Lateral ferruginous groundwater transfer as the origin of the iron crusts in caves: A case study

The iron crusts appearing in all sections of the lower gallery of the Majada del Cura cave (Soria, Spain) consist of iron oxides, hydroxides, and to a lesser extent, manganese; the predominant mineral is goethite. The genesis of goethite is associated with the hydrogeomorphological evolution of the hypogeum river, which has received, upstream from explored cavities, lateral transfer of slightly ferruginous groundwater from the anaerobic environment of the Utrillas facies. The Utrillas facies connects to the cave only through the cave's lower gallery but not from the high gallery. Based on preexisting knowledge of the geomorphological history and the decrease in karstic base level, this connection has existed since at least the Middle Pleistocene. The torrential character of the river and the aerobic conditions cause precipitation by oxidation of Fe+2 with little bacterial participation, and the formation of mineral species is compatible with this oxidizing environment.