Three-dimensional fracture pattern of the Marnoso Arenacea Formation in the Palazzuolo anticline, Northern Apennines, Italy

Abstract

This Ph.D. thesis was performed in the framework of the FRASI project, funded by Petrobras, with the aim of investigating the pattern of fracturing in folded siliciclastic turbidites. The study anticline is exposed in the Romagna Apennines and is known in the literature as Coniale, Diaterna, or Palazzuolo anticline. The research was done with a multidisciplinary approach that included field work, photogrammetric surveys, and numerical modelling. In the photogrammetric study of the vertical cliff exposed along the Santerno river, attention was paid to the statistical study of the joint spacing parameter, because the distribution laws of this important parameter remains a controversial topic in the literature. Our results show that the gamma distribution is the best density function for describing joint spacing, and that fracture clusters and corridor can form before spacing value patterns reach a normal distribution, as previously proposed. The studied area is characterized by two orthogonal bed perpendicular sets of fractures, which are parallel and orthogonal to the fold hinge. To study the influence of the structural controls on fracture attributes, the two sets were investigated in different structural positions within the fold, through the linear scanlines method. The Fracture Spacing Index (FSI) was used to compare the state of fracturing in different structural stations. Results show that the hinge-parallel joints and veins can be interpreted as folding-related features whereas the hinge-perpendicular ones may have formed either before or during folding. Folding-related strain is higher in the central transect of the fold compared to the northern periclinal termination, where the anticlinal crest is the most deformed fold sector. During a research period of almost three months in the research center of Petrobras in Rio de Janeiro (Brazil), a 3D geological model of the fold was developed by using Petrel software. The 3D geological reconstruction of the Palazzuolo anticline, was then used as input for numerical modelling of folding-related strain with Dynel3D. Field evidences and numerical model results were integrated to explain the structural evolution of the fold. In particular, our data show that the “classical” interpretation of an anticline formed above a thrust detached at the base of the Marnoso Arenacea formation is not able to explain numerical results. It follows that a much deeper position of the basal décollement has to be assumed to allow matching numerical model predictions with field structural evidence.