Calcite Microspar Research Articles

Molar-Tooth Structure as a Window into the Deposition and Diagenesis of Precambrian Carbonate

Molar-tooth structure (MTS) is an unusual carbonate fabric that is composed of variously shaped cracks and voids filled with calcite microspar. Despite a century of study, MTS remains enigmatic because it juxtaposes void formation within a cohesive yet unlithified substrate with the penecontemporaneous precipitation and lithification of void-filling carbonate microspar. MTS is broadly restricted to shallow marine carbonate strata of the Mesoproterozoic and Neoproterozoic, suggesting a fundamental link between the formation of MTS and the biogeochemical evolution of marine environments. Despite uncertainties in the origin of MTS, molar-tooth (MT) microspar remains a popular target for geochemical analysis and the reconstruction of Precambrian marine chemistry. Here we review models for the formation of MTS and show how detailed petrographic analysis of MT microspar permits identification of a complex process of precipitation and diagenesis that must be considered when the microspar phase is used as a geochemical proxy. ▪ Molar-tooth fabric is an enigmatic structure in Precambrian sedimentary rocks that is composed of variously shaped cracks and voids filled with carbonate microspar. ▪ Time restriction of this fabric suggests a link between this unusual structure and the biogeochemical evolution of marine environments. ▪ Petrographic analysis of molar-tooth fabric provides insight into fundamental processes of crystallization.

Deposition or diagenesis? Probing the Ediacaran Shuram excursion in South China by SIMS

The Ediacaran Period (ca. 635–541 Ma) witnessed the earliest paleontological evidence for macroscopic animals (i.e., Ediacara biota) and geochemical observations of the largest carbon cycle anomaly in Earth history (i.e., Shuram Excursion, SE). Numerous hypotheses have been proposed for the origins of the SE, ranging from primary seawater anomaly to syn- or post-depositional diagenesis. Despite intensive geochemical and theoretical work published in the past decade, empirical evidence that is strictly based on fundamental petrographic results at the micrometer scale is still limited. To evaluate depositional compositions and diagenetic effects on samples from the SE, we investigated the EN3 interval in the Doushantuo Formation of South China via integrated cathodoluminescence (CL), secondary ion mass spectrometry (SIMS), and scanning electron microscope (SEM). Detailed petrographic observations reveal that the EN3 limestone is dominated by calcite microspar, with minor but variable amounts of disseminated zoned dolomite crystals. The former likely formed via neomorphism of depositional micrite, while the latter was the result of progressive post-depositional dolomitization. The mean values of paired SIMS δ13Ccalcite and δ13Cdolomite compositions are indistinguishable in each sample and consistent with published micro-drilled bulk-powder δ13C values, which we interpret to represent depositional “background” signals of seawater dissolved inorganic carbon (DIC). We also observed μm-scale variability of SIMS δ13C data that may reflect a variable diagenetic overprint after deposition. Our integrated petrographic and geochemical results are consistent with a depositional origin of the SE and provide little evidence for the hypothesized isotope alteration by meteoric and mixing-zone diagenesis or late burial diagenesis. In light of this study, we propose that the SE indeed represents a marine carbon cycle anomaly that bears a close temporal link to the Ediacaran surface environment.

Experimental constraints on nonskeletal CaCO3 precipitation from Proterozoic seawater

Abstract Precambrian carbonates record secular variations in the style of CaCO3 nucleation and growth, yet the geochemical conditions recorded by some enigmatic textures remain poorly quantified. Here, we performed CaCO3 nucleation experiments in synthetic seawater in order to constrain the mineralization pathways of synsedimentary calcite microspar cement, a prolific component of Proterozoic carbonates. We found that dissolved PO4 above ∼12 μmol/L (µM) inhibits the nucleation of aragonite and calcite and permits the formation of an amorphous Ca-Mg carbonate (ACMC) precursor once CaCO3 supersaturation (Ωcal) is ≥ 45. Depending on seawater Mg/Ca, ACMC then rapidly recrystallizes to monohydrocalcite and/or calcite. This precipitation mechanism is consistent with sedimentological, petrographic, and geochemical characteristics of Proterozoic synsedimentary calcite microspar cement, and it suggests that kinetic interactions among common seawater ions may open nontraditional CaCO3 mineralization pathways and sustain high CaCO3 supersaturation.

