STRONTIUM ISOTOPE STRATIGRAPHY AS A CONTRIBUTION FOR DATING MIOCENE SHELF CARBONATES (S. MARINO FM., NORTHERN APENNINES)

This paper provides new data on strontium isotope stratigraphy applied to the Miocene heterozoan shelfal carbonates of the S. Marino Fm. (Marecchia Valley, northern Apennines). Sr isotopic analyses were carried out on oyster shells, bryozoans and bulk-rocks from the lower-middle carbonate portion of the section. In the upper part of the succession that shows evidence of detrital influx, 87Sr/86Sr analyses were performed on foraminifera tests, separating planktonic and benthic forms. Results were compared with calcareous nannofossil biostratigraphic data from the same levels, in order to test the reliability of Sr dating in mixed carbonate-siliciclastic sediments. Mean ages obtained from oysters range between 16.9 Ma and 16.3 Ma. Very similar results are obtained using bryozoans (16.5 Ma to 16.1 Ma) and bulk-rocks (16.8 Ma to 16.2 Ma). These results allow to better constrain the age of the massive carbonate shelf, referable to the upper Burdigalian. In the upper carbonate-siliciclastic portion of the shelf, numerical ages obtained from planktonic and benthic foraminifera are in good agreement with nannofossil biozones (mean ages respectively around 15.3 Ma and 14.5 Ma) although they display wide confidence intervals. These wide age uncertainties depend on the slow rate of change of marine 87Sr/86Sr through time that characterizes the interval between ~15 and ~13.5 Ma

Evolution of a Miocene carbonate shelf (northern Apennines, Italy) revealed through a quantitative compositional study

The evolution of the Miocene San Marino carbonate shelf (Torriana outcrop), developed on the accretionary prism of the northern Apennines, has been interpreted through a stratigraphic and compositional study. Modal analysis allowed to quantify the framework components and to identify four microfacies through which the main steps of the carbonate ecosystem were traced. The healthy phase of the carbonate shelf, dominated by bryozoans and echinoids, originated in a high-energy transgressive setting and evolved during a warm period characterized by a progressive increase of nutrients. The transitional stage is marked by a reduction of carbonate productivity and by terrigenous intermittent pulses associated with bioclast fragmentation. The drowning succession corresponds to deepening upward facies formed by fine-grained hybrid arenites to sandy marls with abundant planktonic foraminifera, glauconitic grains and clay matrix. The demise of the carbonate shelf might have resulted from a combination of regional and global factors that interplayed controlling the detrital input, the nutrient budget and the deepening of the basin. Synsedimentary tectonics triggered subsidence of the basin and enhanced terrigenous discharge. Moreover, the superposition of paleoclimatic and paleoceanographic events (Monterey and Middle Miocene Climate Optimum) could have contributed with the intense weathering and remarkable detrital and nutrients supply

High-resolution chemostratigraphy of a Miocene wedge-top carbonate shelf (San Marino Fm., Northern Apennines, Italy): The major role of the Monterey global fertility event

The evolution of the San Marino carbonate succession, developed on a wedge-top basin of the northern Apennines during the Middle Miocene (Torriana outcrop, Marecchia Valley), was studied through a high-resolution stable isotope analysis on different carbonate components. A marked positive carbon isotopic excursion is identified at ca. 16 Ma. The excellent correlation of the San Marino \u3b413C carbonates with the global \u3b413C reference curve from Zachos et al. (2008) allows to link the marked positive \u3b413C present in the San Marino section with the global carbon isotope maximum of the Monterey event. Subordinate long-eccentricity-driven \u3b413C cycles (~405 kyr) as recorded by Holbourn et al. (2007) were also identified. The correlation with carbon signatures of coeval successions of the Mediterranean region shows that this main carbon isotopic excursion at the Burdigalian\u2013Langhian boundary is widely recorded and predates the crisis of these heterozoan shelves. The demise of the San Marino shelf resulted from a combination of global and regional factors that controlled the nutrient budget, the detrital input and the subsidence of the basin. The high-resolution chemostratigraphy of San Marino succession shows that even though the shelf evolved in the complex setting of a wedge-top basin, which should be largely influenced by local factors (i.e. tectonic subsidence and detrital input), it records the Monterey event and its eccentricity paced rhythms with high precision. This highlights the strong connection of the Monterey event with the development and subsequent demise of shallow water carbonate depositional systems in the Mediterranean, irrespective of variable and changing local conditions

