characteristics of sedimentation and facies distribution in the jay field area have previously been compared with a modern analog from joulters cay in the bahamas the trucial coast of the persian gulf in the abu dhabi region may be a better model the style of deposition and distribution of carbonate and evaporite sediments and diagenetic characteristics in the grainstone barriers and lagoons closely fit the sedimentation and diagenetic pattern in the jay field area end of article last page 950

calcretes from a palaeosinkhole in jurassic palaeokarst subbetic southern spain abstract calcretes have been recognized in the bottom of a palaeokarstic sinkhole etched into early jurassic carbonates from the subbetic southern spain these are 3 to 10 cm thick and they show an internal structure in which three units can be distinguished the lower unit is composed of alternate irregular bands of unfossiliferous micrite and sparry calcite in the micrite several features point out the activity of endolithic microorganisms as fungi and bacteria the middle unit which has a laminated structure with palisade calcite spar and some vadose silt is interpreted as a speleothem and finally the upper unit shows calcrete pisoids and rhizocretions the isotope ratio analysis gives results in which 13 c ranges from 10 1 to 10 9 and 18 o ranges from 5 7 to 7 0 these values are very similar to those from palaeosols of other ages and localities which is an important argument favouring emersion stages with continental deposition in an island with an arid or semi arid climate nevertheless these calcretes outcrop in association with a palaeokarstic relief reflecting wet climate before the calcrete formation

palaeoenvironmental and diagenetic reconstruction of a closed lacustrine carbonate system the challenging marginal setting of the miocene ries crater lake germany chemostratigraphic studies on lacustrine sedimentary sequences provide essential insights on past cyclic climatic events on their repetition and prediction through time diagenetic overprint of primary features often hinders the use of such studies for palaeoenvironmental reconstruction here the potential of integrated geochemical and petrographic methods is evaluated to record freshwater to saline oscillations within the ancient marginal lacustrine carbonates of the miocene ries crater lake germany this area is critical as it represents the transition from shoreline to proximal domains of a hydrologically closed system affected by recurrent emergent events representing the boundaries of successive sedimentary cycles chemostratigraphy targets shifts related to subaerial exposure and or climatic fluctuations methods combine facies changes with 13c 18o chemostratigraphy from matrix carbonates across five closely spaced temporally equivalent stratigraphic sections isotope composition of ostracod shells gastropods and cements is provided for comparison cathodoluminescence and back scatter electron microscopy were performed to discriminate primary syn depositional from secondary diagenetic features meteoric diagenesis is expressed by substantial early dissolution and dark blue luminescent sparry cements carrying negative 13c and 18o sedimentary cycles are not correlated by isotope chemostratigraphy both matrix 13c and 18o range from ca 7 5 to 4 0 and show clear positive covariance r 0 97 whose nature differs from that of previous basin oriented studies on the lake negative values are here unconnected to original freshwater lacustrine conditions but reflect extensive meteoric diagenesis while positive values likely represent primary saline lake water chemistry noisy geochemical curves relate to heterogeneities in primary porosity resulting in selective carbonate diagenesis this study exemplifies that ancient lacustrine carbonates despite extensive meteoric weathering are capable to retain key information for both palaeoenvironmental reconstruction and the understanding of diagenetic processes in relation with those primary conditions also it emphasizes the limitation of chemostratigraphy in fossil carbonates and specifically in settings that are sensitive for the preservation of primary environmental signals such as lake margins prone to meteoric diagenesis this article is protected by copyright all rights reserved

environmental effect and genetic influence a regional cancer predisposition survey in the zonguldak region of northwest turkey the cretaceous eocene volcano sedimentary units of the zonguldak region of the western black sea consist of subalkaline andesite and tuff and sandstone dominated by smectite kaolinite accessory chlorite illite mordenite and analcime associated with feldspar quartz opal ct amphibole and calcite kaolinization chloritization sericitization albitization fe ti oxidation and the presence of zeolite epidote and illite in andesitic rocks and tuffaceous materials developed as a result of the degradation of a glass shards matrix enclosed feldspar and clinopyroxene type phenocrysts due to alteration processes the association of feldspar and glass with smectite and kaolinite and the suborientation of feldspar edged subparallel kaolinite plates to fracture axes may exhibit an authigenic smectite or kaolinite increased alteration degree upward in which al fe and ti are gained and si na k and ca are depleted is due to the alteration following possible diagenesis and hydrothermal activities micromorphologically fibrous mordenite in the altered units and the presence of needle type chrysotile in the residential buildings in which cancer cases lived were detected in addition the segregation pattern of cancer susceptibility in the region strongly suggested an environmental effect and a genetic influence on the increased cancer incidence in the region the most likely diagnosis was li fraumeni syndrome which is one of the hereditary cancer predisposition syndromes however no mutations were observed in the p53 gene which is the major cause of li fraumeni syndrome the micromorphology observed in the altered units in which cancer cases were detected may have a role in the expression of an unidentified gene but does not explain alone the occurrence of cancer as a primary cause in the region

