Morphometry and morphology of the gypsum tumuli from the Sorbas karst region, SE Spain

Abstract

In the Sorbas region of Spain, the gypsum karst is known for occurrences of gypsum tumuli, the peculiar weathering landforms that appear on gypsum rocks in semi-arid areas. Tumuli are small-scale domes (diameter: < 15 m, height: < 1.35 m) with empty internal chambers formed by local detachment and uplift of the surface layer of gypsum bedrock. Their origin is attributed to cyclic wetting and drying and expansive re-precipitation of gypsum in near-surface voids and pores of the rocks. Morphometric and morphological analyses of these gypsum landforms were conducted to verify and supplement data regarding their morphology. Based on linear regression equations, the geometry of the tumuli was analyzed quantitatively using pairs of the following parameters: horizontal long and short axes, relative height, thickness of the uplifted rock layer, height of the internal chamber, and others. The tumuli are limited in size – they rarely exceed 4.0 m in horizontal extension and 0.5 m in height above the surface of the surrounding terrain. Regardless of their size, they tend to preserve a rather constant plan shape, from circular to elliptical. The relative height of the tumuli is as a rule 7 to 8 times smaller than the length of their long horizontal axis, and ca. 6 times smaller than the length of the short axis. Aerial photogrammetry was used to estimate the destruction degree of gypsum tumuli, which helps to interpret their morphological changes during erosion. The appearance of a hole at the top of the form leads to the development of narrow-crater and then wide-crater tumuli. The side inlet opening leads to the formation of half-moon and then sickle shaped tumuli.