Reconstructing Tonian seawater 87Sr/86Sr using calcite microspar

Abstract The Tonian Period followed a long interval of relative stasis and led into the climatic extremes and biological radiations of multicellular life during the Cryogenian and Ediacaran Periods, respectively. However, despite its pivotal situation, it remains relatively understudied, in large part due to the lack of robust age constraints. A combination of fossil evidence, radiometric ages, and isotopic constraints reveal that carbonate strata on the North China craton were deposited between ca. 980 and ca. 920 Ma, thereby filling a gap in marine archives. Here we present 87Sr/86Sr data from selected calcite microspar cements, which filled early diagenetic “molar tooth” cracks, along with data from demonstrably well-preserved bulk carbonate samples. These new data show that seawater 87Sr/87Sr rose in stages from ∼0.7052 at ca. 980 Ma to ∼0.7063 by ca. 920 Ma, after which a return to low values coincided with the eruption of the Dashigou large igneous province across the North China craton. We also present a new Neoproterozoic seawater 87Sr/86Sr curve, which reveals that the general trend toward higher 87Sr/87Sr during the Tonian Period was checked repeatedly by the input of less-radiogenic strontium from a series of eruptive events, both coincident with and prior to the main breakup of Rodinia. The weathering of Tonian volcanic provinces has been linked to higher carbon burial, glaciation, and oxygenation due to the high phosphorus content of flood basalts. Here we show that the weathering of major volcanic provinces affected material fluxes and ocean chemistry much earlier than previously envisaged.

Nanoscale petrographic and geochemical insights on the origin of the Palaeoproterozoic stromatolitic phosphorites from Aravalli Supergroup, India.

Stromatolites composed of apatite occur in post-Lomagundi-Jatuli successions (late Palaeoproterozoic) and suggest the emergence of novel types of biomineralization at that time. The microscopic and nanoscopic petrology of organic matter in stromatolitic phosphorites might provide insights into the suite of diagenetic processes that formed these types of stromatolites. Correlated geochemical micro-analyses of the organic matter could also yield molecular, elemental and isotopic compositions and thus insights into the role of specific micro-organisms among these communities. Here, we report on the occurrence of nanoscopic disseminated organic matter in the Palaeoproterozoic stromatolitic phosphorite from the Aravalli Supergroup of north-west India. Organic petrography by micro-Raman and Transmission Electron Microscopy demonstrates syngeneity of the organic matter. Total organic carbon contents of these stromatolitic phosphorite columns are between 0.05 and 3.0 wt% and have a large range of δ(13) Corg values with an average of -18.5‰ (1σ = 4.5‰). δ(15) N values of decarbonated rock powders are between -1.2 and +2.7‰. These isotopic compositions point to the important role of biological N2 -fixation and CO2 -fixation by the pentose phosphate pathway consistent with a population of cyanobacteria. Microscopic spheroidal grains of apatite (MSGA) occur in association with calcite microspar in microbial mats from stromatolite columns and with chert in the core of diagenetic apatite rosettes. Organic matter extracted from the stromatolitic phosphorites contains a range of molecular functional group (e.g. carboxylic acid, alcohol, and aliphatic hydrocarbons) as well as nitrile and nitro groups as determined from C- and N-XANES spectra. The presence of organic nitrogen was independently confirmed by a CN(-) peak detected by ToF-SIMS. Nanoscale petrography and geochemistry allow for a refinement of the formation model for the accretion and phototrophic growth of stromatolites. The original microbial biomass is inferred to have been dominated by cyanobacteria, which might be an important contributor of organic matter in shallow-marine phosphorites.

Molar Tooth Structure: a Contribution from the Mesoproterozoic Gaoyuzhuang Formation, Tianjin City, North China

