Micritic Crusts Research Articles

Episodic massive release of methane during the mid-Cretaceous greenhouse

Abstract Methane-derived carbonates (MDCs) are common along modern and ancient continental margins, and the majority of such formations are associated with seafloor cold seeps. Here, we document petrographic, rare earth element + yttrium (REE + Y), carbonate clumped isotope temperature (TΔ47), and carbon-isotopic evidence from a shale succession in southern Tibet spanning a ~28 m.y. interval (ca. 113–85 Ma) that coincided with the mid-Cretaceous greenhouse event. At least nine limestone nodule-bearing horizons exhibit seep-associated sedimentary structures, including carbonate fabrics (e.g., micritic crusts, crystal fans, and botryoidal textures) and 13C-depleted isotopic compositions (δ13Ccarb < –32.3‰), which are indicative of methane-derived carbon sources. Along with sedimentary evidence, the patterns of TΔ47–δ13Ccarb–δ18Ocarb support precipitation of these MDCs over a large temperature range. The REE + Y compositions and europium (Eu) anomalies indicate that the release of methane was associated with hydrothermal fluids. Methane may have been derived from both thermogenic and biogenic sources based on the inorganic carbon isotopic signatures of the carbonate. These nodular carbonate horizons document multiple episodes of seafloor methane release during the mid-Cretaceous and represent exceptionally long-lived, active methane seepage. Massive methane release events may have played a role in generating the greenhouse climate of the mid-Cretaceous.

Earliest Carboniferous stromatolites from the Qianheishan Formation, Dashuigou section, northwestern China: Implications for microbial proliferation after the end‐Devonian mass extinction

Earliest Carboniferous stromatolites were found in the Qianheishan (QHS) Formation in the Dashuigou section, Gansu Province, northwestern China. These stromatolites are exposed in a conglomerate‐dominated succession of about 22.5 m in thickness, which can be traced laterally for more than 200 m in outcrop. They consist mainly of micrite, peloids, oncoids, silt‐sized quartz grains, and sparry calcite with rare fine to coarse quartz grains and bioclasts. The occurrence of marine fossils (e.g., bryozoans) and fenestral structures in the stromatolites suggests that they developed in intertidal environments. Three types of stromatolite laminae were distinguished including micritic laminae, grain‐dominated mixed laminae, and micrite‐dominated mixed laminae. These laminae are separated by thin micritic crusts and form two lamination styles of repetitive lamination and alternating lamination. The development of grain‐rich and micrite‐rich laminae in the stromatolites implies that they were formed by the combination of microbial baffling, binding, and calcification. Facies analysis suggests that growth and demise of the stromatolites were controlled by relative sea‐level changes. They grew during a continuous relative sea‐level rise, indicated by variations in their thickness and morphology. Stromatolites change from thin‐bedded laminar forms in the lower part of the studied interval to medium‐bedded wavy‐laminar forms and thick‐bedded domal forms in the middle and upper parts, respectively. Their demise was triggered by dramatic relative sea‐level fall, evidenced from the sandstone facies overlying the stromatolite interval. The occurrence of the QHS stromatolites provides an excellent example for microbial proliferation and profound changes in the marine biosphere after the end‐Devonian Hangenberg extinction event.

The Importance of Microbial Binding In Neogene–Quaternary Steep Slopes

Abstract: The geometry and inclination of carbonate-platform slopes are typically described as the result of the sediment texture, components, and grain size occurring in the slope. Three carbonate slopes from the Holocene of the Tongue of the Ocean (Bahamas) and three slopes from an upper Miocene carbonate platform in SE Spain are investigated to identify the factors controlling dip angles. Although platforms differ in size and elevation with regard to the surrounding basin, both locations present slopes with linear profiles and steep angles over 35°. The slope facies in both locations are very similar, with the matrix and some bioclasts presenting microfabrics that evidence microbial activity during deposition. These include (1) clotted micrite patches locally connecting bioclasts or infilling primary pores, (2) porostromate structures in the micrite, (3) dense micritic masses, (4) binding structures, (5) micritic crusts, and (6) peloidal textures. The Holocene and Miocene depositional geometries and facies distribution are the response to various sedimentary processes including rockfalls, gravity flows, and in situ carbonate production. Extensive microbial binding is identified as the controlling factor for early stabilization of the slope, which in combination with subsequent rapid cementation produces and maintains the steep slope angles. Consequently, microbial-induced stabilization of modern and Neogene slopes is as significant as in Paleozoic and Triassic examples.

