Transported Rhodoliths Witness the lost carbonate factory: A case history from the miocene pietra da cantoni limestone (Nw Italy)

Inner-platform bioclasts may have a remarkable preservation potential when transported toward deeper and quieter basin environments. Rhodoliths can bind skeletal grains during their growth, thereby storing information on their native environment even after transport. The Burdigalian Pietra da Cantoni Group (Piedmont, NW Italy) has been analyzed as an example of lost carbonate factory, to stress the rhodolith potential in studies of displaced sediments and within this framework an overview of the main mechanisms of rhodolith transport is provided. The examined successions testify the progressive sediment starvation of the middle and outer ramp caused by the interruption of shallow-water sediment supply due to sea-level rise. Four facies have been identified: Facies 1 is a coarse limestone characterized by inner-middle ramp rhodalgal skeletal assemblages deposited on the lower middle-ramp; Facies 2 is an impure limestone linked to the inception of sediment starvation; Facies 3 is a glauconitic-phosphatic marly-limestone that marks the end of sediment supply from shallowwater to the lower middle-ramp; Facies 4 is a marly limestone that represents the beginning of hemipelagic sedimentation. The sediment has been displaced toward a deeper setting from its source area, reconstructed as a high-energy and turbid shallow-water environment. The skeletal debris trapped inside the rhodoliths are related to an innermiddle ramp setting and have faithfully witnessed the environment of rhodolith formation.