Micritic Matrix Research Articles

Diagenesis of the lower Eocene Thebes Formation, Gebel Rewagen area, Eastern Desert, Egypt

Abstract The diagenesis of lower Eocene shallow water carbonates with flint was studied in the Gebel Rewagen area, Eastern Desert, Egypt. The carbonates are mainly wackestones to packstones with benthic bioclasts embedded in a dark red luminescent micrite matrix. The studied succession displays a complex diagenetic history that involves syngenetic and late diagenetic processes. Silica, which exists either as persistent bands, nodules and/or silicified benthic bioclasts shows a distinctive pattern regarding its distribution, source, depositional environments and timing. Three lines of evidence support a syngenetic origin of the chert bands: (1) they alternate in a cyclic manner within the host carbonates and (2) they exhibit noticeable lateral persistence throughout the investigated area following the strata boundaries and (3) there is a lack of any carbonate dissolution in limestone adjacent to chert bands. The deposition of silica bands in association with shallow water carbonates is possibly related to eustatic sea-level changes, which were accompanied by episodic variations in silica and carbonate productivities. With a relative sea-level fall and the establishment of a lowstand period at the end of the early Eocene, a basinward shift of the groundwater zones is expected within the carbonate platform. During this period some late diagenetic processes took place, which involve: (1) the formation of siliceous and carbonate concretionary growths, (2) partial silicification of bioclasts, (3) neomorphic stabilization of the CaCO3 bioclasts and (4) the formation of equant calcite cement. Siliceous and carbonate concretions are believed to have taken place within microenvironments created and controlled by sulphate-reducing bacteria and physico-chemical and kinetic factors near a marine-meteoric water mixing zone. This is inferred from the distribution of iron sulphides, the non-ferroan nature of all concretions and the depleted δ13C (−5.4‰ to −6.0‰ PDB) and δ18O (−5.8‰ to −6.8‰ PDB) values of the carbonate concretions. The silica fabrics of all the partly silicified benthic bioclasts, with abundant pyrite cubes, argue for a delayed silicification process within the sulphate reduction zone. The close and relatively depleted δ18O values of the low−Mg calcite micrite matrix (−3.0‰ to −4.4‰ PDB) and benthic bioclasts (−3.4‰ to −4.8‰ PDB) indicate that both matrix and bioclasts have probably suffered neomorphic stabilization from freshwater-dominated solutions. The initial marine δ13C values of both low-Mg calcite matrix (+0.2‰ to −2.2‰ PDB) and altered benthic bioclasts (−0.2‰ to −0.9‰ PDB) are retained and not affected by diagenesis. The luminescence character and the extremely depleted oxygen values (−8.3‰ to −10.1‰ PDB) of the equant void-filling calcite mosaics are consistent with their formation within a meteoric phreatic realm.

Conceptual models for burrow‐related, selective dolomitization with textural and isotopic evidence from the Tyndall Stone, Canada

ABSTRACTThe formation of dolomite is generally explained using models that reflect larger‐scale processes that describe the relationship between the supply and transport of Mg, and geochemical conditions that are amenable to the formation of dolomite. However, heterogeneities in the substrate, such as those made by bioturbating infauna, may play a more important role in dolomitization than has been previously considered. Burrow‐facilitated dolomitization is evident in the Ordovician Tyndall Stone (Red River Group, Selkirk Formation) of central Canada. The diagenetic fabrics present are attributed to dolomitizing fluids that both flowed through and evolved within burrow networks. Petrographic analysis suggests that two phases of dolomite formation took place. The first formed a fine‐grained, fabric‐destructive type that probably accompanied early burial; the second is a fine‐ to medium‐grained, locally sucrosic dolomite that is interpreted to have precipitated during later burial. Isotopic analysis supports the proposed paragenetic history: (1) an apparent linking of the stable isotopes 13C and 18O strongly suggests that the micrite matrix formed during very early diagenesis and was derived from seawater; (2) the initial phase of dolomitization is potentially microbially mediated, as evidenced by the enrichment of 13C; and (3) isotopic values for the second generation of dolomite reflect the mixing of ground water and resorbed early dolomite. This paper conceptualizes the physical and chemical conditions required for the formation of dolomite in association with burrow fabrics. The proposed model reveals a composite of biological and inorganic reactions that demonstrates the interdependence of sediment fabric, organic content and microbial interactions in the development of burrow‐mottled dolomitic limestone. It is suggested that where burrow‐associated dolomite occurs, it is most likely to develop in two stages: first, the byproducts of the degradation of organic material in burrows locally increase the permeability and porosity around burrow fabrics in shallow diagenetic depositional environments; and, second, the passing of burrowed media into deeper dysaerobic sediment is accompanied by the establishment of fermenting micro‐organisms whose byproducts mediate dolomitization.