Abstract:Molar‐tooth (MT) structure is an enigmatic sedimentary structure consisting of variously‐shaped cracks and voids filled with a characteristically uniform, equant calcite microspar. It is globally distributed but temporally restricted to rocks from Neoarchean to Neoproterozoic age. The origin of MT structures has been debated for more than a century and the topic continues to be highly contentious. Some features of MT structure occurring in micritic limestones of the Mesoproterozoic Gaoyuzhuang Formation (ca. 1500 Ma to ca. 1400 Ma), Jixian section, Tianjin City, North China show that: 1) there is a definite interface or lining, rich in organic material and pyrite, between the MT crack‐filling calcite microspar and the micritic host rock, which is also rich in organic matter; 2) the micritic host rocks are notable for the absence of stromatolites and microbial laminites; 3) distinctive conglomeratic lag deposits made up of intraclasts of MT microspar result from storm reworking of the MT structures; 4) the MT structure is associated with possible algal megafossils such as Chuaria; 5) the MT microspar is made up of the larger calcite crystal and the MT crack is marked by the diversity of configurations; 6) both the TOC content and the carbon‐isotopic value (δ13CPDB) among the host rock, the MT microspar and the possible algae fossil are obviously different. For the forming mechanism of the Gaoyuzhuang MT structure, these features can still indicate that: A) the MT microspar was formed by rapid precipitation and lithification; B) the MT microspar precipitated directly within the cracks; C) the decomposition of organic matter within the host micrite might be the chief mechanism producing gas bubbles; D) microscale gas‐sediment interaction led to the generation of the MT cracks and the precipitation of microspar therein; E) the MT cracks might represent the track of migration and expansion of gas bubbles, and that the recrystallization of host micrites cannot be eliminated during forming process of the MT microspar; F) the MT structure is occurred in early diagenetic period; and G) the formation of MT microspars is a complex diagenetic process. Therefore, model of the microbially‐induced gas‐bubble expansion and migration is the best interpretation for the formation of the MT structure. Effectively, MT structures are a type of sedimentary structure that is formed in the early diagenetic period and is related to microbial activities and organic matter degradation.

Influence of oxic diagenesis on source potential and lithofacies cyclicity: insight from Cenomanian Natih-B Member intrashelf basinal carbonates, Oman

Investigation of core and outcrop samples of the Cenomanian Natih-B Member (North Oman) indicates that the different lithofacies present experienced rather different early diagenesis shortly after deposition. Transmitted-light, cathodoluminescence and backscattered scanning-electron microscopy, as well as stable-isotopic, X-ray diffraction and total organic carbon (TOC) analyses were employed to delineate the major controls on the cyclic pattern of early diagenesis and hydrocarbon source potential. The Natih-B intrashelf basinal carbonates are composed of pelagic sediments that exhibit high-frequency cyclicity marked by decimetre-thick lithofacies alternations, mainly between: Lithofacies A compacted, partially bioturbated, skeletal, organic carbon-rich mudstone to wackestone; and Lithofacies B uncompacted, extensively bioturbated, skeletal, sparry-calcite rich wackestone to packstone. Individual units are composed variously of authigenic and biogenic calcite (58.1–97.6%, average 78.5%) and organic carbon (0.3–13.7% TOC, average 3.6%), together with minor quartz, clay, pyrite, dolomite and phosphatic material (fish debris). Lithofacies A contains relatively more organic carbon, clay, pyrite and dolomite than Lithofacies B and constitutes an excellent source rock. Diagenetic textures of Lithofacies A are dominated by compactional deformation of burrow fabrics, faecal pellets and solution seams, in addition to zoned/bright luminescent, non-ferroan sparry and isopachous calcite cement in and around uncompacted foraminifer tests, in an uncemented matrix. In contrast, Lithofacies B does not show any signs of compaction other than microstylolites and is dominated by zoned/dull luminescent, non-ferroan calcite microspar replacement, in addition to pore-filling, predominantly dull-luminescent, non-ferroan, sparry calcite cement. Moreover, Lithofacies B shows evidence of isopachous and meniscus-style cementation, together with geopetal structures and mictritic peloids. Stable-isotopic compositions of both lithofacies were determined from whole-rock samples (
13C =
0.9 to 0.9‰, average 0.3‰;
18O =
5.6 to
3.7‰, average
4.8‰) and sparry calcite (both cement and matrix) subsamples (
13C =
0.6 to 1.2‰, average 0.6‰;
18O =
5.7 to
3.7‰, average
4.3‰); all results being relative to Vienna Pee Dee Belemnite. These petrographic and isotopic characteristics suggest that the Natih-B abundant calcite cements and replacements were precipitated early, prior to compaction, mainly from ‘normal’ (open, oxic) seawater at slightly elevated depositional temperatures. Some of the slightly negative
13C values, however, indicate an addition of isotopically light carbon, probably derived from organic-matter oxidation by organisms living in marine pore waters. Based on evidence of extensive seafloor bioturbation and cementation, and their position within the depositional succession, the tops of Lithofacies B (wackestones to packstones) are interpreted as ‘discontinuity surfaces’ that cap shallowing-upward, fifth-order cycles, formed as a function of sediment starvation and increased bottom-current activity during relative sea-level stillstand/turnaround. In contrast, Lithofacies A (mudstones to wackestones) is believed to reflect high organic production coupled with high sedimentation rate and rapid burial. These conditions limited total infaunal colonization and extensive calcite precipitation, and preserved organic matter together with some escape burrows and in-place fauna, suggesting episodic sediment influx when more accommodation was available and seafloor diagenesis was minimized during relative sea-level rises. The relatively higher amounts of pyrite and dolomite in Lithofacies A likely indicate organicmatter degradation by bacterial sulphate reduction in anoxic pore waters during shallow burial.

Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis

AbstractSeptarian concretions in the Staffin Shales Formation (Kimmeridgian, Isle of Skye) allow controls on concretion rheology and septarian cracking to be investigated. Stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite. Intergranular volumes range from 77% to 88%, and calcite δ13C and δ18O values in most concretion bodies range from −10·0‰ to −17·3‰ and +0·3‰ to −0·6‰ respectively, consistent with rapid and pervasive cementation in marine pore fluids. Septarian rupture occurred during incipient cementation, with a sediment volume reduction of up to 43%. Crack‐lining brown fibrous calcite records pore fluid re‐oxygenation during a depositional hiatus, followed by increasing Fe content and δ13C related to bacterial methanogenesis. Brown colouration results from an included gel‐like polar organic fraction that probably represents bacterially degraded biomass. A new hypothesis for concretion growth and septarian cracking argues that quasi‐rigid ‘proto‐concretions’ formed via binding of flocculated clays by bacterial extracellular polysaccharide substances (EPS). This provided rheological and chemical conditions for tensional failure, subcritical crack growth, volume contraction, calcite nucleation, and incorporation of degraded products into crack‐lining cements. Bacterial decay of EPS and syneresis of host muds provided internal stresses to initiate rupture at shallow burial. Development of septarian (shrinkage) cracks in muds is envisaged to require pervasive in situ bacterial colonization, and to depend on rates of carbonate precipitation versus EPS degradation and syneresis. Subsequent modification of septarian concretions included envelopment by siderite and calcite microspar, hydraulic fracturing associated with Cretaceous shallow burial or Palaeogene uplift; and cementation by strongly ferroan, yellow sparry calcite that records meteoric water invasion of the host mudrocks. An abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products, and δ13C data suggest a predominantly methanogenic bicarbonate source. However, the wide δ18O range for petrographically identical cement (−1·3‰ to −15·6‰) is difficult to explain.

Sandstone-hosted concretions record evidence for syn-lithification seismicity, cavitation processes, and Palaeocene rapid burial of Lower Cretaceous deep-marine sandstones (Outer Moray Firth, UK North Sea)

Syn- and post-lithification cemented fractures are abundant in concretions formed during shallow burial of Hauterivian turbidites in the Outer Moray Firth. Early fractures are irregular, bifurcating, and contain entrained sand grains. They are confined to the concretion bodies, but commonly splay into narrower fracture swarms at the margins. Multiple, cross-cutting fills of calcite microspar have similar isotopic composition to adjacent concretionary cements, suggesting that rupture and cementation cycles occurred as host concretions lithified. Expulsion of cementing pore fluids from the fractures produced cement protrusions at concretion margins. A hydraulic fracture mechanism resulting from localized permeability heterogeneity and episodic seismic activity is proposed. Rapid seismic loading created high pore fluid pressures for fracture dilation in lithifying concretions and, through cavitation and CO 2 degassing, a trigger for rapid homogeneous nucleation of calcite. Many of the early structures were subsequently reactivated as brittle tensile fractures, and host a variety of mesogenetic cements. Reactivation probably occurred in response to stress amplification during rapid Palaeocene burial. By this time lithification of the concretions had dramatically changed their rheology and failure style. Minor permeability remained in the concretion cements adjacent to reactivated fractures, permitting invasion of intercrystalline voids by late diagenetic fluids.

