Original Carbonate Mineralogy Research Articles

Resetting of Shallow-Water Carbonate Boron Isotope Values During Marine Burial Diagenesis

The boron isotopic composition (δ11B) of bulk carbonates may provide an archive to reconstruct changes in ocean pH. Reconstructions from ancient carbonates typically assume that no significant resetting of δ11B occurred during marine burial diagenesis. However, our understanding of B isotopic behavior associated with this process remains limited. Here we provide measurements of B/Ca and B isotopic composition (δ11B) from a modern peri-platform carbonate sequence near the Great Bahama Bank that has undergone marine burial diagenesis. Our results reveal significant decreases in both δ11B (~13 ‰) and B/Ca (~80 %) of bulk carbonates with depth. We attribute this pattern to the release of isotopically light B (δ11B ~20 ‰) to porewater during aragonite dissolution, with uptake of substantially isotopically lighter borate ions (δ11B ~-1 ‰) from porewater by newly forming low-Mg calcite. A quantitative model adds further support for this interpretation and provides an estimate of average neomorphism rate ( k0) in the range of 1×10-6 to 5×10-6 yr-1, which is comparable to previous rate estimates for neomorphism and/or recrystallization during meteoric diagenesis. Our results demonstrate the strong potential for resetting δ11B signatures in bulk carbonates during recrystallization, which must be considered in future attempts to reconstruct pH and pCO2 from these records and may require reinterpretation of existing records. Our results also suggest the potential of B isotopes as a proxy for carbonate recrystallization/neomorphism and original carbonate mineralogy.

Re-evaluation of original carbonate mineralogy, an integration of petrographic and geochemical data, Southwestern margin of Neo-Tethys Ocean

Re-evaluation of original carbonate mineralogy, an integration of petrographic and geochemical data, Southwestern margin of Neo-Tethys Ocean

238U/235U in calcite is more susceptible to carbonate diagenesis

The uranium isotopic composition (δ238U) of bulk marine calcium carbonates has been extensively explored as a promising paleoredox proxy to track the extent of global oceanic anoxia in deep time. Multiple studies have examined whether primary calcium carbonates can directly capture seawater δ238U and whether bulk measurements of recent and ancient carbonates preserve seawater U isotope signatures. Here we assess the role of diagenesis in altering δ238U signatures in carbonates sediments that have a primary calcitic mineralogy at the Paleocene-Eocene Thermal Maximum (PETM), an interval with rapid global warming and oceanic deoxygenation at ∼56 million years ago.Although primary abiotic and biogenic calcium carbonates (aragonite and calcite) can directly capture seawater δ238U with small offsets (<0.1‰) relative to modern seawater, diagenetic alteration of Bahamian shallow-water platform carbonate sediments that have a predominantly primary aragonitic mineralogy resulted in significantly larger offsets (up to 0.6‰). Since U concentration in primary aragonite is at least one order of magnitude higher than primary calcite (>1 ppm vs. <0.1 ppm), δ238U in calcite should be even more susceptible to diagenesis than that in aragonite.We find strong evidence of this effect in analysis of δ238U in PETM shallow-water carbonate sediments from Drilling Project (ODP) Hole 871C (Limalok Guyot, Pacific Ocean). Our results reveal large fluctuations in bulk carbonate δ238U from −0.69 to +0.71‰ around the PETM boundary but consistently heavier δ238U (between −0.14 and +0.47‰) than modern seawater outside of this interval. The significantly lighter δ238U values than modern seawater were interpreted to result from the operation of a Mn oxide shuttle. The heavier δ238U values are most likely caused by authigenic reductive accumulation of U(IV) in pore waters below the sediment-water interface. We found that carbonate δ238U values higher than modern seawater tend to increase with increasing U/Ca. This relationship is well-explained by an authigenic reductive accumulation model that simply assumes addition to primary calcite during diagenesis of calcitic cements containing isotopically heavier U(IV).Our work confirms expectations that δ238U in primary calcite is more susceptible to the amount of diagenetic cementation compared to primary aragonite, and that variations of δ238U in carbonate sediments with a primary calcitic mineralogy would more dominantly reflect the local redox state of depositional and early diagenetic environments. It is essential to identify the original carbonate mineralogy, the diagenetic history, and constrain the redox state of local deposition environments of sedimentary carbonate rocks when applying bulk carbonate δ238U as a global proxy for oceanic anoxia in deep time.