Diagenetic Processes and Their Progression in The Rocks of Injana Formation, Borehole (KH8/9), NW Iraq.

Several diagenetic phenomena have affected the sandstones of Injana Formation. These include: compaction, cementation (manifested by dominance of carbonate cement with minor silica, ferruginous, and gypsum cements), replacement, recrystallization which noticed by the transformation of micritic matrix and fragments to sparry calcite whereas the alteration appeared through change of feldspar to clay minerals. Most of these diagenetic phenomena are related to eodiagenesis and minor rule of mesodiagenesis (alteration and partial recrystallization and cementation ).

Microbial boundstone dominated carbonate slope (Upper Carboniferous, N Spain): Microfacies, lithofacies distribution and stratal geometry

The Carboniferous, particularly during the Serpukhovian and Bashkirian time, was a period of scarce shallow-water calcimicrobial-microbialite reef growth. Organic frameworks developed on high-rising platforms are, however, recorded in the Precaspian Basin subsurface, Kazakhstan, Russia, Japan and Spain and represent uncommon occurrences within the general trend of low accumulation rates and scarcity of shallow-water reefs. Sierra del Cuera (Cantabrian Mountains, N Spain) is a well-exposed high-rising carbonate platform of Late Carboniferous (Bashkirian-Moscovian) age with a microbial boundstone-dominated slope dipping from 20° up to 45°. Kilometer-scale continuous exposures allow the detailed documentation of slope geometry and lithofacies spatial distribution. This study aims to develop a depositional model of steep-margined Late Paleozoic platforms built by microbial carbonates and to contribute to the understanding of the controlling factors on lithofacies characteristics, stacking patterns, accumulation rates and evolution of the depositional architecture of systems, which differ from light-dependent coralgal platform margins. From the platform break to depths of nearly 300 m, the slope is dominated by massive cement-rich boundstone, which accumulated through the biologically induced precipitation of micrite. Boundstone facies (type A) with peloidal carbonate mud, fenestellid and fistuliporid bryozoans, sponge-like molds and primary cavities filled by radiaxial fibrous cement occurs all over the slope but dominates the deeper settings. Type B boundstone consists of globose centimeter-scale laminated accretionary structures, which commonly host botryoidal cement in growth cavities. The laminae nucleate around fenestellid bryozoans, sponges, Renalcis and Girvanella-like filaments. Type B boundstone typically occurs at depths between 20–150 m to locally more than 300 m and forms the bulk of the Bashkirian prograding slope. The uppermost slope boundstone (type C; between 0 and 20–100 m depth) includes peloidal micrite, radiaxial fibrous cement, bryozoans, sponge molds, Donezella, Renalcis, Girvanella, Ortonella, calcareous algae and calcitornellid foraminifers. From depths of 80–200 m to 450 m, 1–30 m thick lenses of crinoidal packstone, spiculitic wackestone, and bryozoan biocementstone with red-stained micrite matrix are episodically intercalated with boundstone and breccias. These layers increase in number from the uppermost Bashkirian to the Moscovian in parallel with the change from a rapidly prograding to an aggrading architecture. The red-stained strata share comparable features with Lower Carboniferous deeper-water mud-mound facies and were deposited during relative rises of sea level and pauses in boundstone production. Rapid relative sea-level rises might have been associated with changes in oceanographic conditions not favourable for thecalcimicrobial boundstone growth, such as upwelling of colder, nutrient-rich waters lifting the thermocline to depths of 80–200 m. Downslope of 150–300 m, boundstones interfinger with layers of matrix-free breccias, lenses of matrix-rich breccias, platform- and slope-derived grainstone and crinoidal packstone. Clast-supported breccias bound by radiaxial cement are produced by rock falls and avalanches coeval to boundstone growth. Matrix-rich breccias are debris flow deposits triggered by the accumulation of red-stained layers. Debris flows develop following the relative sea-level rises, which favour the deposition of micrite-rich lithofacies on the slope rather than being related to relative sea-level falls and subaerial exposures. The steep slope angles are the result of in situ growth and rapid stabilization by marine cement in the uppermost part, passing into a detrital talus, which rests at the angle of repose of noncohesive material. In the Moscovian, the aggradational architecture and steeper clinoforms are the result of increased accommodation space due to tectonic subsidence and due to a reduction of slope accumulation rates (from 240±45−605±35 m/My to 130±5 m/My). The increasing number of red-stained layers and the decrease of boundstone productivity are attributed to environmental changes in the adjacent basin, in particular during relative rises of sea level and to possible cooling due to icehouse conditions. The geometry of the depositional system appears to be controlled by boundstone growth rates. During the Bashkirian, the boundstone growth potential is at least 10 times greater than average values for ancient carbonate systems. The slope progradation rates (nearly 400–1000 m/My) are similar to the highest values deduced for the Holocene Bahamian prograding platform margin. The fundamental differences with modern systems are that progradation of the microbial-boundstone dominated steep slope is primarily controlled by boundstone growth rates rather than by highstand shedding from the platform top and that boundstone growth is largely independent from light and controlled by the physicochemical characteristics of seawater.

