Sedimentation in shallow to deep water carbonate environments across a sequence boundary: effects of a fall in sea-level on the evolution of a carbonate system (Ladinian-Carnian, eastern Lombardy, Italy)

In the Concarena-Pizzo Camino Massif (Lombardy Basin, Southern Alps, Italy) the lateral transition from Ladinian-Carnian carbonate platforms to coeval intraplatform basins is preserved. The succession records the sedimentological evidence of a sea-level fall on a flat-topped platform with a narrow marginal reef rim and its effects in the adjacent deeper-water basin. Repeated high-frequency exposures of the platform top are recorded by a peritidal-supratidal succession that overlies subtidal inner platform facies of the former highstand system tract (HST). On the slope and in the basin, the sea-level fall is recorded by a few metre thick succession of bioclastic packstones. These facies directly lie on coarse clinostratified breccia bodies (slope facies of the former HST) or on resedimented, well-bedded, dark laminated limestones (basinal facies of the HST). This facies distribution indicates that during the sea-level fall carbonate production on the platform top decreased rapidly and that sedimentation in the basin was mainly represented by condensed facies. Microfacies record an enrichment, during low stand, in pelagic biota (packstones with radiolarians and spiculae), whereas the occurrence of platform-derived, shallow-water materials is limited to thin lenses of reworked and micritized Fe-rich oolites and bioclasts (mainly pelecypods and echinoderms). The facies association in the Concarena-Pizzo Camino Massif demonstrates that a highly-productive carbonate factory was almost completely turned off during the emergence of the platform top at a sequence boundary, leading to low-stand starvation in the basin. The reconstruction of the stratigraphic evolution of the Concarena-Pizzo Camino carbonate platform therefore represents a significant case history for the study of the behaviour of ancient carbonate systems during a fall in sea-level, independent of its origin (eustatic or tectonic).