Microfacies, geochemistry and sequence stratigraphy of the Sarvak Formation (Mid Cretaceous) in the Kuh-e Siah and Kuh-e Mond, Fars area, southern Iran

During Middle Cretaceous (Late Albian-Early Turonian) period, the Sarvak Formation prevailed in the coastal Fars area (Zagros), southern Iran. This formation is one of the main hydrocarbon reservoirs in the Zagros Basin which has gradually been deposited on the Kazhdomi Formation and unconformably covered by the Laffan shales. This study used thin section descriptions and geochemical analyses to determine the depositional environment of the Sarvak carbonates. Based on the petrographic studies, fifteen microfacies were recognized. These microfacies were deposited in the lagoon, shoal, back reef, reef, fore-reef, shallow open marine and deep open marine facies belts. Facies analysis indicates that the Sarvak limestones were deposited in a carbonate ramp environment which changed to the rimed shelf environment towards the top of the sequence. The minor elements (such as Sr and Mn) and petrographic studies indicate that microfacies could be separated from each other by using trace element values. The aragonite was original carbonate mineralogy These carbonates were affected by the meteoric diagenesis in a semi-closed system. Six transgressive-regressive third-order sequences have been identified. The sequence boundaries have been determined with a sudden change in the depositional and important diagenetic processes related to the relative sea level fall. The maximum flooding surfaces have been recognized by the high value gamma ray log and abundant bioturbation related to the deep facies which separates TST from RST.

The impact of the Cretaceous–Paleogene (K–Pg) mass extinction event on the global sulfur cycle: Evidence from Seymour Island, Antarctica

The Cretaceous–Paleogene (K–Pg) mass extinction event 66 million years ago led to large changes to the global carbon cycle, primarily via a decrease in primary or export productivity of the oceans. However, the effects of this event and longer-term environmental changes during the Late Cretaceous on the global sulfur cycle are not well understood. We report new carbonate associated sulfate (CAS) sulfur isotope data derived from marine macrofossil shell material from a highly expanded high latitude Maastrichtian to Danian (69–65.5 Ma) succession located on Seymour Island, Antarctica. These data represent the highest resolution seawater sulfate record ever generated for this time interval, and are broadly in agreement with previous low-resolution estimates for the latest Cretaceous and Paleocene. A vigorous assessment of CAS preservation using sulfate oxygen, carbonate carbon and oxygen isotopes and trace element data, suggests factors affecting preservation of primary seawater CAS isotopes in ancient biogenic samples are complex, and not necessarily linked to the preservation of original carbonate mineralogy or chemistry. Primary data indicate a generally stable sulfur cycle in the early-mid Maastrichtian (69 Ma), with some fluctuations that could be related to increased pyrite burial during the ‘mid-Maastrichtian Event’. This is followed by an enigmatic +4‰ increase in δ34SCAS during the late Maastrichtian (68–66 Ma), culminating in a peak in values in the immediate aftermath of the K–Pg extinction which may be related to temporary development of oceanic anoxia in the aftermath of the Chicxulub bolide impact. There is no evidence of the direct influence of Deccan volcanism on the seawater sulfate isotopic record during the late Maastrichtian, nor of a direct influence by the Chicxulub impact itself. During the early Paleocene (magnetochron C29R) a prominent negative excursion in seawater δ34S of 3–4‰ suggests that a global decline in organic carbon burial related to collapse in export productivity, also impacted the sulfur cycle via a significant drop in pyrite burial. Box modelling suggests that to achieve an excursion of this magnitude, pyrite burial must be reduced by >15%, with a possible role for a short term increase in global weathering rates. Recovery of the sulfur cycle to pre-extinction values occurs at the same time (∼320 kyrs) as initial carbon cycle recovery globally. These recoveries are also contemporaneous with an initial increase in local alpha diversity of marine macrofossil faunas, suggesting biosphere-geosphere links during recovery from the mass extinction. Modelling further indicates that concentrations of sulfate in the oceans must have been 2 mM, lower than previous estimates for the Late Cretaceous and Paleocene and an order of magnitude lower than today.