Questioning carbonate diagenetic paradigms: evidence from the Neogene of the Bahamas

Carbonate diagenetic models have been heavily influenced by numerous studies of exposed Quaternary limestones. As a result, meteoric diagenesis is often assumed to be the principle means of altering aragonite-rich sediments into calcitic limestones. However, these models are limited by the scarcity of examples of aragonite-rich sediments buried in seawater that have never been influenced by meteoric fluids. The Bahamas transect cores recovered originally aragonite-rich sediments deposited in deep water beyond the easy reach of meteoric waters and provide an opportunity to test current diagenetic paradigms. The Bahamas transect consists of seven cores drilled in the prograding western margin of Great Bahama Bank. The two proximal cores (Clino and Unda) were drilled on the platform top and recovered shallow-water platform to reef facies overlying deeper margin and proximal slope facies. The five distal cores were drilled by ODP Leg 166 in up to 660 m of water and recovered carbonate slope facies. All studied sections are Neogene to Pleistocene in age. Diagenetic environments were identified based on petrographic and scanning electron microscopy (SEM) observations, XRD mineralogy, carbon and oxygen stable isotopic data, and trace elements. The upper 100–150 m of the two proximal cores were altered in meteoric to mixing-zone diagenetic environments but all other intervals were altered exclusively in marine pore fluids during seafloor, marine-burial, and deep-burial diagenesis. Several of the findings of this study question current carbonate diagenetic paradigms. These include: (1) large-scale sea level lowstands may not have chemically active meteoric lenses as we found no meteoric alteration at the −120 m elevation of the latest Pleistocene lowstand. Rather, phreatic meteoric diagenesis appears restricted to within ≈10 m of the land surface. (2) Mixing-zone diagenesis includes aragonite dissolution and minor LMC cementation but does not show the cavernous porosity or dolomitization predicted by mixing-zone diagenetic models. Current models are based on coastal mixing zones, which do not appear to be applicable to these more inland, and perhaps more typical, locations. (3) Marine-burial diagenesis produces a mature limestone with fabrics formerly considered diagnostic for meteoric diagenesis such as moldic porosity, aragonite neomorphism, blocky calcite spar and calcite microspar. However, oxygen stable isotopic data (average δ 18O=+1‰) indicate alteration in marine pore fluids only. The character of marine-burial diagenesis is partially controlled by the nature of the sediment being altered. We have identified two end-member styles, an open-system style characterized by dissolution of aragonite without significant cementation and a more closed-system style with aragonite dissolution accompanied by calcite cementation. The sediments examined were deposited well above the aragonite compensation depth, so seawater entering the sediment is saturated with respect to aragonite. The under-saturation needed to drive diagenesis is likely the result of bacterial oxidation of organic matter using sulfate. (4) Microspar forms in these sediments as a cement based on petrographic and SEM examination of partly to completely altered samples. This contradicts the common assumption that microspar forms by aggrading neomorphism of micrite. (5) Strontium content of sediments altered in marine pore fluids can show an extreme range of values, formerly thought to indicate different environments. The opportunity to finally examine the diagenesis of aragonite-rich sediments buried in seawater challenges current diagenetic paradigms and emphasizes the importance of integrated studies.

Molar-tooth structure in Proterozoic carbonate rocks: Origin from synsedimentary earthquakes, and implications for the nature and evolution of basins and marine sediment

Proterozoic argillaceous lime mudstones commonly contain molar-tooth structure, a curious synsedimentary feature consisting of crumpled, generally vertically oriented veins filled with calcite microspar. Long enigmatic, these veins are interpreted here as earthquake-induced deformation structures. A model is proposed whereby violent ground motion caused shrinkage, dewatering, and fissuring of the colloidal sea-floor sediment of clay and lime mud. A slurry of equant particles of lime mud, segregated from the clay platelets trapped in the matrix, was expelled into these voids. The host bed underwent as much as 80% compaction as well as shear, causing the lime mud-filled fissures to be ptygmatically folded and shingled both vertically and horizontally, and in places pulled apart. This lime mud lithified rapidly by high-Mg calcite grain growth, such that subsequent earthquakes shattered the folded veins. In the Belt basin of western North America, tsunamis from different events created currents that transported ooids and coarse sand from shoals into deeper water, and scoured deeply into the now-consolidated sediment, leaving behind a lag of vein fragments. Molar-tooth structure is a signature of basin tectonism recorded specifically in argillaceous lime mudstones deposited in low-energy settings above the thermocline in supersaturated marine water. Syneresis cracks are the counterpart in terrigenous facies, in that silt and sand were injected into the shrinkage fissures. This process points to the importance of seismically induced compaction under negligible burial. Smectite might have made up a significant proportion of the sediment, making it more conducive to shrinkage. The absence of molar-tooth structure in Phanerozoic strata is ascribed mainly to changes in sediment rheology brought about by increased organic binding due to diversification of the microbiota.

