Structure and mineralogy of bedding-parallel fibrous veins (‘beef’) in the Lagonegro basin succession, Southern Apennine, 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). Bedding-parallel fibrous veins ('beef') are found at the base of Lagonegro sequence in the Southern Apennine of Italy, within interbedded shales and limestones of Triassic age. Structural and microscopical (SEM) analyses have been used to constrain the kinematic and growth 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 most of the veins separate beds of contrasting lithology, particularly limestone and shale, some of them are located in apparently homogeneous shale. Veins mostly consist of calcite, in fibrous to columnar crystal oriented perpendicular to the vein walls. At the optical microscope, the calcite fibers show twinning developed at low temperature, consistent with Type I (BURKHARD, 1993; FERRIL, et alii, 2004). The commonest mineral in the fibers is calcite, but also quartz, minor pyrite and iron oxides/hydroxides may be present. Vein opening is interpreted to be the result of vertically oriented stressed generated by hydraulic overpressuring. The possible causes of overpressure are a matter of debate. The most popular would be mechanical compaction, hydrocarbon generation and the onset of horizontal compressive stress. Vein-wall normal calcite columns and fibers provide a more convincing evidence that bedding parallel veins formed before the onset of contractional deformation in the study area. The main evidences are that bedding-parallel fibrous veins appear as the first structures formed in the Lagonegro sediments, since they are disrupted by all structures related to the later contractional deformations and mostly appear coeval to other overburden structures, such as stylolites and bed-perpendicular veins. We suggest that the bedding-parallel fibrous veins ('beef') formed during burial in the Lagonegro basin, when sediments were already compacted and lithified. They formed before the onset of Miocene folding and thrusting, marking the early stage of tectonic inversion of the Lagonegro Basin. Future studies of fluid inclusions and stable isotopes may help to constrain better the timing and depth of formation of the 'beef'.