Neoproterozoic marine carbonates and their paleoceanographic significance

Abstract The primary mineralogy of marine carbonate precipitates has been a crucial factor in constraining the major element composition of ancient oceans. Secular changes in Phanerozoic marine chemistry, including Mg/Ca, have been well-documented using the original carbonate mineralogy of ooids, marine cements and biominerals. However, the history of Precambrian seawater chemistry is not as well constrained, partially due to the prevalence of dolomitisation in the Precambrian geological record. The Neoproterozoic (~ 1000 Ma to ~ 541 Ma) record of primary carbonate mineralogy is documented here using a combination of literature data and new analysis of marine carbonate precipitates from the Otavi Fold Belt, Namibia, the Death Valley succession, USA and the Adelaide Fold Belt, Australia. These data suggest that the last ~ 460 million years of the Proterozoic were dominated by aragonite and high-Mg calcite precipitation in shallow marine settings. In contrast, low-Mg calcite has only been recognised in a small number of formations. In addition to aragonite and calcite precipitation, marine dolomite precipitation was widespread in Neoproterozoic oceans, including mimetic (syn-sedimentary) dolomitisation and primary dolomite marine cementation. The combination of marine aragonite, high Mg-calcite and dolomite precipitation during the Neoproterozoic suggests extremely high seawater Mg/Ca conditions relative to Phanerozoic oceans. Marine dolomite precipitation may also be linked to widespread marine anoxia during this time.

Microfacies Analysis, Geochemical and Petrographic Evidences for Recognition of Original Carbonate Mineralogy of Dalan Formation in the South Pars gas field

Microfacies Analysis, Geochemical and Petrographic Evidences for Recognition of Original Carbonate Mineralogy of Dalan Formation in the South Pars gas field

Inorganic control on original carbonate mineralogy and creation of gas reservoir of the Upper Jurassic carbonates in the Kopet-Dagh Basin, NE, Iran

The Upper Jurassic Mozduran Formation (Oxfordian–Tithonian) is the main petroleum reservoir in the Kopet-Dagh Basin, northeast Iran, which consists predominantly of carbonate rocks with subordinate evaporites and siliciclastics deposited in platform to deep marine settings of a subtropical sea. Detailed field surveys, petrographic investigations, facies and wire line log analyses were carried out at eight surface sections and four wells in the Kopet-Dagh Basin. Integration of petrographic and isotopic data suggests primary low-Mg calcite mineralogy of Oxfordian and Tithonian ooids. On the other hand, in the wells, Kimmeridgian ooids and cements are dominantly aragonitic and high-Mg calcite mineralogy (HMC). Marine cements with isopachous, fibrous and isopachous bladed fabrics indicate original aragonite and HMC mineralogy, respectively. The domination of aragonite mineralogy could be related to increased hypersalinity, evaporite precipitation and consequently an increase in Mg/Ca ratio, which resulted in formation of aragonite in preference to calcite. Preserved ooids with radial and concentric cortices in shallow-water settings that are nearby siliciclastic source, together with aragonitic and HMC ooids accompanied by evaporites in the drilled fields, suggest original mineralogy was probably controlled inorganically following local salinity variations. This study suggests that Kimmeridgian pay zones are mainly controlled by depositional facies, aragonitic and HMC mineralogy, and diagenetic processes such as dolomitization and dissolution.

Geochemistry, microfacies and diagenetic evidences for original aragonite mineralogy and open diagenetic system of Lower Cretaceous carbonates Fahliyan Formation (Kuh-e Siah area, Zagros Basin, South Iran)

