Mud volcanoes and synsedimentary deformation structures in Miocene lacustrine deposits over evaporites: Evidence of seismite interpretation

In continental tectonic basins, evaporite paleokarst and soft sediment deformation structures can occur together. However, distinguishing between the effects of tectonism and karst-related dissolution is challenging. Highly deformed beds overlying an evaporite paleokarst in the Miocene deposits of Madrid Basin have been studied in exceptional and laterally continuous outcrops. The paleokarst overburden is mudstone-dominated and displays closely packed fluid seepage structures in the form of mud breccias, tubes, fractures and veins networks connected to surface. At fault intersections, these features dip inwards decametric bowl-shaped to funnel-like structures bounded by circular fractures. This supports that these depressions represent collapsed mud volcanoes built at the surface by the fluid seepage features. They developed in periods of high pore pressures favoured by compressional regimes. Associated fluidization structures including in situ breccia, plastic intrusions, convolute and load forms support that earthquakes are the triggering mechanism.

The network of conduits is lithified by carbonate cements, including slickenfibers of aragonite, as well as by oxides of rare earths and iron‑manganese. The calcite and aragonite cements are highly depleted in ¹⁸O in comparison with the hosts suggesting overpressured aquifers as the source of freshwater fluids.

This analogue provides clues to identifying ancient, sagged beds as evolved fluid seepage systems and help to recognizing seismic events in lacustrine environments. Interpretations of sagging structures, common in worldwide-evaporitic paleokarst as purely dissolution-driven should be revisited considering the potential tectonic controls behind their rapid and extensive formation.