Microspar development during early marine burial diagenesis: a comparison of Pliocene carbonates from the Bahamas with Silurian limestones from Gotland (Sweden)

ABSTRACTComparison 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.

Coastal lithofacies and biofacies associated with syndepositional dolomitization and silicification (Draken Formation, Upper Riphean, Svalbard)

The Draken Formation (120–250 m) of northeast Spitsbergen (Svalbard) forms part of a thick Upper Proterozoic carbonate platform succession. It consists predominantly of intraformational dolomitic conglomerates, with excellent textural preservation. Six main lithofacies were recognized in the field: quartz sandstones, stromatolitic mats, conglomerates with silicified intraclasts, dolostone conglomerates with desiccated mudrocks, oolitic/pisolitic grainstones and fenestral dolostones. A series of five main gradational biofacies were recognized from silicified (and rare calcified) microfossils. Biofacies 1 represents low-energy subtidal benthos (erect filaments) and plankton (acritarchs and vase-shaped microfossils) whereas biofacies 2 to 5 are microbial mat assemblages (with filamentous mat-builders, and associated dwellers and washed-in plankton) ranging from basal intertidal to high intertidal/supratidal. Colour values (a measure of the lightness of the colour shade) of sawn rock samples were quantified using a Munsell chart, and exhibit a pronounced variation (means of major groups varying from 4.0 to 5.95) across the spectrum of subtidal to supratidal sediments as inferred from other criteria. The lightening in progressively more exposed sediments is related to lowering of organic carbon contents, probably mainly by oxidation. Six types of early cement have been recognized. Calcite microspar (type 1) is common as a subtidal cement in many Proterozoic formations, whereas types 2 (subtidal isopachous fringes), 3 (subtidal hardground dolomicrite) and 4 (intertidal meniscus dolomicrite) are very similar to Phanerozoic examples except for their dolomitic mineralogy. Types 5 and 6 are complex and variable dolomite growths associated with expansion and replacive phenomena. They characterize the fenestral lithofacies and compare with modern supratidal cements. Consideration of diagenetic fabrics and truncation textures of intraclasts indicates that leaching, dolomitization, silicification were all significant syndepositional processes altering the original metastable carbonates. The data set provides evidence for a spectrum of peritidal environments including ooid shoals, protected subtidal, tidal sandflats and protected carbonate mudflats. Different sections show a preponderance of particular facies. The coastal lithofacies continuum was completely dolomitized, unlike offshore to ooid shoal facies of adjacent formations. Dolomitization thus bears a relationship to depositional bathymetry. Although hydrodynamics clearly have a role, the potential importance of whiting precipitation in raising Mg/Ca in marginal marine environments is also stressed.

The diagenetic history of some septarian concretions from the Kimmeridge Clay, England

ABSTRACT Large septarian concretions from the Kimmeridge Clay, up to 1.2 m in diameter, have centres comprising anhedral calcite microspar passing into margins of radiating fibrous calcite microspar, with a pyrite‐rich zone at the transition. Septarian veins formed and were lined with brown calcite synchronously with fibrous matrix growth, with white calcite precipitated in septarian cavities after concretion growth ceased. Septarian veins, filled only with white calcite, formed later, at the same time as the outermost calcite microspar crystals were enlarged.The concretions were buried in the Late Jurassic to about 130 m, and in the Late Cretaceous to about 550 m, with uplift between. Oxygen isotopes show that the concretion grew throughout the first burial, with septarian veins forming from about 30 m depth onwards. Later septarian veins formed between about 200 and 500 m during the second burial.Carbon isotopes show that the compact inner matrix grew in the sulphate reduction zone, the end of which is marked by the pyrite‐enriched zone. Dissolving shells, and possibly minor methanogenic carbonate, slowly diluted sulphate reduction‐zone carbonate during deeper burial. During early concretion growth, Mg and Sr were depleted in the pore water. During later stages of the first burial, Mg, Sr, Mn and Fe all increased, especially after concretion growth ceased. During the second burial, Fe, Mn and Mg decreased as calcite precipitated, implying relatively closed systems for these elements.Synchronous formation of septarian fractures and fibrous calcite matrix shows that the Kimmeridge Clay became overpressured during the later stages of both burials.