The Fahliyan Formation is an important reservoir unit of Cretaceous strata in the south of Iran (Zagros Mountains). This formation with the age of Lower Cretaceous overlies unconformably the Hith Formation and is conformably covered by the Gadvan Formation in the studied area. To evaluate the original carbonate mineralogy, paleotemperature, and diagenetic environment in Kuh-e Siah anticline in Boushehr Province, the Fahliyan Formation was investigated by petrographic and geochemical analyses. Petrographic studies led to the recognition of 25 microfacies that were deposited in four facies belts: tidal flat, lagoon, and shoal in inner ramp and shallow open marine in mid-ramp environment. Of these microfacies, twenty-one belong to inner ramp and four are located in the mid ramp. Calcareous algae and benthic foraminifera are abundant in the shallow-marine carbonates of the Fahliyan Formation. Evidences such as absence of calciturbidite deposits and reefal facies, the occurrence of widespread tidal flat deposits, gradual facies changes, and abundant micrites indicate that the Fahliyan Formation was deposited in a homoclinal carbonate ramp environment. Abundant aragonite skeletal and non-skeletal components and presence of dolomites in Fahliyan carbonates indicate original aragonite mineralogy. Radial ooids generated in low-energy environments similar to modern aragonitic forms. Deformed and spalled ooids as well as shattered micritic envelopes indicate aragonite dissolution during meteoric diagenesis. Isopachous and fibrous sparry calcite cements resemble modern aragonite morphologies. Micritization, geopetal fabric, bioturbation, neomorphism, compaction, cementation, dissolution, dolomitization, dedolomitization, and fracturing are diagenetic processes in the Fahliyan Formation, occurring in marine to meteoric and burial diagenetic environments. Cementation, dissolution, and dolomitization are the main diagenetic processes that affected the original texture. Values of major (Ca and Mg) and minor (Sr, Na, Fe and Mn) elements (such as high Sr value) and δ18O and δ13C designate that the Fahliyan carbonates were deposited in a shallow warm-water sub-tropical environment and aragonite was the original carbonate mineralogy. Variations of Sr/Ca and δ18O values versus Mn suggest that diagenetic alteration must have occurred in an open diagenetic system, with high water–rock interaction. Dolomitization, dissolution and cementation are the main diagenetic features observed in the Fahliyan Formation of Kuh-e Siah and confirm high water–rock interaction in open diagenetic system. Dissolution as leaching is the most important diagenetic event in the evolution of porosity, particularly where green algae and other benthic foraminifera are abundant. This may indicate that the original carbonate mineralogy of the mentioned bioclasts might be aragonite, rather than low-Mg calcite. Temperature calculation based on the oxygen isotope value of the least-altered sample, using Anderson and Arthur (Stable isotopes in sedimentary geology, vol 10. Society of Economic Paleontologists and Mineralogists. Short Course, pp 1–151, 1983) equation, indicates paleotemperature to be around 29.1 °C.

Original Mineralogy and Recognition of Upper Boundary of the Sarvak Formation Based on Geochemistry and Isotope Studies

Carbonate sequence of upper cretaceous (Cenomanian) of Sarvak Formation is a part of Bangestan Group with the thickness of 760 m in Ahvaz oil field (well no. 63). This formation is overlain by the Kazhdumi Formation and is uncomfortably underlain by the Ilam Formation. In this study major and minor elements and carbon and oxygen isotope values and bivariate plot of them indicate that aragonite was the original carbonate mineralogy of the Sarvak Formation. Variations of Sr/Ca and δ18O values versus Mn also illustrate that they were affected by nonmarine diagenesis in a nearly closed system. Recognition of the exact boundary between the Sarvak and Ilam Formations is difficult, due to similar lithologies. So in this study, elemental and oxygen and carbon isotope analysis were used to determine the exact boundary between these Formations. Geochemical data clearly indicate a sub aerial exposure surface, below which meteoric diagenesis effected the sediments.

Petrography and geochemistry of Paleocene–Eocene limestones in the Ching-dar syncline, eastern Iran

The Ching-dar syncline is located to the west of the city of Birjand, in the east of Iran. The ca. 500 m thick studied section at the eastern flank of the syncline contains a sequence of almost continuous shallow-marine limestones that exhibit no major sedimentary breaks or evidence for volcanic activity. Skeletal grains consist of large benthic foraminifera and green algae whereas non-skeletal grains are mostly peloids and intraclasts. They were deposited on a shallow-marine carbonate ramp. The limestones have undergone extensive diagenetic processes with varying intensities, the most important of which are micritization, cementation, compaction (chemical and mechanical), internal filling and stylolitization. Chemical analysis of the limestone samples revealed high calcium and low magnesium content. Major and minor element values were used to determine the original carbonate mineralogy of these limestones. Petrographic evidence and elemental values indicate that calcite was the original carbonate mineral in the limestones of the Ching-dar syncline. The elemental composition of the Ching-dar carbonates also demonstrates that they have stabilized in a meteoric phreatic environment. Variation of Sr/Ca vs. Mn values suggests that diagenetic alteration occurred in an open geochemical system.

