Bedding-parallel veining and fluid overpressure at the base of the Lagonegro sequence, Southern Apennines, Italy

Hydraulic fractures along bedding planes are common in compacted shale sequences of numerous sedimentary basins (MARSHALL, 1982; AL-AASM, et alii, 1992; PARNELL et alii, 2000; SUCHY et alii, 2002; COBBOLD et alii, 2007; RODRIGUEZ et alii, 2009). Where mudrocks and shale predominate, bedding-parallel fibrous veins ('Beef') are found at the base of Lagonegro sequence in the Southern Apennine of Italy. Structural and microscopical (SEM) analyses have been used to constrain the kinematic history of the bed-parallel fibrous calcite veins outcropping in a key sector of the high Agri Valley. They represent the earliest generation of vein found in the rocks. The veins range from 1 to 8 cm. in thickness. Although some veins separate beds of contrasting lithology, particularly limestone from superimposed shale, the majority are located in apparently homogeneous shale. Veins mostly consist of calcite, in fibrous to columnar crystal oriented perpendicular to the vein walls, at optical microscope, the calcite fibrous also seem to be twinned in Type I (BURKHARD, 1993; FERRIL, et alii, 2004). The commonest mineral in the fibres is calcite, other mineral present in the veins are quartz and minor pyrite, iron oxides/hydroxides. Vein opening is interpreter to be the result of vertically oriented stressed generated by hydraulic overpressuring. The possible causes of overpressure are a matter of debate. The popular would be mechanical compaction, hydrocarbon generation and the onset of horizontal compressive stress. However, vein-wall normal calcite columns and fibres provide a more convincing evidence for a pre-folding rather than an early-folding origin of bedding parallel veins of fibrous calcite in the study area. We suggest that the bedding-parallel fibrous veins ('beef') formed during burial processing, in sediments that were compacted and lithified. They formed before the onset of Miocene folding and thrusting, marking an early stage of tectonic inversion of the Lagonegro Basin. Future studies of fluid inclusions and stable isotopes may help to constrain better the time and depth of formation of the 'Beef'.