From axial-parallel to orthogonal groundwater flow during fold amplification: insights from carbonate concretion development during the growth of the Quattro Castella Anticline, Northern Apennines, Italy

This contribution is based on a multidisciplinary field and laboratory study of carbonate concretions developed in poorly lithified Quaternary, syn-kinematic sediments along the Quattro Castella Anticline, Northern Apennines (Italy). Studied concretions consist both of tabular (parallel to bedding) and elongate single to coalescent concretionary bodies oriented at different angles to bedding throughout the exposed stratigraphic succession. Concretion dimensions range from a few centimetres for single elongate concretions, up to several metres for tabular to coalescent ones. Field observations and petrophysical data indicate that concretions developed preferentially in sediments characterized by 90–290 µm mean grain sizes and permeability spanning from 7×10² to 7×10⁴ mD. Carbon and oxygen stable isotope analysis in conjunction with petrographic investigations indicate that the precipitation of concretionary calcite occurred in a meteoric vadose realm during early eogenesis, and subsequently in a meteoric phreatic environment. Diagenetic data and concretion patterns in syn-tectonic sediments suggest they formed during lateral propagation of the anticline that, in turn, promoted a change in the local topographic-hydraulic gradient from fold axial-parallel to fold-orthogonal. Accordingly, the integrated analysis of carbonate concretions can provide a useful tool to unravel palaeo-fluid flow history and, then, to predict fluid circulation patterns in folded siliciclastic rocks.