In situ brecciation and fluid migration in seep carbonates : the case study of the messinian limestones in the western mediterranean basin

Abstract

Brecciated fabrics, coupled with authigenic carbonate precipitation, extensively affect the carbonatic units developed during the Messinian Salinity Crisis (MSC) of the Mediterranean Sea. These limestones represent a complex unit in terms of genesis and stratigraphy: they show a rich facies inventory and are intimately related to a major basin-scale unconformity (the MES, Messinian Erosional Surface). The brecciated limestones are known as “Calcare di Base” Fm. (basal limestone) and show sulphur-bearing lithofacies at places. They have been long investigated and the attempt to group them on the basis of sedimentological, mineralogical and geochemical datasets results in still fragmentary models, where the fabric aspects have not been highlighted yet, nor the trigger for the wholesale brecciation hasn’t been clearly defined. Two different triggers for brecciation have been so far claimed: evaporitic collapse and mass wasting processes. The relation between the upper Messinian breccias and fluid migration, to date only speculated, is tested in the present study using a suite of sedimentary techniques (outcrop facies analysis, natural radioactive measurements, facies and fabric investigations) and geochemical techniques (XRD and EDS analyses, stable isotopes δ18O and δ13C, organic geochemistry). The brecciated limestones widely crop out in the Western Mediterranean area: some key-sections located in Italy (Maiella area; Calabrian Arc; Sicily) have been chosen in this thesis to obtain an exhaustive stratigraphic framework and a genetic model for the Upper Messinian breccias. The overall challenge is to validate the role of investigating brecciated fabric as: 1) a sedimentological tool for detecting ancient cold seeps, offering a contribute to the “paleo-seep search strategy”; 2) an opportunity to achieve an outstanding insight into the processes taking place below the seafloor in modern seepage settings. The Messinian breccias are made up of limestones exhibiting high variability in facies and thickness: their geometry varies from a patchy distribution within a host sediment to massive or stratified thick bodies interbedded with pelitic horizons. The carbonatic beds consist of locally brecciated marly lime mudstones, highly cemented, devoid of gravity segregation and any preferential distribution. Generally brecciation overprints the primary fabric, resulting in fabric-retentive breccias (in situ brecciation). Breccias are typified by monomictic subangular clasts, showing scale-invariant fabric and lithology-independent patterns. Brecciation, accompanied by fluid migration pathways occurring at any scale of observation, is a diffused phenomenon at the mesoscale, but localized at the microscale. This possibly accounts for variation in the energy input of the fluids, supposed to have triggered brecciation in such environments. - 2 - The co-occurrence of plastic and brittle behaviour proxies point to fluidification and associated brecciation phenomena taking place in a partly unlithified sedimentary column, typified by differential diagenesis. Both the carbonates and the pelitic fraction show an intense natural radioactivity (up to 63 Cps), mostly related to authigenic 238U. The geochemical dataset is complex and shows wide ranges both in δ18O (+7.74 down to -9.64 ‰ PDB) and δ13C values (+4.14 down to -43.7 ‰ PDB). This is the result of: 1) originally mixed carbon sources involved in authigenesis; 2) a complex fluid-rock interaction; 3) different fluid composition in the different basins (in the Maiella Basin and at Capodarso - in the Caltanissetta basin - they were enriched in hydrocarbons; in the Calabrian Arc and in Centuripe - in the Caltanissetta basin - the nature of the fluids, was possibly saline). In addition, the coupled positive δ18O - negative δ13C widely typifying the Calcare di Base both in Sicily and Calabria, could be related to gas hydrate destabilization processes, since textures resembling gas-hydrate bearing sediments were observed as well. A revisiting of the term “seep limestone” is proposed herein, since the aforementioned facies and fabric association enucleates important sedimentological proxies for the detection of seep limestones: (a) irregular geometries of the geobodies; (b) primary fabric overprinting; (c) peculiar textural characters of breccias; (d) scale-independent and lithology-independent patterns; (e) complex rheology; (f) fabric resembling gas-hydrate infilling sediments. The upper Messinian brecciated limestones occur in correspondence to different phases of the Messinian Salinity Crisis, but are strictly related to the regional unconformity: the MES played a critical role in the brecciation processes. There is no evidence to consider the upper Messinian breccias as a stratigraphic unit: the processes of brecciation and widespread fluid migration took place, involving different stratigraphic levels, according to their rheology and to the structural local setting. The age of this processes is constrained to the early upper Messinian phase, before the development of the Lago-Mare event. The major drawdown of the Mediterranean Sea recorded by the MES corresponds to a huge sea level drop: this most likely represents the primary trigger for fluid migration: the high depressurization experienced by the sedimentary column after the removal of at least 1 km of water column, could have likely favoured a catastrophic migration of overpressured fluids from below.