Construction of the earliest stromatoporoid framework: Labechiid reefs from the Middle Ordovician of Korea

The Great Ordovician Biodiversification Event (GOBE) involved a remarkable rise of skeletal reefs during the late Middle Ordovician. A new type of skeletal patch reef built by labechiid stromatoporoids is reported from the Yeongheung Formation (Darriwilian) of the Korean Peninsula, part of the eastern North China Block, a hot-spot for late Middle Ordovician diversification of early labechiids. The attachment of laminar labechiids, subordinate peloidal micritic crusts and spiculate sponges, and minor bryozoans on peloidal–intraclastic packstone to grainstone substrates, and upward growth of these organisms by repeated encrustation, resulted in the construction of centimeter-scale domal tight frame structures. These skeletal frames were in turn veneered by larger labechiids, forming sub-meter-scale laminar skeletal reefs with primary cavities. Such skeletal reefs that formed by the encrustation and binding of labechiid stromatoporoids are analogous to the bindstone frameworks of coeval Laurentian reef mounds and skeletal reefs, though they differ markedly in composition, as is apparent in the absence of tabulate corals and markedly lower contribution of bryozoans, both of which were key elements of the Chazy reefs of Laurentia. The Yeongheung reefs described herein represent some of the oldest labechiid skeletal reefs known to date, and formed mainly by the aggregation of early stromatoporoids prior to the arrival of corals and other robust constructors in the region. This discovery provides new information on the evolutionary trends of early skeletal reefs in response to the appearance of new reef-building organisms and their regional distributions during the Ordovician radiation.

Microbial community from the Lower Permian (Artinskian–Kungurian) paleoclimatic transition, mid-Panthalassan Akiyoshi atoll, Japan

Large oncoids (up to 14cm) and microbialites are abundant in the Artinskian (Lower Permian) section of mid-Panthalassan Akiyoshi atoll carbonates. The oncoids and microbialites consist mainly of a tubular microproblematicum, girvanellid cyanobacterial filaments, microbial micrite crusts, and pore-filling sparry calcite cements. They are surrounded by intraclastic–bioclastic grainstone/rudstone, indicative of moderate- to slightly high-energy subtidal conditions. The microbial community was the primary boundstone-forming organisms on the Akiyoshi atoll during this time. It represents a transitional stage in a mid-Panthalassan reef succession between a cooler-water autotrophic Palaeoaplysina–microencruster community in the Gzhelian–Asselian and a warmer-water heterotrophic calcareous sponge–microencruster community in the Middle Permian. The flourishing mid-Panthalassan microbial community during the late Early Permian is related to enhanced alkalinity, increasing nutrient levels, elevated sea-surface temperatures, and the absence of major reef-building metazoans, which resulted from Gondwanan deglaciation, climatic changes, and a pulse of active volcanism.

