"Dolomites and dolomites" in Mesozoic platform carbonates of the Southern Apennines: Geometric distribution, petrography and geochemistry

A multidisciplinary study of the dolomitized bodies present among the Late Triassic to Late Cretaceous carbonate deposits of the Monti Lattari belt (Southern Apennines of Italy) compares the different dolomitization styles present within this succession of these Mesozoic platform carbonates and furnishes an overview of their formative genetic models. The revealing traces of the diverse modes of diagenesis in this region include both surface and subsurface mechanisms, utilizing: a geometric reconstruction of the dolomitized bodies, an integrated facies and petrographic analysis of the components, and a geochemical characterization of the sediments including detailed mineralogy and associated stable isotope values.

The data presented demonstrate that some intervals are characterized by surface-related dolomitization processes (middle late Triassic and Cretaceous), while others (latest Triassic to Lower-Middle Jurassic), and are mainly dominated by later, large-scale, subsurface dolomitization processes.

All of the middle late Triassic (Norian) successions are pervasively dolomitized, exhibit fabric-preserving textures and have a distinctly heavy stable isotope signature. Their origin has been related to diagenesis due to particular climatic and palaeoceanographic conditions, leading to the formation of extended blankets of shallow, warm saline seawater. This setting was terminated by climatic and palaeoceanographic changes during the latest Triassic which dramatically reduced the significance of surface-related dolomitization on a regional scale.

The overlying, latest Triassic (Rhaetian) outcrops consist of a widespread coarse, fabric-destructive, massive dolomite with a lighter isotopic signature. These dolomites seem genetically related to the ones of the Lower-Middle Jurassic, which share the same geometrical, petrographic and geochemical characteristics caused by large-scale fluid circulation. This diagenetic pattern apparently started in the Early Jurassic, during burial, driven by thermal convection due to the different temperatures between the oceanic waters and the warmer carbonate platform.

The Middle-Upper Jurassic units feature local episodes of early dolomitization that partially replaced the muddy carbonate facies due to tidal pumping and reflux mechanisms. Dolomitization is not a pervasive feature during this phase of deposition.

In the Cretaceous portion of the succession, dolomites have a totally different and distinct character and are generally related to bedding and show variability in textures and Mg/Ca ratios. Local association with evaporites, geometry of dolomitized bodies and stable isotope geochemistry suggest processes comprising evaporative pumping and reflux, all related to the surface environment. This evidence points to a renewal of a climatic context more favourable for early dolomitization.

Thus this study permits recognition of the presence of three quite different dolomitization styles within the carbonate platform succession of the Southern Apennines fold–thrust belt: one ascribed to the Norian, one to the Rhaetian/Lower Jurassic and one to the Cretaceous.

Iannace, A, Capuano, M and Galluccio, L. (2011) "Dolomites and dolomites" in Mesozoic platform carbonates of the Southern Apennines: Geometric distribution, petrography and geochemistry. Palaeogeography, Palaeoclimatology, Palaeoecology 310: p324-339

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