Abstract

Middle Triassic platforms (Formazione di Contrin, Upper Anisian) in the Italian Dolomites, southern Alps, record large changes in carbonate production and depositional geometry. The changes include variation in the abundance of detrital micrite, i.e., allochthonous calcareous mud, relative to the autochthonous micrite, which formed in situ and was syndepositionally lithified. Carbonate factory dynamics controlled the geometry of the clinostratified slope units and were probably associated with changes in oxygenation. Early Anisian low-relief platforms were followed by late Anisian, high-relief buildups, associated with basinal dysoxic-anoxic environments. The geometric evolution of the platform slopes records a progressive increase in the dip angle of clinostratifications, matched by a deepening evolution of the basinal environments. At the same time, the slope sediments record a gradual change from loose detrital micrite, characterizing the lower portion, to autochthonous micrite, dominating the upper part and recording massive syndepositional lithification. The development of the autochthonous micrite was associated with the preservation of significant amounts of organic matter. This sharp increase in automicrite formation was probably induced by a rapid change from oxic to suboxic conditions in the carbonate slope and margin environments, while anoxic conditions developed in the adjacent organic-rich basin (Moena Formation). The low oxygen level promoted the preservation of organic matter and the activity of sulfate-reducing bacteria, which in turn induced in situ deposition of autochthonous micrite through biomediated processes. This pervasive early cementation and lithification induced the development of steep platform slopes. The conceptual model distilled from this Middle Triassic case can support the interpretation of analogous buildups where the skeletal framework is subordinate to the micrite component.

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