A NEW MODEL FOR THE FORMATION OF DOLOMITE IN THE TRIASSIC DOLOMITES, NORTHERN ITALY

The Pale di San Martino and Pale di San Lucano (referred to together as the “Pale”) are remnants of an originally more extensive carbonate platform in the Dolomite Mountains of northern Italy. The platforms are composed of Middle Triassic dolomites and limestones up to 1.6km thick. Limestones comprise 2–3% of the platform carbonates and are restricted to narrow corridors (tens to a few hundred metres wide, hundreds of metres long and high) within the dolomite. The mainly sucrosic dolomites of the Pale are interpreted as the result of recrystallization of a depositional, nearly stoichiometric Mg calcite under burial temperatures of ca. 40–70°C. The principal arguments are: The quantitative composition indicates that all platform carbonates are composed mainly of micritic crusts (45%; boundstone fabric prevails) and early cement (35%; microcrystalline, fibrous). The platform carbonates were probably mainly bacterial precipitates and tight at the sediment-water interface (porosities <5%, permeabilities in the micro-Darcy range). The limestone-dolomite transitions (centimetres to decimetres wide) lack dolomite gradients. The lack of evidence for flowing fluids causing dolomitization suggests stagnant pore waters. The δ13C of average dolomite is 1.3‰ heavier than that of coeval limestone (666 analyses). The difference corresponds to a primary difference of 50mol% MgCO3 and is interpreted as the result of fractionation. It suggests a dolomite precursor of very high Mg calcite, whereas present-day limestone of the Pale was probably deposited as a basically Mg-free polymorph (aragonite and/or calcite). The dolomite δ18O (+1 to −11‰ VPDB) values show a scatter over the platform thickness and preserve randomly distributed values around 0‰. The scatter is probably due to selective re-setting of δ18O near pore spaces and is mainly a sampling effect. The observation that 87Sr/86Sr ratios (77 analyses) of limestone and dolomite are either slightly higher or lower than Middle Triassic seawater, but almost never “normal marine”, suggests that the platform carbonates of the Pale were deposited from seawater contaminated with artesian freshwater. The limestone corridors are probably caused by artesian springs of somewhat higher than ambient depositional temperature, with low Mg calcite and/or aragonite deposited in or near fracture zones. The volumetrically subordinate cycle-cap dolomite is possibly a primary precipitate.

Origin and significance of lamination in Lower Cretaceous stromatolites and proposal for a quantitative approach

Stromatolite lamination is typically defined as alternation of dark and light laminae. Study of Lower Cretaceous stromatolites from the Leza Fm (N Spain) supports this statement, but recognises additional complexities in lamination that have implications for interpreting accretion processes. These stromatolites are partial analogues of present-day coarse-grained carbonate stromatolites in the Bahamas and Shark Bay (Australia) that mainly form by trapping and binding carbonate sand. The Leza examples contain both grain-rich and micrite-rich laminae with scarce particles, suggesting that they accreted both by trapping and not trapping grains. Lamination in modern and ancient coarse-grained stromatolites is commonly defined by thin micritic crusts that formed during interruptions in accretion and separate contiguous grainy laminae (repetitive lamination). Leza examples also contain these thin hiatal crusts and locally show repetitive lamination, but their conspicuous macroscopic lamination is defined by thicker alternating grain-rich and micrite-rich laminae (alternating lamination). This indicates that, although hiatuses in accretion occurred, change in accretion process was the main cause of macroscopic lamination. These differences in accretion processes and lamination styles between Leza examples and modern coarse-grained stromatolites may reflect differences in their environmental settings. Modern examples occur in shallow marine tidal environments, whereas Leza Fm coarse-grained stromatolites developed in tide-influenced water-bodies in coastal-wetlands that experienced fluctuations in water salinity and hydrochemistry. Analysis of lamina-thickness in these Cretaceous stromatolites and similar published examples provides a quantitative approach to the processes that underlie stromatolite lamination.

Geologia e paleontologia do biohermito da Formação Pirabas (Mioceno Inferior)

O presente trabalho compreende o estudo geológico e paleontológico detalhado da litofácies recifal da Formação Pirabas, Estado do Pará, aflorante na Praia do Maçarico, município de Salinópolis, como um corpo pequeno, isolado e lenticular, que deve ter se desenvolvido na rampa interna da plataforma do Mar de Pirabas. O conteúdo biótico é rico e variado, composto por três grupos de algas coralíneas, poríferos, corais, briozoários, equinoides, foraminíferos, ostracodes, e moluscos. Foram reconhecidos neste recife em mancha três estágios de sucessão ecológica, estabilização, colonização e diversificação, onde as finas crostas micríticas reconhecidas na parte superior do estágio diversificação indicam degradação das condições ambientais favoráveis à acreção recifal, provavelmente devido a continentalização do sítio deposicional. O exame petrográfico revelou apenas uma microfácies deposicional, biomicrito, sem consideráveis variações texturais e composicionais. A cimentação marinha é predominante, representada por massas microcristalinas e cimentos micríticos, localmente neomorfisados para bioesparito, e detritos carbonáticos advindos do fluxo biogênico inconsolidado, resultantes principalmente da dispersão de cristais de aragonita dos fragmentos algálicos, acumulados sob a forma de lama micrítica. Para a gênese da bioconstrução as algas foram fundamentais na consolidação e modelagem do arcabouço, principalmente as macroalgas vermelhas, decisivas para a proteção e cimentação dos demais organismos.