Microfacies and geochemical evidence for original aragonite mineralogy of a foraminifera-dominated carbonate ramp system in the late Paleocene to Middle Eocene, Alborz basin, Iran

The Ziyarat Formation is a Late Paleocene to Middle Eocene carbonate sequence, located in the north of Tochal Village (south-east of Tehran). The Ziyarat Formation with a total thickness of 212.5 m conformably overlies the Fajan conglomerate and is overlain by greenish tuffaceous siltstone of the Karj Formation. Petrographic studies have led to the recognition of 11 microfacies. Of these microfacies, five belong to inner ramp, four belong to middle ramp and two are located in the outer ramp. Due to the great diversity and abundance of large benthic foraminifera (e.g., Nummulites, Alveolina, Discocyclina), the Ziyarat Formation represents an example of “foraminifera-dominated carbonate ramp settings”. Paleobathymetry, light level and water energy were determined during deposition of Ziyarat carbonates based on the test shape and size variation of different types of benthic foraminifera and other components in different facies. Micritization, cementation, compaction, dissolution, dolomitization and fracturing are important diagenetic processes in the Ziyarat Formation, occurring in marine to meteoric and burial diagenetic environments. Geochemical studies indicate that Ziyarat carbonates were deposited in a shallow warm-water sub-tropical environment and aragonite was the original carbonate mineralogy with mean Sr values of 3,470 ppm. It should be noted that the maximum Sr content in abiotic calcite is 1,000 ppm. In Ziyarat limestone, dissolution as leaching is the most important diagenetic event in the evolution of porosity, particularly where Nummulite and other benthic foraminifera are abundant. This may indicate that the original carbonate mineralogy of Nummulites might be aragonite, rather than low-Mg calcite. In the Jahrum Formation of Iran and in the El Garia Formation in north-east of Libya, the widespread moldic porosity in Nummulite beds enhanced the reservoir potential of the carbonate sequence. Thus, it is proposed that the original carbonate mineralogy of nummulitid shells may be varied in different environments, and those with metastable mineralogy are excellent for forming hydrocarbon reservoirs. Bivariate plots of Mn versus Sr/Ca and δ18O values illustrated that Ziyarat limestones were affected by open diagenetic system with high water/rock interaction. Early burial diagenetic temperature calculation based on heaviest oxygen isotope values of micrite and δw of Eocene seawater of −0.85 SMOW showed that temperature was around 39 °C. Cathodluminescence studies of carbonate cements illustrated dull luminescence, because these carbonates were affected mainly by burial diagenesis. This statement is confirmed by δ18O and δ13C isotope evidences. Based on different facies and evidences such as absence of reefal facies, calciturbidite deposits, the occurrence of widespread tidal flat deposits and abundant micrites in all microfacies indicate that the Ziyarat Formation was deposited in a homoclinal carbonate ramp environment. In addition, many researchers suggested that due to extreme global warming during the Paleocene–Eocene time, platform margin coral reefs declined in low latitude and heterozoan calcitic foraminifers became abundant and, thus, widespread carbonate ramp systems developed.

Depositional facies and geochemistry of the Kangan formation in the South Pars Field, Persian Gulf (Iran)

The Kangan Formation is one of the largest carbonate reservoirs located in the South Pars Field, which is one of the major gas fields in the central basin of the Persian Gulf. Facies analyses indicate that the Kangan carbonates are formed in four microfacies belts: tidal flat, lagoon, shoal, and shallow open marine, in a platform ramp environment. The predominance of tidal flat sediments along with the oolitic shoal facies and lack of a large shoal reef indicate a homoclinal-ramp system. Major and minor elements and carbon and oxygen isotope values were used to determine the original carbonate mineralogy of the Kangan Formation. The data indicate aragonite carbonate mineralogy in a closed diagenetic system. The Kangan Formation displays a gradual upward increase in δ13C values and variation in δ18O with changing lithologies. Palaeotemperature calculation, based on the heaviest δ18O values in limestone micrites and dolomicrites, range from 36 to 44 °C, respectively. The isotopic compositions of the Kangan carbonates indicate that they were deposited in shallow tropical waters. Stratigraphic analysis indicates four major transgressive and regressive events of nearly two million years’ average duration, which controlled the development of two depositional sequences and their systems tracts.