the formation of the pinnacle karst in pleistocene aeolian calcarenites tamala limestone in southwestern australia abstract a spectacular pinnacle karst in the southwestern coastal part of western australia consists of dense fields of thousands of pinnacles up to 5 m high 2 m wide and 0 5 5 m apart particularly well exposed in nambung national park the pinnacles have formed in the pleistocene tamala limestone which comprises cyclic sequences of aeolian calcarenite calcrete microbialite and palaeosol the morphology of the pinnacles varies according to the lithology in which they have formed typically conical in aeolianite and cylindrical in microbialite detailed mapping and mineralogical chemical and isotopic analyses were used to constrain the origin of the pinnacles which are residual features resulting mainly from solutional widening and coalescence of solution pipes within the tamala limestone the pinnacles are generally joined at the base and the stratigraphy exposed in their sides is often continuous between adjacent pinnacles some pinnacles are cemented infills of solution pipes but solution still contributed to their origin by removing the surrounding material although a number of pinnacles contain calcified plant roots trees were not a major factor in their formation pinnacle karst in older better cemented limestones elsewhere in the world is similar in morphology and origin to the nambung pinnacles but is mainly influenced by joints and fractures not evident at nambung the extensive dissolution associated with pinnacle formation at nambung resulted in a large amount of insoluble quartz residue which was redeposited to often bury the pinnacles this period of karstification occurred at around mis 5e and there was an earlier less intense period of pinnacle development during mis 10 11 both periods of pinnacle formation probably occurred during the higher rainfall periods that characterise the transition from interglacial to glacial episodes in southern australia the extensive karstification around mis 5e indicates that the climate was particularly humid in southwestern australia at this time

record of late pleistocene oxygen isotopic stage 5 climate changes during episodes of karst development on the northern coast of crete sequence stratigraphic implications abstract in late pleistocene times during oxygen isotopic stage ois 5 aeolian calcareous sand dunes were deposited along the northern coast of crete where they now discontinuously crop out wedged between thin shallow marine tyrrhenian deposits late pleistocene marine sediments are scarce and amalgamated because the combination of slow uplift movement and low elevation of ois 5 highstand shorelines close to present mean sea level promoted their erosion in the shallow marine and subaerial environments the studied succession is organized in transgressive regressive t r sequences bounded below and above by transgressive surfaces of erosion superimposed on subaerial unconformities the vertical stacking of sedimentary facies between consecutive key surfaces demonstrates that the preservation potential of marine transgressive deposits tst was higher than that of regressive sediments similarly aeolian strata and palaeosols located landward were well preserved detailed sedimentological and petrographic studies of the aeolianites and their framing units from several sites revealed a complex microstratigraphy of diagenetic features that relate to 1 the succession of highstand shorelines during ois 5 and 2 episodes of subsurface karstification interpreted to result from soil cover development on limestone substrate during relative sea level falls a complex sequence of climate changes during karst formation is suggested by solution pipes that cross cut or contain calcified root systems and whose inner walls are covered with carbonate crusts interpreted as calcretes the repetition during ois 5 of this transition from rhizocretion formation to karstification to calcretization in the studied deposits is argued to occur in response to contrasting climate conditions at times of sea level lowstand it is proposed that palaeosols and rhizocretions formed during the falling limb of the relative sea level curve under semi arid conditions karstification developed at the turn between falling and rising sea level when climate was at its wettest and calcretization occurred during the rising limb of the relative sea level curve under arid conditions identification of wave cut surfaces of erosion on top of transgressive marine sediments and below aeolian deposits suggest that the latter are contemporaneous with falling sea level marine units deposited basinward below the lowstand shoreline namely forced regressive systems tracts frst otherwise not recognized in the field the complex suite of alteration processes recorded within the palaeosols above aeolian deposits are interpreted to represent periods extending from the latest stage of sea level fall to the earliest stage of sea level rise including lowstand systems tracts lst this proves a satisfactory complement to reconstruct palaeosea level fluctuations in settings where the shallow marine nature of sedimentary environments means that most regressive and lowstand deposits are not recognized in the stratigraphic record