Origin and Significance of Isotope Shifts in Pennsylvanian Carbonates (Asturias, NW Spain)

ABSTRACT The primary variability of the composition and properties of seawater is much greater in the shallow coastal zones than in the main body of ocean water. An inadequate understanding of this variability, as well as different diagenetic environments, severely limit the interpretation of the stable-isotope record of shoalwater carbonates. In order to investigate this primary and diagenetic variability along a Bashkirian-Moscovian platform-to-basin transect, 13C and 18O analyses have been performed on more than 1000 matrix micrite, carbonate cement, and brachiopod shell samples. In isotope analysis, these different carbonate materials tend to complement each other, inasmuch as they have different advantages and shortcomings. The resulting data reveal spatial trends in 13C and 18O signatures from platform top (lower values) to basin (higher values). In the case of 13C from pristine brachiopods, this trend can be explained by the long residence time (aging) of platform-top water masses. In the case of brachiopod 18O, this variance is interpreted to reflect temperature differences between warm surface and colder bottom water separated by a permanent thermocline at about 150 to 200 m beneath the shelf break. Micrite and marine cement isotopic values from the platform interior were reset (lowered) during pervasive early meteoric diagenesis. In contrast, micrite and marine cement isotopic values from the outer platform, slope, and basin show higher values close to the assumed Pennsylvanian seawater isotopic composition. This implies that isotopic data from shoalwater carbonates (including pristine brachiopod shells) might not necessarily reflect paleoceanographic trends of the open-ocean water masses because of changes in coastal water-mass isotope signature and interaction with early meteoric fluids.

An exhumed Palaeozoic underwater scenery: the Visean mud mounds of the eastern Anti-Atlas (Morocco)

About one hundred carbonate mud mounds, covering an area of 440 km 2 in the eastern Anti-Atlas of Morocco, constitute one of the largest mound agglomerations known so far from Lower Carboniferous settings. They occur within a 4000-m-thick succession of shales with intercalated bedded limestones, sandstones, and siltstones. According to conodont and goniatite biostratigraphy, mound formation started in the early Gnathodus texanus Zone and terminated during the G. bilineatus Zone of the Visean stage. Individual mounds are a few metres to 30 m high, have base diameters of up to 300 m and are concentrated in several parallel, WNW–ESE running belts. From their lithology and facies relationships, four types of mounds can be distinguished: (1) massive crinoidal wacke- or packstones without stromatactis; (2) massive crinoidal wacke- or packstones with rare stromatactis; (3) similar to (2), but allochthonous; and (4) biodetrital (skeletal) grainstone mounds. While carbonate deposition in types (1) to (3) was probably triggered by microbial precipitation, type (4) is the result of a predominantly mechanical accumulation of skeletal debris. Biota in the four types comprise a great variety of invertebrates, among which crinoids, sponges, and bryozoans are most common. Diagenesis of the mound carbonates was dominated by recrystallization of micritic matrix and organic remains and late burial cementation. Oxygen and carbon isotope data of brachiopod and crinoid ossicles, matrix, and early marine cements plot in a large field and do not allow definite conclusions about the composition of the ambient seawater. Microbial activity and the absence or scarcity of green algae, colonial corals and coralline sponges suggest deposition of the mounds in moderate water depth close to the lower limit of the photic zone. D 2001 Elsevier Science B.V. All rights reserved.

Bioconstructions a spongiaires siliceux dans le Lias terminal du Bassin lusitanien (Portugal); stratigraphie, sedimentologie et signification paleogeographique

