Carbonate facies dominated by syndepositional cements: a key component of Middle Triassic platforms. The Marmolada case history (Dolomites, Italy) |
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Authors: | Prof Dr Franco Russo Dr Adelaide Mastandrea Dr Marco Stefani Prof Dr Claudio Neri |
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Institution: | (1) Dipartimento di Scienze della Terra, I-87036 Arcavacata di Rende (Cosenza);(2) Dipartimento di Scienze Geologiche e Paleontologiche, Corso Ercole I d'Este 32, I-44100 Ferrara |
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Abstract: | Summary This article deals with the discussion of the role of the syndepositional cementation for the growth of the Middle Triassic
pre-volcanic carbonate platforms of the Dolomites (Southern Alps, Northern Italy). The study is concentrated on the Marmolada
Buildup, which escaped the facies destroying dolomitization which affected many surrounding platforms. The investigations
took place within an almost isochronous uppermost Anisian palcogeographic transect, ranging from the platform-top to the margin
and the upper slope. Methods used include geological mapping, sedimentological and paleontological studies, evaluation of
the microfacies, as well as SEM and EDS epifluorescence analyses.
The well bedded platform-top succession consists of intra-bioclast calcarenites and calcirudites, interbedded with subordinate
boundstones, and organized in shallowing upward, meter scale depositional cycles, sometimes capped by subaerial surfaces.
The platform margin belt is rich in boundstones and lacks a primary framework formed by organisms; metazoan skeletons form
less then 5% of the rock volume. The outer margin and the uppermost slope are characterized by decimeter-scale boundstone
blocks, coated and linked to each other by huge amounts of radiaxial fibrous calcite cements, arranged in concentric crusts.
These cements (“evinospongiac”) represent the main component of the margin and upper slope facies.
Epifluorescence analyses suggest the existence of abundant organic residual matter associated not only with the bioclasts
and peloids, but also with the syndepositional cements. Organic matter likely played a significant role in carbonate cementation
and was a key factor for the early lithification of the platform as well as for the sediment production. Minor element microanalyses
reveal an uniform Mg content in different calcite types (2–4 Mole % MgCO3), independently from the primary nature of the components. Late diagenetic sparry calcites exhibit similar Mg values but
no iron. These data point to a homogenization of minor element distribution, probably associated with a slow but long-lasting
semi-closed fluid circulation, possibly related with the Neogene uplifting of the Dolomite Mountains. |
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Keywords: | Carbonateplatforms Microfacies Cementation Dolomites (Italy) Middle Triassic |
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