carboniferous calcretes in the canadian arctic calcrete palaeosols have been found in the upper carboniferous canyon fiord formation of southwestern ellesmere island canadian arctic these calcretes were developed in fluvial and shallow marine sediments that accumulated within two adjacent subbasins in which the tectono sedimentary environment led to the deposition of five different sedimentary facies i floodbasin sandstones ii alluvial fan sandstones iii alluvial fan conglomerates iv braided fluvial sandstones and v shallow marine limestones nodular massive palaeosol profiles consisting of cryptic nodular massive and laminar horizons occur within the floodbasin sandstone and alluvial fan sandstone facies plugged palaeosol profiles consisting of cryptic plugged and laminar horizons are restricted to the alluvial fan conglomerate facies massive brecciated palaeosol profiles consisting of cryptic massive brecciated and laminar horizons occur essentially within the shallow marine limestone facies the relationships between calcrete profiles and sedimentary facies suggest that profile types were controlled mainly by the texture and composition of the parent material nodular massive profiles are restricted to silicate rich sandstone hosts plugged profiles are restricted to carbonate rich conglomerate hosts and massive brecciated profiles are restricted to limestone hosts important relationships also exist between the maturity levels of nodular massive profiles and sedimentary facies profiles are generally mature in the floodbasin sandstones invariably immature in the alluvial fan sandstones and absent from the braided fluvial sandstones these different maturity levels were probably controlled mainly by exposure time vegetation and substrate composition

petrography and environmental controls on the formation of phanerozoic marine carbonate hardgrounds early marine seafloor lithification of carbonate sediments leads to the formation of hardgrounds and is known from rocks as old as the proterozoic hardground surfaces however are more commonly recorded in the phanerozoic sedimentological archive while ecological studies of hardground biota abound the environmental and physico chemical parameters leading to the development of seafloor lithification remain in many cases poorly understood and documented this paper reviews published evidence on the petrography mineralogy and geochemistry of phanerozoic carbonate hardgrounds within a process oriented context of their environmental controls hardgrounds typically develop in warm and shallow agitated tropical waters that are saturated with respect to caco3 but are also reported from hemi pelagic to bathyal realms and from cool water temperate settings a range of early marine cement types are documented from present day hardgrounds whereas preserved cement fabrics and related mineralogies are less diverse in early phanerozoic hardgrounds carbonate hardgrounds are widespread during calcite sea periods e g ordovician and cretaceous as opposed to some preservation bias ancient aragonite seas e g permian and triassic obviously the relation between seawater chemistry and hardground abundance and attributes is not an easy one here the concept of aragonite versus calcite seas serves as a first order template for the sake of a structured discussion whereas modern aragonite mode oceans document a significant diversity in spatial and bathymetric seawater properties this spatio temporal complexity is perhaps reflected in the scarce record of calcite sea hardgrounds from the devonian or paleogene rock record contrasted by the abundance in aragonite holocene and recent seas holocene hardgrounds allow for the direct assessment of rates of and reasons for early subaquatic lithification and in most cases escaped subsequent non marine diagenetic overprint conversely studies of ancient hardgrounds are often biased by early biological and mechanical erosion the degree of diagenetic overprint and tectonic to orogenic processes the formation distribution and physical properties of hardgrounds depend on the global climatic context greenhouse icehouse modes on sea level changes on ocean stratification on the spatial extent of epicontinental seas and carbonate depositional environments and on the spatially different mineralogies and rates of carbonate production the fabrics and mineralogies of early hardgrounds cement seem to substantially hinge on variations in atmospheric co2 thus the application of lessons from modern to ancient hardground case examples is not straightforward a holistic model explaining the full variability of controls on submarine hardground formation and their cement petrography throughout the phanerozoic is as yet lacking a more critical view on secular changes in hardground formation patterns is clearly needed future work should rely on the combination of sedimentological stratigraphic palaeoecological petrographic and geochemical approaches specific emphasis should be given to time intervals triassic carboniferous cambrian etc where the bulk of reported case studies is at best scarce

