Palaeo-fluid Flow History of an Extensional Fault System from Carbonate Concretions in Shallow Marine Sediments, Italy

We present a multidisciplinary study on the relationships between the structural architecture of an extensional fault system in poorly lithified shallow marine sands and the associated pattern of diagenetic carbonate concretions. Based on their shape, spatial distribution, cement texture and chemistry and isotopic signature, carbonate concretions are grouped into (i) tabular concretions developed within the fault zones, and (ii) elongate and coalescent strata-bound concretions formed adjacent and mostly parallel to fault zones. Tabular concretions formed during early diagenesis in the vadose mixing marine-meteoric zone, possibly during coseismic rupture propagation and, in the interseismic periods, due to slow capillary suction along the low-permeable fault zones. On the other hand, elongate concretions formed after regional uplift and during telodiagenesis by precipitation from meteoric, phreatic water flowing parallel to the fault zones which acted as hydraulic barrier. The concretion patterns record the evolution of fluid flow orientation and the changing chemistry of fault-zone fluids that were fundamentally driven by the the fault zones architecture during two major diagenetic stages.