Selective alteration of arkose framework in Oligo-Miocene turbidites of the Northern Apennines foreland: impact on sedimentary provenance analysis

Abstract

Abstract Upper Oligocene-Middle Miocene turbidites of the Macigno and Cervarola Formations fill post-collisional elongate basins of the Northern Apennines foreland; they crop out axially for distances up to 300 km and reach a total thickness of 4000 m. Sandstones are arkosic with quartzose grains averaging 50% and fine-grained lithic clasts from 5 to 25%; K-feldspar and plagioclase abundances are similar, and rock fragments are dominated by igneous and metamorphic types. The Macigno sandstones reach a much higher diagenetic grade than the younger Cervarola sandstones. The latter (mature stage of diagenesis), have a clay mineral assemblage with ordered I/S interlayers and illite crystallinity (I.K.) averaging from 8.0 to 10.1, while in the Lower Unit of the Macigno Formation clay crystallinity averages 3.8 (Kübler’s epizone). The evolution of diagenetic processes is unusual because of the abundance of unstable grains. Physical compaction is the main cause of pore reduction. Post-depositional mineralogical changes are relevant in both formations; ignoring authigenic overgrowths and late calcite replacements, they commonly affect 10–20% of the framework grains. Several mineralogical-textural changes influence provenance determinations, including: dissolution processes, which almost totally eliminate unstable species, such as amphibole and pyroxene, originally dominating the heavy mineral population; replacement processes, mainly represented by albite after plagioclase, and by clay or mica materials after unstable grains; intense deformation plus replacement of ductile or brittle grains with the authigenesis of chlorite, white mica and pseudomatrix.