stable isotope signatures associated with palaeosols pennsylvanian holder formation new mexico the holder formation pennsylvanian virgilian of southern new mexico usa consists of limestones interbedded with siliciclastics it was deposited during times of glacio eustatic sea level change and was exposed subaerially during multiple sea level lowstands microcomponents and whole rock samples of limestones were analysed for 13c and 18o values to examine the method of whole rock isotopic analysis for detecting subaerial exposure events and to determine the diagenetic processes acting during subaerial exposure whole rock isotopic shifts are not consistently present across petrographically identified subaerial exposure surfaces apparently whole rock isotopic shifts do not result from wholesale replacement of the host sediment during soil formation however the isotopic shifts are present in calcareous soil precipitated microcomponents such as rhizoliths laminated crusts and soil precipitated cements the components are heterogeneous in isotopic composition but converge on a meteoric calcite line at about 18o 5 5 these microcomponents are heterogeneous in distribution and may either dominate or be a minor constituent of the whole rock at a single stratigraphic horizon without petrographic selection of palaeosol components the detection of whole rock isotopic shifts may depend on the selection or chance sampling of a rock containing abundant microcomponents precipitated in a soil environment only minor whole rock isotopic shifts come from those rocks bearing no evidence of exposure and bearing lithological characteristics suggesting subaerial exposure was unlikely

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

paleosols in shallow marine carbonate sequences abstract shallow water carbonate sequences in the geological record commonly exhibit numerous exposure surfaces reflecting emersion of sediment due to progradation processes and sealevel oscillations some ancient platform sequences contain hundreds of such surfaces associated with paleosols fossil soils typically calcretes a wide range of macro and microstructures are associated with these calcretes which are reviewed profile types and stages in profile development can be defined the nature of the exposure surface and associated paleosols can be used to identify the nature of the prevailing climate although hydromorphic effects related to marine flooding of the platforms can modify the paleosols a potentially important use of paleosols is to define the nature of sequence boundaries several studies have identified sets of paleosols with varying maturities reflecting changes in rates of accommodation space creation during sealevel rises and falls

podogenic columnar calcite from the oolite group lower carboniferous south wales abstract calcrete palaeosols in the oolite group have microspar hardpan layers overlying clay rich nodular layers which in turn overlie the degraded host oolite within and immediately below hardpan layers there are extensive sheet like and fan developments of columnar calcite with individual crystals up to 30 cm long crystals range from those with straight extinction and normal growth zonation through feathery crystals to those showing sweeping extinction the calcretes accumulated within a seasonal climate with hardpan accumulation probably occurring through evaporation during the dry season the columnar calcite precipitated from more dilute solutions with maximum growth probably occurring during the wet season precipitation may have been partly or wholly bacterially mediated or induced by the reduction in p co 2 of fluids as they migrated out of the extremely restricted microporosity of the hardpan columnar fabrics reflect low supersaturations relative to those at which the hardpan accreted inhibition of crystal nucleation and of growth on x axis parallel faces and an excess of co 2 2 availability over ca 2 growth and nucleation may have been inhibited by the presence of particular organic compounds or clays the range in crystal form is related to the degree of poisoning of crystal growth faces by organic compounds

a holocene calcrete from north yorkshire england implications for interpreting palaeoclimates using calcretes glacial gravels of late devensian dimlington stadial age 26 000 13 000 years bp at west tanfield north yorkshire england have been cemented by carbonate rich solutions to produce a strongly indurated calcrete horizon the low mg cements occur as drusy spar needle fibres alveolar septal structures micrite and micropinnacles indicative of vadose zone cementation some complex pore partition structures attributed to precipitation along meniscus films also occur these partitions separate air dominated and water dominated microenvironments of the vadose zone the abundance of vadose fabrics shows that the accumulation is not a groundwater calcrete in addition much of the carbonate appears to have been precipitated by biological mediation carbon and oxygen isotopic data suggest that the carbonate did not form as a result of freezing as has been suggested for some arctic soil carbonates the pollen history of the area since the devensian suggests that this calcrete precipitated at low temperatures this contrasts with widely reported occurrences of calcrete in soils of hot arid or semi arid regions and suggests that palaeo calcretes should not be used as absolute palaeoclimatic indicators the unusual occurrence albeit localized of a thick calcrete under a cool and wet climate probably reflects the well drained nature of the gravels the abundance of caco3 as limestone clasts in the gravel and a high degree of biological activity beneath a forest cover which created a local environment favouring carbonate precipitation