From birth to maturity: evolution of depositional processes, architecture and facies association in a Triassic high-relief carbonate platform (Southern Alps of Italy)

The evolution of high-relief carbonate systems may be characterized by changes in the type and distribution of facies associations that reflect changes in depositional processes. The detailed reconstruction of the changes in... [ view full abstract ]

The evolution of high-relief carbonate systems may be characterized by changes in the type and distribution of facies associations that reflect changes in depositional processes. The detailed reconstruction of the changes in geometry and facies distribution, since the initial onset of the platform, is required in order to define the factors that control sediment production and distribution at different evolutionary stages of a high-relief carbonate platform.
Whereas the final geometry of mature carbonate systems in the geological record is commonly preserved in outcrop, the inception stage of high-relief carbonate platforms is rarely exposed, as it is commonly covered by inner platform facies and bordered by prograding slope facies. Outcrop conditions that expose the different parts of high-relief carbonate platforms are present in the western Southern Alps of Italy: here the Triassic Esino Limestone, up to 800m thick, recorded the evolution of a high-relief platform with a platform-to-basin relief that reaches 500 m, during a period characterized by tectonic quiescence.
Two main evolutionary stages have been identified. The nucleation of this platform, occurring after deposition of ammonoid-bearing mudstones and marls, is characterized by a 25-110 metres-thick, shallowing upward succession mostly consisting of locally clinostratified intra-bioclastic burrowed packstone. The mature stage of the platform evolution, volumetrically dominant, is instead characterized by the definition of a wide inner platform (bedded peritidal limestones with stromatolites, oncoids, fenestrae and dasycladaceans) bordered by a narrow reef (Tubiphytes and microbial mounds associated with coral framestones with calcisponges and intrabioclastic packstone) that sourced the breccias of the steep slopes.
The evolution from the bioclastic sands of the inception stage to the peritidal limestone of the mature platform is rapid. The progradation of the platform is the result of the interplay between sea-level change and carbonate production, able to exceed the rate of creation of accommodation space. Facies distribution reflects the coalescence of the initial nucleation places of the platform leading to the onset of a single, larger carbonate platform facing a basin becoming deeper with time. This rapid facies and architectural change (from small low-relief bioclastic platforms to a wide, rimmed carbonate platform) records a major, physiological event of reorganization of the depositional environments within the carbonate system, with important implications on the distribution of petrophysical properties.