Early Ordovician reef construction in Anhui Province, South China: A geobiological transition from microbial- to metazoan-dominant reefs

The Lower Ordovician records a transition from microbial- to metazoan-dominant reefs immediately prior to the Great Ordovician Biodiversification Event. The Hunghuayuan Formation (late Tremadocian-early Floian) of the Jianxin section in Anhui Province, South China, includes well-preserved reefs, which consist of a combination of microbial boundstones and microbial-lithistid sponge-receptaculitid boundstones. The microbial boundstones are characteristically made up of nodular or columnar stromatolites produced by an irregular alternation of Girvanella-rich layers and thick micrite layers in which the filamentous calcimicrobe Girvanella and bioclasts occur sporadically. Microbial biofilms, including the filamentous microbes, may have trapped and bound lime mud and/or precipitated micrite during their growth and metabolic activities. These activities contemporaneously contributed to substrate stabilisation, encrustation, and the construction of microbialite frameworks (stromatolites). In contrast, the microbial-lithistid sponge-receptaculitid boundstones are characterised by more micritic constituents than skeletal reef-building constituents. Thin and/or domal crusts produced by peloid and dense micrite, together with various proportions of Girvanella, occur upon bioclastic sediments and reef-building skeletal organisms. These micritic crusts with Girvanella therefore played roles as binders and stabilisers, and as consolidators by encrusting reef-building skeletal organisms. In the boundstones, it is remarkable that the degradation of lithistid sponges, due to microbial decay, produced micrite in the sponge bodies. As a result, lithistid sponges played an apparently inconspicuous but potentially significant role as micrite producers as well as automicrite framework constructors. These Lower Ordovician reefs, in which microbialites (e.g., stromatolites) dominate but coexist with metazoans (e.g., sponges), were widespread at the time. They provide excellent examples for understanding reefal palaeoecology during the transition from microbe-dominant reefs to reefs enriched in metazoans.

Lithified and Unlithified Mg-Calcite Precipitates in Tropical Reef Environments

Abstract Push cores from the branching-coral framework of lagoonal reefs in Belize and Panama indicate that the micritic Mg-calcite in the unlithified mud matrix is similar in composition to the well documented submarine lithified micritic crusts and infillings found in open reef framework. The sediment matrix of the lagoonal framework contains significant concentrations of 12 to 13 mole % MgCO3 calcite that shows almost no signs of skeletal origins, as indicated by a fairly consistent mineralogy and independent distribution patterns in sand-, silt-, and clay-size fractions. Its sedimentological and mineralogical similarity to cemented micritic crusts and infillings suggests that the precipitation of micritic Mg-calcite is widespread in tropical-reef waters and becomes lithified when precipitated in open frameworks but remains unlithified when trapped in impermeable lagoonal muddy sediments.

Sedimentologic Role of Microproblematica Donezella in a Lower Pennsylvania Donezella-Siliceous Sponge-Dominated Carbonate Buildup, Frontal Ouachita Thrust Belt, Oklahoma, U.S.A.