Deposition and Diagenesis of Flippen Limestone (Wolfcampian), Fisher and Jones Counties, Texas: ABSTRACT

The Flippen carbonate (Wolfcampian) was deposited during a high stillstand of sea level under subtidal to intertidal conditions. Eleven cores in Fisher and Jones Counties have been subjected to megascopic and petrographic analysis with special interest given to the Alkali Creek SW field in Fisher County. Five distinctive limestone facies, designated according to their most outstanding characteristics are: (1) constructional phylloid algal buildups, (2) crestal boundstones, (3) flanking bed packstone-wackestone, (4) foremound pelloidal grainstone, and (5) capping grainstones. Early diagenesis occurred after deposition of the sediments in the marine environment. This is evident in micritization, submarine cements, and effects attributed to binding and encrusting habits of algae, particularly Archaeolithoporella and Tubiphytes. Primary intergranular and intrabiotic porosity is best developed and preserved in biograinstones along the shallow flanks of the constructional mound. Stabilization and lithification of originally deposited sediments began early during subaerial exposure and subsequent freshwater diagenesis. Secondary porosity was formed by the dissolution of aragonitic phylloid algae and pelecypods, forming hollow micrite envelopes or biomolds. Aragonitic lime mud was replaced by calcite micrite and microspar. Calcitization and dissolution resulted in the precipitation of crusts of calcite scalenohedra in primary and secondary voids. Dolomite cement crusts also line primary and secondary voids, and these rhombs subsequently were dedolomitized. Precipitation of blocky equant nonferroan calcite, ferroa calcite, and ferroan dolomite cements partially to completely filled primary and secondary voids. A late stage of dissolution, which presumably occurred at depth, enhanced existing primary and secondary porosity. No cementation followed the late stage of diagenesis. End_of_Article - Last_Page 265------------

Stable isotopic and elemental relationships of ancient shallow-marine slope carbonates, Cambro-Ordovician Cow Head Group, Newfoundland; implications for fluid flux

ABSTRACT Carbon and oxygen stable isotopic and trace-elemental variations in diagenetic calcite and dolomite from slope and platform-margin carbonates of the Cambro-Ordovician Cow Head Group, western Newfoundland, are explained by calculations that consider variations in the water: rock ratio during burial history of a reconstructed cross-section of the continental margin. Platform-margin deposits are preserved as blocks of limestone with calcified algae in slope strata. The blocks contain neomorphic, radial (18O = -9.3 to -5.8 PDB; 13C = 0.7 to + 1.4 PDB) and blocky spar (18O = -8.3 to -6.7 PDB; 13C = 0.6 to + 1.2 PDB) calcite, sparry dolomite (x18O = +5.8 PDB; x13C = +0.6 PDB), and lutecite (length-slow chalcedony; 18O = +27.9 to +29.7 SMOW). Reaction of these almost isotopically homogeneous rocks with sea water, meteoric water, and upward-migrated basinal water is suggested by their distinctive fabrics and isotopic compositions. The beds of slope limestone contain abundant calcite microspar and pseudospar (18O = -9.2 to -6.6 PDB; 13C = -5.6 to +0.6 PDB), dolomite (18O = -9.3 to -2.8 PDB; 13C = -3.0 to +1.0 PDB), and minor chert (18O = +23.4 to +29.1 SMOW). The three dolomite fabrics are fine-crystalline rhombohedra that grew in clay-rich carbonate mudstone, medium-crystalline replacive mosaics that are nonfabric selective, and coarse, white, sparry rhombohedra associated with stylolites and fractures. The microspar and pseudospar grew by widespread neomorphism of 1) fine-grained, detrital calcite or aragonite in lime mudstone, and 2) calcite cement in calcarenite. Cathode-luminescent patterns of calcites and dolomites suggest a prolonged history of recrystallizations (dissolution-precipitation on a fine-scale) during burial diagenesis. With increasing distance basinward from the platform margin, diagenetic calcite and dolomite are systematically depleted in the heavy isotopes of oxygen and carbon, and diagenetic calcite is enriched in magnesium, suggesting crystal growth and recrystallizations in evolved disgenetic fluids. These data, along with the burial history, indicate that the limestones of the Cow Head Group were systematically altered as greater amounts of burial fluids were either focused along preferred permeability pathways during compactive expulsion or recirculated during possible convective flow through strata farther from the platform margin.

