FAULT ZONE CHARACTERISTICS IN TIGHT SILICICLASTIC TURBIDITES: AN OUTCROP STUDY ON THE MACIGNO FORMATION, TUSCANY, ITALY

Faults in siliciclastic rocks are characterized by great variability of fault zone architecture and relative permeability properties. This is because siliciclastic rocks (i.e. turbidite successions) are commonly represented by alternating layers of various thickness and grain size, forming successions with contrasting mechanical properties. For example, the alternation of sandstone and mudstone layers is responsible for the simultaneous occurrence of brittle and ductile deformation, as “clay smear structures.”

We present the results of field and laboratory analyses of the deformation of the Macigno Formation, which crops out along the western coast of Tuscany. Here, the Macigno Formation is represented by tight, Late Oligocene foredeep siliciclastic rocks dominated by turbidite sandstones with minor siltstones and black shales. These rocks are heavily fractured and affected by several sets of normal to strike-slip faults with different degrees of development (from incipient simple discontinuities with mm-offset to fault zones with tens of meters offset and well-developed fault cores and damage zones). We have documented varying fault zone properties in different turbidite lithofacies associations.

The thickness of sandstone and siltstone/shale beds varies from tens of centimeters up to 4 meters and from several centimeters to tens of centimeters, respectively. Thin section and 3D X-ray Synchrotron microtomography allow us to characterize grains, especially their geometrical and morphological properties (e.g. size, shape, specific surface area). We document how thickness, grain size and mechanical properties of alternating beds strongly control fault zone architecture, especially in terms of fault core composition and texture, damage zone thickness and fracture intensity and height. Furthermore, we observe how the development of clay smear structures is enhanced by the presence of thin siltstone/shale layers alternating with sandstone ones.