F. Russo et al., Carbonate facies dominated by syndepositional cements: a key component of Middle Triassic platforms. The Marmolada case history (Dolomites, Italy), FACIES, 42, 2000, pp. 211-226
This article deals with the discussion of the role of the syndepositional c
ementation for the growth of the Middle Triassic pre-volcanic carbonate pla
tforms of the Dolomites (Southern Alps, Northern Italy). The study is conce
ntrated on the Marmolada Buildup, which escaped the facies destroying dolom
itization which affected many surrounding platforms. The investigations too
k place within an almost isochronous uppermost Anisian paleogeographic tran
sect, ranging from the platform-top to the margin and the upper slope. Meth
ods used include geological mapping, sedimentological and paleontological s
tudies, evaluation of the microfacies, as well as SEM and EDS epifluorescen
ce analyses.
The well bedded platform-top succession consists of intra-bioclast calcaren
ites and calcirudites, interbedded with subordinate boundstones, and organi
zed 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 les
s then 5% of the rock volume. The outer margin and the uppermost slope are
characterized by decimeter-scale boundstone blocks, coated and linked to ea
ch other by huge amounts of radiaxial fibrous calcite cements, arranged in
concentric crusts. These cements ("evinospongiae") represent the main compo
nent 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 t
he 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 micro
analyses reveal an uniform Mg content in different calcite types (2-4 Mole
% MgCO3), independently from the primary nature of the components. Late dia
genetic sparry calcites exhibit similar Mg values but no iron. These data p
oint to a homogenization of minor element distribution, probably associated
with a slow but long-lasting semi-closed fluid circulation, possibly relat
ed with the Neogene uplifting of the Dolomite Mountains.