Abstract The Upper Liassic series in the western border of Iberia (Lusitanian Basin, Portugal), show an important lutitic sedimentation, characterized generally by a monotonous marl/limestone alternation. Small scale siliceous sponge mudmounds occur in these deposits from Middle Toarcian to Lower Aalenian age. The scope of this work is to pinpoint the stratigraphical and sedimentological context and to characterize controlling factors of the spongioliths. Stratigraphic and facies analysis. Relevant sections were observed and investigated in different locations of the Lusitanian Basin (e.g., Alvaiazere, Porto de Mos, Rabacal, Coimbra and Cantanhede) (fig. 1). The siliceous sponge facies correspond to the upper part of the S. Giao Unit and to the lower part of the Povoa da Lomba Unit (fig. 2). Considering the sequential scheme of Duarte [1997], the sediments correspond to groups of third-order depositional sequences MST3 and MST4 (mainly in the upper part of this sequence: MST4B). The sedimentary evolution of these units shows a stacking pattern composed of shallowing upward sequences deposited in an outer homoclinal ramp setting, dipping northwestwards. Both units increase in thickness from south to north (fig. 3) and their vertical facies associations correspond to a very bioturbated (Chondrites, Zoophycos, Planolites and Thalassinoides) marl/limestone succession (figs. 4 and 5). MST3 is demonstrably more marly than MST4B. The base of MST4 [MST4A in Duarte, 1997] corresponds to a marl/marly limestone alternation, very poor in siliceous sponge mudmounds. The first unit (MST3) which includes sponge mudmounds is dated as uppermost Bifrons zone through the base of the Bonarellii zone. The majority of the siliceous sponge mudmounds occur within this time slice. These mounds are characterized by a great diversity of accompanying fauna mainly composed of brachiopods (rhynchonellids and terebratulids), crinoids and bivalves. The initial growth of the sponge build-ups can be correlated basin-wide to the intra Bifrons regional flooding surface (MST2/MST3 boundary). The second unit (MST4), particularly its upper part (MST4B), corresponds to the top of the Meneghinni-Opalinum interval and is related to a carbonate progradational phase. In the eastern part of the basin, the calcareous facies of MST4B are more bioclastic. Siliceous sponge mudmounds. The Toarcian mudmounds of the Lusitanian Basin are usually only a few decimetres thick and most display irregular knob-like to flat lenticular morphologies. Some build-ups are round and can reach 1,5 metres in thickness and ten metres in diametre. Also worth mentioning is a siliceous sponge biostrome developed at the base of MST3 in the Porto de Mos section (figs. 3 and 6). The upper mound surface is normally rough and uneven. In both sequences they are always related laterally with carbonate beds, which corresponds to the top of fourth order sequences. The mudmounds consist of mostly brownish iron-rich calcified siliceous sponges and a greyish, sometimes peloidal allochthonous micritic matrix. In general, the sponges themselves consist of dense leiolitic microbolites [automicrites sensu Reitner and Neuweiler, 1993]. The sponge spicules are diagenetically transformed into calcite. The great majority of the sponge specimens belong to the Hexactinosa (Class Hexactinellida) and are unknown and undescribed to date. "Lithistides" (polyphyletic desma-bearing demosponges) are very rare and only occur as forms encrusting Hexactinosan sponges. The benthic macrofauna is abundant and consists of monospecific crinoids, rhynchonellids, terebratulids and bivalves (mainly pectinids and ostreids). Encrusting organisms are serpulids, bryozoans and foraminifera, as well as "Lithistids" mentioned above. They are entirely restricted to the stratonomical surfaces of the siliceous sponges. The sponge bioherms consist of several microfacies types (wackestones, packstones, floatstones and boundstones). All of them are micrite dominated and represent low energy environments. They differ mainly in the amount of siliceous sponges, micrite, microbialites and the accompanying fauna. Palaeoenvironmental significance. The amount of microbial induced carbonate clearly mirrors the importance of microbial activity in respect of the reef building potential. Furthermore, three other controlling factors played an important role in the initiation of the siliceous sponge mudmounds of the Lusitanian Basin: bathymetry, sea-floor morphology and sedimentation rate. The role of the first two factors is evident because the siliceous sponge mudmounds are particularly important (abundance and volumetric expression) in the eastern part of the basin (Rabacal-Alvaiazere region). They are practically absent towards the west (essentially in MST4B) where the series show hemipelagic sedimentologic features (figs. 7 and 8). Reduced sedimentation rate is a precondition for the settlement of siliceous sponges and Hexactinosa in particular. Compared to all other Toarcian sequential units, MST3 and MST4B are the thinnest and therefore reflect the lowest sedimentation rates (fig. 8).

Diagenetic evidence for an epigenetic origin of the Courtbrown Zn–Pb deposit, Ireland

Mineralisation at the Courtbrown deposit in south-western Ireland is concentrated in the basal section of the Chadian Waulsortian Limestone, immediately above the Courceyan Ballysteen Limestone. Two episodes of sulphide deposition have been identified: an early stage of minor pyrite precipitation, and a later base-metal-rich mineralisation event. Sphalerite, galena and pyrite of the later mineralisation event occur predominantly as replacement phases along stylolites, dissolution seams, and within the micritic matrix of the host limestone. These sulphide minerals also occur as cements within late stage fractures.

