Genesis and evolution of gypsum tumuli

Tumuli are hollow subcircular domes of the most superficial stratum of gyps um, principally found in outcrops of macrocrystalline gypsum. They vary fro m a few centimetres to several metres in diameter and reach maximum heights of a little more than 1 m. The relationships between the morphostatistical parameters that define these formations are: h = r/3 and e = r/9, where h is the elevation of the raised layer, e its thickness and r the mean radius . Their genesis has caused some controversy over the involvement of phenome na such as hydration of anhydrite, or tectonic processes capable of explain ing this folding. This paper shows their genesis linked to the dissolution of macrocrystalline gypsum and reprecipitation of microcrystalline gypsum w ithin the same gypsiferous layer. It has been calculated that to reach the theoretical saturation within the few centimetres' thickness of the cap of the tumulus, water infiltration velocities are required of between 0.002 cm s(-1) for an uplifted stratum of 2 cm thickness, and 0.03 cm s(-1) for 30 cm thickness. These velocities imply the existence of very slow rates of in filtration and/or capillary movement of water within the gypsiferous layer. The secondary microcrystalline gypsum is precipitated in the intercrystall ine and intracrystalline voids of the gypsum crystals, producing an increas e in porosity and associated volume that causes the doming of the gypsifero us layer. The development of tumuli is a cyclic process which is favoured b y a sequence of short wet and dry intervals which, in turn, facilitate the almost simultaneous processes of dissolution and precipitation. These condi tions predominate in arid and semiarid climates where intense evaporation c an occur suddenly following sporadic infiltration. Copyright (C) 1999 John Wiley & Sons, Ltd.