Carbonate Deposition In Restricted Basins: A Pliocene Case Study From the Central Mediterranean (Northwestern Apennines), Italy

Abstract

Carbonate depositional models are often informed by the study of platforms of good lateral continuity
\nand sizable thickness, because of their significance in petroleum geology. However, spatially restricted and more
\nephemeral carbonate accumulations can be an important but frequently overlooked component of otherwise
\nsiliciclastic-dominated or mixed carbonate–siliciclastic systems. Pliocene successions of Tuscany and the Tyrrhenian
\nshelf (Northern Apennines, Italy) record a regional pulse of nontropical carbonate deposition across several restricted
\nbasins that has not yet been precisely constrained in its genesis and correlation. This study investigates the
\nstratigraphic expression of these carbonates to extract general aspects applicable to carbonate sedimentation across
\ntectonically structured and physiographically complex shelves. Analyses of the extension, composition, facies, and
\nsequence stratigraphic architecture of the studied Piacenzian carbonate units are complemented by new
\nbiostratigraphic and magnetostratigraphic data, which document the slightly diachronous development of the
\ncarbonates within Mediterranean planktonic foraminifera Subzones MPL4b and MPL5a and magnetic polarity
\nChron C2An. The carbonate units are part of a regionally defined, tectonically controlled sequence, and represent the
\nfirst transgressive unit overlying the basal sequence boundary, but are also found in the regressive portion of the
\nsequence. Collectively, the units document the establishment of a shallow-marine carbonate factory, dominated by
\ncalcareous red algae, the larger benthic foraminifer Amphistegina, and heterozoan skeletal components, along the
\nmargins of a complex, archipelago-like coastal domain. A review of the studied successions and other examples from
\nthe literature indicates that carbonate accumulations in settings with varied coastal physiography and active
\npartitioning of depocenters are generally characterized by: a) spatial discontinuity; b) an overall skeletal association
\nconsistent across separate basins, but with local variability in dominant skeletal components; c) preferential
\naccumulation along basin margins or over isolated structural highs not in direct proximity to a hinterland with major
\ndrainage systems; d) predominance of low- to moderate-energy facies; e) preferential onset of deposition during
\ntransgression, with possible development of mixed carbonate–siliciclastic facies; and f) differing internal stratigraphic
\narchitecture tied to the specific subsidence and uplift history of coeval individual basins. These observations built upon
\nthe study of mid-Piacenzian carbonate units from the central Mediterranean contribute to the refinement of carbonate
\nfacies models and their application in the fields of sequence stratigraphy and basin analysis.