Microfacies and geochemistry of the Ilam Formation in the Tang-E Rashid area, Izeh, S.W. Iran

The Ilam Formation (Santonian–Campanian in age), part of the Bangestan Group, is disconformably overlain by the Sarvak Formation and underlain by the Gurpi Formation in the Tang-E Rashid, Peyon area, Izeh (Zagros), southwest of Iran. Facies analyses indicate that the Ilam carbonates formed in four microfacies belts: tidal flat, lagoon, shoal and open marine, in a platform ramp environment. Major and minor elements and carbon and oxygen isotope values were used to determine the original carbonate mineralogy of the Ilam Formation. Petrographic evidence and elemental and oxygen and carbon isotope values indicate that aragonite was the original carbonate mineralogy in the Ilam Formation. The elemental and isotopic compositions of the Ilam carbonates also illustrate that they have stabilized in the marine phreatic environment. Variations of Sr/Ca and δ 18O values versus Mn suggest that diagenetic alteration occurred in a closed system. Temperature calculation based on the oxygen isotope value of the least-altered sample indicates that the very early shallow burial fluid temperature was around 28 °C. Recognition of the exact boundary between the Ilam and Sarvak Formations is difficult, due to similar lithologies and the absence of the Surgah Formation in the study area. However, elemental and oxygen and carbon isotope analysis were used to determine the boundary between these formations. The δ 18O and δ 13C values, along with elemental results, clearly indicate a subaerial exposure surface, below which meteoric diagenesis affected the sediments.

Cortical fabrics of Upper Jurassic ooids, Arab Formation, Saudi Arabia: Implications for original carbonate mineralogy

This study reports the occurrence of partly leached ooids, interpreted to have been originally aragonitic, that still preserve intra-cortical fabric details in the Upper Jurassic of Saudi Arabia. A thin dark layer of insoluble material (probably organic matter) separates adjacent carbonate lamellae. Selective leaching of carbonate occurs in each lamella, yielding intra-cortical moldic porosity. It is the presence of the thin dark layers that allows fabric details to be preserved during and after leaching of the ooids. Locally, the intra-cortical moldic pores may be infilled by finely crystalline equant calcite cement that is interpreted to be of meteoric origin. In all cases, this equant calcite cement has nucleated on top of the underlying dark layer of insoluble material and has grown upward and outward until reaching the next layer. If allowed to go to completion, crystal growth in this fashion will produce a radial fabric, similar to or even mimicking that seen in many ancient ooids. This is an important observation because well-preserved radial fabrics seen in ancient ooids have previously been interpreted to represent original calcite mineralogy. Results from this study suggest that a well-preserved radial calcite fabric may be indicative of either originally aragonite or calcite mineralogy.

