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

Pliocene stratigraphic paleobiology in Tuscany and the fossil record of marine megafauna

“…Other parts of the S1-S3 succession were drawn based on available lithostratigraphic literature (see below). Studies integrating sedimentary facies analysis, biostratigraphy and magnetostratigraphy (Nalin et al, 2016) have helped correlating carbonate bodies typical of S4. The reader is referred to Benvenuti et al (2007Benvenuti et al ( , 2014 for details on facies analysis and sequence stratigraphic interpretation of synthems S3-S6.…”

“…The Pliocene has been traditionally subdivided into three main informal lithostratigraphic units: continental conglomerates and sandstones at the base, overlain by the "Blue Clay Formation" (Argille Azzurre: Zanclean-lower Piacenzian), marking the post-Messinian Mediterranean marine transgression and forming the thickest part of the basin infill (e.g., Bossio et al, 1994Bossio et al, , 1997Ghinassi and Lazzarotto, 2005), and the "Upper Sands" (Sabbie superiori: Piacenzian-Gelasian) and conglomerates, deposited during the ensuing regression. Several finer lithostratigraphic units have been introduced to define the local stratigraphy, resulting in a complex and largely informal lithostratigraphic terminology which includes Zanclean lower "Blue Clays" and Zanclean-Piacenzian upper "Blue Clays" (Capezzuoli et al, 2005), the latter eventually further separated by the widespread occurrence of Piacenzian carbonates (Nalin et al, 2016). The lower Zanclean (OOB: Ghinassi, 2007;Nalin et al, 2010) and the Piacenzian, are characterised by the high-frequency alternation of coarse-grained and fine-grained facies, ranging from fluvial to marine shelf settings (Benvenuti et al, 1995a(Benvenuti et al, , 1995b(Benvenuti et al, , 2007(Benvenuti et al, , 2014Martini et al, 2011, Fig.…”

“…In EB, S4 comprises a lower interval dominated by richly fossiliferous, massive mudstone or very-fine-grained sandstone (prodelta-inner shelf), overlain by bioclast-rich sandstones recording prograding mixed carbonateclastic ramp, outcropping in the southeastern part of the Elsa valley. Equivalent deposits, also comprised in biozone MPL4b, are patchily distributed in FB, VEB, SRB, OOB and other basins of southern Tuscany (Ghinassi and Nalin, 2016). The upper interval of S4 is formed by a succession of delta front sandstones, passing in EB eastern margin to a few tens of m-thick fluvial succession, hinting at an original depositional gradient.…”

“…Facies types are fluvial conglomerate and sandstone, and alluvial mudstone (F1); intertidal to very shallow subtidal mudstone and muddy sandstone (F2); shallow subtidal coarse-and medium-grained sandstone with sparse conglomerate (F3); deep subtidal muddy finegrained sandstone (F4); open shelf sandy mudstone (F5); outer shelf to upper bathyal mudstone (F6); outer shelf and bathyal turbidite sandstone and conglomerate (F7). An eighth group is formed by facies deposited subtidally in limited mixed carbonatic-siliciclastic ramps (F8), irrespective of depth (Nalin et al, 2016). Facies F6-F7 are restricted to Zanclean deposits (synthems S1-S3); facies F5 and F8 characterise the upper Zanclean-middle Piacenzian interval (synthems S3-S4); facies F1-F4 characterise the upper Piacenzian (synthems S5-S6).…”

“…6). At shallower settings, in all basins to the East and South of FB, EDS of synthem S4 take the form of an alternation of mudstone and carbonate (facies F5 and F8: Nalin et al, 2016), or mudstone and sandstone (facies F5 and F3-F4: Benvenuti et al, 2014; biozone MPL5a). Middle and upper Piacenzian EDSs form an alternation of coastal mudstone and sandstone (facies F1-F2-F3: Benvenuti and Dominici, 1992;Dominici, 1994), with MF interval and HST marked by a shell bed topped by a lower shoreface sandstone, or a shelf mudstone (facies F4-F5: Benvenuti et al, 2007;Dominici et al, 2008).…”