The 'Brecciated Limestones' of Maiella, Italy: Rheological implications of hydrocarbon-charged fluid migration in the Messinian Mediterranean Basin

摘要

The upper Messinian "Brecciated Limestones" of the Maiella area (Central Italy) reveal clues to the rheological behavior of the sedimentary column during fluid migration processes, as well as the timing of a major phase of methane release. Seep limestones have been studied worldwide in ancient and present-day settings. However, investigations of the complex rheologic behavior shown by the wide array of brecciated facies have received less attention. In this work special emphasis was placed on fabric and facies analyses, comparisons of the various scales of observation, and integrated stable isotope analyses (δ~(18)O and δ~(13)C), all within the context of the precisely-dated stratigraphy of the late Messinian sediments of the Mediterranean Basin. Above the basin scale Messinian Erosional Surface (MES), widespread brecciated limestone buildups and minor concretions constitute microbial carbonate bodies laterally embedded or passing to a marly-pelitic succession. In situ fluid-driven brecciation, together with the occurrence of fluid migration paths, are interpreted to represent vestiges of a mud volcano feeder system. However, chaotic textures testify to complex rheological behavior, where brittle and plastic responses in the same micro-environments co-occurred during fluid migration through a partially lithified sedimentary column. Different patterns of tar-bearing facies and degrees of brecciation, coupled with a geochemical dataset yielding wide ranges of δ~(18)O (from +4 to -10‰ PDB-1) and δ~(13)C values (from +4 to -40‰ PDB-1), point to two distinct stages of ascending hydrocarbon-charged fluids: in an earlier phase, fluids were mostly typified by low-molecular-weight hydrocarbons and induced widespread brecciation in the sedimentary column; in a later phase, fluid migration was accompanied by crude oil migration. During the early phase, fluid migration through the sedimentary column developed both: (a) with large vigorous fluxes, creating neoformed feeder channels and causing high degrees of brecciation; and (b) via slow seepage, favouring authigenic carbonate precipitation in the host sediment and causing negligible brecciation. In our proposed scenario, the Maiella area witnessed a hydrocarbon-rich fluid migration event (mainly methane) during the late Messinian, accompanied by authigenic carbonate precipitation and widespread brecciation. We suggest that this event was triggered by the sudden depressurization induced by the major drawdown of the Mediterranean Sea in late Messinian times.