AHARON, PAUL, Department of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA; G. TERZI, F. RICCI LUCCHI, and G.B. VAI, Universita degli Studi di Bologna, Italy; and M. TAVIANI, Istituto di Geologia Marina, GNR, Bologna, Italy

ABSTRACT: A Fossil Record of Hydrocarbon and Fluid Venting Imprinted in the Miocene-Age Lucina Limestones of the Northern Apennines, Italy

Investigations of modern hydrocarbon vents occurring on the sea floor along passive and active continental margins have excited persistent interest in the geological community ever since their first discovery less than a decade ago. Whereas modern occurrences of hydrocarbon vents and associated chemosymbiotic fauna have been extensively studied, the question of identification of fossil hydrocarbon vents in the geologic record only recently started to be addressed.

Low relief, richly fossiliferous, carbonate blocks (up to 8 m height, 3 to 5 m wide) containing large bivalves of Lucinids, Mytilids, and Vesicomyids occur in association with middle to late Miocene age turbidite wedges in the fore land basins of the Northern Apennines of Italy (e.g., Romagna and Toscany). These carbonate blocks locally called "Calcari a Lucina" (i.e., Lucina Limestones) are normally found in discontinuous, irregularly shaped masses lacking in internal bedding with brecciated and generally disordered macro and micro-structures. Vertical teepee structures, calcite-filled tubular pipes, and goethite-filled fractures offer evidence for fossil fluid conduits. The presence of sub-littoral bivalves led early authors to interpret the Lucina limestones as shelf deposits that ere incorporated in slumps and transported to the deeper parts of the basin. Here we present evidence derived from field observations, petrology, and stable oxygen and carbon isotopes supporting the hypothesis that Lucina limestone blocks represent fossil equivalents of modern carbonate buildups forming around cold hydrocarbon vents ("chemoherms"), and that the large bivalves contained in the carbonates represent Miocene deep sea chemosymbiotic fauna fueled by methane and hydrogen sulfide.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.