Miocene Seep-Carbonates of the Northern Apennines (Emilia to Umbria, Italy): An Overview

The natural emission of methane-rich fluids from the seafloor, known as cold seepage, is a widespread process at modern continental margins. The studies on present-day cold seepages provide high-resolution datasets regarding the fluid plumbing system, biogeochemical processes in the sediment, seafloor seepage distribution and ecosystems. However, the long-term (hundreds of thousands to millions of years) evolution of cold seepage remains elusive. The identification and study of outcrop analogous now exposed on land represent a valuable method for better understanding the effects of geological processes and climate forcing on the development of cold seepage systems. Here, we provide an overview on Miocene seep-carbonate deposits of the northern Apennines (from Emilia to the Umbria-Marchean sector, Italy), based on decades of field research integrated with detailed sedimentological and geochemical investigations. We report a total of 13 seep-carbonate outcrops, which formed in three different structural settings of the paleo-accretionary wedge corresponding to wedge-top basins, outer slope and intrabasinal highs at the deformational front. We discuss the recurring lithostratigraphic occurrence of seep deposits and the main compositional features (carbonate facies, carbon and oxygen stable isotopes) in order to interpret the seepage dynamics, duration and infer the contribution of methane-rich fluids released by paleo-gas hydrates. The datasets presented in this study represent a valuable complete record of cold seepage spanning ~12 Myr, that can be used to better understand factors controlling the regional-scale spatial and temporal evolution of cold seepage systems at modern active continental margins

History of Miocene temperate-type carbonate shelf in a compressive setting (northern Apennines), constrained by a chemostratigraphycal, microfacies and compositional study

A detailed chemostratigraphic study associated with microfacies and compositional analysis has been performed on a early middle Miocene shallow-water shelf deposited on a wedge-top basin in the northern Apennines. More than 140 samples were collected in the representative outcrop of Torriana (Val Marecchia valley, Romagna Apennines) in order to identify factors controlling the inception and crisis of the shelf. The succession is constituted by 100 m thick mixed siliciclastic-carbonate rocks, unconformably overlying the allochthonous Ligurian units. The evolution of the carbonate shelf passed true four main phases, as shown by the detailed microfacies and compositional study, that evidence the progressive decrease of the carbonate productivity, gradually replaced by detrital sedimentation. The basal portion consists of rhodalgal rudstones and grainstones dominated by echinoids, bryozoans, coralline algae and benthic foraminifera. The progressive increase of terrigenous components and planktonic and benthic taxa marks the gradual transition to a mixed carbonate-siliciclastic shallow water-facies. The top of the succession is characterized by fine-grained sediments rich in glaucony. The main factors controlling the evolution of the shelf are increased subsidence of the basin, related to the thrust migration, and terrigenous contributions from the erosion of the rising Apennines areas. The stable isotope study show that the carbonate production has been influenced not only by regional factors, but also by a global fertility event, as the Monterey Event, recorded in the Mediterranean during the late Burdigalian - Serravallian

Miocene carbonate shelf in the northern Apennines (S. Marino Fm): insights from microfacies, petrography and strontium isotopes

A detailed stratigraphic study associated with microfacies and strontium analysis has been performed on middle Miocene shallowwater carbonates developed in a wedge-top basin in the northern Apennines. More than 140 samples were collected in the representative outcrop of Torriana (Marecchia valley, Romagna Apennines). It covers the entire carbonate succession from the basal unconformity on Ligurian units to the terrigenous-rich sediments that mark the demise of the shelf. The modal analysis allowed to quantify the main framework components: an intrabasinal carbonate fraction (biogenic), a terrigenous fraction (largely siliciclastic) and an authigenic intrabasinal component mainly constituted by glauconitic grains. The carbonate succession starts with massive carbonates consisting of rudstones and grainstones dominated by bryozoans and echinoids. The basal few meters are rich in rhodolithes. The succession passes to planar-bedded grainstones to packstones with the same fossiliferous composition. The upper part is characterized by low angle cross-bedded grainstones to packstones with high detrital content, that gradually passes to mixed siliciclastic-carbonate sediments and finegrained hybrid arenites. An overall increase of the terrigenous input to the basin and the progressive deepening from a shallow shelf to outer shelf-slope environment caused the drowning of the shelf, marked by relatively high concentration of glauconitic grains. Our data indicate an almost exclusive intrabasinal supply during deposition of the basal portion. A vertical evolution is documented by a progressive increase in the terrigenous supply that replaces the bioclastic fraction. Preliminary 87Sr/86Sr data obtained from 7 carbonate samples allowed us to obtain absolute ages for the lower massive portion of the succession. For each sample we analyzed mollusk shells, bryozoans and bulk rock. Results indicate ages between 16,52-15,91 Ma, corresponding to the Burdigalian-lower Langhian interval. Comparing the values obtained from mollusk shells, bryozoans and bulk rock from samples at the same stratigraphic level, we observe similar results. This, according to petrographic analysis, permitted us to suppose a low diagenetic imprint of the rock