Iron bacterial and fungal mats, Bajocian stratotype (Mid-Jurassic, northern Normandy, France)

The Oolithe ferrugineuse de Bayeux Formation is located at the historical Bajocian stratotype of Sainte-Honorine-des-Pertes, north of Bayeux, Normandy. The condensed formation ranges from the base of the Humphriesianum Zone to the Parkinsoni Zone and is divided into four beds of decimetric scale. Three main microfacies are present: (1) oncoid rudstones, (2) ooid bioclastic packstones and (3) silty burrowed wackestones/packstones. Sedimentation took place in a very quiet environment, below the photic zone and below or near the storm wave base. The general setting is a distal carbonate ramp, its lower part characterized by hemipelagic sedimentation indicated by the presence of planktonic foraminifers. The inferred depth is around 100 m. Free oxygen concentration was low. Dysaerobic conditions are indicated by a scarcity of benthic macrofauna. Ferruginous structures are numerous in the first two microfacies, and absent in the last. Hematite staining is not uniform and follows many sedimentary patterns. Among the more widespread Fe structures are perforation infillings with endolithic micro-organisms, microstromatolites, oncoids, ooids, blisters, coatings and hardgrounds. These structures can be associated and none are mutually exclusive. Hematite-coated filaments of different sizes and shapes are observed in the micrite matrix: the walls of various organisms; the calcite crystals associated with the Fe cortical laminations; the perforations and burrow; the hardgrounds; and microstromatolites. Petrographical and SEM examinations suggest that the laminated crusts (oncoids and hardgrounds) are formed by microbial iron mats dominated by filamentous bacteria and fungi. Seven types of microbes are recognized: filaments (five morphotypes), spheroidal bodies and stalked bodies. Filamentous microfossils of type 1 to 4 resemble the present-day filamentous bacteria (Beggiatoales and Cytophagaceae). Because of their large diameter and their branching nature, filaments of type 5 are possibly filamentous fungi. Another argument in favor of fungi is the presence of stalked and spheroidal bodies that resemble zoosporangia and oogonia of some Oomycota. In deep, calm and dysaerobic waters, many interfaces (e.g. between aerobic and dysaerobic waters) are present in the sediments. The stability of the soluble reduced state of iron (Fe2+) is higher at such interfaces, and many ferric iron-encrusted microbial fossils are observed. Iron could thus serve as an electron donor for microbial iron-oxidation processes. Other microbial iron deposition pathways are also possible. It appears that, regardless of geological age (Paleozoic, Mesozoic) and geographical location, the same microbiological mechanisms are probably responsible for the red color in calcareous stratified or unstratified bodies. The presence of fossilized iron-encrusted bacteria and fungi at interfaces may therefore serve as an indicator of anoxic to dysaerobic conditions in various paleo(micro)environments.

New Geochemical Support for Mixing-Zone Dolomitization at Golden Grove, Barbados

Dolomite occurs in the Golden Grove reef terrace of late Pleistocene age in southeastern Barbados, where fore-reef lithologies have been partially to completely dolomitized. Interpretation of the possible modes of formation of this dolomite has been a subject of disagreement for over a decade. Among the points of contention are the timing of deposition, dolomitization and dolomite cement precipitation, and the relationship of replacement dolomite with dolomite cement. Data from this study indicate that dolomitization occurred during a single relative sea-level fall following reef development at marine oxygen isotope stage 7.3, at approximately 216 ka. Replacement dolomitization affected both micritic matrix and skeletal grains. During or after replacement dolomitization, dolomite and calcite cements filled primary and secondary pores. The cement paragenesis comprises early dolomite cement, complexly banded cements of alternating calcite and dolomite, late dolomite cement of near-stoichiometric composition, and a final phase of blocky calcite cement. Replacement dolomitization and cement precipitation represent a continuum of processes resulting from progressive modification of diagenetic fluids. Stable carbon and oxygen isotope and strontium elemental compositions of replacive dolomite, dolomite cements, and calcite cements indicate that dolomitization occurred in a mixing zone between marine and meteoric groundwaters occupying the reef terrace shortly after deposition. Dolomite oxygen isotope values range from +0.18 to +3.85‰ PDB, and become progressively more depleted throughout the paragenesis, indicating a greater influence of meteoric water through time. Remarkably depleted carbon isotope compositions, for both replacement dolomite and dolomite cement (−9.53 to −23.04‰ PDB), originated from oxidation of thermogenic methane advected from the underlying accretionary complex. Methane was oxidized in marine waters that subsequently mixed with meteoric waters, as evidenced by a progressive enrichment in carbon isotope composition throughout the paragenesis. Dolomite strontium contents are substantially elevated above those expected for marine dolomitization, ranging from 640 to 1650 ppm, and increasing throughout the paragenesis. Meteoric water provided the excess strontium, as aragonite-to-calcite transformation in the exposed reef terrace increased the fluid Sr/Ca ratio. The continuum of geochemical compositions between the replacement dolomite and dolomite cement argues for their genetic association. These new data provide additional support to the mixing-zone interpretation for Golden Grove, and that the model of mixing-zone dolomitization remains viable.

