Sandstone diagenesis and petrophysics of San Mauro formation (Cilento group), Southern Apennines, Italy

The Cilento Group (Langhian-to-Tortonian) is an upward thickening and coarsening turbiditic megasequence (1200-2500 m) of the Southern Apennines foreland region that unconformably overlain the Lucanian oceanic terranes. In the studied stratigraphic section (Monte della Stella), we focus the post-depositional history of the upper portions of the Cilento Group, the San Mauro Formation (SMF). The lower portion of the SMF, 300–500 m in thickness, consists of thin bedded sandstone and thick mudstone and marls of the fan fringe and basin plain turbidite facies associations. Several intrabasinal carbonaticlastic turbidites and a coarse grained volcaniclastic layer are interbedded with distal fan turbidites. An impressive intrabasinal carbonate turbidite megabed, of 65 m in thickness, separates the lower and the upper portions. The upper portion of SMF, about 1100-1200 m in thickness, consist of sandstone lobe turbidites and thin fan-fringe facies to channel-mouth and channelized turbidite sandstone and conglomerate for over 700 m in thickness, passing upward to, 400–500 m in thickness, coarse-grained sandstone and conglomerate turbidite beds of the channel mouth and channelized facies association. A second impressive intrabasinal carbonate megaturbidite bed, 35 m in thickness, is interbedded within inner channelized turbidite facies (Critelli et al., 2011). Sandstones are quartzolithic, volcanolithic and quartzofeldspathic, whereas hybrid arenites and calcarenites characterize the carbonate megabeds. Several diagenetic processes reducing porosity and permeability, such as compaction and cementation, occurred within the SMF. Framework grains are generally compacted, as testified by the common deformation of ductile grains (mainly micas) and detrital matrix. Authigenic minerals occur as both pore-filling cements or replacement on detrital grains. The main cements are: 1) carbonate, such as calcite and less dolomite, occurring mainly as replacement and also as pore-filling cement; 2) small amount of authigenic quartz represented by little overgrowths or mosaic quartz; 3) phyllosilicates, mainly developing in the upper part of SMF as pore filling chlorite cement or as small and incomplete illite coatings on skeletal grains; 4) Fe-oxides occurring as patchy or pore-filling cements. The pore system consists of inter-intra-granular pores with a small pore radius and fractures. The porosity and permeability values are highly variable, and are on average higher in the lower SMF. The relationship between the compactional porosity loss (COPL) and the cementational porosity loss (CEPL) testifies that compaction is the main reduction porosity process for both upper and lower SMF sandstones. Burial analysis (Corrado et al., 2019) suggests two phases of intense subsidence, interpreted as the result of syn-orogenic extension at shallow crustal levels intermitted by periods of low sedimentation rates, linked to gravitational instability of a vertically growing orogen.