Abstract The problematic organism Donezella emerged worldwide as one of the main carbonate mound builders in the middle Carboniferous. Donezella is composed of thin (70–160 μm in diameter), bifurcating (45 to 90 degrees) tubes. Microfacies analysis of a Donezella buildup from the Wapanucka Formation, Frontal Ouachita thrust belt, Oklahoma, U.S.A., was undertaken to examine the role of Donezella in a carbonate buildup. Donezella is the predominant biotic component of the Wapanucka buildup, and the buildup is composed of irregular pockets of boundstone dominated by Donezella, worm tubes, peloids and micritic crusts, siliceous sponge spicules, less common siliceous sponge boundstone, and peloidal wackestone with other bioclasts. In the lower part of the buildup, Donezella colonies are robust, convex upward, and up to several centimeters in diameter. These colonies are composed of delicately branching Donezella with an enclosed constructional pore system. These robust, upward-growing Donezella structures formed a substrate for other binders; therefore it appears that, on a microscale, Donezella, together with siliceous sponges and worm tubes, played the role of a constructor. Donezella occurs as a long, semicontinuous row of thalli when encrusting other framework grains, and encrusts siliceous sponges, Donezella colonies, corals, and worm tubes; thus, it also acted as a member of the binder guild. Deciphering these various roles of Donezella is a crucial first step toward documenting a possible spectrum of Donezella-dominated lithologies that constitute key elements in carbonate buildups in the Early to Middle Pennsylvanian.

Microbial structures and microencrusters in the Upper Jurassic - Lower Cretaceous deposits from Buila-Vânturariţa massif (South Carpathians)

The Upper Jurassic – Lower Cretaceous limestones from Buila-Vânturariţa massif partly consisting of reef build-ups, are characterised by an important component of microbial organisms. This paper describes and illustrates some of the most important structures interpreted as having a microbial nature: “Tubiphytes” and Bacinella-type structures, cyanobacterial structures, micritic crusts, and peloidal structures. Most of these structures are related to different shallow-water environments, from intertidal, high-energy environment, to protected low energy subtidal environment.

TEM analysis of microbial mediated sedimentation and lithification in modern marine stromatolites

Three sedimentary processes are involved in the growth of living stromatolites at Highborne Cay, Bahamas: (1) trapping of oolitic sands, (2) formation of surface micritic crusts, and (3) formation of fused-grain laminae. The microbial role in each process was investigated by examining the stromatolites using transmission electron microscopy. Species composition and physiological state of the bacteria were discerned by ultrastructure. A well-dispersed population of Schizothrix gebeleinii was observed in rapidly accreting surface layers. Their filaments produce copious quantities of amorphous exopolymer and condensed sheath that surround individual, and pairs of cells. Both fresh and degraded sheaths are, however, devoid of carbonate precipitates. This suggests that the primary roles of S. gebeleinii are the trapping and binding of unconsolidated sediment and the production of extracellular polymeric secretions. Surface micritic layers are composed primarily of needle-shaped crystals of aragonite. The uppermost surface of the micritic crust is coated with a biofilm comprised primarily of small Gram negative bacteria (i.e., sulfate reducing bacteria) that range in size from 250 to 500 nm in diameter and up to 1 μm in length. Empty cyanobacterial sheaths and occasional Gram positive spores were also observed. Thin sections through resin-casts of ooid microborings in the fused-grain laminae show cells of the endolithic cyanbacterium Solentia sp. and evidence of an organic matrix. The micritic crusts and fused-grain layers became lithified laminae that were preserved at depth, although the organisms that formed them were not. This suggests that morphological remains of these organisms (i.e., microfossils) in ancient stromatolites should not be expected.

Formation of lithified micritic laminae in modern marine stromatolites (Bahamas); the role of sulfur cycling

