Carbonate facies dominated by syndepositional cements: a key component of Middle Triassic platforms. The Marmolada case history (Dolomites, Italy)

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.