A re-evaluation of aragonite versus calcite seas

Some workers have argued that the mineralogy of ancient carbonates may have been different from that of modern sediments, with calcite being considered the dominant mineral during the Ordovician, Devonian-mid Carboniferous, and Jurassic-Cretaceous to Early/Middle Cenozoic (e.g. Sandberg 1983; Wilkinson and Algeo 1989). Variation in carbonate mineralogy has been related to the position of global sea level (emergent or submergent modes, Wilkinson et al. 1985), change in rates of seafloor spreading (e.g. Mackenzie and Pigott 1981; Hardie 1996),PCO 2 level (e.g. Sandberg 1985; Mackenzie and Morse 1992; Hallock 1997) and Mg/Ca ratios related to spreading rate (e.g. Stanley and Hardie 1998). However, other researchers suggest that the assumption of change of original carbonate mineralogy through time needs to be re-evaluated in the light of mineralogical change that is related to water temperature or latitudes (e.g. Nelson 1988). Evaluation of Ordovician (Arenig to Ashgill) Gordon Group carbonates of Tasmania (Australia), based on petrographic (e.g. abundantChlorozoan biota, and oomold texture) and geochemical criteria (such as high Sr/Na ratios) indicated that aragonite (not calcite) was the predominant mineral in these warm water, subtropical carbonates (Rao 1990). Petrographic (e.g. presence or absence of aragonite relicts, abundant acicular to fibrous isopachous marine cement, presence of diffuse laminae and a number of spalled ooids) and geochemical evidences (such as elevated Sr) in the Upper Jurassic Mozduran limestone, in Kopet-Dagh Basin in northeast Iran, showed variation in carbonate mineralogy, in spite of similar atmosphericPCO 2 level, global sea-level and tectonic setting. This is evidenced by aragonite occurring in the shallowest part of the basin (Adabi and Rao 1991) and mainly calcite with some aragonite forming in the relatively deeper water (below wave base) (Adabi 1997). Carbonate mineralogy in Recent shallow marine carbonates, and in experimental studies, varies with seawater temperature. In the Recent, aragonite is the predominant mineral in warm, shallow marine carbonates and calcite the dominant mineral in marine cool water carbonates (James and Clarke 1997). Therefore, variations in carbonate mineralogy in the Mozduran limestone are attributed to seawater temperature assuming invariant seawater chemistry prevailed in the Upper Jurassic. Several Jurassic examples show variations in ooid carbonate mineralogy, such as the Upper Jurassic Smackover oolite of the Gulf Coast region (southern Arkansas and northern Louisiana) and Upper Jurassic ooids in the Purbeck limestones of Swiss and French Jura. These results are not in agreement with the concept of a “calcite sea” during the Ordovician and the Upper Jurassic periods. Very recently, Westphal and Munnecke (2003) showed that in spite of the tendency of abiotic precipitates (Sandberg 1983) and skeletal mineralogy (Stanley and Hardie 1999) to follow the general trend of calcite seas and aragonite seas, organisms with calcite and aragonite mineralogy coexisted throughout the Phanerozoic. They have examined the temporal and spatial distribution of limestone-marl alternations in Ordovician, Jurassic and Cretaceous (times of calcite seas). Limestone-marl alternations are most abundant in settings that favored aragonite production and accumulation analogous to the modern environment (Westphal and Munnecke 2003). If the above observations confirmed, the proposed secular variation in Phanerozoic carbonate mineralogy needs to be re-evaluated.

Alterations in guano phosphates and mio-pliocene carbonates of table mountain Santa Barbara, Curacao

The Table Mountain Santa Barbara, consisting of the Mio-Pliocene Seroe Domi Limestone Formation, was probably covered with a guano layer in the Pleistocene (?). Solutions, carrying guano-derived phosphate, percolated downwards, resulting in partial phosphatization of the underlying limestones. Only two phosphate minerals are present in the Table Mountain: apatite, and whitlockite. Phosphatization includes both replacement of the original carbonate or (pre-phosphate) dolomite, and cementation in primary- as well as secondary porosity (cryptocrystalline, isotropic phosphate cement, (micro)crystalline apatite, isopachous apatite fringe cement, multiple-zoned apatite crusts, rhombic whitlockite cements). Two factors controlled the final distribution of the phosphate within the Table Mountain. Firstly, changing positions of the sea level (and, therefore, of the diagenetic environments) determined the overall distribution of the phosphatized interval, a horizontal layer (98–128 m above sea level) sandwiched between two non-phosphatized limestone units. Original carbonate facies and -mineralogy was the second controlling factor causing the final variations in degree of phosphatization. The limestones comprise two lithofacies: (a) coralliferous limestone lenses erratically distributed amidst; and (b) micritic limestones. These coralliferous limestones are phosphatized preferentially forming so-called “phosphate pockets”, sharply outlined within a non-phosphatized micritic limestone “host”. These pockets are characteristically organized into five zones which are described in detail. Higher original porosity/permeability of the coralliferous limestone lenses as compared to the micrites, additionally enhanced by a phase of pre-phosphate dolomitization, determined preferential phosphatization of this facies type. A second controlling factor was the original carbonate mineralogy and resulting dia-