Pall Corp, USA

Plumbing systems of marine seeps are complex pathways along which hydrocarbon-rich fluids move upward through the sedimentary column. In the shallow subsurface, seep plumbing systems may become successively filled by methane-derived authigenic carbonates resulting in mineralized conduits. This work investigates mineralized conduits in the uppermost plumbing network of Oligocene seep deposits from the Lincoln Creek Formation in Washington State that consist of a paragenetic sequence of authigenic mineral phases. At two study sites the pipes grade into the base of seep deposits. The studied samples of concentrically-mineralized conduits are cylindrical in shape, measure 2–3 cm in diameter, and are up to 15 cm in length; the mineralized conduits are referred to as pipes hereafter. The earliest mineral phase in the paragenetic sequence is matrix micrite, making up the outer part of the pipes. Toward their centers the pipes are filled by two intercalated cement phases, (1) clear, banded and botryoidal aragonite and (2) micro-to cryptocrystalline yellow aragonite. The innermost portions of the pipes are filled by brownish calcite, microspar, or pipe-filling micrite. The observed paragenetic sequences archive successions of biogeochemical processes. The δ13C values of clear and yellow aragonite cements range from −50.6 to −41.2‰, suggesting that their main carbonate source was provided by anaerobic oxidation of methane. In contrast, the later phases (brownish calcite, microspar, pipe-filling micrite) are relatively enriched in 13C (δ13C values: −2.3 to 3.9‰), pointing to carbonate precipitation from fluids affected by methanogenesis. The size and morphology of the pipes suggest that initial excavation was caused by burrowing organisms. The burrows subsequently acted as preferred fluid pathways, suggesting that the transport of fluids within near-surface sediments at the Oligocene seeps was strongly influenced by the activity of the local fauna. Possible producers of the burrows were sediment-dwelling crustaceans, specifically callianassid decapods, as well as bivalves.

Bacterial mediation, red matrices diagenesis, Devonian, Montagne Noire (southern France)

Two Devonian red carbonate rock sections are studied in the Montagne Noire, at Coumiac (Frasnian/Famennian) and at the Pic de Vissou (Eifelian/Givetian). The sediments are grey-red mudstones and wackestones rich in pelagic fossils. They are characteristic of an outer ramp. The Coumiac sequence is condensed with numerous hardgrounds and hiatuses. The Pic de Vissou succession is more complete and of shallower origin. In both cases, the origin of the red coloration of the micritic matrix is probably linked to bacterial activity which produced submicronic hematite. Both iron and manganese concentrations are low (average 0.2%). Bacteria form ferruginous microstromatolites, blisters, microtufts, `hedgehogs' filling sponge perforations and thin continuous mineralized films (probably biofilms). Hardgrounds are underlined by ferruginous microstromatolites. The origin of the matrix color is probably related to the destruction of these bacterial constructions, the submicronic hematite ultimately coating the crystal faces of the calcite mosaic. During early lithification, microfissures appeared and were invaded by microbial colonies. Scanning electron microscopy (SEM) shows that these colonies are composed of spheroidal beads. This suggests continuity of the bacterial activity during early diagenesis. Later on, these early fissures were cut by burrows. Subsequently a secondary fissure network transected all the previously mentioned sedimentary structures. This late fissure network is characterized by diagenetically remobilized hematite and/or calcite. The latest alterations are stylolites and ultimate tectonic fractures.

Rôle des organismes microbiens dans la formation des matrices rougeâtres paléozoïques: exemple du dévonien, Montagne Noire

The Middle and Upper Devonian red ≪ griottes ≫ limestones of the Montagne Noire (South of France) are deposited on a distal hemipelagic outer ramp, well below the storm wave base and the photic zone, by more than about a hundred meters of water, in poorly oxygenated environments. The red coloration of the micritic matrix is probably related to bacterial activity, and more specifically to iron-bacteria linked to the Siderocapsaceae. These bacterial communities formed benthic microtufts and mats which trapped the ferrous iron. The destruction of these mineralogical-microbial communities allowed dispersion of the submicronic hematite in the micritic matrices. In other European Paleozoic red carbonate matrices (≪ griottes ≫), the red pigmentation is also related to iron-bacteria similar to the Recent Beggiatoales and Siderocapsaceae.