Radiolarites and their relation to subjacent "oceanic crust" in Liguria, Italy

Radiolarites of the Ligurian sequence are rhythmic alternations of generally red chert and shale beds from 1 to 10 cm thick that overlie ophiolite rocks of presumed oceanic crust and underlie pelagic and redeposited limestone. The rhythmic bedding is the product of deposition of radiolarian sand beds and some overlying mud by turbidity currents and possibly other marine bottom currents that alternated with episodes of slow deposition of hemipelagic red clay. Evidence of current deposition of radiolarian sand beds includes: tightly packed Radiolaria; presence of hydraulically-equivalent clasts of red clay and ophiolite debris; beds with grading, laminations, current-ripple cross-beds, and flute and groove casts. Chert beds formed principally by dissolution of the more delicate Radiolaria and reprecipitation of silica cement as either microcrystalline quartz or opal C-T around more resistant Radiolaria during burial diagenesis. Radiolarite has abrupt lateral differences in thickness (from absent to 200 m), which reflects deposition on a highly uneven sea floor. An interpretation of the depth of water during radiolarite deposition depends in part on how one interprets the origin of ophicalcite that locally underlies radiolarite. Ophicalcite consists of clasts of serpentinite and locally gabbro and peridotite in a pink calcite microspar matrix, the whole cut by white vein-filling calcite of younger tectonic origin. Ophicalcite can be interpreted as a breccia formed by total recrystallization, in part, of pelagic carbonate ooze deposited along ocean spreading ridges or as paleocaliche that developed upon ophiolite exposed above sea level. Evidence to support a pedogenic origin of ophicalcite includes textures and structures identical with caliche, silicified serpentinite masses (jasper) whose texture and mineral composition mimic modern soils on serpentinite, presence of a flat ?wave-cut surface beneath radiolarite, upward increase within ophicalcite of both calcite and oxidized serpentinite debris, and absence or thinness of radiolarite on paleotopographic highs. Many unanswered problems exist to both the above hypotheses. The deep-water hypothesis (favored by EFM) is that deposition took place on oceanic crust in water deeper than wave base (>200 m) and probably deeper than 2500 m; the latter depth implies the existence of a CCD during Late Jurassic time to account for the absence of carbonate in radiolarite. The shallow-water hypothesis (favored by RLF) is that deposition took place on previously exposed subsiding oceanic crust in water depths from a few meters to a few hundred meters; the absence of carbonate in radiolarite is attributed to local scarcity of carbonate-producing plankton compared to the overwhelming supply of clay from weathered ophiolite. Both hypotheses involve the interpretation that radiolarian-rich sediment, initially deposited on topographic highs, was transported into adjacent troughs by turbidity and other marine currents; the deeper troughs were floored by pillow lavas.

Phreatic and Vadose Diagenetic Modification of Pleistocene Limestone: Petrographic Observations from Subsurface of Barbados, West Indies

Data from a shallow cored borehole through 105,000-year-old reef-tract sediments on the south coast of Barbados indicate that the carbonate sediments have been altered more rapidly and more extensively in freshwater phreatic diagenetic environments than in the vadose or marine phreatic diagenetic environments. The upper part of the borehole section has been exposed only to subaerial and shallow vadose processes since emergence from the marine depositional environment about 105,000 years ago. These sediments are recrystallized only partly. Total amounts of cement, mainly needle-fiber cement and dense micrite coatings, are high. Porosity is low. Dissolution, a minor feature in this part of the section, is associated with localized development of needle-fiber fabrics. Those parts of the borehole section which have been occupied by a freshwater phreatic lens have been cemented by calcite microspar. Aragonite grains have been dissolved on a massive scale, resulting in a very well-developed moldic porosity. Neomorphic grain alteration has been volumetrically important. Packstones and grainstones in the lowest part of the borehole are largely unaltered; metastable carbonate mineralogy dominates. This part of the sedimentary column has been subjected alternately to vadose conditions during low stands of sea level (> 70,000 years total) and to marine phreatic conditions during high stands of the sea (< 35,000 years total). Despite prolonged exposure to vadose conditions, diagenetic modification is slight. If the recrystallized lime muds contain greater than 25 percent cement (micrite), then mass-balance calculations suggest large amounts of calcium carbonate have been transported to that part of the sedimentary section exposed to freshwater phreatic diagenetic processes. The source of the additional carbonate may be updip in the aquifer, from mixing of underlying marine pore fluids with the freshwater pore fluids of the coastal water lens, from the overlying shallow vadose zone, or from marine water soon after deposition. Processes which affect early diagenetic modification of carbonate sediments on the south coast of Barbados operate more rapidly and more effectively in the freshwater phreatic environment than in vadose or marine phreatic environments.