Messinian seep-carbonates marking the transition to the evaporite deposits in the Romagna sector of the northern Apennines (Italy)

Abstract

Numerous outcrops of Messinian authigenic carbonates mark the passage between euxinic shales of the Ghioli di letto Fm at the closure stage of the Miocene foredeep, and the evaporites of the Gessoso-solfifera Fm in the Romagna sector of the northern Apennines. Carbonates have a lenticular to stratiform shape up to 20 m thick and are aligned parallel and concordant to the contact with the evaporites, indicating their primary position. The study of the carbonate succession has evidenced 12 different chemofacies (vuggy and mottled fabric, vein network, conduits, breccias) with chemosymbiotic taxa (lucinids, bathymodiolins, vesicomyids) and negative 13C isotopic signatures (δ 13 C from -36,92‰ up to -54,25), indicating that they are the product of hydrocarbon richfluids seepage at the sea-floor. Facies of the upper succession, close to the contact with evaporites, are typically laminated, with millimetric alternances of peloidal and dark micritic levels. They still preserve seepage imprint testified by 13 C depletion (-22, 37‰) and occurrence of small lucinids and serpulids, and are here interpreted as microbial mats related to vigorous fluid release. The occurrence of seep-carbonates at direct contact with selenitic gypsum offers new data on the transition from marine conditions to evaporitic environment in the framework of the Messinian Salinity Crisis.

The transition from normal marine to evaporitic conditions recorded in a cold seep environment: The Messinian succession of Northern Italy

“…During the closure stage, foredeep deposits were capped by silty-clayey hemipelagic marls with synsedimentary slide deposits and finally by sporadic diatom-rich layers and thin marls rhythmically interbedded with organic-rich mudstones informally referred as euxinic shales of the Ghioli di letto Fm (about 150 m thick). The last deposits enclose numerous seep-carbonate bodies, lens-like to stratiform, late Tortonian to early Messinian in age (Terzi et al, 1994;Conti et al, 2022). The extension of seepcarbonate bodies ranges from 1 to 100 m and the thickness varies from some decimeters to 30 m and are concentrated between the Lamone and Sintria valleys.…”

“…Recently, new petrographic and isotopic analyses on Messinian pre-evaporitic carbonates from the Tuscan-Romagna area revealed that AOM contributed to the formation of these carbonates (Conti et al, 2022), providing the unique opportunity to investigate in great detail the evolution of cold seep environment during a key transition period in Earth history which is still debated. Here, we use a multiproxy approach including field facies investigation, carbonate petrography (including optical and scanning electron microscopy), mineralogical (XRD), and isotope analyses (δ 13 C, δ 18 O) to describe for the first time seep-impacted carbonates displaying evidence of intense microbial activity sustained by methane in the Messinian pre-evaporite setting of northern Apennines.…”