Other| December 01, 1998 Formation of lithified micritic laminae in modern marine stromatolites (Bahamas); the role of sulfur cycling Pieter T. Visscher; Pieter T. Visscher University of Connecticut, Department of Marine Sciences, Groton, CT, United States Search for other works by this author on: GSW Google Scholar R. Pamela Reid; R. Pamela Reid Search for other works by this author on: GSW Google Scholar Brad M. Bebout; Brad M. Bebout Search for other works by this author on: GSW Google Scholar Shelley E. Hoeft; Shelley E. Hoeft Search for other works by this author on: GSW Google Scholar Ian G. Macintyre; Ian G. Macintyre Search for other works by this author on: GSW Google Scholar John A. Thompson John A. Thompson Search for other works by this author on: GSW Google Scholar American Mineralogist (1998) 83 (11-12_Part_2): 1482–1493. https://doi.org/10.2138/am-1998-11-1236 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Pieter T. Visscher, R. Pamela Reid, Brad M. Bebout, Shelley E. Hoeft, Ian G. Macintyre, John A. Thompson; Formation of lithified micritic laminae in modern marine stromatolites (Bahamas); the role of sulfur cycling. American Mineralogist 1998;; 83 (11-12_Part_2): 1482–1493. doi: https://doi.org/10.2138/am-1998-11-1236 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Microbial mats on the surfaces of modern, marine stromatolites at Highborne Cay, Bahamas, were investigated to assess the role of microbial processes in stromatolite formation. The Highborne Cay stromatolitic mats contain Schizothix as the dominant cyanobacterium and show millimeter-scale lamination: Some layers in the mat are soft (unlithified), whereas other layers are crusty (lithified). Lithified layers within the mats correspond to micritic horizons composed of thin (20-50 mu m) micritic crusts, which commonly overlie truncated, micritized carbonate sand grains. These features are identical to lithified laminae in the underlying stromatolite; the micritic crusts are similar in thickness to micritic laminae in many ancient stromatolites. Biogeochemical parameters in a representative stromatolitic mat from Highborne Cay were measured to identify the role of bacteria in precipitation and dissolution of CaCO 3 . Depth distributions of O 2 , HS (super -) , and pH were determined with microelectrode measurements in the field. Oxygen profiles were used to calculate photosynthesis and aerobic respiration. Sulfate reduction was determined using 35 SO 4 (super 2-) and sulfide oxidation potential was measured in homogenized samples. Our results indicate that cyanobacterial photosynthesis, sulfate reduction, and anaerobic sulfide oxidation in stromatolitic mats at Highborne Cay are responsible for CaCO 3 precipitation, whereas aerobic respiration and aerobic sulfide oxidation cause CaCO 3 dissolution. A close coupling of photosynthesis and aerobic respiration in the uppermost few millimeters of the mats results in no, or very little, net lithification. Sulfur cycling, on the other hand, shows a close correlation with the formation of lithified micritic layers. Photosynthesis, combined with sulfate reduction and sulfide oxidation results in net lithification. Sulfate reduction rates are high in the uppermost lithified layer and, on a diel basis, consume 33-38% of the CO 2 fixed by the cyanobacteria. In addition, this lithified layer contains a significant population of sulfide-oxidizing bacteria and shows a high sulfide oxidation potential. These findings argue that photosynthesis coupled to sulfate reduction and sulfide oxidation is more important than photosynthesis coupled to aerobic respiration in the formation of lithified micritic laminae in Highborne Cay stromatolites. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Growth History of Stromatolites in a Holocene Fringing Reef, Stocking Island, Bahamas

ABSTRACT A stromatolite and algal ridge reef complex 2.1 m thick fringes the east coast of Stocking Island, Exuma Cays, Bahamas. This reef was established on a Pleistocene calcarenite terrace about 4500 yr B.P. Stromatolites, which occur in back-reef and reef-flat zones, are up to 1 m thick and were constructed by cyanobacterial-dominated communities. Study of the growth history of these stromatolites, ranging in scope from facies analyses to details of microfabric construction, presents new perspectives on stromatolite formation. Lithologies identified in eight cores from across the Stocking Island reef complex, together with plots of 13 radiocarbon dates in relation to a Bahamian sea-level curve, indicate that this reef began as an intertidal vermetid gastropod buildup. Subsequent flooding of the Pleistocene terrace allowed the branching coralline alga Neogoniolithon strictum to overgrow the vermetids and eventually form an emergent algal ridge about 1500 years ago. Shifting sands accumulated in the lee of this ridge and excluded most benthic communities and herbivores, thereby promoting growth of cyanobacterial mats that formed stromatolite buildups. With a decrease in wave energy over the last 500 years, possibly due to the growth of offshore patch reefs, the urchin Echinometra lucunter coloni ed the algal ridge. Resultant bioerosion by this urchin destroyed the emergent part of the ridge and is now undercutting the seaward edge of the stromatolite buildups. Lamination in the Stocking Island stromatolites results from early lithificafion processes in cyanobacterial mats, possibly in response to biogeochemical changes in the mats during hiatuses in sediment accretion. These processes, which create partially indurated laminae with a distinct microstructure, involve precipitation of thin micrite crusts, intense microboring along a surface below this crust, micritization of sediment grains, and precipitation of point-contact cement between micritized grains. Introduction of turf algae to the cyanobacterial mat community disrupts formation of the lithified laminae, thereby inhibiting stromatolite development.