Chemical and mechanical processes during burial diagenesis of chalk: an interpretation based on specific surface data of deep‐sea sediments

Burial diagenesis of chalk is a combination of mechanical compaction and chemical recrystallization as well as cementation. We have predicted the characteristic trends in specific surface resulting from these processes. The specific surface is normally measured by nitrogen adsorption but is here measured by image analysis of scanning electron micrographs. This method concentrates on the micritic matrix alone. Deep‐sea sediments are ideally suited to the study of burial diagenesis because they accumulate in a relatively conservative tectonic setting. We used material from the Ontong Java Plateau in the Pacific, where a > 1 km thick package of chalk facies sediments accumulated from the Cretaceous to the present. In the upper 200–300 m the sediment is unconsolidated carbonate ooze, throughout this depth interval compaction is the principal porosity reducing agent, but recrystallization has an equal or larger influence on the textural development. In the chalk interval below, compaction is not the only porosity reducing agent but it has a larger influence on texture than concurrent recrystallization. Below 850 m grain‐bridging cementation becomes important resulting in a lithified limestone below 1100 m. This interpretation is based on specific surface data alone, and modifies current diagenetic models.

Palaeontological response to sea-level change: Distribution of fauna and flora in cyclothems from the Lower Pseudoschwagerina limestone (Latest Carboniferous, Carnic Alps, Austria)

Microfacies and palaeontological analysis in cyclothems from the Lower Pseudoschwagerina Limestone (Latest Carboniferous, Carnic Alps, Austria) show characteristic features in content and distribution of biota in the different stages of sea level. Intervals of transgressions favor fast-growing organisms. The deposits of the early transgression are dominated by oncoids and bioclasts. The following, later stage of transgression is dominated by the dasycladacean, Anthracoporella, building massive reef mounds. The maximum flooding surface interval lacks sessile biota; deeper water fauna is common herein. The shallow-water organisms included in a micritic matrix occurring in deposits of the maximum flooding zone are probably shed to deeper water. Intervals of regression are characterized by reworked organisms from the underlying deposits from the maximum flooding interval. The environmental conditions are not favorable to biota development. Settings with high-frequency sea-level changes, as the Upper Paleozoic of the Carnic Alps in this paper, provide excellent data on palaeontological response to sea-level changes.

Selective Dolomitization in Arabian Carbonate Rocks

Selective dolomitization is a distinctive petrographic feature observed in the Upper Permian Khuff and Upper Jurassic Jubaila and upper Hanifa carbonate rocks exposed in Central Saudi Arabia. In broader terms, an intimate affinity exists between dolomite replacement and original micrite matrix in the limestone, so that the former steadily increases with the increased abundance of the latter. In closer terms, however, each one of the carbonate constituents in limestone (micrite matrix, allochems and sparry calcite cement) has shown variable susceptibility to dolomite replacement. In general, the Khuff and Jubaila limestones were differently affected by dolomitization in both intensiveness and texture of the replacin~ dolomite. The original depositional constituents of the Khuff rocks were usually affected by complete (indiscriminate) dolomitization resulting in extremely fine-textured dolomite, whereas those of the Jubaila and adjacent rocks were affected by partial (selective) or complete dolomitization resulting in coarser-textured dolomite. It seems that the general contrast in apparent dolomite behaviour in the examined rocks is mainly attributed to whether dolomitization occurred earlier or later than the diagenetic cementation of carbonate sediments. Subsequently, early and late phases of dolomitization might be envisaged to have respectively antedated and postdated the neomorphic transformation of carbonate sediments from aragonite into calcite. Seli:ctive dolomitization could be explained in the light of certain factors which may separately or collectively influence the dolomite replacement of each carbonate constituent. The three main factors are: pre-dolomitization minenalogy, grain size and permeability of the original calcium carbonate sediments and rocks. A general order for the sequence of variable susceptibility to dolomitization of the various carbonate constituents is established for the examined rocks.

Nacre in an early gryphaeid bivalve (Mollusca)

McRoberts (1992, figs. 4.13, 4.14, 6.8) illustrated the shell microstructure of late Triassic Gryphaea (Gryphaea) arcuataeformis Kiparisova, 1936, and Gryphaea (Gryphaea) nevadensis McRoberts, 1992. McRoberts (1992, p. 33) described the left valve of G. arcuataeformis as showing “neomorphosed calcite with multiple laminae of ?prismatic structure perpendicular to [the] outer shell surface ….” He described the left valve of G. nevadensis as consisting of two distinct layers of neomorphosed calcite:“…an outer layer with ?prismatic structure occasionally with bands of dark material (?micritic matrix), and a much thinner inner layer with ?cross-foliated structure ….” Subsequent study has shown these microstructural diagnoses to be inaccurate. They are revised as follows.

