Distinctive microfabrics and radiometric age of seep carbonates from the Romagna Apennines (San Vernicio, Italy): Insights into seepage dynamics

Massive hydrocarbon emissions in marine environments can produce climatic shifts and influence ecological communities. The late Miocene in the Mediterranean area was a period of drastic geological and climatic transformations that occurred in combination with widespread hydrocarbon emissions. This study aims to characterize in detail seep carbonates from San Vernicio (Romagna, Italy), describing distinctive microfabrics and providing a radiometric age to get new insights on hydrocarbon seepage and its sedimentary products during the late Miocene in the studied area. Features herein reported include limpid spar-filled micro-domains (interpreted as a newly discovered gas hydrate-related texture), dissolution cavities, botryoidal cements, and former organic-walled tube worms. In situ U-Pb dating of seep-related calcite cements within former conduits yielded a lower intercept age of 6.8 ± 0.9 Ma (Tortonian – Messinian). This radiometric age postdates the deposition of the hosting sediments (< ca. 10.5 Ma, based on biostratigraphic proxies) by at least ca. 2.8 Ma, underlying the unreliability of indirect age proxies in these extremely dynamic environments. Low δ ¹³ C values (> -38.8‰ Vienna PeeDee Belemnite) and the presence of spar-filled spheres resulted from oxidation of oil droplets point to a thermogenic origin of the hydrocarbons. These findings testify that a detailed characterization of the seep-related deposits is pivotal in determining duration and modality of seepage activity.