Data di Pubblicazione:
2012
Citazione:
High-relief microbial boundstone platforms / G. Della Porta, T. Playton, N. Preto, J.A.M. Kenter, J.R. Bahamonde, O. Merino Tome’, P.M. Harris. ((Intervento presentato al convegno AAPG HEDBERG Conference Microbial carbonate Reservoir Characterization tenutosi a Houston nel 2012.
Abstract:
Carbonate platforms with high-relief, steep depositional slopes and margins dominated by microbial boundstone do not occur in present-day tropical marine sedimentary environments. These types of platforms (defined as M-type carbonate factory by Schlager, 2003) are, however, well-represented in the Phanerozoic geologic record. They accumulated following mass extinction events of reef-building marine organisms with a rigid carbonate skeleton, in particular during the late Paleozoic (post Frasnian) and Mesozoic. This study aims to outline some common characteristics of high-relief microbial platforms with respect to platform geometry, facies belts and lithofacies types based on the evaluation of several case studies from outcrops and published literature of Late Devonian, Carboniferous, Permian, Middle Triassic and Early Jurassic age.
The term microbial boundstone is here meant to include those reef lithofacies consisting of variable proportions of: 1) microbial micrite and microspar, 2) early marine cement, and 3) skeletal biota.
1) Laminated and clotted peloidal micrite and microsparitic fabrics are precipitated in situ by microbially mediated processes such as biologically induced and influenced carbonate precipitation in association with cyanobacteria, heterotrophic bacteria and their EPS-rich (extracellular polymeric substances) biofilms. These fabrics are gravity defying and build a rigid framework that isolates primary cavities up to centimeters in size. 2) Early marine cement fills primary boundstone voids and generally consists of botryoidal aragonite and/or radial and radiaxial fibrous cement. Marine cement can constitute more than 50% of the boundstone lithofacies. 3) Skeletal biota can range from scarce to abundant and vary in composition according to the geological period; they do not act as rigid framework builders although they might contribute to it. Common biota include calcareous and siliceous sponges, calcareous green and red algae, bryozoans, brachiopods, mollusks, foraminifers, echinoderms and corals.
Microbially mediated precipitation can take place through various organomineralic processes, which are mostly light independent and associated with degradation of organic matter from biofilms and sponge tissues in dysoxic conditions. Thus, the microbial boundstone carbonate producing factory is not limited by light penetration within the water column and can extend well beyond the depth of the photic zone. This constitutes a fundamental difference with respect to the coralgal reef-building facies of the modern tropical carbonate realms.
Observations from multiple examples show that the microbial platforms are characterized by high-relief (several hundred meters) geometry with steep slope (> 30⁰), lack of a raised rim at the margin, a flat platform interior and a gradual transition from the platform-top interior into the slope through a set of outermost platform beds, slightly dipping basinward. The slope clinoforms vary from planar to exponential and the cement-rich microbial boundstone can accumulate in situ on the slope contributing to the stability of the steep clinoforms.
In terms of lithofacies belts, the well-bedded meter-scale cyclic platform interior is basinward transitional to subtidal facies. In such outer platform subtidal facies, cyclicity is less pronounced and microbial boundstone lenses can often alternate with coated grain and skeletal grainstone and packstone. The outer platform facies belt can be a few hundred meters or only tens of meters wide. At the platform break and along the upper slope, microbial boundstone lithofacies dominate and can accumulate in situ on the slope to depths of 300-400 m. At these depths, corresponding to the upper to lower slope transition, microbial boundstone to
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Keywords:
carbonate factory ; microbial boundstone ; slopes
Elenco autori:
G. Della Porta, T. Playton, N. Preto, J.A.M. Kenter, J.R. Bahamonde, O. Merino Tome’, P.M. Harris
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