Facies types and architecture of a Triassic high relief carbonate system terminated by subaerial exposure (Lombardy, Southern Alps, N Italy)

Abstract

The goal of the field itinerary is to illustrate the facies and architecture of a Triassic high-relief carbonate platform (lithostratigraphically known as Esino Limestone; Lombardy, N Italy), similar in age and evolution to the more renowned coeval carbonate platforms of the Dolomites (NE Italy) and the coeval basinal facies. In the central Southern Alps dolomitization is less pervasive with respect to the Dolomites, so that the facies preservation is spectacular, allowing for the observation of detailed depositional and diagenetic features. Furthermore, the visited platform outcrops provide the unique opportunity to observe the sedimentological record of its demise, freezing the architecture of a platform that lasted for about 5 Myr (from close to the Anisian-Ladinian boundary to the Ladinian-Carnian boundary), from the platform top to the basin. The complete section of this Ladinian-Carnian (Middle-Upper Triassic) high-relief carbonate platform is exposed along the Brembana Valley, north of the city of Bergamo. Facies types can be observed in selected outcrops during the field trip, whereas seismic scale geometric and stratigraphic relationships (from the platform top to the slope and basinal setting), can be observed from selected viewpoints. The carbonate platform system reaches a thickness of up to 800 m, with a platform-basin relief of more than 600 m at the end of its evolution. The field itinerary crosses the entire system, from the inner platform to the basin, of one of the best-preserved Triassic carbonate platforms of the Southern Alps of Italy. Inner platform (subtidal to peritidal cycles consisting of oncoidal-bioclastic packstone to grainstone capped by stromatolitic beds), reef (mostly microbial boundstone), slope (clast-supported, early-cemented poorly-selected breccias produced by collapses of the reef-upper slope belt) and basinal facies (dark, well-bedded limestone) facies are exposed in the visited outcrops. During a ‘geological dive’, from the platform top to the basin floor, the diverse subenvironments of the carbonate system can be observed, appreciating the variability of facies along the depositional profile. Each observation is framed in the seismic-scale geometry of the platform that can be appreciated from easily accessible viewpoints. The exceptionally well-preserved facies, as the facies-destructive dolomitization that heavily affects the spectacular coeval platforms of the Dolomites is here rare, permit to document in detail the depositional, early and late diagenetic events. The visited high-relief carbonate system is characterised by a rapid demise, recorded by changes in the facies associations that are exposed in some of the stops representative of different depositional environments: platform top, reef and slope. The abrupt demise of this carbonate system is marked by a major (probably earliest Carnian) sea-level fall associated with a climate change recorded in the different parts of the depositional system by major facies changes. On the platform top the demise is marked by regressive carbonate facies that have different sedimentological characteristics and thickness in the inner platform and in the reef belt. In the basin and on the slope the demise of the carbonate platform is associated with the abrupt input of clay in the basinal setting facing the progradational platform: the seismic-scale onlap relationships between the last prograding clinoform of the Esino Limestone (clast-supported breccias) and the overlying basinal clay can be observed in a spectacular outcrop along the platform slope.