Search results for "Micrite"
showing 10 items of 19 documents
Microbial-mediated pre-salt carbonate deposition during the Messinian salinity crisis (Calcare di Base fm., Southern Italy)
2017
Abstract A multi-scale analysis of sedimentary carbonate facies and post-sedimentary diagenetic features of the Calcare di Base Formation, the precursor to evaporites in Upper Messinian successions of Northern Calabria and Central Sicily, has revealed their microbial bio-mediated origin. Massive to laminated microbial boundstones represent the most common sedimentary facies forming flat to low relief cm to m scale stromatolitic and thrombolitic bodies. The fabric of the micrite varies from peloidal to aphanitic, and almost always preserves filamentous bacteria which characterized the original microbial mat. The mat was dominated by sulphur-oxidizing bacteria belonging to the Thiotrichaceae,…
Carbonate deposits from the ancient aqueduct of Béziers, France — A high-resolution palaeoenvironmental archive for the Roman Empire
2016
Abstract Carbonate deposits from a Roman aqueduct in Beziers, southern France, record environmental conditions during the late first century C.E. These deposits formed in a steep section of the aqueduct with a high flow velocity, which caused rapid deposition of up to 11 mm of calcite per year over a period of 22–24 years. The microstructure, trace element and stable isotope composition show that regular deposition was interrupted by high-discharge events, probably in response to heavy rainfall during autumn and winter, transporting colloidally- and particle-bound elements and depositing calcite with elevated δ 18 O values. Individual autumn high-discharge events coincide with abrupt decrea…
Influence of a major exposure surface on the development of microporous micritic limestones - Example of the Upper Mishrif Formation (Cenomanian) of …
2017
18 pages; International audience; Microporous platform top limestones of the Cenomanian Mishrif Formation (offshore Qatar) were studied in order to investigate the diagenetic processes associated with the top-Mishrif subaerial unconformity and its influence on the development of microporosity in underlying carbonates. Petrographical and stable isotope results indicate that complex diagenetic changes occurred during subaerial exposure of the Mishrif Formation, including pervasive dissolution and meteoric cementation, as well as neomorphism of the micritic matrix. Micrites at the top of the Mishrif Formation are coarse (i.e. > 2 μm), sub-rounded and very dull luminescent under cathodoluminesc…
The aqueduct of Gerasa – Intra-annual palaeoenvironmental data from Roman Jordan using carbonate deposits
2021
Abstract Calcium carbonate (CaCO3) deposits from Roman aqueducts are an innovative archive to obtain local high-resolution palaeoenvironmental and archaeological data in interdisciplinary studies. Deposits from one of the aqueducts of the Roman city of Gerasa provide a record of 59 years during the 1st to 3rd centuries CE, divided into three sequences separated by plaster layers. Annual carbonate layers show an alternation of sparite, formed in winter, and micrite, formed in summer. Brown bands at the base of many sparite layers probably correspond to large rainstorms in early winter. A fine lamination present in the brown bands may be diurnal in origin. Stable isotope and trace element dat…
Spherulites in Calcrete Laminar Crusts: Biogenic CACO3 Precipitation as a Major Contributor to Crust Formation
1995
ABSTRACT Spherulites (calcitic fibro-radial spherulitic polycrystals) are a major component of calcite layers in Quaternary calcrete laminar crusts. To explain the formation of spherulites, petrographic and chemical studies were performed on Pleistocene calcrete laminar crusts, Holocene biological crusts, and laboratory (in vitro) cultures of cyanobacterial strains. Spherulites were found to be formed of acicular and radiating crystals, either smooth and regular-edged, or twisted and irregular-edged. Spherulites were composed of low-Mg calcite, the concentration of Mg in the spherulites increasing from nucleus to periphery. The shape and chemistry of the in vitro spherulites and laminar cru…
Burial dissolution of micrite in Middle East carbonate reservoirs (Jurassic–Cretaceous): keys for recognition and timing.
2006
14 pages; International audience; This paper discusses burial diagenesis and especially a burial dissolution phase and its effects on micrite microtexture in reservoirs in the Middle East. Three microporous reservoirs were selected: the Cenomanian Mishrif Formation in Iraq; the Kimmeridgian–Tithonian Arab D and the Barremian Kharaib Formations, both in Abu Dhabi. Staining, cathodoluminescence and scanning electron microscopy were used for petrographic observations. In the selected reservoirs, three typical micrite microtextures are distinguished: micro-rhombic, compact anhedral, and rounded. The chronology of diagenesis, based on petrographic observations, shows that mineralogical stabilisa…
Carbonate stromatolites from a Messinian hypersaline setting in the Caltanissetta Basin, Sicily: petrographic evidence of microbial activity and rela…
2010
Lower Messinian stromatolites of the Calcare di Base Formation at Sutera in Sicily record periods of low sea-level, strong evaporation and elevated salinity, thought to be associated with the onset of the Messinian Salinity Crisis. Overlying aragonitic limestones were precipitated in normal to slightly evaporative conditions, occasionally influenced by an influx of meteoric water. Evidence of bacterial involvement in carbonate formation is recorded in three dolomite-rich stromatolite beds in the lower portion of the section that contain low domes with irregular crinkly millimetre-scale lamination and small fenestrae. The dominant microfabrics are: (i) peloidal and clotted dolomicrite with c…
The beast burrowed, the fluid followed – Crustacean burrows as methane conduits
2015
Abstract An extensive pockmark field with associated methane-seep carbonates has recently been reported from the late Albian (Lower Cretaceous) Ubidepea Mudstone of the Black Flysch Group in the Basque Country, northern Spain, but the exact pathways of the migrating methane-rich fluids remained elusive. Here we provide petrographic, stable carbon and oxygen isotope evidence that abundant crustacean burrows in the surrounding mudstone, preserved as the trace fossil Thalassinoides, and the seep carbonates themselves have acted as long-lasting fluid conduits in this system. The Thalassinoides infill generations show a diagenetic parasequence often starting with a distinctive lining, followed b…
Partial high-pressure aragonitization of micritic limestones in an accretionary complex, Tavşanlı Zone, NW Turkey
2006
Pelagic micritic limestones within an upper Cretaceous accretionary complex in the Tavsanli Zone, NW Turkey, preserve textures indicating incomplete prograde transformation of micritic calcite to aragonite, representing the only known example of this type. Aragonitization starts at the central parts of the micritic limestone beds and advances towards the lower and upper parts of the layers at the expense of micrite. Micrite is very fine grained (<0.003 mm) and contains radiolaria, foraminifera and thin shell fragments. Aragonite forms large crystals, up to 3 cm across, with straight grain boundaries and c-axis mostly subparallel to the carbonate beds. Relict micritic portions are devoid of …
Calcitic radial palisadic fabric in freshwater stromatolites: diagenetic and recrystallized feature or physicochemical sinter crust?
1999
Abstract When favorable conditions occur for carbonate precipitation (physicochemical processes) in tufas and travertines, organisms intervene either directly (the framework created by organisms acts as a crystal support) or indirectly (organic products influencing the mineralogy, nucleation, growth and habit of primary crystals). In the literature, fluvial and travertine crystalline sparite crusts with palisadic or prismatic crystals are generally interpreted as the result of early recrystallization of micrite in algal buildups (mostly composed of Schizothrix). Therefore, biological mediation is essential for the construction of continental bioherms and biostromes, even if organic remains …