Columnar joints in the Patino Formation sandstones, Eastern Paraguay： a dynamic interaction between dyke intrusion, quartz dissolution and cooling-induced fractures

The Patiho Formation sandstones, which crop out in Aregud neighborhood in Eastern Paraguay and show columnar joints near the contact zone with a nephelinite dyke, have as their main characteristics the high pro- portion of syntaxial quartz overgrowth and a porosity originated from different processes, initially by dissolu- tion and later by partial filling and fracturing. Features like the presence of floating grains in the syntaxial cement, the transitional interpenetrative contact between the silica-rich cement and grains as well as the intense fracture porosity are strong indications that the cement has been formed by dissolution and reprecipita- tion of quartz from the framework under the effect of thermal expansion followed by rapid contraction. The increase of the silica-rich cement towards the dyke in association with the orthogonal disposition of the columns relative to dyke walls are indicative that the igneous body may represent the main heat source for the interstitial aqueous solutions previously existing in the sediments. At macroscopic scale, the increasing of inter- nal tensions in the sandstones is responsible for the nucleation of polygons, leading to the individualization of prisms, which are interconnected by a system of joints, formed firstly on isotherm surfaces of low tem- perature and later on successive adjacent planes towards the dyke heat source.