Modern marine stromatolites in the Exuma Cays, Bahamas: Uncommonly common

Modern stromatolites in open marine environments, unknown until recently, are common throughout the Exuma Cays, Bahamas. They occur in three distinct settings: subtidal tidal passes, subtidal sandy embayments and intertidal beaches. These stromatolites have a relief of up to 2.5 m and occur in water depths ranging from intertidal to 10 m. Surfaces near the sediment-water interface are typically colonized by cyanobacterial mats, whereas high relief surfaces are commonly colonized by algal turf and other macroalgae such asBatophora, Acetabularia, andSargassum. The internal structure of the stromatolites is characterized by millimeter-scale lamination defined by differential lithification of agglutinated sediment. In thin section, the lithified laminae appear as micritic horizons with distinct microstructures: they consist of thin micritic crusts (20–40 μm thick) overlying layers of micritized sediment grains (200–1000 μm thick); the micritized grains are cemented at point-contacts and are trucated along a surface of intense microboring. The Exuma stromatolites are built by cyanobacterial-dominated communities. These laminated prokaryotic structures grade to unlayered thrombolites built by eukaryotic algae. The variety of sites, settings and shapes of stromatolites in the Exuma Cays present excellent opportunities for future studies of stromatolite morphogenesis.

High energy coralgal‐stromatolite frameworks from Holocene reefs (Tahiti, French Polynesia)

ABSTRACTDrill cores from Holocene reefs on Tahiti (French Polynesia) reveal a framework composed of massive branching acroporids encrusted by coralline algae associated with sessile vermetid gastropods and arborescent foraminifers. Laminated micritic crusts form coatings over coral branches or, more commonly, over related encrusting organisms throughout the cored reef sections; these crusts appear as a major structural and volumetric component of the reef framework. The microbial nature of these micritic crusts is inferred from their typical organic growth forms and geometry, the occurrence of microbial remains and stable isotope measurements. The reef communities accumulated at depths less than 5 m below mean sea level in a high energy environment throughout vertical growth from 7140 ± 170 yr bp to the present. The nature of the involved benthic communities, stable isotope data and high calcification rates of microbially encrusted corals strongly suggest that local environmental conditions have been optimal for reef development for the last 7000 years. The causes of the predominance of microbial communities over actual encrusters (red algae, foraminifers) remain problematic and could be related to short term fluctuations in ecological parameters. Microbial micritic crusts seemingly played a prominent role in protecting the coralgal colonies from bioeroders and grazers and, possibly, in strengthening the framework, due to rapid lithification. The record of similar microbial crusts in other Quaternary reef tracts suggests that microbial communities may have played a more prominent role in Quaternary reefs than presently recognized.

Diagenesis of Pleistocene reef-associated sediments from the Red Sea coastal plain, Egypt

Pleistocene reef-associated sediments from the Red Sea coastal plain of Egypt consist of carbonate to mixed carbonate-siliciclastic sediments. The recognized facies are calcareous sandstone, skeletal rudstone, grainstone, wackestone and micritic crusts. Evidence from cement fabrics (aragonite, high-Mg calcite and gypsum) suggests that cementation mainly occurred in the intertidal zone under marine vadose conditions. However, the interaction of these sediments with meteoric waters resulted in the modification and stabilization of the metastable carbonates, the replacement of detrital feldspars by carbonates and the silicification of carbonates.

Micritic crusts in carbonate platforms

Micritic crusts in carbonate platforms

Laminar micrite crusts and associated foreslope processes, Red Sea; discussion and reply

Laminar micrite crusts and associated foreslope processes, Red Sea; discussion and reply