Upper Cretaceous Peritidal Deposits of Olib and Ist Islands (Adriatic Sea, Croatia)

Upper Cretaceous carbonate deposits of Olib and Ist islands are characteristic of peritidal sediments. They consist of shallow subtidal deposits alternating with intertidal laminites (shallowing-upward cycles). Subtidal beds with micritic matrix predominate over peritidal sediments. Determination of the micro- and macrofauna revealed two distinctive assemblages: one of Middle to Upper Cenomanian and the other of Upper Turonian to Lower Santonian age. The relatively high proportion of subtidal over intertidal sediments (subtidal/intertidal ratio 2.72) indicates that the Middle to Upper Cenomanian beds were deposited during a fall in the third order relative sea-level curve (late HST to LST). Sediments of the next carbonate sequence (Upper Turonian to Lower Santonian) with a higher proportion of the subtidal over intertidal sediments (subtidal/intertidal ratio 4.57) indicate deposition during relative sea-level rise (TST) and highstand (HST). Senonian limestones are overlain by sediments of Lower Lutetian age.

The facies properties and depositional environments of nodular limestones and red marly limestones (Ammonitico Rosso) in the Ankara Jurassic sequence, central Turkey

ABSTRACTThe nodular limestones and red marls of the Ankara region, deposited during the early to middle Jurassic, show similar palaeontological and sedimentological characteristics to those of the red nodular limestones form the Northern Alps (Adnet limestones) and from the Southern Alps (Ammonitico Rosso).The nodular limestones appear to be hardground breccias drowned into the red marly limestones due to the instability of the bottom. The association of sponge spicules, crinoid fragments, small ostracods, benthic foraminifers, shell debris and common micrite matrix suggests a subtidal environment. The subsequent formation of red marly limestones consists of the partial dissolution of the shells; this suggests that a low sedimentation rate and/or sedimentological breaks took place during the precipitation of the ammonite‐bearing marls.The nodular limestones (hardground breccias) and the Ammonitico Rosso‐type facies of the Ankara Jurassic succession were formed in a deeper subtidal environment and/or deeper shelf extending into the basin. The hardground layers drowned into the Ammonitico rosso were likely formed on a local carbonate shelf, that deepened increasingly through the early to middle Jurassic. Development of a local submarine clastic fan within the carbonate succession of the Ankara Jurassic basin indicates an irregular bottom topography induced by the syn‐sedimentary faults.

Depositional environment and diagenesis of carbonates at the Mamu/Nkporo formation, anambra basin, Southern Nigeria

At Leru-Okigwe, the Enugu-Port Harcourt expressway cuts through the Campanian Nkporo Shales which pass upwards into the cyclic, ripple laminated sandstones and shales of the Lower Maastrichtian Mamu Formation. The Mamu Formation at this locality consists of a 60 m thick shale-sandstone sequence with the basal and middle part of the section consisting of a total of 9 carbonate units. These carbonate units vary from 10 to 70 cm in thickness, cyclically interbedded with shales and are overlain by coarsening upwards sandstone bodies. Detailed mapping and petrographic studies indicate that the carbonate units are divisible into a lower finely laminated mudstone which passes upwards into oolitic packstone/grainstone in the middle and is overlain by an upper set of laminated mudstones. The lowest mudstone unit (dark grey to greenish rock) is finely laminated, pelleted, oncolitic and sparsely fossiliferous. The oolitic packstone/grainstone consists of oolites cemented together by siderite microspar. Identifiable bioclasts include tests of small size benthic foraminifera, pelecypods and rare ostracod carapace. This unit attains a maximum thickness of about 70 cm. The upper mudstone units consist essentially of uniformly recrystallised siderite microspar. Intraclasts include micritised pelecypod fragments and small foraminifera tests. Ovoid, flat bottomed and biconvex vugs developed good geopetal structures in the mudstone. Petrographic, Xray diffraction and microprobe analyses indicate that the carbonate constituent in these units consists of solid solutions of FeCO 3MgCO 3CaCO 3 and minor MnCO 3. Sideritization, the dominant replacement process has led to the recrystallization of the micritic matrix and microcrystalline siderite is commonly associated with goethite, chamosite relics and quartz. The carbonates with associated chamosite are thought to have formed in a shallow marine subtidal to intertidal environment developed during periods of rise and fall in sea level. Formation of chamosite-bearing oolites record periods of increasing wave energy corresponding to storm conditions between quiet shallow marine sedimentation. At least five diagenetic stages involving micritization, dissolution of the primary chamosite, replacements of chamosite by siderite cement, growth of blocky calcite and a continuing replacement of the preexisting minerals by goethite were establised from textural and compositional evidence. The recognition of shallow marine subtidal to intertidal environments for carbonates at the Mamu/Nkporo Formation transition supports continuous marine influences with periodic subaerial exposure of sediments in the Mamu deltas after the deposition of the prodeltaic Nkporo Shales.