Inception and crisis of a Miocene temperate-type carbonate shelf in a compressive setting (northern Apennines)

A detailed study including stratigraphy and composition has been performed on shallow-water carbonates of early-middle Miocene age deposited in a wedge-top basin in the northern Apennines. In the representative outcrop of Torriana (Val Marecchia valley, Romagna Apennines) more than 140 samples were collected in order to identify factors controlling the inception and crisis of the shelf. The succession unconformably rests on the allochthonous Ligurian units and is constituted by up to 100 m thick carbonates and mixed siliciclastic-carbonate rocks. The basal portion consists of rhodalgal rudstones and grainstones dominated by echinoids, bryozoans, coralline algae and benthic foraminifera. It gradually passes into mixed carbonate-siliciclastic shallow water-facies, characterized by an increase in terrigenous components and planktonic and benthic taxa. The occurrence of glaucony-rich packstones with abundant planktonic foraminifera marks the transition to the upper fine-grained sediments. Results of a detailed compositional study evidence four main phases in the platform evolution, and a progressive decreasing of the carbonate productivity, gradually replaced by detrital sedimentation. Regional factors linked to the Apenninic tectonics seem to primarily support the eu- to mesotrophic conditions in the examined carbonate shelf. However this time interval (late Burdigallian - Serravallian) is characterized by the global fertility event, the Monterey Event, recorded also in the Mediterranean. The stable isotope study shows that this global event also influenced the carbonate production. The inception and demise of these temperate rhodalgal-foramol carbonate sediments located in a wedge-top basin is primarily controlled by synsedimentary tectonics related to the thrust migration, through the combined effect of two processes: increasing the subsidence of the basin, and triggering the terrigenous discharge from the erosion of the uplifted Apenninic areas

Paleoenvironmental evolution in a high-stressed cold-seep system (Vicchio Marls, Miocene, northern Apennines, Italy)

Excellent exposures of Miocene seep-carbonates enclosed in marine marly sediments (Vicchio outcrops in the northern Apennines, Italy) offer the opportunity to highlight the evolution of a fossil seep ecosystem and the response of benthic communities to high-stressed environmental conditions. For this purpose, seep-related facies and molluscs within carbonate bodies have been studied, coupled with benthic foraminiferal assemblages and carbon and oxygen isotopes in the enclosing marls. The integrated planktonic foraminiferal and calcareous nannofossil biostratigraphy has allowed us to constrain the seepage within well-calibrated bioevents and to stress the relationships with paleoclimatic and paleoceanographic variations during the middle Miocene in the Mediterranean area. Our biostratigraphic data indicate that the onset of the seepage approximates the Mi3b cooling event (13.82 Ma) and the seepage system lasts for 400 kyr. The evolution of the Vicchio cold-seep system passes through four phases: (1) The onset of the seepage, characterized by a pervasive flow of methane-rich fluids, is inferred by δ13C depletion of marly sediments and by prevailing benthic foraminifera indicative of suboxic conditions at the sea-floor. (2) The methane flow becomes focused causing the precipitation of wide pinnacle-like carbonate bodies which contain giants lucinids. Enclosing marls indicate well-oxygenated conditions, possibly enhanced by paleoceanographic variations connected to the Mi3b cooling event. (3) The appearance of the vesicomyid Christineconcha cf. C. regab and the absence of lucinids in seep-carbonates suggest stable methane-rich fluid emissions; higher flow rates locally favoured the flourishment of bacterial mats. Benthic foraminifera show abundance peaks of organic matter depending taxa. (4) The reduced intensity of methane-rich fluid flows favours the precipitation of stratiform carbonate bodies along strike; the macrofauna is characterized by the presence of both Vesicomyidae and giant lucinids. Foraminiferal assemblages in the enclosing marls indicate the restoration of well-oxygenated conditions